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. • Reduction of Cr(VI) to Cr(III) by agricultural waste materials. • Complex mechanism called "adsorption-coupled reduction". • Interpretation of experimental data with reduction reaction concept. [ABSTRACT FROM AUTHOR]