Kinetics and equilibrium of Cu(II) adsorption onto chemically modified orange peel cellulose biosorbents

Hydrometallurgy 95 (2009) 145–152

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Hydrometallurgy j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / h yd r o m e t

Kinetics and equilibrium of Cu(II) adsorption onto chemically modified orange peel cellulose biosorbents Dandan Lu a, Qilin Cao b, Xiaomin Li a, Xiuju Cao a, Fang Luo a,⁎, Wenjing Shao a a b

Key Laboratory of Polyoxometalates Science of Ministry of Education, College of Chemistry, Northeast Normal University, Changchun, 130024, PR China Duoluoshan Sapphire Rare Metal Co. Ltd of Zhaoqing, Sihui, 526200, PR China

A R T I C L E

I N F O

Article history: Received 30 December 2007 Received in revised form 13 May 2008 Accepted 16 May 2008 Available online 23 May 2008 Keywords: Orange peel cellulose Chemically modified Biosorption Copper Kinetics Langmuir isotherm Freundlich isotherm

A B S T R A C T A series of orange peel cellulose biosorbents has been specifically prepared by different chemical modifications to understand the mechanism of copper adsorption from chloride solutions. The different biosorbents and raw orange peels were characterized using elemental analysis and Fourier transform infrared spectroscopy (FTIR). The acidic and basic sites and pH of zero charge were also determined. The influences of pH, contact time, initial copper concentration and solid/liquid ratio on copper removal were examined. The maximum adsorption capacity of copper was 1.22 mol/kg, using orange peel esterified by 0.6 mol/L citric acid at 80 °C after 0.1 mol/L NaOH saponification. A comparison of different isotherm models revealed that the combination of Langmuir and Freundlich (L–F) isotherm model fitted the experimental data best. Results indicate that the chemically modified orange peel cellulose can provide an efficient and costeffective technology for eliminating copper from aqueous solution. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Heavy metal pollution represents an important environmental problem due to its toxic effects to the environment and accumulation throughout the food chain. The main sources of heavy metal pollution are mining, milling and surface finishing industries, discharging a variety of toxic metals such as Cu, Zn, Cd and Pb into the environment. The conventional technologies for the removal of toxic metals from the wastewater include ion exchange, chemical reduction, chemical precipitation, electrochemical treatment, membrane separation etc. In general, most have high cost and are ineffective when heavy metals are present in the wastewater at low concentrations. Therefore an efficient and very cost-effective treatment method is required to treat large volumes of industrial heavy metal-bearing wastewaters (Volesky 2001; Leusch and Volesky 1995). Biosorption removes metal ions by biological materials and biomaterials and have been considered as potentially important sorbents for heavy metal removal. Of particular interest are the abundant biomass generated as a waste by-product of large-scale industrial processes and vegetable biomass, such as marine algae (Luo et al., 2006), rice husk (Chockalingam and Subramanian 2006), sawdust (Larous et al., 2005), crop milling waste (Saeed et al., 2005), corncob (Leyva-Ramos et al., 2005), cellulose/chitin beads (Zhou et al., 2005), etc. ⁎ Corresponding author. Tel.: +86 431 8509 9667. E-mail address: [email protected] (F. Luo). 0304-386X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2008.05.008

In this Journal, studies have been reported on the adsorption of heavy metals by biomaterials derived from the marine algae (Feng and Aldrich, 2004); the biosorption of heavy metals by Sphaerotilus natans (Esposito et al., 2001); the adsorption of lanthanum and cerium on leaf (Sert et al., 2008) and the copper adsorption on calcium alginate beads (Veglio et al., 2002). Such studies provide information on the fate of metal ions in effluents in marine or natural aquatic environments. In our previous work, we have investigated in detail the removal of Pb(II), Cd(II), Zn(II), Ni(II) and Co(II) in hydrochloric systems by orange peel adsorbents and in this work we consider the adsorption behavior of Cu(II) ions. Orange peel residue cellulose was chosen as biosorbent due to its special structure, insolubility in water, chemical stability and local availability. This orange product in China is the third largest in the world (Xuan et al., 2006; Li et al., 2007). Orange peel principally consists of cellulose, hemi-cellulose, pectin substances, chlorophyll pigments and other low molecular weight compounds like limonene. These components contain various interesting functional groups such as carboxyl, hydroxyl and amido-cyanogen, which play an important role in removing the heavy metals (Gross 1977). In this study, a series of orange peel cellulose adsorbents were was purposefully prepared by means of different chemical modifications to explain the adsorption mechanism. The effects of different alkaline saponification (NaOH, NH4 OH and CaCl2) and esterification by citric acid, oxalic acid and phosphoric acid on the character of the adsorbents were examined under different conditions, including acid concentration and temperature. Various

