Masters thesis of applied science pb (ii) removal from aqueous solution by biochar produced from giant reed

Over the past years, the interest in production of biochar from different biomass samples for the purpose of treating aqueous phase contaminated with toxic trace metal has gained popular

Pb (II) Removal from Aqueous Solution by Biochar Produced

from Giant Reed

A thesis submitted in fulfilment of the requirements for the degree of Master of Applied Science

Declaration

Declaration for candidates submitting a thesis

I certify that except where due acknowledgement has been made, the work is that of the author alone; the work has not been submitted previously, in whole or in part, to qualify for any other academic award; the content of the thesis is the result of work which has been carried out since the official commencement date of the approved research program; any editorial work, paid or unpaid, carried out by a third party is acknowledged; and ethics procedures and guidelines have been followed

I acknowledge the support I have received for my research through the provision of an Australian Government Research Training Program Scholarship

Eric York

Date: 18 November 2020

Dedication

I would like to dedicate this thesis to my late dad, who inspired my interest in science and motivated me to undertake this research

Acknowledgements

If it had not been the Lord on my side, let Israel say (Psalm 124:1) My greatest thanks and appreciation go to the almighty God for the love, protection and numerous blessings He has bestowed on me Father, my mouth will continually sing your praises This work would not have been possible without my supervisors A/Prof Samantha Richardson and Dr James Tardio, whose guidance and support were unwavering throughout the duration of my studies

I would also like to thank all my colleagues and collaborators, who have contributed in some way towards me completing my master’s thesis

I also thank the Commonwealth of Australia for an Australian Postgraduate Award I acknowledge RMIT University for the Research Stipend Scholarship Award for this Research Lastly, I would like to thank my partner, Florence Appiah, whose unconditional support and patience enabled me to complete this degree

Table of Contents

Declaration i

Dedication ii

Acknowledgements iii

List of Figures vi

List of Tables vii

Abstract 1

Chapter 1: Introduction 3

1 Background 3

1.1 Trace Metals 6

1.2 Sources of Trace Metals in Aqueous Solution 7

1.3 Lead 9

1.4 Biochar Production 10

1.5 Feedstock for Biochar Production 11

1.6 Biochar Production by Slow Pyrolysis 15

1.7 Giant Reed (Arundo donax) 17

1.8 Arundo donax: Weed Potential 21

1.9 Use of Arundo donax in Constructed Wetlands 23

1.10 Generation of Biochar from Giant Reed 26

1.11 Use of Biochar for Removal of Trace Metals from Aqueous Phase 28

1.12 Mechanisms of Adsorption by Biochars 29

1.13 Adsorption Isotherm Models 30

1.15 Adsorption Kinetics 34

1.16 Properties of Biochar 34

1.17 Factors Affecting Adsorption of Contaminants on Biochar 35

1.18 Management of Spent Biochar 37

1.19 Potential Biochar Improvement Techniques 37

1.20 Environmental Impact of Biochar use for Trace Metal Decontamination 38

1.21 Sustainable Use of Biochar 39

1.22 Potential Application of Biochar 39

1.23 Conclusion 40

Chapter 2: Experimental 40

Materials and Methods 41

2.1 Chemicals 41

2.2 Sample Preparation 41

2.3 Production of Biochar by Slow Pyrolysis 42

2.4 Biochar Properties 42

2.6 Measurement of Adsorption of Pb 2+ with Microwave-Plasma Atomic Adsorption Spectrometer (MP-AES) 51

2.7 Calculation of Adsorption Efficiency of Trace Metal 52

2.8 Data Modelling using the Langmuir, Freundlich and Temkin Equations 53

2.9 Statistical Analysis 54

Chapter 3: Characterisation of Biochars and Preliminary Adsorption Studies 55

3.1 Introduction 56

3.2 Experimental 57

3.3 Results and Discussion 58

3.4 Conclusions 70

Chapter 4: Adsorption of Pb(II) on Biochars – Studies on the Influence of some key Adsorption Test Parameters 71

4.1 Introduction 71

4.2 Experimental 72

4.3 Results and Discussion 72

4.3.5 Rates of Reaction: 82

Pseudo First Order Model 82

4.4 Conclusions 84

Chapter 5: Conclusions and Recommendations 86

5.1 Conclusions 86

5.2 Recommendations 87

5.3 Summary of Study 88

References 89

Appendix I: 102

List of Figures

Figure 1.1 Trace metal distribution 7

Figure 1.2 Source of trace metals in the aqueous phase 9

Figure 1.3 (a) Biomass feedstock for production of biochar; (b) biochar produced from the feed stocks 12

Figure 1.4 Slow pyrolysis set- up for biochar production 16

Figure 1.5 Illustrations of giant reed 27

Figure 2.1 (A) Raw biomass of giant reed; (B) Dry biomass of giant reed 41

Figure 2.2 Production of biochar by slow pyrolysis 42

Figure 2.3 Perkin Elmer 2400 Carbon, Hydrogen, Nitrogen, Sulphur and Oxygen Analyser 43 Figure 2.7 Perkin Elmer ATR Fourier Transformed Infra-Red Spectroscopy 46

Figure 2.9 500MHz Agilent DD2 nuclear magnetic resonance spectrometer 47

Figure 2.10 HANNA pH meter 48

Figure 2.11 Centrifuge tube on orbital shaker, Chiltern model 50

Figure 2.12 Agilent 4200 microwave plasma - atomic emission spectrophotometer 52

Figure 3.1 Weight loss of GR 500, GR 300 and GR biomass using TGA800, Perkin Elmer 59 Figure 3.2 Scanning electron micrograph of giant reed biochar produced at (A) 300 ℃ and (B) 500 ℃ 60

Figure 3.3 (A) BET Isotherm Linear Plot for GR 300 ℃; (B) BET Isotherm Linear Plot for GR 500 ℃ 61

Figure 3.4 Fourier transform infrared spectroscopy of giant reed biochar produced at 300 ℃ and 500 ℃ 63

Figure 3.5 Carbon-13 NMR of biochar produced at 300 ℃ 64

Figure 3.6 Carbon-13 NMR of biochar produced at 500 ℃ 65

Figure 3.7 XRD patterns of GR 500 and GR 300 Major diffraction lines for specific phases are marked 66

Figure 3.8 SEM-EDS analysis of GR 300 ℃ 67

Figure 3.9 EDS spectra for selected regions of GR 300 68

Figure 3.10 Pb2+ adsorption versus time for GR 300 and GR 500 69

Figure 4.1 Adsorption capacity versus initial Pb(II) concentration 73

Figure 4.2 Adsorption capacity versus solution pH 75

Figure 4.4 Adsorption capacity versus biochar particle size 76

Figure 4.5 Ce/qe versus Ce for GR 300 77

Figure 4.6 Ce/qe versus Ce for GR 500 78

Figure 4.7 Log Qe versus Log Ce for GR 300 79

Figure 4.8 Log Qe versus Log Ce for GR 500 80

Figure 4.9 Schematic diagrams for cooperative adsorption 81

List of Tables

Table 1.1 The application of biochar produced from different feedstock and techniques in

aqueous solution 13

Table 1.2 Advantages and disadvantages of A donax with respect to its use as a plant in constructed wetlands for wastewater treatment 24

Table 1.3 Environmental impact of the use of biochar and activated carbon 38

Table 1.4 Economic impact of biochar and activated carbon for trace metals adsorption 38

Table 2.1 Experimental design for adsorption and characterization of biochar produced in this study 48

Table 3.1 Elemental analysis of giant reed biochar 58

Table 3.2 Total surface area of biochar as attained by BET analysis 62

Table 3.3 pH of biochars from giant reed in deionised water 67

Table 4.1 Comparative results for Pb(II) removal using biochars prepared from different types of biomass 84

The sorption of Pb (II) by biochar produced from giant reed at different temperature profiles was studied Two biochars, produced at a temperature of either 300 ℃ or 500 ℃, were studied The prepared biochars were characterized using the following: an X-Ray Diffractometer; BET Surface Area and Pore Analyser; Scanning Electron Microscope; Carbon, Hydrogen & Nitrogen Analyser;13C Nuclear Magnetic Resonance Spectrometer; and Thermogravimetry Analyser

