Chromium geochemistry and speciation in natural waters, Iceland

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• 作者：Andri Stefá ; nsson^a ; ^{as@hi.is" class="auth_mail" title="E-mail the corresponding author} ; Ingvi Gunnarsson^b ; Hanna Kaasalainen^a ; Stefá ; n Arnó ; rsson^a
• 刊名：Applied Geochemistry
• 出版年：2015
• 出版时间：November 2015
• 年：2015
• 卷：62
• 期：Complete
• 页码：200-206
• 全文大小：1182 K

文摘

Natural waters in Iceland were collected and analyzed for chromium concentration and speciation (Cr^III, Cr^VI and Cr_TOT). The water sampled included non-thermal surface and spring water, surface geothermal water, and single and two-phase geothermal well discharges with sampling temperatures of 0–178 °C, pH of 2.0–9.5, and total dissolved solids (TDS) of 35–4030 ppm. The total Cr concentration was between <0.01 and 660 ppb with highest concentrations in waters with the lowest pH. At pH > 4 the measured Cr^III concentration was low, generally <1 ppb but with decreasing pH higher Cr^III concentrations were observed reaching values of hundreds of ppb. At pH < 6 no measurable Cr^VI was detected whereas in neutral to alkaline waters measured Cr^VI concentrations were as high as 3 ppb, frequently dominating over Cr^III. The Cr chemistry in natural waters associated with mafic rocks in Iceland is largely influenced by the water pH. At low pH Cr-containing minerals are unstable and Cr leaches into the waters as Cr^III. Possible oxidation of Cr^III to Cr^VI is minimized thermodynamically and the mobility of Cr^VI is further reduced by surface complexation reactions. At neutral to alkaline pH the primary Cr-containing mineral phases like titanomagnetite and chromite may be stable, limiting Cr-rock leaching. Solubility of secondary Cr^III–Fe^III-mineral phases may further reduce and/or limit the Cr^III availability. In contrast, Cr^VI mobility is enhanced at a pH > 8 associated with decreasing importance of mineral surface complexation. Hence, Cr^VI becomes an increasingly dominant form of dissolved Cr at pH above 7–8. Many groundwater drinking supplies associated with mafic rocks are characterized by moderately alkaline pH resulting in Cr^VI concentrations of a few ppb.