ANC BNC and mobilization of Cr from polluted sedi

Authors: A Shtiza R Swennen V Cappuyns A Tashko

Publish Date: 2008/03/11

Volume: 56, Issue: 8, Pages: 1663-1678

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Abstract

During the manufacturing of chromate salts 1972–1992 large quantities of Chromite Ore Processing Residue COPR were released into a decantation pond east of the former chemical plant of PortoRomano Durres Albania giving rise to yellow colored pond sediments These CrVI bearing sediments were deposited upon Quaternary siltyclay lagoonal sediments rich in iron oxides and organic matter The pH values in these lagoonal sediments vary around 66 while in the pond sediments it is mainly acidic due to the presence of the sulfur stock piles in the area and the release of the H2SO4 from the activity of the former chemical plant varying between 14 and 38 Continuous leaching of the COPR waste resulted in yellowcolored surface water runoff The prediction of pH changes in the different types of sediments based upon acid/base neutralizing capacity ANC/BNC jointly with the quantitative data on release of heavy metals and especially Cr is considered an important advantage of the pHstat leaching test if compared to conventional leaching procedures Thus factors controlling the leaching of CrVI CrIII Ca Al Fe Mg from the COPR were investigated by means of pHstat batch leaching tests and mineralogical analysis Moreover mathematical and geochemical modeling complemented the study The COPR in the area contain very high concentrations of chromium 24409 mg/kg which mainly occurs as CrIII 75–90 as well as CrVI 25–10 The leaching of CrVI occurs in all the range 2–10 of the tested pH values however it decreases under acidic conditions Beside some reduction of CrVI to CrIII the CrVI content of the leachtes remains relatively high in the acidic environment while the limning of CrVI pond sediments will increase the release of the latter specie The leaching of the CrIII occurs strictly under acidic conditions whereby limning of these sediments will give rise to the lower solubility of CrIII The key mineral phases responsible for the fast release of the CrVI are the chromate salts ie sodium chromate and sodium dichromate while sparingly soluble chromatite CaCrO4 and hashemite BaCrO4 release CrVI very slowly Thus pH and mineral solubility have been identified as key factors in the retention and the release of the hexavalent CrO42− and Cr2O7− from the COPRrich pond sediments

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