Remediation Mechanism And Ecological Risk Assessment Of Cd,Pb And Zn Contaminated Soil By Biodegradable Chelator

Soil washing with chelating agents has recently become one of the hotspots and key techniques in soil heavy metal–polluted remediation field with the advantages of high efficiencies,time–saving,and operational flexibility.Among these chelators,ethylenediaminetetracetic acid（EDTA）has been extensively proposed for the extraction of toxic metals from contaminated soils because of its broad spectrum and strong complexation ability for different metals.Unfortunately,it is poorly photo–,chemo–and biodegradable and easily retains in the soil environment,resulting in limiting its application.Therefore,screening of novel chelating agents with high efficient and low toxicity（non–toxic）and great application potential instead of EDTA has become the core and challenge of the development of soil washing technology.Moreover,soil washing will inevitably have a certain influence on soil properties,but little information about the effect of soil washing on the mechanism of the dynamic changes of soil enzyme activity is is available.In addition,more work are highly needed for supporting whether the washed soil has the potential to be reused in agriculture.Aim to deal with the issues mentioned above,four biodegradable chelating agents（BCAs）including iminodisuccinic acid（ISA）,glutamate–N,N–diacetic acid（GLDA）,glucomonocarbonic acid（GCA）,and polyaspartic acid（PASP）were employed.The removal mechanism of soil Cd,Pb,and Zn,soil properties,and potential environmental effects of washed soils are discussed,which provided the theoretical and scientific basis for the remedition of heavy metal–contaminated soil.The main conclusions of this study are as follows:（1）The factors and their interactions affecting the metal removal efficiencies from contaminated soil by washing with BCAs were systematically investigated.The BCAs could effectively remove Cd,Pb,and Zn from polluted soil with the removal efficiencies increasing with the increase of BCAs concentration and washing time,and decreasing with the addition of competitive cation(Ca²⁺)and the decrease of washing solution of pH and liquid–soil ratio.GLDA and ISA achieved significantly higher metal removal efficiencies than that of PASP and GCA（P<0.05）.It is recommended to select a soil–liquid ratio of 1:10 and no electrolyte addition as a follow–up study condition for BCAs washing based on the comprehensive consideration of metal removal efficiencies and economic cost.Kinetic studies suggested that the removals of Cd,Pb,and Zn with BCAs washing were fitted well with pseudo–second–order kinetic models.Response surface methodology（RSM）based on Box–Behnken Design（BBD）was further applied to analyze the experimental variables which all showed a positive effect on metal removal efficiencies.The optimization results showed that the initial BCAs concentration was the dominant factor controlling the metal removals with the contribution rate more than50%,followed by washing solution of pH and washing time.The interactions between BCAs concentration and washing time were significant（P<0.05）,while the interactions of washing solution of pH with BCAs concentration or washing time were not pronounced（P>0.05）.The ANOVA results verified that the proposed models were precise and reliable for the examination of the variables of BCAs washing treatment（P<0.05）.The optimum values of selected variable and their mutual interactions were detected by RSM.The results illustrated that metal removal of 7.66–93.16%was predicted under optimum conditions of BCAs concentration of 75 mmol L^-1,washing solution of pH 4.0,and 120 min of reaction time,which was close to the experimentally validated value（7.11–83.20%）in BCAs washing process.Among the four BCAs,the Cd,Pb and Zn removal efficiencies by GLDA and ISA washing were comparable to the traditional chelating agent EDTA,which could be a potential and suitable candidate to substitute for the conventionally–used EDTA in the field of soil pollution control.