Interface Structure Control And Application Of Ferric Oxide-based Composite Materials For Sb(?) Removal In Textile Wastewater

In order to make the concentration of antimony in the textile wastewater after treatment reach the discharge requirement,the iron oxides with a high affinity for Sb(?)were selected as the applicable adsorbent based on the fact that the main form of antimony in textile wastewater was Sb(OH)₆^-(Sb(?)).Amorphous iron oxide was proved to have higher Sb(?)adsorption capacity among the normal iron oxide forms.Hence,the metal(Mn)oxides and nonmetallic(Si)oxides were doped into the amorphous iron oxide to adjust and control the forms of iron oxides.Then the sintering stabilization method and soft-template adjustment method were used to modify and adjust the structure and morphology of the amorphous iron oxide-based composites for further improving their Sb(?)adsorption performance.In this way,the better Sb(?)adsorption performance and the enhanced stability and separability of amorphous iron oxide-based composites were obtained under the influence of coexisting organic and inorganic substances.The amount of dissolved Fe was also decreased,which was beneficial for the subsequent advanced treatment of textile wastewater.This study provided several novel amorphous iron oxide-based composites for the efficient Sb(?)removal in the real textile wastewater.After comparing the Sb(?)adsorption capacity of crystal hematite,goethite,magnetite and amorphous ferrihydrite,the amorphous ferrihydrite was considered to have a higher Sb(?)adsorption capacity.Hence,in order to enhance the stability of amorphous iron oxide,Mn oxide was added into the iron oxide to prepare the Mn-doped iron oxide composite(FMBO).Results indicated that the addition of Mn oxide accelerated the formation of amorphous iron oxide in FMBO,retarded the phase transformation from amorphous iron oxide to crystal goethite,and improved the stability of amorphous phase to some extent.Thus,the Sb(?)adsorption capacity of FMBO was improved after the doping of Mn.As compared to other FMBO,FMBO₃with Fe/Mn molar ratios of 3:1 exhibited the larger content of amorphous Fe(43.7%),the higher adsorption rate and capacity,the wider range of adaptable pH(5 to 9),and the lower influence of coexisting substance.Hence,Fe/Mn molar ratio of 3:1 was determined as the optimal doping ratio in FMBO.When initial Sb(?)concentration was about 20 mg/L,the highest Sb(?)adsorption capacity of FMBO₃ was 118 mg/g.The residual Sb(?)concentration in the real textile wastewater could meet the discharge standard after treated by FMBO₃ with dosage of 0.1 g/L.Additionally,the ability of dynamic adsorption and cyclic adsorption-desorption towards Sb(?)by FMBO₃ was strong.However,due to the dissolution and reprecipitation of Fe oxide in FMBO₃,the amorphous iron oxide gradually transformed to crystal goethite,which could decrease the Sb(?)adsorption capacity of FMBO₃.To further retard the crystal transformation of amorphous Fe oxide during adsorption,sintering stabilization method was used for the further modification of FMBO(SFMC).With the increased sintering temperature and time,the reversible dehydroxylation became nonreversible after the crystal transformation,while the surface area,total pore volume,and the content of amorphous Fe oxide in SFMC was firstly increased and then decreased.Compared with other SFMC,the composites under the sintering temperature of 350oC and sintering time of 2 h possessed the larger surface area and total pore volume,the higher Sb(?)adsorption capacity,and the lower cost,which was determined as the optimal SFMC.The highest Sb(?)adsorption capacity of optimal SFMC was 95 mg/g.As compared to the unsintered pristine FMBC,although the Sb(?)adsorption rate and capacity of optimal SFMC was slightly decreased,optimal SFMC exhibited better separation performance and stability,a 60%reduction in the amount of dissolved Fe and Mn,and higher cyclic adsorption efficiency.Additionally,the phase transformation rate and extent in optimal SFMC was also greatly retarded.SFMC could make the Sb(?)concentration in the real textile wastewater attain the discharge standards after the dynamic and static adsorption.Since the iron oxide composite was still aggregated after doping with Mn,CTAB,Brij-58,and SDS were used as the soft-template to further adjust the structure and morphology of Mn-doped iron oxide composite(SMFO).Sintering method was also used to remove the residual templates and enhance the stability of the composites.During the preparation,the addition of soft-templates changed the