Wet Milling of Zerovalent Iron in Sulfide Solution: Preserving and Securing the Metallic Iron

Abstract

Herein, a critical problem regarding the milling of zerovalent iron (ZVI) to produce fine ZVI particles is solved. During milling, ZVI is prone to be rapidly oxidized by oxygen, water, and other potential oxidants in the milling environment, which results in significant metallic iron (Fe(0)) loss and raises safety concerns. In this work, a sulfide solution (using Na2S) is introduced as the milling solvent; as benchmarks, ZVI milling under a pure aqueous solution (ZVI­(W)) and organic solvents (pure ethanol, ZVI­(E)) is also investigated. Produced ZVIs are characterized with six different techniques to assess their Fe(0) content, safety, reactivity, and reductive capacity. The obtained results show that the sulfide solution is effective in minimizing Fe(0) oxidation during milling; the Fe(0) content (∼63%, w/w) of ZVI milled in sulfide solution (ZVI­(S)) is more than 10 times that of ZVI­(W) after 5 h of milling. ZVI­(S) is comparable to ZVI­(E) in their Fe(0) contents, reductive reactivity, and capacity, but ZVI­(S) is more stable and safer in use; an optimal Na2S concentration of 10 g Na2S/L is recommended. A mechanistic study shows that Fe(0) preservation in a sulfide solution results from a combined effect of the alkaline pH of the solution and the coating of iron sulfides on ZVI. This exploratory study demonstrates an original milling solution for a highly scalable, economical, and safe scheme to produce ultrafine and reactive ZVI with a high Fe(0) content for environmental applications.

In situ characterization of aggregates of nanoscale zero-valent iron (nZVI) in water: an engineering aspect

“…the complete particle suspension positively correlates with the density difference between the particle and the mixed solution, as described by the Zwietering equation: 24 R ∼ υ 0:1 d 0:2 p ρ p − ρ ρ 0:5 (9) where υ and ρ are the kinematic viscosity and density of the mixed solution, and d p and ρ p are the diameter and the density of the mixed solid, respectively. The low aggregate density thus implicates that the aggregates have a low critical rotation speed for complete suspension, which is consistent with the observations that the aggregates appear as light flocs.…”

“…[4][5][6][7]10 For example, 4400 kg nZVI, in the form of concentrated nZVI slurry, were shipped to a wastewater treatment plant in a 28 day period 8 (shipping nZVI in aqueous solution can reduce the safety risk because fresh dry nZVI is highly pyrophoric upon exposure to air). 9 In the plant, the slurry was dispersed using a high-speed mixer and then dosed via This journal is © The Royal Society of Chemistry 2022 pumping to a mixing reactor (Fig. 1A), where the dosed nZVI was mixed with 60 m 3 wastewater at ∼3 g nZVI per L using a 5 kW blade mixer.…”

“…1–8 In large-scale applications using nZVI, the nZVI nanoparticles are unexceptionally in aqueous solution and concentrated nZVI slurries are common in nZVI production, storage and delivery (Fig. 1); 3–6,9 the slurries are processed via unit operations such as high-speed mixing for dispersion, mechanical mixing for reaction, pipeage for transportation or dosing of the slurry, settling for the nZVI separation or condensation (Fig. 1A).…”

Unveiling the Mechanistic Role of Surface Nitrogen and Sulfur in Boosting the Dechlorination Performance of Zero-Valent Iron

“…This technology has been used to remediate contaminated groundwater over the last three decades. − ZVI also reduces water to H 2 via the hydrogen evolution reaction (HER): Fe 0 + 2H 2 O → Fe 2+ + 2OH – + H 2 (g). The competition between the reduction of target CHCs and the HER has been the focus of recent studies since it controls ZVI’s long-term performance. , These studies have employed electron efficiency (ε e ) to assess the proportion of electrons produced by ZVI oxidation that are utilized for contaminant reduction relative to total reduction reactions. , The modification of ZVI by reduced sulfur (i.e., sulfidation) − has been shown to improve the ε e of ZVI. This is achieved by more selectively transferring electrons from Fe 0 to the target CHC, with dechlorination rates comparable to or higher than those of the unsulfidated ZVI.…”