Simultaneous Extraction Of Nitrogen And Phosphorus From Biogas Slurry And Preparation Of Nitrogen And Phosphorus Sustained-Release Materials Based On Composite Adsorbent

With the rapid development of society and the continuous acceleration of human production activities,organic solid waste has become a serious environmental pollution problem facing the world today.Anaerobic digestion is one of the mainstream technologies for organic solid waste recycling.However,anaerobic digestion will produce a large amount of biogas slurry with high concentrations of nitrogen,phosphorus and refractory organic matter.Improper disposal can easily cause soil and water pollution.However,nitrogen,phosphorus,etc.in biogas slurry are also plant nutrients,so biogas slurry recycling is a hot research topic in recent years.Among many resource recovery methods,adsorption has received extensive attention due to its low cost,environmental friendliness,and high efficiency.Since nitrogen（NH₄⁺）and phosphorus(PO₄^3-)in biogas slurry exist in different forms,it is very difficult to efficiently adsorb nitrogen and phosphorus in biogas slurry at the same time.Therefore,to adsorb nitrogen and phosphorus in biogas slurry at the same time,different characteristic functional groups are required,and The two adsorption operating conditions do not affect each other.In this paper,a multifunctional composite adsorbent capable of simultaneously adsorbing nitrogen and phosphorus in biogas liquid was synthesized using two environmentally friendly raw materials,fulvic acid and hydrotalcite.Fulvic acid（FA）is one of the main components of natural macromolecular humic substances.It has a variety of active functional groups and can adsorb cationic pollutants.Hydrotalcite（Layered double hydroxide,LDH for short）has a special layered structure,has anion exchange properties and is easy to synthesize.In this paper,a multifunctional composite adsorbent of fulvic acid anchored magnesium aluminum hydrotalcite（Mg₂Al LDH-FA）was assembled and synthesized through the coagulation of LDH crystal nuclei in the coprecipitation process and the adsorption bridging effect with fulvic acid macromolecules（FA）.The large-sized FA molecules are anchored on the surface of the LDH layer and can adsorb cationic pollutants,and the LDH interlayer domains are still exchangeable anions,so they can simultaneously adsorb anionic and cationic pollutants.Firstly,through single factor experiment,the paper sets up nitrogen and phosphorus single adsorption and nitrogen and phosphorus composite adsorption system respectively,and investigates the effect of solution p H,Cl^-concentration and adsorbent dosage on the adsorption of nitrogen and phosphorus in aqueous solution by composite adsorbent.The adsorption mechanism of nitrogen and phosphorus was preliminarily explored by analyzing the isothermal adsorption and adsorption kinetic characteristics of the composite adsorbent,and the corresponding adsorption mechanism was further analyzed by means of XRD,Zeta potential and other characterization methods.In addition,the isothermal adsorption characteristics of the composite adsorbent on nitrogen and phosphorus in biogas slurry and the effect of adsorbent dosage on the adsorption of nitrogen and phosphorus in biogas slurry by the composite adsorbent were investigated.At the same time,the preparation of Mg₂Al LDH-FA in biogas slurry and the effects of solid-liquid ratio,material ratio,initial p H and other conditions on the adsorption of nitrogen and phosphorus in biogas liquid were also investigated.Then,the product that adsorbed nitrogen and phosphorus in biogas slurry during the synthesis process was applied to the soil,and the release law of nitrogen and phosphorus in the soil was studied,and the pot experiment of cabbage was carried out.The main conclusions are as follows:（1）The fulvic acid-anchored magnesium-aluminum hydrotalcite multifunctional composite adsorbent（Mg₂Al LDH-FA）was prepared by a simple co-precipitation method.The combination of LDH and FA was firm,and no FA was dissolved in the range of p H=3-11.The interlayer anion of Mg₂Al LDH-FA is still chloride ion,which has an anion exchange function,and the loaded FA can adsorb cationic pollutants.The maximum adsorption capacities of Mg₂Al LDH-FA for Orange II^-,Cu²⁺,Pb²⁺,Ni²⁺and Cd²⁺were 1.78,1.83,3.19,2.00 and 2.09 mmol/g,respectively.（2）The adsorption behavior of Mg₂Al LDH-FA on single-component phosphate and ammonia nitrogen solution conformed to the Langmuir isotherm adsorption model,and the maximum adsorption capacity for phosphate was 19.8 mg/g,and the adsorption mechanism was ion exchange.The maximum adsorption capacity of ammonia nitrogen was only 8.22 mg/g,and the adsorption mechanism was electrostatic attraction.In the mixed solution of nitrogen and phosphorus,Mg₂Al LDH-FA showed excellent nitrogen and phosphorus synergistic adsorption performance,the adsorption capacity for phosphorus increased by about 1 times compared with the single component,and the ammonia nitrogen adsorption capacity increased by 2-3 times.In the actual biogas slurry,Mg₂Al LDH-FA also showed excellent nitrogen and phosphorus synergistic adsorption performance.The adsorption capacity of ammonia nitrogen in biogas liquid can reach 221.64 mg/g,and the removal rate is 22.83%.The adsorption capacity of total nitrogen was 387.74 mg/g,and the removal rate was 28.73%.The high concentration of nitrogen in the biogas slurry promoted the adsorption of nitrogen by the material.（3）The synthetic process of the composite adsorbent has an excellent effect of synergistically adsorbing nitrogen and phosphorus in biogas liquid.When the solid-liquid ratio is 4:1000,the phosphorus removal rate can reach 98.37%.When the solid-liquid ratio is 1:1000,the removal rate of ammonia nitrogen in biogas slurry is as high as about 45%,and the adsorption capacity can reach about 450 mg/g,which is higher than the removal rate of ammonia nitrogen in biogas slurry by directly using LDH-FA material（24%）.nearly twice as high.The loading of FA is beneficial to the adsorption of ammonia nitrogen by the composite adsorbent,but has almost no effect on the adsorption of phosphorus.The removal rate of TP decreases slightly with the increase of LDH:FA material ratio,but the removal rate is always greater than 90%.The p H has little effect on the adsorption of nitrogen and phosphorus.When the p H is in the range of 8-11,the removal rate of TP decreases slightly with the increase of p H.When the p H is 8-10,the removal rate of ammonia nitrogen does not change much,and p H=11 The removal rate was significantly reduced.（4）The total removal rate of total phosphorus,ammonia nitrogen and total nitrogen after three times of synthesis and adsorption under the conditions of solid-liquid ratio 1:1000,LDH:FA material ratio 4:2,and p H=9 by adding phosphorus twice externally The residual phosphorus concentration in the filtrate was only 3.25mg/L,and the ammonia nitrogen concentration was only 8.47 mg/L.The loading of total phosphorus in the solid product is 16.5 mg/g,the loading of ammonia nitrogen is163.2 mg/g,and the loading of total nitrogen is 312.8 mg/g,which is consistent with the slow-release fertilizer specified in the national standard（GB/T 23348-2009）.In contrast,the total nutrient content（N+P₂O₅+K₂O）of nitrogen and phosphorus extracts was 35.06%,which met the requirement of medium concentration≥30%.（5）The nitrogen and phosphorus in the nitrogen and phosphorus adsorption products have the characteristics of slow release.When the cumulative amount of water added was 12 L,the nitrogen and phosphorus in the material were almost completely released.The application of nitrogen and phosphorus slow-release materials in the soil can promote the growth of pakchoi,and the biomass of pakchoi gradually increases with the increase of the application amount.However,with the increase of biogas slurry application,the growth of pakchoi was inhibited.