Biogenic Goethite-Mediated Production Of Reactive Oxygen Species And Degradation Of Sulfanilamide Antibiotics

Iron minerals have been employed to activate oxidants like O2,H2O2,and persulfate to produce highly oxidizing radicals such as ·OH and ·SO4-for pollutants degradation.Microbes are important driving force for the(trans)formation of iron minerals in natural environments.Biogenic iron minerals formed through microbial activity widely exist in the environments.In comparison to abiotic counterparts,the biogenic iron minerals generally have different morphologies,surface area values and redox activities,which influence their electron transfer activity,bioavailability and pollutant adsorption capacity.However,the removal of recalcitrant organic pollutants based on activation of different oxidants by Fe(Ⅲ)/Fe(Ⅱ)cycling of biogenic iron minerals has not been investigated.Therefore,biogenic goethite(Gtbio)and coprecipitates between goethite and humic acid(HA)or reduced graphene oxide(rGO)(Gtbio-HA and Gtbio-rGO)that possessing electron shuttling or Fe(Ⅲ)/Fe(Ⅱ)complexing activities were prepared by Acidovorax sp.BoFeN1,a typical nitrate-dependent Fe(II)oxidation strain.The application of these biogenic goethites in activating O2,H2O2,or peroxydisulfate(PDS)for the degradation of sulfanilamide(SA)as well as other organic pollutants was also investigated.The main research contents and results are as follows:(1)The impacts of HA on ·OH production during redox transformation of and Gtbio and hydrolysis-prepared abiogenic goethite(Gtchem)under fluctuating anaerobic-aerobic conditions were studied.After 144 h of anaerobic reduction and 32 h of aerobic oxidation,the cumulative ·OH concentration in the Gtbio/HA systems(47.1-1.3 μM)was much higher than those in the Gtchem/HA systems(17.0-29.2 μM).The cumulative ·OH concentration in the Gtchem and the Gtbio systems was respectively increased by 40.5%-241%and 1.7%-54.0%with the addition of 10-100 mg/L HA.Partial mineralization of the free and adsorbed HA molecules occurred under fluctuating anaerobic-aerobic conditions,resulting in the decrease of their average molecular size and increase of their oxygen-containing functional groups.Acting as electron shuttle and iron complexing agent,HA could promote dissimilatory Fe(III)reduction in the anaerobic stage and ·OH accumulation in the aerobic stage during the redox transformation of Gtchem and Gtbio.(2)To improve the stability of HA during goethite-mediated ·OH generation,Gtbio-HA with different C:Fe ratios(0.16-0.99)were biologically prepared with growing BoFeNl cells.For system containing Gtbio-HA with C:Fe ratios of 0.30,0.55 and 0.99,91.1%,79.0%and 77.3%of SA(10 mg/L)could be degraded in 2 h,which was much higher than the removal efficiency obatined with system containing Gtbio alone(44.8%).Efficiencies of H2O2 decomposition,·OH production,and organic carbon removal in the Gtbio-HA systems were also more efficient than those in the Gtbio system.Higher carbon moieties stability and lower micropore surface area of Gtbio-HA decreased the competition for ·OH and H2O2,thus helped to improve SA degradation efficiency.The coprecipitated HA could serve as electron shuttle and complex with Fe(Ⅲ)via carboxyl groups to improve Fe(Ⅲ)/Fe(Ⅱ)cycling.(3)Based on the electron shuttling and Fe(Ⅲ)/Fe(Ⅱ)complexing abilities of rGO,Gtbio-rGO with different rGO contents(2%-10%)were biologically prepared by BoFeN1 and used to efficiently catalyze H2O2 activation for SA degradation.In 4 h,95.4%of SA(60 μM)could be efficiently degraded in the Gtbio-rGO mediated system.The SA degradation rate in the Gtbio-rGO/H2O2(0.97 h-1)system was 6.70,15.4,and 162 folds higher than those in the control rGO/H2O2,Gtbio/H2O2,and H2O2 systems,respectively.·OH generation,H2O2 decomposition,as well as Fe(II)accumulation in the Gtbio-rGO systems were all faster than those in the control Gtbio or rGO systems.Higher Fe(Ⅲ)/Fe(Ⅱ)cycling rates were obtained in the Gtbio-rGO systems,which might be attributed to the strong Fe-C coordination effects and the decreased aggregation of rGO and the lowered Gtbio particle sizes in the coprecipitates.Moreover,the embedded Gtbio particles in Gtbio-rGO exposed more defects as active sites for H2O2 activation,which would be in favor of efficient Fenton-like reactions.(4)SA as well as other organic pollutants could be decomposed during the Gtbio-rGOmediated PDS activation.In 4 h,about 92.5%of SA(20 μM)could be degraded in the Gtbio-rGO/PDS system.The pseudo-first-order rate constant of SA degradation in the Gtbio-rGO/PDS system(0.68 h-1)was 6.30 22.7,and 25.3 folds higher than those in the control rGO/PDS,Gtbio/PDS,and PDS systems,respectively.On the one hand,·O2-and 1O2 were generated in the Gtbio-rGO/PDS system for pollutants oxidation.On the other hand,Gtbio-rGO was recognized as a facile electron transfer mediator to directly accelerate electron transfer in a ternary electron donor(pollutants)-mediator(Gtbio-rGO)-acceptor(PDS)system.Based on the non-radical oxidation mechanism,a selective reactivity toward organic pollutants possessing different ionization potentials was shown in the Gt-rGO/PDS system.Collectively,the results of this study showed that biogenic goethite exhibited higher ability than the abiogenic one during the activation of different oxidants.HA and rGO could further increase the catalytic activity of the biogenic goethite via enhancing its Fe(Ⅲ)/Fe(Ⅱ)cycling.This study improves our understanding of reactive oxygen species generation and fate of pollutants under natural redox fluctuation conditions.It also lays the foundation for the use of biogenic minerals in the removal of recalcitrant organic pollutants.