近日，華中農業大學資源與環境學院邱國紅教授課題組在重金屬污染場地土壤修復領域取得系列新進展，相關成果以“Highly efficient removal of Cu-organic chelate complexes by flow-electrode capacitive deionization-self enhanced oxidation（FCDI-SEO）：Dissociation，migration and degradation”“Remediation of As-contaminated soils using citrate extraction coupled with electrochemical removal”和“Photoinduced self-organized precipitation in leachate for remediation of heavy metal contaminated soils”為題分別發表在Chemical Engineering Journal、Science of the Total Environment和ACS ES&T Engineering期刊。
土壤中重金屬具有高毒性、環境持久性和弱遷移性，嚴重威脅生態環境和人類健康；尤其是針對重金屬污染程度高的場地土壤修復技術研發迫在眉睫。各種物理、化學和生物方法（如電動修復、鈍化、淋洗、植物修復和微生物選取等）已被用於降低土壤中重金屬總量或生物可利用組分含量，以緩解其在食物鏈中的累積，而淋洗以其快速高效的修復速率已成為最常用的土壤修復方法之一。然而，化學選取劑的使用可能對土壤造成二次污染，同時產生含重金屬的淋洗廢水需進一步處理，新增了修復成本，這些不利因素限制了淋洗科技在土壤修復中的應用。
課題組先前研究表明環境友好的小分子有機酸（如檸檬酸和草酸等）可通過溶解、氧化還原和絡合管道將土壤中重金屬有效浸提到淋洗液中，其用於重金屬污染場地土壤選取劑具有好的應用前景（Xiong Yang，Lihu Liu，Wenfeng Tan，Chengshuai Liu，Zhi Dang，Guohong Qiu*，Environ. Pollut.，2020，264，114745）。基於此結果，課題組進一步發展了小分子有機酸淋洗-電化學/光化學氧化聯合科技用於重金屬污染場地土壤修復。
首先，構建了流動電極電化學體系，並考察了其用於處理含有機酸絡合態重金屬土壤淋洗液的可行性及機理（圖1）。結果表明流動電極電化學體系可高效去除類比土壤淋洗液中有機絡合態重金屬。在流動電極電化學體系中，電場下較穩定的有機絡合態重金屬離子主要以帶負電荷的形態遷移至陽極室；而亞穩態的有機絡合態重金屬離子則極易發生解離，並以游離態陽離子遷移至陰極室。帶負電的絡合態重金屬離子遷移到陽極室後解絡，有機小分子可被電化學及電化學介導生成的•OH氧化降解。在初始pH 3.5–5.5範圍內，恒定電流10 mA時，類比土壤淋洗液中絡合態重金屬（初始濃度1.0 mmol L−1）去除率接近100%。流動電極電化學體系也具有優异的穩定性，經5次連續運行及電極再生後，對類比土壤淋洗液中絡合態重金屬的去除率沒有明顯降低。該研究證實了電化學吸附和氧化體系用於土壤淋洗液有機絡合態重金屬去除的可行性，相關結果發表在期刊Chem.Eng. J.。
圖1流動電極電化學體系去除土壤淋洗液中有機酸絡合態重金屬離子示意圖
其次，構建了小分子有機酸淋洗-氧化錳電極電化學氧化體系修復實際高濃度砷（As）污染場地土壤（圖2）。經檸檬酸鈉溶液淋洗和以水鈉錳礦為陰極的不對稱電解池中（槽壓1.5 V）電解11天后，土壤中As總量和有效態含量分別由1980和242 mg kg−1降低至1260和152 mg kg−1，淋洗出的As可被完全去除。淋洗及電化學去除過程中，土壤中非晶質和晶質鐵鋁氧化物結合態As含量均顯著降低，而其他形態存在的As變化不大。經三輪連續修復後土壤中As總量和有效態含量分別進一步降低至563和86 mg kg−1。檸檬酸鹽通過溶解、配體交換及間接還原As（V）三種方式浸提去除了土壤中的As，而在電化學體系，淋洗液中As的去除主要歸因於陽極上形成水鐵礦和δ-MnO2及陰極上形成菱錳礦的吸附作用。相關結果發表在期刊Sci.Total.Environ.。
圖2小分子有機酸淋洗-氧化錳電極電化學氧化體系修復As污染場地土壤示意圖
再次，發展了有機酸浸提與光輻照聯合的方法用於污染場地土壤中多種毒性重金屬的快速去除與回收（圖3）。經檸檬酸選取後土壤中As和Cd（含量分別為1601和104 mg kg−1）的去除率達到73.0%和67.3%；從土壤中選取的Fe與檸檬酸結合形成Fe（III）-檸檬酸絡合物，UV光照射下Fe（III）-檸檬酸絡合物中配體向金屬的電荷轉移過程及伴隨該過程產生的O2•−，導致檸檬酸氧化降解及重金屬被沉澱或絮凝去除（60 h去除率高達97.0%以上）。小白菜盆栽實驗結果表明經修復後土壤中重金屬生物毒性降低，其中小白菜地上部分As和Cd降低率分別為22.0%和15.9%，根中As和Cd降低率分別為35.9%和58.6%。太陽光照射處理土壤提取液實驗結果進一步證實了該方法潜在的應用性。相關結果發表在期刊ACS EST Engg.（Environ. Sci. Technol.姊妹刊）。
圖3有機酸浸提及光輻照結合的方法用於重金屬污染場地土壤修復機理示意圖
以上研究結果有望降低重金屬污染場地土壤淋洗修復成本，同時擴展了電化學及光化學科技在重金屬污染場地土壤修復領域中應用。
三篇論文（Chem. Eng. J.、Sci. Total. Environ.和ACS EST Engg.）的第一作者分別為華中農業大學資源與環境學院碩士生尹浩宇、博士生楊雄和博士後劉立虎，通訊作者分別為博士後劉立虎、博士後劉立虎和邱國紅教授（共同）、邱國紅教授。華中農業大學譚文峰教授、曹夢華副教授、廣東工業大學馬金星教授和美國康涅狄格大學Steven L. Suib教授等在工作開展和論文撰寫等方面提供了寶貴的意見和建議。以上研究得到了國家自然科學基金、國家重點研發計畫、中組部“萬人計畫”科技創新領軍人才及博士後創新人才支持計畫等項目的資助。
【英文摘要】
（發表於Chemical Engineering Journal）
It is generally difficult to remove copper（Cu）from industrial wastewaters through traditional processes because Cu can form stable complexes with organic chelating agent.Here，the flow-electrode capacitive deionization-self enhanced oxidation（FCDI-SEO）system was applied to achieve highly efficient removal of two typical Cu-organic complexes（ethylenediaminetetraacetic acid-Cu（EDTA-Cu）and citric acid-Cu（CA-Cu）），and the underlying mechanism was also investigated.At the initial concentration of 1 mmol L−1，Cu removal efficiency reached 99.3% and approximately 100.0% respectively in FCDI-SEO treated EDTA-Cu and CA-Cu containing wastewaters after 75 min. Stable EDTA-Cu mainly migrated into the anode chamber as chelated Cu，while CA-Cu was easily dissociated and migrated into the cathode chamber as free Cu2+.After entering the anode chamber，the degradation rate of