The results showed that biochar produced at 300 ℃ was more effective than that produced at 500 ℃ in the removal of lead from aqueous solution Solution pH showed a strong effect on the adsorption ability of giant reed biochar produced at 300 ℃ (GR 300 ℃)

to adsorb Pb (II) ions The maximum adsorption capacity (19.2 mg/g) was found to occur under the following conditions: Temp 25 ℃, Pb (II) conc 40mg/L, stirring rate 60 rpm, biochar dosage 0.1g / 100 mL, 24 h The equilibrium data were fitted to Freundlich and Langmuir models The Freundlich Isotherm gave the best fit for GR 300 ℃ with a value of 0.88 FTIR analysis and batch experiments results suggested that Pb (II) adsorption mechanisms were dominated by complexation with active surface groups, precipitation and cationic exchange Experimental model results suggested that giant reed- derived biochar has

a good adsorption capacity for Pb (II) in aqueous solution compared to other plant biochars reported in the literature

The application of biochar in remediation of ground waste water is still warranted and future research should consider that the outcome of remediation of ground waste water could

be achieved with biochar Under a holistic approach to assessing remediation options, the environmental outcome of the application of giant reed biochar will help equip researchers and industry alike in their endeavour to reduce the burden posed by contaminated groundwater into the future

Chapter 1: Introduction

1 Background

Pollution is regarded by many in modern, western industrialized societies as a series

of current environmental issues Of the many types of chemicals / compounds that are known, pollutants include toxic heavy metals such as cadmium, lead, mercury and arsenic, which are

of significant concern This is made evident by the fact that these toxic heavy metals all appear on the World Health Organisations list of the top ten chemicals of major public concern (WHO 2011) As a long-standing concern, pollution does not receive the same political or media attention as climate change, biodiversity or manifestations of genetic research but pollution investigation into anthropogenic (human) impacts on local, regional and global scales has the power to alter lifestyle and economic production (Demirbas 2004)

In most third world countries, untreated agricultural, residential and industrial pollutants are discharged into open streams and rivers leading to gross pollution of the natural aqueous phase (Ahmad, Rajapaksha et al 2014, Baltrėnas and Baltrėnaitė 2020) Wastewater effluent is normally composed of multiple harmful compounds that pose a huge risk to human health and to the aquatic habitat Therefore, a low-cost solution to remediate wastewater is an indispensable need in these countries Adsorption using low cost material which is locally available should be an effective and economical means to remove various contaminants from the aqueous phase (Baltrėnas and Baltrėnaitė 2020)

Trace metals such as lead have become a worldwide issue due to resulting toxicity to aquatic organisms and when ingested by humans(Chowdhury, Mazumder et al 2016, Nagel, Cuss et al 2020) Although lead is often considered to be one of the most toxic metal pollutants, it is important to recognize that it is a natural substance (Garrett 2000) In most cases, lead becomes a pollutant when human activity releases it from rocks (i.e from ores, mainly through mining and smelting) (Garrett 2000, ATSDR 2007) Sources of lead moving into the aquatic environment include mining, agriculture, fossil fuel combustion, battery manufacturing and metal plating (Bradl 2005).Lead can cause disruption of vital organs in aquatic organisms when its concentration increases beyond permissible limits (Chandurvelan, Marsden et al 2015) This has called for the treatment of lead-contaminated aqueous phase through low-cost means (Li, Dong et al 2017)

Biochar is a porous carbonaceous solid material that is produced in pyrolysis units such as kiln reactors and furnaces by thermal decomposition of the precursors in limited air

or in an inert environment(Wang, Jiang et al 2020) Another definition describes biochar as a pyrogenic black carbon produced from biomass such as wood and seed when heated in an oxygen-depleted environment and is rich in minerals, such as Mg, K and Ca (Lehman and Joseph 2009, Ahmad, Rajapaksha et al 2014, Schmidt, Kammann et al 2014) Biochar production is increasing gradually with soil mitigation, CO2 sequestration and control of pollutants such as trace metals (Kajitani, Tay et al 2013) The high carbon content, porosity, cationic exchange capacity and large surface area of biochar makes it a suitable product for various application such as climate mitigation, energy, heat and soil amendment (Ahmad, Rajapaksha et al 2014, Adekiya, Olayanju et al 2020)

Over the past years, the interest in production of biochar from different biomass samples for the purpose of treating aqueous phase contaminated with toxic trace metal has gained popularity due to the low cost in getting the precursor for its production However, little has been done regarding converting invasive biomass, such as giant reed and others into biochars for treating aqueous phase that is contaminated with toxic pollutants (Kumar, Singh

et al 2020)

Conventional water treatment techniques such as ion-exchange and chemical precipitation have been reported by many in the literature as being efficient for lead removal from polluted aqueous phase but it is not cost-effective (Demirbas 2008).The limitations of these methods include the production of toxic sludge and a high energy requirement (Barakat 2011) Adsorption by commercially produced activated carbon has been widely used for the removal of lead in water treatment processes (Martin 1980, Cao, Ma et al 2009, Liang, Guo

et al 2009, Liu and Zhang 2009, Shin, Lee et al 2020) However, the cost of raw materials for its production makes it disadvantageous to use (Zhou, Chen et al 2017) Krasucka et al (2021) reported a biochar production cost of 350–1200 USD/ton, which is lower than the value for activated carbon (USD1100–1700 per ton) Some studies showed lower biochar production prices as low 90 USD/ton in the Philippines Higher values were reported for those produced in the UK at 8850 USD /ton (Ahmed et al 2016) From an economic point of view, the price of the biochar listed above depends on the effectiveness of the generated material It is also an important factor correlated with the biochar cost Several aspects related

to biochar production, such as the pyrolysis production conditions, selected feedstock, and transport have a considerable effect on the production costs Homagain et al (2016), through life cycle assessment reported that most of the production cost is related to the pyrolysis

process He also emphasized that the cost is related to the storage, processing of the biochar and feedstock Also, Keske et al.(2020) the evaluated economic analysis of biochar production from Canadian black spruce forest They reported that the pyrolysis stage is the most costly, that is 36% of the cost, followed by feedstock collection (12%) and transportation cost (9%) These resulted in a biochar cost of 762.20 USD/ton That is the reduction in the cost is taken to rely on the use of production technologies and the selection

of better low-cost materials The use of wastes aligns with the economy principles and the aims of “2030 Agenda for Sustainable Development” used by the United Nations (UN) for favouring the reuse of materials, avoiding the generation of waste and reducing carbon emissions,

Biochar has been found to be a promising material for the removal of inorganic contaminants such as trace metals from wastewater (Tan, Liu et al 2015, Ahmed 2016, Li, Dong et al 2017, Sizmur, Fresno et al 2017), particularly by methods of adsorption (Cao,

Ma et al 2009, Regmi, Moscoso et al 2012, Shahrokhi-Shahraki, Benally et al 2020), due to its surface area, mineral component and functional groups (Cha, Park et al 2016)

Previous research has shown biochar’s capacity in adsorbing trace metals such as copper, lead and mercury (Ameloot, De Neve et al 2013, Shen, Tian et al 2017, Wang, Huang et al 2020) For instance, Carrier, Hardie et al (2012) showed that biochar was effective as a commercially produced activated carbon when sugar cane was pyrolyzed in a vacuum pyrolysis unit Jenks (2014) added biochar from hardwood to sand in a ratio of 30-70

% to create a filtration medium for the removal of trace metals such as Cu and Zn from a point source and non-point pollution sources The results from the experiment showed a 92% reduction in Zn, 81% reduction in Cu, 94% reduced turbidity and 85% reduced suspended solids Biochar formed a small part of the project, but it showed the potential as an active component that can improve the system as a whole Cooks Jr (2014) showed that biochar produced from pine and oak was able to remove 46% of organic salts and 95% of hydrocarbons from fracking wastewater

The use of biochar in wastewater contaminated with toxic trace metals involves the exchange of existing ions in the biochar with trace metals in the aqueous phase This shows that there is a link between the content of biochar and its potential for removal of trace metals from the aqueous phase (Chen, Cui et al 2013) However, more research is needed to explore other low-cost biomass precursors for the production of biochar for removal of trace metals from the aqueous phase (Tan, Liu et al 2015, Li, Dong et al 2017)

