Environ Monit Assess (2015) 187: 470 DOI 10.1007/s10661-015-4692-2

Kinetics of cadmium, chromium, and lead sorption onto chemically modified sugarcane bagasse and wheat straw M. Mahmood-ul-Hassan & V. Suthar & E. Rafique & R. Ahmad & M. Yasin

Received: 8 December 2014 / Accepted: 16 June 2015 / Published online: 27 June 2015 # Springer International Publishing Switzerland 2015

Abstract In this study, cadmium (Cd), chromium (Cr), and lead (Pb) adsorption potential of unmodified and modified sugarcane bagasse and ground wheat straw was explored from aqueous solution through batch equilibrium technique. Both the materials were chemically modified by treating with sodium hydroxide (NaOH) alone and in combination with nitric acid (HNO3) and sulfuric acid (H2SO4). Two kinetic models, pseudo-first order and pseudo-second order were used to follow the adsorption process and reaction fallowed the later model. The Pb removal by both the materials was highest and followed by Cr and Cd. The chemical treatment invariably increased the adsorption capacity and NaOH treatment proved more effective than others. Langmuir maximum sorption capacity (qm) of Pb was utmost (12.8–23.3 mg/g of sugarcane bagasse, 14.5–22.4 mg/ g of wheat straw) and of Cd was least (1.5–2.2 mg/g of sugarcane bagasse, 2.5–3.8 mg/g of wheat straw). The qm was in the order of Pb > Cr > Cd for all the three adsorbents. Results demonstrate that agricultural waste materials used in this study could be used to remediate the heavy metal-polluted water.

Keywords Cadmium . Chromium . Lead . Sorption . Agricultural waste materials M. Mahmood-ul-Hassan (*) : V. Suthar : E. Rafique : R. Ahmad : M. Yasin Land Resources Research Institute, National Agricultural Research Centre, Islamabad 45500, Pakistan e-mail: [email protected]

Introduction Improper management of industrial and municipal discharges is one of the main causes of environmental pollution and degradation, especially in developing countries. Although most of these countries have industrial and municipal discharge regulations and protocols for their safe disposal; their realistic implementation is not strictly ensured. Further, rapid industrialization, particularly cartage industries like metallurgical processing, surgical equipment, cutlery, polishing, pharmaceuticals, galvanizing, and electroplating and dry battery cell, has aggravated the problem and has been excessively releasing heavy metals into the environment through their discharges (Mahmood-ul-Hassan et al. 2012; Kadirvelu et al. 2001). This leads to environmental degradation, destruction of the ecosystem and ultimately poses great health risk, particularly those living in and around the cities. Unlike organic wastes, heavy metals are nonbiodegradable and they can be accumulated in living tissues, causing various diseases and disorders; therefore, they must be removed before discharge to water bodies and arable land. Accumulation of these heavy metals in arable land not only has detrimental effects on the ecosystem functioning but also poses potential health risks for animals and human beings due to transfer of these contaminants into the food chain (KabataPendias 2011; Giller et al. 1998). Removal of heavy metals from industrial and municipal discharges can reduce their hazardous impact substantially on environment, biota, and human and animal health. Conventional techniques, which are being

