Study On Treatment Of Cr(Ⅵ) Contaminated Soil By Washing With Organic Acid And EDTA

Chromium and its compounds are widely used in various industries as important chemical raw materials,but a large amount of chromium-containing waste residues will be produced in the production process,and the unstandardized disposal of these waste residues is one of the main sources of chromium-contaminated soil at present.It is the most toxic and harmful substance among them.If it is not repaired,the hexavalent chromium will accumulate through the food chain and endanger human health.This research is mainly to screen and optimize the effect of organic acid and EDTA on the washing of hexavalent chromium-contaminated soil under different conditions,and then conduct compound experiments of organic acid and EDTA to find the best combination of leaching and leaching.The effect of the washed soil was evaluated,and the mechanism of the washing agent to remove hexavalent chromium in the soil was explored.The research conclusions are as follows:（1）From the screening of the eluent and the optimization experiment of the conditions,it is concluded that the removal effect of oxalic acid and citric acid is better,and the optimal elution conditions of oxalic acid are the concentration of 1.2 mol/L and the solid-liquid ratio of 1:8,the p H is 4,the washing time is 6h,the removal rate under this condition is 76.38%,the optimal washing condition of citric acid is the concentration of0.3 mol/L,the solid-liquid ratio is 1:10,the p H is 4,the washing time is 6 h.The removal rate under this condition was 73.52%.The p H value of the eluent had a great influence on the speciation distribution of chromium in the soil.When the p H value of the washing solution is low,it will promote the increase of exchangeable chromium content in soil.Therefore,the appropriate p H value of the eluent should be selected during washing.The migration and stability analysis of chromium in soil washed with oxalic acid and citric acid under different p H conditions showed that when the p H of the washing agent was 4,the mobility of chromium in the soil washed with oxalic acid decreased and the stability increased compared with the citric acid.（2）In the compound washing experiment of organic acid and EDTA,the best washing combination was screened out.The washing method was to first use EDTA to rinse under the optimal conditions（concentration is 0.16 mol/L,solid-liquid ratio is 1:10,p H is 4）for 3hours,and then use oxalic acid to rinse under the optimal conditions.After washing for 3h,the washing removal rate can reach 86.02%,which is 9.64%higher than that of single oxalic acid washing.For the soil treated with the best compound washing combination（EDTA 3h+oxalic acid 3h）,the leaching concentration of hexavalent chromium measured by acetic acid buffer solution method（HJ/T 300）is 0.16 mg/L,which meets the admission requirements in the Pollution Control Standard for Domestic waste Landfill（GB16889-2008）.Namely,the leaching concentration of hexavalent chromium<1.5 mg/L.The soil bioavailability after washing decreased significantly,and the soil bioavailability decreased most obviously after the optimal compound washing combination（EDTA 3h+oxalic acid 3h）treatment.Compared with the original soil（0.81%）,the content of organic matter in the soil after washing was greatly increased,regardless of whether it was a single agent washing or a compound agent washing.（3）In the mechanism research experiment of organic acid and EDTA to remove hexavalent chromium in soil,the desorption kinetics of citric acid,malic acid,oxalic acid,tartaric acid,and EDTA were fitted by Elovich equation and pseudo-second-order kinetic equation.From the fitting results,the pseudo-second-order kinetic equation model is more in line with the desorption process of the above eluent.XRD pattern analysis showed that the soil stability enhancement after treatment with compound washing（EDTA 3h+oxalic acid 3 h）was related to Al₄[Si O₁₀]（OH）₈.FTIR analysis showed that the absorption peaks of Si-O-Si,C=O,-OH in the soil all exhibited different degrees of stretching vibration after washing,indicating that these functional groups had complex reactions with hexavalent chromium.The morphological analysis after treatment with the best compound washing combination shows that the compound washing of EDTA and oxalic acid can promote the conversion of the exchangeable state and the bound state of iron and manganese oxides to the residue state in the soil.The results of SEM and EDS showed that the optimal composite washing combination had less erosion effect on soil particles.After washing,the weight percentage of Cr,Ca,Mn and Fe on the soil surface decreased significantly from2.92%,16.50%,1.69%,and 27.19%to 1.49%,5.93%,1.36%,and 25.06%,respectively,while the weight percentage of Al increased from 51.71%to 66.16%.The results showed that composite washing had an obvious effect on the elution of heavy metals in soil and the stability of chromium in soil after washing was enhanced.