In this study,Schwertmannite,Akaganéite and ammoniojarosite were biosynthesized by different bacteria and characterized.The results showed that bacteria are critical in mediating the mineral formation process:the morphology,crystallinity,grain size and specific surface area of each mineral varied upon different bacteria and culturing conditions.In addition,the formed minerals’elemental composition and group disparity lead to different morphology,crystallinity and subsequent adsorption performance.In particular,adsorption difference existed in iron minerals biosynthesized by different bacteria.The maximal adsorption capacities of Akaganéite,Schwertmannite and ammoniojarosite were 26.6 mg/g,17.5 mg/g and 3.90 mg/g respectively.Cr(VI)adsorption on iron-minerals involves hydrogen bonding,electrostatic interaction,and ligand exchange.The adsorption only occurred on the surface of ammoniojarosite,while for Akaganéite and Schwertmannite,the tunnel structure greatly facilitated the adsorption process and improved adsorption capacity.Thus,the molecular structure is the primary determining factor for adsorption performance.Collectively,the results can provide useful information in selecting suitable bacteria for synthesizing heavy-metal scavenging minerals according to different environmental conditions.
The migration and transformation of hexavalent chromium(Cr(VI))in the environment are regulated by pyrite(FeS2).However,variations in pyrite crystal facets influence the adsorption behavior and electron transfer between pyrite and Cr(VI),thereby impacting the Cr(VI)reduction performance.Herein,two naturally common facets of pyritewere synthesized hydrothermally to investigate the facet-dependent mechanisms of Cr(VI)reduction.The experimental results revealed that the{111}facet exhibited approximately 1.30–1.50 times higher efficiency in Cr(VI)reduction compared to the{100}facet.Surface analyses and electrochemical results indicated that{111}facet displayed a higher iron-sulfur oxidation level,which was affected by its superior electrochemical properties during the reaction with Cr(VI).Density functional theory(DFT)calculations demonstrated that the narrower band gap and lower work function on{111}facet were more favorable for the electron transfer between Fe(II)and Cr(VI).Furthermore,different adsorption configurations were observed on{100}and{111}surfaces due to the unique arrangements of Fe and S atoms.Specifically,O atoms in Cr_(2)O_(7)^(2−)directly bound with the S sites on{100}but the Fe sites on{111}.According to the density of states(DOS),the Fe site had better reactivity than the S site in the reaction,which appeared to be related to the fracture of S-S bonds.Additionally,the adsorption configuration of Cr_(2)O_(7)^(2−)on{111}surface showed a stronger adsorption energy and a more stable coordination mode,favoring subsequent Cr(VI)reduction process.These findings provide an in-depth analysis of facet-dependent mechanisms underlying Cr(VI)reduction behavior,offering new insights into studying environmental interactions between heavy metals and natural minerals.
The capability of traditional ligand in countering rapid passivation on nanoscale zero-valent iron(nZVI)surface is inadequate,and the precise electron transfer mechanism remains elusive.In this study,we reported that myo-inositol hexakisphosphate(IHP),a redox-inactive organophosphorus in soil,could highly enhance Cr(VI)reduction and immobilization in comparison with typical ligands(TPP,EDTA,oxalate and phosphate).And the effects of IHP concentration,Cr(VI)concentration and initial pH were systematically investigated.Cr Kedge XANES and XPS analysis revealed that Cr(III)was the exclusive form in solid products regardless of IHP existence.Results of ATR-FTIR and FESEM inferred that IHP was adsorbed on nZVI surface via inner-sphere complexation,thus averting encapsulation of[Fe,Cr](OH)_(3)coprecipitate and impeding solid particles agglomeration.Additionally,IHP expedited the production of surface-bound Fe(II),primarily attributable to the interaction between nZVI and oxygen.These surface-bound Fe(II)species played a pivotal role in Cr(VI)reduction.Electrochemical analysis unveiled that IHP lowered redox potential of Fe(III)/Fe(II),thereby facilitating reaction between Fe(II)and Cr(VI),whereas inhibited direct electron transfer from nZVI core to Cr(VI).Our findings proposed a novel potential ligand for alleviating nZVI passivation in Cr(VI)removal and deepened our understanding in the process of electron transfer.