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practiced to remove the heavy metal, include lime precipitation, ion exchange, adsorption into activated carbon (Dean et al. 1972), membrane processing, and electrolytic methods (Braukmann 1990). Among them, adsorption is a simple and relatively cost-effective method and thus has been widely used (Leung et al. 2000; Kurniawan and Babel 2003a). Basically, adsorption is a mass transfer process by which a substance is transferred from the liquid phase to the surface of a solid, and becomes bound by physical and/or chemical interactions (Kurniawan and Babel 2003b). The sorption rate is of the utmost importance when designing batch sorption systems; consequently, it is important to establish the time dependence of such systems under various process conditions (Ho and Ofomaja 2006). The uses of low-cost agricultural by-products as possible metal sorbents have attracted attention of researchers in recent years. The process is called biosorption and is emerging as a promising technique for the removal of heavy metals from aqueous environments. The biosorption, a fast and reversible reaction of the heavy metals with non-living biomaterials, is called biomass. The biomasses have the ability to bioadsorb heavy metal ions and can be used as low-cost and environmentally friendly absorbents. Structurally, these materials consist of cellulose, hemicellulose, and lignin. The presence of these three biological polymers make them rich in hydroxyl, phenolic, and carbonyl groups which are expected to interact with polyphenols, heavy metal ions by hydrogen bonding, hydrophobic interaction, or complexation (FitzPatrick et al. 2010; Osman et al. 2010). Laszlo and Dintzis (1994) and Osman et al. (2010) have shown that lignocellulosics have ionexchange capacity and general sorptive characteristics, which are derived from their constituent polymers and structure, i.e., extractives, cellulose, hemicelluloses, pectin, lignin, and protein. Agricultural by-products vary greatly in their ability to remove metals from solution. Further, pretreatment of plant waste materials with base solution (sodium hydroxide, calcium hydroxide, sodium carbonate) mineral and/or acid solutions (hydrochloric acid, nitric acid, sulfuric acid, citric acid) removes soluble organic compounds and increases efficiency of metal adsorption (Tarley et al. 2004; Kumar and Bandyopadhyay 2006; Chen et al. 2007; Suradi et al. 2009; Tan and Xiao 2009). The pretreatment can remove lignin, hemicellulose, reduce cellulose crystallinity, and increase the porosity or surface area which, in general, enhances metal adsorption capacities (Chen

Environ Monit Assess (2015) 187: 470

et al. 2007; Suradi et al. 2009). This study aims to investigate the Cd, Cr, and Pb adsorption behavior and kinetics of adsorption on to chemically treated and untreated ground sugarcane bagasse and wheat straw from aqueous solution.

Materials and methods Preparation of agricultural biosorbents Locally available agricultural waste materials, i.e., sugarcane bagasse and wheat straw were collected, washed thoroughly with tap water and then rinsed with deionized distilled water. Then, the samples were sundried and afterward oven-dried at 70 °C. The dried samples were milled to 250 μm. The milled materials were treated with 0.5 M sodium hydroxide (NaOH) to activate the surface groups for 24 h, and then the treated material was divided into three parts. One part was kept as such and second and third parts of all the three sorbents were further treated with 1 M sulfuric acid (H2SO4) and with 1 M nitric acid (HNO3) for 4 h at room temperature, respectively. Subsequently, the samples were rinsed with distilled water several times and dried at 70 °C to a constant weight. Batch sorption experiments Batch sorption experiments were carried out in polyethylene centrifuge tubes to study the effect of contact time and initial metal concentrations. The effect of contact time was studied by equilibrating 1 g of modified and unmodified materials of all three sorbents separately with 50 mL of aqueous solution of Pb, Cd, Cr, Ni, and Cu at 50 mg/dm3. The suspension was shaken constantly on a mechanical shaker at 175 rpm at room temperature for the time period of 10, 20, 40, 60, 90, 120, 150, and 180 min each. The equilibrated suspensions were filtered and the filtrates were collected separately. In another set of experiments, the effect of initial metal ion concentration on adsorption was evaluated. Similarly, 1 g of all the treated and untreated sorbents were equilibrated with 10, 15, 20, 25, and 30 μg/mL solution of Pb, Cd, Cr, Ni, and Cu for 90 min. The equilibrated solutions were filtered and the filtrates were collected separately. The filtrates were analyzed for Pb, Cd, Cr, Ni, and Cu concentrations using atomic absorption spectrometer with graphite furnace (Perkin Elmer AAnalyser