EDTA and CA was respectively 66.1% and 60.2%.Compared with the isolated closed-cycle mode，the short-circuited closed-cycle mode showed insignificant difference in Cu removal efficiency but higher energy efficiency.With pH increasing from 3.5 to 5.5，Cu removal efficiency showed little difference between the EDTA-Cu and CA-Cu system.With the addition of 180 mg L−1extra Na+，Cu removal efficiency was about 80% and 90% in the EDTA-Cu and CA-Cu system，respectively.After operation for five cycles and regeneration of electrode，FCDI-SEO could still achieve stable removal efficiency.This work provides a new pathway for the removal of aqueous Cu-organic complexes.
論文連結：https://doi.org/10.1016/j.cej.2022.136811
（發表於Science of the Total Environment）
Arsenic（As）pollution of soils poses serious threats to the ecological environment.In this study，organic acid（citrate）washing and electrochemical removal（manganese oxide cathode）were combined to remediate highly As-contaminated soils，and the effect of voltage was investigated as well.Citrate could extract the As bound to iron and aluminum oxides and enhance As mobility by indirectly reducing As（V）to As（III）in the soils.During the electrochemical removal of As，therhodochrositeproduced from the reduction ofbirnessiteat the cathode，the birnessite generated from the re-oxidation of released Mn（II）and theferrihydriteformed from the hydrolysis of Fe（III）at the anode together contributed to the adsorption and fixation of As in the leachate.After three successive rounds of combined remediation by citrate（0.1molL−1）washing and electrochemical removal with birnessite electrode at 1.5V，the As was totally removed in the leachate and the content of As bound to iron and aluminum（hydr）oxides was reduced by 84.2% in soils.Correspondingly，the contents of total and bioavailable As in the soil decreased from 1981.4 and 242.0 to 563.2 and 86.0mgkg−1，respectively.The As removal efficiency from the leachate and soil increased with increasing voltage from 0 to 1.5V.This study provides a new method for the effective treatment of As-contaminated soils.
論文連結：https://doi.org/10.1016/j.scitotenv.2022.153042
（發表於ACS ES&T Engineering）
Soil pollution by heavy metals has become an urgent environmental problem worldwide.However，existing remediation technologies have various deficiencies such as high energy consumption，long operation time，and secondary pollution，which largely limit their practical engineering application.Here，we propose an environmentally friendly method combining citrate washing and subsequent light irradiation for rapid and simultaneous removal of toxic heavy metals from soils.At initial concentrations of 1601 and 104 mg kg–1in the soil，the As and Cd percentage removal reached 73.0% and 67.3%，respectively.The ligand-to-metal charge transfer in Fe（III）-citrate complexes and formation of reactive intermediate HO2•/O2•–contributed to the citrate degradation and high-efficiency heavy metal removal（above 97.0% after 60 h）via coagulation and precipitation in the leachate under ultraviolet irradiation.For the soil with a low content of Fe，the photochemical removal efficiency of heavy metals could be significantly improved by the addition of Fe3+in leachate.Pakchoi cultivation showed an obvious reduction in biotoxicity of the contaminated soil after remediation.Solar irradiation experiments demonstrated the potential applicability of the green and efficient approach for remediation of heavy metal contaminated soils.
論文連結：https://doi.org/10.1021/acsestengg.1c00483