6 research outputs found
Effects of Nano Zero-Valent Iron on Oxidation−Reduction Potential
Oxidation−reduction potential (ORP) measurements have been
widely used to assess the results of injection of nano zerovalent
iron (nZVI) for groundwater remediation, but the significance of these
measurements has never been established. Using rotating disk electrodes
(RDE) in suspensions of nZVI, we found the electrode response to be
highly complex but also a very sensitive probe for a range of fundamentally
significant processes. The time dependence of the electrode response
reflects both a primary effect (attachment of nZVI onto the electrode
surface) and several secondary effects (esp., oxidation of iron and
variations in dissolved H<sub>2</sub> concentration). At nZVI concentrations
above ∼200 mg/L, attachment of nZVI to the electrode is sufficient
to give it the electrochemical characteristics of an Fe<sup>0</sup> electrode, making the electrode relatively insensitive to changes
in solution chemistry. Lower nZVI concentrations give a proportional
response in ORP, but much of this effect is mediated by the secondary
effects noted above. Coating the nZVI with natural organic matter
(NOM), or the organic polymers used to make stabile suspensions of
nZVI, moderates its effect on ORP measurments. Our results provide
the basis for interpretating ORP measurements used to characterize
the results of injecting nZVI into groundwater
Kinetics of Heavy Metal Dissociation from Natural Organic Matter: Roles of the Carboxylic and Phenolic Sites
We
developed a unifying model for the kinetics of heavy metal dissociation
from natural organic matter (NOM) in this study. The kinetics model,
integrated with the equilibrium model WHAM 7, specifically considered
metal ion reactions with various NOM sites formed by the carboxylic
and phenolic sites. The association and dissociation rate coefficients
for metal reactions with various NOM sites were constrained by WHAM
predicted equilibrium distribution coefficients at specific reaction
conditions. We developed the relationship for the dissociation rate
coefficients among different binding sites for each metal, which was
internally constrained by the metal binding constants. The model had
only one fitting parameter, the dissociation rate coefficient for
the metal complexes formed with two weak carboxylic sites, and all
other parameters were derived from WHAM 7. The kinetic data for metal
dissociation from NOM were collected from the literature, and the
model was able to reproduce most of relevant data analyzed. The bidentate
complexes appeared to be the predominated species controlling metal
dissociation under most environmental conditions. The model can help
to predict the reactivity and bioavailability of heavy metals under
the impact of multiple competing ligands including NOM
A General Model for Kinetics of Heavy Metal Adsorption and Desorption on Soils
In this study, we
propose a general kinetics model for heavy metal
adsorption and desorption reactions in soils when soil organic matter
(SOM) is the dominant adsorbent. The kinetics model, integrated with
the equilibrium speciation model WHAM VI, specifically considers metal
reactions with SOM and dissolved organic matter (DOM) and accounts
for the variations of solution chemistry. Metal reactions with SOM
are associated with two groups of sites, one from the monodentate
sites and another one from the bidentate and tridentate sites. There
are three model parameters, desorption rate coefficients of the two
groups of SOM sites for each metal and reactive organic carbon (ROC)
for each soil. The applicability of the kinetics model was mainly
examined with three elements, Cu, Pb, and Zn, which demonstrate different
binding ability with organic matter. The kinetic data were collected
with a stirred-flow reactor covering a wide range of experimental
conditions, including varying SOM, DOM, Ca, and metal concentrations,
reaction pHs, and different flow rates. The kinetics model has been
successfully applied to describe heavy metal adsorption and desorption
on soils under various reaction conditions
Kinetics of Cation and Oxyanion Adsorption and Desorption on Ferrihydrite: Roles of Ferrihydrite Binding Sites and a Unified Model
Quantitative
understanding the kinetics of toxic ion reactions with various heterogeneous
ferrihydrite binding sites is crucial for accurately predicting the
dynamic behavior of contaminants in environment. In this study, kinetics
of As(V), Cr(VI), Cu(II), and Pb(II) adsorption and desorption on
ferrihydrite was studied using a stirred-flow method, which showed
that metal adsorption/desorption kinetics was highly dependent on
the reaction conditions and varied significantly among four metals.
