At least 20 metals are classified as toxic and half of these are emitted into the environment in quantities that pose risks to human health. Chromium has both beneficial and detrimental properties. Two stable oxidation states of chromium persist in the environment, Cr (III) and Cr (VI), which have contrasting toxicities, mobilities and bioavailabilities. Whereas Cr (III) is essential in human nutrition (especially in glucose metabolism), most of the hexavalent compounds are toxic. Chromium and its compounds are widely used in electroplating, leather tanning, cement, dyeing, metal processing, wood preservatives, paint and pigments, textile, steel fabrication and canning industries These industries produce large quantities of toxic wastewater effluents.

A wide range of physical and chemical processes is available for the removal of Cr (VI) from wastewater, such as electro-chemical precipitation, ultra filtration, ion exchange and reverse osmosis. A major drawback with precipitation is sludge production. Ion exchange is considered a better alternative technique for such a purpose. However, it is not economically appealing because of high operational cost. Adsorption using commercial activated carbon (CAC) can remove heavy metals from wastewater, such as Cd, Ni, Cr and Cu. However, CAC remains an expensive material for heavy metal removal.

Natural biopolymers are industrially attractive because of their capability of lowering transition metal-ion concentration to parts per billion concentrations. Natural materials that are available in large quantities or certain waste from agricultural operations may have potential to be used as low cost adsorbents, as they represent unused resources, widely available and are environmentally friendly. In Malaysia, oil palm is the most important commercial crop. It was reported that Malaysia currently produces about 30 million tonnes annually of oil palm biomass, including trunks, fronds, fruit waste and empty fruit brunches. Of these, about two million tonnes of fruit shell (or endocarp) is generated annually. Preliminary studies have shown that it is feasible to prepare chars with sufficient densities and high porosity from oil palm fruit waste. The exchange/sorption properties of palm oil shell are due to the presence of some functional groups, such as carboxylic, hydroxyl, and lactone, which have a high affinity for metal ions. In recent years, development of surface modified activated carbon has generated a diversity of activated carbon with far superior adsorption capacity. The use of palm oil shell with surface modification to improve its metal removal performance would add its economic value, help reduce the cost of waste disposal, and most importantly, provide a potentially inexpensive alternative to existing commercial activated carbon.

Among the many other low cost absorbents identified chitosan has the highest sorption capacity for several metal ions. Chitin (2-acetamido-2-deoxy-b-D-glucose-(N-acetylglucan)) is the main structural component of molluscs, insects, crustaceans, fungi, algae and marine invertebrates like crabs and shrimps. Worldwide, the solid waste from processing of shellfish, crabs, shrimps and krill constitutes large amount of chitinaceous waste. Chitosan (2-acetamido-2-deoxy-b-D-glucose-(N-acetylglucosamine)) is a partially deacetylated polymer of chitin and is usually prepared from chitin by deacetylation with a strong alkaline solution. Chitosan chelates five to six times greater amounts of metals than chitin. This is attributed to the free amino groups exposed in chitosan because of deacetylation of chitin. The biosorbent material, chitosan, is slightly soluble at low pHs and poses problems for developing commercial applications. It is also soft and has a tendency to agglomerate or form a gel in aqueous solutions. In addition, the active binding sites of chitosan are not readily available for sorption. Transport of the metal contaminants to the binding sites plays a very important role in process design. Therefore, it is necessary to provide physical support and increase the accessibility of the metal binding sites for process applications.

In the present investigation an attempt was made to overcome these mass transfer limitations by synthesizing a biosorbent by coating chitosan on the surface of palm oil shell charcoal and evaluating its equilibrium adsorption properties. The combination of the useful properties of oil palm shell char and that of natural chitosan, could introduce a composite matrix with many application and superior adsorption capabilities. Using synthetic wastewater, the Cr removal by oil palm shell charcoal coated with chitosan and acid treated oil palm shell charcoal adsorbents were compared. Oil palm shell has been successfully used to produce high quality activated carbon because of their inherent high densities and carbon content. In this work char from oil palm shell was prepared according to the method described by Guo and Lua, 2000. No attempt was made to determine the solid density and apparent densities of both the starting material and the char. 

