Biosorbents for toxic heavy metals - A review

The present investigation shows that the agricultural by-products like bengal gram husk, tur dal husk, and tamarind husk can be used as an effective adsorbent for the treatment of wastewaters containing metals like chromium (VI), iron (III), nickel (II) and mercury (II). The term biosorption commonly refers to the passive binding of metal ions or radioactive elements by dead biomass. It has to be distinguished from bio-accumulation which is usually understood to be an active, metabolically mediated metal-accumulation process occurring specifically in living organisms. The focus in early studies has been exclusively on the toxicological aspects of biosorption. Recently, however, efforts are being made to harness this phenomenon into a technique for the detoxification of metal-bearing industrial effluents by removing and/or eventually also recovering the metals. Compared to conventional wastewater “polishing” techniques such as ion-exchange, activated carbon treatment or membrane technologies (electro-dialysis, reverse osmosis), and the advantage of biosorption is not only in that it can be operated under a broad range of conditions (pH, temperature) but especially that it appears to be economically attractive due to the cheap raw materials that can be used as biosorbents. The effects of pH adsorbent dosage on the removal of metals for all the adsorbates were examined. The optimum pH level for the removal of chromium (VI), iron (III), mercury (II) and nickel (II) was found to be 2.0, 2.5, 5.5 and 6.0 respectively. The review shows that the amount of the metal removed at optimum pH increased with increase in initial metal concentration but the percentage absorbed decreased with increase in initial metal concentration.