Research On The Removal Of Phosphorus From Wastewater By Microwave-assisted FeCl₃ Modified Biological Ceramsite

The excess of nitrogen and phosphorus is the main cause of water eutrophication,while the content of phosphorus is one of the main factors to control the eutrophication of fresh water,lower N/P ratio in water is more likely to cause eutrophication.The removal of phosphorus by adsorption method has the advantages of simple process,small footprint,small sludge yield and friendly environment.How to improve adsorption capacity of adsorbents,and at the same time,control the cost of preparation of adsorbents,is the focus of research on phosphorus removal by adsorption method.Seven kinds of common adsorbent materials were compared and selected through the experiment of adsorption treatment of simulated phosphorus wastewater with a concentration of 3mg/L.Chemical method and microwave irradiation method were used to modify the adsorbent.The phosphorus removal effects of 24 combinations of single modification and compound modification were compared.Finally,the optimal modification method of the adsorbent was determined and the optimal preparation conditions of the modified adsorbent were studied.The adsorption mechanism of phosphorus removal by modified bio-ceramics was analyzed by means of scanning electron microscopy,specific surface area and pore size analysis,Fourier transform infrared spectroscopy,X-ray diffraction,isothermal adsorption curve fitting,and adsorption kinetic model fitting and so on.The desorption regeneration test of adsorbent and the dynamic adsorption test with real wastewater containing phosphorus were carried out.This paper draws the main conclusions as follows:（1）100ml of simulated phosphorus-containing wastewater with a concentration of 3mg/L was oscillated for 5h,at room temperature and adsorbent dosage of 20g/L.Among the seven kinds of pretreated adsorbent materials:zeolite,biological ceramsite,volcanic stone,coconut shell activated carbon,commercial activated carbon,quartz sand and diatomite,biological ceramsite had the best adsorption effect on total phosphorus,and the removal rate was 61.15%.Therefore,biological ceramsite was selected as the adsorbent in this experiment.（2）Single modification:1mol/L of eight chemical reagents（HCl,NaOH,NaCl,MgCl₂,CaCl₂,FeCl₃,Al₂（SO₄）₃,ZnCl₂）were used to hydrothermal modification of biological ceramsite respectively.The adsorption effect of FeCl₃ modified ceramsites on total phosphorus was the best.For 100ml of 3mg/L phosphorus-containing wastewater,20g/L of adsorbent was added at room temperature,the total phosphorus removal rate was 90.24% when the adsorption was oscillated for 180min,and 97.76% when the adsorption took 240min.Compound modification:（1）among the 8 chemical reagents+high-temperature roasting（400℃3h）modified ceramsites,the FeCl₃+high-temperature roasting modified ceramsite had the best adsorption effect on phosphorus,the total phosphorus removal rate was 70.56% when the adsorption was oscillated for 180min,and 89.38% when the adsorption took 240min.（2）Among the 8 chemical reagents+microwave irradiation（406W 10min）modified ceramsites,the FeCl₃+microwave irradiation modified ceramsite had the best adsorption effect on total phosphorus,the total phosphorus removal rate was 98.27% when the adsorption was oscillated for 180min,and up to 100% when the adsorption took 240min.Among the 24modification methods,FeCl₃+microwave radiation-modified ceramsite had the best adsorption effect on total phosphorus,so the microwave-assisted FeCl₃ modification was chosen as the best modification method of biological ceramsite.According to the results of single-factor test,the optimal preparation conditions of FeCl₃+microwave irradiation modified bio-ceramics are:the concentration of FeCl₃ solution is 0.6mol/L,the ratio of solid to liquid is 1:5,the power of microwave irradiation is 567w,and the time of microwave irradiation is 10min.