Development of a new Cr(VI)-biosorbent from agricultural biowaste

Abstract

Among useless but abundant agricultural biowastes such as banana skin, green tea waste, oak leaf, walnut shell, peanut shell and rice husk, in this study, banana skin was screened as the most efficient biomaterial to remove toxic Cr(VI) from aqueous solution. X-ray photoelectron spectroscopy (XPS) study revealed that the mechanism of Cr(VI) biosorption by banana skin was its complete reduction into Cr(III) in both aqueous and solid phases and partial binding of the reduced-Cr(III), in the range of pH 1.5–4 tested. One gram of banana skin could reduce 249.6 (±4.2) mg of Cr(VI) at initial pH 1.5. Namely, Cr(VI)-reducing capacity of banana skin was four times higher than that of a common chemical Cr(VI)-reductant, FeSO₄ · 7H₂O. To diminish undesirable/serious organic leaching from the biomaterial and to enhance removal efficiency of total Cr, its powder was immobilized within Ca-alginate bead. The developed Cr(VI)-biosorbent could completely reduce toxic Cr(VI) to less toxic Cr(III) and could remove almost of the reduced-Cr(III) from aqueous phase. On the basis of removal mechanisms of Cr(VI) and total Cr by the Cr(VI)-biosorbent, a kinetic model was derived and could be successfully used to predict their removal behaviors in aqueous phase. In conclusion, our Cr(VI)-biosorbent must be a potent candidate to substitute for chemical reductants as well as adsorbents for treating Cr(VI)-bearing wastewaters.

Introduction

Chromium has long been used in electroplating, leather tanning, metal finishing and chromate manufacturing industries. Effluents from these industries contain both trivalent chromium, Cr(III), and hexavalent chromium, Cr(VI), with concentrations ranging from tens to hundreds of mg/L (Barnhart, 1997). Cr(VI) occurs as highly soluble and toxic chromate anions (${\text{HCrO}}_4^{-}$ or ${\text{Cr}}_2{\text{O}}_7^{2-}$), which are suspected to be carcinogens and mutagens (Costa, 2003). In contrast, Cr(III), having a limited hydroxide solubility and lower toxicity, is generally regarded as a much less dangerous pollutant (Anderson, 1997). Because of these differences, the discharge of Cr(VI) to surface waters is regulated to below 0.05 mg/L by the US EPA, while total Cr, including Cr(III), Cr(VI) and its other forms, is regulated to below 2 mg/L (Baral and Engelken, 2002).

A chemical treatment method has been generally used to treat Cr(VI)-bearing wastewaters (Qin et al., 2005). This method involves the aqueous reduction of Cr(VI) into Cr(III) using chemical reductants under strong acidic conditions (<pH 2), with the subsequent adjustment of the solution pH to near-neutral conditions, for the precipitation of the Cr(III) ions produced. Considering economic efficiency and sustainable environment, however, this method is undesirable due to the use of large amounts of expensive and harmful chemicals and due to the production of a large amount of secondary pollutant, i.e., metal-bearing chemical sludge (Mohan and Pittman, 2006). To solve these problems, many researchers have developed sorption-based processes using synthetic resins (Gode and Pehlivan, 2005), activated carbons, inorganic sorbent materials, or the so-called biosorbents derived from nonliving biomaterials (Mohan and Pittman, 2006). Of these, biosorbents have been considered as the cheapest, most abundant, and environmentally friendly option (Volesky and Holan, 1995, Volesky, 2007).

Since a report on the use of sawdust as a biosorbent for Cr(VI) removal (Srivastava et al., 1986), a great many researchers have tested various biomaterials, such as nonliving bacteria, algae, fungi, seaweed, industrial byproducts and agricultural biowastes, in order to remove toxic Cr(VI) from aqueous solutions (Mohan and Pittman, 2006). For the last a few decades, an anionic adsorption of Cr(VI) to cationic functional groups of the biomaterials has long been misunderstood as an absolute mechanism of Cr(VI) biosorption (Mohan and Pittman, 2006, Park et al., 2006b). Nowadays ‘adsorption-coupled reduction’ is widely accepted as the true mechanism of Cr(VI) biosorption by natural biomaterials under acidic conditions (Elangovan et al., 2008a, Elangovan et al., 2008b, Gao et al., 2008, Kumar et al., 2008, Yang and Chen, 2008) (Note that the mechanism of Cr(VI) biosorption by biomaterials will be mentioned in detail later).

