Effect Of Lanthanum-based And Lanthanum/Iron-based Remediation Materials On Transport And Transformation Of Phosphorus In Sediments

At present,eutrophication is still one of the important environmental problems in the word,and phosphorus?P?is the main limiting factor of water eutrophication management.In recent years,the input of exogenous phosphorus has been effectively controlled,but the eutrophication problem has not been improved.The main reason is that the accumulation of internal phosphorus load?the releasing of phosphorus from sediment?results in the weakning of water treatment effectiveness.In recent years,more and more scholars have realized that in-situ amendment/covering technology is a very high potential method,which is used to control endogenous phosphorus releasing.In-situ amendment/covering technology is a contaminated sediment remediation method that passivating agents were directly added to the sediment or was placed onto the interface between sediment and overlying water to reduce the endogenous phosphorus loading.The controlling efficiency of in-situ amendment/covering technology for the endogenous phosphorus is closely related to the selection of phosphorus-sorbents.Therefore,the selection of phosphorus-sorbents with strong passivation ability,low economic cost and little environmental pollution is the key to the application of in-situ amendment/covering technology.Lanthanum-based materials are the best phosphorus-sorbent for in-situ amendment/covering technology,because lanthanum-based material located in sediments not only absorbs the dissolved phosphorus in the overlying water,but also increases the stability of phosphorus in the sediments.However,after lanthanum-based materials are added into sediments,it is generally difficult to recycled from the sediments,which will lead to the substantial consumption of non-renewable La resource.The combination of lanthanum-based materials with magnetite to create a lanthanum/iron-based composite is expected to be a feasible solution for both effectively controlling phosphorus release from sediment and recyclability.Hence,it is necessary to understand the effect of lanthanum-based and lanthanum/iron-based remediation materials on the transport and transformation of phosphorus in sediments,which could increase the feasibility of materials in practical application.For this purpose,firstly,this work prepared lanthanum hydroxide?La-OH?and magnetite/lanthanum hydroxide composite?Mag-La-OH?,and then La-OH and Mag-La-OH were used as sediment amendments to immobilize phosphorus?P?in sediments.The immobilization efficiency of mobile P?Mob-P?and bioavailable P?BIO-P?in sediments by La-OH and Mag-La-OH was investigated.Results showed that the addition of La-OH into sediment resulted in the transformation of loosely adsorbed P?LA-P?and redox sensitive P?RS-P?to sodium hydroxide extractable P?OH-P?and hydrochloride extractable P?H-P?in the sediment,while the addition of Mag-La-OH into sediment led to the transformation of LA-P,RS-P and H-P to OH-P and residual P?RES-P?in the sediment.Both La-OH and Mag-La-OH can effectively immobilize Mob-P?LA-P+RS-P?in sediments,but La-OH had a higher Mob-P immobilization capacity than Mag-La-OH.The amendment of sediments with La-OH and Mag-La-OH both can reduce the amounts of different types of BIO-P including water soluble P?WA-P?,algal available P?AL-P?and Fe oxide paper extractable P?FE-P?in the sediments,and La-OH had a higher BIO-P immobilization capacity than Mag-La OH.The immobilization of Mob-P in sediments by Mag-La-OH could be described by the equation:W=0.333×??Mob-P?-14.4,where?Mob-P?mg/kg?is the amount of Mob-P bounded in sediments and W?%?is the Mag-La-OH dosage.The immobilization of FE-P in sediments by Mag-La-OH could be described by the equation:W=0.380×??FE-P?-1.14,where?FE-P is the amount of FE-P bounded in sediments.Considering that Mag-La-OH can be retrieved from the water bodies under the action of external magnetization fields after its application,Mag-La-OH could have high potential to be used as an amendment for the immobilization of Mob-P and BIO-P in sediments.Secondly,in this study,the adsorption characteristics of phosphate on zirconium-modified zeolite?Zr MZ?,lanthanum-modified zeolite?La MZ?and lanthanum/zirconium-modified zeolite?La Zr MZ?were comparatively investigated,and the effect of Zr MZ,La MZ,and La Zr MZ addition on the mobilization of phosphorus?P?in sediments was comparatively studied.Results showed that the phosphate adsorption capacity decreased in the order of La Zr MZ>La MZ>Zr MZ.The addition of La Zr MZ,La MZ,and Zr MZ in sediments