Therefore, biochar is a promising novel material for metal removal from wastewater (Lehman and Joseph 2009) However, its application to real ground waste water is warranted due to the complexity of predicting the actual sorption capacity of an adsorbent for a target trace metal among other pollutants (Li, Dong et al 2017) Due to its large surface area and high abundance of functional groups, biochar may have a high metal sorption capacity, where the mechanism of sorption of trace metals may vary due to the intrinsic property of the specific target metal in its mono or multi-metal aqueous phase (Ahmad, Rajapaksha et al

2014, Park, Ok et al 2016, Li, Dong et al 2017)

In this study, we aimed to carry out an investigation into adsorption characteristics of

biochar produced from giant reed (Arundo donax) for adsorption of lead in aqueous phase

Arundo donax is a potential bioenergy crop that can be used to produce biochar or activated carbon(Sagehashi, Fujii et al 2010, Sun, Yue et al 2013, Hou, Huang et al 2016).According

to Angelini, Ceccarini et al (2009),giant reed has been found to be high above-ground biomass, which yields (over a period of 10 years) with a better productive performance as compared to Miscanthus With such characteristics, it can be proposed as a biomass with a significant environmentally compatible contribution to energy needs

1.1 Trace Metals

Trace metals are a set of elements such as lead, chromium, cobalt and copper which are required in low amounts in animals and plants but become toxic at high levels (Srivastava and Majumder 2008, Mao, Liu et al 2020) The term “Trace metals” has been preferred of late as a better definition regarding the toxicity of these elements in place of the term “heavy metals”(Fu and Wang 2011) Alternatively, trace metals can be defined as set of elements which have high atomic number and specific gravity greater than 5 kg m-3 (Srivastava and Majumder 2008) Unlike organic compounds, trace metals cannot be broken down to less harmful products (Fu and Wang 2011)

Effluents from industries and residual domestic waters are the main source of trace metal pollution existing in water streams (Förstner and Wittmann 2012, Joseph, Jun et al 2019) Generally, industrial waste contains trace metals such as cadmium arsenic, copper, lead and mercury (Dunbabin and Bowmer 1992).Trace metals go through different stages and ultimately end up in waste (Mulligan, Yong et al 2001) However, the distribution of the trace metals may vary when they are present at different concentration levels in the aqueous phase (Hart 1982) Trace metal removal efficiency can vary depending on the solubility of the metal in the aqueous phase (Bailey, Olin et al 1999) Numerous studies have been

conducted on the analyses of the impact of trace metal contamination of water bodies (Galloway, Thornton et al 1982, Dunbabin and Bowmer 1992, Förstner and Wittmann 2012)

In mammals, trace metal toxicity can result in renal impairment, skin blisters, anaemia, tissue edema and liver dysfunction (Nriagu 1990, Goher, Ali et al 2019) Therefore, it is vital

to reduce the level of trace metals to levels that are safe for water sustainability and for the survival of living species in them This has created a global concern for the water community worldwide to find an efficient way of removing trace metals to the limits acceptable for preventing toxicity

Figure 1.1 Trace metal distribution

(retrieved from Amiard Alloway (2012)

1.2 Sources of Trace Metals in Aqueous Solution

Trace metals are naturally found in the environment However, due to anthropogenic activities, their concentrations have increased in many aquatic environments (Alloway 2012) Some of the major processes that contribute to trace metal accumulation in the aquatic environment are run-off from metal contaminated waste, soil erosion and the application of fertilizer and agrochemicals (Nriagu 1990, Mishra, Bharagava et al 2019) All these sources leach trace metals that are eventually washed off into nearby aqueous environments Anthropogenic activities such as electroplating and battery manufacturing have contributed to the increase of trace metal concentrations in the environment (Dehdashti, Amin et al 2020) Poor disposal of domestic wastes and mining activities have also contributed to contamination of aqueous phases polluted with toxic trace metals and Australia is no exception (ABC 2014) According to the ABC (2014), the nearby environment in Costerfield,

Australia, where antimony is mined, has been polluted with trace amounts of antimony beyond permissible limits as per the national Australian drinking water quality guidelines (NHMRC 2015)

Effluents from nuclear generating facilities in some developing countries have been reported to be one of the major sources of trace metal discharge into the environment (Pacyna, Scholtz et al 1995, Diarra and Prasad 2020) The large amount of wastewater generated from operations in industries are usually not free from trace metal contamination and are consequently discharged into groundwater and surface water (Chowdhury, Mazumder

According to Duruibe, Ogwuegbu et al (2007), human exposure to trace metals occurs from direct exposure to polluted food and water Some trace metals such as Pb (II), As (III) and Sb (V) can be lethal even when ingested at low concentrations (around µg/ L) into the human body (Jaishankar, Tseten et al 2014) Studies have shown that diseases such as those resulting in skeletal damage in humans can result from ingestion of toxic trace metals (Järup 2003) Vital organs such as the kidney can easily mal-function when trace amounts of these toxic chemicals accumulate, sourced from food such as vegetables that are consumed

on a daily basis (Patočka and Černý 2003) Trace metals are toxic to humans due to the inability of the body to metabolise them and hence they readily bioaccumulate in the tissues (Akpor and Muchie 2010)

Figure 1.2 Source of trace metals in the aqueous phase

(Adopted from Frank Galadima and Garba (2012)

1.3 Lead

Lead (Pb) was one of the first known metals, which is very resistant to corrosion so it does not biodegrade, instead, it can bioaccumulate in the human body where it serves no beneficial purpose (Philip and Gerson 1994)

A significant amount of Pb (II) has been released into the environment For example,

in 1998, Pb (II) was suggested to have been released more than any other heavy metal (Nriagu 1998) In the environment, Pb (II) generally exists in a +2 oxidation state in the form

of galena (PbS), anglesite (PbSO4) or Cerussite (PbCO3) Pb (II) is relatively insoluble at common environmental pH and readily adsorbs to the surface of particles (Morrison and Murphy 2010)

Speciation of lead in an aquatic environment is mainly influenced by chlorates, carbonates and organic occurring natural ligands (Zuluaga Rodríguez, Gallego Ríos et al 2015) The proportion of lead in the inorganic complexes is mainly determined by the pH of the water This complex gradually increases in concentration with increasing total lead metal loading in sediments, which pose a potential threat to aquatic biota and benthic organisms in the aquatic system (Chakraborty, Babu et al 2012)

Lead toxicity is as a result of accumulation that causes a decrease in hemoglobin production and can adversely affect the reproductive organs and kidneys, and cause long-term damage to the central and peripheral nervous systems (Galadima and Garba 2012) Lead exists as Pb (IV) and Pb (II) oxidation states Anthropogenic activities such as battery manufacturing haveincreasedits concentration in the environment by~1000-fold and the highest occurrence was reported between 1950 and 2000 (Nriagu 1998)

Once absorbed, lead is transported in the bloodstream to other tissues and can accumulate in high concentrations in the brain, bones, liver, lung, kidney, spleen, teeth, and can pass through blood-brain and placental barriers (Casarett 1991).In humans, lead has a probable average life of 35 days in blood, 40 days in soft tissues and 20 – 30 years in bones The average Pb biological life can be substantially higher in children than in adults (Patrick 2006) The chronic toxicity of lead in humans regularly produces apathy, convulsions, coma, constipation, epigastric pain, low attention capacity, irritability, vomit and death (Zuluaga Rodríguez, Gallego Ríos et al 2015, Engwa, Ferdinand et al 2019)

In children, lead accumulation can lead to encephalopathy with lethargy, anoxeria, mental illness and vomiting Persistent lead exposure can reduce cognitive function and can cause disorders such as aggression, confusion, mental deficit and psychosis (ATSDR 2007)

or CO2 as the gasifying environment, preferably from 500 ℃ to 800 ℃, although low temperature derived biochar (biochar generated below 500 ℃ ) has been reported to bind well with certain inorganic compound in the aqueous phase (Kwak, Islam et al 2019)

