Study On Competitive Adsorption Mechanism Of Arsenic,Antimony,and Phosphorus On Typical Goethite

In recent years,arsenic（As）and antimony（Sb）in the environment have become two representative metal-like elements in ecological risk research due to their diversity of valence,binding,and toxicity.In addition,As and Sb often coexist in the environment and are interfered by other coexisting ions,which further complicates the pollution status,existence form,and regression behavior.Goethite,as a widely distributed and stable iron oxide hydrate in nature,is often used for pollutant removal,which has an important effect on the migration and transformation behavior of As and Sb in polluted environment.In this study,the adsorption properties of goethite with different crystal surface exposure ratios for arsenic（As（Ⅴ））and antimony（Sb（Ⅴ））were studied.The adsorption mechanism of goethite for As（Ⅴ）and Sb（Ⅴ）was preliminarily explored by analyzing the effects of different pH,initial concentration,adsorption time,and co-existing phosphates.Secondly,the competitive adsorption relationships of As（Ⅴ）,Sb（Ⅴ）,and pentavalent phosphorus（P（Ⅴ））on goethite were investigated by using the CD-MUSIC model and experimental data,and the potential competitive adsorption mechanism was studied by using characterization analysis technique and density functional theory calculation.Finally,through the competitive adsorption experiment and CD-MUSIC model simulation of the three under different pH,as well as the competitive adsorption kinetics under different pH,a comprehensive characterization analysis of the change mechanism of the competitive adsorption relationship between the three under different pH was carried out.The results of this study provide a theoretical basis for understanding and predicting the behavior of fixation,migration and existence of As（Ⅴ）,Sb（Ⅴ）,and P（Ⅴ）in the environment under the coexistence system.The main research results are as follows:（1）By controlling the addition rate and aging time of sodium hydroxide（Na OH）,Goe-A and Goe-B with different{021}/{110}crystal surface exposure ratios were prepared.The specific surface areas were 92.76 m²·g^-1 and 46.78 m²·g^-1,respectively,and the exposure ratio of{021}/{110}crystal surface was 1.8%and 4.9%,respectively.According to the isothermal adsorption and kinetic experiments,the adsorption of As（Ⅴ）and Sb（Ⅴ）on the surface of goethite is more consistent with Freundlich and the quasi-second-order kinetic model,indicating that the adsorption process is mainly composed of multiple molecular layers and chemical adsorption.After normalized calculation of specific surface area,it is found that the adsorption capacity of goethite containing different proportions of{021}/{110}crystal faces is different,which proves that different exposed crystal faces of goethite play a crucial role in the strength of its adsorption performance.The adsorption rates of As（Ⅴ）and Sb（Ⅴ）decreased significantly with the increase in pH.Moreover,the existence of co-existing phosphates will compete with As（Ⅴ）and Sb（Ⅴ）for adsorption sites,which seriously affects the adsorption process.XRD analysis shows that As（Ⅴ）and Sb（Ⅴ）are mainly adsorbed on the surface of goethite,but not combined into the crystal structure of goethite.The analysis of FTIR and XPS showed that As（Ⅴ）and Sb（Ⅴ）were mainly absorbed on the surface of the goethite by forming a complex（≡Fe-O-As/Sb）with≡Fe-OH on the surface of the goethite through ligand exchange.