Study On Remediation Technology Of Biogas Residue With Ferrous Sulfate In Hexavalent Chromium Contaminated Soil

The remediation of soil contaminated by hexavalent chromium(Cr(VI))has received extensive attention from the state and society.At present,the common ferrous sulfate reduction technology for Cr(VI)contaminated soil has some problems,such as secondary pollution,limited remediation capacity and poor long-term effect.There is an urgent need to develop Cr(VI)contaminated soil ecological restoration and treatment technologies.At the same time,a large number of organic solid wastes such as biogas residue can not be utilized and pollute the environment.Aiming at the above technical bottlenecks,the remediation technology of biogas residue with ferrous sulfate in Cr(VI)contaminated soil was studied.Through stabilization experiment and continuous extraction experiment,the effects of process parameters such as Fe(II)/Cr(VI)molar ratio,biogas residue dosage and moisture content,as well as carbon source,electron donor/acceptor and liquid-solid ratio on Cr(VI)stabilization effect were studied.The long-term stability of the biogas residue and ferrous sulfate remediation technology on Cr(VI)was explored through long-term monitoring experiments,and the in-situ characterization and analysis calculation methods were used to reveal the microscopic mechanism of the biogas residue and ferrous sulfate remediation of Cr(VI)contaminated soil.Seed germination test and pot experiment clarified the phytotoxicity of biogas residue and ferrous sulfate to repair Cr(VI)contaminated soil.In addition,field verification was carried out on Cr(VI)contaminated soil.The research in this paper can provide important theoretical reference and scientific basis for the remediation of Cr(VI)contaminated soil.The main research conclusions of this paper are as follows:(1)The promoting effect of biogas residue on the stabilization of Cr(VI)contaminated soil by ferrous sulfate was studied through batch experiments.The process parameters such as different Cr(VI)content,Fe(II)/Cr(VI)molar ratio,biogas residue dosage and moisture content,as well as the effects of carbon sources and electron supply/receptor factors such as glucose,sulfate and humus on the stabilization effect of Cr(VI)were analyzed.Through one-year continuous monitoring,the longterm stability of biogas residue combined with ferrous sulfate in the remediation of Cr(VI)contaminated soil was evaluated.The results showed that biogas residue and ferrous sulfate have a good stabilizing effect on Cr(VI)contaminated soil.When the content of Cr(VI)was 500～2500 mg/kg,the stabilization efficiency of biogas residue combined with ferrous sulfate was more than 99.5%;when the content of Cr(VI)was1000 mg/kg,the optimal remediation conditions of biogas residue and ferrous sulfate were Fe(II)/Cr(VI)molar ratio of 3:1,water content of 40%,biogas residue dosage of5.2%,under these conditions,the efficiency was 99.92%,and the hexavalent chromium content was 0.80 mg/kg after stabilization,which could meet the requirements of the second type of land use risk screening value(≤5.7 mg/kg)in China’s "Soil environmental quality Risk control standard for soil contamination of development land"(GB36600-2018).The results of leaching toxicity can be known that the total chromium and hexavalent chromium leaching concentration meets the requirements of the "Identification standards for hazardous wastes-Identification for extraction toxicity"(GB 5085.3-2007)after the stabilization treatment of biogas residue and ferrous sulfate.The results of the stabilization experiment for one year showed that the content of Cr(VI)met the requirements of the second type of land use risk screening value in the soil pollution control standard,no re-oxidation phenomenon occurs,and the long-term stability of chromium was achieved.(2)By means of large-scale instruments and continuous extraction experiments,the occurrence states of chromium such as valence and morphology,as well as the changes of main phase composition and mineral characteristics of soil in the process of biogas residue cooperating with ferrous sulfate to restore Cr(VI)were characterized.Molecular biological methods such as high-throughput sequencing and functional gene sequencing reveal the microscopic mechanism of biogas residues and ferrous sulfate in remediation of Cr(VI)contaminated soil.The results showed that more than 95% of Cr(VI)in the soil samples was converted to Cr(III)and amorphous chromium oxide or chromium hydroxide precipitate was formed when the biogas residue was treated with ferrous sulfate for 15 days.Continuous extraction experiment result,the renewal synergy ferrous sulfate pollution soil samples after stabilizing treatment of unstable state(exchange state and carbonate bound state)and steady state Iron and manganese oxide binding state,organic binding state and residue state)chrome content respectively by 76% and 24% of the original soil into 12% and 86%,better coordination treatment group stabilization continued stability;the risk analysis further showed that the risk level of chromium after biogas residue combined with ferrous sulfate stabilization treatment was reduced from very high risk to low risk;after biogas residue combined with ferrous sulfate stabilization,the abundance and diversity of bacterial communities in soil increased significantly,and the sulfate functional groups,iron respiratory functional groups and nitrate respiratory functional groups related to Cr(VI)reduction increased from 0.08%,0.11% and 0.12% in original soil to more than 0.5%,0.23% and 3.02% respectively;the biogas residue plays a vital role in the reduction and stabilization of Cr(VI).The reducing substances in the biogas residue contribute to the reduction and stabilization of Cr(VI)at the initial stage of the reaction,and the directly reduce Cr(VI)to Cr(III).With the increase of organic matter and nutrients in the biogas residue,the metabolic activity of sulfate-reducing bacteria and iron-reducing bacteria in the soil was enhanced,which promotes the further conversion of Cr(VI)to Cr(III)to form stable Cr(III)deposits or minerals;the conversion of Cr(VI)to Cr(III)in the process of stabilization of biogas residue and ferrous sulfate is achieved through chemical reduction,direct and indirect microbial reduction.(3)The phytotoxicity and accumulation of biogas residue combined with ferrous sulfate in the remediation of Cr(VI)contaminated soil were analyzed by seed germination experiment and pot experiment.The results showed that: from the seed germination experiment,it can be seen that the root length,relative root length and seed germination index of Chinese cabbage increased significantly after the biogas residue and ferrous sulfate stabilization treatment.The root length of Chinese cabbage increased from 8.62 mm in the original contaminated soil to more than 30 mm.It shows that the toxicity is significantly reduced after the treatment of biogas residue and ferrous sulfate;The results of pot experiment show that when the molar ratio of Fe(II)/Cr(VI)is 2.5:1+biogas residue 6% and the molar ratio of Fe(II)/Cr(VI)is 3:1+biogas residue4%,the content of Cr(VI)meets the requirements of the second type of land use risk screening value,and the growth of Chinese cabbage is significantly improved,and the phytotoxicity is alleviated.The transfer coefficient of chromium to pakchoi was less than 1,and the chromium was mainly accumulated in the root of pakchoi.The results of seed germination and plant pot experiment indicated that the co-stabilized treatment group with biogas residue had certain advantages in improving soil fertility and promoting plant growth,significantly reducing the phytotoxicity of Cr(VI)and improving the ecology of remediation technology compared with the ferrous sulfate group alone.(4)Combining the advantages and disadvantages of the soil remediation process in the current project,according to the characteristics of the combined remediation technology of biogas residue and ferrous sulfate,an ex-situ remediation process and an in-situ high-pressure rotary injection remediation process were developed;The on-site remediation pilot study shows that the effect of biogas residue and ferrous sulfate to remediate the actual chromium-contaminated soil meets the requirements of the second type of land screening value(≤5.7 mg/kg)in GB36600-2018;which has obvious environmental and economic dual benefits and potential for promotion and application.