Leaching Feasibility Of Heavy Metals In Contaminated Soil Using Sequenced Reagents Of Oxalic Acid,Citric Acid,and A Copolymer Of Maleic And Acrylic Acid Instead Of Ethylenediaminetetraacetic Acid

With the expansion of industries,soils contaminated by metal(loid)s from mining and smelting are widespread in some districts of China.The release of heavy metals into the soil negatively impacts human health and ecological safety.Despite several developments and improvements in earlier technologies such as phytoremediation,vitrification,and immobilization,these remediation methods still have limitations;low performance,high cost,long duration,and new contaminants.In terms of permanently removing heavy metals from soils,soil washing is one of the few treatments available.The simplicity of this technology,its short operation time,cost-effectiveness,and high efficiency make it a practical option for heavy metal removal.A wide range of washing reagents used include surfactants,inorganic acids,chelating agents,and organic acids.Inorganic acids have irreversible effects on soil physical and chemical features,including the depletion of soil organic matter(SOM)and vital nutrients.An alternative to inorganic acids is chelating agents such as ethylenediaminetetraacetic acid(EDTA),which bind multiple heavy metals to form soluble and stable complexes.However,its usage has potential damage to the soil ecosystem due to its unfavorable biodegradability.[S,S]-stereoisomer of acid ethylenediamine succinic(EDDS),as a possible EDTA replacement,was comparatively costly and ineffective in removing Cd and Pb.Therefore,it is critical to find better remediation alternatives to EDTA in cleaning soils contaminated by anionic and cationic metal pollutants.In this study,we compared oxalic acid(OA),citric acid(CA),and a copolymer of maleic and acrylic acid(PMAA)to wash contaminated soil collected around an abandoned smelter in the Central South of China.The optimum conditions(p H,concentration,washing time,and soil to liquid ratio,S:L)of the reagents for removing metals were studied.The individual and combined removal efficiencies of the three reagents were compared against EDTA in the removal of arsenic(As),cadmium(Cd),copper(Cu),lead(Pb),and zinc(Zn).We also compared the influence of the washing reagents on soil properties.A pot experiment was also carried out on washed soils to determine the influence of reagents on the usability of washed soils for plant growth.Finally,metal contents determination was carried out on plant materials harvested from washed soils.The major conclusions that were drawn from the study are as follows;(1)The consecutive washing of the sample with the individual acids at optimum p H,concentration,washing time,and S:L ratio significantly improved the removal of heavy metals to efficiencies greater than the performance of EDTA in the soil.The optimum leaching conditions for OA were p H-3,concentration-80 g/L,washing time-60 min,and an S:L ratio-1:15.CA optimum performance was at p H-5,concentration-60 g/L,washing time-60 min,and S:L ratio of 1:10.PMAA optimum performance was at p H-5,concentration-100 g/L,washing time-90 min,and S:L ratio of 1:10.The removal of As,Cd,Cu,Pb,and Zn in the sequence of CA-OA were 65.6%,79%,59.1%,64.6%,and 63.5%,respectively and that of CA-PMAA-OA were 64.7%,93.3%,65%,76.7%,and 76.8%,respectively.The increased removal efficiencies with additional reagents in sequence resulted from the liberation of metals from soil particles bound within the soil matrix which were difficult to remove with the first reagent of the sequence.EDTA at optimum conditions(p H-3,concentration-60 g/L,washing time-60 min,and an S:L ratio-1:10)resulted in 10.2%,71.3%,29.8%,61.6%,and 52.4%removal for As,Cd,Cu,Pb,and Zn,respectively.Washing the contaminated soil with CA-OA and CA-PMAA-OA sequences could remove cationic and anionic metals to efficiencies greater than EDTA.