Biosorption of chromium and nickel by heavy metal resistant fungal and bacterial isolates

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Abstract

Microorganisms play a significant role in bioremediation of heavy metal contaminated soil and wastewater. In this study, heavy metal resistant fungi and bacteria were isolated from the soil samples of an electroplating industry, and the bioaccumulations of Cr(VI) and Ni(II) by these isolates were characterized to evaluate their applicability for heavy metal removal from industrial wastewaters. The optimum pH and temperature conditions for both the growth and heavy metal removal were determined for each isolate. The optimal pH for fungal isolates was lower (5–5.2) than that for bacterial isolates (7). The observed effect(s) of pH was attributable mainly to organism-specific physiology because in all the tested cases the cellular growth positively correlated with heavy metal removal. Batch and tolerance experiments provided information for solid retention time (SRT) design and the lethal tolerance limits for the isolated microorganisms. Experimental results indicated that expanded SRTs (stationary phase) can be recommended while using the fungal and bacterial Cr-resistant isolates for removing chromium. In the case of Ni-resistant bacterial isolate, a non-expanded SRT was recommended for designing continuous-flow completely stirred (CFCS) bioreactor so that a mid-log phase of cellular growth can be kept during the bioaccumulation process. The tolerance data with a high range of heavy metal concentrations revealed the Cr-resistant isolates, especially the fungal one, could tolerate chromium toxicity at up to 10,000 mg L−1 chromium. Result indicates the applicability of the isolated Micrococcus sp. and Aspergillus sp. for the removal of chromium and nickel from industrial wastewater.

Introduction

Chromium and nickel are released into the environment by a large number of processes such as electroplating, leather tanning, wood preservation, pulp processing, steel manufacturing, etc., and the concentration levels of chromium and nickel in the environment widely varies. These two metals are of major concern because of their larger usages in developing countries and their nondegradability nature. Hexavalent chromium is highly soluble in water and carcinogenic to human. Ni(II) is more toxic and carcinogenic metal when compared with Ni(IV). Due to their toxic effects on living systems stringent limits have been stipulated for the discharge of chromium and nickel into the environment. According to ISI: Bureau of Indian Standard (BIS) the industrial effluent permissible discharge level of Cr(VI) and Ni(II) into inland water is 0.1 and 3.0 mg L−1, respectively.

Conventional physicochemical methods such as electrochemical treatment, ion exchange, precipitation, reverse osmosis, evaporation, and sorption [1], [2] for heavy metal removal from waste streams are not cost effective [3] and hence biological approach has been considered as an alternative remediation for heavy metal contamination. Recently microbial systems like fungus, bacteria and algae have been successfully used as adsorbing agents for removal of heavy metals [4], [5], [6], [7]. Microbial populations in metal polluted environments adapt to toxic concentrations of heavy metals and become metal resistant [8]. Different species of Aspergillus, Pseudomonas, Sporophyticus, Bacillus, Phanerochaete, etc., have been reported as efficient chromium and nickel reducers [9], [10]. The response of microorganisms towards toxic heavy metals is of importance in view of their interest in the reclamation of polluted sites. In the present investigation, the ability of isolated fungal and bacterial strains towards remediation of chromium and nickel was evaluated by characterizing the bioaccumulation of these metals. Effect of temperature, pH, and tolerance to the heavy metals by the isolated organisms were carried out.

Section snippets

Sampling

Soil samples were collected from a 30-year-old small scale electroplating industry at Sipcot, Vellore district, Tamil Nadu, India that uses chromium and nickel for metal plating. In the soil sample collected, the chromium and nickel concentration were approximately between 100 and 50 mg L−1, respectively. Other reported literature values for contamination in soil are chromium: 4700 mg kg−1 and nickel: 5100 mg kg−1 [11]. The collected samples were stored at −80 °C before analysis.

Isolation of metal-resistant microorganisms

Chromium and Nickel

Optimal pH and temperature for heavy metals removal by the isolated species

In the pH range studied (3–11 for 100 mg L−1 of chromium and 50 mg L−1 of nickel), maximum removal of Cr(VI) (92%) and Ni(II) (90%) were observed around pH 5 in the case of Aspergillus sp. (Fig. 1, Fig. 2). Micrococcus sp. reported a maximum removal for Cr(VI) (90%) and Ni(II) (55%) at pH 7.0 (Fig. 3, Fig. 4). With increase in pH from two to four almost no bioaccumulation occurred in the case of both the metals for the isolated Aspergillus and Micrococcus sp. Above pH 5, the percent removal for

Conclusion

In this study, Chromium- and Nickel-resistant microorganisms were isolated from heavy metal contaminated environments, and the applicability of their heavy metal removal from industrial wastewater was evaluated at a laboratory scale. The optimum conditions for both the growth and heavy metal removal were determined for each isolate. The optimal pH for fungal isolates was lower (5–5.2) than that for bacterial isolates. The observed effect(s) of pH on bioaccumulation was attributable mainly to

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