Removal of Cadmium from Aqueous Solutions by Charcoals Prepared from Giant Reed (Arundo donax)

ABSTRACT Giant reed (Arundo donax) is a perennial, gramineous plant that has high biomass production potential. In this study, we focused on its applicability as a raw material for the production of adsorbents for the purification of cadmium-contaminated water. Charcoals were prepared from the stalk of the giant reed at various temperatures (400 - 700°C) under nitrogen stream. Analysis of pore size distribution based on methanol vapor adsorption revealed that the mesopores within the range of 20 to 100 Å were abundant in charcoals prepared under 400°C and 500°C. Using these charcoals, the removal capacity of cadmium from aqueous solution was investigated. As a result, high removal capacity under low concentration of cadmium was observed. High yield of cadmium with 0.01 N hydrochloric acid treatment was also clarified. X-ray diffraction (XRD) analysis revealed that Cd(OH)2 existed on the surface of the cadmium-adsorbed charcoal. The possible mechanism of the “apparent adsorption” was discussed

Removal of Cadmium from Aqueous Solutions by

Charcoals Prepared from Giant Reed (Arundo donax)

Masaki SAGEHASHI*, Takao FUJII**, Hong Ying HU***, Akiyoshi SAKODA**

*Center of Education for Leaders in Environmental Sectors, Tokyo University of Agriculture

and Technology, Tokyo 184-8588 Japan

**Institute of Industrial Science, The University of Tokyo, Tokyo 153-8505 Japan

***Environmental Simulation and Pollution Control State Key Joint Laboratory, Department of

Environmental Science and Engineering, Tsinghua University, Beijing 100084, PR China

ABSTRACT

Giant reed (Arundo donax) is a perennial, gramineous plant that has high biomass production

potential In this study, we focused on its applicability as a raw material for the production of adsorbents for the purification of cadmium-contaminated water Charcoals were prepared from the stalk of the giant reed at various temperatures (400 - 700°C) under nitrogen stream Analysis

of pore size distribution based on methanol vapor adsorption revealed that the mesopores within the range of 20 to 100 Å were abundant in charcoals prepared under 400°C and 500°C Using these charcoals, the removal capacity of cadmium from aqueous solution was investigated As a result, high removal capacity under low concentration of cadmium was observed High yield of cadmium with 0.01 N hydrochloric acid treatment was also clarified X-ray diffraction (XRD) analysis revealed that Cd(OH) 2 existed on the surface of the cadmium-adsorbed charcoal The possible mechanism of the “apparent adsorption” was discussed

Keywords: cadmium, charcoal, giant reed, water treatment

INTRODUCTION

The shortage of freshwater and its declining quality have been one of the serious issues

at present On the other hand, the progress of global warming as well as the depletion of fossil fuels are other important environmental concerns Thus, the development of energy-saving water purification methods is essential

Constructed wetlands to purify eutrophicated water (Sagehashi et al., 2009a; 2009b), or

water containing harmful compounds and heavy metals are examples of such methods because they are based fundamentally on natural energy (e.g solar energy), and

potentially produce biomass resources Giant reed (Arundo donax), a perennial,

gramineous plant having high biomass production potential, can be employed in constructed wetlands The harvested giant reed biomass can be used as a non-wood raw material for pulp (Shatalov and Pereira, 2002), and, recently attracted attention as an

energy crop (Angelini et al., 2009)

To use such characteristics, we proposed a sustainable energy and material cycles

system using giant reed (Sagehashi et al., 2009a) Meanwhile, the adsorptive

purification of polluted water using inexpensive adsorbents is an energy and cost-saving method A number of researches have been conducted regarding the preparation of adsorbents from low-cost materials such as agricultural wastes Wetland plants including giant reed can be used as raw materials for the production of adsorbents This leads to the rational, on-site utilization of the biomass produced in constructed wetland

of which the contaminated water is nearby The important factors to be considered are the preparation of the adsorbent, and its removal performance for the contaminants Among harmful heavy metals, cadmium was selected and used as a pollutant in this study, because of its toxicity Many studies on cadmium removal with constructed wetlands were reported (e.g., Pimpan and Jindal,2009) Moreover, cadmium can be

removed with various adsorbents (e.g., Yadanaparthi et al., 2009) Basso and Cukierman

