Removal Of Nitrate By Anion Adsorption Activated Carbon Combined With Electron Shuttle

Nitrogen pollution in water environment has always been a hot and difficult topic in environmental research.Because nitrate nitrogen is not easy to be adsorbed and removed,it is easy to migrate with water,which requires anaerobic denitrification and the removal conditions of sufficient carbon source supply,resulting in the increase of its concentration in surface water and groundwater,and thereby causing harm to the aquatic ecological environment.Therefore,it is of practical significance to study the packing that can quickly adsorb nitrate and accelerate the removal of low concentration nitrate by enhancing the electron transfer in the process of biochemical denitrification.In this paper,polydiallyl dimethyl ammonium chloride（pDADMAC）was simply modified activated carbon to prepare an efficient and low-cost nitrate adsorbent,and the feasibility of combining adsorption with biological nitrogen removal to improve the efficiency of nitrogen removal was explored.On the other hand,the immobilized electron shuttle was prepared by using anthraquinone-2-sodium sulfonate（AQS）to investigate the effect of enhancing biological nitrogen removal.Further,the synergistic effect of immobilized electron shuttle and nitrate adsorbent on biological nitrogen removal was discussed.The results are as follows:The prepared nitrate adsorbent（pDADMAC-GAC）had good adsorption capacity for NO₃^-,and the equilibrium adsorption capacity was 3.36-5.06 times of that of unmodified activated carbon（GAC）.The optimal modified concentration of pDADMAC is 350 g·L^-1.When the adsorbent dosage is 1 g·L^-1 and p H is 6,the NO₃^-equilibrium adsorption capacity reaches 3.21 mg·g^-1 at 25℃.The adsorption process of pDADMAC-GAC on nitrate conforms to the quasi second-order kinetics and Langmuir model,which is essentially a single molecular layer chemical adsorption.The adsorption-desorption experiment showed that the adsorbent had the ability to be reused.Coexisting anions(SO₄^2-,H₂PO₄^-,Cl^-)have some interference on the adsorption of nitrate.After adsorbed on the surface of activated carbon,the immobilized electron shuttle（AQS-GAC）was obtained.The adsorption process of NO₃^-was consistent with the quasi second-order kinetic model（R²=0.9915）and Langmuir isotherm model（R²=0.9632）,and the single molecule chemisoradsorption was the main method.The electron accepting capacity（EAC）and electron donating capacity（EDC）of AQS-GAC were 1.26±0.32 mmole^-g^-1 and-1.37±0.51 mmole^-g^-1,and the electron transfer capacity（ETC）of AQS-GAC was 2.63±0.83mmole^-g^-1,indicating a good electron transfer capacity.When the influent NO₃^-concentration is 5 mg·L^-1 and the hydraulic retention time is 2 d,the removal rates of pDADMAC-GAC adsorption filter column（PDM₀）,AQS-GAC adsorption filter column（AQS₀）and mixed material adsorption filter column（PDM₀+AQS₀）are about 2.62-3.36 times of that of GAC adsorption filter column（CK₀）.With the increase of operation cycle,the removal rate of each device decreases.Furthermore,the removal of low concentration nitrate by the modified material in the micro biological filter was studied.When C/N=5 and HRT=1 d,the nitrate removal rates of GAC biofilter（CK₁）,pDADMAC-GAC biofilter（PDM₁）,AQS-GAC biofilter（AQS₁）and mixed material biofilter（PDM₁+AQS₁）were significantly affected by influent NO₃^-concentration.As HRT increased,nitrate desorbed from the surface of the adsorbents,while NO₃^-desorbed could not be captured by microorganisms in time and entered the body for denitrification,and the nitrate removal effect of the four devices became worse.The removal rate of PDM₁ decreased the most,which was 62.67%.The nitrate removal rates of PDM₁ and AQS₁ devices were higher than those of CK₁ when NO₃^-concentration was 25 mg·L^-1 and HRT was 1 d for 22 days,indicating that the coupling of adsorption and biological methods could improve the efficiency of biological nitrogen removal,and the immobilized electron shuttle could accelerate the electron transfer rate in the process of biological nitrogen removal.The nitrate removal rate of PDM₁+AQS₁ was 37.44%,which was higher than the average removal rate of PDM₁ and AQS₁,indicating that the application of AQS-GAC in adsorption and biological coupling system could further strengthen the biological denitrification process.Finally,the removal mechanism of nitrate nitrogen by micro-biofiltration was analyzed through microbial community diversity.The order of microbial community abundance and diversity in the four micro biological filters is AQS₁>PDM₁+AQS₁>PDM₁>CK₁.The unique OUT quantity of PDM₁,AQS₁ and PDM₁+AQS₁ is 1.54-2.99 times of the total OUT quantity.The addition of pDADMAC-GAC and AQS-GAC could promote the growth of Burkholderia-Caballeronia-Paraburkholderia and Saccharimonadales.In addition,AQS-GAC also promoted the enrichment of Run-SP154.Thus improve the nitrogen removal performance of the device.In conclusion,pDADMAC-GAC has the potential to be an excellent NO₃^-anion adsorbent.It is feasible to combine the adsorbent with microorganism for denitrification.AQS-GAC can promote the electron transfer between the electron donor and the denitrizing microorganism,and increase the rate of nitrate reduction.This paper provides a new idea for the research and development of functional fillers for ecological denitrification facilities in tailwater of sewage treatment plant and functional fillers for ecological restoration facilities containing low concentration of nitrate in surface water and groundwater.