Reactivity, Selectivity, and Long-Term Performance of Sulfidized Nanoscale Zerovalent Iron with Different Properties

Abstract

Sulfidized nanoscale zerovalent iron (SNZVI) has desirable properties for in situ groundwater remediation. However, there is limited understanding of how the sulfidation type and particle properties affect the reactivity and selectivity of SNZVI toward groundwater contaminants, or how reactivity changes as the particles age. Here, SNZVI synthesized by either a one-step (SNZVI-1) or two-step (SNZVI-2) process were characterized, and the reactivity of both fresh and aged (1d to 60 d) nanoparticles was assessed. The measured S/Fe ratio was 5.4 ± 0.5 mol % for SNZVI-1 and 0.8 ± 0.1 mol % for SNZVI-2. XPS analysis indicates S2–, S2 2–, and S n 2– species on the surface of both SNZVI-1 and SNZVI-2, while S2 2– is the dominant species inside of the SNZVI nanoparticles. SNZVI-1 particles were hydrophobic (contact angle = 103 ± 3°), while the other materials were hydrophilic (contact angles were 18 ± 2° and 36 ± 3° for NZVI and SNZVI-2, respectively). SNZVI-1, with greater S content and hydrophobicity, was less reactive with water than either NZVI or SNZVI-2 over a 60 d period, resulting in less H2 evolution. It also had the highest reactivity with TCE and the lowest reactivity with nitrate, consistent with its higher hydrophobicity. In contrast, both NZVI and SNZVI-2 were reactive with both TCE and nitrate. Both types of SNZVI remained more reactive after aging in water over 60 d than NZVI. These data suggest that the properties of the SNZVI made from a one-step synthesis procedure may provide better reactivity, selectivity, and longevity than that made from a two-step process.

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

“…Adding more S ([S/Fe] measured > 0.07) decreased the water contact angle (Figure 1k; Figure S8, Supporting Information). Although the morphology, sulfur distribution, and/or hydrophobicity of SNZVI made using a sulfur dose of 0.4 S/Fe molar ratio [ 47 ] and 0.14 S/Fe molar ratio [ 26 ] have been reported, the present study indicates that the actual (measured) S content affects the distribution and speciation of S in the structure of SNZVI, the hydrophobicity, and the crystallinity as discussed below.…”

“…Briefly, SNZVI was synthesized by dropwise addition (≈7 mL min −1 ) of 200 mL of a 34 g L −1 NaBH 4 (98%, Fisher Scientific) and 1.6 g L −1 Na 2 S 2 O 4 (≥88%, Sinopharm Chemical Reagent Co., Ltd., China) solution into 200 mL of a 29 g L −1 FeCl 3 (97%, Sigma‐Aldrich) solution while stirring at 600 rpm under nitrogen. [ 26 ] This theoretically provides a [S/Fe] dosed ratio of 0.10. SNZVI particles with different [S/Fe] dosed ratios (0, 0.035, 0.05, 0.07, 0.10, 0.14, 0.17, 0.20, 0.24, 0.28, and 0.34) were synthesized using the same procedure, except changing the concentration of Na 2 S 2 O 4 .…”

“…Recently, it was shown by us and others that the sulfidation of NZVI lowers its reactivity with water and other non‐target hydrophilic contaminants (e.g., NO 3 − ), while increasing its reactivity with target contaminants like chlorinated solvents and antibiotics. [ 25–31 ] Several mechanisms have been hypothesized for the enhanced reactivity and selectivity of sulfidized nanoscale zerovalent iron (SNZVI) compared to NZVI. First, SNZVI is more hydrophobic than NZVI, resulting in lower interaction with water and charged solutes, and greater interaction with hydrophobic contaminants.…”

“…First, SNZVI is more hydrophobic than NZVI, resulting in lower interaction with water and charged solutes, and greater interaction with hydrophobic contaminants. [ 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.…”

Adsorptive and Reductive Removal of Chlorophenol from Wastewater by Biomass-Derived Mesoporous Carbon-Supported Sulfide Nanoscale Zerovalent Iron

“…It should be noted that although supported nZVI can prevent the agglomeration and improve the stability of nZVI, it is easily oxidized by water, O 2 and other coexisting solutes, which would impede electron transfer from nZVI core to target contaminates [17]. In view of the above issue, recent research has found that sulfidation of nZVI is the most promising approach to suppress surface oxidation and increase reactivity of nZVI [18][19][20]. On the one hand, the iron sulfide shell on the surface of nZVI acts as a barrier to prevent nZVI contacting water and mitigating the its surface corrosion.…”