Eco-Friendly Alkali-Free Arginine-Assisted Hydrothermal Synthesis of Different Layered Double Hydroxides and Their Chromate Adsorption/Reduction Efficiency

A new one-pot synthesis of highly crystalline MgAl-, CoAl- and ZnAl-layered double hydroxides (LDHs) containing NO₃ ⁻ as interlayer anion along with the surface functionalization of L-arginine was demonstrated by the ecofriendly alkali-free arginine-assisted hydrothermal method to examine the adsorptive reduction of chromate (CrO₄ ²⁻) from aqueous solutions. The amino acid mediated water hydrolysis resulted abundant OH⁻ in the medium, which served as precipitant, and the NO₃ ⁻ from metal precursor acts as an interlayer anion along with surface functionalization of amino acid. Adsorption studies revealed that the removal of CrO₄ ²⁻ by anion exchange was the greatest in MgAl-LDH with 1.619 mmol-Cr/g of LDH. The mechanism of CrO₄ ²⁻ removal was due to the ion-exchange of NO₃ ⁻ from the interlayer as well as the counter anions present on the functionalized arginine cation. The adsorbed CrO₄ ²⁻ was partially reduced to nontoxic Cr³⁺ on the surface of LDH due to the presence of amino acid functionalization. The present technique provides several advantages from the aspect of sustainable chemistry: (i) avoiding hazardous alkali for the preparation of LDH, (ii) removal of CrO₄ ²⁻ by ion-exchange and (iii) partial detoxification of CrO₄ ²⁻ by reduction to Cr³⁺ on the surface of the LDH.