Advances in Environmental Research 7 (2003) 311–319 1093-0191/03/$ - see front matter  2002 Elsevier Science Ltd. All rights reserved. PII:S1093-0191 Ž 02 . 00004-7 Comparative studies on the microbial adsorption of heavy metals N. Goyal , S.C. Jain , U.C. Banerjee * a a b, Department of Chemical Engineering and Technology, Panjab University, Chandigarh 160 014, India a Institute of Microbial Technology, Sector – 39 A, Chandigarh 160 036, India b Accepted 5 December 2001 Abstract A process of competitive biosorption of Cr(VI) and Fe(III) ions on Streptococcus equisimilis, Saccharomyces cerevisiae and Aspergillus niger is described and compared to single metal ion adsorption in solution. The ability of these three microorganisms to adsorb metal ions wCr(VI) and Fe(III)x, is shown as a function of metal concentration, pH, temperature, growth medium composition, culture age and contact time with the biosorbents. The effect of addition of an extra energy source in the form of glucose, fructose and sucrose in the adsorption medium is studied for the biosorption of metal ions by microorganisms. Freundlich constants are determined from the Freundlich adsorption isotherms for all the organisms. The adsorbed metals from the sorbents can be regenerated in situ with 0.1 M sodium hydroxide.  2002 Elsevier Science Ltd. All rights reserved. Keywords: Biosorption; Streptococcus equisimilis; Saccharomyces cerevisiae; Aspergillus niger; Biosorbents; Freundlich constants; Isotherms 1. Introduction The toxic heavy metals cause serious threat to the environment, animals and humans. Many industries such as mining, iron-sheet cleaning, plating, metal processing, automobile parts manufacturing, dyeing, textile, fertil- izer and petroleum industries release heavy metals such as chromium and iron in the environment (Rapoport and Muter, 1995). Like all transition metals, chromium can exist in several oxidation states from Cr(0), the metallic form to the hexavalent form, Cr(VI). However, only the trivalent and hexavalent forms are environmen- tally important; the latter being of particular concern because of its greater toxicity. Iron present in industrial *Corresponding author. Department of Biotechnology, National Institute of Pharmaceutical Education and Research, Sector 67, S.A.S. Nagar, Mohali 160 062, Punjab, India. Tel.: q91-172-214682–87; fax: q91-172-214692. E-mail address: niper@chd.nic.in (U.C. Banerjee). wastes is primarily in the form of the trivalent Fe(III) ion. Chromium and ferric ion concentrations in indus- trial waste water approach 200–500 and 10–50 mgyl, respectively. According to the US Environmental Pro- tection Agency (EPA) the acceptable value of Cr is 0.05 mgyl and for Fe, it is less than 1 mgyl (Sag et al., 1998). Chemical oxidation–reduction, precipitation, adsorp- tion, solidification, electrolytic recovery and ion exchange are some of the physico-chemical wastewater treatment processes which are being used for metal removal. Application of such processes, however, is sometimes restricted because of technical or economical constraints. Precipitation by alkali addition usually pro- duces large quantities of solid sludge for disposal. Adsorption and ion exchange processes are expensive when the heavy metal concentrations are in the range of 10–100 mgyl (Eccles, 1999). Therefore, there is a need for the development of low cost processes where metal ions can be removed economically. The search