Removal of heavy metals from aqueous solution by apple residues

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All rights reserved. Printed in Great Britain. 0032-9592/98 $19.00 + 0.00. S0032-9592(97)00055-1. Removal of heavy metals from aqueous solution by apple ...

Process Biochemistry Vol. 33, No. 2, pp. 205-211, 1998 © 1998 Elsevier Science Ltd All rights reserved. Printed in Great Britain 0032-9592/98 $19.00 + 0.00

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S0032-9592(97)00055-1

Removal of heavy metals from aqueous solution by apple residues Sung Ho Lee, a* Chong H u n Jung, b Hongsuk Chung, ~' Moo Yeal Lee r and Ji-Won Yang c ~'Basic Technology Development, Korea Atomic Energy Research Institute, P.O Box 105, Yusong, Taejon 305-600, Korea bNuclear Fuel Cycle Research, Korea Atomic Energy Research Institute, P.O. Box 105, Yusong, Taejon 305-600, Korea CBioprocess Engineering Research Center, Department of Chemical Engineering, Korea Advanced Institute of Science and Technology, Taejon 305-701, Korea (Received 11 March 1997; revised version received 12 May 1997; accepted 20 May 1997)

Abstract

The removal of copper, lead and cadmium by apple residues (AR) was investigated to evaluate cation exchange capacities. The effects of solution pH, ionic strength, ligands and co-ions were studied in batch experiments. Apple residues were modified with phosphorus (V) oxychloride to improve their physicochemical properties and greatly enhance the capacity of metal removal. Adsorption equilibria were established rapidly initially and decreased markedly after 1 h. Column experiments were carried out in a glass column filled with AR and modified AR to evaluate the metal removal capacity. The influences of the feed concentration, chemical treatment and ligand were also studied. After exhaustion of the residues, the columns were regenerated successfully by a simple elution procedure. ~) 1998 Elsevier Science Ltd

Keywords: apple residues, heavy metals, metal binding, removal.

Other available processes include ion exchange, reverse osmosis, adsorption on activated carbon, and solvent extraction. These methods are relatively expensive, involving either elaborate and costly equipment or high operation costs and energy requirements. The ultimate disposal of the contaminants may also be a problem in these techniques [8]. Recently, the recovery and recycling of organic residues (agricultural, urban industry, etc.) have become main areas of investigation in developed countries. Agricultural wastes such as tree bark, peanut skin and hull, and growing plants (tobacco and tomato root tissue) have been used to remove heavy metals from water [9-11]. Because apple residues (AR) are readily available, their use as adsorbents seems appropriate. In the present study, AR were used as an adsorbent for the removal of metals. The cation exchange properties of these residues can be attributed to the presence of carboxylic and phenolic functional groups, which exist in either the cellulosic matrix or in the materials associated with cellulose, for example, hemicellulose and lignin [12]. To improve their structural strength on prolonged use and their low ion exchange capacity compared with synthetic resins, AR were submitted to phosphation reactions. Batch and

Introduction

Public concern over heavy metal pollution has grown constantly since the outbreak of Minamata disease caused by mercury in Japan [1]. Man's awareness of the hazards of heavy metals now covers a wide spectrum of metals such as lead, cadmium, chromium, copper and zinc [2]. Among these lead- and cadmium poisoning in humans cause severe dysfunction of the kidneys, reproductive system, liver, and brain and central nervous system [3]. To remove heavy metals effectively from metalladen wastewater, engineers and scientists have developed processes and measurements for the treatment and disposal of metal-containing wastes, namely, chemical precipitation, ion exchange, membrane separation and adsorption [4-7]. Among these methods, the most common is chemical precipitation. However, this method may be costly: it requires a relatively large amount of space for the clarifier, it typically produces a wet, bulky sludge, and generally requires final filters for polishing if small residual levels of metals are required. *To whom correspondence shuld be addressed. 205

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column experiments were carried out to investigate the cation exchange capacities and removal characteristics of metals from aqueous solution.