(2006) reported the adsorption of nickel on an adsorbent prepared from giant reed However, there are only a few studies regarding cadmium adsorption on charcoal prepared from giant reed with simple methods In this study, to estimate the ability of a simple carbonization process, charcoals were prepared from the stalk of giant reed under various temperatures with nitrogen stream without adding any activating reagents Physical characteristics of these charcoals were clarified as well as their cadmium removal capacity

MATERIALS AND METHODS

Carbonization

Fertilized giant reed grown at a managed area in the Kanto Plain, Japan, was used for experiments The stalk was milled by a mortar, and sieved into a fraction of 20 to 100

μm The fraction was carbonized in a quartz tube-equipped electrical furnace under nitrogen stream for 1 hour The electrical furnace was set at a temperature between 400 and 700ºC Charcoals were then obtained and used in the following tests

Physicochemical analysis of charcoals

The vapor-phase methanol adsorptions on the charcoals were measured using the quartz-balance adsorption measurement equipment (Fujii and Sakoda, 2008) The pore

size distribution was estimated based on the methanol adsorption (Urano et al., 1970)

The BET specific surface area was calculated by the methanol adsorption results The elemental component (i.e., carbon, nitrogen and hydrogen) was measured with a CHN analyzer (model 2400-II, Perkin-Elmer, USA) For the mineral analysis, the following methods were employed The stalk was incinerated under 550ºC, and washed with 1 N hydrochloric acid (HCl) The concentrations of Na, K, Ca, and Mg in the HCl solution were measured by an atomic absorption analyzer (AAnalyst 200, Perkin-Elmer, USA), and the contents of these metals were calculated The X-ray diffraction (XRD) analysis was performed with an X-ray diffractometer (RINT2000, Rigaku, Japan) for the estimation of the form of Cd on the charcoal

Adsorption of Cadmium

Two-hundred milliliters (for adsorption rate estimation; results are shown in Fig 2) or

100 mL (for adsorption capacity measurement; results are shown in Fig 3) of cadmium chloride solutions (20 mg-Cd/L) and a prescribed amount of the carbonized stalk (charcoal) of giant reed (10 - 100 mg in each) was placed in an Erlenmeyer flask The flask was kept under 25ºC and shaken at 350 rpm by a rotary shaker (NR-20, TAITEC, Japan) After the prescribed contact time, the solution was filtrated (Millex GV, pore size = 0.22 μm, MILLIPORE, USA), and the concentration of cadmium was measured

by the atomic absorption analysis described above, and the amount adsorbed was calculated The variation of pH in the solution was also measured for the adsorption rate

estimation (PH1500, EUTECH INSTRUMENTS, Singapore)

Desorption of Cadmium

An adsorption experiment using the charcoal prepared under 700ºC was performed

basically in the same manner as mentioned above, in which the initial concentration of

cadmium was 10 mg-Cd/L, the volume of solution was 50 mL, and the amount of

charcoal was 20 mg Three days from the start of contact (concentration of cadmium

was 1 mg-Cd/L, amount of cadmium adsorbed was 22 mg/g), the charcoal with

adsorbed cadmium was transferred into a 0.01 N HCl solution After 6 hours of contact

time, the concentration of cadmium in solution was measured and the amount desorbed

was evaluated

RESULTS AND DISCUSSION

Components of the charcoal

Minerals contained in giant reed are shown in Table 1 The minerals contained in the

charcoal were calculated with reference to the weight of the giant reed used for the

carbonization Especially, the potassium (K) content was high The chemical

components of a charcoal prepared under 700ºC are shown in Table 2

Pore structure of the charcoals

The pore size distributions obtained by the methanol adsorption are shown in Fig 1

