Biosorption of chromium by Termitomyces clypeatus

https://doi.org/10.1016/j.colsurfb.2007.05.021Get rights and content

Abstract

The manuscript describes removal of chromium from aqueous solution by biomass of different moulds and yeasts. The biomass of Termitomyces clypeatus (TCB) is found to be the most effective of all the fungal species tested. The sorption of hexavalent chromium by live TCB depends on the pH of the solution, the optimum pH value being 3.0. The process follows Langmuir isotherm (regression coefficient 0.998, χ2-square 5.03) model with uniform distribution over the surface which gets strong support from the X-ray elemental mapping of chromium adsorbed biomass. The amino, carboxyl, hydroxyl, and phosphate groups of the biomass are involved in chemical interaction with the chromate ion forming a cage like structure depicted by scanning electron microscopic (SEM) and Fourier transform infrared spectroscopic (FTIR) results. Desorption and FTIR studies also exhibited that Cr6+ is reduced to trivalent chromium on binding to the cell surface. The level of chromium concentration present in the effluent of tannery industries’ is reduced to a permissible limit using TCB as adsorbent.

Introduction

Chromium, a toxic heavy metal, dissipates into environment as a result of various industrial activities [1], [2] such as steel manufacturing, metal plating, mining, leather tanning, textile dying, cement industries etc. Of all the different oxidation states trivalent and hexavalent chromium exist as stable species. The hexavalent chromium species exists in aqueous solution as oxyanionic entities like chromate (CrO4−2), bichromate (HCrO4) and dichromate (Cr2O7−2), the relative distribution of which depends on the solution pH [3], [4]. Two other forms of chromium, e.g. Cr3O10−2, and Cr4O13−2 have also been detected in highly acidic medium [4]. The oxyanionic entities of Cr6+ do not bind to the negatively charged mineral surfaces, e.g. silica or clay, become highly mobile in the environment and soluble in a solution of neutral pH. In comparison, Cr3+ forms stable hydroxo complexes [e. g. Cr(OH)n(3−n)+] and the cationic Cr3+ having strong affinity for particle surfaces yields insoluble Cr(OH)3 at neutral pH, and becomes almost immobile in the environment [5], [6]. Cr3+ is also an essential micronutrient compared with the toxic, mutagenic and carcinogenic hexavalent chromium [7] in addition to its reduced toxicity due to its low bioavailability. In view of toxicity and related environmental hazards the level of chromium in wastewater must be reduced to a permissible limit (5.0 mg/L and 0.5 mg/L for trivalent and hexavalent chromium, respectively) [8] before discharging into water bodies. The removal of chromium employing conventional methodologies [9], [10] like ion exchange, chemical precipitation or reverse osmosis suffer from limitations like high operating cost, incomplete precipitation, sludge generation, etc. On the other hand biosorption is receiving increasing attention as an emerging technology for the removal of heavy metals from contaminated effluents. The process is based on the adsorption behavior of certain biological materials towards organic or inorganic substances from their solution. Different types of adsorbents [8], [11], [12], [13], [14], [15], [16], [17] including activated carbon, sawdust, cactus leaves, lignin, spent grain, chitin, chitosan, jacobsite (MnFe2O4), etc. used for the removal of chromium results in low removal requiring prolonged equilibrium time. Recently many efforts have been directed towards the development of specific biosorbents, the performance of which depends on the biomass characteristics and microenvironment of target metal ion solution. The research focus for new biosorbents has in recent years converged to microbial biomass [18], [19], [20], [21] because the cell walls of these types of biosorbents contain polysaccharides and proteins having various functional groups such as amine, carboxyl, hydroxyl, sulphates, and phosphates responsible for interacting with the metal ions. Understanding the adsorption behavior is a necessary prerequisite to the development of biosorption process for industrial applications. This paper deals with our investigations on the sorption behavior of chromium on Termitomyces clypeatus biomass, a mold strain available in large quantities as a byproduct of enzyme and fermentation industries. The role of different physicochemical parameters associated with the sorption of chromium from aqueous solution by T. clypeatus biomass (TCB) through batch method is described. Information regarding the binding sites and mechanism of sorption at molecular level has been explored employing Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and energy dispersive X-ray analysis (EDAX).

Section snippets

Materials

Dehydrated microbiological media and ingredients were procured from Himedia, India. All other reagents were of analytical grade and purchased from Merck, Germany and Sigma, USA.

Metal solution and analysis

A stock solution of chromium (1000 mg/L) was prepared by dissolving potassium dichromate (K2Cr2O7) and chromic chloride (CrCl3, 6H2O) separately in double distilled water and diluted to get the desired concentration. Total chromium and hexavalent chromium concentrations were measured by flame atomic absorption

Screening of microorganisms

The biomass of different nonpathogenic molds and yeasts was initially screened to study their uptake capacity of chromium from its solution. Live biomass of T. clypeatus was found to be the most potent of all the biosorbents used (Table 1). It was observed that the adsorption capacity of TCB was more than threefold larger in comparison to yeast biomass. It is reported [27], [28] that the type of biomass has a significant effect on the sorption process due to the variation in the cell size,

Conclusions

The present study has revealed new insight into the initial cell surface binding of chromium on live biomass of T. clypeatus. The sorption process was found to depend on the pH of the solution, pH 3.0 being the optimum value. The process follows the Langmuirian isotherm model exhibiting uniform distribution of chromium over the surface of TCB. The results of FTIR study demonstrate the interaction of functional groups of the biomass with the chromium. The adsorbed chromate is reduced to

Acknowledgement

We thank Mr. S. Dey (Indian Institute of Chemical Biology, Kolkata) for his cooperation during Electron Microscopic analysis. Thanks are due to Ms. Mousumi Basu (Institute of Environmental Studies and Wetland Management, Kolkata) and Prof. A. Dasgupta (Department of Biochemistry, Calcutta University, Kolkata) for providing Atomic Absorption Spectroscopic analysis and Zeta potential measurement facility, respectively. We specially thank Dr. P.C. Banerjee (Indian Institute of Chemical Biology,

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