Organics and Heavy Metals Co-Contamination: Issues and Recent Approaches for Co-Remediation of Phenol and Hexavalent Chromium by Using Microorganisms

Co-contamination of the environment with organic and inorganic pollutants (mainly heavy metals) is on the rise due to rapid urbanization and industrial growth. Bioremediation of co-contaminated systems is difficult as microbes must survive or perform efficiently under the combined toxicity of organics and metals. Additionally, the presence of one type of contaminant often hinders the removal of others, and vice versa. Hence, for bioremediation of such systems, microbial strains are required not only to tolerate the simultaneous presence of these toxicants but also to have the capability for their co-removal. Microbial strains endowed with adaptive and detoxification mechanisms toward both heavy metals and organics are of particular interest for the remediation of such co-contaminated matrices. Among the organic and heavy metal pollutants, phenol and hexavalent chromium (Cr) are noteworthy. Both are considered extremely toxic and highly mobile pollutants, and their co-presence is prevalent in various industrial effluents and natural contaminated systems. In the present review, recent approaches for using microorganisms in the co-remediation of phenol and Cr (VI) are presented. A broad perspective on toxicity and adaptive measures used by microorganisms to cope with individual toxicants and the effect of metal contaminants on the removal of organics and vice versa are also highlighted.