Chromium vi removal from aqueous solution by using silver nano activated carbon

THAI NGUYEN UNIVERSITY UNIVERSITY OF AGRICULTURE AND FORESTRY HA THI LAN ANH CHROMIUM VI REMOVAL FROM AQUEOUS SOLUTION BY USING SILVER NANO-ACTIVATED CARBON BACHELOR THESIS Study

THAI NGUYEN UNIVERSITY

UNIVERSITY OF AGRICULTURE AND FORESTRY

HA THI LAN ANH

CHROMIUM (VI) REMOVAL FROM AQUEOUS SOLUTION BY

USING SILVER NANO-ACTIVATED CARBON

BACHELOR THESIS

Study Mode: Full-time

Major: Environmental Science and Management

Faculty: Advanced Education Program Office

Batch: 2014 - 2018

Thai Nguyen, 25/09/2018

Thai Nguyen University of Agriculture and Forestry

Degree Program Bachelor of Environmental Science and

Management

solution by using Silver nano-activated carbon

Environment and Earth Science, Thai Nguyen University of Sciences) Abstract:

Chromium (Cr(VI)) is a heavy metal that can cause a serious impact on the environment and human – being The treatment of Cr(VI) was reported through several methods such as chemical precipitation, adsorption, membrane filtration, coagulation/flocculation, ion exchange and absorption However, absorption is considered one of the most idea method for Cr(VI) removal Activated carbon is a low-cost material derived from wood or other organic waste from the shell and coir

As the main constituent of coal is carbon, so all the carbon-rich fuels can be used to make activated carbon Besides, silver nano particles as a catalyst for modifying activated carbon to increase the adsorption capacity of activated carbon In this study, the activated carbon loaded silver nanoparticle (AgNPs-AC) was used as a low-cost adsorbent to remove Cr (VI) from the aqueous solution Batch absorption

experiments were conducted to evaluate the effects of pH, initial concentrations of Cr(VI), contact time and dose of AgNPs-AC on Cr(VI) removal efficiency The results showed that at pH = 4, contact time of 180 min, 20mg AgNPs-AC/25mL of

K2Cr2O7 solution with initial Cr(VI) concentration at 5 mg/L were the most suitable conditions for adsorption of Cr VI) from aqueous solutions The optimum adsorption capacity achieved after processing was 27.70mg/g at 20 mg/25mL of AgNPs-AC dose and 40 mg/L initial Cr(VI) The adsorption kinetic data were found

to fit well with the pseudo-first and second order models with very high correlation coefficients From this study, it can be concluded that AgNPs-AC is an interesting adsorbent, saving, easy to remove Cr (VI) from the aqueous solution

Chromium, Adsorption capacity, Activated carbon

ACKNOWLEDGEMENT

First of all, I would like to thank you teachers at University of Agriculture and Forestry – University of Thai Nguyen has dedicated teaching me during the period of study at the school

I would like to express deep gratitude to the teachers Dr Van Huu Tap whose

guidance, encouragement, suggestion and very constructive criticism have contributed immensely to the evolution of my ideas during the project Without his guidance, I may not have this report

At the same time, I also want to express my deep gratitude to Dr Vu Xuan Hoa,

who gave me a chance to interact with a nanotechnology field I also thank faculty of Environment and Earth Science – Thai Nguyen University of Sciences - Thai Nguyen University has facilitated me throughout the course of the thesis

Finally yet important, I took this opportunity to express my deepest appreciation

to my families, relatives, friends who encouraged and supported me unceasingly and all who directly or indirectly, have lent their helping hand in this venture

Thank you all very much!

