Experimental study and surface complexation modeling of fluoride removal by magnesium hydroxide in adsorption and coprecipitation processes

High contaminated levels of fluoride (F) in groundwater and drinking water are of significant concern for human health in many countries. This study examined adsorption and coprecipitation for effective F- removal from high F-containing water using inexpensive magnesium hydroxide, Mg(OH)2. Fluoride solutions were reacted with Mg2+-or Mg(OH)2-containing solution at different F/Mg molar ratios of 0.05-2, and the F- removal performance and mechanisms were investigated in both processes. As a result, the residual amount of F- in the solution decreased with an increase in Mg2+ concentrations for both processes, and the sorption isotherms followed the Langmuir type. Mg(OH)2 was precipitated only after F- removal, which suggests that adsorption to Mg(OH)2 was the main removal mechanism in both processes. However, the saturated adsorption capacity in the coprecipitation process was two times higher than that in the adsorption process. The chemical equilibrium calculation results implied that the surface complexation of MgF° and MgOH2F° was the dominant mechanism in the adsorption process whereas multiple complexations of Mg-MgF3° and Mg-MgF4- occurred in the coprecipitation process. This complex formation improves F- removal; hence, coprecipitation with Mg(OH)2 exhibited better potential as an efficient process for the treatment of industrial wastewater that contains F-.