Electrochemical membrane filtration is widely reported
to enhance
water contaminants’ degradation or rejection via anodic oxidation
or cathodic repulsion. Despite their advances, electrochemical membranes
or electrocatalysts often suffer from corrosion or passivation, especially
under strong electrode potentials or reactions. Moreover, the formation
of toxic byproducts, such as chlorinated organic compounds and oxyhalides
(e.g., ClO4
–) is another major concern.
This study investigated the membrane aging processes of two types
of conductive membranes, multiwalled carbon nanotubes (MWCNTs) and
ferrite/graphitic carbon nitride hybrids (Fe3O4@g-C3N4) coated on ceramic membranes. Under
high current densities (∼20 mA·cm–2)
with anodic potentials (∼10 V), MWCNTs and Fe3O4@g-C3N4 catalysts underwent evident
oxidation as indicated by an increase of the intensity ratio of the
Raman spectral bands (I
D/I
G) and charge transfer resistance (R
ct) of two electrochemical membranes. Under variations of electrode
potentials, chloride or bromide were shown to be oxidized to bromate
(BrO3
–) and chlorate (ClO3
–) at levels of 1–10 mmol·L–1. The formation of BrO3
– and ClO3
– was dependent on the solution pH, current
densities (1–20 mA·cm–2), and initial
concentrations of Br or Cl ions. To warrant a safe and rational design
and operation of electrochemically reactive membrane processes, membrane
aging and toxic byproduct’s formation deserve careful characterization
under relevant water filtration environments.
Oxidation is a corrosion reaction where the corroded metal forms an oxide. Prevention of oxidation at the nanoscale is critically important to retain the physicochemical properties of metal nanoparticles. In this work, we studied the stability of polyethylene glycol (PEG) coated copper nanoparticles (PEGylated CuNPs) against oxidation. The freshly-prepared PEGylated CuNPs mainly consist of metallic Cu which are quite stable in air although their surfaces are typically covered with a few monolayers of cuprous oxide. However, they are quickly oxidized in water due to the presence of protons that facilitate oxidation of the cuprous oxide to cupric oxide. PEG with carboxylic acid terminus could slightly delay the oxidation process compared to that with thiol terminus. It was found that a solvent with reducing power such as ethanol could greatly enhance the stability of PEGylated CuNPs by preventing further oxidation of the cuprous oxide to cupric oxide and thus retain the optical properties of CuNPs. The reducing environment also assists the galvanic replacement of these PEGylated CuNPs to form hollow nanoshells; however, they consist of ultra-small particle assemblies due to the co-reduction of gold precursor during the replacement reaction. As a result, these nanoshells do not exhibit strong optical properties in the near-infrared region. This study highlights the importance of solvent effects on PEGylated nonprecious metal nanoparticles against oxidation corrosion and its applications in preserving physicochemical properties of metallic nanostructures.
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