Thai Nguyen, 25/09/2018

Student

Ha Thi Lan Anh

TABLE OF CONTENTS

ACKNOWLEDGEMENT iii

TABLE OF CONTENTS iv

LIST OF FIGURES vi

LIST OF TABLES vii

PART I INTRODUCTION 1

1.1 Research rationale 1

1.2 Research's objectives: 2

1.3 Research hypotheses: 3

1.4 Limitations 3

1.5 Definitions……….3

PART II LITERATURE REVIEW 4

2.1 Chromium 4

2.1.1 Electronic and molecular structure of hexavalent chromium compounds 4

2.1.2 Sources of Chromium 5

2.2 Routes of exposure (Chromium) 5

2.2.1 Air 5

2.2.2 Drinking-water 6

2.2.3 Food 6

2.3 Coconut shell activated carbon 7

2.4 Silver nanoparticles 7

2.5 Silver nano-activated carbon (AgNPs-AC) 8

3.1 Materials 9

3.1.1 Chemicals 9

3.1.2 Adsorbent materials 9

3.1.3 Laboratory instruments 10

3.2 Location and research time 11

3.3 Research Contents 11

3.4 Adsorption experiments of Chromium (Cr6+) onto (AgNPs –AC) 11

3.4.1 Measurements 15

3.4.2 Data analysis 15

PART IV RESULTS AND DISSCUSSION 16

4.1 Characterization of the nano-activated carbon 16

4.2 Effect of impregnation ratio (AgNPs/AC) on Cr(VI) adsorption capacity 18

4.3 Effect of pH 20

4.4 Effect of contact time 21

4.5 Effect of adsorbent dose 23

4.6 Effect of initial Cr(VI) concentrations 24

4.7 Adsorption isotherm 25

4.8 Adsorption kinetics of AgNPs-AC 31

PART V CONCLUSION 35

REFERENCES 36

LIST OF TABLES

Table 1: Levels of daily chromium intake by humans from different routes of

exposure 7 Table 2: Adsorption isothermal parameters and correlation coefficients of Langmuir, Freundlich and Temkin models for sucrose adsorption on Chromium 31 Table 3: Calculated kinetic parameters of models for adsorption of Chromium onto AgNPs-AC 34

PART I INTRODUCTION

1.1 Research rationale

Chromium, named for its multicolored compounds, is a transition metal, number 24 in the periodic table of elements This element is found in combination, mainly in chromite ores, and, even if in lower abundant amounts, as crocoites (PbCrO4) and chrome ochre (Cr2O3) Cr is a major element that exists primarily in two different oxidation states, hexavalent and trivalent These oxidation states are denoted

as Cr(VI) and Cr(III), respectively The rarely found naturally occurring element has zero oxidation, Cr(0), other oxidation states of Cr are not stable and therefore, are not found in the natural environment Cr(VI) is more flexible than Cr(III) and dificult to remove in water (Elisabeth L Hawley et al, 2004)

This chromium (VI) detoxification leads to increased levels of chromium (III) (ATSDR, 1998) Air emissions of chromium are predominantly of trivalent chromium, and in the form of small particles or aerosols (ATSDR, 1998) and (SAIC.PM, 1998) The most important industrial sources of chromium in the atmosphere are those related

to ferrochrome production Ore refining, chemical and refractory processing, producing plants, automobile brake lining and catalytic converters for automobiles, leather tanneries, and chrome pigments also contribute to the atmospheric burden of chromium (U.S Environmental Protection Agency, 1998) The general population is exposed to chromium (generally chromium [III]) by eating food, drinking water and inhaling air that contains the chemical The average daily intake from air, water, and food is estimated to be less than 0.2 to 0.4 micrograms (µg), 2.0 µg, and 60 µg,

cement-respectively (ATSDR, 1998) Dermal exposure to chromium may occur during the use

of consumer products that contain chromium, such as wood treated with copper dichromate or leather tanned with chromic sulfate (ATSDR, 1998) Occupational exposure to chromium occurs from chromate production, stainless-steel production, chromium plating, and working in tanning industries; occupational exposure can be two orders of magnitude higher than exposure to the general population (ATSDR, 1998) People who live in the vicinity of chromium waste disposal sites or chromium manufacturing and processing plants have a greater probability of elevated chromium exposure than the general population

Several technologies have been applied to remove Cr(VI) from aqueous solutions including precipitation, reverse osmosis, ion exchange, filtration, sand filtration, chemical reduction/oxidation, electrochemical precipitation, membrane filtration, solvent extraction, and electrochemical deposition and adsorption (Chi-Chuan-Kan, 2017) Adsorption is an effective and low cost method Particularly, the problem of chromium pollution in water resources is causing concern in major cities and industrial parks; therefore, it is necessary to have a method to remove Cr from the water environment In this study, Cr was treated by adsorption with the adsorbed material is activated carbon coconut shell

