Sulfur Dose and Sulfidation Time Affect Reactivity and Selectivity of Post-Sulfidized Nanoscale Zerovalent Iron

Abstract

Exposing nanoscale zerovalent iron (NZVI) to dissolved sulfide species improves its performance as a remediation agent. However, the impacts of sulfur dose and sulfidation time on morphology, sulfur content, reactivity, and selectivity of the resulting sulfidized NZVI (SNZVI) have not been systematically evaluated. We synthesized SNZVI using different sulfur doses and sulfidation times and measured their properties. The measured S/Fe molar ratio in the particles ([S/Fe] particle ) was 10−500 times lower than [S/Fe] dosed but was predictable based on [S/Fe] dosed × t sulfidation . The low sulfur content (0.02−0.65 mol % S/Fe) inhibited the reaction of SNZVI with water (up to 13-fold) and increased its reactivity with trichloroethene (TCE) (up to 14-fold) and its electron efficiency (up to 20-fold). A higher [S/ Fe] particle (0.86−1.13 mol % S/Fe) led to complex particle structures and lowered the resistance to electron transfer but did not improve the benefits realized at the lower S/Fe ratios. Adding small amounts of sulfur into NZVI led to more accumulation of acetylene, especially for low Fe/TCE conditions, suggesting that sulfur lowers the rate of hydrogenation of acetylene to ethene. These results show that [S/Fe] dosed × t sulfidation can be used to predict the measured S content in the particles and that affects reactivity, longevity, and electron selectivity, for post-SNZVI.

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

“…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.…”

Quantifying the efficiency and selectivity of organohalide dechlorination by zerovalent iron

“…It is important to note that not all of the added S in the synthesis procedure is incorporated into the suldized particles, and this is especially true for nZVI particles that are suldized aer the nZVI is already formed, where less than 8% of the dosed sulfur was incorporated into the particles. 33 As shown in Fig. 3A, the effect of the S/Fe ratio on 3 e varies with the type of nZVI (e.g., polymer coated or not).…”

“…As discussed above, many laboratory studies have used high TCE concentrations (e.g., >200 mg L À1 ) relative to the ZVI dose in order to simulate source treatment with nZVI. These conditions generally produce higher 3 e for dechlorination than lower TCE concentrations (depending on the ZVI type), 29,30,33,39 but as the TCE is degraded (e.g., to <10 mg L À1 ) a notable decrease in 3 e is also observed. This trend is expected because lower contaminant concentrations make it less competitive with water (or other species that contribute to NRD) for reactive sites on ZVI.…”

“…To determine the quantity of electrons used in any contaminant reduction reaction from experimentally measured values of reactants and products, the corresponding redox halfreaction(s) must be specied. Using TCE dechlorination as an example, a generic half-reaction can be written as in eqn (3) (Liu et al 30 ), which has been used in many studies with minor variations: 29,33 TCE + ne À + mH…”

“…40 On the other hand, if the sulfur is present during particle formation, it is incorporated into the structure of the material to form an Fe(0)-FeS composite. 1 The operational conditions under which suldation is conducted and the amount of sulfur incorporated into the material inuence the reactivity of S-ZVI, 1,33 including its electron efficiency. It is important to note that not all of the added S in the synthesis procedure is incorporated into the suldized particles, and this is especially true for nZVI particles that are suldized aer the nZVI is already formed, where less than 8% of the dosed sulfur was incorporated into the particles.…”