Metals and Living organism

Metals play an integral role in the life processes of living organisms. Some metals (e.g. Ca, Co, Cr, Cu, Fe, K, Mg, Mn, Na, Ni and Zn) are essential, serve as micronutrients and are used for redox-processes, to stabilize molecules through electrostatic interactions; as components of various enzymes; and regulation of osmotic pressure. While many other metals have no biological role (i.e. Ag, Al, Cd, Au, Pb, and Hg), and are nonessential  and potentially toxic to living organism specially microorganisms. Toxicity of nonessential metals occurs through the displacement of essential metals from their native binding sites or through ligand interactions. In addition, at high levels both essential and nonessential metals can damage cell membranes; alter enzyme specificity; disrupt cellular functions, and damage the structure of DNA.

Heavy metals

Heavy metals are metals with  densities higher than 5 g/cm³ . Heavy metals in wastewater come from industries and municipal sewage, and they are one of the main causes of water and soil pollution. Accumulation of these metals in wastewater depends on many local factors such as type of industries in region, people's way of life and awareness of the impacts done to the environment by careless disposal of wastes. Therefore the presence of heavy metals in wastewater is not only of great environmental concern but also strongly reduces microbial activity, as a result adversely affecting biological wastewater treatment processes. Moreover the toxicity of heavy metals in wastewater was shown to be dependant on factors like metal species and concentration, pH, wastewater pollution load and solubility of the metal ions.

Metal removals

Biological removal processes has been attracting considerable attention for removing heavy metals from aqueous wastes and screening for microorganisms having higher potential for removing heavy metals from wastes. Microbial removal of heavy metals offers the advantages of low operating cost, minimizing secondary problems with metal-bearing sludge and high efficiency in detoxifying very dilute effluent.

Biosorption

Biosorption, biopreciptation, and uptake by purified biopolymers derived from microbial cells provide alternative and/or additive processes for conventional physical and chemical methods. Intact microbial cells, live or dead, and their products can be highly efficient bioaccumulators of both soluble and particulate forms of metals. The cell surfaces of all microorganisms are negatively charged owing to the presence of various anionic structures. This gives the bacteria the ability to bind metal cations. Various microbial species, mainly Pseudomonas, have been shown to be relatively efficient in metal uptake from polluted effluents.

Biosorption is a promising method for removal of toxic ions from wastewater. Its advantage is especially in the treatment of large volumes of effluents with low concentration of pollutants. In the past few years a lot of effort has been made on screening of efficient biomass types, its preparation and the biosorption mechanism determination. Biosorption is a fast and reversible process which resembles adsorption and in some cases ion exchange.

Apparently there are many modes of non-active metal uptake by (microbial) biomass. Any one or a combination of them can be functional in immobilizing metallic species on biosorbents. A number of anionic ligands participate: phosphoryl, carbonyl, sulfhydryl and hydroxyl groups can all be active to various degrees in binding the metal.

Adsorption Isotherms

Two important physicochemical aspects of evaluation of the adsorption process as a unit operation are the equilibria of the adsorption and the kinetics. Equilibrium studies give the capacity of the adsorbent. The equilibrium relationship between adsorbent and adsorbate are described by adsorption isotherms, usually the ratio between the quantity adsorbed and that remaining in solution at a fixed temperature at equilibrium. Most often biosorption equilibria are described with adsorption isotherms of Langmuir or Freundlich types.