Precipitation of anions: Chemistry, prediction, and environmental applications

Anions such as arsenate, borate, fluoride, phosphate, and silicate are of environmental significance. Precipitation-dissolution is an important reaction that governs their fate, transport, and concentration in the environment. Precipitation is also a physico-chemical process in water and wastewater treatment. This paper reviews precipitation of anions, including its chemistry, equilibrium prediction, and the environmental implications as applied in natural systems and wastewater treatment. The application of precipitation process to remove anions from wastewater is assessed. Critical parameters of anion precipitation, such as pH, degree of saturation, metal salts, and interferences of other ions are examined. Thermodynamic modeling can provide the theoretical analysis of main parameters and evaluate the effect of solution condition in precipitation process. The review compares several models, such as GEOCHEM, WATEQ4F, MINTEQ, VIS. MINTEQ, and PHREEQC that are commonly used to predict the speciation and SI (saturation index) in aqueous solution. It is aimed to provide a summary of recent developments in precipitation of anions in aqueous systems.