Mechanism And Performance Of Sulfur-enhanced Iron-based Advanced Oxidation Process For The Removal Of Organic Pollutant In Water

Water shortage caused by pollution has been the biggest challenges for the sustainable development of human beings.Recent years,the advanced oxidation processes（AOPs）based on H₂O₂ and persulfate（PS）attraced increasing attentions on water pollution control due to their advantages of easy operation,practicability,low cost and high effiency,etc.H₂O₂ and PS have similar chemical structure,both of which have a-O-O-bond.Through energy absorption or electro ns capture,-O-O-bond can be activated and produce reactive oxidant species（ROS）such as high valent metal-oxo species and radicals,which have stronger activity and oxidizing capacity than H₂O₂ and PS.Iron-based materials is one of the best electron donors for H₂O₂and PS activation due to its excellent characteristics,such as high electron transfer efficiency,cheap and widely available,diversified forms,strong interactivity and environmental-friendly.However,three problems impede the applacations of iron-based AOPs in wastewater treatment.Fistly,Fe（Ⅱ）,as the electron donor of H ₂O₂ and PS,can activate these oxidants to produce ROS,but Fe（Ⅱ）can also compete the ROS with target organic contaminant,which suppress the contaminant removal and the utlilization efficiency of oxidants.Secondly,as reaction proceeding,the regeneration of Fe（Ⅱ）from Fe（Ⅲ）reduction（Fe（Ⅲ）/Fe（Ⅱ）cycle）is too slow to provide suffient electrons for oxidants activation,which result in the decrease of ROS production and suppress the removal of target contaminant.Meanwhile,the accumulation of Fe（Ⅲ）bring a large amount of iron sulge.Finally,the iron-based AOPs always present an excellent degradation efficiency in acdic condition and the performance under netural or basic conditions is unstisfactory,which limited its wide application.In this paper,aimed at above three problems,four iron-based AOPs system were proposed by using different iron materials and oxidants.Their performances for organic pollutants removal in water were investigated.Meanwhile,the corresponding parameters were optimized and the redox mechanisms were investigated and clarified.Besides,the effects of ambient fators on the performance of these sysems were studied,and the toxicity,cost and efficiency were evaluated.The details are summarized briefly as follows:（1）In order to suppresse the scavenging effect caused by Fe（Ⅱ）,n anoscale zero-valent iron（NZVI）modification material,sulfide modified NZVI（NZVI-S）which could release Fe（Ⅱ）to solution effectively and slowly were fabricated by a liquid phase reduction method.Then a comparative study of NZVI and NZVI-S for peroxydisulfate（PDS）activation to degrade trichloroethene（TCE）was proceeded.The NZVI-S had a core-shell structure that made up with iron core and semiconductor FeS shell,which could effectively improve the electrons transfer efficiency form iron core to NZVI-S surface,accelerated the≡Fe（Ⅲ）/≡Fe（Ⅱ）cycle,improve d the Fe（Ⅱ）releasing efficiency,aggrandized the specific surface area and provided more active sites.Therefore,compared with bare NZVI,NZVI-S showed a better p H application range（p H=2.32-9.58）,stronger contaminant removal ability and higher contaminant mineralization rate when it was used to activate PDS.The NZVI-S/PDS system also exhibited good resistance to coexistence ion or natural organic materials（NOMs）that usually appeared in the wastewater.The Fe/S molar ratio of NZVI-S significantly influenced the PDS activation efficiency,and the highest TCE degradation was observed at Fe/S molar ratio of 25-30.Scavenging experiments and chemical probe tests indicated that SO₄^·-,·OH and Fe（IV）all participated in the TCE degradation.The quantitative evaluation of the contribution of these ROS to TCE removal indicated that SO₄^·-and·OH played a major role in TCE degradation while the role of Fe（IV）was minor.Finally,the possible pathways of TCE degradation in AOPs was revealed.