Insights into simultaneous adsorption of orthophosphate (PO43−) and 1-hydroxyethane 1,1-diphosphonic acid (HEDP) by kaolin/lanthanum carbonate composites: Experimental analysis and DFT calculations

Orthophosphate and phosphonate often co-exist in industrial wastewater, and it is a challenge to remove the above inorganic and organic phosphorus simultaneously to meet the stringent total phosphorus (TP) discharge standard. The kaolin-based lanthanum carbonate (LC) composites have been demonstrated to decontaminate TP (PO43− and HEDP) from reverse osmosis (RO) concentrate effectively, while the individual and simultaneous adsorption behavior and mechanisms are still not clear. In this study, the individual and simultaneous PO43− and HEDP adsorption were compared. The adsorption kinetics, isotherms, initial pH and matrix effects were performed. The results showed that PO43− exhibited a nice adsorption performance, and was severely suppressed in the presence of HEDP. The maximum adsorption capacities were obtained at initial pH of 4.0, and the co-existing Ca2+ significantly enhanced both PO43− and HEDP removal, while HCO3−/CO32− markedly interfered with their adsorption process. LaCO3OH was the main composition in the composites for PO43− and HEDP adsorption, and no La-P compounds formed after adsorption. A variety of characterization methods were adopted to explore the adsorption mechanisms, and electrostatic attraction, inner-sphere complexation and hydrogen bond were responsible for PO43− and HEDP adsorption. The density functional theory (DFT) calculations were applied to distinguish the PO43− and HEDP adsorption from the molecular level, and the binuclear bidentate complexation configuration of H2HEDP2− species (H2HEDP-BB) with higher adsorption energy (-165.42 KJ/mol) resulted in the predominant species of HEDP in the competitive adsorption. In the fixed-bed column experiments, both PO43− and HEDP adsorption performance also matched the results of batch experiments and DFT calculations. This study provides insights into the competitive adsorption of orthophosphate and phosphonate by LC-based adsorbent.