Advances in Surface Passivation of Nanoscale Zerovalent Iron: A Critical Review

Abstract

Nanoscale zerovalent iron (NZVI) is one of the most extensively studied nanomaterials in the fields of wastewater treatment and remediation of soil and groundwater. However, rapid oxidative transformations of NZVI can result in reduced NZVI reactivity. Indeed, the surface passivation of NZVI is considered one of the most challenging aspects in successfully applying NZVI to contaminant degradation. The oxidation of NZVI can lead to the formation of Fe II-bearing phases (e.g., Fe II O, Fe II (OH) 2 , Fe II Fe III 2 O 4) on the NZVI surface or complete oxidation to ferric (oxyhydr)oxides (e.g., Fe III OOH). This corrosion phenomenon is dependent upon various factors including the composition of NZVI itself, the type and concentration of aqueous species, reaction time and oxic/anoxic environments. As such, the coexistence of different Fe oxidation states on NZVI surfaces may also, in some instances, provide a unique reactive microenvironment to promote the adsorption of contaminants and their subsequent transformation via redox reactions. Thus, an understanding of passivation chemistry, and its related mechanisms, is essential not only for effective NZVI application but also for accurately assessing the positive and negative effects of NZVI surface passivation. The aim of this review is to discuss the nature of the passivation processes that occur and the passivation byproducts that form in various environments. In particular, the review presents: i) the strengths and limitations of state-of-the-art techniques (e.g., electron microscopies and X-ray based spectroscopies) to identify passivation byproducts; ii) the passivation mechanisms proposed to occur in anoxic and oxic environments; and iii) the effects arising from synthesis procedures and the presence of inorganics/organics on the nature of the passivation byproducts that form. In addition, several depassivation strategies that may assist in increasing and/or maintaining the reactivity of NZVI are considered, thereby enhancing the effectiveness of NZVI in contaminant degradation.

Aging of zero‐valent iron‐based nanoparticles in aqueous environment and the consequent effects on their reactivity and toxicity

“…To date, many researchers have put great efforts into answering to the above questions. There have been some reviews summarizing the transformations of nZVI in aqueous environment (Bae, Collins, Waite, & Hanna, ; Lei, Sun, Tsang, & Lin, ; Liu, Gu, Wang, Liu, & Zhang, ). However, these reviews mainly focused on the nZVI particles, without much attention given to the transformations of the other modified nZVI particles (e.g., sulfide‐modified nZVI, nZVI‐based bimetallic particles, surface‐coated nZVI), especially for the consequent impacts of the transformations on their reactivity and toxicity.…”

Evaluation of Nano Zero-Valent Iron (nZVI) Activity in Solution and Immobilized in Hydrophilic PVDF Membrane for Drimaren Red X-6BN and Bisphenol-a Removal in Water

“…Size distribution ( Figure 1A) revealed nZVI/CMC particles with two major peaks at the size of 75.6 nm (width: 32.8 nm) and 417.7 nm (width: 187.1 nm), as well as the z-average diameter of 109.0 nm (Polydispersity Index-PDI: 0.483). As CMC polymer acts as a stabilizer to avoid nanoparticles' aggregation by implying negative charges in the upper layer [3,9,10], its presence as a nonreacted polymer in the sample solution could generate the double peak. This was confirmed ( Figure 1B) by the peak at 624.6 nm (width: 102.0 nm) and z-average diameter of 1067 nm (PDI: 0.633) for just CMC particles in the solution.…”

“…nZVI has been widely studied for the oxidation of pharmaceuticals and herbicides because of its capability of reducing dissolved oxygen (O 2 ) and producing H 2 O 2 , which reacts with the iron leached from the material to generate hydroxyl radicals (•OH) [3,8,30]. Generally, this approach was followed by deactivation of the catalyst in a short period, diminishing the efficiency of the system.…”

“…When this process occurs at pH ≥ 7, OH − ions are in abundance and react with iron ions in the solution to form hydroxides or oxygenized species (e.g., Fe(OH) 2 , Fe(OH) 3 , Fe 2 O 3 , FeO, Fe 3 O 4 , and FeOOH), decreasing the efficiency of radicals' generation [1,2,30]. Yet, the application of nZVI to these systems is suggested as a catalyst capable of inducing •OH formation in pH near to neutral by the corrosion of the nanoparticle surface, which is mediated by the presence of O 2 [3,8]. Furthermore, Harada et al depict the behavior of nZVI and the gradual enhancement of •OH formation when the pH is decreased from 7 to 3 in oxygenated water [30].…”

“…Fenton reactions that involve nano zero-valent iron (nZVI) as a catalyst has shown high promise in the degradation of various recalcitrant contaminants (such as pharmaceuticals, personal care products, and pesticides) in water by achieving their mineralization via •OH radical generation [1][2][3]. Due to its characteristic as a strong reducing agent, nZVI has been also applied for the dechlorination of trichloroethylene and polychlorinated biphenyls, as well as decolorization of dyes (reactive black 5 and reactive red 198) and removal of antibiotics (tetracycline) [4][5][6][7].…”