Impact of Ca 2+ and Mg 2+ on the Removal of F − by Magnesium Potassium Phosphate

This study investigates the influence of Ca 2+ and Mg 2+ 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 − , Ca 2+ , and Mg 2+ ) after 1 min, respectively. Thus, the F − removal efficiencies are found to increase by about 53% with the co‐active effect of Ca 2+ and Mg 2+ in the solution. Moreover, Ca 2+ and Mg 2+ are almost completely removed in the F − , Ca 2+ , and Mg 2+ system. According to the pseudo‐first‐order modeling, the rate constants k for F − , Ca 2+ , and Mg 2+ are 0.00348, 0.0106, and 0.0159 min −1 respectively; thus, Mg 2+ > Ca 2+ > F − . In the ternary system, the removal efficiencies are 53.29–66.03% for F − , 99.99–100% for Ca 2+ , and 87.21–95.19% for Mg 2+ with initial pH 5–10. The removal efficiencies of F − increases with increases in initial concentrations of F − , Ca 2+ , and Mg 2+ . The removal of F − is governed by two routes: 1) adsorption by electrostatic interactions and outer sphere surface complexation; 2) co‐precipitation with Ca 3 (PO 4 ) 2 , CaHPO 4 , Mg 3 (PO 4 ) 2 , and Mg(OH) 2 .