High resolution scanning transmission electron microscopy coupled
with energy-dispersive X-ray spectroscopy showed that all four metals
were distributed within the ferrihydrite aggregates homogeneously
after adsorption reactions. Based on the equilibrium model CD-MUSIC,
we developed a novel unified kinetics model applicable for both cation
and oxyanion adsorption and desorption on ferrihydrite, which is able
to account for the heterogeneity of ferrihydrite binding sites, different
binding properties of cations and oxyanions, and variations of solution
chemistry. The model described the kinetic results well. We quantitatively
elucidated how the equilibrium properties of the cation and oxyanion
binding to various ferrihydrite sites and the formation of various
surface complexes controlled the adsorption and desorption kinetics
at different reaction conditions and time scales. Our study provided
a unified modeling method for the kinetics of ion adsorption/desorption
on ferrihydrite
Mechanisms of Synergistic Removal of Low Concentration As(V) by nZVI@Mg(OH)<sub>2</sub> Nanocomposite
In
this work, by using Mg(OH)<sub>2</sub> nanoplatelets as support
material for nanoscale zerovalent iron (nZVI), nZVI@Mg(OH)<sub>2</sub> composite was prepared and found to have super high adsorption ability
toward As(V) at environmentally relevant concentrations. It was revealed
that the variation of corrosion products of nZVI in the presence of
Mg(OH)<sub>2</sub> and Mg<sup>2+</sup> is an important factor for
increase in the adsorption ability toward As(V). X-ray diffraction
(XRD) analysis indicated that the weakly basic condition induced by
Mg(OH)<sub>2</sub> decreases the lepidocrocite (γ-FeOOH) and
increases the magnetite/maghemite (Fe<sub>3</sub>O<sub>4</sub>/γ-Fe<sub>2</sub>O<sub>3</sub>) content in the corrosion products of nZVI,
and the latter has better adsorption affinity to As(V). Moreover,
extended X-ray absorption fine structure spectroscopy (EXAFS) indicated
that the coordination between arsenic and iron minerals is influenced
by dissolved Mg<sup>2+</sup>, leading to probable formation of magnesium
ferrite (MgFe<sub>2</sub>O<sub>4</sub>) which has considerable adsorption
affinity to As(V). This work provides an important reference not only
for the design of pollution control materials but also for understanding
arsenic immobilization in natural environments with ubiquitous Mg<sup>2+</sup> ion
Effects of Sulfidation, Magnetization, and Oxygenation on Azo Dye Reduction by Zerovalent Iron
Applications
of zerovalent iron (ZVI) for water treatment under
aerobic conditions include sequestration of metals (e.g., in acid
mine drainage) and decolorization of dyes (in wastewaters from textile
manufacturing). The processes responsible for contaminant removal
can be a complex mixture of reduction, oxidation, sorption, and coprecipitation
processes, which are further complicated by the dynamics of oxygen
intrusion, mixing, and oxide precipitation. To better understand such
systems, the removal of an azo dye (Orange I) by micron-sized granular
ZVI at neutral pH was studied in open (aerobic) stirred batch reactors,
by measuring the kinetics of Orange I decolorization and changes in
“geochemical” properties (DO, Fe(II), and Eh), with
and without two treatments that might improve the long-term performance
of this system: sulfidation by pretreatment with sulfide and magnetization
by application of a weak magnetic field (WMF). The results show that
the changes in solution chemistry are coupled to the dynamics of oxygen
intrusion, which was modeled as analogous to dissolved oxygen sag
curves. Both sulfidation and magnetization increased Orange I removal
rates 2.4–71.8-fold, but there was little synergistic benefit
to applying both enhancements together. Respike experiments showed
that the enhancement from magnetization carries over from magnetization
to sulfidation, but not the reverse