Environ Monit Assess (2015) 187: 470

800). Each data point was obtained from an individual filtrate separately for each experiment. Classical sorption model Langmuir equilibrium isotherm equation was used to calculate the sorption parameters, i.e., maximum sorption and binding strength from the slop of the line and intercept. A common form of the Langmuir equation is as follows: x KCb ¼ m 1 þ KC where C is the equilibrium concentration of adsorbate in question, x/m is the weight of adsorbate per unit weight of adsorbent, K is a constant related to the binding strength, and b is the maximum amount of adsorbate that can be adsorbed (i.e., a complete monomolecular layer). Linear form of the above equation is as follows: C 1 1 . ¼ þ C Kb b x m A plot of C/x/m versus C yields a straight line with a slope l/b and intercept 1/Kb. The Langmuir constant K is obtained by dividing the slope (1/b) by the intercept (1/Kb). In other batch experiments, metal sorption parameters (i.e., maximum sorption capacity and binding strength) were determined. One gram of modified and unmodified biosorbent materials were equilibrated with 50 mL of aqueous solution of varying Pb, Cd, Cr, and Cu concentrations (0, 50, 100, 200, 300, and 400 μg/ mL) by shaking at 175 rpm at room temperature for 3 h. As described above, the suspensions were filtered and filtrates were analyzed for Pb, Cd, Cr, and Cu concentrations using atomic absorption spectrometer with graphite furnace. Mass balance technique was used to calculate the amount of metal sorbed by the biosorbent and metal sorption parameters were calculated using Langmuir model. Filtrates of all the above experiment were analyzed for Pb, Cd, and Cu concentrations with atomic absorption spectrophotometry. Kinetic study The adsorption kinetics, solute uptake rate that controlled by the residence time, is an important characteristic in defining the efficiency of an adsorption process and to understand the behavior of the adsorbent. The

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kinetics of Pb, Cd and Cr adsorption onto the analyzed using pseudo first-order and pseudo-second-order kinetic models. The pseudo-first-order equation The pseudo-first-order kinetics may be expressed as (Ho and Mckay 1999). dqt ¼ k ðqe −qt Þ dt where qe and qt are the amount of metal ion adsorbed at equilibrium and time t, respectively (mg/g), and k is the rate constant of pseudo-first-order adsorption (dm3/ mg min). The pseudo-second-order equation The pseudo-second kinetics rate equations can thus be written as follows: dqt ¼ k ðqe −qt Þ2 dt Rearranging the variables in Eq. (2) gives the following: dqt ðqe −qt Þ2

¼ kdt

Integrating Eq. (2) for the boundary conditions t = 0 and t = t and q = 0 and q = q, gives the following: 1 1 ¼ kt ðqe −qt Þ qe which is the integrated rate law for pseudo-second reaction. However, Eq. (4) can be linearized as four different forms (Table 1). The experiments were carried out at 50 mg/dm3 of Pb, Cd, and Cr concentrations for all studies. Sample (2.5 mL) was withdrawn at suitable time intervals and filtered, and the filtrate was analyzed for the remaining Pb, Cd, and Cr concentrations with atomic absorption spectrophotometry (AAS).

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Environ Monit Assess (2015) 187: 470

Table 1 linearized forms of the pseudo-second kinetic equation Type 1

2

3

Linear form 1 qt

1 qt

Plot

¼ kq12 þ q1 t

¼



1 kq2e

qt ¼ qe −

1

 qt

e

e



1 t

þ

1 kqe

1

1 qe

 

Parameters

 qt

qe = 1/slope

vs:t

vs:t

K2 = slope2/intercept 1

h = 1/intercept



qe = 1/intercept

t

K2 = intercept2/slope h = 1/slope

qt vs:qtt

qt t

qe = intercept k = −1/(intercept × slope) h = −intercept/slope

4

qt t

¼

kq2e −kqe

=t vs:qt

qe = −intercept/slope

qt

qt

k = slope2/intercept h = intercept

Results and discussion Effect of contact time and initial concentrations Effects of contact time on adsorption of Cd, Cr, and Pb were evaluated by conducting batch experiments, and results are presented in Fig. 1 (average of modified and unmodified sugarcane bagasse and wheat). The Cd, Cr, and Pb had similar adsorption trend; a rapid increase in sorption (≈80 % of equilibrium) was attained during the initial phase (20 min), since the active biosorption sites were more available and the metal ions could interact easily with these sites (Tan and Xiao 2009). This rapid initial metal sorption rate has great practical significance in column and continuous process where the contact time between the metal solution and the sorbent is 5.0 Amount adsorbed (mg/g)

