Sulfidation of Iron-Based Materials: A Review of Processes and Implications for Water Treatment and Remediation

Abstract

Iron-based materials used in water treatment and groundwater remediation-especially micro- and nanosized zerovalent iron (nZVI)-can be more effective when modified with lower-valent forms of sulfur (i.e., "sulfidated"). Controlled sulfidation for this purpose (using sulfide, dithionite, etc.) is the main topic of this review, but insights are derived by comparison with related and comparatively well-characterized processes such as corrosion of iron in sulfidic waters and abiotic natural attenuation by iron sulfide minerals. Material characterization shows that varying sulfidation protocols (e.g., concerted or sequential) and key operational variables (e.g., S/Fe ratio and sulfidation duration) result in materials with structures and morphologies ranging from core-shell to multiphase. A meta-analysis of available kinetic data for dechlorination under anoxic conditions, shows that sulfidation usually increases dechlorination rates, and simultaneously hydrogen production is suppressed. Therefore, sulfidation can greatly improve the efficiency of utilization of reducing equivalents for contaminant removal. This benefit is most likely due to inhibited corrosion as a result of sulfidation. Sulfidation may also favor desirable pathways of contaminant removal, such as (i) dechlorination by reductive elimination rather than hydrogenolysis and (ii) sequestration of metals as sulfides that could be resistant to reoxidation. Under oxic conditions, sulfidation is shown to enhance heterogeneous catalytic oxidation of contaminants. These net effects of sulfidation on contaminant removal by iron-based materials may substantially improve their practical utility for water treatment and remediation of contaminated groundwater.

Sulfur Loading and Speciation Control the Hydrophobicity, Electron Transfer, Reactivity, and Selectivity of Sulfidized Nanoscale Zerovalent Iron

“…[ 26,29 ] Second, the incorporated sulfur lowers electron transfer resistance from Fe 0 to the contaminant. [ 32,33 ] Third, the incorporated sulfur blocks adsorption sites for atomic hydrogen and inhibits the water reduction reaction and H 2 evolution. All are possible, but evidence for these proposed mechanisms is limited to a few observations, and the limited characterization of SNZVI makes differentiating between these mechanisms difficult.…”

Water Treatment Using Metallic Iron: A Tutorial Review

“…Contaminants are transformed and removed by the same mechanisms [133,134]. An affordable bimetallic system that has been successfully tested is one entailing the addition of sulfur [123,135,136].…”

Palygorskite-supported sulfide-modified nanoscale zero-valent iron for Congo red removal

“…Orange I removal k obs rate of S-nZVI is 71.8 times higher than the one without sulfidation [21]. Many results showed that sulfidation conditions had great effects on target pollutants degradation efficiency [15,17,19,20]. For example, Cao et al found that the removal of antibiotic florfenicol was strongly dependent on S/Fe and the surface-areanormalized reaction rate constant of antibiotic florfenicol on S-nZVI at the optimal S/Fe molar ratio (0.14) was 48 times higher than nZVI [22].…”

“…Recently, many studies have revealed that surface sulfidation can control the corrosion and improve the conductivity of nZVI [15][16][17]. Because FeS has a higher electronegativity (5.02 V) than nZVI (4.04 V), the electrons generated from the corrosion process of nZVI can transfer to FeS.…”