Controlling Efficiency And Mechanism Of Iron And Zirconium Co-modified Attapulgite For Phosphorus Release From Sediments

Since the effective control of exogenous phosphorus cannot solve the problem of eutrophication of water bodies,sediment remediation methods is used to control the release of P from sediment to the overlying water,which is a key step in alleviating eutrophication problems.In situ passivation of sediment is regarded as a promising sediment remediation strategy.The key to the successful application of this technology lies in the selection of a suitable phosphorus-inactivation agents(PIAs).In recent years,the use of iron-based and zirconium-based materials to control the release of phosphorus from the sediment of water bodies has attracted the attention of scholars in China and abroad.However,the use of pure iron oxides and zirconia is too costly to be suitable for practical engineering applications.Natural mineral materials are abundant in nature,cheap and easy to obtain,and have the advantages of unique structure of natural materials,suitable for use as support materials for loading metals,which can solve cost problems and be environmentally friendly.Compared to other natural materials,attapulgite has a unique layered chain structure suitable as a supporting material for iron and zirconium.However,there are few reports in China and abroad on the control of phosphorus release from sediment by iron-modified attapulgite,zirconium-modified attapulgite and zirconium-modified attapulgite.In addition,non-magnetic iron-based,zirconium-based and iron/zirconium-based materials are difficult to recycle after application,which increases the cost of sediment remediation.Giving magnetism to materials is expected to solve the defects of non-magnetic materials.Depending on the supporting material,the performance of magnetic iron-based materials to control the release of phosphorus from the sediment may be different.However,there are few reports of comparative studies in China and abroad on the use of iron tetroxide to control the release of phosphorus from the sediment using different natural mineral materials.In addition,the performance and mechanism of magnetic iron zirconium combined with modified attapulgite to control the release of phosphorus from the sediment are not well understood.First,the sorption characteristics and mechanism of phosphate on zirconium-modified attapulgite(Zr-ATP),iron-modified attapulgite(Fe-ATP)and zirconium/iron co-modified attapulgite(Zr/Fe-ATP)prepared by a simple impregnation method was studied,and the impacts of Zr-ATP,Fe-ATP and Zr/Fe-ATP amendment and capping on the migration of phosphorus(P)from sediments to overlying waters was investigated.The results showed that Zr-ATP and Zr/Fe-ATP possessed stronger adsorption ability for phosphate in aqueous solution than Fe-ATP.The ligand replacement of hydroxyl group with phosphate anion and formation of the inner-sphere phosphate complex played a crucial role in the adsorption process of phosphate on Zr-ATP,Fe-ATP and Zr/Fe-ATP.Compared with phosphate-loaded Fe-ATP,Zr-ATP and Zr/Fe-ATP-bound phosphates are mostly present in a more stable form,and it is hard for these P species to be re-released into water under the anoxic environment and normal p H(5-9).The ratio of mobile P to total P of Fe-ATP loaded with phosphate was much higher than those of Zr-ATP and Zr/Fe-ATP loaded with phosphate,indicating that Fe-ATP-bound phosphate has higher re-releasing risk than Zr-ATP-bound and Zr/Fe-ATP-bound phosphate.Zr-ATP,Fe-ATP and Zr/Fe-ATP amendment all can reduce the releasing risk of P from sediments to overlying waters.The amendment of sediment with Zr-ATP and Zr/Fe-ATP can both induce the conversion of redox-sensitive P(BD-P)to Na OH-IP and Res-P in the sediment,making the phosphorus in the sediment more stable.However,the amendment of sediment with Fe-ATP had a negligible effect on the inorganic P activity in the sediment.Zr-ATP,Fe-ATP and Zr/Fe-ATP capping all can reduce the risk of P release from sediment into the overlying water,and Zr-ATP and Zr/Fe-ATP capping had a better reduction efficiency of internal P liberation to the overlying water than Fe-ATP capping.Zr-ATP,Fe-ATP and Zr/Fe-ATP capping all can give rise to the reduction of pore water SRP and diffusive gradient in thin-films(DGT)-labile P in the upper sediment.This is beneficial to the control of P releasing from sediment into the overlying water by the Zr-ATP,Fe-ATP and Zr/Fe-ATP capping.The findings of this work suggest that Zr-ATP and Zr/Fe-ATP are promising active capping or amendment materials for internal P loading management in surface water bodies.Secondly,in order to solve the problem of recycling,magnetic zeolite(M-Ze),magnetic bentonite(M-BT),magnetic activated carbon(M-AC),magnetic attapulgite(M-ATP)and magnetic medical stone(M-MS)were prepared and passivation effect of these five materials was studied.Studies have shown that M-Ze,M-BT,M-AC,M-ATP and M-MS capping can effectively control phosphorus in the sediment and significantly reduce the concentration of phosphorus released from the sediment to the overlying water.M-Ze,M-BT,M-AC,M-ATP and M-MS covering can all lead to the reduction of SRP and DGT available phosphorus in the upper sediment pore water,which helps to control phosphorus release from sediments into overlying water by M-Ze,M-BT,M-AC,M-ATP and M-MS capping.The proportions of Na OH-IP,HCl-P and Res-P in M-Ze,M-BT,M-AC,M-ATP and M-MS after phosphate adsorption were 71.0%,65.2%,60.8%,69.8% and 85.0%,respectively.It was shown that M-Ze,M-BT,M-AC,M-ATP and M-MS bound phosphorus had a lower risk of re-release.The maximum phosphate adsorption for M-ATP was much higher than those for M-Ze,M-BT,M-AC and M-MS.Thus,M-ATP is more suitably used as an active capping material to reduced the risk of phosphorus migration from sediments to the overlying water.Finally,iron and zirconium were used to modify attapulgite and give it magnetic properties to prepare a zirconium-modified attapulgite/magnetite composite(M-Zr Fe ATP)and use it as an active covering material to resist phosphorus(P)in sediments release into the overlying water.The adsorption characteristics of phosphate onto M-Zr Fe ATP was evaluated.The effectiveness of direct M-Zr Fe ATP capping and fabric-wrapped M-Zr Fe ATP capping with and without sediment covering on the transferring of sedimentary P was evaluated.The results indicated that the resultant M-Zr Fe ATP had a outstanding phosphate adsorption performance with the maximum phosphate monolayer adsorption capability of 11.5 mg/g.The direct M-Zr Fe ATP capping could evidently intercept the mobilization of sedimentary P into the overlying water under anoxic condition,and the immobilization of pore water soluble reactive P(SRP),diffusion gradient in thin film(DGT)-labile P and mobile P in the upper sediment as well as the precipitation of phosphate with zirconium should play the important part in the inhibition of sedimentary P liberation into the overlying water.The covering of the M-Zr Fe ATP capping layer with a certain thickness(less than 30 mm)of sediment had a certain adverse result on the interception of sediment-P release into the overlying water.The fabric-wrapped M-Zr Fe ATP capping still could resist the release of P from sediment into the overlying water,but its efficiency was less than that of the direct M-Zr Fe ATP capping.The presence of sediment covering had a much larger negative effect on the efficiency of the fabric-wrapped M-Zr Fe ATP capping to control sedimentary P liberation into the overlying water than that of the direct M-Zr Fe ATP capping.The adsorption of phosphorus in pore water by M-Zr Fe ATP and the passivation of phosphorus in the sediment are important mechanisms for controlling the release of phosphorus in the sediment.In summary,the iron and zirconium co-modified attapulgite is a promising active covering/improving material for controlling phosphorus release from water sediments.