Environmental applications of immobilized and bio-resourced redox mediators: A Review

Redox mediators (RMs), also known as electron shuttles, have been widely reported to promote both biotic and abiotic reductions of oxidized pollutants in water, soil, biogeochemical cycles, and wastewater treatment systems. However, the continuous addition of dissolved RMs is unaffordable and the potential environmental risks remain unknown because most applied RMs are synthetic chemicals. Immobilization technology enables RMs to be attached on non-dissolved supports, avoiding wash-out from the treatment systems. This realizes the reuse of RMs in scaled-up engineering applications and the in-situ remediation. Moreover, renewable natural biomass and their derivatives, such as biochar, have also aroused increased interest because they provide an economical and feasible way to solve the shortcomings of applying soluble RMs. This review presents different RM immobilization methods, which include entrapment, adsorption, and surface modification, as well as the use of bio-resourced RMs. The immobilization procedures and reaction mechanisms of the immobilized RMs and bio-resourced RMs in environmental applications are critically compared and summarized.