Enhanced Cr(<sc>vi</sc>) removal by Co and PPy co-modified Ca-Al-layered double hydroxides due to adsorption and reduction mechanisms

Co and polypyrrole co-modified hierarchical CaAl-LDH microspheres (CCALP) were synthesized via hydrothermal and in situ polymerization methods. The synergistic effect of PPy and Co endowed CCALP with higher surface area and more reduction sites than CaAl-LDHs modified by Co or PPy alone, maintaining good recyclability for Cr(vi) removal efficiency over four cycles without any treatment. Compared to Co, PPy doping was the dominant reason for Cr(vi) reduction on CCALP. Under optimized conditions, the theoretical maximum adsorption capacity reached 845.25 mg g-1, and the removal efficiency of Cr(vi) achieved 98.83%. The Langmuir model fitted well with the Cr(vi) adsorption on CCALP, supporting the monolayer adsorption hypothesis. The adsorption process followed the Avrami fractional kinetics (AFO) model, suggesting complex and multiple kinetic stages. Thermodynamic experiments confirmed that the adsorption was a spontaneous exothermic process. The density functional theory (DFT) and electrostatic potential (ESP) calculations confirmed that the oxygen-containing parts of Cr2O72- and HCrO4- were the affinity sites, and the co-doping of Co and PPy significantly improved the Cr(vi) adsorption energy on CCALP. Therefore, the Cr(vi) removal mechanism on CCALP was proposed with electrostatic interaction, ion exchange, complexation and reduction.