Nitrogen And Phosphorus Recovery From Wastewater By Mg(OH)2 Modified Biochar And Application For Heavy Metal Immobilization In Contaminated Water

The magnesium hydroxide modified biochar（Mg（OH）₂-BC）was prepared by a precipitation-thermal modification method.Ammonia nitrogen and phosphate were simultaneously recovered from the through simulated biogas slurry by the adsorption-struvite crystallization deep coupling technique.The recovered product,struvite supported biochar（MAP-BC）,was applied to immobilized Cu（Ⅱ）and Pb（Ⅱ）in simulated heavy metal polluted wastewater.This study provided a feasible path for ammonia nitrogen and phosphorus synchronously recovery and resource utilization in livestock and poultry biogas slurry.The main conclusions of the study are as follows:（1）The presoma was prepared by direct precipitation method.Biochar was used as a raw material and reacted with magnesium chloride and sodium hydroxide.The precipitation was washed,filtered and dried.After that,the product was calcined in a muffle furnace at 450°C for 2 h to obtain Mg（OH）₂-BC.（2）The adsorption capacity of ammonia nitrogen and phosphate by Mg（OH）₂-BC could reached 130.0 mg/g and 58.83 mg/g respectively in a Mg（OH）₂-BC dosage of0.3 g/L,initial nitrogen and phosphorus concentrations of 120 mg/L and 60 mg/L,initial solution pH of 8,and reaction temperature of 298.15 K.The results showed that the optimal solution initial pH was 8;kinetic and thermodynamic studies showed that the adsorption of ammonia nitrogen and phosphate in solution by Mg（OH）₂-BC was more fitted with the pseudo-second order kinetic model and Langmuir model.Which indicated that the adsorption of ammonia nitrogen and phosphate Mg（OH）₂-BC were both spontaneous and exothermic.A low temperature could favor the adsorption.（3）Applied Mg（OH）₂-BC to recover nutrients in sludge-digestion liquid.The optimal Mg（OH）₂-BC dosage was 0.7 g/L,and the maximum adsorption capacity of ammonia nitrogen and phosphate were 35.71 mg/g and 182.14 mg/g,respectively.The removal efficiency of ammonia nitrogen and phosphate could reach 35.97%and82.26%,separately.（4）The results of batch Pb（Ⅱ）and Cu（Ⅱ）adsorption experiments showed that the immobilization efficiency of MAP-BC on Pb（Ⅱ）and Cu（Ⅱ）was both increased while the raising of solution pH.The surface morphology and structure of the final product would change at different initial solution pH.And the fixation effect of MAP-BC on Pb（Ⅱ）and Cu（Ⅱ）is positively related to the initial pH of the solution.MAP-BC could significantly increase the pH of the solution,removing heavy metal ions by generating metal hydroxide precipitation.In addition,Pb（Ⅱ）could be immobilized by MAP-BC by forming metal-phosphate precipitate.Cu（Ⅱ）could be removed by forming copper-ammonia-phosphate precipitate and copper（II）hydroxide.（5）The kinetics and thermodynamics studies showed that the adsorption of Pb（Ⅱ）and Cu（Ⅱ）by MAP-BC was more suited with the pseudo-second order kinetic model and Langmuir model.Adsorption was endothermic and spontaneous.And high temperature might advance the adsorption.The maximum adsorption capacity of Pb（Ⅱ）and Cu（Ⅱ）could reach 694.44 mg/g and 176.68 mg/g,separately.For coexisting ions,Fe（Ⅲ）had a positive effect on Pb（Ⅱ）and Cu（Ⅱ）immobilization while the Al（Ⅲ）had a negative effect.Chloridion had almost no effect on the immobilization of both Pb（Ⅱ）and Cu（Ⅱ）.Sulfate radical had a positive effect on the immobilization of Pb（Ⅱ）but had little effect in Cu（Ⅱ）immobilization.