Study On The Mechanism Of Na₂S₂O₈/Na₂FeO₈-modified Montmorillonite For Synergistic Control Of Soil Hg Release In Greenhouse

Mercury is listed by the World Health Organization as one of the"Ten Chemicals of Major Concern".Soil mercury is released into the atmosphere through the soil-air interface,and plants absorb gaseous elemental mercury（GEM）through the respiration of leaf stomata,which is amplified by the food chain and eventually enriched in the human body,thus endangering human health.Greenhouses are in a relatively closed environment and soil mercury release is the main source of GEM in greenhouses and the main source of contamination of vegetable leaf parts,Inhibition of soil mercury release can reduce atmospheric GEM concentration and thus the uptake of mercury by vegetable leaves.Accordingly,this experiment simulates an agricultural vegetable shed by adding mercury nitrate[Hg（NO₃）₂]solution to the natural soil so that the THg concentration of mercury-contaminated soil is 15.0 mg/kg,After application of montmorillonite[thymine montmorillonite（T-MMT）and Tris（hydroxymethyl）methyl aminomethane（Tris-MMT）]and oxidants[sodium persulfate（Na₂S₂O₈）and sodium perchlorate（Na₂Fe O₄）]to the mercury-contaminated soil,Exploring the variation of total mercury（THg）concentration in cabbage and garlic（roots,stems and leaves）;Analysis of the effects of GEM and soil mercury release flux on the accumulation of mercury in cabbage and garlic;Combination of soil Hg release flux,GEM,different valence states of Hg[(Hg（0）,Hg（I）,Hg（II）],environmental conditions（light intensity inside and outside the shed,temperature inside and outside the shed,soil temperature,soil water content）,Soil physicochemical properties[redox potential（Eh）,p H,organic matter（SOM）,total soil mercury（THg）],sorption-desorption characteristics,and microstructural characterization were used to analyze the mechanisms influencing the release of mercury from soils.The main findings are as follows:（1）The biomass of cabbage in natural soil（THg=0.22 mg/kg）and mercury contaminated soil（CK）was 9.90 g and 11.9 g.The treatment with Na₂S₂O₈,Na₂Fe O₄,T-MMT,Tris-MMT,Na₂S₂O₈+T-MMT and Na₂Fe O₄+Tris-MMT in Hg-contaminated soil increased respectively cabbage biomass by 2.41%,30.2%,60.4%,62.3%,98.4%,and 72.5%.Garlic biomass respectively was 8.80 g and 10.2 g in natural soil and CK,and the Na₂Fe O₄+3%Tris-MMT treatment was recorded the highest biomass（11.5 g）and plant height（85.0 cm）.It can be seen that the addition of MMT and oxidant mixture of remediation agents significantly increased the biomass of cabbage and garlic,probably because the mixture contains elements such as C,Fe,S and N required for plant growth,which in turn promotes the accumulation of cabbage and garlic biomass.（2）After the application of MMT with oxidant,the total mercury（THg）concentrations in cabbage roots in the treatments of CK,Na₂S₂O₈,Na₂Fe O₄,T-MMT,Tris-MMT,Na₂S₂O₈+T-MMT and Na₂Fe O₄+Tris-MMT were 1.10,0.28,0.74,0.88,0.76,0.64,and 0.66 mg/kg.The lowest THg concentrations were found in cabbage and garlic roots in the 200 mg/kg Na₂S₂O₈ treatment,with 11.0 and 1.38 fold reduction;THg concentrations in cabbage stems and garlic stems were low in all treatments except for 200 mg/kg Na₂Fe O₄,5%T-MMT and Na₂Fe O₄+1%T-MMT treatments in which THg concentrations in cabbage stems were higher than in CK;The THg of both cabbage and garlic leaves in the MMT and oxidizer treatments did not exceed that of the Hg-contaminated soil（0.45 mg/kg）,indicating that both MMT and oxidizer reduced the uptake of GEM by cabbage and garlic leaves;The lowest concentration（0.14 mg/kg）was found in cabbage leaves in the 100 mg/kg Na₂S₂O₈ treatment,which was the most effective in reducing THg concentration in cabbage leaves;the lowest concentration（0.11 mg/kg）was found in garlic leaves in the Na₂Fe O₄+5%Tris-MMT treatment,which was the most effective in reducing THg concentration in cabbage leaves;The enrichment capacity of cabbage and garlic for Hg was in the order of root>leaf>stem,and cabbage leaf（0.27 mg/kg）>garlic leaf（0.21 mg/kg）,but cabbage root（0.66 mg/kg）<garlic root（1.32 mg/kg）.