| According to the general national survey of soil contamination in 2014,the rates of soil samples exceeded the soil quality standard for cadmium(Cd)and arsenic(As)ranked first and third,respectively.Some researchers pointed that the soils in many areas suffered from the co-contaminations of As and Cd.Compared with other heavy metals(HMs),As and Cd are more comfortable accumulating in paddy rice.Therefore,both of them posed high health exposure risk to the human body along the food chain(e.g.,dietary intake).Thereby,it is necessary to adopt efficient remediation methods to decrease the As and Cd accumulation in the food and reduce exposure risk to humans.However,it is often challenging to meet the remediation requirements in As and Cd contaminated soils through a single remediation material due to their opposite chemical behaviors in the soil environment(such as paddy soil).Among the various passivation materials,biochar(BC),as a resource product,has abundant raw materials and is expected to solve agricultural waste disposal,if it can be applied to remedy the As and Cd co-contaminated soils.Previous studies have illustrated that BC has high removal or adsorption efficiencies for As and Cd in the aqueous solution,which lays a foundation for the simultaneous passivation for the As and Cd co-contaminated paddy soils.For realizing the potential of BC on the remediations of farmlands polluted by As and Cd,this study conducted a Metaanalysis based on the published literature.The meta-analysis performed the quantitative analysis for the effects of BC on reducing the As and Cd bioavailability in soils and inhibiting As and Cd accumulation in plants.This comprehensive analysis also illustrated the impact of BC and soil properties on the bioavailability and bioaccumulation of As and Cd via the SVM classification method.The results could provide a reference for the raw material selection and the modification of BC.Based on the comprehensive analysis,four kinds of BC were modified by ferrihydrite(Fh),layer double hydroxides(LDHs),and layer double hydroxides doped silicon(LDHs-Si)to improve their adsorption capacities for As and Cd.Afterward,performances of MBC for As(III)and Cd(II)were conducted in the mixed solution,and the associated adsorption mechanisms were systematically studied by means of experimental and characterization methods(such as BET,SEM-EDS,FTIR,XRD,and XPS).Furthermore,the effects and mechanisms of simultaneous passivation of MBC for As and Cd co-contaminated soils were systematically investigated by culture experiments(i.e.,the flooding culture test and material stability test).Finally,MBC was applied to the pot experiment and field experiment to clarify the inhibition effects of MBC on the uptake and accumulation of As and Cd in rice and analyze the impacts of MBC on the growth(e.g.,biomass,height,tiller number)and yield of paddy rice.This study is expected to provide the reference and basis for the remediation of As and Cd co-contaminated soils.The main results of this study are as follows:1)Remediation potential of BC or MBC on As and Cd in co-contaminated soils,and the key factors.The meta-analysis was conducted with 1030 observations considering the difference of the BC,plants,and trial characteristics.This part aims to clarify the following questions:(a)Whether BC or MBC application can inhibit the bioavailability of soil As and Cd;(b)Whether BC or MBC application can reduce the accumulation of As and Cd in plants or plant tissues;(c)the effects of soil,plant,and biochar properties on the As and Cd bioavailability and bioaccumulation.The results showed that BC or MBC application could significantly reduce the bioavailability of Cd in soils by 50.12%[40.03%,58.45%],but could slightly increase the bioavailability of As in soils by 2.39%[-5.16%,10.57%].In the field experiments,BC or MBC application significantly decreased the soil As and Cd bioavailabilities,about 23.28% [8.47%,35.70%] and 47.40%[20.10%,65.38%],respectively.In the non-field experiment,BC or MBC application significantly increased the soil As bioavailability by about 10.17% [1.08%,20.08%];but dramatically increased the As concentration in porewater by approximately 62.29% [-9.62%,191.40%].Fortunately,BC or MBC application could simultaneously inhibit the plant uptake for As and Cd by 25.48% [22.06%,28.76%] and 38.66% [33.18%,43.69%],respectively.The effect of BC or MBC on reducing the bioaccumulation