Removal of hexavalent chromium by heat inactivated fungal biomass of Termitomyces clypeatus: Surface characterization and mechanism of biosorption

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Abstract

Mechanism of Cr(VI) biosorption by heat inactivated fungal biomass of Termitomyces clypeatus was studied by analyzing its pH profile and surface chemistry. The biosorption efficiency enhanced with acid pretreatment and reduced by alkali. The pretreatment with CaCl2 increased the biosorption capacity but decreased with NaCl. Surface chemistry was characterized by potentiometric titration pH of zero charge and SEM–EDX analysis. The acidic and basic sites for the biomass were quantified as 7.75 and 3.25 mmol/g, respectively, and concluded that surface of the biomass was acidic. Acidic [carboxyl (pKa 3.45 and 4.29), imidazole (pKa 5.98), and phosphate (pKa 6.75)] and alkaline [amines (pKa 9.96, 11.92, and 12.47), sulfhydryl (thiol) (pKa 8.48), hydroxyl (pKa 11.12)] functional groups were identified and confirmed by FTIR analysis. The roles played by functional groups in chromium biosorption were found to be in the order: carboxyl > phosphates > lipids > sulfhydryl > amines. Integrative analysis of surface chemistry, functional group modification and FTIR showed that the Cr(VI) biosorption involved more than one mechanism such as physical adsorption, ion exchange, complexation and electrostatic attraction and followed in two subsequent steps – Cr2O72− biosorption at the protonated active sites (amino, carboxyl and phosphate groups) and reduction of Cr(VI) to Cr(III) by reductive groups (hydroxyl and carbonyl groups) on the biomass surface.

Highlights

► The surface of the heat inactivated biomass was acidic. ► Cr(VI) removal by adsorption, ion exchange, complexation, and electrostatic attraction. ► Acidic/alkaline functional groups, imidazole, –Cdouble bondO, –PO4, –NH2, –SH, –OH on cell wall. ► SEM–EDX showed new shiny bulky particles on the surface and Cr peaks in the spectra. ► Cr(VI) sequestration by potentiometric titration, PZC, functional gp modification, FTIR.

Introduction

Increased industrialization and human activities have created impact on the environment through the disposal of waste containing heavy metals. The existence of heavy metals in the environment represents a significant and long-term environmental hazard. Even at low concentration these metals can be toxic to organisms, including humans. Chromium is a contaminant that is a known mutagen, teratogen and carcinogen [1], [2]. Hexavalent chromium, Cr(VI), is contained in wastewaters produced by industrial processes, such as those employed in the electroplating, metal finishing, metallurgical, leather tanning, dye, wood preservation and battery manufacturing industries. Among the several oxidation states (di, tri, penta and hexa), the hexavalent state, together with trivalent chromium, can be the main forms present in aquatic environments [3]. Chromate (CrO42−) and dichromate (Cr2O72−), are the prevalent species of Cr(VI) in natural aqueous environments, as major pollutant from chromium-related industries which create significantly higher levels of toxicity than the other valency states of the metal [4], [5]. The maximum permissible limit of Cr(VI) in natural water is only 0.05 mg/L by the U.S. Environmental Protection Agency [4].

The removal of Cr(VI) from aqueous solution has received considerable attention in recent years. Traditionally, these removals are made by electrochemical treatment, chemical precipitation, membrane process, reverse osmosis, ion exchange, liquid extraction, electro dialysis, evaporation and sorption [6]. However, the application of these treatment processes has been found to be sometimes restricted, because of expensive investment, operational costs, potential generation of secondary pollution, and its disposal is not eco-friendly [7]. Furthermore such processes may be ineffective or extremely expensive when the initial heavy metal concentrations are in the range of 10–100 mg/L. At present emphasis is given to the utilization of biological adsorbents for the removal and recovery of heavy metal contaminants. Biomass involving pure microbial strains has shown high capacities for the selective uptake of metals from dilute metal bearing solutions [8]. Several investigations have been reported on metal binding efficiencies of various strains of bacteria, algae, fungi and seaweed [8], [9], [10], [11]. Among them fungi are fast growing; low cost (less nutrient requirement) has adaptability to natural environments, available as industrial/laboratory byproduct [12], [13], [14], [15], [16]. Fungi can tolerate and detoxify metals by several mechanisms including passive accumulation processes which may include ion exchange, complexation, adsorption, extra and intracellular precipitation, valence transformation and also active uptake [15], [17]. The cell wall of fungi is composed typically of chitin (a long linear homopolymer of beta-1,4-linked N-acetylglucosamine), glucan, mannan, proteins and other polymers [18] that possess carboxyl, phosphoryl, hydroxyl, amino, amine and imidazole functional groups at the surface.