（2）To more accurately study the competitive adsorption behavior of As（Ⅴ）,Sb（Ⅴ）,and P（Ⅴ）on goethite,the isothermal adsorption of As（Ⅴ）,Sb（Ⅴ）,and P（Ⅴ）on goethite with main exposed{110}crystal surface was analyzed.The results showed that the saturated adsorption capacity（pH=7）of As（Ⅴ）,Sb（Ⅴ）,and P（Ⅴ）reached 0.205 mmol·g^-1,0.212 mmol·g^-1,and 0.209mmol·g^-1,respectively,and the adsorption behavior was consistent with Freundlich model,indicating that the adsorption process was dominated by multimolecular layer adsorption.With the increase of pH,the adsorption rate of the three decreased from about 98%to about 8%.Under acidic and neutral conditions,when the initial concentration is 0.04 mmol·L^-1,the adsorption rates of all three are about 92%.When the initial concentration was 0.16 mmol·L^-1,the adsorption rate decreased significantly with the increase of pH.The adsorption kinetics showed that the adsorption rates of As（Ⅴ）,Sb（Ⅴ）,and P（Ⅴ）were all about 98%in the single system at pH 4,but only 40.5%,33.5%,and 36.0%respectively at pH 10,indicating that the adsorption rates and adsorption capacity of goethite were seriously affected by the change of pH.In the ternary system,when pH is 4,As（Ⅴ）,Sb（Ⅴ）,and P（Ⅴ）are affected to the same degree,and their adsorption rates are 44.0%,38%,and 36.5%,respectively,and 38.5%,14.5%,and34.0%,respectively,when pH is 7.At pH 10,only 20.5%,3.5%,and 17.5%,respectively.Competition is further intensified by the effects of pH and coexisting ions in the ternary system compared to the single system.The experimental results of different adsorption order of As（Ⅴ）-Sb（Ⅴ）and P（Ⅴ）-Sb（Ⅴ）systems show that the addition of As（Ⅴ）and P（Ⅴ）will lead to the desorption rate of Sb（Ⅴ）is 20.3%and 17.3%.However,As（Ⅴ）and P（Ⅴ）did not desorption with the addition of Sb（Ⅴ）.In the As（Ⅴ）-P（Ⅴ）system,the two can desorption each other,and the desorption rates of As（Ⅴ）and P（Ⅴ）are 8.8%and 15.3%,respectively.Comprehensive analysis shows that the order of binding affinity between goethite and the three is As（Ⅴ）>P（Ⅴ）>Sb（Ⅴ）.（3）According to the results of characterization analysis,the content of≡Fe-OH decreases and the content of≡Fe-O increases in the adsorption process,indicating that a new complex is formed in the adsorption process（≡Fe-O-As/Sb/P）.As（Ⅴ）,Sb（Ⅴ）,and P（Ⅴ）showed different competitive abilities for goethite surface sites in the multiplex system.The main difference is that As（Ⅴ）and P（Ⅴ）can compete for both single and double dentate sites,while Sb（Ⅴ）mainly competes for double dentate sites.Therefore,the overall competitive adsorption capacity of As（Ⅴ）and P（Ⅴ）is greater than that of Sb（Ⅴ）.In addition,due to the different adsorption mechanisms and morphology of the three,the adsorption of Sb（Ⅴ）is more dependent on protonated hydroxyl sites,while As（Ⅴ）and P（Ⅴ）can still form bidentate and single-dentate complexes by deprotonating hydroxyl sites.Therefore,with the increase of pH value,the adsorption gap between the three gradually increases.Especially under neutral conditions,the adsorption effect of goethite on As（Ⅴ）and P（Ⅴ）is significantly greater than that of Sb（Ⅴ）.In terms of competitive adsorption at specific sites,As（Ⅴ）and Sb（Ⅴ）showed stronger competition at the bidentate sites,with about 40%of As（Ⅴ）and Sb（Ⅴ）adsorbed to the surface via the bidentate sites under acidic conditions（pH=4）,while only 25%of P（Ⅴ）was adsorbed this way.On the contrary,P（Ⅴ）showed stronger competition at the single tooth site.P（Ⅴ）is more inclined to occupy the single dentate site for adsorption in the case of intense competition between the two dentate sites,and this phenomenon is more obvious at neutral and basic pH conditions.The DFT calculation also confirmed that the protonated didentate complex of As（Ⅴ）has more stable adsorption energy and has an advantage in the competition with P（Ⅴ）.P（Ⅴ）mainly exists as monodon complex,and its monodon complex has higher adsorption energy than the monodon complex of As（Ⅴ）,which is close to the protonated bidentate complex of P（Ⅴ）,which also indicates that P（Ⅴ）exists more in the form of monodon complex in the coexistence system.