Removal And Recovery Of Phosphorus From The Secondary Effluent Of Wastewater Treatment Plant By LDH/BCs

Phosphorus is an important element for agricultural and life activities,but is currently facing the dual problem of excess phosphorus in water bodies and tight agricultural demand.The use of sorbents to enrich phosphorus and reuse it as phosphorus fertilizer in agricultural systems could alleviate this problem.In this thesis,three typical materials of layered double hydroxides(LDHs)compounded with biochar(BC),Zn-AlLDH/BC,Mg-Al-LDH/BC and Mg-Fe-LDH/BC,were investigated to investigate the adsorption properties and principles and to evaluate the feasibility of the adsorbents as phosphorus fertilizer after adsorption.The main results of the study are as follows.The results of adsorption experiments showed that adsorption capacity of LDH/BCs ranged from 36.19 to 58.41 mg/g,and the adsorption capacity increased with the increase of temperature.The adsorption isotherm model fitting showed that the Fredulich model was a better fit for all three LDH/BCs.For the adsorption rate,all three materials were able to reach adsorption-desorption equilibrium within 720 min for a phosphate solution with an initial concentration of 25 mg/L.The adsorption kinetic model found that the quasi-secondary adsorption kinetics simulated the adsorption process best,and thus the adsorption process was mainly controlled by chemisorption.The Elovich adsorption kinetic model showed that the adsorption rate of Zn-Al-LDH/BC was much higher than that of Mg-Al-LDH/BC and Mg-Fe-LDH/BC at the early stage of adsorption.In terms of the mechanism study,by the zeta potential XPS studies showed a decrease in chloride ions before and after adsorption,accompanied by a decrease in metal hydroxyl groups.Therefore,the adsorption process of LDH/BCs includes three mechanisms: electrostatic attraction,ion exchange and complexation.Batch experiments with simulated secondary effluent showed that increasing the concentration of LDH/BCs could increase the phosphate removal rate,but the optimum adsorbent dosage was 0.2 g/L when considering the cost.Meanwhile,the adsorption effect of LDH/BCs was sensitive to the solution p H,and the phosphate removal rate was less than 50% at p H < 5 or p H > 10,while at p H = 7,the phosphate removal rate was more than 95%.The removal rates were all able to exceed 95%.In terms of adsorption selectivity,the adsorption of phosphorus by Zn-Al-LDH/BC was less affected by coexisting anions,while the adsorption effect of Mg-Al-LDH/BC and Mg-Fe-LDH/BC was easily affected by NO3-and SO42-.In the fixed-bed experiments,when the flow rate was 5 m L/min and the adsorption layer height was 0.5 cm,the penetration times of ZnAl-LDH/BC,Mg-Al-LDH/BC and Mg-Fe-LDH/BC were 45,37 and 64 min,respectively.decreasing the flow rate and increasing the adsorption layer height could enhance the phosphorus removal effect of the fixed-bed experiments,and the Thomas model for the fixed-bed The experiments of treating the actual secondary effluent showed that the penetration time of the fixed bed was reduced by 22%-75% of that of the simulated secondary effluent.A graded study of phosphorus in LDH/BCs by the Hedley method revealed that 23%,45% and 46% of the phosphorus adsorbed by Zn-Al-LDH/BC,Mg-Al-LDH/BC and MgFe-LDH/BC was readily available to plants.In terms of long-lasting phosphorus release,all three LDH/BCs released phosphorus stably during the 30-day experimental cycle,but the phosphorus release of Zn-Al-LDH/BC was only 4.2 mg/g,which was lower than that of Mg-Al-LDH/BC and Mg-Fe-LDH/BC at 18.08 and 29.62 mg/g.Experiments with hydroponically grown plants revealed that Mg-Al-LDH/BC and Mg-Fe-LDH/BC significantly enhanced the height and quality of plants,while Zn-Al-LDH/BC inhibited the growth of plants.Mg-Al-LDH/BC and Mg-Fe-LDH/BC elevated the chlorophyll and carotenoid content of plants,while Zn-Al-LDH/BC inhibited photosynthetic pigments of plants.In terms of plant phosphorus content,Zn-Al-LDH/BC inhibited the phosphorus content of above-ground plant parts,while Mg-Al-LDH/BC and Mg-Fe-LDH/BC increased the phosphorus content of above-ground plant parts.All three LDH/BCs increased the phosphorus content in the belowground parts of plants.In conclusion,LDH/BCs-like materials have good adsorption capacity and fast adsorption rate,and the adsorption process involves a variety of mechanisms.Therefore,LDH/BCs can effectively adsorb phosphate from secondary effluent of wastewater treatment plants,and the removal process is significantly influenced mainly by the coexisting anions.The adsorbed phosphorus LDH/BCs can be used as phosphorus fertilizer to promote plant growth,but some of the materials can cause potential toxicity to plants.In conclusion,LDH/BCs can effectively recover phosphate from secondary effluent of wastewater treatment plants.