Fe(III)-polyhydroxy cations supported onto K10 montmorillonite for removal of phosphate from waters

Since phosphate is strongly related to eutrophication of environmental waters, several research groups quest for materials that can efficiently remove phosphate from wastewaters before it contaminates lakes and reservoirs. In the present work, a commercial clay mineral (K10 montmorillonite) modified with Fe³⁺ polyhydroxy cations was investigated as an adsorbent for phosphate. The incorporation of the polycations did not alter the main conformational characteristics of the montmorillonite, as verified by specific surface area measurements, X-ray diffractometry, FTIR, electron microscopy, and zeta potential titrations. On the other hand, the materials supporting Fe³⁺ polyhydroxy cations exhibited a significant enhancement of adsorption capacity, as determined by Langmuir-Freundlich isotherms, from 39 ± 2 to 104 ± 15 μmol g^-1. The different ratios of OH^- to Fe³⁺ did not affect the adsorption capacities. The adsorption kinetics was best described by the pseudo 2^nd order model, approaching the equilibrium after 120 min of contact time. A variation of pH between 4.6 and 8.5 did not affect the adsorption percentages. The adsorption capacities increased with the increase of the ionic strength, thus suggesting that the formation of inner-sphere complexes prevails over electrostatic interactions as the adsorption mechanism. The materials removed phosphate from three polluted water samples having phosphate concentrations between 0.0919 and 1.211 mg L^-1. The remaining phosphate concentration was below the limit of quantification of the analytical method (0.063 mg L^-1 in P, or 2.0 μmol L^-1). The presence of 10 mg L^-1 humic of fulvic acid did not affect the performance of the materials. In conclusion, the modification of clay minerals with Fe³⁺ polyhydroxy cations is useful in producing low-cost adsorbents for phosphate.