| The pollution of hexavalent chromium（Cr（Ⅵ））in soil is becoming more and more serious.Cr（Ⅵ）has strong migration in soil and has carcinogenic effect,while trivalent chromium（Cr（Ⅲ））is a trace element necessary for human body.The soil can exist as a stable hydroxide,and the Cr（Ⅵ）is reduced to Cr（Ⅲ）and fixed by adsorption,precipitation,etc.,which is a common Cr（Ⅵ）contaminated soil treatment method.Ferrous sulfide（FeS）nanoparticles can provide both Fe（II）and S（-II）reducing components as electron donors,and transfer electrons to Cr（Ⅵ）,which can improve material use efficiency and reduce investment.Addition,in recent years,has proven to be a new material for the efficient treatment of Cr（Ⅵ）contaminated soil.However,defects such as easy agglomeration and oxidative deactivation make FeS nanoparticles limited in practical engineering applications.It is of great research significance and application value to modify FeS nanoparticles to enhance their dispersibility and oxidation resistance.Sodium alginate（SA）is a biodegradable polymer with a large amount of carboxyl and hydroxyl groups in the molecule,which can play a good steric hindrance and help to enhance the dispersion and oxidation resistance of the nanoparticles.Therefore,the paper studied the modified FeS nanoparticles with SA as a coating agent,and prepared SA-FeS nanoparticles with high dispersibility and oxidation resistance by co-precipitation method.The preparation and characterization of particles,particle stability,factors affecting Cr（Ⅵ）removal and reaction mechanism were studied.On the basis of this,the treatment properties of SA-FeS nanoparticles on Cr（Ⅵ）contaminated soil were discussed.It provides theoretical support for the application of SA-FeS nanoparticles as active materials in Cr（Ⅵ）contaminated soil treatment projects.Mainly got the following conclusions:（1）The preparation,characterization and stability of SA-FeS nanoparticles were studied.The effects of SA dosage and preparation temperature on the removal performance of Cr（Ⅵ）were investigated by batch experiments.The structural characteristics of SA-FeS nanoparticles were characterized by SEM,TEM,XRD and FT-IR.The sedimentation experiments and passivation were carried out.The sedimentation distance of FeS and SA-FeS nanoparticles and the removal effect of Cr（Ⅵ）in each time period were compared.The results showed that the optimum SA dosage and temperature for the preparation of SA-FeS nanoparticles were 0.20 wt%and 35°C,respectively.The removal rate of Cr（Ⅵ）by SA-FeS nanoparticles is 18.58%higher than that of unmodified FeS nanoparticles,SA-FeS nanoparticles have a spherical or ellipsoidal shape with a particle size of about 100 nm.SA mainly binds to the FeS nanoparticles through the double-tooth bridging of the carboxyl groups in the molecule and the intermolecular hydrogen bonds of the hydroxyl groups,and coats the surface of the nanoparticles.Electrostatic repulsion,steric hindrance and oxygen barrier make it more dispersible and oxidation resistant.（2）The factors affecting the removal of Cr（Ⅵ）by SA-FeS nanoparticles were investigated.The effects of dosage of SA-FeS nanoparticles,initial concentration of Cr（Ⅵ）,reaction temperature,initial pH and humic acid on Cr（Ⅵ）removal performance were investigated by batch experiments.The results show that increasing the dosage of SA-FeS nanoparticles,decreasing the initial concentration of Cr（Ⅵ）and decreasing the initial pH will promote the removal of Cr（Ⅵ）.In the range of 15°C-30°C,increase the temperature to Cr（Ⅵ）.The removal rate has no obvious effect.The presence of humic acid（HA）will compete with Cr（Ⅵ）to adsorb the surface active sites of nanoparticles and reduce the removal rate of Cr（Ⅵ）.（3）The reaction kinetics and reaction mechanism of SA-FeS nanoparticles with Cr（Ⅵ）were discussed.The experimental data were fitted by reaction kinetics model,and the parameters such as apparent rate constant and half-life were obtained.The phase and element changes of the reaction products were characterized by XRD and XPS.The results show that the process of reducing Cr（Ⅵ）by SA-FeS nanoparticles conforms to the first-order reaction kinetics equation.Increasing the dosage of nanoparticles,increasing the reaction temperature,lowering the initial concentration of Cr（Ⅵ）and initial pH are all beneficial to improve the reaction.Rate;the reaction of SA-FeS nanoparticles with Cr（Ⅵ）is mainly based on reduction,and with a small amount of adsorption,Cr（Ⅵ）is reduced to more stable forms of Cr2O3 and Cr（OH）3,and also produces Fe（Ⅲ）-Cr（Ⅲ）coprecipitation adsorbed on the surface of nanoparticles,and Fe（II）and S（-II）were oxidized to Fe（OH）3 and elemental S,respectively.（4）The reaction parameters of SA（Fe）nanoparticles treated with Cr（Ⅵ）contaminated soil were optimized.The effects of the dosage of SA-FeS nanoparticles and the presence of humic acid on the treatment properties of Cr（Ⅵ）contaminated soil were investigated by batch experiments.The results show that the molar ratio of SA-FeS nanoparticles to Cr（Ⅵ）=3.0:1 is the optimal nanoparticle dosage;the presence of humic acid（HA）will compete with Cr（Ⅵ）for the adsorption of nanoparticle surface active sites.Reduce the treatment effect of Cr（Ⅵ）contaminated soil.（5）The treatment properties of SA-FeS nanoparticles on Cr（Ⅵ）contaminated soil were studied.The effects of SA-FeS nanoparticles and FeSO4 solution on Cr（Ⅵ）contaminated soil and the changes of soil pH and Cr morphology before and after treatment were compared by comparison experiments.The results show that the SA-FeS nanoparticles show better treatment ability for Cr（Ⅵ）-contaminated soil.The pH of the soil treated with SA-FeS nanoparticles does not change much,but the soil treated with FeSO4 solution is obviously acidified.Treatment of Cr（Ⅵ）contaminated soil with SA-FeS nanoparticles can convert high-yield exchangeable（EX）and carbonate-bound（CB）into more stable Fe-Mn oxidation（OX）and The organic binding state（OM）can effectively fix the chromium element in the soil and reduce the probability of the generated Cr（Ⅲ）being reoxidized. |
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