| Heavy metal pollution in agricultural soils is considered a serious environmental concern due to their toxicological effects on soil-plant systems and humans.Particularly,cadmium contamination is a concern in huge area of the world,rendering productive cropland into non-cultivated areas.Therefore,amelioration of Cd polluted soil is an important task to minimize its accumulation by edible plant parts and,thereby,reduces its mobility in agricultural soils.Thus,cheap and less destructive technique(in-situ immobilization)has been established to remediate Cd-contaminated soil by using organic and inorganic amendments.In the present studies,the immobilizing effects of rice straw(RSB),rice hull(RHB),maize stover(MSB),KOH modified rice straw biochar,zeolite and rock phosphate were evaluated on Cd mobility,adsorption,distribution among different fractionations,leaching characteristics,bioaccessibility,plant growth,metal uptake by plants in a combination of incubation and green house experiments.These findings could offer basis for exploring the fundamental technical and scientific issues appropriate to Cd-polluted acidic soil by immobilizing amendments.Firstly,a laboratory incubation study was conducted to assess the effectiveness of rice straw biochar(RSB),zeolite(ZE),and rock phosphate(RP)stabilizers on the restoration of cadmium(Cd)polluted soils.Sequential extraction procedure,the European Community Bureau of Reference(BCR),the toxicity characteristics leaching procedure(TCLP),and extraction with ammonium nitrate were carried out.Results showed that with a raise of soil pH,the exchangeable fraction of Cd in soil was significantly reduced by 29.41%,12%and 13%for RSB,ZE,and RP,respectively at 3%application rate.According to the Langmuir adsorption isotherm,BC-amended soil showed higher adsorption capacity "Qm" of Cd from 8.38 mg g-1 to 19.85 mg g-1.The results of FTIR,SEM and zeta potential clearly indicated the immobilization mechanism of Cd in contaminated soil through adsorption.Overall,BC offered better results as compared to other amendments.Plant growth experiment was conducted to explore the effects of RSB,ZE and RP on Cd immobilization by single extraction CaCl2 and leaching by means of TCLP,and phytoavailability with water spinach(Ipomoea Aquatic)in Cd-contaminated soil.Concentration of Cd in TCLP and CaCl2 extract gradually decreased with the increase of amendments(RSB,ZE and RP)rates.The increase in BC addition from 1.5%to 3%significantly decreased CaCl2-extractable Cd by 65.8%-72.9%and TCLP extractable Cd by 31.2%-37.7%respectively,over control.Whereas,RP addition decreases 53.4%-65.2%and 11.7%-19.5%in CaCl2 and TCLP extractable Cd respectively,as compared to control soil.The addition of RSB,ZE and RP decreased Cd uptake by 22.9%-61.8%with 1.5%and 3%application rate,respectively.Moreover,the antioxidant enzymes activity i.e.,superoxide dismutase(SOD)and peroxidase(POD)decreased with the addition of BC,ZE and RP under Cd stress.In conclusion,rice straw biochar could be highly recommended as a safe stabilizer to immobilize Cd in polluted agricultural soils.Additional experiment was done to sort out the best biochar by possible combinations of RSB,RHB,and MSB amendments on Cd mobility by growing Chinese cabbage(Brassica chennises L.)in contaminated soil.Results demonstrated that application of RSB at 3%was more effective among all applied biochar and prominently increased Cd fraction from acid-soluble to residual portion and increased the geochemical stability in acidic soil.The acid-soluble portion of Cd decreased by 24%-32%,19.3%-23.4%,and 22%-27%,but the residual portion of Cd increased by 33.4%-37.7%,32%-35.4%and 33%-36.3%with the incorporation of RSB,RHB and MSB at 1.5%and 3%application rate,respectively.The SBET techniques showed significant decrease in Cd by 30.5%,20.6%and 27.5%for RSB,RHB and MSB,respectively at 3%rate.A decrease in CaCl2-extractable Cd was observed from RSB at 58.4%,from RHB at 39%and from MSB at 46.4%and for the TCLP extract,it was decreased from RSB at 42.85%,from RHB at 32.65%and from MSB at 36.73%after a 3%application rate of biochar.In the similar manner soil's chemical properties significantly increased with the incorporation of BC,especially soil pH,soil organic carbon(SOC)and total nutrient contents.Soil microbial biomass(C and N)and inorganic nitrogen forms(NH4 and NO3)were significantly increased among all the applied biochar.Overall,these results indicated that applications of RSB can proficiently stabilize Cd and thereby,reduce its phytoavailability to Chinese cabbage in Cd polluted soil to mitigate food security risks.In the adsorption study,isotherm investigations were performed to examine the effect of KOH modified and non-modified rice straw derived-biochar for Cd removal from aqueous solution.Results showed that the highest adsorption capacity of pristine rice straw biochar was 12.17 mg g-1.The chemically modified rice straw biochar showed greater Cd adsorption capacity of 41.9 mg g-1,which was more than 3 times that of pristine biochar.Increase of surface area and changes of porous structure,especially the functional groups on the surface of modified biochar were the major contributors to its more efficient adsorption of Cd.The possible mechanisms for Cd adsorption by biochar mainly involves(1)surface precipitation by forming insoluble Cd compounds in alkaline condition,and(2)ion exchange for Cd with exchangeable cations in the biochar,such as calcium ions,which were confirmed by FTIR,XRD,SEM and Zeta potential determination.Additional study was conducted to test the efficiency of modified(BCK)and non modified(BC)rice straw biochar for Cd solubility and bioavailability in Cd-contaminated soil.Cd-contaminated soil was incubated for 60 days with 1.5%and 3%of BC and BCK biochar.Results showed that,both biochar have significant effect on reducing soluble fractions of Cd by 40.42%and 34.88%with BCK and BC at 3%rate,respectively.Similarly,bioaccessibility of Cd to the human gastric phase(SBET)extractable was decreased by 32.4%and 25.2%with the addition of BCK and BC,respectively,relative to control.In addition,both biochar significantly reduced Cd in TCLP leaching and NH4NO3-extractable in amended-soil relative to control.The reduction in Cd solubility and bioavailability/bioaccessibility by BCK and BC may be due to significant increment in soil pH and surface complexation.While,the significant reduction in TCLP leachable and acid-soluble phase may be due to the distribution of Cd from more available to less available/residual portion.In summary,the application of alkaline amendments in acidic red soil could ameliorate the acidification of contaminated soils and reduce Cd mobility and phytoavailability in contaminated soil.Among all the amendments rice straw biochar offered the better results to reduce Cd mobility and increase soil adsorption capacity.In addition,RSB could reduce Cd solubility in soil and reduced its translocation from plant(water spinach and Chinese cabbage)shoot to root.Furthermore,modification of RSB with alkali showed greater Cd adsorption from aqueous solution and increases its fraction from soluble to most stable residual form in soil.Application of BCK has ability to decrease Cd bio-accessibility in acidic red soil as well as improve soil chemical properties due to its greater surface properties and thereby,mitigate food security risks. |
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