Sulfide Drives Hydroxyl Radicals Production In Oxic Fe(?)/NOM Systems

Although the production of hydroxyl radicals?·OH?in oxic environments has been mainly attributed to the photo-irradiation of dissolved organic carbon?DOC?,these areas contain large amounts of Fe???minerals and NOM-Fe???coprecipitates in addition to DOC.Increasing researches have been devoted to the identification of new potential sources of·OH in the last decade.Consequently,it was discovered that in addition to photolytic processes,biotic and abiotic processes can produce·OH in dark environments.Many studies showed that O₂ perturbation of anoxic environments was a very important source of abiotic·OH production in dark environments.Whereas this type of redox perturbation gained very high attention,the impact of the perturbation of oxic environments by reduced species from anoxic environments on·OH production remains unexplored.However,such perturbations occur widely in natural and engineered systems with sulfide being an important component of these species.We hypothesized that sulfide perturbation of oxic environments may produce·OH from sulfide interaction with oxic Fe???oxyhydroxides,NOM and NOM-Fe???coprecipitates or sediments in these areas under dark and neutral pH conditions.·OH production at sediment anoxic-oxic interfaces could be highly significant due to the proximity of sediments and anoxic environments.The objectives of this thesis were to?1?ascertain whether and how·OH can be produced when sulfide perturbs the oxic systems containing Fe???oxyhydroxides,NOM,their coprecipitates or real sediments.?2?To evaluate whether the·OH produced in above systems could oxidize coexisting organic contaminants.To reach these goals,sulfide was reacted with oxic Fe???oxyhydroxides,HA,HA-Fe???coprecipitates or sediments in the presence of sodium benzoate?BA?.·OH production was quantified from the hydroxylation of BA to p-hydroxybenzoic acid?p-HBA?.The environmental significance of produced·OH from the interaction of lepidocrocite with sulfide was evaluated through its reaction with phenol.Wherever possible,spectroscopic analyses were used to strengthen the understanding of reaction mechanism.As hypothesized,sulfide reaction with oxic systems containing oxic Fe???oxyhydroxides,HA,HA-Fe???coprecipitates or sediments produced significant and variable·OH concentrations.The addition of 0.5 mM S?-II?into lepidocrocite,ferrihydrite and goethite suspensions?11.2 mM Fe,pH 7?produced 14.2,14.3 and 22.4?M·OH within 120 min,respectively.The addition of 0 to 7.5 mM S?-II?to lepidocrocite suspensions?11.2 mM Fe?increased·OH concentration from 0 to 63.1?M within 120min,respectively.At a fixed sulfide dosage of 5 mM,increasing the number of sulfide additions enhanced the production of·OH.The mechanisms of·OH production were attributed to the generation of surface-bound Fe?II?,most likely in the form of>Fe^IIOH₂⁺,and Fe?II?in the solid phase or FeS from the reactions between sulfide and Fe???,followed by O₂ activation.·OH production could take place until depletion of sulfide.Finally,we found that the generated·OH could oxidize the coexisting redox-active substances like phenol under neutral and oxic conditions.In addition to·OH production from sulfide interaction with oxic Fe???oxyhydroxides suspensions,·OH was produced from its interaction with HA.Both sulfide and HA concentrations had a prominent influence on·OH production,with sulfide having a high impact.When HA was controlled at 16 mM C under oxic and pH-neutral conditions,the cumulative·OH concentration increased from 0 to 72.2?M within 120 min with increasing sulfide concentration from 0 to 5 mM.·OH accumulation showed good consistency with sulfide oxidation.However when sulfide was maintained at 2.5 mM,the cumulative·OH concentration increased from 0 to 44.8?M with increasing HA concentration from 0 to 16 mM C.During oxygenation,O₂ consumption increased with increasing HA concentration for a fixed sulfide concentration.For a fixed HA concentration,O₂ consumption increased with increasing sulfide concentration.These results suggested the prominent role of available HA reactive sites for sulfide on·OH production.Both UV-Vis and FTIR results suggested the addition of sulfur into the HA molecule structure.They suggested further the implication of both produced sulfur and phenolic moieties in·OH production.In addition to the above characterizations,3DEEM fluorescence suggested the reaction of sulfide and HA in oxic conditions as a reversible reduction-oxidation process with production of labile chromophoric intermediates.In this reaction,both HA sulfur and phenolic moieties acted as redox-active moieties.In addition to Fe???oxyhydroxides and HA,·OH was also produced from the reaction of sulfide and oxic HA-Fe???coprecipitates or sediments.In these reactions,C/Fe molar ratios of the coprecipitates and sulfide concentrations were the key driving factors for·OH production in HA-Fe???coprecipitates whereas sediment composition defined the trend of·OH production in the sediments.With increasing sulfide concentrations from 0.5 to 5 mM,the cumulative·OH concentrations increased from 8.2to 71.8?M·OH for 11.2 mM Fe in HA-Fe???coprecipitates with a C/Fe molar ratio of1.44 under oxic conditions and pH 7,within 120 min.Sulfide concentrations higher than2.5 mM enhanced·OH production when compared to lepidocrocite suspensions.Although C/Fe molar ratios influenced slightly the cumulative·OH concentrations at 120min for 2.5 mM sulfide,·OH accumulation during the reaction was affected.The addition of sulfide into oxic sediments produced·OH as well.The addition of 2.5 mM sulfide into100 g/L of sandy or clayey sediments produced 29.2 and 55.5?M·OH within 120 min,respectively.·OH production in the coprecipitates could be attributed to the production of HA-Fe?II?and FeS from the reduction of HA-Fe???coprecipitates by sulfide,followed by O₂ activation for·OH production.The interaction of FeS and desorbed HA,reduced desorbed HA and HA-Fe???may explain the enhancement of·OH production at higher sulfide concentrations.Our findings reveal that sulfide perturbation of oxic systems containing Fe???oxyhydroxides,NOM,their coprecipitates or sediments is a significant source of·OH which has been previously overlooked.Therefore,the oxidation of contaminants by·OH produced by these perturbations necessitates consideration in these environments.As sulfide may be discharged into oxic environments from numerous sources such as the discharge of anoxic groundwater,springs,and seafloor black smokers,·OH production from this type of perturbation may be widespread in the environment.Sulfide perturbation of oxic environments is an important source of·OH production in dark natural environments in addition to photo-irradiation.