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D. Lu et al. / Hydrometallurgy 95 (2009) 145–152

Fig. 1. Adsorption of Cu2+ at different pH by chemically modified biosorbents and raw orange peels (initial copper concentration = 0.001 mol/L; s/l ratio = 1.6 g/L).

factors such as initial pH of the solution, contact time, initial copper concentration and solid/liquid ratio on copper removal were investigated to clarify and compare the metal adsorption behaviors of each of the adsorbents. 2. Materials and methods

2.1.4. Modified with citric acid 5 g AOP was directly mixed with 100 mL of 0.1 mol/L citric acid and stirred for 2 h at room temperature. After vacuum filtered, washed with distilled water to neutral pH and then was kept for drying in an oven at 55 °C about 24 h, hereafter abbreviated as 0.1CA. Change citric acid concentration into 0.6 mol/L, another biosorbent was obtained as 0.6CA.

2.1. Biosorbent preparation 2.1.1. Raw orange peel Crude washed orange peel, abbreviated as OP, was dried for 72 h at 50 °C then ground to approximately 100–500 μm. Its average pore diameter was 30.5 Å and specific surface area was 128.7 m2/g (Quantachrome NOVA 1000). Elemental analysis of orange peel was carried out using GmbH Elementar by heating the sample from 25 °C to 1000 °C at a heating rate of 10 °C/min and analyzing the gases generated by an electrical conductivity detector. The results showed that orange peel is composed of 42.2% carbon, 5.4% hydrogen, 51.4% oxygen and 1.0% nitrogen. 2.1.2. Modified with different alkali saponification 20 g OP was stirred in 20% aqueous isopropyl alcohol for about 24 h at room temperature. The sample was vacuum filtered, repeatedly washed with 20% isopropyl alcohol until it had no color in the filtrate, then dried in an oven at 55 °C for 24 h. This sample (now designated AOP) was stirred with 0.1 mol/L NaOH (10% w/v) for 1 h at room temperature, then filtered, dried at 55 °C for 24 h and washed with distilled water to neutral pH (now designated SNa). Here, 0.1 mol/L NaOH was utilized to further remove hemi-cellulose, pigments etc. Other samples, abbreviated as Sam and CaC, were similarly obtained by substituting 0.1 mol/L NaOH with 0.1 mol/L NH4OH or saturated CaCl2, respectively. 2.1.3. Modified with different acids after saponification 10 g SNa was mixed with 70 ml mL 0.6 mol/L oxalic acid and stirred for 2 h at 80 °C. The acid/peel slurry was filtered and dried overnight at 50 °C, then washed with distilled water until the pH was neutral. The dried residue (7.34 g) was designated as SOA. Another adsorbent (SPA) was similarly obtained using phosphoric acid instead of oxalic acid.

2.1.5. Modified with citric acid after alkali saponification 5 g SNa was mixed with 100 mL of 0.1 mol/L citric acid and stirred for 2 h at ~ 20 °C, 50 °C and 80 °C. After filtering, washing and drying as above, the samples were designated 0.1SCA2, 0.1SCA5 and 0.1SCA8, respectively. Other biosorbents (0.6SCA2, 0.6SCA5 and 0.6SCA8) were similarly obtained by using 0.6 mol/L citric acid. 2.2. Determination of active sites Acidic and basic sites on both raw and chemically modified orange peels were determined by the acid–base titration method proposed byBoehm (1994). The total acid sites were neutralized using 0.1 mol/L NaOH while the basic sites were neutralized with 0.1 mol/L HCl. The acidic and basic sites were determined by leaving 50 mL of 0.1 mol/L titration solution and 0.2 g of orange peel for 5 days at room temperature with occasional shaking, before titrating a 10 mL sample with 0.1 mol/L HCl or NaOH solution.

