Ecotoxicity study of reduced-Cr(III) generated by Cr(VI) biosorption.

Hexavalent chromium (Cr(VI)) is a known carcinogen in living organisms, and many studies have investigated Cr(VI) removal methods. Biosorption, one of the Cr(VI) removal methods, is dominated by chemical binding, ion exchange, physisorption, chelation and oxidation-reduction. Among these mechanisms, Cr(VI) can be removed by nonliving biomass through a redox reaction, which has been recognized as 'adsorption-coupled reduction'. Cr(VI) is reduced to Cr(III) during biosorption, but there are no studies on the property and toxicity of the reduced-Cr(III). In this study, the harmfulness of reduced-Cr(III) was identified through assessment of mobility and toxicity in nature. Pine bark, a low-cost biomass, was used to remove Cr(VI) from aqueous solution. Reduced-Cr(III) was characterized by structural features using X-ray Absorption Near Edge Structure (XANES) spectra, mobility assessed using precipitation, adsorption and soil column test, and toxicity using radish sprouts and water flea. XANES analysis confirmed that reduced-Cr(III) has an unsymmetrical structure, and the reduced-Cr(III) showed low mobility and was virtually non-toxic, and helpful for plant growth. Our findings demonstrate that Cr(VI) biosorption technology using pine bark is a groundbreaking technology for Cr(VI) detoxification.

Comment on the review paper "agricultural waste materials for adsorptive removal of phenol, chromium(VI) and cadmium(II) from wastewaters: A review" by Othmani et al. (2022).

Review papers help researchers understand the direction and flow of research and help beginners quickly acquire relevant knowledge. Therefore, the review paper should describe only accurate contents and should be written with only core and important matters. Recently, Othmani et al. (2022) reviewed more than 200 papers related to the adsorptive removal of three harmful pollutants: toxic organics (phenols), anionic heavy metal (Cr(VI)) and cationic heavy metal (Cd(II)) by agricultural waste materials. However, Cr(VI)-related section of this review is missing something important that should be provided to the readers of this journal. In fact, the removal mechanism of Cr(VI) by nonliving biomass including agricultural waste materials under acidic conditions is not a simple anion adsorption but a complex mechanism involving redox reaction called "adsorption-coupled reduction". Therefore, when interpreting the factors affecting Cr(VI) removal, it should be interpreted in terms of the redox reaction concept, not anion adsorption.