Preparation Of Fe/C Catalysts Based On Ferric Citrate And Its Activation Performance In Degradation Of Sulfadiazine By Persulfate

Sulfadiazine（SDZ）is a representative antibacterial drug of the sulfonamides and is widely used in many industries including medicine,agriculture and farming,but in recent years,it has been frequently reported that sulfonamide antibiotic contamination,especially sulfadiazine contamination,has become increasingly serious and has posed a serious threat to biosafety and environmental stability.The efficient removal of SDZ is extremely necessary and urgent,while the traditional treatment technologies generally have problems such as incomplete removal and difficult practical application.Advanced oxidation methods based on sulfate radicals have been widely studied and focused on for their excellent performance in degrading pollutants.Finding an efficient persulfate catalyst is the key to this method,among which the non-homogeneous Fe/C catalysts are favored by researchers due to their high catalytic activity and easy separation and recovery.However,how to prepare a Fe/C catalyst with environmentally friendly and easy to obtain raw materials,simple and effective preparation,and excellent catalytic effect still needs to be explored.Therefore,in this paper,Fe/C catalysts were prepared from inexpensive and environmentally friendly ferric citrate,which is rich in iron-carbon elements.Fe/C catalysts were characterized and used to activate peroxodisulfate（PDS）and peroxymonosulfate（PMS）for the degradation of SDZ.The effects of reaction factors on the degradation were investigated,and the mechanism of SDZ degradation in the system was investigated to provide some reference for the removal of SDZ in the aqueous environment.The main research findings are as follows:（1）Four Fe/C catalysts were prepared by high-temperature carbonization method at four pyrolysis temperatures（700,800,900 and 1000°C）,and characterized by SEM,EDS,BET,XRD,FTIR,XPS,TG and VSM,respectively.The pyrolysis temperature affected the morphological structure and composition of the final catalyst materials,etc.Both too high and too low pyrolysis temperatures were not conducive to the enhancement of their ability as persulfate catalysts.The pore collapse and agglomeration of the materials occured at high temperatures,and the formation of related iron oxides is less likely to occur at low temperatures.Among them,Fe/C-800 had the highest specific surface area,and it had the same loose and porous mesoporous structure as Fe/C-900.Both XPS and XRD characterization proved the abundant content of Fe²⁺and Fe⁰ in them,which contributed to the activation of persulfate and the degradation of SDZ.Both Fe/C catalysts had excellent magnetic properties and could be quickly separated for recycling.（2）The effects of different pyrolysis temperatures on the activation of PDS by Fe/C catalysts for the degradation of SDZ were investigated,and the catalyst Fe/C-800 at 800℃was determined as the target catalyst of this system for in-depth study,and its performance and mechanism of activation of PDS for the degradation of SDZ were analyzed.The results showed that 0.05 g/L Fe/C-800 could degrade 98.8%of SDZ（10 mg/L）at PDS dosage of 1mmol/L and an initial p H of 7.The system had a wide p H range and could efficiently degrade different concentrations of SDZ at low catalyst dosage with good recycling performance.The inhibition of SDZ degradation by coexisting anions and humic acid was very weak,indicating that the system has strong anti-interference ability.（3）Comparing the effects of four Fe/C catalysts for activation of PMS to degrade SDZ,the best catalyst for PMS system was determined to be Fe/C-900,and the effects of each reaction factor on the degradation of SDZ were investigated in depth.The optimal reaction conditions were as follows:Fe/C-900 at 0.1 g/L,PMS at 0.5 mmol/L and p H=7,which could degrade 98.7%of SDZ（10 mg/L）.Too high or too low Fe/C-900 and PMS concentrations were not conducive to the improvement of the degradation effect.The system performed well at p H 3～9 and the initial concentration of SDZ 5～40 mg/L,especially it had 98.6%removal effect on SDZ at p H=9.The degradation rate of SDZ by Fe/C-900 decreased only 11.2%after three cycles,which had good stability.The dissolved amount of iron ion was only 0.350 mg/L,and the secondary pollution was small.The inhibition effect of coexisting substances on the degradation of SDZ was not significant.（4）Comparative analysis of degradation performance and degradation mechanism of Fe/C catalyst activated PDS/PMS systems were as follows.From the viewpoint of degradation performance,due to the advantages of larger specific surface area and higher iron content,Fe/C-800 can achieve efficient SDZ degradation in PDS system with low concentration,while the dosage of Fe/C-900 in PMS system was doubled.The asymmetric structure and the strong oxidative pollutant removal ability of PMS made the dosage of PMS only half of that of PDS.PMS system was more adaptable to treat alkaline SDZ wastewater than PDS system.The reuse performance of Fe/C-900 in PMS system was better than that of Fe/C-800 in PDS system.Both systems had excellent treatment effect on SDZ in a wide concentration range.In terms of degradation mechanism,both systems had free radical pathways and non-radical pathways acting together in the degradation process of SDZ.The degree of contribution of each reactive substance in the PDS system was¹O₂>O₂^-·>SO₄^-·>OH·,and in the PMS system was ¹O₂>SO₄^-·>O₂^-·>OH·,both based on the non-radical-dominated degradation process of ¹O₂,while SO₄^-·and O₂^-·were the main reactive substances in the free radical pathway.Fe⁰ and C in the catalysts could promote the conversion of Fe³⁺to Fe²⁺to achieve the continuous and efficient degradation of SDZ in a benign cycle.Both systems were activated by the release of Fe²⁺,Fe⁰and others from Fe/C catalysts to activate PDS/PMS or the decomposition of PDS/PMS itself to produce multiple active substances to remove SDZ synergistically.