Study On Preparation Of Calcium-modified Attapulgite And Its Adsorption Mechanism Of Phosphorus Removal

With the rapid development of China’s social economy,the amount of sewage generated by production and living activities continues to increase,and the standard treatment of low-concentration phosphorus-containing sewage has become a hot topic of widespread concern.Due to the advantages of low fixed investment,stable effluent quality and a wide range of applicable concentrations,adsorption phosphorus removal technology has been widely used in treating medium and low concentrations of phosphorus-containing sewage.Attapulgite（GAT）is a crystalline hydrated magnesium aluminum silicate clay mineral,the crystal is needle-like,fibrous or fiber collection,and the theoretical chemical formula is Mg₅Si₈O₂₀（OH）₂（H₂O）₄·4H₂O,with unique layer chain structure,high-temperature resistance and adsorption capacity,is an ideal natural mineral phosphorus removal adsorbent.However,based on clarifying its phosphorus removal adsorption mechanism,how to improve its phosphorus removal performance in treating low-concentration phosphorus-containing sewage,improve the adsorption capacity,and eliminate the secondary pollution caused by phosphorus-containing adsorbents is still a bottleneck problem that limits its practical application.For this,a new phosphorus removal adsorbent with high adsorption capacity,good regeneration performance and low secondary pollution was prepared using natural GAT as the substrate.Its phosphorus removal adsorption behavior and regeneration mechanism were elucidated and based on molecular simulation technology,combined with Scanning Electron Microscopy（SEM）,X-ray Diffraction（XRD）,X-ray Photoelectron Spectroscopy（XPS）analysis and other instrumental analysis techniques,the phosphorus removal adsorption mechanism was revealed at multiple levels and angles,in order to enrich the relevant theoretical knowledge of phosphorus removal adsorption technology.To provide a new way for deep phosphorus removal of low-concentration phosphorus-containing sewage,the main research contents and results are as follows:（1）Preparation and property characterization of calcium-modified attapulgite.When the heat treatment temperature was 700°C,the Ca Cl₂concentration was 0.25 mol/L and the adsorbent dosage was 1:500,the effect of treating 2.0 mg/L simulated phosphorus-containing sewage was the best,and the removal rate of phosphorus could reach 89.11%,and compared with the unmodified,the removal efficiency and adsorption capacity of phosphorus increased by 81.68%and 91.69%,respectively.The results of SEM,Inductively Coupled Plasma Emission Spectroscopy（ICP-OES）,X-Ray Fluorescence Spectrometer（XRF）,XRD and Fourier Transform Infrared Spectroscopy（FTIR）showed that the surface of calcium-modified attapulgite（Ca-GAT）was a relatively ordered and compact rod-like or fibrous structure,the zeolite water in the pores,the coordination water at the edge of the octahedron,and the structural water in the middle of the octahedral layer were removed,and dolomite decomposed to produce hydroxycalcite and clinocalcite,and the Ca content of the obtained Ca-GAT adsorbent increased from 26.891 mg/g before modification to 52.347 mg/g,the adsorption site increased significantly.However,the specific surface area and pore volume of Ca-GAT were reduced from 127.990 m²/g before modification to 51.273 m²/g,and the pore volume decreased from 0.200 cm³/g to 0.157 cm³/g.In addition,the surface Zeta potential of the obtained adsorbent increased from-18.650 m V to-12.175 m V.The positive surface charge increased,which was conducive to the electrostatic adsorption removal of phosphate anions in sewage.Its zero-charge point p H_pzcrose from 8.63 before modification to 10.67,indicating that the Ca-GAT prepared in this study can treat low-concentration phosphorus-containing sewage on a broader p H range broadens the application field of this adsorbent.The improvement of Ca-GAT adsorption performance cannot be explained by the pore reaming mechanism caused by dehydration,and the calcium ion content and surface Zeta potential are important to improve the adsorption performance.