| Hydroxyl radical(HO·)is a reactive oxidant with a strong oxidizing capacity(standard reduction potential:2.8 V)that can react unselectively with most organic pollutants and redoxsensitive elements.Pyrite is the most abundant iron sulfide on the earth’s surface and the dominant Fe-S mineral in estuarine environments,especially in salt marsh sediments.Under reducing conditions,pyrite can remain stable in the sedimentary environment for a long time.however,wind and hydrodynamic disturbances,tectonic processes,as well as anthropogenic activities result in the oxidation and corrosion of pyrite in terrestrial weathering environments.Pyrite has been proven to be an inexpensive natural Fenton-like reagent that is capable of reducing O2 to produce reactive oxygen species(ROS)such as hydrogen peroxide(H2O2)and HO· under oxic conditions due to the large amount of Fe(Ⅱ)contained in its structure.At present,the HO-production from pyrite oxidation in estuarine areas is still poorly understood.Therefore,this study focused on the mechanism of the effects of estuarine environmental conditions on HO· production by abiotic oxidation of pyrite.The dominant electron donor and processes of electron transfer of HO· production were demonstrated using masking and quenching experiments.The mechanism of the effects of estuarine environmental conditions including aging(in air,in water),seawater anions(Cl-,Br-and HCO3-)and UV irradiation on the HO·production by pyrite oxidation was investigated by a combination of batch experiments and characterization techniques.The contents and conclusions of this research were as follows:The mechanism of HO· production by pyrite oxidation under dark conditions,the main electron donor and processes of electron transfer of HO· production were investigated,and the effects of released SO42-,pyrite dosages and specific surface areas on HO· production were also elucidated.Sodium benzoate was used as a probe for quantifying the HO·production during pyrite oxidation.The cumulative concentration of HO·under oxic conditions increased linearly with reaction time,while no HO·was observed in the anoxic suspension,indicating that O2 was a necessary factor for HO·production by pyrite oxidation.It has been demonstrated by quenching experiments that one-electron transfer dominated the proceeding of the conversion from O· to HO·induced by pyrite,where Fe(Ⅱ)was the major electron donor in the oxidation process.SO42-released with pyrite oxidation facilitated the formation of Fe2O4 on pyrite surface,which can promote the HO·production by accelerating the surface electron transfer.The cumulative concentration of HO·in suspension under circumneutral conditions was much lower than that under acidic conditions,and the higher the pH.the stronger the inhibitory effect of HO generation.Pyrite particles with larger specific surface areas exposed more active reaction sites which facilitated the HO· production during pyrite oxidation.Taking into account the actual estuarine environmental conditions,pyrite aged in air and deionized water was prepared separately to explore the mechanism of HO·production by oxidation of aged pyrite.Pyrite aged in air and in water has similar dual effects on HO·production,with short-term aging(0-6 d)showing an inhibiting effect,while long-term aging(6-12 d)exhibiting a promoting effect.There were two inhibiting factors in short-term aging.First.the Fe(Ⅱ)/Fe(Ⅲ)(oxyhydr)oxides on the surface of aged pyrite can be considered as catalase-like centers that consumed the generated adsorbed H2O2 to limit HO· production.Second,it did not yet constitute a complete and functionally stable coating on pyrite surface during short-term aging,so the contact between the active reaction sites and O2 was inhibited by the blockage of surface oxides.In contrast,a functional Fe(oxyhydr)oxide layer dominated by highly reactive oxides was formed on surface of long-term aged pyrite.which can accelerate the pyrite oxidation by mediating the electron transfer from the interior of pyrite to dissolved O2.Pyrite aged in water has a stronger promoting effect compared to pyrite aged in air,where the HO· production was mainly from the oxidation of surface Fe(Ⅱ).Salinity is one of the most important variable features of estuaries.Cl-,Br-and HCO3-were selected as influencing factors to examine the effects of the three typical seawater anions on the HO production by pyrite.Cl-and Br-suppressed the HO· production in suspension due to their ability to in situ destroy the Fe(oxyhydr)oxide layer on pyrite surface and interfere with the process of surface electron transfer.Besides,both Cl-and Br-are important HO· scavengers that can cause the consumption of HO· through free radical reactions.The addition of HCO3led to the appearance of induction period of HO·production for the following three possible reasons.First,as a buffer substance,HCO3-was capable of consuming H+produced by pyrite oxidation,thus inhibiting the generation of O2·-and H2O2.Second,HCO3-can react with HO·and be converted to CO3·-.leading to the consumption of measurable HO·.Third,HCO3-was able to replace OH-and occupy the Fe(Ⅱ)active sites on pyrite surface,thus suppressing the conversion of Fe(Ⅱ)to Fe(Ⅲ).In addition,HCO3-caused an elevated average rate of HO·production in the later stage of oxidation as a result of the accelerated Fe(Ⅱ)oxidation by the formation of Fe(Ⅱ)-CO2 complexes on pyrite surface.Based on the electronic and optical properties of pyrite,the effect of illumination on the HO· production by pyrite oxidation was investigated by comparison and characterization analysis.UV irradiation greatly accelerated the oxidation process of pyrite,and the average rate of HO· production in the UV light was about 4.6 times higher than that in the dark.UV irradiation induced the formation of hole-electron pairs on pyrite surface,where the excited photoelectrons reacted rapidly with dissolved O2 to promote the extensive production of HO·through the process of one-electron transfer.This research revealed the mechanism of the effects of typical estuarine environmental conditions on HO·production by pyrite,which provides new insights into the environmental chemical behavior of pyrite and the selfpurification procedure of estuarine pollution. |
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