（3）The mean mercury release flux from soil（CK treatment）was 187[ng/（m²·h）],higher than the Manitoba mercury mine site of 29.0[ng/（m²·h）],much higher than the US urban bare ground of 6.50[ng/（m²·h）],and lower than the indiscriminate wood mill mercury mine site of 2283[ng/（m²·h）;The average GEM concentration of 74.8 ng/m³is higher than that of the southwest coast of Taiwan Island（6.70±2.00 ng/m³）and much higher than that of Beijing,China（2.42±0.73 ng/m³）during the non-haze period;When montmorillonite and oxidizer were applied,the mean values of soil mercury release fluxes for CK,Na₂S₂O₈,Na₂Fe O₄,T-MMT,Tris-MMT,Na₂S₂O₈+T-MMT,and Na₂Fe O₄+Tris-MMT respectively were 181,110,122,164,87.1,157,and 109[ng/（m²·h）.It can be seen that Tris-MMT treatment showed the best inhibition of soil Hg release flux,with a 48%decrease compared to CK treatment.GEM-flux,flux-cabbage leaves were significantly positively correlated（p≤0.01）,flux-garlic leaf,garlic leaf-cabbage leaf positive correlation（p≤0.05）,GEM-cabbage leaves and GEM-garlic leaves were positively correlated,but the correlation was low.It is known that soil Hg release fluxes are the main cause of Hg accumulation in plant leaves.（4）With the increase of incubation time,SOM gradually decreases and soil Eh increases,The mean values of Na₂S₂O₈,Na₂Fe O₄,T-MMT,Na₂S₂O₈+T-MMT,Tris-MMT and Na₂Fe O₄+Tris-MMT treatments were 494,504,496,502,508 and 514 m V,respectively,which were higher than those of CK（483 m V）,Soil THg declines,The variation of soil THg concentration in CK,Na₂S₂O₈,Na₂Fe O₄,T-MMT,Na₂S₂O₈+T-MMT and Na₂Fe O₄+Tris-MMT treatments ranged from 13.9-14.9,13.4-14.8,13.9-15.4,13.0-14.8,13.1-15.3,13.5-14.7,13.9-14.9 mg/kg,Soil temperature,shed temperature and light intensity all increased and then decreased,with water content at about 15%.（5）Correlation analysis showed that after the application of MMT and oxidant,GEM-soil temperature,GEM-flux,GEM-SOM,GEM-Hg（0）,GEM-Hg（I）,GEM-inside and outside shed temperature,flux-soil temperature,flux-Hg（0）,flux-Hg（I）,flux-inside and outside shed light intensity,and flux-inside and outside shed temperature were highly significantly and positively correlated（p≤0.01）.GEM-Eh,flux-Eh were significantly negatively correlated（p≤0.01）.It is known that in Hg-contaminated soils,the application of MMT and oxidant can increase soil Eh,which in turn affects the flux of Hg release,and Hg（0）in soil can significantly affect soil Hg release flux and GEM concentration,and soil Hg release in greenhouses is the only source of GEM in greenhouses,which are closely related.（6）Tris-MMT reaches maximum adsorption when p H is 8;Na-MMT reaches maximum adsorption when p H is 3;T-MMT reaches maximum adsorption when p H is4.The adsorption time has little effect on the amount of mercury adsorbed,and the adsorption process is rapid.The increase in the initial concentration of mercury increases the adsorption of MMT.The adsorption amounts respectively were:Na-MMT（8.65 mg/g）<soil（13.7 mg/g）<T-MMT（48.0 mg/g）<Tris-MMT（53.5 mg/g）.The adsorption processes of Na-MMT,T-MMT and Tris-MMT were in accordance with the quasi-secondary kinetic model,and the theoretical adsorption amounts fitted by the model were in general agreement with the experimental adsorption amounts.Na-MMT,Tris-MMT and T-MMT all conform to the Freundlich and Langmuir models;according to the microstructural analysis,T-MMT and Tris-MMT contain-OH,-C=O,-NH and-NH₂ groups that can complex or precipitate with mercury.（7）The application of both MMT and oxidants promoted the oxidation process of Hg（0）/Hg（I）and Hg（I）/Hg（II）in the soil;The mean values of Hg（0）concentration in soil were KB（blank natural soil,2.04 ng/g）>Na₂Fe O₄（1.31 ng/g）>Na₂Fe O₄+Tris-MMT（1.29 ng/g）>Na₂S₂O₈（1.22 ng/g）>T-MMT（1.19 ng/g）>Tris-MMT（1.13 ng/g）>Na₂S₂O₈+T-MMT（1.11 ng/g）;The ratios of Hg（II）/Hg（I）and Hg（I）/Hg（0）were greater in the soil with MMT and oxidizer application than in the KB group,which in turn presumably led to the conversion of Hg（0）to the higher valent state of mercury in the soil.This finally led to a decrease in soil Hg release flux.