Fig. 1 Effect of contact time on Cd, Cr, and Pb adsorption by agricultural waste materials (average of sugarcane bagasse and wheat straw)

generally short. The high initial sorption phase was then followed by a gradual increase to approach an equilibrium in about 60 min. Similarly, Noeline et al. (2005) have also reported 60 min time for Pb adsorption equilibrium from an aqueous solution (10–10 mg/dm3) by polymerized banana stem and Serencam et al. (2013) reported 60 min equilibrium time for Cd from 50 mg/ dm3 Cd solution with Morus alba L. pomace. While Tan and Xiao (2009) attained quicker Cd adsorption equilibrium (in 30 min) when ground wheat stems was exposed to 22.5 mg/dm3 Cd solution. Although, Cd, Cr, and Pb were equilibrated at the same contact time (60 min), the high Pb ions sorption at each given contact time was most probably due to ions size—smaller sizes are heavily hydrated and make the size larger and bulkier than the less hydrated like Pb ion. This process

4.0

3.0

2.0 Cd

1.0

Cr

Pb

0.0 0

25

50

75 100 125 Time (minutes)

150

175

200

Environ Monit Assess (2015) 187: 470

Page 5 of 11 470 100

Metal removal (%)

Fig. 2 Metal removal percentage of agricultural by-products sorbents from aqueous solution

90

Raw

NaOH

80

HNO3

H2SO4

70 60 50 40 30 20 10 0 Cd

Cr

Pb

Cd

Sugarcane bagasse

enhances their sorption chances to the reactive sites as the heavily hydrated ions migrate slowly in aqueous solutions (Chen et al. 2010). Results (average of all three modified and unmodified materials) showed that an increase in adsorption was observed when 0.5 g sorbent was exposed to incremental initial metal concentration. The increase in metal adsorption capacity with increasing initial metal concentrations may be due to higher probability of collision between metal ions and adsorbent particles (Tijani et al. 2011). The variation in the extent of adsorption may also be due to the fact that initially, all sites on the surface of adsorbent were vacant and the solute concentration

The amount of metal uptake (biosorption capacity) per gram of the biomass q (mg/g) was calculated as follows:   C i −C f ⋅V q¼ ms 25

1.5 1.0 0.5

Pb adorpon (103 μg/g)

2.0

5.0 4.0 3.0 2.0 1.0 0.0

0

a

b adsorpon (103 μg/g)

Cr adsorpon (103 μg/g)

2.5 2.0 1.5 1.0 0.5 0.0

5.0 4.0 3.0 2.0

50 100 150 200 250 300 350 400 Equilibrium Concentraon (μg/mL)

Untreated NaOH HNO3 H2SO4

5

50 100 150 200 250 300 Equilibrium Concentraon (μg/mL)

20.0 16.0 12.0 Untreated NaHO HNO3 H2SO4

8.0 4.0

1.0

0.0

0.0 0

10

24.0

6.0

3.0

15

0

50 100 150 200 250 300 350 400 Equilibrium Concentraon (μg/mL)

7.0

3.5

20

0 0

50 100 150 200 250 300 350 400 Equilibrium Concentraon (μg/mL)

4.0

Cd sorpon (103 μg/g)

Effect of chemical pretreatment on metal removal

6.0

0.0

b

Pb

gradient was relatively high. Although, high initial concentrations lead to an increase in the affinity of the metal ions towards the active sites, at low concentrations, adsorption sites took up the available metal more quickly (Saifuddin et al. 2005).

7.0

Cr adsorpon (103 μg/g)

Cd adsorpon (103 μg/g)

2.5

Cr Wheat straw

0

50 100 150 200 250 300 350 400 Equilibrium Concentraon (μg/mL)

0

50

100

150

200

250

300

Equilibrium Concentraon (μg/mL)

Fig. 3 Langmuir isotherm plot for Cd, Cr, and Pb removal using untreated and treated sugarcane bagasse (a) and wheat straw (b) at 25 °C

470 Page 6 of 11 0.30

Environ Monit Assess (2015) 187: 470 0.16

Sugarcane bagasse

C/(x/m) (g/mL)

Wheat straw

0.14

0.25

0.12 0.20

0.10 0.08

0.15

0.06

0.10

0.04 0.05

0.02 0.00

0.00 0

50

100

150

200

250

300

350

0

400

50

100 150 200 250 300 350 400

a 0.10

0.08 Sugarcane bagasse

0.08 C/(x/m) (g/mL)

Wheat straw

0.07 0.06 0.05

0.06

0.04 0.04

0.03 0.02

0.02

0.01 0.00

0 0

50

100

150

200

250

300

350

400

0

50

100 150 200 250 300 350 400

b 0.030

0.024

C/(x/m) (g/mL)

