Removal Of Nitrate And Phosphate From Aqueous Solution And Immobilization Of Tungsten And Chromium In Soil By FeCl₄^-/Gemini Surfactant-modified Montmorillonite

Water eutrophication and soil contamination by heavy metals are important environmental issues.To effectively and simultaneously separate and/or immobilize multiple pollutants in complex environmental systems,such as nitrogen and phosphorus in water and tungsten and chromium in soil,has been a leading direction of water and soil pollution prevention and control technology.In order to expand the application of environmental functional materials to meet the need of the efficient treatment of complex environmental systems,the design of multi-functional material and its application in remediation of complicated environmental systems attracts more and more attention.In this work,the research objective has shifted from the remediation of single pollutants in an environmental system to the treatment of multiple pollutants in several environmental systems.Gemini surfactant-modified montmorillonite（Mt）with tetrachloroferrate（FeCl4^-）（FeOMt）was prepared by ex-situ modification of Mt with butane-1,4-bis（dodecyl dimethyl ammonium bromide）（g BDDA）and FeCl4^-,respectively.The physicochemical characteristics and structural changes of FeOMt in the modification process were analyzed,and its application performance and mechanism in the removal of nitrate and phosphate from aqueous solution and immobilization of tungsten and chromium in soil were studied.The main research results are summarized as below:（1）The characterization of XRD,Raman,FTIR,TG,XPS,and CS indicates the successful intercalation of g BDDA and loading of FeCl4^-in FeOMt.Accompanied with the sequential modification of Mt by two modifiers,the interlayer space was gradually expanded with the increase of the filling density,decrease of the specific surface area,and enhancement of the hydrophobicity.The stable layered structure was maintained for FeOMt similar to that of the original Mt.The paraffin-type configuration of g BDDA was proposed in the interlayer space of FeOMt,where a small fraction of g BDDA was anchored on the Mt surface with one quaternary ammonium head group（-R4N⁺）neutralized the negative charge of Mt,while the other existed in a“free state”.Most of the g BDDA were in form with two free-R4N⁺and bonded with the Mt-anchored through weak hydrophobic force.FeCl4^-was loaded in FeOMt by electrostatic interaction with free-R4N⁺and protonated hydroxyl groups on the Mt surface.（2）Removal of nitrate and phosphate in single and/or binary systems by FeOMt showed that with the increase of FeOMt dosage,the removal of nitrate-nitrogen（NO3^--N）increased proportionally while that of phosphorus(PO4^3--P)increased at first and then decreased.The removal of N is negligibly affected by pH,but the alkaline condition was beneficial to the adsorption of PO₄^3--P.Equilibrium of N and P uptake by FeOMt was reached rapidly in 5 min,and the increase of initial concentration enhanced the uptake of N and P.The adsorption isotherms of N and P were well fitted to Langmuir and Freundlich models,showing the adsorption capacities of N and P in the binary system of 8.77 and 28.1 mg/g,respectively.Compared with the polymerized ferric chloride,FeOMt showed a similar performance in terms of P removal,and the N uptake was also observed.The adsorption of NO3^--N by FeOMt was readily affected by the concentrated coexisting ions,while the removal of PO4^3--P was affected by Fe³⁺-philic ligands.FeOMt was renewable and maintained more than 50%N and P uptake after continuous regeneration for three times.Dissociation of FeCl4^-released-R4N⁺and provided adsorption sites for NO3^-,while the further hydrolysis of FeCl4^-produced Fe（OH）3 which removed PO4^3-through precipitate and ligands exchange.As a result,the simultaneous removal of nitrate and phosphate was achieved.Due to the removal of N and P by FeOMt was ascribed to different adsorption sites,there was no significant difference in the uptake of NO3^--N and PO4^3--P between the single and the binary systems,and no antagonism or synergism was observed.（3）The immobilization of W（VI）and/or Cr（VI）by FeOMt in the single and binary soil systems showed that the available concentrations of W and Cr leached from the simulated soil were increased with the initially spiked concentrations of WO4^2-and Cr O4^2-into the soil.The immobilization of W and Cr by FeOMt was completed rapidly in 5 min.pH and common coexisting ions in the leachate had slight effects on the immobilization of W and Cr,and the existing forms of W and Cr in the systems were maintained as initially added,i.e.,WO4^2-and Cr O4^2-,respectively.Compared with OMt and Mt,FeOMt showed a more favourable performance in terms of simultaneous immobilization of W and Cr,and which was also suitable for remediation of W and Cr co-contaminated soil systems at low concentrations.In the W/Cr-coexisted binary soil system immobilizationed by FeOMt,some of the weakly adsorbed W and Cr were released slowly with time,accompanied with the release of some adsorption sites which could selectively adsorb a small amount of released W and Cr in return.After immobilization by FeOMt,exchangeable and carbonate W and Cr were transformed into more stable oxidizable and reducible fractions,respectively,which effectively reduced the bioavailability.The plants growing in W/Cr-bearing soil spiked with FeOMt showed better growth than the control group,including the increased seed germination rate,the greater biomass,and the promoted elongation and reduced W and Cr enrichment of vegetative organ.Based on the mechanism of simultaneous removal of nitrate and phosphate by FeOMt from aqueous solution,the immobilization sites for W and Cr in FeOMt were mainly derived from-R4N⁺and Fe（OH）3 produced by FeCl4^-dissociation and hydrolysis,in which-R4N⁺and Fe（OH）₃ had higher selectivity for Cr O₄^2-and WO₄^2-,respectively.However,because W and Cr were competitively immobilized by these two sites,the significant antagonism was observed,showing the poorer immobilization efficiency for W and Cr in the binary system with respect to the single system.In this work,the simultaneous removal of nitrate and phosphate in water and immobilization of tungsten and chromium in soil were achieved using a gemini surfactant-modified Mt through introducing functional charge-balancing anion（FeCl4^-）.This will provide meaningful reference for the design of modified clay materials and their application in the remediation of contaminated water and soil systems.