Recently, brown seaweed Ecklonia sp. and fungal Rhizopus sp. have been suggested as efficient biomaterials capable of treating Cr(VI)-bearing wastewaters, which were selected among eight seaweeds and four fungi, respectively (Park et al., 2004, Park et al., 2005a). These biomaterials completely removed toxic Cr(VI) through direct and/or indirect reduction reaction(s) into Cr(III) under acidic conditions, and efficiently adsorbed the reduced-Cr(III) ions at elevated pHs; i.e., it could be used as a reductant like FeSO₄ · 7H₂O as well as an adsorbent like ion exchange resin. Especially the Cr(VI)-reducing capacity of protonated Ecklonia biomass was 3.7 times higher than that of FeSO₄ · 7H₂O at pH 2 (Park et al., 2004). In a feasibility test with actual electroplating wastewater (1.5 mg/L Cr(III), 35.4 mg/L Cr(VI), 4.1 mg/L Zn(II) and other trace metals below 0.2 mg/L), ‘two-stage biosorption process’ showed efficient removal efficiencies of two different chromium species and other metals (Park et al., 2006a). In spite of these remarkable advantages, there are still a few unsolved drawbacks to industrial applications of the biomaterials as organic reductant as well as adsorbent; (i) Reduction rate of Cr(VI) by the biomaterials were much slower than that by chemical reductant. Thus there is a need for searching new superior biomaterials capable of reducing Cr(VI) more fast even under weak acidic conditions (>pH 4). (ii) Natural biomaterials are generally biodegradable. It means that it can release soluble organic compounds into the aqueous phase during biosorption, which must be further removed before being discharged into the environment. (iii) Not only Cr(VI) but Cr(III) must be efficiently removed by same biomaterial, but optimum conditions for their removals are different from each other. As solution pH decreases, the removal efficiency and rate of Cr(VI) increase since protons participate in abiotic reduction reaction of Cr(VI) by biomaterial (Park et al., 2004, Park et al., 2005b, Park et al., 2008). On the contrary, those of Cr(III) decrease with increasing the pH since protons inhibit adsorption reaction between cationic Cr(III) ions and negatively charged functional groups of biomaterial (Park et al., 2004, Yun et al., 2001). To the best of our knowledge, nobody has ever seriously considered these un-negligible problems until now. Most of researchers have only focused on fundamental kinetic and/or equilibrium studies of Cr(VI) biosorption by unreported or less-researched biomaterials (Abbas et al., 2008, Elangovan et al., 2008a, Elangovan et al., 2008b, Ertugay and Bayhan, 2008, Gokhale et al., 2008, Gupta and Rastogi, 2008, Pehlivan and Altun, 2008, Wang et al., 2008, Zubair et al., 2008). To make matters worse, not a few researchers have still misunderstood Cr(VI) biosorption mechanism under acidic conditions (Abbas et al., 2008, Ertugay and Bayhan, 2008, Gode et al., 2008, Gokhale et al., 2008, Gupta and Rastogi, 2008, Pehlivan and Altun, 2008, Rawajfih and Nsour, 2008, Wang et al., 2008, Zubair et al., 2008).

The aims of this study were (i) to screen new superior biomaterials capable of reducing Cr(VI) more fast from un- and/or less-researched agricultural biowastes, (ii) to characterize the mechanism of Cr(VI) biosorption by the agricultural biowastes, (iii) to develop a simple method to solve the problems mentioned above, and (iv) to develop a kinetic model to describe Cr(VI) and total Cr removal behaviors in aqueous solution. To address these issues, Cr(VI)-removal capacities of banana skin, green tea waste, oak leaf, walnut shell, peanut shell and rice husk were examined by batch experiments and compared to those of well-known biomaterials, i.e., brown seaweed Ecklonia and Sargassum biomasses. Mechanism of Cr(VI) biosorption by a selected biowaste was characterized by a X-ray photoelectron spectroscopy (XPS). The biomaterial was immobilized within Ca-alginate bead, and then effects of biomaterial content, solution pH and initial Cr(VI) concentration on Cr(VI) and total Cr removals were examined in detail. Finally, a kinetic model was derived on the basis of the removal mechanisms of Cr(VI) and total Cr by the Cr(VI)-biosorbent developed in this study.