（2）Study on static phosphorus removal adsorption behavior of Ca-GAT adsorbent.When the initial total phosphorus concentration was 2.0 mg/L,the adsorption capacity of GAT and Ca-GAT was 0.074 and 0.891 mg/g,respectively,which increased by about 12 times.The higher p H deprotons the surface of Ca-GAT,the negative charge increases,and the adsorption reaction between the surface adsorption site and the phosphate is weakened,which is not conducive to the adsorption removal of phosphorus by attapulgite.When competing anions below 100 mg/L are present in the solution simultaneously,the adsorption and phosphorus removal performance of Ca-GAT is hardly affected.Compared with the pseudo-first-order,intra-partical diffusion and Elovich models,the pseudo-second-order model can better describe the adsorption process of attapulgite on phosphate in sewage before and after modification,and the activation energy of Ca-GAT is 72.112 k J/mol,and its adsorption reaction to phosphate is chemical adsorption.The Sips adsorption isotherm model can better fit the isothermal adsorption process of the obtained adsorbent,indicating that the adsorption process of Ca-GAT on phosphate is more complex,affected by a variety of mechanisms,including physical adsorption and chemical adsorption,and the adsorption process of attapulgite before and after modification occurs on a non-uniform surface.At 298 K,308 K and 318 K,the Gibbs free energies of the Ca-GAT adsorption reaction were-18.280,-20.021and-21.687 k J/mol,respectively,indicating the presence of physical adsorption and chemical adsorption.Before and after modification,the enthalpy of the adsorption reaction was 47.906and 32.496 k J/mol,showing that the adsorption process was an endothermic reaction,and the higher the temperature,the easier the adsorption process was to carry out.（3）Study on the breakthrough curve and phosphorus removal adsorption behavior of Ca-GAT fixed-bed column.When the influent flow rate and initial total phosphorus concentration decrease and the bed height increases,the breakthrough time of the obtained Ca-GAT fixed-bed adsorption column is prolonged,which is conducive to improving the phosphorus removal efficiency.When the influent flow rate was 1 m L/min,the initial total phosphorus concentration was 2.0 mg/L and the bed height was 6 cm,the breakthrough time was 62 h,36 h and 56 h,the removal rates of total phosphorus were 78.52%,71.59%and73.83%,and the adsorption capacity was 0.55 mg/g,0.78 mg/g and 0.74 mg/g,respectively.Under different fitting conditions,the correlation coefficient R²of the Thomas model and Yoon-Nelson model did not change much,0.896-0.976 and 0.858-0.976,respectively,which can be well fitted to the dynamic adsorption process,indicating that internal diffusion and external diffusion are not rate-limiting steps that lead to the reduction of adsorption rate.Yoon-Nelson model can accurately predict the breakthrough time required to adsorb 50%adsorbate with less than 3%error between theoretical value and actual value.Compared with HCl and Na OH+Na Cl,the desorption agent Na OH had the best desorption and regeneration effect on saturated Ca-GAT adsorbents,after three cycles,the desorption efficiency and regenerated adsorption capacity were 54.61%and 0.30 mg/g,respectively.The desorption mechanism is a high concentration of OH^-substituted Ca-GAT adsorbed phosphate,the adsorbent can be regenerated,but the Ca²⁺content of the adsorbent does not change much.The desorption efficiency of the desorption agent HCl is as high as 94.09%,but while desorbing phosphate,the Ca²⁺on the surface is synchronously desorbed,due to the reusability of the adsorbent is poor.The salt effect of Na Cl and Ca CO₃in the desorption agent Na OH+Na Cl increases the solubility of Ca CO₃in the adsorbent,so the regeneration effect is not ideal.