Stabilization Mechanism And Risk Assessment For Remediation Of Arsenic Contaminated Soils Using Zero-Valent-Iron Embedded Biochar

Arsenic(As)contaminated soil has attracted continuous attention due to its wide distribution and risk.A variety of technologies have been used to remediate As contaminated soil,Among them,stabilization remediation technology has been widely used because of its low cost,short period,and easily operation characteristics.Biochar(BC),a carbonaceous material prepared by pyrolysis of waste biomass,has been proven as an effective and friendly amendment for remediation of heavy metal contaminated soil.However,BC is less effective for soil arsenic(As)immobilization in most cases and in contrast,mobility or bioavailability of As was enhanced with the addition of BC in some cases.In order to improve the arsenic immobilization ability of biochar,various iron modified biochar materials were used.Among them,zero-valent-iron loaded biochar(ZVI/BC)have been attracted wide attention in recent years.However,traditional ZVI/BC is difficult to be applied in large scale due to its high cost and complex preparation procedure.Meanwhile,As immobilization by ZVI/BC mainly focus on waste water,application of ZVI/BC in As-contaminated soil is limited.Meanwhile,there was little information about the process of rhizosphere interface between As and rice roots during the rice culture,long-term stability and human health risk assessment of As after remediation by ZVI/BC.Therefore,in our study,ZVI/BC was rapidly prepared through simple one-step pyrolysis of biomass sawdust and iron salt,and the As immobilization mechanisms associated with iron transformation was investigated in As contaminated soil amended with ZVI/BC.Meanwhile,the process of rhizosphere interface between rice roots and As after remediation during the rice culture was further studied.Finally,the long-term stability and human health risk assessment of the amended soil were carried out.The main results were as follows:(1)Iron-loaded biochar was prepared by pyrolysis sawdust which loaded different iron salt under different temperature.The results showed that Fe species in iron loaded biochar prepared by Fe Cl₃,Fe(NO₃)₃,and Fe₂O₃ loaded biomass in 500?was mainly Fe₂O₃ and Fe₃O₄.When pyrolysis temperature increased to 800?,Fe species in Fe Cl₃/BC800 prepared by Fe Cl₃ loaded biomass was mainly Fe₃C.However,Fe species was mainly Fe⁰ in Fe(NO₃)₃/BC800 and Fe₂O₃/BC800 prepared by Fe(NO₃)₃and Fe₂O₃loaded biomass,respectively.According to the iron species transformation under different pyrolysis conditions,Fe was first transformed into Fe₂O₃ and then to Fe₃O₄,and then reduced to Fe⁰ with the formation of Fe₃C at 800?.Among Fe(NO₃)₃/BC800 and Fe₂O₃/BC800,Fe₂O₃/BC800 presented the higher As adsorption capacity and better stabilization of As contaminated soil.Therefore,zero valent iron loaded biochar could been rapidly prepared through one-pot pyrolysis of Fe₂O₃ loaded biomass,and the prepared Fe₂O₃/BC800(ZVI/BC)was a potential remediation amends for As contaminated soil.(2)Two mining As-contaminated soils were collected from Guangxi and Jiangxi Provinces,respectively and amended with ZVI/BC.Pristine sawdust biochar(BC)and ZVI alone or in combination(BC+ZVI)were included for treatment comparison.Results show that All treatments except BC reduced As concentration in(NH₄)₂SO₄extraction and the remediation effect presented as ZVI<BC+ZVI<ZVI/BC.Particularly,ZVI/BC reduced the labile As in GX and JX soils by 93.7%and 97.7%,respectively.Fe⁰ on the surface of ZVI/BC was oxidized into amorphous Fe OOH which adsorbed or co-precipitated with As.Oxidation of ZVI/BC mainly occurred on the exterior of biochar porous,accompanied with which,a portion of As(VI)was reduced into As(III).Meanwhile,Ca-Fe-As-O and Al-Fe-As-O co-precipitated at the interface between ZVI/BC and two soils enriched with Ca and Al,respectively.