Pathways of phosphate uptake from aqueous solution by ZnAl layered double hydroxides

ZnAl layered double hydroxides (LDHs) were prepared by urea hydrolysis-based coprecipitation for removing phosphate from aqueous solutions. The chemical formula of the product was determined as Zn_5.54Al_3.02(OH)_8.73(CO₃)_0.57Cl_5.66·7.84H₂O. Chloride ion was the major interlayer anion of the ZnAl LDHs. Adsorption of phosphate onto the ZnAl sorbent over the entire study period was not in close agreement with pseudo-first-order or pseudo-second-order models. The adsorption can be divided into two steps. A fast adsorption was observed during the first 10 h with a marked increase in the concentration of Cl⁻ in the bulk solution. This indicated that the adsorption of phosphate was largely attributed to the ion exchange between phosphate and the interlayer Cl⁻. A second fast adsorption of phosphate occurred after 10 h. During this period, the pH increased slowly, whereas the Cl⁻ concentration was stable. The uptake of phosphate was likely attributed to OH⁻–H₂PO₄⁻/HPO₄²⁻ ion exchange as well as surface adsorption/complexation. Acidic conditions favored adsorption of phosphate by ZnAl LDHs, which is consistent with the pH increases during the adsorption. Coexisting anions, e.g., SO₄²⁻ and CO₃²⁻, are competitive ions for the adsorption of phosphate. The results verify the contribution of ion exchange and surface adsorption/complexation in the removal of phosphate by ZnAl LDHs.