Selectivity of Nano Zerovalent Iron in In Situ Chemical Reduction: Challenges and Improvements

Abstract

Nano zerovalent iron (nZVI) is a promising remediation technology utilizing in situ chemical reduction (ISCR) to clean up contaminated groundwater at hazardous waste sites. The small particle size and large surface area of nZVI result in high reactivity and rapid destruction of contaminants. Over the past 20 years, a great deal of research has advanced the nZVI technology from bench‐scale tests to field‐scale applications. However, to date, the overall number of well‐characterized nZVI field deployments is still small compared to other alternative remedies that are more widely applied. Apart from the relatively high material cost of nZVI and questions regarding possible nanotoxicological side effects, one of the major obstacles to the widespread utilization of nZVI in the field is its short persistence in the environment due to natural reductant demand (NRD). The NRD for nZVI is predominantly due to reduction of water, but other reactions with naturally present oxidants (e.g., oxygen) occur, resulting in situ conditions that are reducing (high in ferrous iron phases and H2) but with little or no Fe(0). This article reviews the main biogeochemical processes that determine the selectivity and longevity of nZVI, summarizes data from prior (laboratory and field) studies on the longevity of various common types of nZVI, and describes modifications of nZVI that could improve its selectivity and longevity for full‐scale applications of ISCR. © 2016 Wiley Periodicals, Inc.

Dechlorination of carbon tetrachloride by Nanoscale Nickeled Zero‐Valent Iron @ Multi‐Walled Carbon Nanotubes: Impact of reaction conditions, kinetics and mechanism

“…This was benefited from MWCNTs as a carbon‐based material was capable of CT adsorption, as shown in Figure S2, CT could be adsorbed rapidly by MWCNTs from aqueous phase to catalyst surface, and MWCNTs without loss the capacity of CT adsorption in five consecutive cycles. Moreover, the pH variations of dechloriantion process were also monitored, Figure shown that 2N 4 F@MWCNTs had a lower pH while with satisfactory reusability, which meant 2N 4 F@MWCNTs could evolve more H 2 under lower pH solution and has an excellent dechlorination performance …”

“…It can be learned from Table that the 2N 4 F@MWCNTs possessed lowest concentration of dissolved cations but highest loss rate after five times reactions compared with nZVI and N 4 F. Lowest dissolution signified a slight negative environment impacts, and the relatively high loss rate may attribute to severe corrosion on the bimetallic surface due to the large reaction specific surface area. Besides, more H 2 evolution by 2N 4 F@MWCNTs means less Fe 0 was directly involved in the process of CT dechlorination …”

Quantifying the efficiency and selectivity of organohalide dechlorination by zerovalent iron

“…As noted above, however, this does not necessarily imply that these reactions occur (only) by electron transfer mechanisms, because various atom-transfer reactions can also contribute to redox reactions. In previous work, 32 we have tried to address this issue by using the more general term "reducing equivalents", but that is not continued in this work for simplicity and consistency with the majority of other work on this topic. Also, for modied ZVI, and in particular suldated ZVI, electrons from iron sulde are not considered here because the dechlorination of organohalides (e.g., TCE) by FeS is typically much slower than dechlorination by suldated ZVI.…”

Relative Permeability Measurements during the Exsolution and Dissolution of Hydrogen Gas Produced by the Hydrolysis of Sodium Borohydride

“…These objectives were addressed using a series of laboratory experiments conducted in a thin flow cell and using the reaction of sodium borohydride (NaBH 4 ) and water to exsolve H 2 as a novel method to investigate the effects of internal drainage. In addition to providing a useful laboratory technique, the exsolution of H 2 is of particular interest because of its importance to the remediation of chlorinated organics by the injection of nanoscale zero‐valent iron (nZVI) (Fan et al, 2016; O’Carroll et al, 2013; Qin et al, 2018; Johnson et al, 2013). The production of H 2 by the reaction of water with both nZVI and NaBH 4 , which has been applied in excess for nZVI synthesis, can result in the alteration of flow paths away from target treatment areas (Johnson et al, 2013) by blocking pores, as has also been reported for zero‐valent iron used in permeable reactive barriers (Ruhl and Jekel, 2014; Jeen et al, 2012).…”