Effects Of Nanomaterials On Removal Efficiencies Of Soil Heavy Metals By Low-molecular-weight Organic Acids

Heavy metal contamination in soils has become one of the most serious environmental issues throughout the world.Low-molecular-weight organic acids?LMWOAs?are feasible and eco-friendly to remove heavy metals in soil,but their single application has limited removal efficiency.Therefore,three nanomaterials including nano-silicon dioxide?nano-SiO₂?,multi-walled carbon nanotube?MWCNT?and nanocrystalline titanium dioxide?nano-TiO₂?,were investigated to improve the removal efficiencies of soil heavy metal by LMWOAs including malic,citric and oxalic acids through soil washing experiments.To evaluate the influence of LMWOAs and nanomaterials leaching on soil,soil nutrient,the fractions of heavy metal and potted toxicity experiments before and after washing were compared.The main research results showed as follows:?1?The removal efficiencies of soil Pb,Zn and Cd were different by three LMWOAs,and single nanomaterial achieved low removals of the heavy metals?less than 2.0%?.Among the LMWOAs used,the removal efficiencies of soil Pb,Zn,and Cd reached 47.38%,40.19%,and17.88%by 0.05 mol L^-11 CA.The removals by CA or MA were significantly higher than those by OA?p<0.05?at the same gradient concentrations.?2?The concentration,solution pH and duration were three crucial factors to significantly influence the removal efficiencies by the combined washing solutions.Compared with LMWOAs alone,the removal efficiencies of soil heavy metals increased obviously?p<0.05?with increasing nano-SiO₂ and MWCNT.The addition of 0.10 mol L^-1nano-SiO₂ and 0.06 mol L^-1 MWCNT drastically improved the removal efficiencies by the LMWOAs of 0.25 mol L^-1,and the maximum improvements reached 19.34-27.56%and17.23-23.73%for the removal of soil Pb,Zn and Cd,respectively.On the contrary,the addition of nano-TiO₂ could significantly decreased the removal efficiencies by LMWOAs?p<0.05?,maximum reducing value reached59.23,35.23 and 34.45%for the removal of soil Pb,Zn and Cd,respectively.The removal efficiencies of soil heavy metals significantly decreased with increasing pH values of washing solutions?p<0.05?,while significantly increased with increasing duration to 2.0 h?p<0.05?,when further prolonging duration from 2.0 h to 6.0 h,the removal efficiencies remained stable or increased marginally?p>0.05?.?3?The three nanomaterials could affect the amount,position and bond strength of function groups involved in washing process,such as hydroxy and carboxylic groups,thus affect removal efficiencies of soil heavy metals by the three LMWOAs.Among the three,nano-SiO₂and MWCNT could promote hydroxy and carboxylic groups chelating with metal ions,enhanced the removal efficiencies by LMWOAs.While nano-TiO₂ could inhibit the chelation between function groups and metal ions during washing process,and significantly decreased the removal efficiencies by LMWOAs?p<0.05?.?4?Washing process has different effects on soil environmental quality.Among the three LMWOAs,the contents of organic carbon were all increased for 0.5-12%by single LMWOA,the contents of available phosphorus were increased for 5.11%by single MA,while the soil pH and the contents of total nitrogen,available nitrogen,total phosphorus,total potassium and available potassium of soils were decreased.The soil enzyme activities were also significantly decreased?p<0.05?.Compared with LMWOAs alone,the addition of nanomaterials could improve the contents of soil nutrients and the maximum improvements reached 3-9%for nano-SiO₂,2-10%for MWCNT and 2-5%for nano-TiO₂.?5?After washing by LMWOAs combined with nanomaterials,the exchangeable Pb,Zn and Cd were all significantly reduced?p<0.05?,maximum reducing value reached 95.4,98.2,and 99.7%,respectively,while the removals of residual Pb,Zn and Cd were the least.Compared with LMWOAs alone,the addition of nano-SiO₂ and MWCNT could significantly improve the removals of the oxidizable Pb,Cd and all kinds of forms of Zn?p<0.05?.The change of fractions of heavy metals in soil by LMWOA was affected marginally by nano-TiO₂.?6?Brassica bara and Lactuca sativa grew well in the soils washed by MA,CA combined with nanomaterials,but considerably less biomass was produced than by plants grown in unwashed soil.SOD,POD and CAT activities in their roots,stems and leaves were significantly reduced?p<0.05?,and the maximum reducing value reached 36.45-57.24%,16.78-38.12%and 13.56-28.45%,respectively.Compared with LMWOAs alone,the addition of MWCNT improved 2.06 times biomass of Brassica bara than those in original soil,and the addition of nano-SiO₂ improved 1.95 times biomass of Lactuca sativa than those in original soil.While the addition of nano-TiO₂ had slight effects on biomass of both Brassica bara and Lactuca sativa.