The residence time for pyrolysis is normally between 30-60 minutes However, some studies have reported shorter times(Zhao, O'Connor et al 2018) According to Ahmad, Rajapaksha et al (2014), the type of feedstock is a key factor when pyrolysis is used in the production of biochar for specific environmental remediation purpose In case of HTC, low temperature and pressure are used to keep the water in the subcritical liquid phase (Huff, Kumar et al 2014, Qian, Kumar et al 2015) In these processes, drying of the feedstock is

not required Therefore, HTC can be considered a viable option for biochar (Qian, Kumar et

al 2015)

Biochar has been found to be a good adsorbent for removal of pollutants, including trace metals, such as Cu (II) and Pb (II) in water due to its large surface area, mineral component and functional groups (Cha, Park et al 2016)

Different techniques to improve the adsorption characteristics of biochar have been investigated over the past decades A review by Li, Dong et al (2017) showed that biochar’s ability to adsorb contaminants from the aqueous phase is based on many factors including the type of contaminants In previous studies conducted on removal of lead, it has been reported that lead is adsorbed by biochar through surface complexation, precipitation and cationic exchange (Yang, Wei et al 2014, Kołodyńska, Wnętrzak et al 2012)

1.5 Feedstock for Biochar Production

Feedstock is the conventional term used for biomass used in biochar production (Verheijen, Jeffery et al 2010, Schmidt, Kammann et al 2014, Hassan, Liu et al 2020) The type of feedstock is an important factor in production of biochar through slow pyrolysis High moisture content feedstock for example needs to be thoroughly dried before slow pyrolysis (Tomczyk, Sokołowska et al 2020)

Biochar has been produced from different biomass feedstocks such as crop residues, woody biomass, perennial grasses, wood wastes, animal litters and solid waste (Askeland, Clarke et al 2019) The intrinsic characteristics of the feedstock such as ash content, proportion of hemicellulose, cellulose, lignin, percentage composition of inorganic substances, bulk density, calorific value, fixed carbon, moisture and volatile content are very important determiners for the heating conversion process and of the final biochar produced (Angın 2013) For instance, lignin and cellulose have been reported to resist thermal degradation at temperatures of 280 ℃ to 500 ℃ and 240 ℃ to 350 ℃, respectively (Demirbas 2004) This shows that the fractional proportions of each component of biomass feedstock determine the structures that will remain during the pyrolysis process at a given temperature This was also confirmed by Winsley (2007) when pyrolysis of feedstock from wood-based generated coarser biomass after addition of resistant biochar with a carbon content of 80% The unbending lignolytic nature of the raw material remained in the biochar residue after pyrolysis

Collison, Collison et al (2009) reported that biomass feedstock can have different compositions due to the environmental conditions they experience during growth Such

factors include soil type, temperature and harvesting times The nature and type of feedstock used may also be dependent on the economic factors such as availability and cost of transport

to production systems and to market

Figure 1.3 (a) Biomass feedstock for production of biochar; (b) biochar produced from the feed stocks Adopted from EBC (2012)

Table 1.1 The application of biochar produced from different feedstock and techniques in aqueous solution

Table 1.1 The application of biochar produced from different feedstock and techniques in aqueous solution

Biomass

Feedstock

Pyrolytic Temperature (℃)

Residence Time

Pyrolysis Technique

Herbicides

(Sun, Keiluweit et al 2011)

Spartina alterniflora 400 2 h Slow pyrolysis Cu (II) (Li, Liu et al 2013)

Carbonization

Miscanthus

sacchariflorus

Table 1.1 The application of biochar produced from different feedstock and techniques in aqueous solution

Biomass

Feedstock

Pyrolytic Temperature (℃)

Residence Time

Pyrolysis Technique

sulfamethoxazole (Zheng, Wang et al 2013)

Brazilian Pepper

Wood

Although the above feedstocks have been used for production of biochar for removal of contaminants for environmental restoration purposes, biochar activation has recently been found as a key method in improving the overall characteristics of biochar, which will lead to improving factors such as the surface area thereby increasing the adsorption capacity of different contaminants (Mohan, Rajput et al 2011)

The most common method of biochar activation includes the use of chemicals such as acids and bases Steam activation as well as nano-particles have been employed in modification of biochar to improve their sorption ability (Tan, Liu et al 2016, Kwak, Islam et

al 2019)

1.6 Biochar Production by Slow Pyrolysis

Biochars from different feedstock and production techniques have been used for treatment of aqueous solutions contaminated with toxic trace metals Slow pyrolysis is one of the commonly used technologies for production of biochar used for the treatment of wastewater This is because slow pyrolysis favours the production of biochar within a few minutes to days of residence time (15 – 89%) (Tong, Li et al 2011, Ahmad, Lee et al 2012, Chen, Chen et al 2012)

Biomass feedstock for the production of biochar is composed primarily of hemicellulose, cellulose and lignin This is gradually pyrolysed with the increase of temperature According to Rutherford, Wershaw et al (2012), lignin is much more recalcitrant than hemicellulose and cellulose during charring The pyrolysis characteristics are determined with a thermogravimetry and derivative thermogravimetry curve (TG-DTG)

of raw biomass under inert condition (Chen and Chen 2009)

The weight loss during the drying process at low temperature includes the evaporation

of water The major decomposition process occurs between 200℃ and 500 ℃ (Chen, Chen et

al 2012) The steps involved in this decomposition process are (i) partial hemicellulose decomposition; (ii) complete hemicellulose decomposition to partial cellulose decomposition; (iii) full cellulose and partial lignin composition; and (iv) the successive decomposition and increasing degree of carbonization (Chen and Chen 2009)

The weight loss in biochar becomes gradual when the temperature is up to 700 ℃ The yields of biochar are approximately 15 – 89% by different feedstocks (Tong, Li et al 2011) This shows the thermal stability of biochar as an adsorbent

Figure 1.4 Slow pyrolysis set- up for biochar production

(Adopted from Seal, Panda et al (2015)

The characteristics of biochar are heavily influenced by the extent of pyrolytic temperature, pressure and exclusively by biomass size, kiln or furnace residence time (Asensio, Vega et al 2013) The vapour residence time is determined by the rate at which volatile compounds and gases are removed from the kiln or furnace (Meyer, Glaser et al 2011) Long residence time has secondary reactions on biochar surfaces and the charring of the tar instead of additional combustion or processing outside the kiln or furnace (Oliver, Pan

et al 2013) Typical pyrolysis product yields produced from gasification and fast pyrolysis processes are considerably lower when compared to that of solid product yield of hydrothermal carbonization, flash carbonization and torrefaction

Downie, Crosky et al (2009) reported that the yield of biochar products ranged from

25 % to 40% and observed slightly decreased yields at higher pyrolysis temperatures, which was attributed to volatilisation of other volatile products from the component of the biomass feedstock The experimental pyrolysis was conducted at a heating rate of about 10 k / min up

to the press cubed pyrolytic temperature ranging from 400 ℃ to 700 ℃ (Downie, Crosky et

al 2009)

Pellera, Giannis et al (2012) also reported that biochar yield of rice husk and compost derived after hydrothermal pyrolysis was about 62.5% and olive pomace and organic waste were quite lower at 37.5% This revealed that an increase in pyrolytic temperature led to a decrease in the yield for all produced material through pyrolysis To be precise, the yield of biochar at 300 ℃ and 600 ℃ were 32.8% and 31.9% for rice husk, 39.6% and 32.8% for

orange waste, 39.3% and 26.7% for olive pomace and 78.7% and 46% for compost, respectively (Pellera, Giannis et al 2012) Producing biochar at a higher efficiency rate via slow pyrolysis by developing integrated systems are still at a developmental stage, hence the need for exploitation of other effective technologies

1.7 Giant Reed (Arundo donax)

Giant reed (Arundo donax) is a perennial, herbaceous plant found in grasslands and

wetlands over a wide range of climatic and habitat conditions It belongs to the family Poacea and considered one of the largest Graminea with a fast growing rate of 0.3 - 0.7 m per week during several months of ideal conditions (o Di Nasso, Angelini et al 2010) Giant reed is considered by some to be native to East Asia (Polunin, Huxley et al (1965), India Dudley (2006)), and by others in the Mediterranean The situation is unclear because for thousands of years, giant reed has been cultivated in Asia, South Europe, North Africa and the Middle East In the 19th Century, giant reed was diffused widely in North and South America and in Australia (Perdue 1958)