size,structure,and aggregation of particles,which greatly increased the surface area of SMFO.Especially for the SMFO-B adjusted by Brij-58,more porous pores were generated,the size of particle aggregates was greatly decreased,and the aggregation extent of particles was remarkably decreased.Hence,the surface area of SMFO-B was higher than other composites,reached to 137.6 m²/g,10 times higher than that of MFOC without the addition of template.These changes were beneficial for generating and exposing more functional hydroxy groups.The isoelectric point of SMFO-B was also improved,contributing to the higher adsorption capacity and rate of SMFO-B.The highest Sb(?)adsorption capacity of SMFO-B was 147 mg/g,2 times higher than that of MFOC.In addition,SMFO-B had a wider adaptable pH range(5-10)and a stronger capacity to reduce the impact of coexisting substances.After the adsorption by SMFO-B(0.1 g/L),the Sb(?)concentration in the real textile wastewater was 25?g/L,much lower than the discharge standard.Hence,the improvement effect of SMFO adjusted by Brij-58was the best.The dynamic adsorption capacity and the cyclic adsorption-desorption performance of SMFO-B towards Sb(?)were also excellent.According to the related theory and experience of Mn-doped iron oxide composite,the nonmetallic(Si)oxide was doped into the amorphous ferrihydrite to improve its related properties(SFOC).A proper amount of added Si oxide could retard the crystal growth of Si and Fe oxide particles in the SFOC,enhance the stability of amorphous iron oxide,and decrease the size of formed particles.Meanwhile,the pore structure was generated and the aggregation extent of particles in SFOC was greatly decreased.Hence,the surface area and pore volume of SFOC were greatly improved and more functional Fe hydroxyl groups were generated on the surface.In this way,the Sb(?)adsorption performance of SFOC was further improved.As compared to pristine ferrihydrite and other SFOC,SFOC₁₀ with Fe/Si molar ratio of 10:1 exhibited the highest surface area of 364 m²/g,which was 1.7 times higher than that of pristine ferrihydrite.SFOC₁₀ also had a larger amount of exposed functional Fe-OH and the higher adsorption capacity(176 mg/g)and rate towards Sb(?).Therefore,Fe/Si molar ratio of 10:1 was determined as the optimal doping ratio in SFOC.The Sb(?)removal efficiency by SFOC₁₀ could reach to 90%when the concentration of coexisting substance in the simulated wastewater were equal to their actual concentration.Under the dosage of 0.1 g/L,the Sb(?)removal efficiency by SFOC₁₀ in the real textile wastewater was 83.6%,20%higher than that of pristine ferrihydrite,which had met the standard discharge.After 5cycles,the Sb(?)adsorption and desorption efficiencies of SFOC₁₀ were still more than85%.Little amount of amorphous iron oxide in SFOC₁₀ was transformed during cyclic adsorption,hence the crystal stability of SFOC₁₀ was better than Mn doped iron oxides.The Sb(?)adsorption process by FMBO₃,optimal SFMC,SMFO-B and SFOC₁₀could be better fitted by Freundlich model and pseudo-second order kinetics model.The amorphous iron oxide in these composites played a main role in the Sb(?)adsorption and the surface functional Fe-OH groups were the dominant adsorption sites.The coordination reaction between Sb(OH)₆^–and the functional Fe-OH was the main adsorption mechanism.The analysis of extended X-ray absorption fine structure indicated two main forms of the inner-sphere complex structure.One was the bidentate mononuclear edge-sharing(²E)complex,the interatomic distance of Sb-Fe was about3.03-3.11?;the other one was the bidentate binuclear corner-sharing(²C)complex,the interatomic distance of Sb-Fe was about 3.52-3.60?.The inner-sphere complexation adsorption belonged to selective adsorption,which guaranteed that the influence of coexisting substances on the Sb(?)adsorption by amorphous iron oxide-based composite was relatively weak.Hence,Cl^-,NO₃^-,CH₃COO^-,CTAB and insoluble disperse dyes had limited influence on the Sb(?)adsorption.Nevertheless,surfactants and dyes with sulfonic groups and the anions like SO₄^2-,CO₃^2-,and PO₄^3-could be adsorbed on iron oxide through inner-sphere complexation and competed with Sb(?)for adsorption sites,which had a negative impact on the Sb(?)removal.All these amorphous iron oxide-based composites proposed in this study could be selected for Sb(?)adsorption according to the treatment requirements of the real textile wastewater.The low-cost Mn-doped Fe oxide could be directly added into the wastewater and the stable Si-doped Fe oxide was suitable for cyclic adsorption-desorption.