Defective multi-element hydroxides nanosheets for rapid removal of anionic organic dyes from water and oxygen evolution reaction

Water pollution by dyes is one of the biggest environmental problems. Adsorption technology has been widely used in wastewater treatment. In this work, high-entropy concept is used to design surface defective hydroxides realizing the rapid removal of dyes from water. Multi-element hydroxides (MEHs) containing three (CoMnNi, MEH-Ternary), four (CoMnNiZn, MEH-Quaternary), and five (CoMnNiZnFe, MEH-Quinary) metal elements are successfully synthesized through a polyol process. These as-synthesized MEHs are composed of nanosheets with a brucite-like structure. Along with the increase in compositional complexity (i.e., configurational entropy), the thickness of the nanosheets in these MEHs decreases, while the degree of surface defects increase. These surface defects are probably the active sites for anionic dyes adsorption, suggesting rapid adsorption kinetics with shortened diffusion path length. For MEH-Quinary in 0.2 mM Congo red (CR) and MEH-Ternary in 0.4 mM methyl orange (MO) aqueous solutions, respectively, high removal efficiency > 99.0% is achieved in the first 30 s. Their pseudo-second-order rate constants are two orders of magnitude higher than that of activated carbon and hydrotalcite. MEH-Quinary has maximum CR and MO adsorption quantity of 546.4 and 404.9 mg g-1, respectively, by Langmuir model. The MEH-Quinary is also a potential electrocatalyst for oxygen evolution reaction.

Automated summary

Water pollution caused by dyes is a significant environmental issue, and adsorption technology has been widely used for wastewater treatment. In this study, surface defective hydroxides designed using the high-entropy concept efficiently remove dyes from water. Multi-element hydroxides (MEHs) with different compositions are synthesized through a polyol process and exhibit rapid adsorption kinetics due to increased surface defects. MEH-Quinary and MEH-Ternary show high removal efficiency of over 99.0% within 30 seconds for Congo red and methyl orange, respectively, with pseudo-second-order rate constants two orders of magnitude higher than activated carbon and hydrotalcite. MEH-Quinary also has a high adsorption capacity and potential as an electrocatalyst for oxygen evolution reaction.