0.025

0.020

0.020

0.016

0.015

0.012

0.010

0.008

0.005

0.004

Untreated NaOH HNO3 N2SO4

0.000

0.000 0

c

Wheat straw

Sugarcane bagasse

50

100

150

200

250

Equilibrium Concentration (mg/L)

300

0

50

100

150

200

250

300

Equilibrium Concentration (mg/L)

Fig. 4 Linearized Langmuir isotherm for Cd (a), Cr (b), and Pb (c) removal by untreated and treated agricultural biomasses ta 25 °C

where Ci is the metal initial (μg/mL) of equilibrating solution, Cf is the final concentration (μg/L) of the solution, V is the volume of the solution (mL), and ms is the mass of sorbent in grams. Metal removal capacity of chemically treated and untreated sugarcane bagasse and wheat straw varied in their ability to remove heavy metal ions from solution (Fig. 2). Results showed that chemical treatment

increased the metal removal capacity of both the materials most likely due to the hydrolysis of hemicellulose with alkaline or acid. During hydrolysis, releases of monomeric sugars and soluble oligomers from the cell wall matrix into the hydrolysate increased porosity and chelating properties of the plant materials and hence enhanced metal sorption capacity and efficiency (Gaballah et al. 1997; Chen et al. 2007). Wheat material

Environ Monit Assess (2015) 187: 470

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Table 2 The Langmuir parameters for cadmium, chromium and lead adsorption on chemically treated and untreated materials Treatments

Cadmium

Chromium

Lead

b mg/g

K L/g

r2

b mg/g

K L/g

r2

b mg/g

K L/g

r2

1.446

0.014

0.94

4.018

0.035

0.99

12.801

0.019

0.98

Sugarcane bagasse Untreated NaOH

2.137

0.068

0.99

6.782

0.033

0.99

21.242

0.059

0.99

HNO3

1.917

0.016

0.99

5.393

0.014

0.94

18.724

0.024

1.00

H2SO4

2.221

0.028

0.98

5.122

0.023

0.99

23.251

0.071

1.00

2.528

0.032

0.99

5.066

0.082

0.99

14.513

0.034

0.99

Wheat Untreated NaOH

3.075

0.032

1.00

7.550

0.025

0.99

21.237

0.078

0.98

HNO3

3.833

0.045

0.99

6.677

0.006

1.00

22.398

0.022

0.99

H2SO4

2.776

0.092

0.99

6.081

0.005

1.00

18.345

0.024

0.99

removed relatively more Cd and Cr than that of sugarcane bagasse, while Pb removal was almost similar. Previously, Wartelle and Marshall (2000) reported that a material having more lignin may block or allow little penetration of treater to reactive sites, hence lowered metal ion uptake than those materials having less lignin content. The increase in Cd and Cr removal was higher when the material was treated with NaOH than those treated with acids. Pretreatment with alkaline eliminates residual lignin which breakdown the fiber bundles led increasing the effective surface area and surface charge exposed further hydroxyl and carboxyl groups (Suradi et al. 2009). This may be because alkaline reacts with cementing materials of the fiber, splitting the fibers into finer filaments. In addition, combination of alkaline with peroxide enhanced the porosity and pore size on the fiber size which improved physical interlocking leading better interfacial bonding between fiber and matrix. Hence, alkaline peroxide enhanced the surface charge, porosity, and pore size on the fiber surface that favors metal uptake. Adsorption isotherms Langmuir metal adsorption isotherms (Fig. 3) showed initially a rapid increase in metal adsorption, most likely associated with surface properties and subsequently a gradual rise, due to diffusion (Tan and Xiao 2009). Langmuir sorption parameters, r2 regression coefficient, b the maximum adsorption capacity, and K binding