A relatively rapid and mild deacetylation method proposed by Coughlin et al. 1990 with slight modification where the deacetylation process was subjected to microwave irradiation was used in this work. During microwave irradiation, polar molecules such as water (in the sample) align with the continuously changing magnetic field generated by microwaves. This is supposed to accelerate various chemical, biological, and physical processes. Microwave treatment brings about greater accessibility of the susceptible bonds and hence a much more efficient chemical reaction.

The practical problems of chitosan solubility at low pH aqueous systems, gel forming behaviour and mass transfer limitations were overcome by coating it on other adsorbents like alumina, charcoal or interacting it with other adsorbents like alginate to form a rigid matrix structure of better mechanical strength. In this study these problems were overcome by coating chitosan on oil palm shell charcoal and the coating process yielded a stable granular composite adsorbent that was stable under acidic conditions. The cationic nature of chitosan and the anionic nature of oil palm shell charcoal yielded stable, granular composite matrix due to interaction between the two oppositely charged materials. Chitosan binds with both anionic and cationic species. Chromium (VI) and some other metals such as arsenic, depending on the pH, are known to exist as anions. Chromium (VI) forms dichromate anion at pH around 4. The amine groups of chitosan are largely responsible for the absorption of Cr (VI) ions from the solution. At low pH, the amine group on chitosan is protonated. This leads to the interaction between NH₃⁺ functional groups in chitosan and Cr₂O₇^2- and the interaction is chiefly electrostatic attraction in nature.

The influence of several operational parameters such as dose of adsorbent, agitation speed, initial pH and contact time was investigated. The result were expressed as the removal efficiency (E) of the adsorbent on Cr, which was defined as 

E (%)  =  [(C_o – C₁) / C_o] x 100 , where

C_o and C₁ are the initial and equilibrium concentration of Cr solution (mg/l), respectively. The Cr (VI) ion concentration was determined colorimetrically according to Standard Methods (Clesceri et al, 1998)

pH is an important parameter for adsorption of metal ions from aqueous solution because it affects the solubility of the metal ions, concentration of the counter ions on the functional groups of the adsorbent and the degree of ionization of the of the adsorbate during reaction. To examine the effect of pH on the Cr removal efficiency, the pH was varied from 1.0 to 9.0. It was shown that the uptake of free ionic Cr depends on pH, where optimal metal removal efficiency occurs at pH 5 and then declining at higher pH. Removal efficiency for chitosan coated acid treated bead (CCAB) increased from 65% to 92% over pH range from 1.0 to 5.0. The other two adsorbents, i.e. chitosan coated oil palm shell carbon and acid treated oil palm shell charcoal also showed similar trends but with much lower removal efficiency and slight different optimum pH value. Chromium (VI) and some other metals such as arsenic, depending on the pH, are known to exist as anions. At low pH (below 4), the amine group on chitosan is protonated to varying degree. The pH_pzc obtained for both CCAB and chitosan coated beads (CCB) was in the range of 6.8 - 7.1. This is consistent with results reported for chitosan from lobster and crab which is in the range of 6.4 - 7.2 . The pH_pzc is a point at which the surface acidic (or basic) functional groups no longer contribute to the pH value of the solution. The pH_pzc value of acid treated oil palm shell charcoal (OPSC) was found to be around 4.7. It can be concluded that oxidization of oil palm shell charcoal with sulphuric acid yielded acidic surface since pH values of point of zero charge for these materials are at a lower pH range compared to the value reported for most chemically untreated commercial activated carbon. The surface acidity was due to the introduction of several oxygen-containing functional groups. Cations adsorption will be favourable at pH value higher than pH_ZPC, and anions adsorption at pH values lower than pH_ZPC. The NH₃⁺ group on the chitosan is chiefly responsible for Cr (VI) adsorption. Chromium (VI) forms stable anions, such as Cr₂O₇^2-, HCrO₄^-, CrO₄^2-, and HCr₂O₇^-, the fraction of any particular species is dependent upon the chromium concentration and pH. From the pK_a value of chitosan, it can be calculated that the extent of protonation is 9, 50, 91, and 99% at pH 7.3, 6.3, 5.3, and 4.3, respectively. This leads to the interaction between NH₃⁺ functional groups and the chromate anions. With increase in pH from 5 to 9, the degree of protonation of the adsorbent functional group decreased gradually and hence removal was decreased. Above the pK_a value the chitosan adsorbent will be negatively charged. X-ray photoelectron spectroscopy spectrum by Tiemann et al. (1999) and Dambies et al. (2001) of chitosan biomass after contact with chromium (VI) at pH around 5, reveals that about 55% of chromium (VI) gets reduced to chromium (III). Similar bioreduction process can also be accomplished using alfalfa, seaweed, and some lyophilized plant tissue. The positively charged Cr (III) ions can bind by way of electrostatic interaction with the negatively charged surface of chitosan, mainly through carboxyl ligands.