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Materials and methods

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Apple residue for these experiments was obtained from an apple-juice processing factory. They consisted of the processed skins, seeds and stems, up to 12% of the wet weight of the original fruit. Apple residue is composed mainly of cellulose (30%) and lignin (19%), both with the capacity to bind metal cations due to carboxylic and phenolic groups. Apple residue was dried overnight at 60°C in a convection oven, ground in a ball mill, and sieved into different fractions. In order to eliminate soluble components such as tannins, resins, reducing sugars and colouring agents, the residues were successively washed with 0.5 M HC1 and distilled deionized water until a constant pH was achieved. Phosphated AR (P-AR) was prepared by chemical treatment of AR with phosphorous (V) oxychloride according to the procedure of Peska et al. [13]. Ethylene diamine tetra-acetic acid (EDTA) and oxalic acid were chosen as organic ligands. The ionic strength in solution was adjusted with NaCI. Batch experiments were performed at room temperature and samples were prepared in duplicate. All the glassware and polyethylene tubes were acid-cleaned and rinsed thoroughly before use with distilled de-ionized water. Various initial metal concentrations were prepared by serial dilution of 1000 ppm of standard solution. Ten millilitres of solution prepared in this way was added to each 15 ml test tube containing pre-weighted AR. The pH in solution was adjusted with 0"IN/1N NaOH and 0"IN/1N HCI to cover a pH range from 2 to 12. The test tubes were then sealed with caps and placed on a rotary shaker (Roto-Torque model 7637). The test tubes were removed after 24 h shaking and centrifuged for 5 min at 3000 rpm. The supernatant was analysed using a flame atomic absorption spectrophotometer (AAS: Perkin-Elmer, Model 3100) for residual metal content. Blank tests were also performed without AR to investigate the removal which might occur via metal precipitation and adsorption on to the glass wall. Kinetic studies of metal removal by AR were carried out with a 200 ml copper solution of 10 ppm with 0.2 g AR. Samples were taken periodically from the suspension and centrifuged immediately to remove adsorbents. Experiments were also carried out in a glass column of 1.5 cm internal diameter and 30 cm length filled with AR and P-AR. Metal solution was percolated through the packed column at a flow rate of 5 ml/min, controlled by a peristaltic pump. Effluent samples were collected every 30 min, measured for pH and analysed by flame AAS to determine metal concentrations. After exhaustion of the adsorbent, the adsorbed metal was recovered by elution with 0.5 N

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HCI. Apple residues and P-AR were reused by washing with water to eliminate any excess mineral acid. Results and discussion

Batch experiments Sorption kinetics The sorption kinetics of the metals with time at an initial metal concentration of 10 ppm at pH 5.9 for copper, 5.7 for lead and 7.0 for cadmium are shown in Fig. 1. The rate of metal sorption was rapid initially and decreased markedly after 1 h. The AR functional groups act initially as coordination sites for metal uptake. A similar initial rapid uptake of cadmium by chitosan has been reported by Jha et al. [14]. From these experimental results, an equilibrium contact time of 24 h was used in all further experiments. Effect of pH pH in solution has been identified as the most important variable governing metal adsorption on biosorbents. This is partly due to the fact that hydrogen ions themselves are strongly competing adsorbates. The solution pH influences the speciation of metal ions and the ionization of surface functional groups. The effects of pH for copper, lead, and cadmium removal in a ligand-free system is shown in Fig. 2. The optimal pH ranges for copper, lead and cadmium were from pH 6-0 to 7.0, 6-5 to 8"0, and 8.0 to 9.5, respectively. The maximum removal was 91-2% for copper, 95-3% for lead, and 91% for cadmium. These experimental results indicate that the binding of copper and cadmium is more pH-dependent than that of lead. Blank tests are also shown to verify that the removal mechanism is purely biosorption. As indicated in Fig. 2, precipitation of copper occurs

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at a pH greater than 5.5. However, if precipitation does contribute to the removal mechanism, the removal capacity should not have decreased at a pH greater than 7"5. The decrease in copper removal capacity at pH > 7.5 may be caused by the complexing of copper with hydroxide. The results of blank tests of lead and cadmium indicate that precipitation does not contribute to the removal mechanism of lead and cadmium. Solution pH can affect the charge of AR surfaces and the speciation of metal ions. The ionization constants (pK,) for various carboxyl groups have been reported to be around 4-5 [15]. At pH values higher than 3-4, carboxyl groups are deprotonated and negatively charged. Consequently, the attraction of positively charged metal ions would be enhanced. Monovalent cations, Me(OH) +, are the dominant ion species at the optimal pH range. Based on experimental results and the speciation of metal ions [12,16], metal removal by AR may have occurred by complexing between the negatively charged functional groups and metal cations such as Me ÷2, Me(OH) +.