Table 1 Concentration of active sites and point of zero charge (PZC) for chemically modified biosorbents and raw orange peel cellulose Biosorbents

Total acidic sites (mmol/g)

Basic sites (mmol/g)

PZC

OP SNa SAm SOA SPA 0.1SCA8 0.6SCA8 0.1CA 0.6CA

2.47 0.05 1.96 2.91 2.20 1.63 2.48 2.26 2.26

0.50 1.25 0.75 0.00 0.00 0.75 0.05 0.25 0.05

4.92 6.39 5.87 3.08 3.20 4.50 3.37 4.39 3.90

D. Lu et al. / Hydrometallurgy 95 (2009) 145–152

2.3. Determination of pH of zero charge The pH or point of zero charge (abbreviated as PZC) for the orange peel was determined by boiling 100 mL deionized water for 20 min to eliminate dissolved CO2 and quickly cooling and capping the solution. 0.2 g of orange peel was placed in 15 mL of the CO2-free water, then sealed and continuously agitated for 48 h at room temperature before measuring the solution pH — taken as the point of zero charge. This method has been used satisfactorily by Moreno-Castilla et al. (2000) and Leyva-Ramos et al. (2005). 2.4. Biosorption experiments All solutions were prepared from analytical grade reagents in distilled water with 0.1 mol/L NaCl added to control the ionic

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strength and hexamethylene tetrammine used to buffer the pH. Stock copper solution was prepared by dissolving CuCl2 in 25 mL 0.1 mol/L NaCl and 25 mL hexamethylene tetrammine buffer solution (pH = 5.0) and diluting to 250 mL. The pH of each solution was adjusted with 0.1 mol/L NaOH and 0.1 mol/L HCl and measured with a pHs-3C Model acidity meter (Shanghai Precision & Scientific Instrument Co. Ltd. China). Batch adsorption tests were carried out to study the effect of various parameters on the adsorption efficiency of copper by adsorbents. Unless otherwise stated, 25 mg of each adsorbent was placed in 15 mL of copper solution and mixed vigorously in a rotary shaker at room temperature for 24 h to reach equilibrium. The concentration of copper in the filtrate was titrated by standard EDTA solution. All determinations were performed in triplicate and the average was used for this work.

Fig. 2. Effects of equilibrium time on Cu2+ uptakes by different adsorbents ((a) fitted by the pseudo-first-order model; (b) fitted by the pseudo-second-order model; s/l ratio = 1.6 g/L; initial copper concentration = 0.001 mol/L; pH = 5.3).

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Table 2 The pseudo-first and second-order kinetics parameters for the biosorption of Cu(II) on chemically modified and raw orange peels

complete monolayer on the surface bound at Ce, respectively; b is the equilibrium constant. Freundlich model is given as:

Sorbent type

qe ¼ Kf Ce :

OP SNa 0.1SCA2 0.6SCA2 0.1SCA5 0.6SCA5 0.1SCA8 0.6SCA8 SPA SAm CaC SOA

The pseudo-first-order

The pseudo-second-order

1=n

qe (mol/kg)

K1 (1/min)

R2

qe (mol/kg)

K2 (kg/(mol min))

R2

0.2748 0.4445 0.4839 0.5153 0.5272 0.4166 0.6268 0.6241 0.4622 0.4087 0.2696 0.4875

0.1070 0.0474 0.1122 0.1535 0.1043 0.1894 0.1926 0.1149 0.2720 0.2056 0.3302 0.5049

0.9429 0.9822 0.9598 0.9796 0.9841 0.9913 0.9990 0.9931 0.8772 0.8507 0.9396 0.9681

0.2922 0.4866 0.5169 0.5430 0.5618 0.4318 0.6389 0.6481 0.4977 0.4439 0.2852 0.5033

0.5951 0.1427 0.3305 0.4745 0.2994 0.7957 0.9674 0.3754 0.8222 0.6882 2.027 2.412