Early reports on the origin of this species are conflicting Some suggested that it is from locations around the Mediterranean and Madagascar However, the consensus is that the species originated from Asia (most probably India), and has been cultivated for thousands of years throughout Asia, Southern Europe, North Africa and the Middle East Giant reed is now

a common weed in Iran, Spain, Argentina, Chile, Dominican Republic, New Zealand and United States of America It was probably introduced into California in the early 1800s (Bell 1997)

Although most commonly known as the giant reed, Arundo donax has many other

common names, including:

English names Other languages

• giant cane • narkhat (Indian)

• river cane

• Spanish reed

Arundo donax is the tallest (up to 10m) of the six perennial, reed-like grass species

that make up the Arundo L genus (Bell 1998) The root system of A.donax consists of fleshy,

compact masses of rhizomes from which rise tough, fibrous roots that penetrate deeply into

the soil Once established, A donax forms large, dense clonal rhizome masses The stems of

A donax have a diameter of 1- 4cm, are hollow with walls 2-7mm thick, and are divided by partitions at the nodes which range in length from 12-30 cm (Perdue 1958) The main stems

of A donax commonly branch during the second year of growth The leaves are 5 – 8cm broad at the base and taper to a fine point Flowers are borne on a large plume-like terminal panicle, dense and erect to 60cm long, produced in summer Seeds are wind dispersed (Forestry 2006)

Giant reed has spread across the world, particularly in areas with a Mediterranean

climate, but it has been reported that A donax does not produce viable seeds in most areas where it has been introduced (Perdue 1958).For instance, there are no records of A donax

seedlings in southern California, where it is generally assumed that it does not reproduce sexually (although this has not been confirmed) Consequently, the species is thought to spread primarily asexually by flood dispersal of stem cuttings and rhizome pieces In this situation, the natural variability in existing populations of clones, as it is known, may occur due to spontaneous mutation followed by natural selection as a response to climatic stresses and to different environment, or by transferring part of the plant through the usual ways of

diffusion Very little under-story vegetation is found under A donax due to its dense growth,

and this does not seem to provide the structure required by riparian birds for perching and nesting

Arundo donax does not readily produce viable seed in many locations but there have been instances where it has grown from seed collected from Indian populations Most reproduction occurs via rhizome, which the root spreads easily (Hoshovsky 2003)

A donax prefers well drained soils with abundant moisture Although it has also been reported to thrive in heavy clays, and can spread from the edge of a water body to past the riparian zone (Hoshovsky 2003, Dudley 2006).It grows well where the water table is at or close to the soil surface, and can tolerate excessive salinity (Perdue 1958, Hoshovsky 2003),

perhaps only at the individual plant level rather than a stand of A donax Arundo donax has

an extremely fast growth rate under optimal conditions and growth rates of up to 5cm per day, and 70cm per week under favourable conditions (Perdue 1958) The giant reed can produce more than 20 tonnes per hectare above-ground dry mass or 8.3 tonnes of oven-dry cane per acre (Perdue 1958)

Concomitant with such fast growth rates is a high-water demand, and A.donax can use

as much as 2,000L of water per meter of plant (Perdue 1958) This is three times as much water as USA native plants, and similar to the water use of rice crops The high-water use may make this species suitable for water treatment applications (Nsanganwimana, Florien, et al 2014) High water use may also make A donax a good species for rapid rotation with

agricultural crops in areas with elevated water tables, provided its roots penetrate deeply enough to dry out the vadose zone to significant depths, and a market can be developed for the biomass produced outside of Australia

A.donax has been used for erosion control, as an ornamental plant as thatching and lining of houses and storage bins and for musical instruments ranging from pan-pipe to bagpipes and bassoons (Hoshovsky 1986, Bell 1997).The leaves, stem and rhizome have many other domestic uses such as to make baskets, fishing rods and arrows, for penning and feeding livestock, and as a medicine (Perdue 1958).In Italy, this species has been utilized industrially since 1930, when Snia-Viscosa registered a trademark to obtain cellulose pasta to produce rayon viscose and paper (Cosentino, Copani et al 2006) Recently, this species has been suggested as one of the most promising for energy and cellulose pasta production for the Southern areas of Europe (Ververis, Georghiou et al 2004, Cosentino, Copani et al 2006).Traits that make such uses possible include its perennial nature, easy adaptation to different environmental conditions, high production of biomass, and low input requirements (Cosentino, Copani et al 2006)

One of the goals of the bioethanol industry is to be able to produce ethanol economically from ligno-cellulose, and this requires knowledge of the ligno-cellulose

composition of the feedstocks Ververis, Georghiou et al (2004) reported that A donax has

satisfactory levels of α-cellulose (~31 – 38%, depending on position in stem, and if node or internode) and Klason lignin content (≤20%) compared to those derived from softwoods and

hardwoods Neto, Seca et al (1997) characterised the polysaccharide composition of A

donax in different morphological regions of the plant Glucose 25 – 33% and xylose (24 –

28%) are the main sugars present in A donax Neto, Seca et al (1997) suggested that A

donax could thus be used as a source of pentosans for the furfural-based industry, since it has

a higher pentosan content than traditional sources such as corncobs, rice hulls and sugar cane bagasse The glucose could, of course, be used for bioethanol production, as could the pentoses when economic C-5 fermenting organisms are found The giant reed has also been evaluated as a non-wood fibre source for pulp mills by Lewis, Jackson et al (2002), who

report that A donax was suitable for direct substitution for hardwoods in existing kraft mills

without major equipment changes For instance, the hand sheet strength, burst, tensile and tear were all comparable with, or better than those of wheat straw, kenaf, and hardwood Aspen kraft pulp

Shatalov, Pereira et al (2002) noted that the stem of A donax is similar to wheat

straw and bamboo spp In that it is morphologically heterogeneous, consisting of two

botanically distinct parts: nodes and internodes In other words, A donax is hollow in

internodes, but solid in nodes further noted that stem heterogeneity has caused problems during pulping for other species During their tests, some differences were observed in node and inter-node based pulp, e.g papermaking properties and the brightness of unbeaten kraft pulps produced from internodes were higher than pulps made from nodes, reflecting the mass

proportion of nodes and inter-nodes in the stem of A donax

Bell (1997) suggested that A donax provides little food for wildlife in California, and

speculates that insects are sparse in sites dominated by giant reed because of the many chemical defences produced by the plant e.g silica, triterpenes, curare-mimicking indoles, hydroxamic acid and other alkaloids

Hoshovsky (2003) reported that the use of Angora and Spanish goats is showing

promise as an effective control agent for A donax in California, so obviously some animals

will eat this plant Goats do, however, prefer woody vegetation over most grasses, so this observation is perhaps not surprising Hoshovsky (2003) also reported that sheep can survive

for extended periods on a strict diet of A donax, and thus they may be a practical alternative

to mowing as a form of weed control Cattle are reported to not find the giant reed very

palatable but will graze it when other sources of fodder are not available, e.g during dry seasons (Wynd, Steinbauer et al 1948)

The giant reed is low in protein but high in phosphorus, even when grown on soils deficient in this nutrient However, crude protein reached about 12% in the upper half of younger plants, being reduced to about 6% in the upper half of older plants Both young and old plants contained about 3% protein in the lower half of the stems Phosphorus shows similar patterns, reaching 0.15 % in the upper half of young plants On the other hand, calcium and magnesium concentrations are greatest in the upper half of older plants Early Australian research suggested the species was a suitable alternative fodder for pigs, cattle and horses, the smaller leafy variegated variety being preferred (Wynd, Steinbauer et al 1948)

