Periodic pH regulation controls the phosphate uptake-release behavior and structural evolution of layered double hydroxides

Layered double hydroxides (LDH) are potentially industry-producible adsorption materials for phosphate removal from wastewater. However, the sustainable desorption strategy to support its application in practical scenarios is still lacking. In this study, we proposed to control the phosphate uptake and release behavior of LDH through periodic pH regulation. After twelve stages of regulation, the restoration rates of phosphate and pH were maintained at 86.5 % and 96.2 %, respectively, indicating that LDH was robust to resist multiple rounds of pH shock. But extremely acidic or alkaline conditions can accelerate metal ion loss from LDH laminates, so the regulation needs to be in a mild pH range (4.0-11.0). Under this condition, an alternating pattern appeared in the surface interaction of LDH and phosphate. Spectroscopic data showed that the formation of metal-phosphate coordination complexes (M-O-P) was the dominant mode of phosphate adsorption at high pH, while the formation of electrostatic attraction complexes dominated at low pH. For the interlayer interaction, it is inter-esting to find the exchanged Cl- ions would not re-enter the interlayer of LDH, suggesting that the charge -balancing anions were almost entirely composed of phosphate. Quantitative analysis revealed that the ion -exchanged phosphate amount kept increasing/decreasing in parallel with the total adsorbed amount, implying the driving effect of ion exchange on the periodic uptake/release of phosphate. Overall, the switching mechanism and reversibility of the binding state of phosphate-LDH were revealed during pH regulation. These findings may provide new insights for the development of sustainable desorption options for LDH.