0.9840 0.9891 0.9926 0.9835 0.9859 0.9818 0.9988 0.9969 0.9579 0.9446 0.9856 0.9851

Where Kf and n are the Freundlich constants which features the system, respectively. Langmuir–Freundlich model (Sips 1948) can be expressed as Eq. (5): 1=n

qe ¼ qm

The adsorption amount (q) and the removal percentage (E%) were calculated according to Eq. (1) and Eq. (2): q¼

ðC0 −Ce ÞV W

Ek ¼

ðC0 −Ce ÞV  100: C0

ð1Þ ð2Þ

Where q is the adsorption amount of metal ion (mol/kg), W is the weight of the adsorbent (g), V is the volume of solution (L), and C0 and Ce are the initial and equilibrium concentrations of Cu2+ in solution, respectively (mol/L).

Langmuir model is presented by the following equation: qm bCe : 1 þ bCe

1=n

1 þ bCe

:

ð5Þ

qe −qt ¼ −K1 t: qe

ð6Þ

Where qt is the amount of the metal ions adsorbed (mol/kg) at time t (min) and K1 is the adsorption rate constant of the pseudo-firstorder equation (1/min). Pseudo-second-order kinetic model (Ho and McKay 1999) was expressed as the following formulation: t 1 t ¼ þ : qt K2 q2e qe

ð7Þ

Where K2 is the adsorption rate constant of pseudo-second-order equation (kg/(mol min)). 3. Results and discussion 3.1. Effect of pH on copper biosorption

2.6. Isotherm and kinetics model

qe ¼

bCe

Pseudo-first-order kinetic model (Cheung et al., 2001; Bayramoglu et al., 2002) was generally expressed as Eq. (6): ln

2.5. Adsorption procedure

ð4Þ

ð3Þ

Where qe and qm are the equilibrium and maximum metal adsorption amount of metal ion per unit weight of adsorbent to form a

The effects of initial pH on adsorption of 0.001 mol/L Cu(II) by different adsorbents at room temperature are shown in Fig. 1. It is clear that the amount of adsorbed copper increased with the increase in pH as similarly reported by Reddad (2002a,b). At pH b 2.5, because the surface active sites of the adsorbent were protonated, the competition of Cu2+ and H+ for the same surface active sites resulted in very low copper uptake. When the pH was 4.5–5.5, the maximum adsorption

Fig. 3. Adsorption isotherms of Cu2+ by different chemically modified biosorbents and raw orange peels (initial pH = 5.3; s/l ratio = 1.6 g/L; contact time = 3 h).

D. Lu et al. / Hydrometallurgy 95 (2009) 145–152

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was obtained for all the adsorbents. With pH further increasing, the Cu2+ uptake decreased due to the formation of soluble hydroxyl complexes and the precipitation of copper hydroxide. The experimental data on the acidic and basic sites and the PZC in this study are shown in Table 1. It is observed that the concentrations of acid sites are larger than that of the basic sites except for SNa, hence the surfaces of these adsorbents are generally acidic (Vaughan et al 2001; Wing et al 1997). The PZC also can be used to explain the effect of solution pH on the adsorption of Cu2+. At pH b PZC, the surface charge of the adsorbents is positive which results in low copper sorption. At pH N PZC, the surface charge of the adsorbents is negative, and the Cu2+ ions in solution are attracted to the surface. Therefore, chemically modified adsorbents with lower PZC attract more Cu2+ ions to the surface at a particular pH as the surface becomes more negative (Leyva-Ramos et al 2005) and maximum sorption is likely to occur at pH values greater than PZC when the adsorbents have a net negative charge (Romero-Gonzalez et al., 2001). 3.2. Biosorption kinetics In order to analyze the biosorption kinetics of metal ions and determine optimum operating conditions, the pseudo-first-order model and the pseudo-second-order model were used as shown in Fig. 2. Copper uptake by the fourteen adsorbents exhibited a similar kinetic pattern for the both models. Within the first 10 min, the