strength, were derived from linearized form of isotherms (Fig. 4) and presented in Table 2. The regression coefficient, r2, values as high as one clearly indicates the goodness of fit for explaining adsorption of Cd, Cr, and Pb (Table 2). The treatment, either with base or acid, increased the Cd, Cr, and Pb maximum adsorption capacity of both the materials. Pretreatment can remove lignin, hemicellulose, reduce cellulose crystallinity, and increase the porosity or surface area and chelating efficiency (Gaballah et al. 1997; Chen et al. 2007). Further, pretreatment also increases the number of the functional groups which enhance the binding capacity of ground plant material (Tan and Xiao 2009). For example, Kumar and Bandyopadhyay (2006) reported that rice husk treated with NaOH enhanced the adsorption capacity of Cd. Tarley et al. (2004) found that adsorption of Cd increase by almost double when rice husk was treated with NaOH. Similar increase in heavy metal adsorption by banana stalk has also been reported by Annadurai et al. (2002). Treatment with NaOH greatly increased maximum adsorption capacity of metal while acid-treated materials showed relatively less increase over the untreated materials. The increase in maximum adsorption capacity of heavy metal after base treatment could be explained by the increase in the amount of galactouronic acid groups after hydrolysis of Omethyl ester groups (Low et al. 2000). Marshall and Johns (1996) observed a 26 % increase in soybean hulls, adsorption capacity after NaOH treatment compared with control.

470 Page 8 of 11

120

Sugarcane bagasse

Wheat straw

100

t/qt (min g/mg)

t/qt (min g/mg)

200

Environ Monit Assess (2015) 187: 470

150 100 50

80 60 40 20

0

0 0

40

80 120 t (min.)

160

200

0

40

80 120 t (min)

160

200

a 2.0

1.2 Sugarcane bagasse

1/qt (g/mg)

1.5 1/qt (g/mg)

Wheat straw

1.0

1.0 0.5

0.8 0.6 0.4 0.2 0.0

0.0 0

0.02 0.04 0.06 0.08 1/t (1/min)

0.1

0

0.12

0.02 0.04 0.06 0.08 1/t (1/min)

0.1

0.12

b 5.0

5.0

Sugarcane bagasse

4.0 qt (mg/g)

4.0 3.0 2.0

t

q (mg/g)

Wheat straw

3.0 2.0 1.0

1.0

0.0

0.0 0

0.05

0.1 0.15 qt/t (mg/g min)

0

0.2

0.04 0.08 0.12 0.16 0.2 0.24 0.28 qt/t (mg/g min)

c 0.25

0.3

Sugarcane bagasse

qt (mg/g)

0.20 qt (mg/g)

Wheat straw

0.3

0.15 0.10 0.05

Cd Cr Pb

0.2 0.2 0.1 0.1 0.0

0.00 0

1

2 3 qt (mg/g)

4

5

0

1

2 3 qt (mg/g)

4

5

d Fig. 5 Linearized pseudo-second-order type 1 (a), 2 (b), 3 (c), and 4 (d) for Cd, Cr, and Pb sortion potential on sugarcane bagasse and wheat straw

Environ Monit Assess (2015) 187: 470

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Table 3 Pseudo-second-order kinetic parameters obtained by using the linear methods at 50 mg/L concentration Type

1

2

3

4

Parameter

Sugarcane bagasse

Wheat straw

Cd

Cr

Pb

Cd

Cr

Pb 4.5085

qe (mg/g)

1.2416

2.3513

4.6253

1.8550

2.8125

k (g/mg min)

0.088

0.04807

0.0295

0.08239

0.03934

0.0431

h (mg/g min)

0.136

0.266

0.630

0.284

0.628

0.876

r2

1.00

0.99

0.922

1.00

1.00

1.00

qe (mg/g)

1.2681

2.6229

5.2000

1.883

2.9572

4.6709

k (g/mg min)

0.0643

0.0215

0.0115

0.0559

0.0398

0.0258

h (mg/g min)

0.103

0.148

0.312

0.198

0.348

0.562

r2

0.97

0.95

0.96

0.97

0.97

0.99

qe (mg/g)

1.2587

2.4464

4.9258

1.8640

2.9227

4.6351

k (g/mg min)

0.06764

0.03079

0.01526

0.06012

0.04343

0.02726

h (mg/g min)

