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  

 

The similar chemical and structure properties of As (V) and P(V) often explain the poor removal efficiency of As (V) by an adsorption process when phosphate ions are present in the water to be treated because phosphates ions compete greatly with arsenate ions for the binding sites.

 

With the pKa values of As (V), a speciation diagram can be calculated (HYDRAQL program)

As shown on this diagram, in the typical pH ranges of natural waters, the species H2AsO4- and HAsO42- are dominant.

 

The anionic forms H2AsO4- and HAsO42- determine the occuring reactions during a removal process in the pH range from 4 to 7.

 

Removal efficiency is often much more lower for As (III) than for As(V). Actually, anionic species are often involved in the surface chemical reaction with the binding sites and the anionic species of As (III) are presents only up to pH 9.2.

In many cases, the sorbent surface is positively charged up to pH 7 and at this pH, As (III) species are uncharged and therefore can't be bound to the surface by electrostic reaction.