Strengthening Effect And Mechanism Of Reducing Agent On Fe(Ⅲ)/Persulfate System

Fe（Ⅱ）activated persulfate（PMS and PDS）advanced oxidation systems have been widely concerned in the field of environmental pollution control because of their ability to produce highly active species and decompose non-biodegradable pollutants.In recent studies,methyl phenylsulfoxide（PMSO）was used as the active species probe,and it was found that PMSO ₂,the characteristic product of iron（Fe（Ⅳ））oxidation of PMSO,was the main product of Fe（Ⅱ）/persulfate degradation of PMSO.Therefore,it is confir med that the main active species in Fe（Ⅱ）/persulfate system is Fe（Ⅳ）,rather than sulfate radical(SO₄^·-),which has long been thought.The low circulating rate of Fe（Ⅲ）/Fe（Ⅱ）greatly limits the efficiency of Fe（Ⅱ）/persulfate removal.Therefore,reducing agents are often added synchronously to accelerate the reduction of Fe（Ⅲ）in engineering.However,most of the previous studies focused on the improvement of the pollution removal efficiency of Fe（Ⅱ）/persulfate system by reducing agents,while t he regulatory mechanism of reducing agents on active species was not studied,especially the effect of reducing agents on the formation of Fe（Ⅳ）in the system has not been reported.This research according to the chemical structure,reducing agent,it can be divided into hydroxyl amine inorganic reductant（hydroxylamine sulfate and hydroxylamine hydrochloride）,phenol quinones organic reductant（pyrocatechol,resorcinol and hydroquino ne and 1,4-benzoquinone）,phenolic acids,organic reductant（gallic acid and protocatechuic acid）amino acids and organic reductant（L-cysteine）and other four classes,The formati on mechanism of active species in Fe（Ⅲ）/PDS and Fe（Ⅲ）/PMS systems reinforced with various reducing agents was studied in detail.PMSO probe and electron paramagnetic resonance（ESR）spectroscopy were used to identify the active species in the system,e valuate the relative yields of Fe（Ⅳ）and SO₄^·-,and clarify the formation paths of Fe（Ⅳ）and SO₄^·-,providing theoretical support for the practical application of reductants enhanced iron activated persulfate system.In Fe（Ⅲ）/persulfate system enhanced by hydroxylamine reductants,the degradation efficiency of PMSO was effectively improved by appropriate hydroxylamine reductants,while excessive hydroxylamine reduced the degradation efficiency of PMSO through competitive consumption of active species.In the Fe（Ⅲ）/persulfate system fortified with hydroxylamine sulfate,the formation rate of PMSO₂（η（PMSO₂））was maintained at 90%-100%,indicating that hydroxylamine sulfate had no effect on the formation pathway of active species,and the main active species in the system was still Fe（Ⅳ）.In hydroxylamine hydrochloride fortified system,η（PMSO₂）decreased with the increase of hydroxylamine hydrochloride dosage,which was due to the formation of^·OH from Fe（Ⅳ）consumed by CI^-competition.The addition of hydroxylamine hydrochloride also significantly enhanced the signal in tensity of^·OH in the ESR spectrum of Fe（Ⅲ）/persulfate system,which further confirmed that hydroxylamine hydrochloride promoted the formation of^·OH in the system.In the Fe（Ⅲ）/persulfate system enhanced by phenolic quinone reductants,in general,the appropriate amount of phenolic quinone reductants increased the degradation efficiency of PMSO system,and excessive amount of phenol quinone reductants would have inhibitory effect,because the reductants competed to consume active species.In addition,η（PMSO₂）remained stable at 90%-100%in each phenolic quinone reducing agent reinforced system,indicating that phenolic quinone reducing agent did not change the species of active species in the system,and Fe（Ⅳ）was still the main production in the system.Meanwhile,no SO₄^·-signal peak was observed in the ESR spectrum of Fe（Ⅲ）/persulfate system enhanced with phenolic quinone reductants,further confirming that Fe（Ⅳ）was the active species in the system.In Fe（Ⅲ）/persulfate system reinforced by phenolic acids and amino acids,a moderate amount of reducing agent can effectively improve the degradation efficiency of PMSO,while excessive reducing agent can reduce the degradation efficiency of PMSO through competitive consumption of active species.At the same time,η（PMSO₂）decreased with the increase of reducing agent,because the complexation of phenolic acid and amino acid reducing agent changed the active species generated in the reaction system from Fe（Ⅳ）to SO₄^·-.ESR experiment also found that the addition of reducing agent enhanced the DMPO-SO₄ signal.This indicates that both Fe（Ⅳ）and SO₄^·-are involved in the reaction system..