The BET specific surface areas calculated from the methanol adsorption are also shown

in the figure as well as the differential pore size distribution This figure indicated that

the relatively large pores, i.e the mesopores ranging from 20 to 100 Å, were abundant

in charcoals prepared under 400°C and 500°C Such adsorbent can be applied for the

adsorption of large molecules, and it is one of the important features of charcoals

prepared from giant reed With the increase in preparation temperature, the mesopores

were diminished, and charcoals with abundant micropores were obtained

In this study, the carbonization time was not changed but was fixed at 1 hour, and the

heating rate was not controlled since temperature dependency was not the focus of the

study Meanwhile, Xia et al (2000) demonstrated the effects of heating rate on the yield,

specific surface area, pore volume, and adsorption capacities of the carbonized

sugarcane baggase under nitrogen stream Therefore, further research about the effects

of heating rate and carbonization time on the yield and characteristics of the fabricated

charcoal might improve the preparation methodology of the carbonization materials

from giant reed

Table 1 - Ash components of the charcoal prepared under 700ºC

Contents [mg/g-giant reed] 0.68 3.32 1.19 1.29

Table 2 - Carbon, hydrogen, nitrogen and ash in the charcoal prepared under 700ºC

100

200

300

細孔半径 [Å]

500 600 700

0.3

0.2

0.1

0

Pore radius [Å]

preparation temperature [ºC]

specific surface area [m 2 /g]

530 230 160 150

0 1 2 3 4 5

細孔半径 [Å]

600 700

preparation temperature [ºC]

Pore radius [Å]

Fig 1 - Pore structures of the charcoals prepared from the stalk of giant reed (Left:

cumulative pore volume, Right: differential pore size distribution)

4

5

6

7

8

9

10

0

20

40

60

0 24 48 72 96 120 144 168

0 10 20 30

Time elapsed [hr]

4 5 6 7 8 9 10

0 20 40

0 24 48 72 96 120 144 168

0 10 20 30

Time elapsed [hr]

Fig 2 - Time courses of pH (◇) in the solution, cadmium concentration (○), and

calculated cadmium adsorbed on charcoal (▲) (The charcoal prepared under 700ºC was used and the dosages of charcoal were 316 mg/L (Left) and 250 mg/L (Right))

Removal of cadmium from aqueous solutions

For estimating the removal rate, time courses of cadmium concentration in aqueous solution and calculated amount adsorbed were measured as well as the pH value of the solution for two cases with different dosage of adsorbent, whereas the initial concentration of cadmium and the volume of solution were the same The results are shown in Fig 2, indicating that the pH value was rapidly increased with time in each case at the beginning, and the concentration of cadmium in the solution was decreased The alkaline metal oxides and alkaline-earth metal oxides such as K2O, Na2O, CaO, and MgO, which were produced by the carbonization, were considered as the cause of this sudden elevation of pH Meanwhile, when pH was decreased as shown on the left side

of Fig 2, the cadmium concentration in the solution increased with the decrease in pH The decrease in pH observed still needs to be clarified in further investigations However, it is elucidated that the amount adsorbed was strongly dependent on the pH of the solution

The existing form of cadmium in aqueous solution is different with its pH value (Babić

et al., 2002) At low pH range, cadmium mainly exists as Cd2+ With the increase in pH, the dominant existing form is changed to the following order: Cd(OH)+, Cd(OH)2, and Cd(OH)3- According to Babić et al (2002), at a concentration of 0.01 mmol-Cd/L,

almost all cadmium ions existed in the form of Cd2+ in pH < 5, while the abundance ratios of Cd2+ / Cd(OH)+ / Cd(OH)2 / Cd(OH)3- were 44 / 56 / 0 / 0, and 7 / 91 / 2 / 0 at

pH 8 and 9, respectively Therefore, there is a possibility that desorption of cadmium observed in Fig 2 (especially on the left side) might be caused by this change in existing form

