Partial Oxidation of FeS Nanoparticles Enhances Cr(VI) Sequestration

Abstract

Iron sulfide nanoparticles (nano-FeS) have shown great potential for in situ remediation of Cr(VI) pollution by reducing Cr(VI) to the less soluble and toxic Cr(III). However, material oxidation that inevitably occurs during storage and application alters its reactivity. Herein, we show that partial oxidation of nanoparticulate mackinawite (FeS) significantly enhances its capability in sequestering Cr(VI). Oxidation of nano-FeS increases its binding affinity to Cr(VI), likely due to preferential inner-sphere complexation of Cr(VI) oxyanions to ferric over ferrous iron in mackinawite/lepidocrocite (FeS/γ-FeOOH) nanocomposites. A trade-off is that oxidation mitigates Cr(VI) reduction by lowering the electron-donating potential of the material and the electron transfer at a solution−material interface and consequently hinders the transformation of adsorbed Cr(VI) to Cr(III). Notably, the rate-limiting step of Cr(VI) sequestration transitions from adsorption to reduction during oxidation, as demonstrated with batch experiments coupled with kinetic modeling. Thus, an optimum oxidation degree exists, wherein the gain in the overall performance from enhanced adsorption overcompensates the loss from inhibited reduction, resulting in maximum sequestration of aqueous Cr(VI) as solid-phase Cr(III). Our findings inform better assessment and design of nanomaterials for Cr(VI) remediation and may be extended to interactions of other oxyanions with natural and engineered nanoparticles during oxidative aging.

Enhanced oxidative and adsorptive removal of arsenite by the heterogeneous interfacial reaction of sulfidated nanoscale zerovalent iron

“…It was reported that the surface FeS x generated by the sufidation of nZVI has lower band gaps than that of FeO x , leading to the redistribution of the electron charge density of S-nZVI by sulfur doping. 34 EIS results verify the above conclusion.…”

Sulfite Poses a Risk of Hexavalent Chromium Rebound in Vadose Zone: A Challenge of the Stability of CrxFe1–x(OH)3

“…The abnormal paucity of SO 4 2– is incongruous with the prevalence of REDOX reactions on the surface of Cr 0.5 Fe 0.5 (OH) 3 , but it does correspond to the decrease in the proportion of Cr(VI). The anomalous decrease in both SO 4 2– and Cr(VI) percentage might be attributed to the competitive adsorption of a significant amount of instantaneously formed CrO 4 2– and SO 4 . − Both CrO 4 2– and SO 4 2– were desorbed from the surface of Cr 0.5 Fe 0.5 (OH) 3 . Subsequently, dissolved CrO 4 2– is reduced by dissolved sulfite under the catalysis of dissolved Fe(III) to form Cr(III) ions, which elucidates the notable release of Cr(III). SO 3 · − + SO 3 · − → normalS 2 normalO 6 2 − normalS 2 normalO 6 2 − + normalH 2 …”

“…In fact, sulfite is widely present in soil, primarily due to bacterial growth on organosulfur compounds and chemolithotrophic growth on inorganic sulfur compounds . Additionally, the utilization of sulfur-containing reducing agents (such as Na 2 SO 3 , Na 2 S 2 O 4 , Na 2 S, FeS, , FeS 2 , and CaS x , ) significantly increases the sulfite content in soil. Our preliminary investigation revealed that the proportion of sulfite in total sulfur increased from 4.81% to 14.04% after 6 months of amorphous FeS 2 remediation .…”

“…Hexavalent chromium (Cr­(VI)) is a primary hazardous contaminant in soil due to its high toxicity, strong solubility, and significant mobility. − A commonly employed remediation strategy for Cr­(VI) contamination involves the reduction of Cr­(VI) to less toxic Cr­(III) precipitates through the use of reductants such as Fe­(II), nZVI, CaS x , , FeS, − and FeS 2 . The primary Cr­(III) precipitates resulting from the remediation of Cr­(VI) are amorphous chromium hydroxide (Cr­(OH) 3 ) and amorphous coprecipitation of Cr–Fe (Cr x Fe 1– x (OH) 3 ).…”

“…10.1021/acs.est.3c00087. Detailed information on chemicals, experiments, and characterization methods; results and XRD patterns for the prepared Cr(OH) 3 , Fe(OH) 3 , and Cr x Fe 1−x (OH) 3 ; optimization of testing method for Cr(VI); figures and tables of characterization results for Cr(OH) 3 (s) and Cr 0.5 Fe 0.5 (OH) 3 (s) at different reaction times (FTIR, XPS and EDS); dissolved Cr(III) and Fe(III) concentrations (PDF)…”