A systematic study of chromium solubility in the presence of organic matter: consequences for the treatment of chromium-containing wastewater

The treatment objective for wastewater containing Cr(VI) and/or Cr(III) is the reduction of Cr(VI) to the less toxic Cr(III) and the sequestering of Cr(III) from the water streams. The presence of organic matter in the wastewater and in a bioreactor environment may alter the solubility behavior of Cr(III), resulting in poor sequestering and Cr(III) removal from the soluble phase. Systematic experimental solubility results for Cr(III) were obtained in the presence of organic molecules often encountered in industrial effluents containing chromium as well as in biological treatment systems. Chromium solubility curves were determined experimentally from synthetic solutions of Cr(III) in contact with organic acids (acetic, oxalic, citric and ascorbic), amino acids (alanine and aspartic), proteins (albumin and casein) and synthetic nutrient media (yeast extract, nutrient broth and peptones). Experimental data sets were generated for each one of the above organics at three different initial organics concentrations in order to quantify the role of the relative abundance of organic matter in the solubility behavior of Cr(III). A progressive increase of trivalent chromium solubility was systematically observed when the relative abundance of organic ligands to Cr(III) exceeds 10, indicating the existence of a threshold above which the solubility of Cr(III) increases significantly. The hexacoordinate and octahedral character of Cr(III) complexes may explain the need for ligand excess also pointing to a treatment strategy suggesting the relative abundance of organic moieties should be kept below the above threshold. (C) 2007 Society of Chemical Industry.