Chromium speciation in groundwater of a tannery polluted area of Chennai City, India

Chromium speciation in groundwater of a tannery polluted area was investigated for the distribution of chromium species and the influence of redox couples such as Fe(III)/Fe(II) and Mn(IV)/Mn(II). Speciation analysis was carried out by ammonium pyrolidinedithiocarbamate (APDC)–methylisobutylketone (MIBK) procedure. The groundwater samples were analyzed for Cr(III), Cr(VI), and Cr(III)-organic complexes. The APDC could not extract the Cr(III)-organic complexes, but HNO₃ digestion of the groundwater samples released the Cr(III)-organic complexes. The groundwater of the area is relatively oxidizing with redox potential (E _h) and dissolved oxygen (DO) ranged between 65 and 299 mV and 0.25 and 4.65 mg L^???1, respectively. The Fe(II) reduction of Cr(VI) was observed in some wells, but several wells that had Fe(II)/Cr(VI) concentrations more than the stoichiometric ratio (3:1) of the reduction reaction also had appreciable concentration of Cr(VI). This could partly be due to the oxidation of Fe(II) to Fe(III) by DO. It appears that the occurrence of Mn more than the Fe(II) concentration was also responsible for the presence of Cr(VI). Other reasons could be the Fe(II) complexation by organic ligands and the loss of reducing capacity of Fe(II) due to aquifer materials, but could not be established in this study.