（3）The result of electron microscope scanning showed that the surface of the modified biological ceramsite presented irregular porous structure,with a large number of holes and gaps,and the average pore size increased.FT-IR analysis showed that after the modification,the types and quantity of saturated alkane groups were less,and the number of carboxyl groups and phenolic hydroxyl acid functional groups increased in the surface functional groups of the bio-ceramsite.Therefore,more adsorption sites are provided for the adsorption of phosphate.At the same time,the ceramsite forms the hydroxylation surface,and the phosphate in water will exchange ions with the hydroxyl group to form the surface coordination complex.XRD analysis showed that before the modification,the ceramsite was mainly composed of SiO₂,Al₂O₃,CaCO₃,Fe₂O₃ and other components.After the modification,the content of CaCO₃ was significantly reduced,and the Al₂O₃ content was also reduced.In addition,nano-sized FeOOH appeared on the surface of the ceramsite.FeOOH improves the isoelectric point of the modified ceramsite,which is conducive to the adsorption of negatively charged phosphate ions in the water.Meanwhile,the phosphate in the water can replace the position of the hydroxyl group and coordinate with Fe³⁺to form a complex,so that phosphorus can be removed.BET test results showed that the structure types of ceramsite before and after modification were all non-uniform slit holes,and the pore size was generally in the range of mesoporous,the most probable pore size was about 4.1nm,the specific surface area of modified ceramsite was 3.26 times larger than that before modification,and the pore volume was also significantly improved,so the adsorption capacity of biological ceramsite to total phosphorus was improved.（4）The adsorption behavior of modified ceramsite to total phosphorus was more in line with Langmuir isotherm adsorption model,with a correlation coefficient of 0.9858,indicating that the adsorption of phosphorus is mainly based on the chemical adsorption of single molecular layer and supplemented by the physical adsorption;the adsorption process of modified ceramsite to total phosphorus had the highest fitting degree with the pseudo-second-order kinetic equation,and the correlation coefficient was 0.9944,indicating that the controlling step of the adsorption rate of total phosphorus is chemical reaction or chemical adsorption by electron gain and loss.（5）when the total phosphorus concentration is 3mg/L,the optimal static experimental conditions for the adsorption of total phosphorus by modified biological ceramsite are as follows:the dosage of modified ceramsite is 30g/L,the pH value of simulated phosphorous wastewater solution is 4,the reaction temperature is 55℃,and the oscillation time is 50min.Under this condition,the adsorption and removal rate of total phosphorus in water by modified bio-ceramsite can reach 100%,and the total phosphorus can be almost completely removed.（6）NaOH solution with 1mol/L concentration as the desorption agent had the best desorption effect on the adsorption of saturated modified ceramsite,and the desorption rate of total phosphorus reached 96.22%,which could realize the effective recovery of phosphorus.The modified ceramsite treated by desorption was used to absorb 3mg/L phosphorus-containing wastewater,and the total phosphorus removal rate was 64.99%.（7）An adsorption column with a height of 700mm and an inner diameter of 50mm was used for dynamic adsorption experiments.The Optimal adsorption conditions are as follows:the thickness of the modified ceramsite filter layer was 0.45m and the inflow rate of wastewater was 4ml/min.Under these conditions,the adsorption penetration time was 570min and the adsorption saturation time was 780min.The thicker the modified ceramsite filter layer is,the smaller the inlow rate is,and the longer the penetration time and saturation time of total phosphorus adsorption are.（8）The average concentration of total phosphorus in the primary sedimentation tank of intermediate water treatment station of a university in kunming was 2.632 mg/L,and the average concentration of total phosphorus decreased to 0.428 mg/L after dynamic adsorption by the modified ceramsite filter column,and the removal rate was 83.73%.The total phosphorus concentration of sewage in the upstream section of Longwangmiao river was1.326 mg/L.After dynamic adsorption,the average total phosphorus concentration decreased to 0.334 mg/L,and the removal rate was 74.81%.（9）The modified biological ceramsite has a well removal effect on the total phosphorus in the wastewater,the price of its raw material is lower than the activated carbon filter material,and the biological ceramsite has the advantages of high mechanical strength,suitable for microbial film hanging,and will not increase the chroma of the effluent,which is suitable to be used as a water treatment filter material.