1. Arsenic occurrence

Arsenic (As) is a chemical element the abundance of which in the continental crust of the Earth is given as 1.5–2 ppm. Most often arsenic appears in nature in the form of Fe, Co and Ni arsenides, arsenic sulfide, and native arsenic. It belongs to the metalloids, in spite that it shows intermediate properties between metals and non-metals.

DOI: 10.5772/intechopen.78399

In general, arsenic forms soluble oxyanions with the predominant oxidation states +3 and +5, while the minor oxidation states are 0 and �3. The main form of As(III) is arsenite existing in solutions with pH between 5 and 9. H3AsO3 and the anions resulting from its dissociation occurring according to Eq. 1 are H2AsO3 �, H2AsO3 <sup>2</sup>�, and AsO3 3�:

$$H\_3\text{AsO}\_3 \xrightarrow{p\text{K}\_s=9.2} H\_2\text{AsO}\_3^- + H^+ \xrightarrow{p\text{K}\_s=14.22} H\text{AsO}\_3^{2-} + H^+ \xrightarrow{p\text{K}\_s=19.22} \text{AsO}\_3^{3-} + H^+ \tag{1}$$

The main form of As(V) is H3AsO4. The anions resulting from its dissociation are H2AsO4 � , HAsO4 <sup>2</sup>�, and AsO4 <sup>3</sup>�. The arsenates have high-ionization capacity due to the presence of the double bond. The H3AsO4 molecule easily loses hydrogen ions by dissociation remaining with a negative charge, forming successively several anions according to Eq. 2:

$$\mathrm{H\_3AsO\_4} \xrightarrow{\mathrm{pK\_s=2.2}} \mathrm{H\_2AsO\_4^-} + \mathrm{H^+} \xrightarrow{\mathrm{pK\_s=6.94}} \mathrm{HAsO\_4^{2-}} + \mathrm{H^+} \xrightarrow{\mathrm{pK\_s=11.3}} \mathrm{AsO\_4^{3-}} + \mathrm{H^+} \tag{2}$$
