Synergetic effects of anhydrite and brucite-periclase materials on phosphate removal from aqueous solution

The objectives of this study were to prepare brucite-periclase material (BPM), and a cost-effective anhydrite-BPM sorbent (ABPM) for the removal of phosphate. The samples were characterized by X-Ray Diffraction (XRD), X-ray fluorescence (XRF), BET surface area, scanning electron microscope (SEM), and transmission electron microscope (TEM). The characterization results show that the BPM was successfully prepared after calcination and hydration, with flaky structure transforming into ellipsoidal structural aggregation. The effects of mixed mass ratios, reaction time, pH, co-existing anions, humid acid and initial phosphate concentration on phosphate removal were investigated by batch experiments. The kinetic adsorption process was well described by pseudo-second-order model with high correlation coefficient (R2 > 0.99). The Langmuir model provided a better described for the adsorption process than the Freundlich model. The maximum phosphate adsorption capacity was 23.66, 24.96 and 26.14 mg·g−1 at 298 K, 303 K and 308 K, respectively, with a 99% removal rate. The P concentration after adsorption was lower than 0.1 mg∙L−1 when the initial P concentration was 10 mg∙L−1.The significant synergetic effects of BPM on anhydrite were observed: firstly, the significant improvement of phosphate removal rate (99.28%) of ABPM can be observed, compared with anhydrite (1.26%) and BPM (45.10%); secondly, a special sustained-release effect was described by the Stumm kinetic mode and an alkaline environment was proved to be made by BPM; finally, the adsorption and precipitation sites were provided by BPM. Thus, the mechanism of phosphorus removal is suggested by two steps: chemical adsorption and Ca-phosphate precipitation. Firstly, the phosphate adsorbed on the BPM by chemical adsorption as the precursor; secondly, the amorphous calcium phosphate compounds formed as the precursors of hydroxyapatite and the phosphate was immobilized by chemical precipitation.