The apparent adsorption isotherms of cadmium on the charcoals are shown in Fig 3 The amounts adsorbed on the charcoals after washing with acid (1N HCl) are also plotted in the figure In this experiment, the amounts adsorbed were determined after three days from the start of contact Furthermore, the amounts adsorbed on other adsorbents found in the literature are included in the figure

As compared to other adsorbents, the cadmium adsorption capacity of the charcoal of giant reed is significantly large at low concentration range As described in Fig 2, it should be noted that desorption was caused by the increase in pH However, it can be stated that charcoals prepared from the stalk of giant reed are high-performance adsorbents for cadmium removal Moreover, in the desorption experiment, 97% of cadmium was eluted into the HCl solution It indicated that the adsorbed cadmium can

be recovered efficiently Therefore, it can be concluded that the charcoal prepared from giant reed is useful for cadmium removal

On the other hand, the amounts adsorbed on acid washed (i.e., mineral dissipated) charcoals were very low compared with non-acid washed charcoals It suggested that the minerals included in the charcoal, which can be removed by acid wash, should play important roles for the “apparent” adsorption of cadmium The alkali metal hydroxides react with cadmium ions in water to form Cd(OH)2, which is insoluble and adhered on charcoal Fig 4 shows the results of the XRD analysis, indicating that the crystallized Cd(OH)2 was observed at the surface of the cadmium-adsorbed charcoals Ichinose et

al., (2004; 2005) reported that cadmium hydroxide nanostrands were abundantly

observed in dilute Cd(NO3)2 solutions containing a small amount of NaOH (pH = 7.0 -

8.5) The nanostrand was extremely positively charged (Ichinose et al., 2004) Therefore,

it was suggested that precipitation of cadmium including such positively charged nanostrand occurred on the negatively charged surface of charcoals Moreover, we confirmed that the removal of manganese also made a very insoluble hydroxide, Mn(OH)2, with the charcoals (data not shown) We suppose that this mechanism also took place in the removal of manganese This is why “apparent” adsorption isotherms is used to describe Fig 3

0.1 1 10 100

Cd [mg/l]

400℃

500℃

600℃

700℃

literature values acid wash

charcoal (giant reed)

Concentration of cadmium in water [mg-Cd/L]

1)Ferro-García et al., (1988); 2)Seki et al., (2006); 3)Petrov et al., (1992); 4)Li et al., (2003) 5)Babić et al., (2002); 6)Minamisawa et al., (2002); 7)Suzuki et al., (2005)

Fig 3 -Apparent adsorption isotherms for the charcoals prepared and other adsorbents

0 200 400 600 800

2θ [deg.]

0 200 400 600 800 1000

charcoal with cadmium adsorbed

Cd(OH) 2

unused charcoal

Fig 4 - XRD results of the charcoal with cadmium adsorbed (Black plots: charcoal with

cadmium adsorbed, and Cd(OH)2 (left axis); Gray plots: unused charcoal (right axis))

CONCLUSIONS

Removal of cadmium from aqueous solutions by the charcoals prepared from the stalk

of giant reed was investigated based on the beneficial utilization of giant reed As a result, its high removal capacity was confirmed The cadmium retained in the charcoals was easily desorbed with hydrochloric acid, revealing that the regenerated charcoals can

be used for various purposes such as solid fuels In addition, the pore size can be controlled upon changing the carbonization temperature Especially, mesopores were developed in low carbonization temperature, suggesting that it can be applied for the adsorption of large molecules

ACKNOWLEDGEMENTS

This work was partly supported by Japan Science and Technology Agency (JST) -National Natural Science Foundation of China (NSFC) Joint Program (Strategic International Cooperative Program)

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