1.2 Research's objectives

The purpose of this study was to load silver nanoparticles into activated carbon deriving from coconut shell and application for removing chromium from aqueous solution Research on finding the appropriate impregnated rate, evaluation of appropriate conditions for adsorption, including: pH, sorption time, adsorbent dosages

and initial concentrations The adsorption kinetics, sorption isotherm and corresponding chromium removal mechanisms were also studied under optimal conditions

1.3 Research hypotheses

With many advantages in the environment, the application of nano materials for wastewater treatment is an effective way to remove heavy metals And this scientific research will seek to answer the central research questions

1 What are the characteristics of the type (Ag - NPs) that can remove Cr from the aqueous solution?

2 What is the optimal condition for the removal of Cr from aqueous solutions?

1.4 Limitations

Because the training time is too short, this research project can not perform other experiments and estimate deeply the factors affecting heavy metal adsorption in water using AgNPs-AC

PART II LITERATURE REVIEW

2.1 Chromium

2.1.1 Electronic and molecular structure of hexavalent chromium compounds

In aqueous solution hexavalent chromium exists as oxoforms in a variety of species depending on pH and the hexavalent chromium concentration (R.M Powell et.al, 1995) For the oxo speciesof hexavalent chromium three main pH regions may

be distinguished:

(1) H2CrO4 (pH<0)

(2) HCr and Cr2 (pH 2–6),

(3) Cr (pH>6)

The abundance of these forms is largely dependent upon concentration In very acidic solutions two other forms have been detected, and (T.L Daulton, B.J, 2006) The equilibrium between protons, water molecules, and the hexavalent chromium species are as follows:

↔ HCr + (1)

HCr ↔ Cr + (2)

+ ↔ 2HCr (3)

H ↔ + (4)

Cr ↔ H + (5)

The most important equilibria in hexavalent chromium aqueous solutions above

pH 1.5 are deprotonation and dimerization reactions, which can be described by equations (ATSDR, 1998) and (SAIC PM, 1998)

The electronic structure of the tetrahedral chromate ion, a predominant oxo species at neutral pH, is second only to the permanganate ion (Mn ) in being thoroughly studied The chromate ion forms a regular tetrahedron with the chromium–oxygen distance being 1.66 A˚ (M Cieglak-Golonka Polyhedron, 1996) The schematic energy level diagram for the higher filled (HOMO) and lower unfilled (LUMO) orbitals in the ground state established the 1 orbital to the highest occupied level and 2e to be the lowest unoccupied (S.I Shupack, 1991) The highest occupied

MO level 1 possesses pure oxygen character and the 2e orbital is mostly of chromium character (H Firouzabadi et al, 1982)

2.1.2 Sources of Chromium

Most chromium is released into the environment from human activities at stationary point sources Combustion and the processing of ore discharge primarily trivalent chromium into the environment as chromium oxide; however small amounts

of hexavalent chromium does appear in fly-ash of coal-fired power plants (Donald G Barceloux, 1999) and from chromate manufacturing sites The highest exposure to hexavalent chromium occurs during chromate production, ferrochrome and chrome pigment production, chrome plating, and stainless steel welding (Donald G Barceloux, 1999)

2.2 Routes of exposure (Chromium)

2.2.1 Air

The bronchial tree is the primary target organ for carcinogenic effects of chromium (VI) Inhalation of chromium-containing aerosols is therefore a major concern with respect to exposure to chromium compounds The retention of chromium compounds from inhalation, based on a 24-hour respiratory volume of 20 m3 in urban

areas with an average chromium concentration of 50 ng/m3, is about 3–400 ng Individual uptake may vary depending on concomitant exposure to other relevant factors, e.g tobacco smoking, and on the distribution of the particle sizes in the inhaled aerosol Chromium has been determined as a component of cigarette tobacco produced in the United States, its concentration varying from 0.24 to 6.3 mg/kg (Lyon, 1990), but no clear information is available on the fraction that appears in mainstream tobacco smoke

