| The surplus sludge produced by urban sewage treatment plants is rich in phosphorus substances.If it can be effectively recovered,the "phosphorus crisis" can be alleviated to a certain extent At present,the recovery of phosphorus from sludge supernatant by crystallization methods such as struvite has been widely concerned,but there arc some problems such as low purity of the recovered product and unstable recovery conditions caused by interference of metal ions.Therefore,study heat acid and alkali pretreatment on the sludge to improve the phosphorus release amount,on the basis of using high adsorption capacity and selectivity of the magnetic zirconium oxide nanomaterials as adsorbent adsorption on the recovery of the sludge supernatant of phosphorus,reuse of struvite and hydroxy calcium phosphate method recovering phosphorus from the stripping liquid,in order to improve the recovery rate and purity.Research with the aid of experimental analysis and theoretical calculation,scanning electron microscope(SEM),energy dispersive X-ray spectrometer(SEM-EDS)and X-ray diffraction(XRD),the magnetic zirconia adsorption/desorption efficiency of phosphorus in sewage sludge,identify the main factors affecting the magnetic zirconia phosphorus adsorption,reveals the cation effects on magnetic zirconia phosphorus adsorption mechanism.The purity of struvite and calcium hydroxyl phosphate crystal products was determined.The main conclusions are as follows:(1)The adsorption efficiency of magnetic zirconia on phosphorus in the supernatant of residual sludge from acid heat treatment and alkali heat treatment was obtained.The isothermal adsorption of phosphorus by magnetic zirconia accorded with the Langmuir model,and the theoretical saturation adsorption capacity Qmax=55.56 mg/g.After the sludge is pretreated by acid heat and alkali heat,the concentration of phosphate in the supernatant can reach up to 320mg/L and 150mg/L respectively,and the magnetic zirconia is used for adsorption.When the optimal dosage is 15 g/L and the optimal reaction time is 8 h and 4 h,respectively,the maximum adsorption rates of phosphorus in sludge after acid heat treatment and alkali heat treatment by magnetic zirconia were 97.44%and 93.76%.Quasi-second-order kinetic model is the most accurate description of the kinetic process of magnetic zirconia adsorption of phosphorus from sludge supernatant,and equilibrium adsorption qe is 22.69 and 9.97 mg/g.(2)The desorption and recycling effects of magnetic zirconia and phosphorus oxide adsorption were determined.After optimization,phosphorus desorption solution with good performance was selected.Phosphorus desorption solution of acid-heat pretreatment was 0.1M NaOH+1M NaCl mixed solution,and phosphorus desorption solution of alkaline heat pretreatment was 0.1M NaOH+0.1M NaCl mixed solution.The adsorption loss is less than 25%after 5 cycles,desorption rate is maintained at 92-97%and 80-90%.The concentration of PO43-in the desorption solution is 230-311 mg/L and 113-126 mg/L.SEM-EDS analysis shows that magnetic zirconia can adsorb Ca2+,Mg2+,Al3+ and other metal ions in addition to PO43-.(3)The main ions that affect the magnetic zirconia phosphorus adsorption are identified,and the mechanism of coexisting ions on phosphorus adsorption is revealed.SO42-,CO32-,Ca2+,Mg2+,NO3-,Fe3+ plasma have a great influence on the adsorption performance of magnetic zirconia and phosphorus.Under the condition of acid-thermal pretreatment,SO42-is the most important anion and Ca2+ is the most important cation.Under the condition of alkaline thermal pretreatment,CO32-was the most important anion and Ca2+ was the most important cation.The experimental results show that the mechanism of cationic inhibition of magnetic zirconia and phosphorus adsorption is that the cationic load forms a occupying position on the surface of the adsorbent,and the change of the solid/solution interface potential and the formation of phosphate tri-element bridging complex on the phosphate loaded adsorbent can promote the phosphorus adsorption.(4)Effectively improve the purity of phosphorus recovery products.Phosphorus was recovered by struvite method and calcium hydroxyl phosphate method from magnetic zirconia phosphorus desorption solution.Characterization analysis using SEM-EDS and XRD showed that the purity of struvite recovered from the acid thermal pretreatment desorption solution was up to 91.3%,which was 2.2 times higher than that from the acid thermal hydrolyzed sludge supernatant.The recoveries of struvite from the desorption solution after acid heat treatment and alkali heat treatment were 95.8%and 97.4%.The recoveries of calcium hydroxyl phosphate were 84.2%and 90.8%,and the purities are 37.5%and 32.4%(calculated by P2O5),which were not much different from the theoretical values of hydroxyapatite(42.41%). |
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