Precipitation - coagulation processes

The main arsenic species present in natural waters are arsenate ions (oxidation state +V) and arsenite ions (oxidation state +III). However As (V) ions are most prevalent in oxygenated waters and As (III) in aerobic conditions like in well water or in groundwater.
Both species As (III) and As (V) have significant differences in the chemical behaviour:the equilibrium constants of dissociation of the two oxidation state of arsenic are quite different and are reported below:

H₃AsO₃ As(III) pKa1= 9.20 (the others constant are not known)

H₃AsO₄ As (V) pKa1= 2.19 pKa2= 6.94 pKa3= 11.5

One important thing had to be underlined here: phosphates ions are very similar chemically with As (V) since orthophoshoric acid have a similar pKa values:

H₃PO₄ pKa1= 2.1 pKa2= 7.2 pKa3= 12.3  

Few studies have been carried out with commercial resin Amberlite IRA 900. The results indicated that the best pH is around 6 and that the exchange is As(V)-specific due to a electrostatic adsorption mechanism. The maximum uptake capacity is about 75 mg/g of dry weight.

The main drawbacks are the strong interferences of competing anions such as sulfates ions and the impossibility to remove the uncharged As (III) species at pH 6.