Southern Cross University, Southern Cross GeoScience, Lismore, NSW 2480, Australia.
Southern Cross University, Southern Cross GeoScience, Lismore, NSW 2480, Australia.
Chemosphere. 2019 May;222:440-444. doi: 10.1016/j.chemosphere.2019.01.172. Epub 2019 Jan 29.
Mixed Cr(III)-Fe(III) (oxy)hydroxides are important Cr-bearing phases in natural, unpolluted soil. Fires frequently affect large areas of land around the world, causing the temporary development of elevated soil temperatures. This study examines the hypothesis that heating Cr(III)-Fe(III) (oxy)hydroxides at temperatures which occur in surface soils during fires can drive rapid oxidation of Cr(III) to hazardous Cr(VI). To test this, poorly-ordered Cr(III)-Fe(III) (oxy)hydroxides, with x spanning 0.1 to 0.9, were heated at up to 800 °C for 2 h. Heating at 400-800 °C produced a highly crystalline hematite-eskolaite solid-solution (FeCrO, where n ranges from 0 to 2). Chromium K-edge X-ray absorption spectroscopy showed that during heating up to ∼40% of the initial Cr(III) was oxidized to Cr(VI), with the greatest extent of Cr(VI) formation occurring at 200-400 °C. At these temperatures, a substantial proportion (17%-70%) of the newly-formed Cr(VI) was exchangeable (i.e. extracted by a pH 7.2, 10 mM PO solution). This suggests that much of the Cr(VI) formed by heating of Cr(III)-Fe(III) (oxy)hydroxides at 200-400 °C is likely to be relatively mobile in fire-impacted soils. The results of this study provide new insights into a potentially-important pathway for the in-situ formation of Cr(VI) in soil.
混合的 Cr(III)-Fe(III)（氧）氢氧化物是天然无污染土壤中重要的含铬相。火灾经常影响世界各地的大片土地，导致土壤温度暂时升高。本研究检验了这样一个假设，即在火灾中地表土壤温度下加热 Cr(III)-Fe(III)（氧）氢氧化物会促使 Cr(III)快速氧化为有害的 Cr(VI)。为了验证这一点，我们用 x 从 0.1 到 0.9 的无序 Cr(III)-Fe(III)（氧）氢氧化物在高达 800°C 的温度下加热了 2 小时。在 400-800°C 的温度下加热会产生一种高度结晶的赤铁矿-eskolaite 固溶体（FeCrO，其中 n 的范围从 0 到 2）。铬 K 边 X 射线吸收光谱表明，在加热过程中，初始 Cr(III)中有高达约 40%被氧化为 Cr(VI)，Cr(VI)形成的最大程度发生在 200-400°C。在这些温度下，相当大比例（17%-70%）的新形成的 Cr(VI)是可交换的（即 pH 7.2、10mM PO 溶液提取）。这表明，在 200-400°C 加热 Cr(III)-Fe(III)（氧）氢氧化物形成的 Cr(VI)中的大部分可能在火灾影响的土壤中具有较高的移动性。本研究的结果为土壤中 Cr(VI)原位形成的一个潜在重要途径提供了新的见解。