2.2.3 Food

The daily chromium intake from food is difficult to assess because studies have used methods that are not easily comparable The chromium intake from typical North American diets was found to be 60–90 µg/day (Health assessment document for chromium, 1984) and may be generally in the range 50–200 µg/day

The chromium content of British commercial alcoholic beverages was reported

to be slightly higher than that of wines produced in the United States, namely 0.45 mg/litre for wine, 0.30 mg/litre for beer, and 0.135 mg/litre for spirits (Health assessment document for chromium, 1984)

Table 1 Levels of daily chromium intake by humans from different routes

2.3 Coconut shell activated carbon

From this analysis it is found out that, the fixed carbon content of coconut shell activated carbon is higher than rice husk and oil cake It shows that the moisture content of activated carbon prepared from coconut shell is high while the volatile and ash content are the least This is due to its high fixed carbon, which is a preferred adsorbent (Susmita Mishra, 2014)

to reduce metals by specific metabolic pathways (S Iravani, 2014)

The most popular chemical approaches, including chemical reduction using a variety

of organic and inorganic reducing agents, electrochemical techniques,

physicochemical reduction, and radiolysis are widely used for the synthesis of silver NPs Most of these methods are still in development stage and the experienced problems are the stability and aggregation of NPs, control of crystal growth, morphology, size and size distribution Furthermore, extraction and purification of produced NPs for further applications are still important issues (S Iravani, 2014)

2.5 Silver nano-activated carbon (AgNPs-AC)

This technique allows producing Ag nanoparticles with the size of 4–30 nm dispersed in a non-toxic solution The Ag nanoparticles were loaded in a high surface activated carbon produced from coconut shell, a popular agricultural waste in Vietnam

by thermal activation The surface area of the best activated carbon is 890 m2/g (Tran Quoc Tuan 2011)

The presence of Ag nanoparticles does not change significantly properties of the activated carbon in terms of morphology The materials are potential for prevention and treatment of microbial infection and contamination for environmental application (Tran Quoc Tuan 2011)

PART III METHOD 3.1 Materials

3.1.1 Chemicals

A stock solution of chromium (Cr(VI)) with a concentration of 1000 mg/L was prepared from dissolving accurately 0.7024 gram of potassium dichromate of (K2Cr2O7) in 250 mL of double distilled water Working solutions were prepared by diluting the stock solution with double distilled water to the desired concentrations

3.1.2 Adsorbent materials

Silver nanoparticles:

Process of making silver nanoparticles:

Add 0.5g flour cassava to a three neck round bottom flask which is covered tightly by straight reflux pipe and rubber corks, containing 250 ml of distilled water Add a magnetic stir bar and place on a stir plate Stir and heat the solution until the liquid is 800C

Add 10 ml of 0.5 M silver nitrate (AgNO3) into the flask After drop wise 10ml

of 8M sodium borohydride (NaBH4), stir and heat for 15 minutes The silver nitrate reduction reaction can be written as:

AgNO3 + NaBH4 → Ag + ½ H2 + ½B2H6 + NaNO3

Loading of Silver Nanoparticles onto the Activated Carbon Granules

o Activated carbon (AC):

AC derived from activated carbon coconut shells, purchased from Tra Bac Company, Vietnam

o Silver nanoparticles (AgNPs):

Silver nanoparticles is made by chemical reduction of silver nanoparticles involves the reduction of a silver salt (silver nitrate) with a reducing agent is sodium borohydride in the presence of colloidal stabilizer.Sodium borohydride has been used with flour casava which is used as stabilizing agents Studied at the physics laboratory, Thai Nguyen University of Sciences, Thai Nguyen University

o AgNPs-AC:

A 5 g of the treated AC granules was impregnated in 100 ml of silver nanoparticles solution of different rate (0.5%, 1.0%, 1.5%, 2%, 2.5% and 3% w/w) under vigorous stirring at room temperature overnight to make sure the loading is complete The activated carbon loaded with silver was then cured in a vacuum oven at 105°C for at least 2h to allow full loading of the silver nano-particles onto the activated carbon Thus, Ag-NPs/AC with different ratio (5 mg/g, 10mg/g, 15mg/g, 20mg/g, 25mg/g, and 30mg/g respectively) was prepared Confirmation of the preparation was done by measuring the difference in weight of the activated carbon granules before and after coating process and measuring the residual silver in solution after loading process Studied at the Environment and Earth Science laboratory, Thai Nguyen University of Sciences, Thai Nguyen University

