Biosorption is the uptake of heavy metals by the biomass (living or dead) by distinctly physical-chemical mechanisms. In general, the biosorption process has been described as a non selective mechanism that permits the removal of the following metals, among others: Ag, Al, Au, Co, Cd, Cu, Cr, Fe, Hg, Mn, Ni, Pb, Pd, Pt, U, Th and Zn (Gadd and White (1993), Microbial Treatment of Metal Pollution —A Working Biotechnology? Trends Biotechnol., 11, 353-359).
At present there is abundant scientific literature in the international sphere on the capacity of some microorganisms to concentrate metal ions by biosorption from diluted solutions, including Castro et al (1992), Biomasa de Rhizopus oligosporus como adsorbente de iones metálicos. Microbiologia SEM 8, 94-105, Cotoras et al. (1992), Biosorption of metal ions by Azotobacter vinelandii. World Journal of Applied Microbiology and Biotechnology 8, 319-323, Cotoras et al. (1992) Sorption of metal ions by whole cells of Bacillus and Micrococcus Environmental Technology 13, 551-559. Research has covered different groups of organisms, including bacteria (for example, E. coli, Zoogloea ramigera, Bacillus subtilis, Azotobacter vinelandii, etc.), fungii (Example, Rhizopus arrhizus, Aspergillus niger) and algae (for example, Chlorella vulgaris, Sargassum sp.). From all these works, microorganisms appear to be capable of concentrating appreciable amounts of metal ions.
Bacteria of the Bacillus genus, among others, have been used as biosorbents of copper, Tunali S, abuk A, Akar T. Removal of lead and copper ions from aqueous solutions by bacterial strain isolated from soil. Chem Eng J 2006; 115:203-11; Nakajima A, Yasuda M, Yokoyama H, Ohya-Nishiguchi H, Kamada H. Copper biosorption by chemically treated Micrococcus luteus cells. World J. Microbiol. Biotechnol. 2001; 17:343-7; Cotoras D., Viedma P., Cifuentes L. and Mestre A. Sorption of metal ions by whole cells of Bacillus and Micrococcus Environmental Technology 13, 551-559 (1992); U.S. Pat. No. 7,326,344; U.S. Patent Application Publication No. 2008/0009054; Enterobacter, Lu W-B, Shi J-J, Wang C-H, Chang J-S. Biosorption of lead, copper and cadmium by an indigenous isolate Enterobacter sp. J1 possessing high heavy-metal resistance. J Hazard Mater 2006; 134:80-6 o Pseudomonas, Savvaidis I, Hughes M N, Poole R K. Copper biosorption by Pseudomonas cepacia and other strains. World J. Microbiol. Biotechnol. 2003; 19:117-21.
Recent efforts have centered on the chemical modification of the biomass by means of the insertion of functional groups such as amine, carboxyl and hydroxyl to achieve considerable increases in the biosorbent capacity and at the same time obtain the stabilization of the biomass used maintaining the biosorption efficiency after repeated cycles of adsorption/regeneration of the biomass, Vijayaraghavan K., Yun Y S. Bacterial Biosorbents and Biosorption. Biotechnology Advances 26 (2008) 266-291; Diaz de Apodaca E, Villarán M C, Rio F, Ramirez C A, Ibarreña L. Utilization of adsorbents based on chitosan and sodium alginate for the elimination of metal ions: Cu2+, Pb2+, Cr3+y Co2+. Revista Iberoamericana de Polimeros Volumen 8(1), 2007; Ghoul M, Bacquet M., Morcellet M., Uptake of heavy metals from synthetic aqueous solutions using modified polyethyleneimine-silica gels, Water Res 37 (2003), pp. 729-734; Deng S, and Ting Y. P. Characterization of polyethyleneimine-modified biomass and biosorption of Cu(II), Pb(II) and Ni(II) Water Res. 39 (2005), pp. 2167-2177; Deng S, Bai R. Removal of trivalent and hexavalent chromium with aminated polyacrylonitrile fibers: performance and mechanisms. Water Research 38 (2004) 2424-2432; Crini G. Recent developments in polysaccharide-based materials used as adsorbents in wastewater treatment. Prog. Polym. Sci. 30 (2005) 38-70.
Commercial-type biosorbents have been developed, but due to the complexity of their production procedures they become transformed into technologies that are expensive and not very environment-friendly, Brierley C. L. and J. A. Brierley, Immobilization of biomass for industrial application of biosorption. In: A. E. Torma, M. L. Apel and C. L. Brierley, Editors, Biohydrometallurgical Technologies, Proceedings of the International Biohydrometallurgy Symposium, The Minerals, Metals and Materials Society, Warrendale, Pa. (1993), pp. 35-44; U.S. Pat. Nos. 6,395,143; 5,824,512; 5,789,204; 5,520,811; 4,769,223; 5,296,293; and 6,355,172; and U.S. Patent Application Publication No. 2008/0169238.
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