1.8Arundo donax: Weed Potential

The giant reed (A donax) has been listed in the top 100 of the worst weeds in the

world on the Global Invasive Species Database compiled by the Invasive Species Specialist Group (ISSG) of the IUCN Species Survival Commission (ISSG 2007).Giant reed has also been considered one of the worst weeds in the world by the Weeds CRC (Weeds 2005) Giant reed is considered to have high weed potential because of its rapid growth rate and competitive nature in the vegetation While its seed dispersal is not considered to be a factor

in the spread of the plant, its rhizome and stems are easily spread during flooding This contributes greatly to its fast dispersal in the ecosystem

Arundo donax is also considered to be in the top 30 worst weeds in the world by the

Weeds CRC That said, A donax is not listed as a noxious or invasive weed in Australia but it

is considered a naturalised invasive species in Queensland (#131 of the top 200 species in this category) and controlled on a regional and/or local basis in New South Wales

Arundo donax is not listed as a noxious or invasive weed in Victoria and does not appear to have been considered as an imminent risk by the current Victoria’s Noxious Weeds

The Arundo donax weed status in New South Wales is considered regionally or locally controlled Arundo donax has also been observed in Western Australia, along the Yarra River

in Victoria, the Torrens River in South Australia and in the Little Para estuary of South Australia (CRC 2005)

A donax was established 150 years ago in South Australia for ornamental and fodder use and in many areas for erosion control and ornamental use (CRC 2005, Williams, Biswas

et al 2006) In Western Australia, A donax is considered to be a garden escapee forming

suckering clones around old settlements on roadsides, creek lines, wetlands and wasteland

from Geraldton to Albany, and very common around Perth, where variegated leaf clones (var versicolour) are frequent There is no published distribution information in Victoria, Although, there are anecdotal reports of widespread distribution of small stands around Melbourne and Port Philip Bay, and elsewhere in the state

Arundo donax is considered to have high weed potential because of its rapid growth rate and vegetative competitive nature These two factors alone can cause it to quickly dominate native vegetation in the USA (Hoshovsky 2003) Whilst its seed dispersal does not appear to be a factor in the spread of this plant (Bell 1997), its fragmented stems and rhizomes readily take root, particularly after a flood where they have been broken off and

dispersed downstream It is suspected that A donax may release toxins into the water to

prevent other plants from establishing their colony there (Bell 1997) The giant reed is considered an invasive pest in the USA, being well established in warm, coastal freshwaters from Maryland to northern California (Bell 1997) This plant is considered the greatest threat

to riparian vegetation in coastal southern California Arundo donax is not known to have any

natural predators in North America, and it is uncertain what limits its population in its native

habitat (Bell 1997) Very little under-story vegetation is found under A donax, due to its

dense growth, and the reed does not seem to provide the structure required by riparian birds for perching and nesting The giant reed out competes species such as willows (Salix spp.) that are native to America Since willows are an exotic pest in Australia, it might also be

assumed that A donax will out compete native Australian riparian trees and shrubs under suitable climate and habitat conditions Although A donax was used to control erosion along

ditches in south-west America, it has escaped cultivation areas and become established in ditches and streams, choking irrigation ditches to the point of reducing the water carrying capacity (Hoshovsky 2003)

Despite this weed potential, Hoshovsky (2003) reported that little had been published regarding control strategies for the species Hoshovsky (2003) suggested that a number of approaches, such as slashing, hand pulling and digging, chopping or mowing, burning, prescribed grazing, biological control and chemical control could be employed to reduce

populations or densities of A donax Bell (1998) found that A donax responds quickly after

fire and can easily out-grow Californian native species, therefore burning would need to occur a number of times whilst ensuring native species were not disadvantaged Hoshovsky (2003) concluded that in order to eradicate the plant, the entire rootstock would need to be removed as it reproduces vegetatively from the rhizome and can be spread by rhizome fragments dispersed along watercourses

Bell (1997) suggested that the best way of achieving this is via chemical control using

systemic herbicides The most effective way of controlling A donax in Australia is to remove

it from the entire river system whilst populations are small and preventing re-establishment through habitat restoration and public education regarding the distribution of the species (CRC 2005) Despite having been present in Australia for at least 150 years and having

several qualities that make it an aggressive invasive species elsewhere in the world, A donax

has not yet achieved formal noxious weed status Certainly, it has not come to dominate riparian zones in the manner that some suggest in Victoria (CRC 2005) This is perhaps due

to climatic and ecological differences between southern Australia and the southern USA The giant reed survives in areas with annual precipitation 300 – 4000 mm, and grows in a wide range of soils, although it prefers well-drained soils with abundant moisture, and pH 5 – 8.7 These requirements would seem to be readily available in Victoria, but since the giant reed does not propagate by seed, but rather spreads rapidly following flood events, perhaps changed river management in southern Australia in the past 100 years (river regulation and fewer flood events) and regular droughts have limited its ability to spread?

Although drought causes no great damage to two- to three-year old stands of giant

reed, A donax can be seriously retarded by lack of moisture during its first year (Perdue

1958) The giant reed’s ability to tolerate extreme drought is due to the development of course, drought-resistant rhizomes and deeply penetrating roots that can reach moisture at depth, but intermittent river flows followed by drying conditions again perhaps limit its ability to spread

1.9Use of Arundo donax in Constructed Wetlands

There is limited information available relating to the use of A donax in constructed wetlands In some ways this is surprising, since A donax has a number of characteristics that

appear to make it suitable for constructed wetlands For instance, the plant is easily propagated from rhizomes and stem plantings The species is reported to tolerate excessive salinity levels (Perdue 1958)

Rhizomes are produced on or near the soil surface, although data and information on the depth of rhizome formation has not been reported Shoots and roots emerge from the rhizomes and the roots are reportedly able to penetrate to significant depths to obtain water contributing to their drought resistance particularly after the first year of growth Pests and diseases are not known to affect the species and the only risk after the first year of growth is frosts early in the growing season which can burn the new season’s growth (Perdue 1958)

Williams, Biswas et al (2006) investigated the potential of using A donax for

wastewater treatment and pulp/paper production in South Australia Although this research

was not conducted in a constructed wetland, rather on an established dryland, planting of A

donax over thirty years of age were able to conclude that the biomass yields exceeded that produced by other irrigated effluent crops, such as cereals, forage and hardwood plantations For instance, Williams, Biswas et al (2006) reported that following clear-felling to 10 cm,

within the first year this established stand of A donax when irrigated with wastewater

produced up to five times the biomass of Eucalyptus hardwoods in southern Australia However, the authors conceded that the long term (20 years) productivity still needs to be determined The weed risk of the species was not discussed, although the conclusion was made that the risk would be minimal if managed appropriately (Williams, Biswas et al 2006)

Table 1.1 Advantages and disadvantages of A donaxwith respect to its use

as a plant in constructed wetlands for wastewater treatment

Fast growth rate

Does not appear to be viable from seed

(reduced risk of off-site dispersal)

Weedy potential

Limited number of pests Threat to riparian vegetation

Appears tolerant of high salinity (still to be

confirmed)

Tolerance to continuous wetting unknown

Regenerates after fire from rhizomes Salinity tolerance unknown

Many potential uses for above ground

biomass

Easily propagated from rhizomes

Karpiscak, Gerba et al (1996) reported the successful incorporation of A donax into

multi-species free water surface wetlands in Arizona However, while authors reported good removal of many of the water quality parameters investigated, they did not mention whether there was any change in species distribution within the raceways This has been seen in other wetlands (e.g in the wetlands described, and from both a risk management and wetland

management perspective, it would be good to know if A donax eventually came to dominate the raceways, or could the cattail (Typha domingensis), bulrush (Scirpusolneyi), black willow (Salix negra) and cottonwood (Populus fremonti) compete with the giant reed, or, indeed,

was active management required to maintain species diversity?)