of As in the nonfield experiment(27.33%)was higher than that of the field experiment(18.09%).In the field experiment,the inhibition effect of BC or MBC on Cd bioaccumulation(52.98%)was greater than that of the non-field experiment group(34.37%).The inhibition order of BC or MBC on As bioaccumulation in plant tissues was root < stem < leaf < grain,and the inhibition order of Cd bioaccumulation in plant tissues was root < leaf < stem < grain,indicating that BC or MBC could inhibit the transfer of As and Cd from the soil to the plant.Besides,via the critical factor analysis,the results indicated that BC derived from the manure,BC with the low application rate,BC with the low-temperature pyrolysis,and soils with the high content of soil organic carbon(SOC)are more advantageous in reducing the bioaccumulation of As and Cd in co-contaminated soils.Although the BC or MBC can simultaneously inhibit the bioaccumulation of As and Cd in plants based on the Meta-analysis,their efficiencies need to be further improved to ensure crop production safety.2)Performances of modified biochars on the simultaneous adsorption for As(III)and Cd(II)and their associated mechanisms.This study selected the wheat straw biochar(WSBC),maize straw biochar(CSBC),pig manure biochar(PMBC),and mixed wood biochar(MWBC)as matrix materials and modified BC through the Fh,LHDs,and LDHsSi modification methods.This study conducted the adsorption performance test to determine the adsorption abilities of MBC for As(III)and Cd(II)in solutions and performed the factor experiments and characteristics(SEM-EDS,XRD,FTIR,and XPS,etc.)to explore adsorption mechanisms on MBC for As(III)and Cd(II).The results showed that MBC is capable of simultaneously adsorbing As(III)and Cd(II)in solutions.Compared with BC,12 kinds of MBC significantly improved the simultaneous adsorption performances for As(III)and Cd(II)in solutions,with theoretical adsorption capacities ranging from 25.13 to 97.13 mg/g and 23.95 to 77.29 mg/g,respectively.MBC,the adsorption capacities of Fh@PMBC for As(III)and LDHs-Si@PMBC for and Cd(II)were 97.13mg/g and 77.29mg/g,respectively.Compared to other studies,the MBC in this study has better As(III)and Cd(II)adsorption capacities.As mentioned above,we look forward to PMBC,and its MBC has excellent potentials to remediate As and Cd cocontaminated soils.The chemical analysis and characteristics indicated that iron compounds were successfully loaded on MBC,and the specific surface areas and pore volumes of the modified materials were increased after the modification.FTIR and the XRD analysis showed that iron(hydro)oxides or layer double hydroxides were formed on the surface or in the pore of MBC.These modifications provided more active sites for As(III)and Cd(II)adsorption.Among The results of p H,adsorption isotherm,competitive ions,desorption,redox,and characterizations showed that synchronous adsorption mechanisms for As(III)and Cd(II)by MBC involved the surface complexation,inner layer complexation,ion exchange,redox,and electrostatic adsorption,and so on.3)Potentials of simultaneous passivation for As and Cd co-contaminated soils by the modified biochars and their corresponding mechanisms.In this study,12 kinds of MBC were selected and prepared to verify their simultaneous passivation effects and mechanisms on As and Cd co-contaminated paddy soils through the flooding culture experiments and material stability test for 160-days.The concentration of As in the porewater increased with the enhancement of soil reducibility.When pe+p H <5,the concentration of As in the porewater increased sharply and reached the peak value of about 1000μg/L.Simultaneously,there is an excellent linear relationship between Fe and As in the porewater,indicating that the dissolution of Fe and As in soils might be synchronous.This phenomenon may be caused by the reduction and dissolution of Asbearing iron minerals under strong reduction conditions.However,with the extension of culture time,soil reducibility enhanced,the concentration of Cd in the porewater showed an increasing trend at first and then decreasing trend.During the culture process,the inflection points of Cd concentration in the porewater were also around pe+p H=5.When pe+p H<5,the Cd in the porewater declined dramatically,related to the combination of more sulfide and iron/manganese(hydr)oxides in the paddy soils.BC(except PMBC)among all treatments increased the activity of As