| Improper disposal of chromium-containing waste residue from industrial production had generated large quantities of chromium contaminated soil.They may pose a threat to the surrounding ecological environment.And contaminated land adjacent to water sources or densely populated area can cause many serious health hazards to human.In this study,a chromium-contaminated soil from an industrial waste site was used as the tested soil.Oscillating washing and soil column leaching were used separately,to study the leaching effect of citric acid,oxalic acid and hydrochloric acid on chromium removal in testd soils,the effect of the remediation,and the changes of the chromium chemical fractions in the soil before and after the leaching.The changes in microscopic interface morphology,the differences in elemental composition and the change of the redox state of chromium on the soil particles surface were analyzed to discuss the remediation effect.(i)A simulated soil washing experiment by the method of indoor oscillation washing was used to study the composition of compound washing eluents and the washing times on the removal of total chromium in the contaminated soil.And the morphological changes of chromium in the soils were analyzed.The results of the research indicated that compared with single washing,the composite washing could significantly improve the removal rate of chromium in chromium-contaminated soil,and the overall effect of sequential washing was better than that of mixed washing(p<0.05).There were three washing methods which had better washing effect.The extraction by first washing with hydrochloric acid for 4 h,followed washing with oxalic acid for 4 h.The extraction by first washing with oxalic acid for 4 h,followed washing with citric acid for 4 h.And the extraction by hydrochloric acid mixed with citric acid equally for 8 h.After being leached by those three methods twice,the most economical and effective result was achieved,the removal rate of total chromium was 77.64%,68.32% and 62.35%,respectively.The analysis of chromium speciation in soil before and after washing showed that the complex eluants could effectively remove chromium in the reducible form and oxidizable form in the soil and increase the ratio of chromium in the acid extractable form,but the effect is very limited on the residual form of chromium.Different leaching methods have different remediation effects on different types of chromium-contaminated soils,and the best leaching method for each soil needs to be selected according to the actual situation.(ii)A simulated in-situ soil column leaching experiments were used to investigate the effect of leaching volumes of different eluents(hydrochloric acid,citric acid,and oxalic acid)on chromium in a chromium-contaminated soils.The best remediation result was achieved by the leaching method of using 15 soil pore volumes 0.5mol/L oxalic acid solution.The cumulative leaching amount of total Cr in the soil was 3985 mg/kg.At the soil depth of 10,20,and 30 cm,the total Cr removal rate were 93%,90%,and 87%,respectively,Cr(VI)removal rates were 96%,92%,and 86%,respectively.Soil Cr(VI)and total Cr removal rates were decreased with the increase of soil depth,had the possibility of bottom accumulation.After leaching,the soil was acidified obviously,and the p H value was reduced from 10.5 to about 3;Oxalic acid had excellent leaching effect on chromium fractions of acid extractable,reducible,oxidizable in the soil.The chromium in soil was converted into water-dissolved chemical forms,which contributes to the leaching of chromium in the soil.(iii)In the process of simulated in situ column leaching experiment,due to the characteristics of acids,some of other metal ions will be accompanied by the leaching,and the physical and chemical state of the soil will change.So in the procress of soils leached by citric acid,hydrochloric acid and oxalic acid,the leaching dynamic of iron(Fe),calcium(Ca),magnesium(Mg),and manganese(Mn)were observed and their correlation with chromium leaching dynamic and kinetics was also studyed.The electron microscopic examination and X-ray spectrum analysis were performed on the soil before and after leaching to explore the leaching mechanism.The bioaccessible of chromium in soil before and after leaching were conducted by in vitro gastrointestinal simulation experiments.The results indicated that leaching with acids facilitated the chemical mobilization of Cr,Ca,Mg,Fe and Mn.In the leachates,the concentration of Cr was highly correlated with Ca,Mg,Fe and Mn.The leaching kinetics analysis showed that the diffusion resistance of the product layer of the complexation or chelation reaction product of the organic acid(oxalic acid,citric acid)with chromium controlled the reaction rate.While in the hydrochloric acid leaching process,the reaction rate was controled by the shrinkage of soil particles due to acid dissolution.The reaction rate of other metal elements was mainly controlled by the shrinkage of the core particles of the soil particles caused by the chemical reaction.Scanning electron microscope with energy dispersive X-ray spectroscopy(SEM-EDS)and X-ray diffractometer(XRD)techniques showed that the Cr mainly interacted with soil mineral components.The mineralogical and morphological of the soil were significantly changed after being leached by acids.Some feldspar and silicate-based mineral crystalline phases disappeared and some new crystalline phases of insoluble minerals formed by the complexation of oxalic acid with calcium and citric acid with aluminum were formed.XPS results showed that the valence state of chromium bound on the soil surface of chromium-contaminated soil is very complicated.Cr(III)exists the highest ratio,along with the presence of Cr(VI)and other valence of chromium.After the acid leaching,all the chrome peaks were decreased.Oxalic acid had the most obvious effect on the leaching of chromium compounds and chelation and reduction on chromium on the surface of soil particles,and had the best leaching effect on all valent chromium in the soil.In summary,statistical analyses combined with SEM-EDS,XRD and XPS analysis is effective tool to check the affinity of the soil components for Cr. |
PDF Full Text Request