| Application of soil amendments,or soil heavy metal passivators,is one of the best way to control pollution of heavy metals in soil in practical remediation of farmland pollution.However,the effects of soil amendments on soil properties and heavy metal behaviors are not well understood,which affect the use of amendments in practice.Soil cultivation,soil column leaching,and soil pot experiment were employed to understand the effect of different passivators on Pb and Cd co-contaminated soil.The research is of great significance in understanding the role and mechanisms of soil amendments in controlling soil heavy metal pollution and guiding the application of soil amendments in agricultural land.The main results were summarized as follows.1)In an incubation study instantaneous effects of different concentrations of biochar(BC),slag(SL)and Fe-Mn ore(FMO)on immobilization of Pb and Cd were evaluated through the Toxicity Characteristic Leaching Procedure(TCLP)by following the European Community Bureau of Reference(BCR),CaCl2 and NH4NO3.The application of all amendments significantly decreased the acid soluble fraction,however the addition of biochar(BC)at the rate of 0.5%and 1%prominently decreased the Pb from 46.6%to 51.3%and Cd from 21.3%to 34.73%respectively than control,while the residual proportions of Pb and Cd were enhanced with increasing concentrations of SL and BC.Addition of BC significantly lowered the extractable fractions of both metals by TCLP,NH4NO3,and CaCl2 as compared to SL and FMO.Among all amendments,BC incorporation into co-contaminated soil offered promising results for Pb and Cd immobilization.Overall,all amendments showed the positive and long-term impact on the reclamation of co-contaminated soil with heavy metals and could deserve advance monitoring studies on a field scale.2)Whereas,in soil column leaching trial the effective roles of passivators were investigated in reducing Pb and Cd leaching.Our outcomes revealed that passivators application decreased the downward movement of metals.BC at the application rate of 6%presently reduced the Pb and Cd contents in the leachate.The cumulative efficiency after BCR extraction in residual fraction was 3%,1.18%,and 1.12%for Pb and 15%,12%,and 11%for Cd in the bottom layer of soil column,when treated with 6%BC,SL,and FMO as compared to control.Similarly,TCLP and CaCl2 extractable Pb(46%)and Cd(54%)in the bottom soil layer were also influenced,mainly by BC 6%treatment.The knowledge on passivators and their effect on heavy metals leaching may help remediating contaminated soils.Field experiment would be obligatory to ratify the efficiency of passivators presentation on heavy metals retention,prior to large-scale application.3)In a greenhouse experiment the effects of passivators on soil physicochemical and biological properties under Pb and Cd stress were investigated.The results revealed that the Pb and Cd have an adverse effect on soil properties.However,the application of passivators showed improved results under Pb and Cd stress soil.BC at the higher application rate cause decrease of 6%in soil bulk density(BD),and significant increase of 7%in total porosity(Pt),3%in soil saturation(θs),9%in field capacity(θFC),25%in permanent wilting point(θPWP),4%in available water content(θAWC),50%in hydrolytic conductivity(Ks),6%in soil organic matter(SOM),48%in pH,56%in EC,91%in total nitrogen(TN),189%in total phosphorus(TP),26%in total potassium(TK),109%in total calcium(TC),226%in total magnesium(TM),62%in soil cation,exchange capacity(CEC),as well as 240%in microbial soil biomass carbon(MBC),197%in nitrogen(MBN)and 45%in phosphorus(MBP),respectively as compared to control.While SL and FMO also showed a slight improvement in soil physicochemical and biological properties.No significant changes were detected in soil texture.Results of this study indicated that passivators use can improve the physical,chemical,biological properties and fertility of the soil.Therefore,use of passivators in agricultural lands is highly recommended.4)The pot experiment examined the putative roles of passivators in improving Pb and Cd tolerance in sesame.Our outcomes revealed that application of passivators significantly enhanced the nutrient level of sesame seedlings when grown under excessive Pb and Cd.The enhanced Pb and Cd-tolerance of sesame prompted by passivators could be ascribed to its ability to regulate Pb and Cd uptake,decrease Pb and Cd-induced oxidative stress as evidenced by reducing levels of hydrogen peroxide,malondialdehyde,and electrolyte leakage in Pb and Cd-stressed plants.The positive roles of passivators against Pb and Cd-toxicity also reflected through its protective effects on photosynthetic pigments,soluble protein,and proline.Passivators decreased the antioxidant capacity in Pb and Cd stressed the plant,particularly SOD,POD and CAT in sesame compared to control.Moreover,the passivators application also downregulated the transcript levels of SOD,POD,and CAT genes in sesame under Pb and Cd-stress.Furthermore,supplementation of passivators to the soil reduced the bioaccessibility(SBET),leachability(TCLP),and mobility(CaCl2)of Pb and Cd.Collectively,our findings conclude that the application of passivators enhanced sesame tolerance to Pb and Cd stress by restricting Pb and Cd accumulation,maintain photosynthetic performance and reducing oxidative damage through improved antioxidant system,thereby suggesting passivators as an efficient,stress regulator in mitigating Pb and Cd-toxicity in economically important sesame,and perhaps in other crop plants. |
