Magnesium Conversion Mechanism And Algae Cultivation In Magnesium Anode Electrolysis System Treating Wastewater Containing Nitrogen And Phosphorus

Agricultural sewage is known for its high content of nutrients,such as phosphorus,nitrogen,and organic matter.Direct discharge into water and soil can cause serious eutrophication problems.It should be recycled using efficient and low energy-cost consumption methods before being discharged into natural water bodies.At the same time,phosphorus is an essential but non-renewable element for living organisms participating in biological activities.Today,most of the world’s phosphate production is used for agricultural production and applications,especially fertilizers.However,limited phosphorus resources and growing demand for phosphorus astrict agricultural development.The electrolysis process with sacrificial magnesium anode based on the traditional magnesium ammonium phosphate（MAP）precipitation method has lots of advantages,such as realizing a low-cost magnesium source,forming an alkaline environment spontaneously,and reducing the input of chemical reagents.This research proposed a new method combining sacrificial magnesium anode with electrodialysis to extend the treatment range of sacrificial magnesium anode.Secondly,to improve the current efficiency of the magnesium anode,an AZ31 alloy was polarized in wastewater containing different concentrations of nitrogen and phosphorus to explore the mechanism of magnesium ions releasing and evaluate the contribution of hydrogen evolution reaction in the total magnesium release.Thirdly,the common magnesium alloys on the market were screened for electrochemical tests to evaluate their performance and stability in continuous long-term electrolysis experiments.Then this research investigated the release of impurity metal ions from magnesium alloys and their influence on precipitates.Finally,to solve the problem of excessive magnesium ions in the treated electrolyte and test the treatment effecicency of the system on the real wastewater,the biogas slurry was selected for electrolysis pre-treatment.Spirulina was cultivated in the medium based on the electrolyte to realize the reuse of the residual magnesium.A system of phosphorus removal,struvite recovery,and residual magnesium reuse was achieved.The main research results are as follows:（1）In the magnesium anode electrodialysis system,the phosphorus and ammonia of the wastewater is transferred from the dilute chamber to the concentrate chamber,then combined with magnesium ions in the anode chamber to generate high-purity MAP precipitates.The optimal parameters of the system were p H 8.8 of influent and 200 L/h of flow rate.In the continuous mode,with the increase of cycle,the phosphate removal efficiency increased,and the precipitation of MAP strengthened.The lower phosphorus concentration limit of wastewater treated by this system reached 10 mg/L.The stable phosphate removal rate of dilute chamber reached 99%,and phosphate concentration in the concentrate chamber stabilized at about 30 mg/L.Furthermore,the initial current efficiency of the magnesium anode reached270%.However,with the increase of treatment time,the current efficiency was significantly reduced.This new electrodialysis system with magnesium anode can promote the struvite precipitation in wastewater containing ammonia and phosphorus,and accelerate the phosphate recovery,improve magnesium utilization.（2）In order to improve the current efficiency of magnesium anode and explore the physical and chemical changes of polarized anode surface,AZ31 alloy was used for the electrochemical test in two kinds of nitrogen and phosphorus synthetic wastewater.In the absence of electricity,the concentration of nitrogen and phosphorus in wastewater determined the degree of hydrogen evolution reaction of magnesium plate.The release of magnesium ions in 100 mg/L-P wastewater by hydrogen evolution reaction is 2.7 times of that in 10 mg/L-P wastewater.High concentration of wastewater and polarization will accelerate hydrogen evolution on the surface of magnesium anode,but the magnesium ions produced gradually decreased with the coating of oxide film on the surface of magnesium anode.Mechanism of magnesium conversion on magnesium anode surface in wastewater containing nitrogen and phosphorus:Magnesium metal lose electron and releases Mg²⁺due to applied current;Hydrogen evolution reaction occurred between bare magnesium metal and water molecules to form a mixed double precipitation layer with Mg O inside and Mg（OH）₂,Mg NH₄PO₄ and Mg CO₃ outside.The formation of Mg²⁺in this part was affected by the double precipitation layer.In addition,the polarization can make Mg NH₄PO₄ less adsorbed on the anode surface and converted to precipitation.（3）In order to explore the interference of alloying elements on the system effect and reduce the cost of magnesium anode electrolytic process,this study selected common pure magnesium,AZ31 magnesium alloy,and AZ91D magnesium alloy for testing.In the single treatment of high-concentration nitrogen and phosphorus wastewater,pure magnesium anode showed the fastest phosphorus removal efficiency,the highest magnesium release,and the most minor impurity ion release,followed by AZ31 and AZ91D.The difference came from the strength of hydrogen evolution reaction of different magnesium materials,and the hydrogen evolution strength was pure Mg>AZ31>AZ91D.However,due to the soft quality of the pure magnesium plate,it could not support long-term continuous experiments,resulting in severe corrosion and bending of the plate.In contrast,magnesium alloy could maintain stable magnesium release to support the continuous experiment.Moreover,due to the high p H environment,a large amount of Mg（OH）₂would be mixed into the product at the later stage of the continuous experiment of pure magnesium anode electrolysis,which would affect the purity of the product.The defect of AZ31 and AZ91D anodes was that a small amount of Al³⁺,Mn²⁺,and Zn²⁺impurity metal ions would released and mixed into precipitates.Al element was the element with the greatest interference in magnesium anode system.The higher the content of Al element in magnesium alloy,the more Al³⁺released into the electrolyte,and the easier it was to precipitation and pollute the product.By comprehensive comparison,AZ31 had outstanding phosphorus removal performance among the three materials and showed excellent sustainability and stability.（4）In order to realize the reuse of residual magnesium ions,the static test was conducted to study the magnesium anode’s nitrogen and phosphorus removal effect on the biogas slurry with different electrolysis times.The phosphorus removal efficiency reached 99.9%,the ammonia removal efficiency reached 53.7%,and the p H after electrolysis reached 10.The alkaline environment was suitable for the growth of spirulina cells.With the help of the flocculation and adsorption of Mg（OH）₂,organic matter and suspended particles in biogas slurry were removed in foam and precipitates.The turbidity of biogas slurry was reduced by61%by a three-hour electrolysis experiment.The magnesium anode showed excellent treatment performances for biogas slurry.The culture medium based on biogas slurry electrolyte was better than the biogas slurry,but there needed to be more phosphorus and nitrogen sources.Adding additional nutrients could promote the growth and development of spirulina cells and improve their ability to absorb magnesium from the medium.The magnesium content of spirulina cultured by biogas slurry electrolyte reached 5.1%,much higher than that of the common spirulina,and had self-flocculation ability,reducing the cost of further harvest,which could realize excessive Mg²⁺reuse generated by the magnesium anode electrolytic system.