| A number of highly degraded lands with severe arsenic(As)contamination have been left in Shimen realgar mine due to the historical extensive mining and smelting activities which caused harm to local population health and ecological safety.Therefore,it is especially important for strict control and ecological restoration of such degraded soil.This study takes severely As-contaminated soils from Shimen realgar mine as the research object,the adsorption performance and reaction mechanism of an air-stabilized nZVI(A-nZVI)for aqueous As were discussed.Furthermore,the immobilization efficiency of As and the effectiveness of soil biological function restoration were studied in this mine soil.Results from this work are expected to provide an efficient pathway for management and ecological function restoration of severely As-contaminated soils.The main conclusions are as follows:(1)The adsorption characteristics of A-nZVI for aqueous As(?)and As(V)could be fitted well by pseudo-first-order kinetic,pseudo-second-order kinetic and Langmuir isotherm model.With initial aqueous As levels at 250 mg/L,adsorption occurred rapidly with As(?)and As(V)removal rate peaking at 2.5 h at 80.6% and 37.8%,respectively;Langmuir model fitting maximum sorption capacities of A-nZVI for As(?)and As(V)were 92.4 and 44.1 mg/g,respectively.(2)XRD and XPS results showed that A-nZVI had a unique dual As redox functionality.When it was exposed to As(?)solution,34.5% of As(?)was oxidized to As(V),4.0% of As(?)was reduced to As(0);when added to As(V)solution,35.8% of As(V)was reduced to As(?).(3)With A-nZVI application at ?0.2%(w/w),44.2–74.7% decline in soil As solubility was achieved by neutral water extraction.Nevertheless,the effectiveness of soil As fixation by A-nZVI tended to be impairedwith intensive acid input as unraveled by synthetic precipitation leaching procedure(SPLP,pH 4.2).Besides,acid rain leaching column test showed that As immobilization with A-n ZVI in the tested soil can remain stable under acid precipitation(800 mL)for at least one monsoon season.Therefore,for long-term stabilization of soil As with A-nZVI especially in areas subject to acidic precipitation,further buffering of proton with CEC-rich ameliorator could be beneficial for inhibiting dissolution of As-bearing Fe oxides and thus maintaining a longer-term effectiveness of A-nZVI.(4)The apparent recovery of dehydrogenase activity with decreased soil As lability supported the effectiveness of 0.2–1% A-nZVI treatment(w/w)in mitigation of soil As toxicity to biota.Compared to adjacent forest soil(F-CK),microbial community of the tested soil was featured by 66% lower abundance of Actinobacteria while an elevated richness of Gemmatimonadetes,both of which then showed a gradual shift toward that of F-CK with A-nZVI amendment,indicating that A-nZVI has a beneficial role in promoting the recovery of soil microbial communities by reducing soil bioavailable As.(5)0.2% A-nZVI has proved most effective in the development of ryegrass and averaged available As in rhizosphere of alfalfa decreased by47.1% with 0.2% A-nZVI by diffusive gradients in thin films(DGT),however,As concentration in both ryegrass and alfalfa was still 1.4–2.0times higher in the A-nZVI treatments than in F-CK.Therefore,efficient immobilization of soil As with A-nZVI can only provide a favorable starting point for plant inhabitation in the initial stage of soil ecosystem restoration.Further attenuation of soil As bioavailability is expected throughout the following community succession.Also,A-nZVI treatments tend to decrease the concentration of some mineral nutrients as represented by K and Zn in ryegrass,therefore,from the perspective ofrevegetation,supplementary application of slow-release fertilizer with fortified mineral nutrition could be necessary to improve soil fertility,so as to promote the growth of pioneering plant species. |
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