Coupling Technology Of Catalytic Ozonation And Precipitation For Treatment Of Arsenic(?)-containing Ammonia-nitrogen Wastewater

China is rich in tungsten resources,and the southern area of Jiangxi province has most of them,which known as"The World Capital of Tungsten".However,the wastewater from tungsten smelting contains a large amount of ammonia-nitrogen and arsenic,causing great damage to the environment.Therefore,it is urgent to research a high-efficienct and economical technology of wastewater treatment.Catalytic ozone oxidation technology is widely favored because of its strong oxidizing ability and convenient usage.The advantages of the chemical precipitation method are low cost and simple operation,but As?III?is more difficult to precipitate than As?V?.Therefore,when As?III?is removed by chemical precipitation,it is generally oxidized to As?V?to improve treatment effect.Based on this,this project intends to treat the arsenic-containing ammonia-nitrogen wastewater by coupling technology of catalytic ozonation and precipitation,which in order to provide a feasible solution for the treatment of tungsten smelting wastewater.The competition mechanism between As?III?and NH₄⁺-N by ozone oxidation was analyzed firstly,which showed that As?III?was oxidized prior to NH₄⁺-N.A series of cerium-manganese composite oxide catalysts were prepared by uniform co-precipitation method,and it was found that the catalyst had the highest catalytic activity prepared by Na₂CO₃ precipitant,Ce/Mn molar ratio of 1?2 and calcined at 400°C for 3 h.The test results indicated that the catalyst existed in the form of spherical particles and fine strips.the specific surface area of the catalyst wasn't the main factor affecting the catalytic activity and the active phase of the catalyst was MnO₂.The most active catalyst was used to optimize the conditions of catalytic ozonation process.The mechanism of catalytic ozonation and the stability of the catalyst were studied.The results showed that the best effect was under the condition of reaction temperature 25?,pH of solution 9,flow of ozone 12 mg/min,catalyst dosage 1.50 g/L and reaction time 45minutes.The conversion rate of ammonia-nitrogen with initial concentration of 50 mg/L in arsenic-containing ammonia-nitrogen solution was 80.69%,and the selectivity to gaseous nitrogen was 40.99%,of which NO₂ accounted for 7.40%,NO accounted for 0.85%,NH₃accounted for 0%,N₂ accounted for 32.74%.The conversion rate of As?III?with an initial concentration of 2 mg/L to As?V?was as high as 99.20%.The experiment showed that co-existing Na⁺,K⁺,Ca²⁺and Mg²⁺had no effect on the oxidation of ammonia nitrogen and the conversion of As?III?to As?V?by catalytic ozone oxidation.SO₄^2-and HCO₃^-could inhibit the oxidation of the ammonia-nitrogen due to their capture of·OH while Br^-and CO₃^2-could promote it,but these four anions had no significant effect on the oxidation of As?III?to As?V?.The mechanism of catalytic ozonation showed that the oxidation of ammonia-nitrogen was mainly caused by·OH produced by the decomposition of ozone,while the oxidation of As?III?to As?V?was mainly caused by the direct oxidation of ozone molecules.When the catalyst was reused 5 times,the conversion rate of ammonia-nitrogen decreased to70.18%,the selectivity to gaseous nitrogen decreased to 35.59%,the efficiency of oxidation of As?III?to As?V?was still as high as 98.52%and the recovery rate of catalyst was 85.83%.On the whole,the stability of the catalyst was good.The optimization of conditions for arsenic removal by precipitation showed that As?V?was much easier to remove than As?III?,and the total arsenic removal effect was up to99.10%when As?III?with initial mass concentration of 2 mg/L was oxidized to As?V?,FeCl₃·6H₂O used as arsenic removal precipitant,n?Fe?/n?As?1.50,pH of solution 7 and the reaction time 10 minutes.The co-existing Na⁺,K⁺,SO₄^2-,CO₃^2-and HCO₃^-had no significant effect on the precipitation reaction.Ca²⁺and Mg²⁺could promote the precipitation reaction due to the formation of insoluble compounds with arsenic,while the PO₄^3-could reduced the removal rate of total arsenic because of the complexation reaction between PO₄^3-and Fe³⁺.The mechanism of arsenic removal by iron salt was that Fe³⁺produced by the ionization of iron salt reacted with AsO₄^3-to form FeAsO₄.Finally,the optimal conditions of coupling technology of catalytic ozonation and precipitation were used to treat the actual wastewater from a tungsten smelting plant in Ganzhou,Jiangxi.The conversion rate of ammonia-nitrogen was 73.72%,the selectivity to gaseous nitrogen was 34.98%,and the removal rate of total arsenic was about 98.20%after diluting the raw water twice.The concentrations of ammonia nitrogen and total arsenic in the effluent were 12.92 mg/L and 0.04 mg/L respectively,which could meet the first-class standard of<Integrated Wastewater Discharge Standard>?GB8978-1996?.