Studies on biosorption using different fungal biomasses were focused on the removal of metal ions from aqueous solutions. However, some studies interpreted and established the mechanism involved in Cr(VI) binding [2], [5], [19], [20], [21], [22], [23], [24], [25]. Still the binding sites for chromium were not specifically identified. The fungal species, Termitomyces clypeatus, used in this study is an edible variety of mushroom, commonly found in near-surface system. The inactivated/dead fungal biomass is of little use and is preferred as a source of biomaterial for biosorption processes with no risk of contamination during biosorption process as well as better easy handling [12], [25]. To study the biosorption of Cr(VI), an investigation of surface characteristics of T. clypeatus was required to understand the mechanisms of the metal biosorption. The objective of the present work was to study the mechanism of biosorption of heat inactivated biomass of T. clypeatus for the biosorption of Cr(VI) using different experimental approaches involving biochemical, FTIR and SEM–EDX surface analysis. The biomass was chemically pretreated by acid, alkali and salts for the assessment of change in biosorption efficiency. The functional groups involved in Cr(VI) biosorption were identified by potentiometric titration, pH of zero charge, modification of functional groups by chemical treatment and FTIR analysis. The results would contribute to a better understanding of biosorption and aid in the development of potential biosorption that possess almost complete removal efficiency for Cr(VI) from aqueous environment.

Section snippets

Reagents

All the reagents used in this study were of analytical grade and purchased from Across, India, Merck, Germany and Sigma, USA. Media ingredients were procured from Himedia, India.

Culture conditions and biomass preparation

The fungal strain of T. clypeatus (MTCC-5091) was routinely cultured in complex medium under submerged condition at 30 °C for 5 days and mycelia were taken as byproduct. The complex medium (%, w/v) comprised of the following ingredients: sucrose 5.0, malt extract 1.0, boric acid 0.057, KH2PO4 0.15, CaCl2·2H2O 0.037, MgSO4

Enhancement of Cr(VI) removal efficiency by chemical pretreatment

Various chemical pretreatments of the biomass were applied to improve the Cr(VI) adsorption capacity and removal efficiency. A series of biosorption batch experiments were carried out with heat inactivated (control) and with acid/alkali/salts pretreated biomass at pH 5.0 and 7.0. The obtained results (Table 1) indicated that HCl and CaCl2 pretreatments enhanced the biosorption efficiency 20–25% than the control biomass. Acid-treatment has been used for washing the cell wall to enhance uptake

Removal mechanism by the heat inactivated (non living) biomass

Higher fungi (mushrooms) are abundantly available in nature and can be exploited as low cost materials for their biosorption properties. Studies on biosorption process characteristics of the heat inactivated T. clypeatus biomass such as pH profile, kinetics and biosorption isotherms concluded that at pH 2.0 the rate of biosorption was faster than at pH 5.0 and 7.0 and with increasing contact time similar biosorption efficiency was reported [44]. The biosorption complied with both the Langmuir

Concluding points/remarks

The following conclusions were reached based on the study:

  • Chemical pretreatment of the biomass by acid and CaCl2 showed improvement of Cr(VI) removal rate at pH 5.0 and 7.0 while reverse was true for alkali and NaCl treatments.

  • The mass loss recorded after each chemical pretreatment due to cleaning of the cell wall of the biomass, which indicated that degradation and solubilization of the biomass caused by the alkali treatment and thereby exhibited negative effect on Cr(VI) biosorption.

Acknowledgment

Funding to LR by CSIR, India is duly acknowledged.

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