The oxidative treatment of OPSC with sulphuric acid will introduce more acidic C = O groups on the surface of OPSC. This enhanced the electrostatic interaction between chitosan and the more negatively charged acid treated oil palm shell charcoal (AOPSC) and this prevents any tendency of chitosan to agglomerate. This helped to increase the availability of active binding sites on the chitosan for adsorption of Cr VI at low pH conditions. It has also been suggested that formation of more acidic surface oxides on the carbon surface enhances its hydrophilic character and hence improve the hydrodynamic flow. On the other hand, chitosan coated beads, showed lower adsorption capacity, probably due to less efficient coating of chitosan on OPSC, which has lesser acidic surface oxides. The interaction may not be very strong and the chitosan may agglomerate to a certain degree and become more soluble at low pH and hence reduces the availability of active binding sites on the chitosan for adsorption of Cr (VI). Adsorption of anionic species will be favoured at pH lower than pH_pzc and vice versa for cationic species. The AOPSC used as comparison showed the lowest removal efficiency. The optimum pH was around 4.0. The removal efficiency reduced drastically from pH 4 to 8. At higher pH, the presence of oxygen-containing functional groups, makes the adsorbent surface negatively charged and hence there is repulsive electroststic interaction between the adsorbent and the anions. At pH greater than 8.5, insoluble chromium hydroxide starts precipitating from the solution, making true sorption studies impossible. 

The dependence of Cr sorption on dose was studied by varying the amount of adsorbents from 1.5 to 30 g/l, while keeping other parameters (pH, agitation speed, and contact time) constant. It was observed in this study that removal efficiency of the adsorbent generally improved with increasing dose. This is expected due to the fact that the higher dose of adsorbents in the solution, the greater availability of exchangeable sites for the ions. All of them showed no further increase in adsorption after a certain amount of adsorbent was added (13 - 30 g/l). At 20 mg/l concentration of Cr (VI), the maximum Cr (VI) removal efficiency was about 86% for CCAB at the dosage of 13.5 g/l, while for CCB it was 64% at the dosage of 18 g/l and commercial activated carbon was 52% at dosage of 24 g/l. This suggests that after a certain dose of adsorbent, the maximum adsorption sets in and hence the amount of ions bound to the adsorbent and the amount of free ions remains constant even with further addition of the dose of adsorbent.

It is interesting to note that the saturated values of Cr (VI) removal efficiency of the three types of adsorbents are different from one to another due to the extent of surface modification. Chromium (VI) and some other metals such as arsenic, depending on the pH, are known to exist as anions. At low pH (below 5), the amine group on chitosan is protonated to varying degree. The NH₃⁺ group on the chitosan is chiefly responsible for interaction with anions and negatively charged surfaces. It has also been suggested that the oxidative treatment of OPSC with sulphuric acid would introduce more acidic C = O groups on the surface of OPSC. This would enhance the electrostatic interaction between chitosan and the more negatively charged AOPSC and this prevents any tendency of chitosan to agglomerate. Chitosan  coated beads, showed  lower adsorption capacity, probably due to less efficient coating of chitosan on OPSC, which has lesser acidic surface oxides. The interaction may not be very strong and the chitosan may agglomerate to a certain degree and hence reduces the availability of active binding sites on the chitosan for adsorption of Cr (VI). AOPSC showed poorer adsorption capability for Cr (VI) as compared with the other two adsorbents. This is consistent with the previous result obtained on effect of pH. A close relationship between the surface basicity of the adsorbents and the anions is evident. This is similar to the findings of others, where the interaction between oxygen-free Lewis basic sites and the free electrons of the anions, as well as the electrostatic interactions between the anions and the protonated sites of the adsorbent are the main adsorption mechanism.

