Arsenic Remediation in Drinking Waters Using Ferrate and Ferrous Ions

As groundwaters used for municipalities become increasingly contaminated with arsenic and as Environmental Protection Agency, EPA, acceptable threshold limits decrease (currently 50 ppb) (Pontius 1993), new methods for inexpensive arsenic removal are needed since existing methodologies are inadequate to meet such requirements. The objective of this study was to determine the ability of ferrate to remediate arsenic and arsenic containing compounds in water.

The basic approach is to produce soluble ion pairs between dissolved arsenic and ferrous ions. Oxidation of these moieties using ferrate produces the extremely insoluble ferric arsenate which precipitates and settles from solution.

Fe ²⁺  + AsO ₄ ^3-  <–> FeAsO ₄ ^{–FeO ₄2-}  –> FeAsO _4(s)  + Fe(OH) ₃

The ferrate and ferrous dose, pH, and ferrous ion source were all optimized. Optimal ferric arsenate removal (to below 5 ppb) was found at pH 5 and a total iron/arsenate ration approaching 8:1.

Removal efficiencies for monomethylarsenate (CH ₃AsO ₃ ^2-) and dimethylarsenate ((CH ₃) ₂AsO ₂ ^–) were also determined using this approach. Due to their decreased charges, these latter two species were less efficiently removed. There is, however, some evidence for oxidation of methylarsenate with resultant increases in arsenic removal. Arsenic removal using ferrate alone also was studied. The potential for nitrate and phosphate ion to interfere with this process was studied. While nitrate was found to be innocuous, the addition of phosphate does lead to decreased removal efficiencies.