Heavy metals are found naturally. Anthropogenic activities and rapid industrialization have led to their unprecedented release into the environment. Being non-biodegradable in nature, they persist in the environment. Prolonged exposure and accumulation of these metals poses a serious threat to the ecosystem. Conventional treatment of contaminated material whether soil or water involves expensive chemical or physical methods which are arduous, energy demanding, and carry the risk of secondary contamination. It is thus necessary to adopt a sustainable remediation process to mitigate this problem. Biological remediation processes are preferable as they are environmentally safe, techno-economically feasible, and do not generate toxic byproducts. Microbial bioremediation is particularly attractive as it allows remediation processes by tapping naturally occurring catabolic capacities to transform, accumulate, and adsorb metals for detoxification. It is a comparatively low-cost technology. Therefore, microbial bioremediation is promising as an alternative to physico-chemical methods.
TopHeavy metals are natural components; however they cannot be biodegraded, tend to persist in nature. Because of this property, the heavy metals are recognized as a major environmental concern. Entry of heavy metals in the environment is either via natural sources and/ or anthropogenic sources that include industrial discharge, automobiles exhaust, and mining (Fergusson, 1990; Bradl, 2002; He et al., 2005). Some human activities such as agriculture practices also contribute to increased heavy metal concentrations, ultimately threatening ecological and human health. Various activities contribute to heavy metal poisoning such as contaminated air, industrial exposure, medicines, etc. Heavy metals in environment are also due to emissions occurring naturally as in the case of volcanic eruptions, forest fires, sprays of sea salt, weathering of rocks, soil erosion, etc. Hence under certain environmental conditions natural weathering will lead to release of metals in the biosphere in bound forms as oxides, sulphates, phosphates, others. Heavy metals along with other salts and minerals comprises the inorganic pollutants (Wong, 2012) and are mostly added due to human indulgence (Shallari et al., 1998; Herawati et al., 2000; Goyer, 2001; He et al., 2005), becoming toxic due to bioaccumulation in the food chains (Salomons et al., 1995). All spheres of environment are affected by toxic nature of the heavy metals, resulting in disturbances in the food chains as well as severe health problems.
Environmental pollution due to industrial, agricultural and household emissions as well as natural emissions including mine tailings, use of paints, fertilisers, pesticides, irrigation using waste water, coal combustion, petrochemical usage and spillage, soil erosion, heavy metal leaching, weathering of soil or rocks, volcanic eruptions, etc, are sources of heavy metal pollution (Nriagu, 1989; Fergusson, 1990; Shallari et al., 1998; Bradl, 2002; He et al., 2005; Zhuang et al., 2013; Ventura et al., 2017; Afzal et al., 2018; Soleimani et al., 2018).
Agricultural runoff containing excess of chemical fertilisers and pesticides as well as runoff from mining areas, add significant quantities of heavy metals to the runoff water that enters the water bodies. The heavy metals may be in trace amounts, but may be toxic to the plants and life in the water bodies. The metals sink and get concentrated in the sediment of the water bodies (Musilova et al., 2016).