A Study On Phosphate Adsorption Behavior Of Thermally Activated Hydrated Sulfoaluminate Cement Granules

Excessive phosphorus content in water is one of the main reasons for water eutrophication.Efficient phosphorus removal from water is an effective method to solve water eutrophication.Among various phosphorus removal methods,adsorption method is favored because of its simple operation,the ability to recycle phosphorus,and the small sludge yield.However,traditional phosphate adsorbents generally have disadvantages such as high price,unobtainable raw materials,and low adsorption capacity,which limits the large-scale application of adsorption methods in water phosphorus removal.In this paper,sulfoaluminate cement（SAC）was used as raw material,and hydrated sulfoaluminate cement granules（HSAC）were thermally dehydrated to prepare new phosphate adsorbent granules（THSACs）.The optimal thermal activation conditions of HSAC were mainly determined by comparing the adsorption performance of THSACs to phosphate and the changes in physical and chemical properties of THSACs under different thermal activation conditions.The adsorption performance and adsorption mechanism of the adsorbent(SAC_T200)prepared under the optimal thermal dehydrated conditions for phosphate under different operating conditions and the regeneration performance of SAC_T200as a phosphate adsorbent were investigated.The conclusions of the paper are as follows:（1）Thermal activation significantly improved the adsorption performance of HSAC for phosphate.In the range of activation temperature from 65℃to 400℃,the optimum thermal activation temperature of HSAC was 200℃,and the optimum thermal activation time was 0.5 h.The initial adsorption capacity（4 h）of SAC_T200was 14.28mg/g.And the equilibrium adsorption capacity（72 h）of SAC_T200was 46.87 mg/g.The analysis results of XRD,SEM,BET and FTIR of HSAC and THSACs showed that thermal activation below 65℃had no obvious effect on the crystal structure and chemical bonds of HSAC.However,thermal activation over 100℃would dehydrate ettringite,the main hydration product of HSAC,into amorphous meta-ettringite,which greatly changed the crystal structure and chemical bonds of HSAC.The surface roughness,specific surface area and pore volume of THSACs were increased when the thermal activation temperature of THSACs was lower than 200℃.The activation temperature was 200℃,the specific surface area（15.22 m²/g）and pore volume（0.28cm³/g）of THSACs were the largest.THSACs produced cracks,and its adsorption performance and strength were reduced when the thermal activation temperature was higher than 200°C.（2）The adsorption capacity of SAC_T200for phosphate gradually decreased with the increase of the initial p H of the solution.It was more suitable for use in acidic or neutral water.The adsorption capacity of SAC_T200was 48.81 mg/g（p H=3）and 27.26mg/g（p H=7）,respectively;The equilibrium time for SAC_T200to adsorb phosphate was72 h.The initial adsorption rate of SAC_T200was relatively fast.At 24 h,the adsorption capacity of SAC_T200at 4°C（14.68 mg/g）and 25°C（30.37 mg/g）reached 65%of the equilibrium adsorption capacity.SAC_T200had good selectivity for phosphate,and the coexistence of anions in water affected the phosphate adsorption of SAC_T200in the order of CO₃^2->SO₄^2->NO₃^->Cl^-.The adsorption amount of phosphate by SAC_T200increased gradually with the increase of ionic strength in solution.And SAC_T200had strong anti-interference ability to the change of ionic strength in solution.SAC_T200is a high-efficiency phosphorus removal adsorbent with fast initial adsorption rate and strong selectivity to phosphorus.（3）The kinetic and thermodynamic analysis of the adsorption of phosphate by SAC_T200showed that the adsorption kinetic data of SAC_T200for phosphate was more in line with the Elovich and pseudo-second-order kinetic models.And the adsorption isotherm of phosphate by SAC_T200was more in line with the Freundilch isotherm model.This indicated that chemical adsorption occurred during the adsorption of phosphate by SAC_T200.And its adsorption of phosphate is multi-layer adsorption of non-uniform surface adsorbents,and the increase of temperature was beneficial to the adsorption of phosphorus by SAC_T200.The analysis of the surface morphology,Zeta potential and chemical bonds of SAC_T200before and after adsorption showed that ettringite crystals were re-formed during the adsorption process of SAC_T200,and Ca₃（PO₄）₂surface precipitation occurred during the process of phosphorus removal,and the adsorption of phosphate decreased the isoelectric point of the SAC_T200.There are chemical adsorption and multi-layer adsorption in the adsorption process of phosphate by SAC_T200,and the removal of phosphate by SAC_T200is mainly achieved by inner-sphere complexion,intercalation of phosphate in ettringite reorganization,and surface precipitation.（4）SAC_T200has good regeneration performance.The desorption rate of SAC_T200in0.1 mol/L HCl for 0.5 h reached up to 85%.The desorption rate in 0.5 mol/L Na Cl for0.5 h reached up to 36%.The SAC_T200desorbed with HCl was regenerated three times.And the adsorption capacity of the third regeneration was 25 mg/g.The SAC_T200desorbed with Na Cl was regenerated twice.And the adsorption capacity of the second regeneration was 16.2 mg/g.At this time,the adsorption capacity of SAC_T200after regeneration was still greater than that of HSAC and most of the existing research adsorbents.