Table 3 Langmuir, Freundlich and L–F model parameters for Cu2+ biosorption equilibrium on chemically modified biosorbents and raw orange peels Biosorbents

qm (mol/kg)

b

0.5584

0.6635

OP 0.5033 0.7581

6.808 13.51

0.7518 0.8908

37.33 3.518

1.097 0.8367

0.6816 10.77

0.8707 0.9878

1.208 4.241

1.101 0.9521

0.2470 11.81

1.149 0.9814

0.5565 3.850

1.031 1.201

0.7150 5.118

1.217 0.5207

2.717 6.222

0.5607 0.5262

0.3462 1.112

0.4940 0.7731

6.101 2.635

0.7164 0.8894

6.993 × 104 1.698

0.8082 0.8313

716.9 31.64

0.9456 0.7646

0.05753 1.445

0.7956

0.6042

SNa

0.1SCA2

0.6SCA2

0.1SCA5

0.6SCA5

0.1SCA8

0.6SCA8

0.6CA

0.1CA

SAm

SPA

SOA

CaC

Kf

n

2.656

2.845 0.7570

1.553

7.416 0.9049

2.681

4.618 1.780

1.936

6.353 1.290

2.946

4.717 1.457

2.198

6.459 2.201

2.801

4.842 1.242

2.866

5.917 1.077

1.140

6.495 1.472

2.098

3.358 0.8194

1.579

6.568 0.43

2.779

4.174 0.5590

1.275

12.56 2.624

2.982

3.532 1.124

R2

Sorption model

0.980 0.931 0.985 0.993 0.940 0.993 0.965 0.966 0.986 0.990 0.956 0.993 0.976 0.950 0.986 0.956 0.972 0.994 0.984 0.938 0.987 0.948 0.908 0.948 0.987 0.968 0.994 0.994 0.937 0.996 0.884 0.832 0.909 0.909 0.857 0.924 0.913 0.917 0.921 0.961 0.932 0.962

Langmuir Freundlich L–F Langmuir Freundlich L–F Langmuir Freundlich L–F Langmuir Freundlich L–F Langmuir Freundlich L–F Langmuir Freundlich L–F Langmuir Freundlich L–F Langmuir Freundlich L–F Langmuir Freundlich L–F Langmuir Freundlich L–F Langmuir Freundlich L–F Langmuir Freundlich L–F Langmuir Freundlich L–F Langmuir Freundlich L–F

Fig. 4. The effect of equilibrium pH on Cu(II) adsorption by 0.6SCA8 at different concentrations of Cu(II).

copper uptake by all the adsorbents was rapid, then reached kinetic equilibrium in about 30 min except for samples OP and SNa, which needed longer time. In this case, the ratios of the active sites on the surface of OP and SNa to the amount of copper ions in the solution are low compared to the other biosorbents. Disparity in equilibrium time among the different biosorbents ranging from 10 to 60 min, relate to the differences of capacity in binding Cu2+ as discussed below. The kinetic model parameters are presented in Table 2. The correlation coefficients, R2 of the pseudo-first-order model were in the range 0.8507–0.9990; and were in the range 0.9446–0.9988 for the pseudo-second-order model. Thus both models fit the experimental data well and the calculated qe values are close to experimental qe values. 3.3. Adsorption isotherms Fig. 3 shows the experimental isotherms obtained for Cu(II) binding. It shows that the adsorption capacity increased with increasing equilibrium concentration of Cu(II), progressively reaching saturation of the adsorbent. The Langmuir, Freundlich and Langmuir– Freundlich (L–F) isotherm models were then applied to the experimental data and the obtained parameters are presented in Table 3. The initial isotherm gradient indicates the sorbent affinity at low metal concentrations. In the Langmuir equation, this initial gradient corresponds to the affinity constant b. Davis et al. (2003) and Murphy et al. (2008) showed that high b values reflect high affinity of the sorbent for the metal. From Fig. 3 and Table 3 it is clear that most of the biosorbents have a high b value and are suitable for Cu2+ binding. They also possess a comparatively large qm. The most suitable biosorbent for Cu(II) biosorption is designated 0.6SCA8 due to its largest qm and comparatively high b values. According to the data given in Table 3, a comparison of different isotherm models with the correlation coefficients R2 revealed that a combination of the Langmuir and Freundlich (L–F) isotherm models fitted the experimental data best. The qm values obtained from the Langmuir and Freundlich (L–F) models for Cu(II) binding decreased in the order: 0.6SCA 8 N 0.6SCA 5 ≈ 0.1SCA 8 ≈ 0.1SCA 5 ≈ 0.1SCA 2 SOA N 0.6SCA2 N SPA ≈ CaC N SNa N SAm N 0.6CA ≈ 0.1CA ≈ OP. The maximum uptake of Cu2+ was achieved by using 0.6SCA8, 0.6SCA5, SOA and was 1.22, 1.15, 0.95 mol/kg respectively, which represents an increase of 144%, 130%, 90% compared to OP. Fig. 4 presents the adsorption isotherms of Cu2+ by 0.6SCA8 at various equilibrium pH values. The adsorption capacity increased with

