ANC,BNC and mobilization of Cr from polluted sediments in function of pH changes |
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| Authors: | A Shtiza R Swennen V Cappuyns A Tashko |
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| Institution: | (1) Geology Department, Faculty of Sciences, Katholieke Universiteit Leuven, Celestijnenlaan 200E, 3001 Heverlee, Belgium;(2) Faculty of Geology and Mining, Geochemistry Department, Polytechnic University Tirana, Rruga Elbasanit, Tirana, Albania |
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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 Porto-Romano (Durres, Albania), giving rise to yellow colored
pond sediments. These Cr(VI) bearing sediments were deposited upon Quaternary silty-clay lagoonal sediments rich in iron oxides
and organic matter. The pH values in these lagoonal sediments vary around 6.6, 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 1.4 and 3.8. Continuous leaching of the COPR waste resulted
in yellow-colored 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 Cr(VI), Cr(III),
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 24,409 mg/kg, which mainly occurs as Cr(III) (75–90%) as
well as Cr(VI) (25–10%). The leaching of Cr(VI) occurs in all the range (2–10) of the tested pH values, however, it decreases
under acidic conditions. Beside some reduction of Cr(VI) to Cr(III), the Cr(VI) content of the leachtes remains relatively
high in the acidic environment, while the limning of Cr(VI) pond sediments will increase the release of the latter specie.
The leaching of the Cr(III) occurs strictly under acidic conditions, whereby limning of these sediments will give rise to
the lower solubility of Cr(III). The key mineral phases responsible for the fast release of the Cr(VI) are: the chromate salts
(i.e. sodium chromate and sodium dichromate), while sparingly soluble chromatite (CaCrO4) and hashemite (BaCrO4) release Cr(VI) very slowly. Thus, pH and mineral solubility have been identified as key factors in the retention and the
release of the hexavalent CrO4
2− and Cr2O7
− from the COPR-rich pond sediments. |
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| Keywords: | COPR Cr speciation Cr(VI) Cr(III) pHstat ANC BNC Leaching Solubility |
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