The construction and operation of another free water surface wetland incorporating A

donax was reported by Manios, Kypriotakis et al (2002).This wetland, built on the island of Crete to service a local village (population ~700, although the wetlands was designed for a

population of 1200) used A donax and Typha domingensis (cattail) for pragmatic reasons:

they were the two most common emergent macrophytes found in local lagoons and rivers However, it became impossible to enter the wetland Perhaps a salutary warning from these authors to those considering using the giant reed in a treatment wetland, particularly if species diversity is important That said, other plants were found in the wetlands, particularly along its margins, and the dense matrix created by the giant reed’s stems (70-90/m2) and leaves, produced an excellent physical barrier for deposition of suspended solids The two

beds planted with A donax were compared with two unplanted controls

After batch feeding raw effluent into the beds for two years, and monitoring inflow and outflow for organic loads, phosphorus and nitrogen over that time, the authors reported

no difference in the removal of TSS or COD This latter is perhaps not surprising since vertical flow removes the effluent from the primary site of biological remediation in wetlands, namely the biofilms that form around the macrophyte stems, and thus COD and TSS removal were primarily due to physical processes The planted bed did, however, facilitate infiltration throughout the experiment, while in winter the control bed became clogged, with the former benefit perhaps a result of the greater porosity imparted by the plant roots Nutrient and pathogen removal in the planted beds was acceptable, but conductivity, sodium and chloride concentration increased through the planted bed, perhaps a result of the giant reed’s high-water use concentrating these materials in the vadose zone Many wastewaters contain contaminants other than nutrients and suspended solids, including metals Some cycling of metals occurs within wetlands, for instance through resuspending of

sediments, soils or peat / litter (within which, or adsorbed to, metals reside), or through uptake (passive and active consumption) by organisms followed by release on death and decomposition Removal can be up to 99% effective in some wetlands (DLWC 1998)

1.10 Generation of Biochar from Giant Reed

The adaptability of the giant reed provides many advantages to different environments including its high biomass content and low input required for it growth when compared to other energy crops(Corno, Pilu et al 2014) During the clearing of bush, giant reed is burnt off in an open field Hence, utilization of this invasive biomass could serve as low-cost adsorbents in wastewater reclamation and reuse It could also provide a two-way advantage to preventing environmental pollution and providing cheap resource for beneficial purposes Giant reed can be introduced as good candidate material to prepare high-quality biochar for trace metal adsorption from aqueous solution because of its physicochemical properties It is

a carbon-rich material with chemical compositions of lignin, cellulose, hemicellulose and hydrocarbons

Ramos, El Mansouri et al (2018) reported that the lignocellulosic structure of giant reed comprises of 22.4 ± 0.2 % lignin, 43.1 ± 0.5 % cellulose and 21.9 ± 0.2 % hemicellulose The lignin content in giant reed shows it has an elemental composition higher in carbon and lower in oxygen, and this makes it more stable (Jibril, Houache et al 2008) Hence, the use of giant reed for production of high-quality biochar is a way of conversion of waste into valuable product for environmental remediation purposes

Recently, giant reed biomass has received much attention as a good source of lignin for preparing activated carbon for adsorption purposes (Sun, Yue et al 2013) However, the use of giant reed biochar as an economical adsorbent for the removal of toxic trace metals form the aqueous phase has not yet been fully investigated

Sagehashi, Fujii et al (2010) used charcoal, which was produced from the stalk of giant reed at various temperatures for removal of cadmium from wastewater The results showed that pH was a dependent factor for the adsorption of cadmium This was observed when the concentration of cadmium in the solution decreased as pH increased rapidly with time

Sun, Yue et al (2013) prepared activated carbon from giant reed and H4P2O7 for the removal of Cr (VI) The sorption performance of the activated carbon for Cr (VI) was investigated under batch experiment with varying conditions such initial concentration, time and pH After using isotherm to fit the adsorption data, the best fit was achieved by

Freundlich isotherm This indicated a multilayer adsorption mechanism The adsorption capacity was found to be 90.92 mg/g and it was reported at a temperature of 30℃ and at equilibrium time of 10 h

Mohmoud (2012) investigated the use of activated carbon produced from giant reed for removal of Fe (II) from aqueous solution using batch experiments The adsorption performance of the activated carbon was obtained by fitting the adsorption data with two isotherms, which were the Langmuir and Freundlich isotherms The data obeyed the Freundlich isotherm compared to the Langmuir isotherm The R2 value of Freundlich isotherm was 0.96 while that of Langmuir was 0.86 Also, the effect of adsorbent dose (0.2-2) g/20mL, concentration (10-80) mg/l and pH of (1-9) were studied The initial concentration affected Cr (V) removal more than solution pH This shows that at low concentration of solution, the ratio of number of moles of ion in the adsorption site is low and the amount adsorbed per unit mass of adsorbent is increased

The invasive giant reed- based activated carbon produced had a strong affinity for Fe (II) and Cd (II) in aqueous solution The development of biochar technology gives the opportunity to satisfy the need for cost-effective and eco-friendly adsorbents for the removal

of trace metals from the aqueous phase The batch experiment showed that biochar produced from giant reed had a strong affinity for lead in the aqueous phase This may be due to the nature of the feedstock Giant reed may be richer in lignocellulosic biomass (cellulose, hemicellulose, and lignin), which is a good characteristic for effective adsorption of trace metal ions such as lead (Ramos, El Mansouri et al 2018) This is supported by Harmsen and Naidu (2013)who reported that the nature, characteristics, quality and potential use of biochar are affected by the type, preparation, form and biomass of feedstock and the type of pyrolysis

Figure 1.5 Illustrations of giant reed

(A) (Adopted from Fernando, Barbosa et al (2016)); (B) Picture of giant reed taken by

G Allinson during summer; (C) Picture of giant reed taken by G Allinson during summer

1.11 Use of Biochar for Removal of Trace Metals from Aqueous Phase

The specific characteristics of a biochar are due to its functional factors which include the size of biomass feedstock, type, temperature and conditions of pyrolysis The wide variation in the characteristics of biochar makes some raw materials more suitable than others for adsorption of different trace metals The choice of biochar for adsorption purposes should not only be based on aqueous environmental nature, trace metals concentrations and multiple contaminations but also on physicochemical properties of the biochar produced The adsorption mechanism could be largely dependent on some properties of biochar that include mineral components, porous structure, surface functional groups and specific surface area (Schmidt, Kammann et al 2014)

The utilisation of biochar includes many methods such as complexation, electrostatic interaction (chemisorption), ion exchange, precipitation and physical sorption in the removal

of trace metals from aqueous solutions (Patra, Panda et al 2017, Askeland, Clarke et al

2019, Niu, Feng et al 2020) Biochar is a modern technology that is evolving due to its friendliness, cost-effectiveness and effective in adsorption In the production of activated carbon, higher temperatures and additional activation processes are required The production

eco-of biochar is less expensive with lesser energy requirements (Shin, Lee et al 2020)

Properties of biochar such as ash, carbon contents, pH and surface area can be affected by post-treatments and therefore impact their ability to immobilise heavy metals (Li, Dong et al 2017).Biochars act on bioavailable fraction of trace metals, which can also reduce their leachability; hence it is necessary to review the mechanisms pertaining to the interaction between biochar and trace metals Trace metals have different and specific mechanisms of adsorption which are largely depended on the differences and the properties of biochars (Tan, Liu et al 2015) The adsorption of trace metals by diffusional movement of the metal ions into sorbent pores devoid of formation of chemical bonds is defined by surface or physical sorption For biochars produced from plant and animal biomass, the temperature increases of carbonisation (> 300 °C) will lead to high surface areas and pore volumes in the biochars(Patra, Panda et al 2017) However, there has been a report of sintering when biochar is produced at a higher temperature > 800°C (Li, Dong et al 2017)

1.12 Mechanisms of Adsorption by Biochars

For checking the efficiency of adsorption by biochar, the identification of the principles in the adsorption process is needed The adsorption behaviour of biochars for different contaminants are different and well correlated with properties of the contaminants The characteristics of biochar are due to its functional factors which are the type and size of biomass feedstock, temperature and conditions of pyrolysis The wide variation in the characteristic of biochars make some popular raw materials more suitable than others for adsorption of different trace metals The choice of biochar for adsorption purposes should not only be based on the aqueous environment, trace metal concentration, multiple contaminations but also on the physico-chemical properties of the biochar produced

Dong, Ma et al (2011) hypothesized that sugar beet tailing biochar effectively removed Cr (V) through electrostatic attraction of Cr (VI) coupled with Cr (V) reduction to

