Effects Of Sulfidated Iron-based Nano-composites On The Removal And Fate Of Contaminants And Basic Mechanisms

Recently,nZVI-based environmental remediation technology has become a significant solution to address environmental pollution issues.However,the corrosion reactions between nZVI and other species result in the passision of nZVI,and thus suppressing the removal of contaminants by nZVI and decreasing the electrons selectivity.Therefore,it is important to propose various methods to improve the reactivity of nZVI.Among them,sulfidation of nZVI can inhibit H2 evolution and preserve the reducing capacity of Fe?0?.As a matter of fact,except artificial sulfidation,the microbial sulfidation of iron?oxyhydr?oxide really occurs in the environment,which will not only exert significant influence on the morphology and geochemical cycle of iron?oxyhydr?oxide,but also control the fate of contaminants.Hence,it is essential to study this sulfidation of iron?oxyhydr?oxide in predicting the fate of contaminants.Furthermore,as one of the main secondary products of sulfidation,mackinawite?FeS?has also been applied to remediate environmental contamunation,but the aggregation and oxidation of FeS seriously limit its practical application.Thus,the technology of stabilization and immobilization are employed to enhance the dispersion and anti-oxidation of FeS,so as to improve its reactivity and removal capacity.Additionally,if FeS is used in water remediation,the co-existence of FeS and algae is inevitable due to the ubiquity of algae in the aquatic environment,so it will provide constructive suggestions regarding the application of FeS in the polluted algae-containing water system through investigating roles of algae in the removal of pollutants by FeS.The main contents and results are listed below:1.A dynamic two-step anoxic/oxic process using sulfidated nanoscale zerovalent iron?S-nZVI?was employed to degrade tetrabromobisphenol A?TBBPA?.In the anoxic stage,TBBPA followed a four-step sequential debromination pathway and was completely transformed to bisphenol A?BPA?with the optimal S/Fe molar ratio of 0.3.S-nZVI inhibited H2 evolution and preserved the reducing capacity of Fe?0?.Fe?0?rather than the formed FeS in S-nZVI was responsible for TBBPA debromination.In the oxic stage,the product BPA was attacked by ·OH that produced via Fenton-like reactions,transformed to dihydroxybenzenes and benzoquinones,and eventually achieved complete mineralization to CO₂ via ring-opening reactions.The sulfidation process facilitated ·OH production through a two-electron transfer pathway by surface-bound Fe?II?,in which structural Fe?II?in FeS and regenerated Fe?II?from Fe???reduction by Fe?0?played significant roles toward total BPA degradation.S-nZVI was transformed to S₈ and ?-FeOOH after the oxic treatment.After these two steps,complete degradation of TBBPA was achieved.This study demonstrated the feasibility that refractory contaminants could be completely degraded in the dynamic two-step anoxic/oxic process,thus broadening the utility of S-nZVI for environmental applications in water treatment2.As a matter of fact,except for artificial sulfidation,the microbial sulfidation of iron?oxyhydr?oxide really occurs in the environment,which will control the fate of contaminants.The fate of p-Nitrophenol?p-NP?in the in the goethite-rich and sulfide-containing dynamic anoxic/oxic environment was investigated.In anoxic environment,the adsorbed p-NP on goethite was sharply released,which was caused by the sulfidation of goethite in the presence of S?-??at 20 and 35 mM?denoted as G1 and G2,respectively?due to lower affinity of p-NP on ?Fe-SH than ?Fe-OH.Following that,the desorbed p-NP in G1 was completely reduced to p-Aminophenol?p-AP?,while p-NP concentration in G2 only decreased by?5%.This difference was ascribed to the generation of Fe?II?-bound goethite with high reactivity toward p-NP in G1.When the environment shifted to oxic condition,the structural Fe?II?of FeS in goethite produced 108 ?M of ·OH in G1 and 220 ?M of ·OH in G2 through Fenton-like reaction.54%of p-AP was attacked by the formed ·OH to form hydroquinone and p-benzoquinone in G1,while the remaining p-NP in G2 was completely degraded to low-molecular weight compounds?e.g.,hydroquinone and p-benzoquinone?.After oxic reaction,Fe?II?in both G1 and G2 were mainly oxidized to goethite?-70%?and lepidocrocite?-25%?Our study demonstrated that the adsorption of p-NP by goethite,goethite sulfidation,and the shift of redox conditions could be crucial factors controlling the fate of p-NP,which should be taken into account during the prediction of the fate of p-NP and other organic contaminants3.As one of the main secondary products of sulfidation,FeS nanoparticles have been applied for selenite?Se????remediation in recent decades.However,the easy aggregation and oxidization of FeS hamper their reactivity.In this study,in situ immobilization technology was applied to prepare FeS nanoparticles-impregnated alginate composite?FeS-SA?for Se???remediation.FeS-SA removed 100%of the Se????0.13 mM?,whereas pure non-stabilized FeS and sodium alginate?SA?beads only eliminated 27%and 20%of the Se???,respectively.The removal efficiency increased to 73%when pure stabilized FeS was used.Therefore,FeS-SA showed superior removal efficiency that was comparable with the joint effect of pure stabilized FeS and SA beads due to the homogeneous distribution of FeS in SA matrix.Furthermore,minor differences were established in the oxidation retardation effect of FeS exerted by SA beads under anoxic and oxic conditions.The biogenic regenerated FeS-SA still showed 40%removal efficiency for Se???after 5 cycles due to the Fe leaching.XPS technique combined with the reference compounds and electron balance revealed that FeSe and metal selenium were the main selenium species after treatment This in situ preparation of stabilized FeS-SA exhibited an excellent application prospect in the remediation of Se???.4.If FeS is used in water remediation,the co-existence of FeS and algae is inevitable due to the ubiquity of algae in the aquatic environment.Chlorella vulgaris?CV?,a remarkable algal specie,was used to prepare the CV-supported FeS?CV-FeS?and to investigate the role that CV plays in the removal of model heavy metal pollutant(i.e.,hexavalent chromium?Cr????by FeS.The stabilized effect from algal extracellular polymeric substance?EPS?could avoid the FeS aggregation and enhance the reactivity of FeS,thus increasing Cr???removal rate from 0.21 min^-1 to 0.79 min^-1.Furthermore,the strong buffering induced by the algal functional groups could effectively prevent the solution pH from increasing,which improved Cr???removal because acidic solution facilitated Cr???reduction by FeS.However,the complexing capacity from algal EPS made Fe???unavailable for Cr???reduction,which led to 35%decrease of Cr???removal.The Fe???was oxidized to ?-FeOOH by Cr???in absence of CV,while the unreacted Fe???was detected as in the form of Fe?OH?₂ in CV-FeS.This work showed the different roles of algae in the removal of Cr???by FeS and provided constructive suggestions regarding the application of FeS in the polluted algae-containing water system.