（2）The potential mechanisms of heavy metal removal with the aid of BCAs washing were proposed based on the spectrum characteristics of washed soil.The Fourier transform infrared spectroscopy of polluted soils before and after BCAs washing showed some characteristic peaks position were shifted and peak intensities were weakened in the functional group region while only slight changes were observed in the fingerprint area,which could be ignored.These results suggested that BCAs washing has quiet a few influence on the chemical environment of the washed soils,but this influence is limited and does not damage the soil structure.The scanning electron microscope–energy dispersive X–ray spectroscopy confirmed that BCAs washing had only slight effect on soil morphology and skeleton.X–ray diffraction patterns further illustrated that BCAs washing had only a slight effect on the soil mineral composition with no disappearance or obvious shift of dominant characteristic peaks.Moreover,X–ray photoelectron spectroscopy displayed that the valence states of Cd,Pb,and Zn combined with soil surface and soil solid are predominantly divalent before and after BCAs washing,implying no oxidation or reduction reaction occurs during the washing process.Based on the above results,it is speculated that the dominant mechanisms for Cd,Pb,and Zn removal from polluted soil by ISA,GLDA,PASP,and GCA were H⁺acid dissolution effect,ion exchange and complexation.（3）The effects of BCAs washing on the quality of remediation soil were comprehensively expounded,and the dynamic response mechanisms of soil basic respiration intensity,microbial biomass and enzyme activity were revealed.Soil pH,conductivity and cation exchange capacity decreased after the contaminated soil washing under the optimal conditions（P<0.05）,while slight decrease in soil organic matter,total nitrogen,total phosphorus,total potassium and mineral beneficial elements.No significant changes in available potassium and plant available nitrogen were observed（P>0.05）.Conversely,the content of available phosphorus in the washed soils increased by 1.19–2.01 times（P<0.05）.Moreover,compared with EDTA washing,the soil properties after BCAs washing were obviously improved,suggesting BCAs alleviated nutrient leaching and promoted soil fertility maintenance.The metal in water–soluble,exchangeable,and bound to carbonates fractions were considerably removed as well as part of bound to Fe–Mn oxides fraction and a small amount of residual fraction were also removed after BCAs washing.These results suggested that BCAs washing increased the stability and reduced the mobility of heavy metals thus decreasing the environmental risks and bioavailability of heavy metals in remediated soil.In addition,the soil respiration intensities,microbial biomass and enzyme activities were enhanced in the BCAs–remediated soils in comparison with initial and EDTA–treated soils.The cumulative respiration of microorganisms in both soils increased continuously with the prolongation of culture time followed the order of GLDA>ISA>EDTA>PASP>GCA>initial soil.While the microbial biomass carbon,nitrogen and phosphorus contents in the washed soils gradually increased with the increase of culture time and reached the peak at 21 d（P<0.05）.The enzyme activities ofβ–glucosidase,sucrase,urease,acid phosphatase,and fluorescent diacetate hydrolase in the BCAs–remediated soils showed an overall upward trend with the extension of culture time.The ISA,GLDA,PASP and GCA washing demonstrated a promoting effect on soil enzyme activities compared with EDTA–treated soils.（4）The potential ecological environmental risks of BCAs washing on remediated soils were assessed.The phytoavailability（20.02–95.78%and 12.26–84.56%,P<0.05）,leachability（7.88%–100.00%and 10.37%–75.70%,P<0.05）,mobility（16.81%–100.00%and 10.45%–100.00%,P<0.05）,and bioaccessibility（18.14%–65.93%and 44.03%–94.87%,P<0.05）of residual metals in mine soil and farmland soil,respectively,after BCAs washing decreased notably in comparison to the original soils,thus mitigating the estimated environmental and human health risks.Two common vegetables（Brassica bara and Lactuca sativa）were further used to verify the fitness of plants grown on the washed soils.B.bara and L.sativa grew well in the remediated soils but considerably less biomass（4.24%––91.47%,P<0.05）was produced than by plants grown in unwashed soil.The photosynthetic capacities,oxidation defense abilities,and nutritional qualities in their leaves have been ameliorated,compared with plants cultivated in the original soils.Although the ISA and GLDA treatments reduced the Cd,Pb,and Zn concentrations in the shoots of B.bara and L.sativa,the concentrations of Cd and Pb in the edible parts still exceeded the legal limits.In conclusion,soil washing with BCAs especially ISA and GLDA successfully reduced environmental risks in remediated soils and allowed the plants to grow well while decreasing the uptake of metals remaining in the remediated soil.However,additional measures are highly required to ensure the safe cultivation of plants in washed soils.