Microbial-assisted heavy metal remediation: Bottlenecks and prospects

After human evolution that led to modern civilization, humans have introduced many persistent anthropogenic substances in the environment that can cause high ecological risk and great harm to living organisms. Generally, untreated industrial effluents from steel casting industries, chemical companies, fertilizer companies, and shipbreaking industries contain abundant amounts of toxic heavy metals and persistent dyes. Heavy metals are among the most potent pollutants, and they cause great negative effects on human health, soil quality, ground and drinking water quality, plant physiology, and other biota. They are very persistent in nature and can only be converted from one oxidized state to another. Among all the heavy metals, arsenic (As), mercury (Hg), lead (Pb), and chromium (Cr) are found to be more carcinogenic and harmful to humans. Most of their oxidized states are comparatively more permeable in cells, which leads to the lethal manifestations in the organism. Hence, the development of heavy metal remediation technologies is highly essential to circumvent the harmful effects on the environment. Generally, researchers employ precipitation techniques, membrane filtration, and absorption methods to remove various heavy metals. However, these techniques will be either expensive or environmentally unfriendly. In this context, bioremediation has been considered one of the safer, cleaner, more cost-effective, and environmentally friendly techniques for the removal of heavy metal contamination. Bacterial species such as Bacillus sp. and Pseudomonas sp. have been reported to remediate heavy metals by biosorption or bioaccumulation methods. These techniques can be implemented either by autochthonous species or augmented species. Though bioremediation techniques have many bottlenecks that need to be overcome before their commercial appearance, the use of genetically modified organisms or augmented species with the intervention of biochemical engineering principles can offer a promising solution for heavy metal remediation.