Investigation Of Phosphorus Adsorption Using Modified Bentonite Immobilizedbycalcium Alginate

Phosphorus is a non-renewable and essential resource,and it is a key factor result in the eutrophication.With the increasing demand for phosphorus removal and recovery in surface water or wastewater treatment plant effluent,the enhanced removal method of low concentration phosphorus in water has important environmental significance.In recent years,the phosphorus adsorption technology by bentonite has attracted much attention due to its low price and environmental friendliness.On the basis of extensive and in-depth literature condensing,this paper optimizes the preparation of modified bentonite and immobilized adsorbent for improving the adsorption capacity and facilitating the separation between solid and liquid.The adsorption experiment with initial phosphate concentration of 0.5⁵0mg/L was carried out to achieve effective removal of phosphate and good reduction of phosphate load in wastewater.At the same time,the adsorbent after use the adsorbent after use does will not cause secondary pollution and has good feasibility of returning to the field.The main contents and conclusions of this paper are as follows:（1）Optimized preparation and adsorption efficiency experiment of modified bentonite adsorbent.The bentonite was modified by three steps of water purification,sodiumation and aluminum pillaring.The adsorption performance was improved by about 85%.In the optimized preparation process,the bentonite was purified by shaking with ultrapure water at a ratio of 5%（W/V）at 210 r/min,and then sodiumated with a 1 mol/L NaCl solution.This stage reduced the adverse effects of bentonite on water swelling and improved its dispersion in water.And through cation exchange,Na⁺ions could exchange with high-priced Ca²⁺and Mg²⁺ions in bentonite to obtain higher cation exchange capacity,further increased the ion exchange efficiency of subsequent polymerization of hydroxyaluminum ions and improve the pillar-supporting effect.It is the main reason for the increase in adsorption capacity of modified bentonite.The raw bentonite was calcium and magnesium bentonite showed by XRay Fluorescence spectroscopic analysis.In X-ray diffraction analysis,after modification,the interplanar spacing d value of the（001）plane in X-ray diffraction peak of bentonite increased from 1.52 nm to 1.70 nm.The surface charge was changed and the interlayer spacing of the bentonite was increased by modification.When the initial phosphate concentration was 20 mg/L,the pH was 3,and the modified bentonite dosage was 1 g/L,the adsorption equilibrium was reached in about 6hours,and the phosphate removal rate was 91.68%.（2）Preparation and adsorption efficiency of modified bentonite immobilized beadsThe modified bentonite immobilized beads was prepared by dropping the mixed gel of sodium alginate and modified bentonite into a 3%（w/v）calcium chloride solution to crosslink and solidify.Numerous bulges on the beads made the surface rough and the bead has a heterogeneous dense inner network structure with high porosity.The cumulative pore volume,specific surface area and average pore size for the modified bentonite immobilized beads were 0.04 cm³g^-1,17.01 m²g^-1and 13.41 nm,respectively,which indicate it is a mesoporous material.The pore structure has an excellent mass transfer effect on phosphate,and phosphate is easily captured by the binding site.The density of the immobilized adsorbent particles is 1.32×10³ kg/m³,which is easy to have excellent solid-liquid separation in water.When the initial phosphate concentration was 0.5⁵0mg/L,the pH was 3,the temperature was 30oC,the dosage of immobilized adsorbent particles was 2g/L,and the rotation speed was 210r/min,the adsorption efficiency was gradually increasing with the decrease of initial phosphate concentration.The adsorption efficiency could both achieve100%when the initial phosphate concentration were 0.5 and 1mg/L,and the time for adsorption to reach equilibrium were only 15 min and 24 min,respectively..（3）Adsorption characteristics of modified bentonite immobilized beadsFor the simulated wastewater with initial phosphate concentration of 5⁵0mg/L,kinetics experiments showed that the pseudo-second-order kinetic model can describe the phosphate adsorption process best.External mass transfer and intraparticle diffusion governed the adsorption process mainly.The Freundlich and Langmuir processes could simultaneously governed phosphate adsorption onto beads and the maximum adsorption capacity was 15.68 mg/g.The process of phosphate adsorption onto modified bentonite immobilized beads was spontaneous and endothermic,and the spontaneity is proportional to temperature.After the phosphate enters the modified bentonite immobilized beads,The adsorption mainly through ligand exchange,electrostatic adsorption,anion exchange occurred.（4）Experiment of phosphate adsorption onto modified bentonite immobilized beads in actual water samples.In the static experiment of treating the effluent from a secondary sedimentation tank of municipal wastewater treatment plant.The adsorption equilibrium time,phosphate removal rate,and the phosphate concentration in the effluent after adsorption was16min,98.81%and 0.01mg/L respectively when the initial phosphate concentration was 0.93mg/L and the immobilized adsorbent particle dosage was 4g/L.In a static experiment to treat polluted water in a reservoir,the initial phosphate concentration was0.52 mg/L.The modified bentonite immobilized beads have a removal effect on phosphate,COD and ammonia nitrogen in the wastewater,and the removal rates of each index were 72.42%,4.75%and 9.18%,respectively.It showed that the modified bentonite immobilized beads have the feasibility of practical application.At present,phosphate adsorption by bentonite in wastewater is mostly in the laboratory research stage,the research on actual water samples and the immobilization technology of bentonite remains to be further explored.In this paper,modified bentonite and immobilized beads were prepared to explore the adsorption efficiency and characteristics of the adsorbent.Lab scale experiment were carried out to treat the effluent of the secondary sedimentation tank in a wastewater treatment plant and reservoir water.The recovery of phosphorus in low strength wastewater was realized.The research conclusion can provide theoretical support for phosphorus resource recovery in water,especially for low phosphorus strength wastewater.