Abstract
Chromium (Cr) (VI) is a well-known toxin to all types of biological organisms. Over the past few decades, many investigators have employed numerous bioprocesses to neutralize the toxic effects of Cr(VI). One of the main process for its treatment is bioreduction into Cr(III). Key to this process is the ability of microbial enzymes, which facilitate the transfer of electrons into the high valence state of the metal that acts as an electron acceptor. Many underlying previous efforts have stressed on the use of different external organic and inorganic substances as electron donors to promote Cr(VI) reduction process by different microorganisms. The use of various redox mediators enabled electron transport facility for extracellular Cr(VI) reduction and accelerated the reaction. Also, many chemicals have employed diverse roles to improve the Cr(VI) reduction process in different microorganisms. The application of aforementioned materials at the contaminated systems has offered a variety of influence on Cr(VI) bioremediation by altering microbial community structures and functions and redox environment. The collective insights suggest that the knowledge of appropriate implementation of suitable nutrients can strongly inspire the Cr(VI) reduction rate and efficiency. However, a comprehensive information on such substances and their roles and biochemical pathways in different microorganisms remains elusive. In this regard, our review sheds light on the contributions of various chemicals as electron donors, redox mediators, cofactors, etc., on microbial Cr(VI) reduction for enhanced treatment practices.
Highlights
Chromium (Cr) is a pervasive toxin that inhabits almost every component of the environment including aerial, terrestrial, aquatic, and biological systems (Rahman and Singh, 2019)
The high magnitude of Cr contamination across the globe is estimated to pour risk on approximately 16 million people (Pure Earth, 2015). Both anthropogenic and natural events can contribute to the global Cr reservoir (Jerábková et al, 2018; Coetzee et al, 2020; Tumolo et al, 2020)
Cr is used as catalyst, oxidizing agent and cooling agent with water (Saha et al, 2013). In another way, hastened dissolution of chromite and other minerals from natural reserves as a natural event instigates the release of Cr into groundwaters upon suitable conditions (Gonzalez et al, 2005; Islam et al, 2020)
Summary
Chromium (Cr) is a pervasive toxin that inhabits almost every component of the environment including aerial, terrestrial, aquatic, and biological systems (Rahman and Singh, 2019). Certain disadvantages such as low efficiency and poor compatibility due to limited availability of electron donors and other inducers often hamper the prominence of Cr(VI) bioremediation in large-scale implementation (Malaviya and Singh, 2016; Beretta et al, 2019; Wang et al, 2020) These limitations can be subdued by introducing low concentrations of suitable chemicals in the medium (Figure 1). Such chemical integration demands a detailed understanding of additive-influenced mechanisms in credible microorganisms Considering this significance, this review recognizes and emphasizes the experimental studies of influences of electron donors, electron mediators, and other chemical additives on microbial Cr(VI) reduction for the enhanced Cr(VI) removal approaches. Biological Cr(VI) reduction activities can occur at extracellular, cell membrane, and intracellular locations in FIGURE 1 | Role of various chemicals other than electron donors and mediators on enhanced Cr(VI) reduction
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