![]() Although various types of tolerance mechanisms have been reported in bacteria for heavy metal stress, Cd detoxification has only been restricted to efflux pumps. Bacteria detoxify heavy metals in a variety of different ways. The interaction of microorganism with metal ions depends on factors like oxidation state of the metal ion, chemical/physical nature of metals, growth phase of microorganism etc.īioremediation offer different advantages such as low operating cost, minimum ratio of disposable sludge volume, high efficiency in detoxifying very dilute effluents and even in situ remediation. Microorganisms acquire resistance to these toxic metals by lateral gene transfer. The toxicity of heavy metal ions starts when their concentration becomes higher in the cells, due to which they form complex compounds. In this way, heavy metals play an important role as ‘trace elements’ (cobalt, copper, nickel, and zinc) in sophisticated biochemical reactions and are important cofactors for metallo-proteins and enzymes. They have incompletely filled ‘d’ orbitals which allow heavy-metal cations to form complex compounds that may or may not be redox-active. In periodic table, transition elements are mostly heavy metals. There are 90 naturally occurring elements in periodic table, 21 are non-metals, 16 are light metals and the remaining 53 (with As included) are heavy metals. The list of essential and non-essential heavy metals is given ( Table 1). Such metals are toxic at any concentration. If the metals have no known biological function, they are called as non-essential metals e.g. ![]() Such metals are required in low concentrations (nM), but at higher concentrations (μM to mM) all heavy metals have detrimental effects to organisms. The metals which are need for normal cellular growth are essential metals e.g. They are classified as essential and non-essential metals. Heavy metals are usually defined as metals having density more than 5 g/cm 3. Industrial effluents are known to contain heavy metals which originate from metal plating, mining activities, smelting, battery manufacture, tanneries, petroleum refining, paint manufacture, pesticides, pigment manufacture, and printing and photographic industries. These metals include arsenic, cadmium, chromium, copper, lead, mercury, nickel, and zinc. Even when the concentration of metals does not exceed these limits, there is still a potential for bioaccumulation and associated chronic toxicity as heavy metals are known to be accumulative within biological systems. Metals exhibit health issues if their concentrations exceed allowable limits. These heavy metals are released directly into the environment. ![]() There are two main sources of introduction of heavy metals in the environment (1) natural sources which includes volcanic emissions, forest fires, deep-sea vents, and geysers and (2) anthropogenic sources which includes mining and smelting sites, metal-manufacturing plants, painting- and coating-industries and tanneries. Due to anthropogenic activities, increasing population, industrialization and urbanization, all spheres have become polluted. The industrial revolution in the past five decades is remarkable. Together these spheres are important for maintaining a balanced ecosystem. Nature has gifted our earth with four spheres biosphere, lithosphere, hydrosphere, and atmosphere. Choice of desorption agent should be carefully selected to prevent alteration of physical properties of a biosorbent. Recovery of biosorbed metals can be done using agents like thiosulfate, mineral acids and organic acids. Various models including Freundlich model and Langmuir model can be used to describe biosorption. Different factors affect the rate of biosorption which includes temperature, pH, nature of biosorbents, surface area to volume ratio, concentration of biomass, initial metal ion concentration and metal affinity to biosorbent. Not only living organisms, but also residuals of dead bodies of microorganisms shows biosorbent properties like agricultural wastes including husk, seeds, peels and stalks of different crops. Biosorption is exhibited by bacteria, algae, fungi and yeasts. One such strategy is biosorption which is binding of metal ions with metal binding proteins present on the cell wall. These strategies can be metabolism dependent or independent. They have adopted different strategies to cope up with the harmful effects of these metals. ![]() Micro-organisms are present in industrial effluents. Heavy metals are known to persist in the environment and become a risk for organisms. Industrialization has led to introduction of heavy metals in the environment. ![]()
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