Effect of ionic strength Figure 3 shows the influence of ionic strength on the removal of copper and lead by AR. The results indicate that, up to 0.1 N NaC1, there was no significant decrease in the removal of copper or lead. However, increasing the ionic strength over 0.1 N results in a dramatic decrease in metal ions removal: a decrease of 30% for copper and about 20% for lead with 1 N. Metal uptake is sensitive to changes in the concentration of the supporting electrolyte if electrostatic attraction is the significant mechanism for metal removal. The electrostatic attraction at low ionic strength appears to play a negligible role in the removal of copper and lead. At high ionic strength, over 0-1 N,

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however, the increased amount of electrolyte can swamp the surface of the AR, decreasing the access of metal ions to the AR surface for metal binding. Thus, metal removal decreased significantly. These results can also be explained by considering the competitive effect of Na ÷ ions for copper and lead binding. The ion exchanger tends to prefer the counter ion of higher valance [17]. Therefore, Cu +2 and Pb +2 ions are more selective than Na + ions for cation exchangers. Apple residues may therefore 15refer' Cu +2 and Pb +2 ions to Na + ions, and the competitive effect of Na + ions for copper and lead removal by AR will be low, especially at low ionic strength.

Effect of ligands The effect of organic ligands was investigated for copper removal by AR because dissolved trace metals in natural water often form complexes with a wide variety of organic and inorganic ligands. Figure 4 shows the effect of ligands (EDTA and oxalic acid) for copper removal by AR. EDTA is a strong chelating agent and forms complexes with copper ions over the pH range 2-12 as indicated in copper ion speciation in the presence of EDTA [12,16]. In this pH range, copper removal by AR dramatically decreased due to copper-EDTA complex formation. The degree of decrease of copper removal capacity increased with the increase in the copper-EDTA molar ratio from 1:1 to 1:10. The influence of oxalic acid for copper removal by AR is also shown in Fig. 4. As with EDTA, the presence of oxalic acid in solution reduced the copper removal capacity due to copper-ligand complex formation. This trend can be also explained by copper ion speciation in the presence of oxalic acid [12,16]. The capacity for copper removal decreased significantly in the pH range 2-7 due to copper-ligand complex formation. However, in the pH region beyond pH 7, the

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molar ratio of 1:1, a small reduction in copper removal was observed. As the molar ratio increased to 1:10, an even greater reduction in copper removal yield was noticed. In the case of lead at a molar ratio of 1:1, the copper removal yield decreased by 36%. These results can be explained by the selectivity sequence of the most common cations in cation exchangers. Pb ÷2 ions are more selective than Ni ÷ ions in general cation exchangers [17]. The Pb +2 ion may therefore be a stronger competitive ion than Ni ÷ ion for copper removal by AR and AR may lbrefer' Pb ÷2 ions to Ni ÷ ions.

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extent of copper-ligand complex formation decreased, and the monovalent cation, Cu(OH) ÷, became the dominant species. Thus, the capacity for copper removal by AR increased gradually. The capacity for copper removal by AR decreased significantly therefore, and was dependent on ligand type and concentration.

Effect of co-ion The effect of co-ions (Ni and Pb) on copper removal by AR is indicated in Fig. 5. In the control, the removal of copper ions was approximately 69% from an initial concentration of 10 ppm at pH 5.2. When nickel ions were present as competing metals at a

Raw AR consisted of processed skins, seeds and stems, contained up to 12% wet weight of the original fruit and was improved by treatment with phosphorus (V) oxychloride [13]. The effect of chemical treatment on metal removal was investigated using AR and P-AR. The capacity of P-AR to remove copper and lead was higher than that of AR, especially at pH 2.0-4-0. This was due to the presence of phosphate groups in P-AR (Fig. 6). This can be explained by the speciation of copper and lead ions [12]. At acidic pHs, the dominant metal ions may be the divalent cation, Me ÷2. The phosphate groups in P-AR may effectively bind the free metal ions. The optimal pH zone for lead removal by P-AR was significantly broader than the narrow optimal pH region with unmodified AR, especially in the case of copper removal. The enhancement of mechanical strength was verified using the swelling test in a test tube. The volume increase of P-AR by swelling was reduced to 1/3 of that of AR. This result could be explained by the fact that phosphorus (V)

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