The production of heavy metals has increased rapidly since industrial revolution¹. Heavy metals usually form compounds that can be toxic, carcinogenic or mutagenic even in low concentrations². Due to their mobility in natural water ecosystems and their toxicity to higher life forms, heavy metals in surface and groundwater supplies have been prioritized as major inorganic contaminants in the environment. Even if they are present in dilute, undetectable quantities, their recalcitrance and consequent persistence in water bodies imply that through natural processes such as biomagnification, concentrations may become elevated to such an extent that they begin exhibiting toxic characteristics.

Apart from environmental issues, technological aspects of metal recovery from industrial wastewaters must also be considered³. Metal resources are non­renewable natural resources and are being depleted at faster rate due to increasing demand. It is therefore imperative that those metals considered environmentally hazardous or which are of technological importance and strategic significance or economic value, be recovered at their source through viable treatment options.

Conventional chemical (precipitation/neutralization) or physical (ion exchange, activated carbon sorption and membrane technology) treatment techniques are inherently problematic in their application to metal bearing wastewaters. Chemical treatment methods can prove costly to the user, as the active agent cannot be recovered for reuse in successive treatment cycles. Also, the end product is usually a low-volume, highly concentrated metal bearing sludge, that is difficult to be dewatered and disposed off.

Biological methods of metal recovery termed as biosorption have been suggested as cheaper, more effective alternatives to existing treatment techniques. The process of adsorption is a well established and powerful technique for treating domestic and industrial effluents.

Biosorption is a collective term for a number of passive, metabolism independent, accumulation processes and may include physical and/or chemical adsorption, ion exchange, coordination, complexation, chelation and micro-precipitation accumulating heavy metals^4-7 such as CU²⁺, Cd²⁺, Pb²⁺, Zn²⁺, Cr²⁺, and Ni²⁺

The mechanism of binding of metal ions by adsorbents may depend on the chemical nature of metal ions (species, size, ionic charge), the type of biomass and environmental conditions (pH, temperature, ionic strength), and existence of competing organic or inorganic metal chelators^8-10

In this regard, low cost biosorbents, such as seaweeds, moulds, yeast, bacteria, crabshells. agricultural products such as wool, rice, straw, coconut husks, peat moss, exhausted coffee¹¹, waste tea leaves¹², walnut skin, coconut fibre¹³, polymerized corn cob¹⁴, melon seed husk¹⁵, defatted rice bran, rice hulls, soybean hulls and cotton seed hulls^16,17, wheat bran, hardwood (Dalbergia sissoo) sawdust, pea pod, cotton and mustard seed cakes, petiolar felt sheath of palm^18-20 have been tried.

Iron exists in two forms, soluble ferrous iron (Fe²⁺) and insoluble ferric particulate iron (Fe³⁺). The presence of iron in natural water may be attributed to the dissolution of rocks and minerals, acid mine drainage, landfill leachate sewage or engineering industries. Iron in water is generally present in the ferric state.

Iron typically enters water bodies in the form of ferrous iron (Fe²⁺), which can be oxidized to ferric iron (Fe³⁺) by the oxygen dissolved in water. The rate of oxidation reaction depends primarily on the pH and on the level of dissolved oxygen in water (DO). At pH<4 and a relatively low dissolved oxygen, the oxidation process to ferric iron is very slow. At pH>4, however Fe²⁺ ions oxidize quickly to Fe³⁺ ions, which then react with water producing ferric hydroxide precipitate and acidity

Fe²⁺ + 1/4 O₂ + H⁺ = Fe³⁺ + 1/2 H₂O

Fe³⁺ + 3H₂O = Fe(OH)₃ (s) + 3 H⁺

If the pH drops below 3, the ferric ions cease to precipitate and remain in water in partially hydrolyzed forms.

The presence of iron at concentrations above 0.1 mg/L will damage the gills of the fish²¹. The free radicals formed by the iron on the surface of the gills will cause oxidation of the surrounding tissue and this will lead to massive destruction of gill tissue and anemia.

The presence of iron in drinking water supplies, is objectionable for a number of reasons. Under the pH condition existing in drinking water supply, ferrous sulphate is unstable and precipitates as insoluble ferric hydroxide, which settles out as a rust colored silt. Such water often tastes unpalatable even at low concentration (0.3 mg/L) and stains laundry and plumbing fixtures.

The objective of the present investigation was to examine the sorption of Fe (III) onto Bengal gram husk (bgh), an agromilling waste available in plenty in a tropical country like India.