The Application Research Of Nitrate Removal From Water Sources Using Modified Magnetic Agricultural Straw Combined With Denitrification

Nitrates are mostly ionic compounds,which are a kind of inorganic pollutants and easily dissolved in water bodies.Nitrate affects the ability of human blood to deliver oxygen,seriously endangering human health.In many water treatment processes,a single water treatment method is difficult to do harmless treatment of nitrate.In this study,adsorption method was used to efficiently adsorb nitrate in water source and combine microbial denitrification to transform nitrate into N2,thus realizing harmless treatment.By reading a lot of literatures,this paper selected wheat straw as adsorbent material,magnetic modified wheat straw adsorbent were prepared through ethylenediamine cross-linking method and the coprecipitation method,then use physical and chemical characterization to explore the modification effect and mechanism.Static adsorption experiments and dynamic adsorption tests were carried out to investigate the influence of different adsorption conditions on the adsorption effect and to analyze the adsorption mechanism.In view of the biological reduction characteristics of nitrate,this study developed denitrifying microorganisms to carry out the regeneration treatment of adsorbent,and discussed the feasibility of microbial regeneration of adsorbent.The main conclusions are as follows:(1)In the ethylenediamine crosslinking method,wheat straw successfully created ethylenediamine modified wheat straw adsorbent.By analysis of different experimental factors,this paper determine the optimum synthesis condition of ethylenediamine modified wheat straw adsorbent is: The dosage of wheat straw 2g;Dosage of ethylenediamine 3ml;Dosage of N,N-Dimethylformamide 10ml;Dosage of triethylamine 10ml;60min etherification process time,temperature of 90℃;Catalytic process time 60 min,temperature90℃;Grafting reaction time of 90 min,the temperature is 90℃.(2)Comparing adsorbent adsorption effect of the wheat straw(WS),attached to the magnetic wheat straw(MWS),ethylenediamine modified wheat straw(AWS)and magnetic modified wheat straw(M-AWS).This study showed that the wheat straw(WS)and magnetic wheat straw(MWS)have low adsorption effect on nitrate.But ethylenediamine modified wheat straw(AWS)and magnetic modified wheat straw(M-AWS)demonstrated good nitrate removal efficiency.This study determined choosing magnetic modified wheat straw asadsorbent.Because both magnetic modified wheat straw adsorbent and ethylenediamine modified wheat straw had good adsorption effect.What’s more,the magnetic modified wheat straw under magnetic field could be quickly separated out after the completion of the adsorbent.(3)The pHpzc of the original wheat straw was 2.1;The pHpzc of ethylenediamine crosslinking method and coprecipitation preparation of magnetic modified wheat straw adsorbent was7.1;The results of XRD and FTIR illustrated that a large amount of positive charged amine groups were attached onto the surface of the magnetic modified wheat straw,which led to the surface of wheat straw changing obvious charge;The modified magnetic modified wheat straw adsorbent surface loaded a lot of amino functional groups and Fe3O4 particles,so the adsorbent had a high positive charge and was helpful to remove the nitrate in the aqueous solution.(4)The adsorption of NO3-by magnetic modified wheat straw(M-AWS)was a quick adsorption process.The adsorption reached the equilibrium within 10-20 mins.The pH was an essential factor that affected the adsorption process.PH within 4 to10,the adsorbent had a high removal rate of nitrate.The adsorption isotherms were fit well with Langmuir model,and the maximum adsorption capacities(Qmax)were 54.5mg/g,51.7mg/g and 49.8mg/g at 20,30 and 40 ℃.The adsorption kinetics indicated that the adsorption of NO3-by M-AWS followed pseudo-second-order kinetics equation.Pseudo-second-order kinetics equation could well describe the adsorption process of nitrate by the magnetic modified wheat straw adsorbent,the fitting curve basic would not happen with the adsorption data migration.It had high correlation coefficient(R2): 0.998-0.999,which indicated that the adsorption process was a chemical adsorption and had a speed limit.(5)Cultivating domesticated denitrifying bacteria microbial process,activated sludge basically adapted to the cultivation of the laboratory environment after 5days.Between 15d-22 d,the growth rate of activated sludge was speeding up.Between 22 d and 30 d,activated sludge biomass basically reached the stable state.Through the cultivation of the activated sludge domestication under laboratory conditions,a certain number of nitrate reducing bacteria in the activated sludge were got.By the flora of DGGE gene fragments of DNA extraction and flora analysis,denitrifying bacteria became the advantage bacterium group andmicrobial community was suitable for the reduction of nitrate.(6)The adsorptions of NO3-by fixed-bed column were controlled under the conditions such as influent concentration,flow rate,influent pH and bed depth.It showed that fixed-bed systems achieved a better uptake of NO3-by M-AWS at a lower concentration,lower flow rate and adsorbent bed-depth.Controlling column velocity,prolonged exposure under certain circumstances could increase the adsorption efficiency,but too long time would affect the actual process.The suitable column adsorption conditions would be helpful for improving the efficiency of the column.(7)Results showed that the reduction adsorbent efficiency by the domesticationed anaerobic microorganisms was about 93% after static adsorption,the second reduction efficiency was about 89% and the fifth reduction efficiency was about 80%;To calculate the penetrating point of fixed-bed column adsorption of NO3-,the reduction efficiency of denitrifying microorganism filled columns was about 89%,the second reduction efficiency was about 83%.