(2)Washing with organic acids had slight reductions in soil acidity(p H)and SOM,the major determinants of the usability of washed soil.The p H of all samples had decreased after washing with values of 6.2,5.6,and 5.3,respectively,for EDTA,CA-OA,and CA-PMAA-OA-washed soils compared to the control(CK)with 6.4.The reduced p H in organic acid-washed soils can be attributed to the number of washings used by each treatment;the lower p H of OA used in the washing experiment;and partly due to the generation of H~+ions by hydrolysis metal cations.The percentage of SOM in all the washed soils were 1.8,1.8,and1.4,respectively,for EDTA,CA-OA,and CA-PMAA-OA.Reduced SOM might be due to fractions being lost in each stage of the washing procedure.The CK sample had 48.1%,43%,and 32.3%in the labile fraction for Cd,Pb,and Zn.Speciation studies of the washed samples reveal that the remaining Cd and Pb in CA-OA washed soils were 58.7%and 59.8%,respectively,in the labile fractions.The remaining Cd and Pb in CA-PMAA-OA washed soils were 37.8%and 72.5%,respectively,in the labile fractions.The increased labile fractions of Cd and Pb in CA-OA and CA-PMAA-OA-washed soils are due to the transformation of metals from the F3-F5 fractions after the F1,and F2 fractions have been removed.Despite the remaining contents of Cd and Pb in CA-OA and CA-PMAA-OA washed soils being lower than EDTA-washed soils,they may be at risk of adsorption because of the high labile fraction.The higher removal and risk that may occur by Cd and Pb in the CA-OA and CA-PMAA-OA washed soils was due to the greater proportion of these metals in the exchangeable and carbonates-bound fractions.Scanning electron microscopy(SEM)revealed that the washed samples had slight morphological changes with no fine grains,as found on the CK sample.However,a slight difference was found in the soil surface structure between EDTA-washed soils and sequenced biodegradable reagents.The degradable reagents washed soils had a large sharped layered structure resulting from the number of washings compared to EDTA.Energy dispersive spectroscopy(EDS)revealed that the weights of Cu,Zn,As,Cd,Pb,and other elements that had decreased in the washed soils compared to CK suggest that the reagents could effectively desorb the investigated metals on the soil particle surface.However,the surface of the washed soils could not adversely affect crop growth.The Fourier transform infrared spectroscopy(FTIR)results showed that the contaminated smelter soil was mainly composed of Si/Al/Fe oxides.The absorption peaks for Al-Al-OH,Al-O-Si,and Si-O-Fe in all the washed soils decreased compared to the CK sample,indicating that the washing reagents can promote the dissolution of crystallized Si/Al/Fe oxides.X-ray diffraction(XRD)determined the mineralogical constituents of the CK and washed soil samples.Quartz and some minerals,such as calcite and hematite,constituted the CK soil sample.The presence of these minerals indicates the dominant component of the soil are silt and sand,with a stable soil structure that has been developed from the weathering of iron-containing minerals.The peaks of the samples did not show any major changes after washing with EDTA and degradable washing agents.It was concluded that CA-OA-washed soil was better comparing the influence on soil properties.(3)Pot experiments were conducted with 1 kg soil samples washed with the sequences CA-OA,CA-PMAA-OA,and EDTA at optimum conditions of the individual acids reported in the batch experiment.Sorghum bicolor was used as a tested plant,taking germination success and other plant morphological features.After 63 d,the CA-OA potted plants outperformed all other treatments with 70%germination recorded in 7 d,an average height of 14.6 cm,three leaves,fresh weight of 2 g,and biomass of 0.3 g.The increased growth in CA-OA potted plants was due to slight physicochemical changes and probably due to the activation of nutrients such as nitrogen and phosphorus for plants but hindered in other treatments.Dried plant samples from various treatments were digested and analyzed for metal contents.Except for Cd and Pb,all other metals were below the world health organization(WHO)permissible limits in the biomass of plants from all treatments.The detection of Cd and Pb in dried plant samples can be explained by higher fractions in the labile fractions of the residual metals.This study revealed that washing contaminated smelter soil with the individual organic acids at optimum conditions in the sequences CA-OA and CA-PMAA-OA was better than EDTA.Comparatively,CA-OA washed soils stand out in using washed soils for plant growth with the following advantages;simultaneous removal of cationic and anionic metals,less harmful impact on soil physicochemical properties,successfully supporting the growth of crops,and accumulation of less metal in biomass.Hence,CA-OA is a better alternative to EDTA in removing multi-metal contaminations from soil and successfully supports plant growth.