The dissolution kinetics of atacamite in the acid range and the stability of atacamite containing soils from Namaqualand, South Africa. (January 2016)

• Record Type: Journal Article
• Authors: Le Roux, S.G.; Miller, J.A.; Dunford, A.J.; Clarke, C.E.
• Is Part Of: Applied geochemistry. Volume 64(2016:Jan.)
• Page Start: 22
• Page End: 29
• Publication Date: 2016-01
• Subjects: Paratacamite -- Reaction kinetics -- Buffels river -- Spektakel mine; Environmental geochemistry -- Periodicals; Water chemistry -- Periodicals; Geochemistry -- Social aspects -- Periodicals; Geochemistry -- Periodicals; 551.9
• DOI: 10.1016/j.apgeochem.2015.09.003
• Languages: English
• ISSNs: 0883-2927

Abstract

Abstract: The Cu hydroxy mineral, atacamite, is commonly associated with saline environments and is generally thought to dissolve rapidly in the presence of fresh water. A Cu contaminated soil from the arid Namaqualand region, South Africa, shows atacamite as the dominant Cu containing mineral. The stability of the Cu phase in this soil was determined through equilibrium and leaching studies using both deionised water (DI) and a concentrated (0.5 M) NaCl solution. Initially a high concentration of exchangeable Cu was released from the soils leached with NaCl. Continued leaching with NaCl resulted in a substantial decrease in Cu release as atacamite equilibria started to control dissolved Cu. This suggests that an initial spike of Cu laden water will leach from the soils at the onset of a large rainfall event. Further additions of water will result in a lower but sustained release of Cu from the soil. The Cu contaminated soils are exposed to acidic sulphate leachate thus the dissolution kinetics of synthetic atacamite in the acidic range (pH 5.5–4.0) was determined in both NaCl and DI solutions. The kinetic data showed that atacamite dissolution rates are significantly higher in DI than in NaCl but the rates converge at pH 4. In comparison to common acid soluble minerals, atacamite displays a moderate dissolution rate (10 −9.55 –10 −7.14 mol m −2 s −1 ) within the acid range (pH 5.5–4.0). The atacamite dissolution reaction order with respect to pH is 1.3 and 1.6 in DI and NaCl solutions, respectively, suggesting that dissolution rates of atacamite are highly pH dependent in the acid range. The type of acid used to lower the pH had no effect on the reaction kinetics, with HNO3 and H2 SO4 resulting in comparable dissolution rates of atacamite at pH 4.5. Highlights: Copper release from atacamite rich soils is higher in pure water than in a NaCl solution. Atacamite shows a moderate dissolution rate in the acidic range (pH 4–5.5). Atacamite dissolution rate is higher in pure water than in NaCl solution. Acid type does not influence dissolution rate of atacamite.