0.107

0.184

0.370

0.209

0.371

0.586

r2

0.94

0.91

0.93

0.93

0.94

0.97

qe (mg/g)

1.2788

2.575

5.0969

1.8887

2.9530

4.6593

k (g/mg min)

0.06177

0.0236

0.01275

0.05502

0.04028

0.02624

h (mg/g min)

0.101

0.157

0.331

0.196

0.351

0.570

r2

0.94

0.91

0.921

0.93

0.094

0.97

The maximum adsorption capacity decreased in the order of Pb > Cr > Cd and seemed to be inversely proportional to the hydrated ionic radii of the metals, being Pb (4.01 Å) > Cr (4.19 Å) > Cd (4.26 Å) (Lide 2005). The smaller ionic radii of Cd compared to Pb means greater tendency of Cd to be hydrolyzed, leading to reduced sorption (Horsfall and Spiff 2005). This is in agreement with results from Danny et al. (2004) and Lee and Rowell (2004). Hillel (1998) explained that the smaller the ionic radius and greater the valence, the more closely and strongly the ion adsorbed. On the other hand, the greater the ions’ hydration, the farther it is from the adsorbing surface and the weaker its adsorption. The proportionate increase in maximum adsorption capacity due to treatment (as compared to untreated) was higher in sugarcane bagasse than that of wheat straw. For example, mean increase in maximum Pb adsorption capacity of sugarcane bagasse was 64 % and in wheat straw was 42 % over untreated materials. The less increase in maximum sorption could be associated with high lignin content of sugarcane bagasse. Previously, Wartelle and Marshall (2000) reported that a material having high bulk density, the lignin can block or allowed little penetration of citric acid to reactive sites, hence lower uptake. Comparison of present study

with few other adsorbents used in past reveals that maximum adsorption capacity of all the materials was comparable (Bulut and Tez 2003; Gupta and Ali 2004), however slightly less than those reported by Horsfall and Spiff (2005). Sorption kinetics Linear regressions were used for determining goodness of fitness of kinetic models and four linearized form of pseudo-second-order kinetic model (Table 1) were used to estimate different parameters (Ho and Ofomaja 2006). Cadmium, Cr, and Pb sorption kinetic parameters, i.e., qe (the amount of metal ion adsorbed at equilibrium), k (pseudo-second-order kinetic model constant) and h (the initial adsorption rate), onto sugarcane bagasse and wheat straw were obtained by plugging experimental data in linear equations of the four pseudo-second-order kinetic models (Fig. 5 and Table 3). High regression coefficient (r2) values indicate that the Cd, Cr, and Pb adsorption onto both materials follows the pseudo-second-order kinetic expression. The calculated qe values were also agreed very well (r2 < 0.98) with the experimentally observed values for both the materials and for all the metals. These suggest the suitability of pseudo-second-order kinetic

470 Page 10 of 11

expression for the experimental data. Similar results were also observed in adsorption of Cd and Pb onto sawdust [Yasemin and Zeki 2007) and in adsorption of Cu on palm kernel fiber (Ho and Ofomaja 2006).

Conclusions Results of the present study demonstrate that both materials, sugarcane bagasse and ground wheat straw, have fairly high metal sorption capacity and chemical modification generally improved the adsorption. The rapid metal sorption, >80 % in 20 min, has great practical significance in column and continuous process where the contact time between the metal solution and the sorbent is generally short. Different variants of the pseudo-second-order kinetic model have good agreement with the Cd, Cr, and Pb sorption behavior onto sugarcane bagasse and ground wheat straw. Further, the metal sorbents used in this study have the potential for the removal of Cd, Cr, and Pb ions from wastewater. Acknowledgments The research work was financially supported by the Pakistan Agricultural Research Council through the BResearch for Agricultural Development Program.^ We thank Saif-ur-Rehman and Riaz-ul-Haq for the assistance in laboratory experimental and analytical work.

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Kinetics of cadmium, chromium, and lead sorption onto chemically modified sugarcane bagasse and wheat straw.

In this study, cadmium (Cd), chromium (Cr), and lead (Pb) adsorption potential of unmodified and modified sugarcane bagasse and ground wheat straw was...
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