The effect of agitation speed on removal efficiency of Cr was studied by varying the speed of agitation from 0 (without shaking) to 200 rpm, while keeping the optimum dose of adsorbents and optimum pH as constant. The Cr removal efficient generally increased with increasing agitation speed. The Cr removal efficiency of CCAB adsorbent increased from 70% to 90% when agitation speed increased from 50 rpm to 100 rpm and the adsorption capacity remained constant for agitation rates greater than 100 rpm. These results can be associated to the fact that the increase of the agitation speed, improves the diffusion of Cr ions towards the surface of the adsorbents. This also indicates that a shaking rate in the range 100 - 200 rpm is sufficient to assure that all the surface binding sites are made readily available for Cr uptake. Then, the effect of external film diffusion on adsorption rate can be assumed not significant.

Results indicate that removal efficiency increased with an increase in contact time before equilibrium is reached. Other parameters such as dose of adsorbent, pH of solution and agitation speed was kept optimum, while temperature was kept at 25^oC. It can be seen that Cr removal efficiency of CCAB increased from 60% to 90% when contact time was increased from 30 to 180 min. Optimum contact time for both CCAB and CCB adsorbents was found to be 180 min, compared to that of AOPSC which was 300 min. Hence the chitosan coated beads require a shorter contact time. Greater availability of various functional groups on the surface of chitosan, which are required for interaction with anions and cations, significantly improved the binding capacity and the process proceeded rapidly. This result is important, as equilibrium time is one of the important parameters for an economical wastewater treatment system.

In this study, however, Langmuir isotherm has a better fitting model than Freundlich as the former have higher correlation regression coefficient than the latter thus, indicating to the applicability of a monolayer coverage of the Cr on the surface of adsorbent. This can be explained due to the fact that activated charcoal has a small surface area for metal adsorption. Therefore, only monolayer adsorption occurred on its surface, in spite of any surface modification. Using Langmuir isotherm, the equilibrium data yielded the ultimate adsorption capacity value for the chitosan coated AOPSC on per gram basis of chitosan as 154 mg Cr/g. This adsorption capacity for chromium is considerably higher compared to the values obtained with other adsorbents (50 to 120 mg/g).

After the adsorbent was saturated the metal ions, it was regenerated with 0.1 M sodium hydroxide. Maximum desorption occurred within 5 bed volumes, while complete desorption occurred within 10 bed volumes.  

In conclusion, it has been shown that the use chitosan coated acid treated oil palm shell charcoal for chromium ion removal appears to be technically feasible, eco-friendly and with high efficacy. Besides that, being composed entirely of agricultural and fishing industry waste, it helps in reduction of waste generation. The adsorbent can be regenerated by using sodium hydroxide, and therefore can be reused. This adsorbent can be a good candidate for adsorption of not only chromium ions but also other heavy metal ions in wastewater stream.  

CLESCERI, L.S.; GREENBERG, A.E. and EATON, A.D. Standard methods for the examination of water and wastewater. 20^th ed. American Public Health Association, Washington, 1998; 1325 p. ISBN 0875532357. 

COUGHLIN, R.W.; DESHAIES, M.R. and DAVIS, E.M. Preparation of chitosan for heavy metal removal. Environmental Progress, 1990, vol. 9, p. 35.

DAMBIES, L.; GUIMON, C.; YIACOUMI, S. and GUIBAL, E. Characterization of metal ion interactions with chitosan by X-ray photoelectron spectroscopy. Colloids and Surfaces, 2001, A., vol. 177, p. 203-214.

GUO, J. and LUA, A.C. Preparation and characterization of adsorbents from oil palm fruit solid waste. Journal of Oil Palm Research Malaysian Palm Oil Board, 2000, vol. 12, p. 64-70.   

TIEMANN, K.J.; GARDEA-TORRESDEY, J.L.; GAMEZ, G.; DOKKEN, K and SIAS, S. Use of X-ray absorption and esterification to investigate Cr (III) and Ni (II) ligands in Alfafa biomass. Environmental Science and Technology, 1999, vol. 33, no. 1, p. 150-155.

Note: Electronic Journal of Biotechnology is not responsible if on-line references cited on manuscripts are not available any more after the date of publication.