（2）The fabrication of NZVI-S faced some drawbacks,including the complex pre-processing step,low yield rate,and high cost,which significantly limited its practical application in AOPs.Based on the research of NZVI-S/PDS system,the principal constituents of NZVI-S shell,i.e.,FeS was utilized as the activator to construct FeS/PDS and FeS/peroxymonosulfate（PMS）systems for the target contaminant bisphenol A（BPA）elimination.It was found that the BPA removal rate in FeS/PMS system was much better than that in FeS/PDS system.The FeS/PMS system could realize the effective remove of BPA under a broad p H range（p H=3.2-9.5）.FeS could release Fe（Ⅱ）into solution slowly which suppressed the scavenging effect of Fe（Ⅱ）on ROS.Besides,the structural S（-Ⅱ）on the FeS surface could reduce≡Fe（Ⅲ）to≡Fe（Ⅱ）,which achieved the PMS activation and subsequent ROS generation process on the FeS surface.Moreover,S（-Ⅱ）could also release from FeS to solution and then accelerate the Fe（Ⅲ）/Fe（Ⅱ）cycle in solution,by which the homogeneous activation efficiency of PMS was obviously elevated.Scavenging experiments,EPR and chemical probe tests indicated that SO₄^·-and·OH dominated the BPA degradation in FeS/PMS system while Fe（IV）was the minor ROS.The cycle tests of FeS for PMS activation revealed that FeS surface passivated after several return reactions,and the passivation shell of FeS suppressed the Fe（Ⅱ）and S（-Ⅱ）release and ultimately restrained the PMS activation and BPA degradation,for instance,the BPA removal decreased to 40%in the FeS/PMS sytsme by 2 cycle using FeS while the orginal FeS could activate PMS to totally remove BPA.（3）In the FeS/PMS system,the dissolved S（-Ⅱ）released form FeS could effectively accelerate the homogenous phase Fe（Ⅲ）/Fe（Ⅱ）cycle.PMS/Fe（Ⅲ）/S（-Ⅱ）homogenous AOPs were constructed and its performance of organic pollutants removal in water was investigated.The PMS/Fe（Ⅲ）/S（-Ⅱ）system could impede the scavenging effect of initial Fe（Ⅱ）and presented a broad p H application range（3.2-9.5）.Meanwhile,it exhibited good resistance to coexistence ion e.g.,Cl^-,NO₃^-and NOMs.Except for taget pollutant BPA,the system also achieved satisfactory removal of other organic pollutants including atrazine（ATZ）,sulfamethoxazole（SMZ）and rhodamine B（Rh B）.Further research revealed that S（-Ⅱ）and its derivatives(e.g.,S₂^2-,S⁰,SO₃^2-and S₂O₃^2-)in PMS/Fe（Ⅲ）/S（-Ⅱ）system all could reduce Fe（Ⅲ）to Fe（Ⅱ）and then enhance the PMS activation to produce ROS for target pollutant removal.Scavenging experiments,EPR characterization,chemical probes and ¹⁸O isotope-labeling techniques showed that Fe（IV）was the major ROS for BPA degradation in PMS/Fe（Ⅲ）/S（-Ⅱ）system while·OH and SO₄^·-played a minor role.It should be noted that S（-Ⅱ）was a common substance in industrial water which content was usually higher than 2 mg/L.Therefore,the constructed PMS/Fe（Ⅲ）/S（-Ⅱ）system would provide an important methods for“waste control by waste”.（4）The S（-Ⅱ）dosage in Fe（Ⅲ）/S（-Ⅱ）/PMS system was much higher than the effluent limit of 1 mg/L for S（-Ⅱ）in China（GB-8978-1996）.Fe（Ⅲ）/S（-Ⅱ）/H₂O₂,a new Fenton-like system,was constructed to insure a high contaminant removal efficiency and simultaneously reduce the dosage of Fe（Ⅲ）,H ₂O₂ and especially S（-Ⅱ）.The results showed that all this aims were ac hieved and the S（-Ⅱ）dosage in Fe（Ⅲ）/S（-Ⅱ）/H₂O₂ system only 1/10 of that in Fe（Ⅲ）/S（-Ⅱ）/PMS system,which is lower than the threshold limit of S（-Ⅱ）in the effluent.The·OH yield of H₂O₂ in Fe（Ⅲ）/S（-Ⅱ）/H₂O₂ system was 44.2%,which was much higher than traditional Fenton systm（28.9%）,and this achived a better pollutant degradation efficiency in the former system.Besides,the corresponding parameters of Fe（Ⅲ）/S（-Ⅱ）/H₂O₂ system were optimized,in which the contaminant could be rapidly degraded in a broad p H range,transformed to harmless substances and even mineralized.The high electron utility efficiency and capacity of S（-Ⅱ）led to its high reusability in the reduction of Fe（Ⅲ）to Fe（Ⅱ）.The Fe（Ⅲ）/S（-Ⅱ）/H₂O₂ system ont only achieved satisfactory removal of different organic pollutants including BPA,ATZ,SMZ,Rh B,and nitrobenzene（NB）,but also exhibited good resistance to coexistence ion e.g.Cl^-,NO₃^-and SO₄^2-or NOMs e.g.humic acid（HA）.A series of methods,including scavenging experiments,electron paramagnetic resonance（EPR）characterization,chemical probes,and ¹⁸O isotope-labeling techniques confirmed that·OH was the main ROS for BPA degradation.The toxicity evaluation showed that both acute and chronic toxicity of the target pollutants were significantly reduced upon degradation.Moreover,S（-Ⅱ）was relatively low-cost and harmfulless compared to other reported reducing agents such as hydroxylamine,salicylic acid,and hydroquinone.