Phosphate aggregation, diffusion, and adsorption on kaolinite in saline solutions by molecular dynamics simulation

Large amounts of phosphates are required for the survival of cells. However, undesirable algae overgrowth requires phosphorus levels to be kept under control. For large bodies of water, cost-effective and ecosystem-friendly ways to export phosphate are needed. The use of clays appears to be the most promising method in seawater. However, for a broader application, the mechanisms of phosphate adsorption on kaolinite, the conditions for effective flocculation, and the impact of salt must first be addressed. In this study, molecular dynamics simulations are used to determine the adsorption and anchoring mechanisms of two species of phosphates that predominate at neutral pH, the dihydrogen phosphate ion H ₂PO ₄ ⁻ and the hydrogen phosphate ion HPO ₄ ²⁻ , in a 1:1 ratio, on each of the three kaolinite principal planes, the basal surfaces, octahedral gibbsite (001) and tetrahedral siloxane (00 1 ¯ ), and the edge sites (010), in aqueous NaCl at a concentration of 0.6 M to mimic seawater. Phosphate aggregation is enhanced by Na bridges, although at very high concentrations, the number of aggregates and the size of the largest cluster decrease. Calculated diffusion coefficients for both phosphate species drop sharply in hypersaline solutions, although the decay rate is higher for the HPO ₄ ²⁻ species. Diffusion results are in close agreement with experimental data. Phosphate adsorption on kaolinite occurs in clusters. The hydrophilic surface (001) adsorbs increasing amounts of each of the phosphate species as the salt concentration increases, more HPO ₄ ²⁻ than H ₂PO ₄ ⁻. The hydrophobic (00 1 ¯ ) surface remains non-reactive to any species. The hydrophilic (010) edge surface increases adsorption of H ₂PO ₄ ⁻ and drastically decreases that of HPO ₄ ²⁻ as salt concentration increases. Bidentate adsorption of HPO ₄ ²⁻ on the much rougher edge surface, even if saturated with adsorbed Na ions, is sterically impeded. The dominant adsorption mechanism is through cationic bridges on the gibbsite surface and at the edges. The adsorption capacity of kaolinite, including all its surfaces, increases with salt concentration, reaching its highest value in synthetic seawater. The values are of the order, although lower than the experimental ones obtained with less-accessible clays with atomic substitutions. The use of clays to control phosphate content in coastal waters is expected to contribute to public health and sustainability of marine industrial activities.