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Fig. 5. The removal efficiencies and the adsorption amount of Cu2+ by OP and 0.6SCA8 and the data was fitted by the Langmuir–Freundlich model (initial pH = 5.3; s/l ratio = 1.6 g/L; contact time = 3 h).

increasing equilibrium concentration of Cu2+ reached saturation at 0.003 mol/L. All the adsorption isotherms in Fig. 4 fitted to the Langmuir model with high correlation coefficients (R2 N 0.95). The qm values at pH 2.0, 3.0 and 4.5 were also calculated using the Langmuir model, and were 0.395, 0.525, 0.656 mol/kg, respectively. This suggests that Cu2+ adsorption occurs primarily via electrostatic interactions between Cu2+ and the negative surface of orange peel cellulose. The adsorption capacities and removal efficiencies of Cu2+ by 0.6SCA8 and OP at different Cu2+ concentrations were also calculated as shown in Fig. 5. Compared with 33% adsorption efficiency of Cu2+ by non-modified OP, it is clear that 0.6SCA8 displayed the best adsorption efficiency of Cu2+ with 100% removed from 0.0005 mol/L solution, decreasing to about 40%

removed from 0.003 mol/L solution, which indicates the suitability of 0.6SCA8 for dilute copper solution treatment. The effect of different temperatures on the adsorbent characteristics was also studied and found to be significant as shown in Fig. 6. Although the concentration of acidic sites is different, the amount of Cu2+ adsorbed increased with increasing the temperature. Temperature appears to be an important factor in the process of modifying adsorbents. The increase of temperature introduced more carboxyl and hydroxy groups to the cellulose and enhanced the amount of the surface functional groups on the cellulose, which resulted in additional metal binding capacity. Fig. 6 also shows the beneficial effect on Cu2+ uptake of increasing citric acid concentration during

Fig. 6. Effects of different crosslinking temperature and different citric acid concentrations on the characters of biosorbents (initial pH = 5.3; s/l ratio = 1.6 g/L; contact time = 3 h).

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Table 5 Comparison of infrared bands in the 4000–400/cm spectral region Functional groups of biosorbents

O–H stretching CH2 vibration COO stretching CfO stretching CfC vibration CH bending COO vibration

Bioadsorbents OP

SNa

SCA

SOA

SPA

3377 2920 1747 1647 1520 1445 1370 1276 1206

3432

3417 2925 1740 1646

3439 2925 1741 1645

3422 2924 1740 1645

1381 1262

1384

1371 1259

C–O–C, C–O–P vibration

1103 1052

Finger print zone

1729 1661 1603

1070

617 519

Fig. 7. Effect of s/l ratio on copper biosorption by chemically modified orange peel cellulose (initial pH = 5.3; initial copper concentration = 0.0025 mol/L; contact time = 3 h).