（4）Research on phosphorus removal adsorption mechanism based on molecular simulation technology.The results of Scanning Electron Microscope-Energy Dispersive Spectrometer（SEM-EDS）analysis showed that the calcium in Ca-GAT reacted with the phosphate in the sewage and generated an unevenly distributed Ca-P precipitate on its surface.The FTIR pattern showed that the peak strength of the CO₃^2-bending vibration peak was weakened after phosphate adsorption,possibly with phosphate substitution.XPS analysis of Ca-GAT after adsorption showed that P 2p peaks appeared at 132.8 e V and 133.6 e V binding energies,respectively,which belonged to Ca-P conjugates.Therefore,the possible phosphorus removal adsorption mechanism of Ca-GAT is proposed,including electrostatic attraction,ligand exchange and surface precipitation,and surface precipitation is the main phosphorus removal mechanism.The results of molecular simulation showed that after adsorption of H₂PO₄^-and HPO₄^2-,the total energy of the Ca-GAT adsorption system decreased from 289.46and-919.39 kcal/mol before adsorption to-110.99 and-1328.67 kcal/mol,respectively,with a large range of change and negative values,indicating that compared with unmodified materials,Ca-GAT prepared in this study was conducive to the stable adsorption of phosphate and improved the adsorption performance of phosphorus removal.The radial distribution function was used to study the non-bonding force between molecules.The results showed that the most vital characteristic peaks of Ca-GAT for H₂PO₄^-and HPO₄^2-occurred at 2.6（?）.The strongest distinct peaks of unmodified attapulgite appeared at 4.8（?）,indicating that the reaction between Ca-GAT and phosphate was carried out by hydrogen bonding.In contrast,the phosphate adsorption by unmodified attapulgite mainly relied on van der Waals forces,which once again proved that the Ca-GAT prepared in this study had a stronger ability to adsorb and remove phosphorus.The results of state density analysis showed that the P,O atoms of phosphate ions and Ca atoms hybridized to form a Ca-O-P bond,which belonged to Ca-P precipitates,namely Ca HPO₄,or some amorphous calcium phosphate salts,and the main orbitals involved were the p orbitals of the Ca atom and the p and O orbitals of the P and O atoms in the phosphate.（5）Study on Ca-GAT enhanced phosphorus removal in simulated domestic sewage.The A/O-MBR effluent was treated with attapulgite fixed-bed columns before and after modification,and when the fixed-bed column effluent reached a total phosphorus breakthrough concentration of 0.5 mg/L,the time required for GAT and Ca-GAT fixed-bed column was 7 d and 23 d,respectively.At the 60 d,the average removal efficiency of total phosphorus from GAT and Ca-GAT fixed-bed column were 18.83%and 56.35%,NH₄⁺-N was8.27%and 20.84%,TN was 3.60%and 7.63%,COD was 12.84%and 18.36%,the Ca-GAT removed total phosphorus and other pollutants better.The mechanism of Ca-GAT removal of total phosphorus is still dominated by Ca-P precipitation,and the removal process of nitrogenous pollutants and COD includes three stages:initial adsorption removal,mid-stage biofilm formation and later biological removal,none of them will have a great impact on the phosphorus removal effect of Ca-GAT.The results of SEM-EDS analysis showed that after the treatment of simulated domestic sewage,the components of saturated Ca-GAT mainly included O,Ca,Si,Mg,Al,P,N,Fe and K,which did not contain heavy metals and toxic and harmful elements to soil and crop growth,in addition,the N and P elements of Ca-GAT after saturation adsorption were 4.44%and 6.14%,respectively,which were higher than 1.97%and1.90%of GAT,and contained 1.31%of K elements.Compared with the blank control,plant height,leaf width,fresh weight and dry weight increased by 83.15%,98.94%,222.68%and220.83%,respectively,after 5 weeks of sowing for rapeseed potted results.The cost of Ca-GAT treatment per ton of phosphorus-containing sewage is about 3.11 yuan,which is lower than most phosphorus removal adsorbents and commercially available Phoslock agent.which has good industrial application potential.The Ca-GAT adsorbent prepared in this study can be used to treat low-concentration phosphorus-containing sewage to achieve deep phosphorus removal in domestic sewage at low cost,and phosphorus-containing Ca-GAT can be used as fertilizer or soil amendment to realize the reuse of phosphorus-containing adsorbents and phosphorus resources.