| China is one of major countries to produce chromate. There were over seventy enterprises producing chromate since the first line developed in 1958, and a large amount of Chromite Ore Processing Residue (COPR) was discharged from chromate industries. The accumulated amount of COPR was more than 6 million tons and 600 thousand tons are being discharged annually in China. Owing to historic reasons, most COPR was stacked simply. Dissolvable hexavalent chromium accounting for 0.3~1.5% of COPR can be leached into soils and groundwater by rainfall and runoff, which incurs a significant risk to the environment and human health. The COPR contanminated sites are considered as the most risk sites by Chinese government. In this paper, the COPR contaminated site in Qingdao Hongxing Chemical Plant was investigated, and pilot tests of remediation technology were studied. The main conclusions are as follows:⑴Site investigation①The site was seriously polluted by chromium. In surface soil, the maximum content of total chromium was 20410.9 mg/kg, and that of hexavalent chromium was 6354.4 mg/kg. And in deeper soil, the maximum content of total chromium was 42229.8 mg/kg, and that of hexavalent chromium was 7481.1 mg/kg. Surface soil and deeper soil were all severely polluted in the site, the chromium content of all the soils exceed the Standard of Soil Environmental Quanlity. The pollution in the south and middle regions was more serious, while the content of chromium was evidently higher in the east than that in the west.②The content of chromium in soil was downtrend from the ground surface to the bottom, and the higher the chromium content of the surface the faster the content decreases, the deepest is up to 8 m.③The groundwater of research area was polluted by chromium. The total chromium content of groundwater was in the range of 0.24~880.16 mg/L, and that of hexavalent chromium was 0.07~806.89 mg/L. Chromium in the groundwater chiefly existed as hexavalent chromium. The hexavalent chromium content of all the sampling points exceeded the Environmental Quality Standard for Soils. With the movement of groundwater, the hexavalent chromium in soil could quickly migrate into groundwater.④In the contaminated soils underneath the COPR, the residual speciation chromium was the predominant fraction. The percentage of weak acid extractive speciation was relatively low, but the content remained high. The distribution of oxidizable speciation chromium in soil showed no apparent trend. The highest reducible speciation chromium content was in the mesosphere of the soil.⑵Feasibility research of remediation technologies①Kinetics of hexavalent chromium desorption from the soil can be described by heterogeneous diffusion equation. The desorption rates decreased with desorption time.②Based on the orthogonal test, the best process condition for soil washing was that solid-liquid ratio was 1:40, washing time was 105min, washing temperature was 50℃, and oscillation frequency was 200r/min. Hexavalent chromium removal efficiency was up to 93.3% under the best condition. The sequence of factors that affect the removal efficiency: washing temperature>solid-liquid ratio>oscillation frequency>washing time was obtained by means of pole difference analysis and variance analysis, and there were significantly differences in washing temperature and solid-liquid ratio.③After five counter-current rinses, the max washing efficiency of soil samples could reach 99.2%. The higher the cleaning series is, the less obvious the cleaning efficiency increased.④The in-situ flushing technology was effective on chromium removal of COPR contaminated soil and the efficiency could reach 88%. The effluent water could meet the Quality Standard for Groundwater and the Integrated Wastewater Dischage Standard. However the unit water consumption was large.⑤The preliminary remediation scheme was proposed as: ex-situ soil washing combined with in-situ soil flushing. |
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