Remobilization of Cr(VI) from Cr(OH)3(s) coupled with heterogeneous Mn(II) oxidation

Chromium(VI) is known to be the 2nd most common inorganic contaminant due to the wide range of applications of chromium in the industry. The most effective way to remove toxic Cr(VI) under natural conditions as well as in the engineered systems is to reduce it to less toxic Cr(III) using various reductants. Under circumneutral pH conditions, Cr(III) readily precipitates as sparingly soluble Cr(OH)3(s). This solid phase is generally considered as one of the most desirable remediation product of soil and groundwater contaminated by Cr(VI) because it is less toxic and less mobile form of chromium. In addition, this solid is usually believed to be relatively inert to natural oxidants such as dissolved oxygen and Mn oxides. The oxidation of Cr(III) by dissolved oxygen is known to be kinetically sluggish. Previous studies showed that dissolved Cr(III) could easily be oxidized by Mn oxides under acidic conditions but the oxidation became ineffective under neutral or higher pH conditions as a result of Cr(OH)3(s) precipitation. This study examines the potential remobilization of Cr(VI) from this solid by oxidation coupled with heterogeneous oxidation of Mn(II) by dissolved oxygen. 1.0 g/L Cr(OH)3(s) was reacted with 50 μM Mn(II) in 50 mM NaNO3 at pH 7 to 9 in the presence or absence of dissolved oxygen. The pH was maintained with 10 or 50 mM buffers (MOPS for pH 7 and 8; CHES for pH 9). For the anaerobic conditions, the solutions were purged with N2 in sealed serum bottles. In the absence of dissolved oxygen, the oxidation of Cr(OH)3(s) did not occur either with or without dissolved Mn(II). When the solutions were open to atmosphere, by contrast, the oxidation of the solid did occur both in the presence and absence of Mn(II) when the pH was higher or at 8.0. The amounts of Cr(VI) released increased with increasing pH and were higher in the presence than the absence of Mn(II). At pH 9, Cr(VI) concentration rapidly increased for the first 130 hr and reached up to 300 ppb in the presence of Mn(II). The results of this study show that Cr(OH)3(s) can be oxidized by the product of heterogeneous Mn(II) oxidation and thereby would possibly become a source of toxic Cr(VI).