Immobilization Of Cadmium And Arsenic In Paddy Soil By Iron-cotaining Materials

Cadmium（Cd）and arsenic（As）co-contaminated soils are distributed in some paddy fields in southern China,which often lead to Cd and As in brown rice exceeding the limits of the national food safety standardand,bringing potential risks to people’s health.How to simultaneously reduce the content of Cd and As in brown rice has become a hard nut to crack because of converse chemical behaviors of Cd and As in paddy soils.In this study,Cd and As co-contaminated soils was selected as the research object.The amendments that can simultaneously immobilize Cd and As in soil were screened out by an pure soil culture experiment.On this basis,pot experiments were carried out to investigate the effects of different iron-containing materials on soil p H and Eh,Cd and As concentrations in pore water,Cd and As fractions in soil as well as Cd and As uptake by rice,aiming to provide reference for remediation of Cd and As co-contaminated paddy soils.The main results are as follows:（1）The effects of sepiolite（Sep）,modified sepiolite（IMS and Sep-FM）and slag（SS）on soil p H,Eh,Cd and As concentrations in soil pore water,and Cd and As fractions in soil were investigated by an pure soil culture experiment.It showed that SEP,IMS,Sep-FM and SS treatments increased soil p H value,and decreased soil Eh value and Cd concentration in soil solution.Compared with control,application of the above amendments promoted the transformation of Cd fraction from exchangeable fraction to that plants are difficult to uptake,i.e.the proportion of Cd in exchangeable fraction decreased,while that in reducible fraction,oxidizable fraction and residual fraction increased.The As concentration in pore water treated with 2.5%IMS or 5%SS was at a low value during the culture period.All IMS and Sep-FM treatments reduced the non-specific adsorbed As fraction in soil,and the high application rates of IMS（2.5%）,Sep-FM（2.5%）and SS（5%）treatments also significantly reduced the specific adsorbed As fraction as well as increased amorphous and poorly crystalline hydrated Fe and Al oxide-bound As.In short,IMS,Sep-FM and SS can effectively immobilize soil Cd and As,and promote their transformation to the fraction that plants are difficult to uptake.（2）The effects of goethite（GT）and sepiolite（SEP）using alone or combined（0.5SEP+0.5GT,SEP+GT）on Cd and As concentrations in soil pore water,Cd and As accumulation in rice,and Cd,As fractions in soil were studied by employing a pot experiment.The results showed that the Cd concentration in pore water was reduced by adding amendments,among which SEP+GT was the most effective,followed by SEP and0.5SEP+0.5GT treatments.Compared with control,goethite treatment alone or goethite combined with sepiolite treatment reduced As concentration in pore water,while SEP increased As concentration.Among them,GT had the best effect,followed by SEP+GT and 0.5SEP+0.5GT treatment.Application of amendments increased the ratio of residual Cd in soil,they also significantly inhibited the Cd uptake by rice,and reduced brown rice Cd contents by 40.0%～72.0%.GT,0.5SEP+0.5GT and SEP+GT treatments reduced the fraction of crystalline iron oxide As and increased residual As.0.5SEP+0.5GT treatment significantly reduced the As content in brown rice（p<0.05）,while other treatments had no significant difference with control（p>0.05）.In a conclusion,0.5SEP+0.5GT can reduce Cd and As contents in brown rice simultaneously.（3）A pot experiment was conducted to evaluate the effect of modified sepiolites on immobilizing Cd and As in paddy soil.The specific surface area（BET）and adsorption sites of sepiolite were significantly increased by iron modification and iron-manganese modification.Compared with natural sepiolite（SEP）,the surface roughness of iron modified sepiolite（NIMS）and iron-manganese modified sepiolite（FMS）obviously increased,and large number of irregular particles were loaded on the surface.Sepiolite and its modified materials adsorbed Cd in the form of Cd（OH）₂.Mn（Ⅳ）in FMS can oxidize As（Ⅲ）to As（Ⅴ）,which enhances the adsorption of As.Applying SEP,NIMS and FMS into soil all significantly reduced the Cd concentration in pore water,and NIMS had the best effect.The amendments promoted Cd fraction in soil converting to not readily available to plants.Adding SEP,NIMS and FMS into soil inhibited the Cd uptake by rice,and significantly reduced the Cd content in brown rice.The order of reducing effect was FMS>NIMS>SEP.NIMS and FMS treatment promoted the transformation of soil As fraction to crystalline iron oxides,so that they can restrict the Cd mobility and bioavailability.Compared with the control,NIMS and FMS reduced the As content of brown rice by 30.4%and 24.8%,respectively.In conclusion,NIMS and FMS reduced the content of Cd and As in brown rice at the same time,and FMS was better than NIMS.（4）The effect of one-time application of modified sepiolite on the uptake of heavy metals by rice in the second year were investigated.It showed that one-time application of SEP,NIMS and FMS still reduced the fraction of exchangeable Cd,and promoted the convertion of soil Cd fraction to Fe and Mn oxide bound Cd,organic bound Cd and residual Cd.In the second year,SEP,NIMS and FMS treatment still significantly inhibited the uptake of Cd by rice,and reduced the Cd content in brown rice by 21.2%～66.3%,Among them,FMS treatment had the best reduction effect,followed by SEP and NIMS.One-time application of NIMS and FMS had no significant impact on the As content of brown rice in the second year compared with control（p>0.05）.