| Cadmium(Cd)contamination of paddy soil had the characteristics of high toxicity,easy accumulation and difficult removal,which posed a serious threat to the ecosystem and human health.Therefore,it was extremely important to develop an efficient and environmentally friendly remediation technology for Cd contamination in paddy soil.In recent years,ironbased nanomaterials had achieved satisfactory results in the field of heavy metal contamination remediation.Among them,the nanoscale zero valent iron(nZVI)had attracted widely attention due to its unique shell-core structure,large specific surface area,high reactivity,strong adsorption reduction performance and other advantages.However,nZVI had some disadvantages such as easy aggregation and oxidation in the environment,which reduced its reaction performance.In recent years,sulfidized nZVI(S-nZVI)was a common method used in nZVI modification.The shell of S-nZVI was a FeSx-dominated layer,which not only inhibited the oxidation of nZVI,but also promoted the transfer of electrons from the Fe0 core to the surface layer,enhancing the reactivity of nZVI.In addition,FeSx also could immobilize heavy metals in soil through adsorption,co-precipitation and ion exchange.Nevertheless,the agglomeration of S-nZVI was still inevitable owing to the hydrophobicity,magnetism and nano characteristics.Therefore,how to effectively inhibit the agglomeration of S-nZVI particles and further improve their dispersion in the environment and the remediation efficiency of heavy metals has been a hot issue in current research,where solid supporting technology is considered one of the most effective methods.Biochar(BC),with rich pore structure and a large number of oxygen functional groups,had a strong adsorption and complexation effect on heavy metals,and is often used as a carrier of nanomaterials for environmental contamination remediation.Herein,the BC-supported S-nZVI(S-nZVI/BC)composite nanomaterials were prepared with BC as the carrier for supported S-nZVI particles.The surface morphologies and structural composition of S-nZVI/BC were analyzed by scanning electron microscopy(SEM),transmission electron microscopy(TEM),fourier transform infrared spectroscopy(FTIR),and X-ray photoelectron spectroscopy(XPS).SnZVI/BC was applied to Cd-contaminated paddy soil,and the remediation effects of SnZVI/BC on the speciation,availability,the leaching toxicity,bioavailability and soil physicochemical properties of Cd were investigated for 49 days treatment.The magnetically separated S-nZVI/BC particles after remediation of Cd contaminated soils were subjected to SEM images and energy dispersive spectrometry(SEM-EDS),FTIR and X-ray diffraction(XRD)characterization analyses,which revealed the remediation mechanism of Cd by SnZVI/BC.The main research results were as follows:(1)The surface properties and structural composition of S-nZVI/BC.The SEM images showed that the nZVI nanoparticles were clustered in a spherically chained together,sulfurization caused the formation of an FeSx-based shell resulting in a rougher surface of SnZVI particles,and the dispersion of S-nZVI particles was significantly improved after BC supporting,which indicated that sulfurization modification and BC supporting effectively improved the agglomeration of nZVI.The TEM images demonstrated that the size of SnZVI/BC ranged from about 60 to 100 nm,and obvious shell-core structure could be able to found.In FTIR,S-nZVI/BC not only retained the unique BC functional groups of C=O,-CH2,C-O and C-H,but also appeared a new FeS2 peak.It was inferred from XPS analyssis that the surface shell of S-nZVI/BC is mainly composed of Fe0,FeS,FeS2,FeSn,sulfur oxides,and a small amount of iron oxide/hydroxide.The above characterization results indicated that SnZVI/BC composite nanomaterials had been successfully prepared.(2)The effects of S-nZVI/BC on Cd speciation,availability,leaching toxicity,and bioaccessibility during the remediation of Cd contaminated paddy soil.The results showed that the percentage of acid soluble speciation of Cd was decreased with time,while the percentages of oxidizable and residual speciation of Cd increased over time in each treatment group during treatment.After 49 days,S-nZVI/BC treatment reduced the percentage of acid soluble of Cd decreased by 36.05%and the percentage of residual of Cd increased by 30.82%compared with the control group,which showed that the application of S-nZVI/BC significantly reduced the mobility of Cd in the soil(P<0.05).The available content,leaching mass concentration,and bioextractable mass concentration of Cd in each treatment group decreased gradually with time.In the S-nZVI/BC treatment group,the available content,leaching mass concentration,and bioextractable mass concentration of Cd decreased from 2.14 mg·kg-1,0.075 mg·L-1 and 0.104 mg·L-1 to 1.70 mg·kg-1,0.035 mg·L-1 and 0.039 mg·L1 after 49 days,respectively,indicating that S-nZVI/BC significantly reduced the availability,leaching toxicity and bioavailability of Cd(P<0.05).(3)The effects of S-nZVI/BC composite nanomaterials applied to Cd contaminated paddy soil on its physicochemical properties.The results showed that the pH of the S-nZVI/BC treatment group increased to 8.36 on the first day,and then varied irregularly,it was remained 0.69 units higher than that of the control group after 49 days.Compared with the Eh of the control group(167 mV),the Eh of the S-nZVI/BC treatment decreased rapidly to-654 mV after the first day due to the hydrolysis and oxidation of Fe0,with a gradual transition from a strongly reduced state to an oxidized state over time,but its value was still lower than the control group after 49 days.The application of S-nZVI/BC increased the content of soil organic matter(OM),which gradually decreased during the restoration process due to the microorganisms consumption,but the content of OM was still 7.46 mg·kg-1 higher than that of the control group after 49 days.Correlation analysis of physicochemical properties,Cd speciations,DTPA,TCLP and PBET showed that the increase of pH could reduce the mobility of Cd and thus the environmental risk of Cd,while the opposite was true of Eh.Whereas the change of soil OM had limited effect on the Cd transformation.(4)The remediation mechanism of soil Cd by S-nZVI/BC composite nanomaterials.The SEM-EDS indicated that the surface of S-nZVI/BC became rougher after remediation,and the distribution of Cd was found to overlap significantly with Fe,O,and S,proving that Fe,O,and S were the key elements for Cd immobilization.The comparison of FTIR patterns before and after the remediation of S-nZVI/BC also showed that the functional groups of S-nZVI/BC after remediation increased and new Cd-O and Fe-O peaks appeared,while the XRD patterns showed that the peaks of Fe0 weakened or even disappeared after the remediation,and the peaks of iron oxide/hydroxide increased and enhanced.The characteristic peaks related to Cd appeared,including Cd(OH)2 and CdFe2O4.Based on the above results,it was speculated that the remediation mechanism of Cd mainly involved adsorption,complexation(FeOCd+,FeOCdOH and CdFe2O4)and precipitation(Cd(OH)2 and CdS).In this study,the remediation of Cd by S-nZVI/BC in contaminated paddy soil was explored from the preparation and characterization of composite nanomaterials,the remediation effect of Cd in paddy soil,the changes in physicochemical properties of paddy soil,and the mechanism of Cd immobilization.S-nZVI/BC composite nanomaterials was used as stabilizers for the remediation of Cd contaminated paddy soil in the study,promoting the practical application of modified nZVI nanomaterials in the field of soil heavy metal contamination remediation. |
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