The Performance And Mechanism Of Sulfadiazine Degradation By Zero-valent Iron/Biochar Composite Activating Persulfate

Antibiotic pollution has become a global environmental health problem.At present,the main rivers,lakes,bays and water together with waste activated sludge in sewage treatment plants of our country are generally polluted by antibiotics,and among which the sulfonamides are one of the most prevalent antibiotics.Compared with traditional methods,the advanced oxidation process of zero-valent iron and biochar composite（ZVI/BC）activated peroxodisulfate（PDS）has great advantages in the removal of antibiotics from water.However,the reported ZVI/BC in the literature has some drawbacks,including small specific surface area（SSA）,low proportion of Fe⁰,unclear formation mechanism of embedded Fe⁰,etc.,resulting in a great to improve the efficiency of ZVI/BC activated PDS in destroying antibiotics in environment.In addition,the existing ZVI/BC/PDS systems mainly focus on the removal of antibiotics in water environment,while the removal of antibiotics in complex media such as sludge is very limited.Based on the above background,this paper prepared a series of ZVI/BC with improved qualities by screening and regulation the iron source in the method of co-pyrolysis to prepare ZVI/BC.Then,the as-prepared ZVI/BC was employed to activate PDS to remove sulfadiazine（SDZ）pollution in the water and sludge system respectively.The main research contents and results are as follows:In order to prepare ZVI/BC material with high SSA and high embedded Fe⁰ratio,K₂FeO₄ was used as iron source in co-pyrolysis to get products marked as BC-Fe_n.The main iron phase isα-Fe⁰ in embedded state,and the content ofα-Fe⁰increased with the increase of K₂FeO₄ dosage when biomass was sufficient.The optimized co-pyrolysis product,BC-Fe_0.05,produced by regulating the ratio of K₂FeO₄ and biomass,has the SSA value of 1086 m²/g,which is the highest in the literature.The oxidation of raw biomass by K₂FeO₄ and the corrosion of biochar（BC）skeleton by K⁺during co-pyrolysis are the two main reasons for the formation of high SSA.Meanwhile,K⁺can promote the production of pyrolytic reduction gas（H₂ and CO）released from biomass,which is conducive to the formation ofα-Fe⁰ phase.In the range of less than 800℃,the increase of pyrolysis temperature is beneficial to the improvement SSA prepared by co-pyrolysis with K₂FeO₄ as iron source.BC-Fe_n/PDS system had excellent SDZ removal ability in water,environment and BC-Fe0.05/PDS system showed the best removal performance,which could completely remove SDZ within 10 min.The degradation process displayed pseudo-first-order kinetic characteristics with pseudo-first-order kinetic constant(k_obs)of 0.6429 min^-1,which is much higher than that of other catalysts reported in the literature under similar conditions.The results of active species masking and free radical trapping experiments demonstrated that SDZ removal in BC-Fe_0.05/PDS system was mainly achieved by oxidation effects of SO₄^·-,·OH,O₂^·-and¹O₂.The huge SSA of BC-Fe_0.05 fully improved the synergistic effect ofα-Fe⁰ and BC in SDZ removal by activating PDS.On the one hand,the huge SSA facilitated the dispersity ofα-Fe⁰ on BC,and subsequently improved its ability of PDS activation.On the other hand,the huge SSA was also able to enrich the SDZ on the surface of BC-Fe_0.05,enhancing the contact probability between SDZ and the above four active species,and subsequently accelerating the degradation process of SDZ.It was confirmed that theα-Fe⁰ and the surface C=O functional groups of BC-Fe_0.05 were the key activation sites for PDS activation.Lower p H,higher temperature,and coexisting Cl^-were beneficial to SDZ removal,whereas the coexisting CO₃^2-and HPO₄^2-had inhibitory effects on SDZ degradation.BC-Fe_0.05/PDS system was able to destroy SDZ via four main degradation pathways and then reduced the ecotoxicity of SDZ polluted water.Although the removal effect of SDZ was inhibited somehow by the complex components of real water,the rapid removal of SDZ in real water would still come true by BC-Fe_0.05/PDS system.In order to realize"waste recycling"and reduce the synthesis cost of ZVI/BC,a series of red mud（RM）based ZVI/BC（RMIS_n）was prepared by co-pyrolysis of RM and waste industrial syrup.The RMIS_nwith main iron phase asα-Fe⁰,has porous pore structures,rich oxygen functional groups and ultra-high magnetic saturation strength.RMIS_n/PDS system showed good performance in SDZ removal in water.The RMIS_1:1/PDS system displayed the best removal performance,in which 99.7%SDZ was able to be removed within 10 min with k_obs of 0.3001 min^-1.Besides,the RMIS_1:1/PDS system was also effective in removing other antibiotics,including carbamazepine,oxytetracyclin,ciprofloxacin and ibuprofen.The generated SO₄^·-,·OH,and O₂^·-and ¹O₂ via PDS activating byα-Fe⁰ and C=O in RMIS_1:1 were responsible for SDZ degradation.The initial SDZ concentration had stronger influence on its degradation performance than the dosages of PDS and RMIS_1:1.Co-existed Cl^-slightly improved SDZ degradation rate,while other anions(NO₃^-/SO₄^2-/HCO₃^-/CO₃^2-)and humic acid（HA）inhibited SDZ removal.The electrical energy per mass of SDZ was about 2.24 k Wh/（g·SDZ）by RMIS_1:1/PDS,which was lower than any other reported advanced oxidation processes,implying its good potentiality in practical application.Considering about that the less research on removal of antibiotics in waste activated sludge（WAS）,this study synthesized ZVI/BC（RSn）through co-pyrolysis of red mud and dewatered sludge,investigated the degradation performance of RSn/PDS system on SDZ in WAS,and revealed the mechanism of SDZ removal process by RSn/PDS how influence WAS anaerobic fermentation.The results demonstrated that SDZ was removed effectively by RSn/PDS system,with removal efficiency was above 90%,which was improved along with increasing dosage of RSn and PDS.The SDZ degradation simultaneously enhanced solubilization of WAS and improved the SCFAs yield and acetate acid percentage,while it inhibited the methane production.The SO₄^·-,·OH,O₂^·-and¹O₂ were dominant contributors for SDZ removal and WAS disintegration.The 16S rRNA results showed the process of SDZ removal via RSn/PDS was beneficial for enrichment of acidogenic microbes,Proteiniclasticum,Petrimonas,and Fermentimonas.Metagenome analysis revealed that the process of SDZ removal via RSn/PDS improved the relative abundance of functional genes regulating on extracellular metabolism of carbohydrate and protein,membrane transport and acetate production,while decreased the relative abundance related with methanogenesis.In conclusion,the results of this study can provide some theoretical basis for optimizing the treatment of SDZ contaminated water and sludge medium by ZVI/BC/PDS system.