Optimization for the removal of orthophosphate from aqueous solution by chemical precipitation using ferrous chloride

Abstract

The precipitation reaction between the orthophosphate and Fe ²⁺ ions was studied to describe the optimum condition for the removal of orthophosphate from the aqueous solution. The effects of pH, Fe:P molar ratio, and alkalinity were evaluated for the initial orthophosphate concentrations in the range from 1.55 to 31.00 mg/L⁻ . The optimum pH was found to be 8.0 in all orthophosphate concentration ranges. When the stoichiometric moles of Fe ²⁺ were added, the removal efficiencies were significantly less than the theoretical values. It is likely that the precipitation of Fe(OH)₂(s) is partially formed. For the initial orthophosphate concentration of 3.10 mg/L or greater, the removal efficiencies with the Fe:P molar ratio of 3.0:1.0 approached to the theoretical values, yielding greater than 98.5%. If the molar ratio of Fe:P was great enough, the precipitation reaction was completed within 1 h. As the alkalinity increases, the experimental removal efficiencies were significantly greater than the theoretical values. This is because the formation of vivianite is favoured over FeCO₃(s). Finally, it was demonstrated that the orthophosphate (1.40–6.80 mg/L in the secondary effluents from wastewater treatment plants was effectively removed by dosing sufficient amount of Fe ²⁺ ions.

Keywords:

• Fe ²⁺ ions
• orthophosphate
• pH
• molar ratio
• alkalinity