Impact of Ca²⁺ and Mg²⁺ on the Removal of F⁻ by Magnesium Potassium Phosphate

Abstract

This study investigates the influence of Ca2+ and Mg2+ on the removal of F− by magnesium potassium phosphate (MPP) from water. The kinetic experiments reveal that the F− concentration decreased from 3.5 to 3.31 mg L−1 in a single (F−) system and to 1.45 mg L−1 in a ternary system (F−, Ca2+, and Mg2+) after 1 min, respectively. Thus, the F− removal efficiencies are found to increase by about 53% with the co‐active effect of Ca2+ and Mg2+ in the solution. Moreover, Ca2+ and Mg2+ are almost completely removed in the F−, Ca2+, and Mg2+ system. According to the pseudo‐first‐order modeling, the rate constants k for F−, Ca2+, and Mg2+ are 0.00348, 0.0106, and 0.0159 min−1 respectively; thus, Mg2+ > Ca2+ > F−. In the ternary system, the removal efficiencies are 53.29–66.03% for F−, 99.99–100% for Ca2+, and 87.21–95.19% for Mg2+ with initial pH 5–10. The removal efficiencies of F− increases with increases in initial concentrations of F−, Ca2+, and Mg2+. The removal of F− is governed by two routes: 1) adsorption by electrostatic interactions and outer sphere surface complexation; 2) co‐precipitation with Ca3(PO4)2, CaHPO4, Mg3(PO4)2, and Mg(OH)2.