all could effectively reduce the concentrations of soluble reactive P?SR-P?in the overlying and pore waters.Furthermore,the addition of La Zr MZ,La MZ and Zr MZ all could decrease the amount of mobile P in sediments,and the reduction rate decreased in the order of La MZ>La Zr MZ>Zr MZ.The amendment of sediments with La Zr MZ,La MZ,and Zr MZ all could lead to the decrease in the amount of water soluble P?WA-P?,algal available P?AL-P?and iron oxide-filter paper extractable P?FE-P?in the sediments.The reduction rate of WA-P and AL-P decreased in the order of La MZ>La Zr MZ>Zr MZ,and the reduction rate of FE-P decreased in the order of La Zr MZ>Zr MZ>La MZ.The addition of La Zr MZ and La MZ both could reduce the content of readily desorbed P?RDP?,and the reduction rate decreased in the order of La MZ>La Zr MZ.La Zr MZ and Zr MZ amendment both could decrease the concentration of Na HCO₃extractable P?Olsen-P?in sediments,and the reduction rate decreased in the order of La Zr MZ>Zr MZ.Then,a novel capping material,i.e.,magnetic lanthanum/iron-modified bentonite?M-LaFeBT?was prepared by loading lanthanum onto a magnetic iron-modified bentonite?M-FeBT?and used to reduce the internal phosphorus?P?loading from sediments.To determine the capping efficiency and mechanism of M-LaFeBT,the impact of M-LaFeBT and M-FeBT capping on the transport and transformation of P in sediments was investigated,and the stabilization of P bound by the M-LaFeBT and M-FeBT capping layers was evaluated.Results showed that M-LaFeBT possessed good magnetic property with a saturated magnetization of 14.9 emu/g,and exhibited excellent phosphate adsorption ability with a maximum monolayer sorption capacity(Q_MAX)of14.3 mg P/g at p H 7.Moreover,M-LaFeBT capping tremendously reduced the concentration of soluble reactive P?SR-P?in the overlying water.Furthermore,M-LaFeBT capping significantly decreased the concentration of SR-P in the pore water and DGT?diffusive gradient in thin films?-labile P in the profile of overlying water and sediment.Additionally,most of P bound P by the M-LaFeBT capping layer?approximately 77%?was stable under natural p H and reducing conditions.The phosphate adsorption ability for M-LaFeBT was much higher than that for M-FeBT,and the Q_MAXvalue for the former was 4.86 times higher than that for the latter.M-LaFeBT capping gave rise to a higher reduction of DGT-labile concentration in the profile of Overlying water and sediment than M-FeBT capping.The P adsorbed by the M-LaFeBT capping layer was more stable than that by the M-FeBT capping layer.In the end,in order to explore the adsorption mechanism of iron-based components in lanthanum/iron-based composite and better apply lanthanum/iron-based composite to remove Phosphorus from natural water bodies which generally exist Ca²⁺and Mg²⁺.A hydrous ferric oxide?HFEO?was prepared and characterized at first and then the impact of coexisting Ca²⁺and Mg²⁺on the uptake of Phosphorus by HFEO was studied.The results showed that,without coexisting Ca²⁺and Mg²⁺,the kinetic data for Phosphorus sorption onto HFEO were better described by the Elovich model?R²=0.953?than the pseudo-second-order?R²=0.838?and pseudo-first-order?R²=0.641?models,and the isotherm data were fitted better with the Dubinin–Radushkevich?R²=0.966?and Freundlich?R²=0.953?models than with the Langmuir?R²=0.924?model.The ligand exchange of the Febound hydroxyl group with Phosphorus and the generation of Fe–O–P bonding played a key role in the uptake of Phosphorus by HFEO in the absence of Ca²⁺and Mg²⁺.Coexisting Ca²⁺and Mg²⁺greatly improved the adsorptive removal of Phosphorus by HFEO,including the adsorption capacity and initial adsorption rate.According to the Langmuir isotherm equation,the predicted maximum Phosphorus adsorption capacity for HFEO at p H 7 in the presence of 2mmol/L Ca²⁺?24.7 mg P/g?or 2 mmol/L Mg²⁺?18.4 mg P/g?was much larger than that without coexisting Ca²⁺and Mg²⁺?10.7 mg P/g?.The formation of aqueous Ca HPO₄⁰and Mg HPO₄⁰species firstly and then the adsorption of the formed Ca HPO₄⁰and Mg HPO₄⁰species on the HFEO surface to generate the HPO₄^2--bridged ternary complexes?i.e.,Fe?OPO₃H?Ca⁺and Fe?OPO₃H?Mg⁺?had an important role in the improvement of Phosphorus adsorption onto HFEO by coexisting Ca²⁺and Mg²⁺.By the way,lanthanum-based and lanthanum/iron-based composite could effectively adsorbe P in the overlying water and increase the stability of P in the sediments.However,from a recyclable point of view,lanthanum/iron-based composite has more potential to be used as an amendment for sediment mulch/improvement technique than lanthanum-based composite...