Mechanistic Studies On Sulfite Activation By Light/Fe³⁺ Process For Efficient Destruction Of Emerging Contaminants In Water

In recent years,various Emerging Contaminants（ECs）,mainly Endocrine Disrupting Chemicals and Pharmaceuticals and Personal Care Products,are difficult to be effectively removed by conventional water treatment technology because of their stable chemical structure and difficult biodegradation,which eventually leads to their entry into the water environment and potential harm to the ecosystem and human health.Sulfate radical-based advanced oxidation technologies（SR-AOTs）are widely used in the field of surface and ground water treatment.All along,the way of generating SO₄^·-was mainly based on the activation of persulfate.However,the high cost of persulfate consumption is one of the major problems that limit the practical application of SR-AOTs.Therefore,developing substitutes of persulfate and corresponding activation technologies can effectively promote the application of SR-AOTs in advanced treatment of organic pollutants.Many activation sulfite technologies have begun to attract attention in the field of wastewater treatment.Sulfite can be obtained from waste liquid of wet flue gas desulfurization.Using sulfite as the donor of reactive oxygen species such as SO₄^·-has the cost advantage of"treating waste with waste".Therefore,exploring cost-effective sulfite activation technology is the key to realize the application of sulfite instead of persulfate.This study focuses on activated sulfite technology,which is mainly divided into three parts:（1）Exploring the efficiency and reaction mechanism of 254nm UV activated sulfite system（UV/S（IV））for degradation of ECs in water;（2）Studying the mechanism of Fe³⁺activated sulfite processes(Fe³⁺/S（IV）)and the limiting factors affecting the degradation of ECs;（3）Exploring the synergistic mechanism of sunlight coupling Fe³⁺/S（IV）process for ECs removal.254nm UV light activation is relatively simple,environmentally friendly and free of secondary pollution.Thence,254nm UV was selected to activate sulfite.Two typical ECs,diethyl phthalate（DEP）and bisphenol A（BPA）showing remarkable differences in photoactivities and dissociating properties were selected as the model pollutants.Exploring the influence of p H and dissolved oxygen（DO）on the efficiency and kinetics of pollutant degradation in UV/S（IV）processes.Identify the key components and action mechanism affecting the conversion of active substances in the processes.The results show that solution p H played the key role in determining the reactive species,and both DEP and BPA were more favorably degraded at more alkaline conditions.Under acidic conditions,the H·,O₂^·-,·OH and SO₃^·-were identified,but the amount of·OH accumulated significantly with the elevation of p H.Moreover,the DO played a key role in deriving·OH from SO₃^·-.The degree of effective utilization of DO varies under acid and alkali conditions,and there is a way to compete DO to generate O₂^·-under acid conditions.Quenching experiments show that the primary species contributing to the degradation of DEP and BPA at alkaline condition were determined to be e_aq^-and·OH,respectively.Besides,DEP showed higher quantum efficiency for the indirect photolysis and mineralization degree than that of BPA at alkali conditions mainly due to the direct use of the primary photoproduct.In the second part of this paper,Fe³⁺,which is widely available and cheap,was selected as the initiator of activating sulfite.The pathway and mechanism of degradation of BPA by Fe³⁺/S（IV）processes were explored.The limiting factors affecting the degradation of BPA in Fe³⁺/S（IV）processes were studied emphatically.The results show that Fe³⁺/S（IV）processes can remove BPA to a certain extent,but the effective p H for Fe³⁺/S（IV）processes are always limited to acidic condition.The lack of DO and S（IV）is the major problem for the stagnation of degradation BPA in the later stage of the system by monitoring the concentration changes of Fe²⁺,sulfite and DO.Supplement of S（IV）and DO can improve BPA removal rate,but the effect is not very obvious.Thence,reducing the intensity of Fe³⁺/S（IV）reaction at the initial stage by reducing the concentration of Fe³⁺.Although this can ensure enough DO in the later stage of the reaction,it also faces the dilemma of reducing BPA degradation efficiency.In addition,·OH and SO₃^·-were detected in the system.Quenching experiments showed that the degradation of BPA was caused by the joint action of SO₄^·-and·OH.The steady-state concentration of SO₄^·-in the system was higher than that of·OH,and the formation of high-valent iron was detected for the first time in Fe³⁺/S（IV）process.In order to further ensure that the Fe³⁺/S（IV）process can continuously degrade BPA at low concentration of Fe³⁺,sunlight was introduced into the process.The influence of sunlight on Fe³⁺/S（IV）process under natural reoxygenation was investigated,mainly focusing on the improvement of O₂utilization and reactive species variation.The results show that both simulated sunlight（SSL）and real sunlight（SL）could enhance the regeneration of Fe²⁺,thereby largely reducing the iron demand and increasing the utilization rate of oxygen and sulfite.Supplementation of S（IV）by stages in SL can completely remove BPA,and the effective p H range could also be broadened to neutral by SSL.Experiments show that UV-A（350 nm centered）was assumed to be the major spectral region in sunlight that contributed to BPA removal in SL/Fe³⁺/S（IV）process.In addition,SO₄^·-and·OH converted from SO₃^·-are still the main contributors to BPA removal in SSL/Fe³⁺/S（IV）process.Although the high-valent iron has also been detected in SSL/Fe³⁺/S（IV）process,they have little contribution to BPA degradation.Different from dark Fe³⁺/S（IV）process,the introduction of sunlight can promote the transformation of ROS and generation of·OH.Based on LC/MS analysis,BPA degradation pathways in SSL/Fe³⁺/S（IV）process could mainly include the hydroxylation of the benzene ring and beta-scission of C-C bond of BPA molecule.To sum up,both UV/S（IV）processes and light/Fe³⁺/S（IV）processes can efficiently degrade ECs in water.The results provide new insights for the mechanism of UV/S（IV）and light/Fe³⁺/S（IV）processes and several promising alternative methods for effectively removing ECs in water.