Hexacyanoferrate(III) oxidation of arsenic and its subsequent removal from the spent reaction mixture

Inorganic arsenic is the most toxic form and has been classified in group 1 as carcinogenic to humans which induces lung, urinary bladder and primary skin cancer. Worldwide concern over its presence in water bodies have prompted much research and policy development focusing on the removal of this chronic human carcinogen. It has been observed that the ash of Unio (Lamellidens marginalis - the fresh water mussel) can be used successfully for the removal of arsenic(V) from the aqueous solutions at low pH (∼9.0). Initially the kinetics of oxidation of arsenic(III) by alkaline hexacyanoferrate(III), both with and without adding iridium(III) chloride was studied. Subsequently after complete removal of ferrocyanide, the removal of arsenic(V) produced in the spent reaction mixture was taken up. Out of the five ashes obtained from different sources, the ash of Unio was found to be the best which results in decreasing arsenic(V) concentration from 1000 ppb to >10 ppb, TDS from 16.9 ppt to 8.5 ppt and conductivity from 33.8 mS to 17.1 mS. Kinetic results show the possibility of graphical separation of the reaction proceeding in the absence of iridium(III) from that proceeding in the presence of iridium(III) chloride.