(3)As accumulation in rice and the dynamics and transfer kinetics of As in paddy soil effected by rice using DGT and DIFS model during rice culture after ZVI/BC remediation was detected.Results show that there was no significant change of As content in rice after BC remediation compare to that culture in unamended soil.However,ZVI,BC+ZVI,and ZVI/BC treated soils decreased As concentration by4.31%,41.2%,and 72.5%in rice roots and 16.3%,51.3%,and 73.4%in rice straw,which was related to the lower As concentration in porewater after remediation.Although As accumulation in rice decrease after remediation,As concentration in porewater in rhizosphere soil were higher than that in bulk soil,which increased by102%,52.9%and 50.0%in ZVI,BC+ZVI,and ZVI/BC treated soils,respectively.It is worth noting that the concentration of As(V)and Fe(II)in rhizosphere soil solution were higher than that in bulk soil solution.Combine As(V),As(III)and Fe species in pore water and microbial community structure in both rhizosphere and bulk soil,higher As concentration in rhizosphere porewater attributed to the As released from solid phase under As(V)and Fe(III)reduction bacterias including Anaeromyxobacter,Pseudomonsa,Streptomyces,Geobacter and Clostridium.Meanwhile,As desorption from solid phase in rhizosphere soil was faster and easier than bulk soil according to the higher R and lower T_c which calculated from DIFS model.The desorption rate constant,k_-1,increased by 6 fold in rhizosphere soil than bulk soil in ZVI/BC treatment.Therefore,it is necessary to pay attention to the potential risk of As remobilization in soils induced by the change of rhizosphere characteristics during the rice cultural after remediation using amend materials contained ZVI.(4)The long-term stability of As contaminated soil after stabilization remediation was evaluated by simulating natural aging process through dry-wet and freeze-thaw cycles.The results showed that As concentration in(NH₄)₂SO₄ extraction in soil did not change significantly in unamend soil and BC amended soil during aging process.However,labile As increased gradually and tended to stable during the wet-dry aging process in ZVI,BC+ZVI and ZVI/BC amended soils.For example,labile As in ZVI/BC treated GX and HN soil increased by 39.48%and 46.4%after 35 cycles of wet-dry aging,respectively.In contrast,labile As decreased gradually and tended to stable during the freeze-thaw aging process.For example,(NH₄)₂SO₄ extractable As in ZVI/BC treated GX and HN soil decreased by 42.9%and 64.0%after 35 cycles of freeze-thaw aging,respectively.For ZVI,BC+ZVI and ZVI/BC treated soils,the proportion of amorphous iron increased after dry-wet aging while the crystalline iron increased after freeze-thaw aging.Meanwhile,with the oxidation of Fe⁰ in ZVI/BC during the aging process,the As(V)adsorbed on ZVI/BC was further reduced to As(III).Therefore,the different aging process have different effect on the long-term stability of As after remediation by ZVI/BC,and the labile As tend to be stable eventually.(6)The bioaccessibility of As in GX soil and JX soil after remediation was evaluated by in vitro assay.The results showed that As bioaccessibility in GX and JX soils were increased after BC treated compared to untreated soil.However,As bioaccessibility decreased after ZVI,BC+ZVI and ZVI/BC remediation,and the decreasing degree of As bioaccessibility were ZVI<BC+ZVI<ZVI/BC.As bioaccessibility in gastric phase in GX soil and JX soil amended by ZVI/BC decreased by 88.6%and 85.4%,respectively,and decreased in intestine phase by 85.6%and 90.5%,respectively.As mainly existed as As(V)and As(III)in gastrointestinal solution with As(V)as the main form.The hazard entropy(HQ)and carcinogenic coefficient(CR)of As in both untreated soils and BC treated soils were higher than 1and 10^-4,respectively,which indicated that As had health risk and carcinogenic risk to both children and adults.The health risk of As in soils decreased after ZVI and BC+ZVI amended.However,As still had carcinogenic risk in acidic gastric environment.For ZVI/BC treated GX soil and JX soil,the HQ decreased to 0.14 and0.15 in gastric and 0.18 and 0.09 in intestine,respectively.Meanwhile,CR was lower than 10^-4 in both soils,suggesting that there was no health risk and carcinogenic risk of As in the two soils after ZVI/BC remediation for children and adults.In summary,zero valent iron loaded(ZVI/BC)could been prepared through simple one-step pyrolysis of biomass sawdust and Fe₂O₃ mixture at 800?.The prepared ZVI/BC showed effective As immobilization through adsorption and co-precipitation between As and iron oxides like as Fe OOH which formed from ZVI.Therefore,the prepared ZVI/BC was a potential and economic remediation material for As contaminated soil.However,it is necessary to pay attention to the potential risk of As remobilization induced by the rice roots during the rice cultural after remediation by ZVI/BC.For the long-term stability of As after amended by ZVI/BC,different environmental aging process have different effects on As bioavailability and the bioavailability of As tends to be stable during aging.Meanwhile,there was no health risk and carcinogenic risk for children and adults after ZVI/BC amendment even for high As contaminated soils.