- Filter paper, dropper, parafilm

 Device

- Melting machine

- Analytical balance

- pH meter (Hanna HI 9025 Romania)

- Shaker machine (HY – 2A, China)

- Atomic Absorption Spectrometry (AAS) (Hitachi Z 2000, Japan)

3.2 Location and research time

Research location: This study was conducted in the laboratory in Faculty of Environment and Earth Science, Thai Nguyen University of Sciences

Study period: This experiment was done in period of 4 months from March

2018 to June 2018

3.3 Research Contents

1 Modified activated carbon coconut shells with nano silve and find the optimum ratio

2 Evaluate the effect of pH on the adsorption process

3 Evaluate the effect of time to adsorption

4 Evaluate the effect of dose absorbed on the adsorption process

5 Evaluate the effect of the initial concentration on adsorption

3.4 Adsorption experiments of Chromium (Cr 6+ ) onto (AgNPs –AC)

Experiments were performed by using batch modes A weight of silver activated carbon (AgNPs-AC) ranged from 10 mg to 100 mg was placed into 50 mL conical flasks containing 25 mL of K2Cr2O7 solution with initial concentrations of

nano-Cr(VI) ranged from 5 mg/L to 80 mg/L The adsorption experiments of nano-Cr(VI) were conducted in batch shake-flasks shaken at 120 rpm orbital for 60 min by shaker machine (PH-2A, China), at room temperature (25±20C)

The experiment was conducted in the laboratory It included experiments along with effect of the AgNPs-AC and activated carbon (AC), pH, contact time, adsorbent dose, initial Cr(VI) concentrations

• Effect of impregnation ratio (AgNPs/AC) and AC have 1 sample of AC and 4 samples of AgNPs-AC and each experiment repeated 3 times, then the total number of

15 samples

• Effect of pH changes from 3 to 10, each pH repeated 3 times and then the total number of 24 samples

• Effect of contact time changes from 5 min to 210 min (5, 10, 15, 30, 60, 90,

120, 150, 180, 210min) , each experiment repeated 3 time and then the total number of

30 samples

• Effect of adsorbent dose changes from 10mg to 100mg (10, 20, 30, 40, 50, 60,

70, 80, 90, 100mg) , each experiment repeated 3 time and then the total number of 30 samples

• Effect of initial Cr(VI) concentrations changes from 5mg/L to 80mg/L (5, 10,

20, 30, 40, 50, 60, 70, 80mg/L) , each experiment repeated 3 time and then the total number of 27 samples

Experiments needed are 126 samples (42 experiments x 3 times = 126 experiments)

Experiment 1: Effect of impregnation ratio (AgNPs/AC) on chromium adsorption

Experiment evaluates of impregnation ratio (AgNPs/AC): There are 6 different denaturation ratios including (0.5%, 1.0%, 1.5%, 2.0%, 2.5%, 3.0%) and rate no silver nano (0%)

The effects of AgNPs-AC were conducted by putting 10 mg of AC (0%) in 25

mL of K2Cr2O7 solution containing 10mg/L of Cr(VI) in 50ml conical flasks; 10mg of AgNPs-AC (0.5%, 1.0%, 1.5%, 2.0%, 2.5%, 3.0%) in 25 mL of K2Cr2O7 solution containing 10mg/L of Cr(VI) in 50ml conical flasks The conical flasks containing Cr(VI) solution were shaken for 60 min at 120 rpm orbital, at room temperature (25±20C)

Experiment 2: Effect of pH

These experiments were conducted after determining the best impregnation ratio of AgNPs/AC The effects of pH on Cr(VI) adsorption by AgNPs-AC were conducted by putting 10 mg of AgNPs-AC in 25 mL of K2Cr2O7 solution containing 10mg/L of Cr(VI) in 50ml conical flasks pH were adjusted between 3 and 10 using

either H2SO4 1M or NaOH 1M solution The conical flasks containing Cr(VI) solution were shaken for 60 min at 120 rpm orbital, at room temperature (25±20C)