Cr (III) and Cr (II) complexation The adsorption process was in three parts: finstly, negatively charged Cr (VI) species were migrated to the positively charged surfaces of biochar (at low pH) with the help of an electrostatic driving force Secondly, Cr (VI) was reduced to Cr (III) by the participation of hydrogen ions and the electron donors from biochar; and finally, part of the Cr (II) reduced from Cr (VI) was released into the aqueous solution

Biochars act on bioavailable fractions of heavy metals, which can also reduce leachability, hence it is necessary to consider the specific mechanism pertaining to the interaction between the biochar and trace metal before application(Paz-Ferreiro, Lu et al 2014)The mechanism of biochar adsorption largely depends on the process of biochar-metal sorption, which is usually dependent on biochar’s point of zero charge (PZC) and the pH of the solution (Liu and Zhang 2009, Dong, Ma et al 2011)

A high carbonization temperature of 400 ℃ also helps in the formation of graphene structures in the chars to favour the mechanism of electrostatic attraction sorption(Kim, Shim

et al 2013) Precipitation is the formation of solid(s) at the time of the sorption process and is either in solution or on a surface and has often been mentioned as an essential mechanism that influences the immobilization of trace metals by means of biochar sorbents (Patra, Panda

et al 2017)

Owing to the optimization of adsorption, biochar carries various surface functional groups, mainly oxygen containing functional groups such as AOH and carboxylate, hydroxyl and ACOOH (Li, Dong et al 2017) These functional groups change and act with the increase

in pH of the solution The functional groups on biochar at low pH are present in the form of

positively charged species In contrast, the pyrolytic temperature has a significant influence

on the elemental, morphological and structural properties of the biochars(Kołodyńska, Wnętrzak et al 2012)

Biochar with pore volumes and high surface area have greater efficiency for trace metal removal from aqueous phase because metal ions can be substantially sorbed on top of the char surface and remains within the pores (Kumar, Loganathan et al 2011)

Most biochar surfaces with negative charges can adsorb heavy metals that are positively charged through ligand specificity and electrostatic attraction Furthermore, different functional groups on biochar can also interact with different trace metals by forming complexes or precipitates of their solid mineral phase (Dong, Ma et al 2011)

Many surface functional group when exhibited on biochar’s surface will likely influence its interaction with trace metals, resulting in electrostatic attraction, ion-exchange and/or surface complexation These effects are demonstrated by the changes in functional groups of biochar before and after the metal ion adsorption (Chen, Cui et al 2013)

The large surface area of biochar implies a high capacity for adsorption of complex heavy metals on their surface Surface sorption of trace metals on biochar has been convincingly demonstrated in a multiple study using scanning electron microscopy (SEM) (Beesley, Moreno-Jiménez et al 2011, Lu, Zhang et al 2012).Lu, Zhang et al (2012) also suggested that the functional groups played an important role in Pb adsorption on biochar produced from sludge This includes metal exchange with K+ and Na+ due to the electrostatic outer-sphere complexation with free carboxyl functional groups and free hydroxyl groups

1.13 Adsorption Isotherm Models

Adsorption isotherms are crucial in elevating the use of adsorbents since they clearly describe the interaction between adsorbents and adsorbate Some mathematical models have been used to describe the adsorption equilibrium of trace metals on biochars and examine experimental data Langmuir, Freundlich and Tempkin equations are the most popular and widely used The results using these models are largely dependent on biochar properties and the target contaminants The Langmuir isotherm usually assumes monolayer adsorption of adsorbate on the adsorbent and homogeneous surface

Chen, Chen et al (2011) studied adsorption isotherms at different initial concentrations of Cu (II) and Zn (II) that ranged from 0.1 to 5.0 mM The Langmuir model (R2 > 0.998) fitted the data better than the Freundlich model (R2was 0.86 to 0.94) Many researchers also reported that the adsorption of trace metals by biochar fitted better with the

Freundlich than the Langmuir isotherm in their experiments (Agrafioti, Bouras et al 2013) The Freundlich isotherm discloses information on the heterogeneous adsorption and it is not limited to the formation of a monolayer(Kim, Shim et al 2013).The study of Cr (VI) and Pb (II) adsorption by biochar pyrolyzed from the municipal wastewater sludge are normally simulated with Langmuir and Freundlich equations

One study revealed Pb (II) adsorption behavior fitted better with the Langmuir equation than with the Freundlich equation(Zhang, Gao et al 2013) However, more research

is needed to bolster these previous studies Biochar also showed a high affinity for organic contaminants (Kumar, Loganathan et al 2011)

In recent times, most laboratory experiments were performed to investigate the potential of biochar as an adsorbent for removal of organic and inorganic pollutants from the aqueous phase Overall, the results from these studies demonstrated that the biomass resultant biochar can be used as a cost effective adsorbent for removal of environmental organic pollutants from the aqueous phase The toxins included pesticides, dyes, herbicide, and antibiotics

1.13.1 Langmuir Model

Undoubtedly, the most extensively used sorption isotherm is the Langmuir model (Gerente, Lee et al 2007) The mechanistic model is usually based on numerous assumptions including monolayer adsorption (adsorbed layer is one (1) molecule of thickness), in which sorption only happens at a finite number of indistinguishable active sites Moreover, there is

no steric hindrance or lateral interaction between adsorbed molecules and the adjacent sites Furthermore, the model of Langmuir implicitly assumes that the binding to an adsorbent (biochar) surface is determined by the physical forces and all sites being actively equivalent with equal affinity for the sorbate The model has been used to explain the sorption of trace metals by biochar

1.13.2 Freundlich Model

The Freundlich model is often used for non-ideal sorption on the heterogenous surfaces in addition to multilayer sorption (Gerente, Lee et al 2007) The Freundlich equation does not assume (unlike the Langmuir model) that the sorption sites are homogeneous and

actively equivalent In contrast, the Freundlich model assumes that the sorption sites of the biochar (adsorbent) are of diverse affinities, and stronger binding sites are engaged first in a way that the strength of binding decreases with an increase in the degree of site occupation Therefore, the quantity adsorbed is a summation of all the sites of adsorption, with each site taking its own bond energy and exponentially the energy distribution sorption sites decays, until completion of the process of adsorption (Gerente, Lee et al 2007).The Freundlich model has been successfully used to explain the sorption isotherm of trace metals on different biochars

1.14 Previous Batch Experiments

The study of systems with a mixture of contaminants is very important, particularly for trace metals, as they usually coexist in the environment and undergo competitive sorption Inyang, Gao et al (2012) performed batch experiment by using dairy waste and sugar beet as feedstock, where the residue materials were first dried at 80℃ prior to the production of the biochar Five hundred grams (500g) of dried feedstock was heated at 600ºC for 2 h in a furnace with an N2 environment The concentration of each trace metals in the solution was then adjusted to 0.01 mmol/l, after which 0.10 g of biochar was added into the 68 ml digested vessels and then mixed with 50 ml of trace metals solution of Pb (II), Cu (II), Cd (II) and Ni (II) at room temperature of 22 ± 0.5 ºC It was shaken in a reciprocating shaker for 24 h The adsorption efficiency of the four heavy metals Pb (II), Cu (II), Cd (II) and Ni (II) by digested whole sugar beet biochar was higher than 97%, indicating that the biochar had a strong affinity for the four heavy metals tested Digested dairy waste biochar also showed high adsorption efficiency for Pb (II) (99%) and Cu (II) (98%), but relatively low adsorption efficiency for Cd (II) (57%) and Ni (II) (26%)

Regmi, Moscoso et al (2012) studied batch experiments, in which they first made 50

ml aqueous cadmium or copper solutions at a temperature of 23± 1ºC for 24 h After contact time of 24 h, the solution was collected and filtered via a 0.45 µm nylon filter at consistent interval in glass tubes The batch experiments were done with initial trace metal concentration of 40 mg /L at pH of 5.0 and contact time of 24 h which resulted in almost 100% cadmium and copper removal by activated hydrothermal carbonization biochar (HTCB) at 2g/L, far greater than that of powdered activated carbon (PAC) (4% and 7.70%) and HTC biochar (16% and 5.60%) Activated adsorption capacities for cadmium and copper removal were 34 mg/g and 31 mg/g, respectively The activated HTCB showed a higher