| Arsenic contamination is a universal environmental problem in the world. To different degrees, many countries and regions have been polluted by arsenic. In China, Hunan Province and Guangxi Zhuang Autonomous Region are the most seriously arsenic polluted areas. Previous studies have shown that geological conditions are the major causes that induced the problem in high arsenic regions. In order to understand the spatial distribution and formation of different arsenic species in typical high arsenic regions, field investigation were carried out on two typical mining derived heavy polluted areas located in Hunan and Guangxi region in this project. Sequential extraction and SR-XAFS methods were introducted to reveal the difference of two types of tailings contaminated soil, the distribution of arsenic species in soils of those contaminated areas and the species of arsenic in soil profiles.μ-XANES and X-ray fluorescence were chosen to analyse the distribution and variation of arsenic speciation in ore and tailings contaminated soil. Results showed that:Soils of Hunan Shimen realgar mining area were mainly contaminated by arsenic. Arsenic concentrations in soils can reach as high as 24020 mg/kg, and the minimum concentrations were also even higher than several thousand mg/kg. The soils close to Diaojiang River were seriously polluted by As, Pb and Zn, and the degree of pullution decreased from the upriver to the downriver. The heavy metal concentrations in surface layer of soils in the upper reach, 16 km from the mining area, were high up to 2.4×104mg/kg, 5.6×103mg/kg and 1.2×104 mg/kg for As, Pb and Zn, respectively. The concentrations were similar to those in mine tailings. The average concentrations of As, Pb and Zn of the two samples collected in the lower reach, 154 and 192 km from the mining area, were 47.05, 1.81, 5.48 times higher than the critical values in the soil environmental quality standards(Ⅲ), respectively.The availability of arsenic in Realgar mine tailings contaminated soil was much greater than that in lead-zinc district tailings contaminated soil. Different patterns were found in the two tailings. In lead-zinc district tailings, the substantial amount of H+ produced by the oxidation processes of some other sulfide minerals induced the soils around the tailings to acidic conditions. The processes led to the movement of arsenite(As3+) to the deeper layers. While the soils of the realgar mine tailings are alkalic, arsenic moved from the topsoil to the subsoil in the form of arsenate (As5+). Arsenic in the soil profile mainly in the form of oxidation states of the movement to the deep soil. The oxidation of arsenopyrite when transport with water in Diaojinag River lead to the proportion of arsenopyrite (FeAsS), arsenite(As3+), organic-bound Arsenic (Org-As), residual (Res-As) decreased with the increase of the distances of the sample points to mining site; while the proportion of arsenate (As5+), loosely bound arsenic, Arsenic aluminum (Al-As), arsenic iron (Fe-As), arsenic calcium (Ca-As) and iron-occluded arsenic (FeOc-As) changed with a contrary trend with the increase of the distances of the sample points to mining site. Arsenic could mainly move in forms of oxidation states in the soil profile, and the proportion of arsenate (As5+) increased as depth of soil layer increased in soil profile; while the arsenite (As3+) changed with a opposite trend. In general, the seven species of bound arsenic decreased with the increase of depth of soil profiles, following power functions. However, decreaseing trends varied among the seven species in the whole Diaojiang River valley.The results of micro-analysis of the concentration and species in ore and tailings contaminated soils revealed that arsenic was released to the environment in the process of the oxidation of the original sulfide ore. |
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