Experiment 3: Effect of contact time

The effects of contact time on adsorption capacity of Cr(VI) were conducted with changing of contact time from 5 to 210 min with pH value determined in above experiments with 10 mg of AgNPs-AC in 25 mL of K2Cr2O7 solution The mixtures were placed in to 50 mL conical flasks and then were shaken for 60 min at 120 rpm orbital, at room temperature (25±20C)

Experiment 4: Effect of adsorbent dose

Effects of AgNPs-AC doses on adsorption of Cr(VI) were performed by putting

a dose of AgNPs-AC varying in range from 10 to 100 mg AgNPs-AC into 25 mL of Cr(VI) solution The experiments also were adjusted to optimal pH value and contact time that was determined as above described experiments The conical flasks were shaken at 120 rpm orbital for 60 min, at room temperature (25±20C)

Experiment 5: Effect of initial Cr(VI) concentrations

The effects of initial Cr(VI) concentrations were evaluated by placing each 25

mL K2Cr2O7 solution containing Cr(VI) with varying of initial concentrations from 5

to 80 mg /L into 50 mL conical flasks The mixtures in conical flasks then were adjusted to an optimum pH value, contact time and AgNPs-AC dose that was determined in above described experiments Finally, the conical flasks were shaken at

120 rpm orbital for 60 min, at room temperature (25±20C)

3.4.1 Measurements

Cr(VI) concentrations in the bulk reactor suspensions of all samples in the above experiments were measured Cr is measured by the colorimetric method on Atomic Absorption Spectrometry (AAS) (Hitachi Z 2000, Japan) The pH was measured by pH meter (Hanna HI 9025 Romania)

3.4.2 Data analysis

All experiments were performed in triplicate All data statistics, comprising means, standard deviations, relative standard deviations and regressions (linear) were computed with tools in MS Excel The highest acceptable deviation was 5%

The amount of chromium taken up by the adsorbent was calculated by applying following equation:

Q: is the amount of chromium ion adsorbed on the adsorb of at any time (mg g-1)

C0: is the input concentrations of Cr(VI) at time t0 (mg L-1)

Ct: is the output concentration of Cr(VI) at time t’ (mg L-1)

m: the mass of the adsorbent sample used (g)

V: the volume of the chromium solution (L)

PART IV RESULTS AND DISSCUSSION

4.1 Characterization of the nano-activated carbon

SEM image

When SEM observation pattern Fig 4.1 showed that the steady and porous surface structures are one of the features affecting the adsorption capacity of AC Fig 4.1 shows that stable and porous surface structures are one of the characteristics that affect the adsorption capacity of AC Fig 4.1a shows the porous AC surface structure with high Brunauer–Emmett–Teller (BET) specific surface area of 691.64 m2/g and total pore volume of 0.062 cm3/g Fig 4.1b indicades nano-acivated carbon (2% w/w), noticeably increased surface area 701.65 m2/g and pore volume almost equal about 0.061cm3/g Which found that indicates that silver nanoparticles were successfully loaded onto activated carbon and sponginess increased significantly

Figure 4.1 SEM image of (a) AC and (b) AgNPs-loaded activated carbon (AgNPs-AC),

EDS spectra of (c) AC and (d) AgNPs-loaded activated carbon (AgNPs-AC)

XRD

The crystal structure and phase purity of the prepared silver oxide nanoparticles were identified by measuring the XRD pattern as shown in Fig 4.2 Most characteristic peaks were well indexed to amorphous with a highly graphite crystal structure in the XRD spectra of AC (Fig 4.2a) and AgNPs-AC (Fig 4.2b) XRD results of AC from Fig 4.2a indicates that the broad peak occurred at 30.09° and 61.93° and at 22.63° in case of AgNPs-AC (Fig 4.2b) It proved that carbon graphite

is belonging to AC and AgNPs-AC In the case AgNPs-AC (Fig 4.2b), the Ag-silver appeared on the AC’s surface at the peak of 38.19° and 44.33° This result was agreed