Study On The Effects Of Ion Aging And Sodium/Calcium Ion Pre-treatment On The Control Of Endogenous Phosphorus Release By Lanthanum-modified Bentonite

Since the 18th National Congress of the Communist Party of China,China has been vigorously promoting ecological civilization construction and has made remarkable achievements.However,some lakes such as Taihu Lake,Chaohu Lake and Dianchi Lake are still slightly polluted,and algae blooms occur from time to time.Therefore,attention should be paid to the eutrophication that causes this phenomenon.Phosphorus is considered to be the key limiting element leading to the eutrophication,which is mainly divided into external input phosphorus and endogenous phosphorus release.With the continuous enhancement of national pollution control capacity,external input phosphorus has been well controlled,so it is particularly important to control the release of endogenous phosphorus.In-situ addition/capping technology is mainly used to inhibit the release of phosphorus in the sediment by scattering the material to form a cover layer or to mix it with the surface of the sediment.Lanthanum modified bentonite（LMB）is a kind of material for controlling phosphorus release from sediments,which has attracted extensive attention in recent years.It has been used in many eutrophic lakes,reservoirs and ponds at home and abroad,proving perfect phosphorus removal effect.In the practical application of in-situ addition/capping technology,not only requires the developed materials to have good control phosphorus ability,but also considers the water condition and the long-term effect of the materials.The widespread presence of anions in water may affect the long-term effect of LMB in controlling phosphorus release from sediments.Therefore this study first explored the effect of ion aging on the control of endogenous phosphorus release by lanthanum-modified bentonite.First,raw lanthanum modified bentonite（LMB）was aged with common ions(Na⁺,Ca²⁺,Cl^-,HCO₃^-and SO₄^2-),a new kind of lanthanum-modified bentonite（ion aging LMB）was synthesized.The characteristics and mechanism of phosphate adsorption by LMB and ion aging LMB in water were analyzed by batch adsorption experiment and XPS characterization.The influence of LMB and ion aging LMB covering and addition on phosphorus migration and transformation in sediments was explored by sediment culture experiment.Furthermore,the effects and mechanisms of the common anion aging treatment on the removal of phosphate from water and the release control of phosphorus from sediments by lanthanum modified bentonite were explored.The results showed that the combined aging treatment of Na⁺,Ca²⁺,Cl^-,HCO₃^-and SO₄^2-could reduce the phosphate adsorption capacity of LMB.The combined aging treatment of Na⁺,Ca²⁺,Cl^-,HCO₃^-and SO₄^2-has limited influence on the LMB phosphate adsorption mechanism.The precipitation of phosphate and lanthanum ions to form La PO₄,and the ligand exchange between carbonate group and phosphate to form the inner lanthanum-phosphate complex are still important mechanisms of phosphate adsorption by ion aging LMB.The addition and capping of LMB and ion aging LMB can effectively inhibit the release of endogenous phosphorus from the overlying water under hypoxia condition.The immobilization of DGT available P and PM-P in sediments plays a key role in the inhibition of endogenous phosphorus release by LMB and ion aging LMB addition/capping.Under hypoxia conditions,the combined aging treatment of Na⁺,Ca²⁺,Cl^-,HCO₃^-and SO₄^2-reduced the blunt phosphorus capacity of LMB addition and capping which is not conducive to controlling the release of endogenous phosphorus in sediments.Secondly,in addition to the ions in the water,the ions contained in the material itself may also affect its phosphate adsorption effect and blunt phosphate capacity.This paper used sodium/calcium ion to pretreat bentonite,and lanthanide modified-bentonite（La-RBT）,sodium ion pretreatment lanthanum-modified bentonite（La-Na BT）and calcium ion pretreatment lanthanum-modified bentonite（La-Ca BT）were prepared and characterized.Then,the adsorption characteristics of phosphate in water by La-RBT,La-Na BT and La-Ca BT were explored by batch adsorption experiments.Finally,the effects of La-RBT,La-Na BT and La-Ca BT capping on phosphorus migration and transformation in sediments were investigated by sediment culture experiments.The results showed the phosphate adsorption processes of three materials were consistent with the Langmuir isothermal adsorption model and pseudo-second kinetic adsorption model.The phosphate adsorption capacity was La-Na BT,La-RBT and La-Ca BT in order from high to low.In other words,sodium ion pretreatment could increase the phosphate adsorption capacity of lanthanum modified bentonite,calcium ion pretreatment can reduce the phosphate adsorption capacity of lanthanum modified bentonite.La-RBT,La-Na BT and La-Ca BT capping can effectively control the release of phosphorus from sediments to overlying water under normal p H and anoxic conditions.The immobilization ability of DGT-labile P is the key to control the release of phosphorus from sediments to overlying water.The immobilization ability of DGT-labile P of the three materials is La-Na BT,La-RBT and La-Ca BT from high to low.In other words,sodium ion pretreatment can increase the immobilization ability of DGT-labile P of lanthanum modified bentonite,while calcium ion pretreatment can reduce the immobilization ability of DGT-labile P of lanthanum modified bentonite.