in paddy soils and promoted the migration from soil to porewater.According to results,we found that MBC could synchronously inhibit the release of As and Cd into the porewater from paddy soils,which indicated that iron and its compounds loaded on the MBC played vital roles in the fixation of As in paddy soils.Culture experiments confirmed that both BC(except CSBC)and MBC could significantly improve the soil p H(P < 0.05).The LDHs-Si@BC improve the soil p H apparently,which increased the soil p H by 0.62～0.70 units.PMBC and its MBC can raise soil p H by about 0.3 units.Besides,BC and MBC can also effectively increase soil SOC contents,approximately 1.15～5.48g/kg.The results of BCR analysis showed that the applications of 5‰ BC or MBC might cause the conversion of Cd from soluble fraction to organically-bound and sulfide fraction due to the combination of available Cd with organic matter from BC or MBC in the paddy soils.According to the stability tests of MBC,soluble organic composition and iron(hydrogen)oxides on MBC changed in paddy soils.MBC increased the concentration of DOC and changed the structure and composition of DOM in soil solution,such as the increase of fulvic acid,and thus further affected the activities of As and Cd in the co-contaminated paddy soils.During the culture process,amorphous iron(hydrogen)oxides on Fh@PMBC were transformed into crystalline iron(hydrogen)oxides,which might provide much more interfaces for fixing As and Cd in paddy soils.Besides,It’s worth noting that the collapse of the layered structures of LDHs@BC and LDHs-Si@BC could speculate that the layered structures do not play a role in the synchronous passivation of As and Cd in paddy soils.4)Inhibitory effect of the modified biochars on As and Cd accumulation in rice.In this study,paddy rice was selected for the pot and field experiments.The impacts of MBC on soil physical and chemical properties,As and Cd in porewater,As and Cd fractions in soils,and As(i As)and Cd accumulation in rice were analyzed to explore the passivating effects and remediation effects.In the soil-plant systems,the peak concentration of As in the porewater under the treatments of Fh@MWBC,Fh@WSBC,Fh@CSBC,and Fh@PMBC decreased from 1032 μg/L to 573 μg/L,697 μg/L,588 μg/L,and 698 μg/L,respectively,indicating that four kinds of Fh@BC could inhibit As availability in paddy soils.The applications of WSBC,CSBC,and PMBC to paddy soils slightly increased the peak concentration of As in the porewater from 1032 μg/L to 1128 μg/L,1181 μg/L,and1074 μg/L,respectively.The applications of PMBC,WSBC,CSBC,and MWBC to paddy soils reduced the concentration of Cd in the porewater from 18.13 μg/L to 15.73μg/L,17.13 μg/L,15.40 μg/L,and 15.33 μg/L,respectively.In terms of As in rice,both BC(except for CSBC)and MBC could inhibit the As accumulation,Fh@PMBC had the highest inhibitory effect on the As accumulation,about 27.63%;Both BC and MBC significantly reduced the inorganic arsenic(i As)contents in the rice(P < 0.05),and BC had a more significant inhibitory effect on the i As accumulation in the rice compared to MBC.The contents of rice i As under the applications of PMBC,CSBC,and LDHsSi@MWBC were 0.19 mg/kg,0.19 mg/kg,and 0.15 mg/kg,respectively,which were slightly lower than the standard limits of 0.2 mg/kg recommended by the GB 2762-2017.For rice Cd,both BC and MBC treatments could significantly reduce Cd contents in the rice(P < 0.05),and the contents meet the standard limits of 0.2 mg/kg recommended by the GB 2762-2017.Among these BC and MBC,LDHs-Si@MWBC,WSBC,PMBC,and CSBC had the highest inhibitory effects on Cd accumulation in the rice,about 60%.Combined with the pot and field experiments,results showed that both PMBC and its MBC could effectively inhibit the i As and Cd accumulation in the rice,indicating that they had a particular potential to remediate paddy soils contaminated by As and Cd.However,it is worth noting that LDHs-Si@BC has an adverse effect on the growths of the roots,stems,and leaves,and grains of paddy rice.Thereby,it is not recommended to be used as a remediation material for the As and Cd co-contaminated paddy soils.In contrast,Fh@BC,with a simple and easy modification method,has a good inhibition effect on As and Cd accumulation in the rice.Thereby,Fh@BC can be recommended as a passivation material for the remediation of As and Cd co-contaminated paddy soils. |
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