adsorbent chemical modification which agrees with our previous work on the adsorption of Cd2+ (Li et al., 2007). 3.4. Effect of solid/liquid ratio on copper adsorption The effect of solid/liquid (abbreviated as s/l) ratio on copper adsorption was studied at room temperature and at pH 5.3. Various s/l ratios were tested, while keeping the volume and initial concentration of copper (0.0025 mol/L) constant. The results of four adsorbents show similar behavior (Fig. 7). The rate of adsorption increased as s/l ratio increased, but changed little when s/l ratio was above 5.0 g/L. The maximum adsorption was N90% when the s/l ratio was N4.5 g/L for 0.6SCA8 biosorbent. Thus the optimum s/l ratio is 4.5 g/L. 3.5. FTIR analysis of adsorbents Table 4 presents the role of different chemical reagents in modifying the orange peel. In order to know the structure of the adsorbents, the FTIR spectra of adsorbents SNa, SCA, SOA, SPA and OP were analyzed as shown as Table 5. Clearly, different functional groups were introduced to the surface of adsorbents via the different chemical modifications. After raw orange peel was pretreated by NaOH, some peaks disappeared, such as those at wave number 2920/ cm for –CH2, 1370/cm for CH bending and 1206–1276/cm for –COO vibration. Moreover, some peaks shifted a little, such as O–H stretch-

Table 4 The role of using different chemical reagents to modify the orange peel The chemical reagents 20% isopropyl alcohol

The role

To discolor the orange peel and remove some polarity compound and organic small molecule NaOH, NH 4 OH or saturation To undergo the crosslinking reaction, increase some solution of CaCl2 active binding sites and remove further hemicellulose, pigment, and so on Oxalic acid and phosphoric acid To oxidize some groups and introduce the hydroxyl to the cellulose of orange peel Citric acid To introduce carboxyl to the cellulose Modified by citric acid after To introduce carboxyl to the cellulose based on the alkali saponification alkali saponification Different concentration of citric To examine the effect of concentration of citric acid acid and temperature on Cu2+ adsorption and find out the optimum temperature to produce condensation product and acid anhydride

1113 1019 829 622 450

1071

ing from 3377/cm to 3432/cm, CfO stretching and COO stretching from 1747/cm to 1729/cm etc. It showed that the alkaline pretreatment can cause degradation of cellular compounds, such as cell wall, proteins and complex organic components of biomass. But after the SNa was activated by the acids, new peaks, such as –CH2, –CH bending and –COO vibration, were obtained again and some peaks shifted back to their original position. This indicated that some new functional groups had been successfully linked to the surface of the adsorbents via the acid activation. The determination of active sites and PZC presented in Table 1 also showed the functional chemical modification on the adsorbents. This, and the previous studies, all indicate that chemical modification of orange peel adsorbents that increase the number of functional groups, are more effective in adsorbing Cu2+ from aqueous solution. 3.6. Comparison among the metal ions of Cu2+, Pb2+ and Cd2+ Comparing the experimental results for Cu2+ with that for Pb2+ and Cd2+ obtained in our previous work (Xuan et al., 2006; Li et al., 2007), the general trend is that the adsorption of the metal ions increases when the pH increases, and also increases when the equilibrium concentration of the metal ions in solution increases. The analysis of the experimental results of Pb, Cu and Cd biosorption at the same equilibrium pH or the same equilibrium concentration show that adsorbent affinity follows the order Pb N Cu N Cd. Similar results have been reported by Esposito et al. (2001) and Yin et al. (1999). The optimum s/l ratio was about 3–4 g/L for Pb(II), Cu(II) and Cd(II) at initial concentrations between 0.001 and 0.0025 mol/L. 4. Conclusions Various chemically modified orange peel cellulose materials were studied as adsorbents for the removal of Cu2+ from dilute chloride solutions. The optimal material was obtained after pretreatment with 0.6 mol/L citric acid at 80 °C, and adsorbed a maximum of 1.22 mol/kg Cu2+. The results of 14 different chemically modified orange peel cellulose all showed reasonable maximum adsorption capacities for Cu2+ compared to untreated orange peel. A comparison of different isotherm models revealed that a combination of the Langmuir and Freundlich (L–F) isotherm models fitted the experimental data best. The copper adsorption was strictly pH dependent, and maximum sorption was found to occur at around pH 4.5–5.5. The kinetic equilibrium was rapidly established in about 30 min. Results indicate that the chemically modified orange peel cellulose increases the number of functional groups on the cellulose and can provide an efficient and cost-effective technology for eliminating copper from wastewater and effluent solutions.

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