18 research outputs found
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
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 AsÂ(V), CrÂ(VI), CuÂ(II), and PbÂ(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
Mineralogical characteristics of sediments and heavy metal mobilization along a river watershed affected by acid mine drainage
<div><p>Trace-element concentrations in acid mine drainage (AMD) are primarily controlled by the mineralogy at the sediment-water interface. Results are presented for a combined geochemical and mineralogical survey of Dabaoshan Mine, South China. Developed sequential extraction experiments with the analysis of the main mineralogical phases by semi-quantitative XRD, differential X-ray diffraction (DXRD) and scanning electron microscopy (SEM) were conducted to identify the quantitative relationship between iron minerals and heavy metals. Results showed that schwertmannite, jarosite, goethite and ferrihydrite were the dominant Fe-oxyhydroxide minerals which were detected alternately in the surface sediment with the increasing pH from 2.50 to 6.93 along the Hengshi River. Decreasing contents of schwertmannite ranging from 35 wt % to 6.5 wt % were detected along the Hengshi River, which was corresponding to the decreasing metal contents. The easily reducible fractions exert higher affinity of metals while compared with reducible and relatively stable minerals. A qualitative analysis of heavy metals extracted from the sediments indicated that the retention ability varied: Pb > Mn > Zn > As ≈ Cu > Cr > Cd ≈ Ni. Results in this study are avail for understanding the fate and transport of heavy metals associated with iron minerals and establishing the remediation strategies of AMD systems.</p></div
SEM images of sediment samples before extraction (Sh: schwertmannite; Jt: jarosite; Gt: goethite; Fh: ferrihydrite; Gp: gypsum; (The scale of all figures is the same)).
<p>SEM images of sediment samples before extraction (Sh: schwertmannite; Jt: jarosite; Gt: goethite; Fh: ferrihydrite; Gp: gypsum; (The scale of all figures is the same)).</p
Percentage of iron removed from synthetic minerals in each step of adapted 7-steps sequential extractions.
<p>Percentage of iron removed from synthetic minerals in each step of adapted 7-steps sequential extractions.</p
Fe- and S-Metabolizing Microbial Communities Dominate an AMD-Contaminated River Ecosystem and Play Important Roles in Fe and S Cycling
<p>Indigenous Fe- and S-metabolizing bacteria play important roles both in the formation and the natural attenuation of acid mine drainage (AMD). Due to its low pH and Fe-S-rich waters, a river located in the Dabaoshan Mine area provides an ideal opportunity to study indigenous Fe- and S-metabolizing microbial communities and their roles in biogeochemical Fe and S cycling. In this work, water and sediment samples were collected from the river for physicochemical, mineralogical, and microbiological analyses. Illumina MiSeq sequencing indicated higher species richness in the sediment than in the water. Sequencing also found that Fe- and S-metabolizing bacteria were the dominant microorganisms in the heavily and moderately contaminated areas. Fe- and S-metabolizing bacteria found in the water were aerobes or facultative anaerobes, including <i>Acidithiobacillus, Acidiphilium, Thiomonas, Gallionella</i>, and <i>Leptospirillum</i>. Fe- and S-metabolizing bacteria found in the sediment belong to microaerobes, facultative anaerobes, or obligatory anaerobes, including <i>Acidithiobacillus, Sulfobacillus, Thiomonas, Gallionella, Geobacter, Geothrix</i>, and <i>Clostridium</i>. Among the dominant genera in the sediment, <i>Geobacter</i> and <i>Geothrix</i> were rarely detected in AMD-contaminated natural environments. Canonical correspondence analysis indicated that pH, S, and Fe concentration gradients were the most important factors in structuring the river microbial community. Moreover, a scheme explaining the biogeochemical Fe and S cycling is advanced in light of the Fe and S species distribution and the identified Fe- and S-metabolizing bacteria.</p
Sequential extractions of sediment samples from sampling sites (S0-S11).
<p>Sequential extractions of sediment samples from sampling sites (S0-S11).</p
The variation of heavy metals concentration in Hengshi River.
<p>The variation of heavy metals concentration in Hengshi River.</p
DXRD and semi-quantitative XRD patterns of sediment samples.
<p>((a) and (b) DXRD patterns refer to samples S0 and S1; (c) and (d) refer to the mixture of NaCl with Sh and Fh; (e) and (f): Semi-quantitative XRD patterns of samples S7 and S8; the Sh: schwertmannite; Fh: Ferrihydrite).</p
Mechanisms of Synergistic Removal of Low Concentration As(V) by nZVI@Mg(OH)<sub>2</sub> Nanocomposite
In
this work, by using MgÂ(OH)<sub>2</sub> nanoplatelets as support
material for nanoscale zerovalent iron (nZVI), nZVI@MgÂ(OH)<sub>2</sub> composite was prepared and found to have super high adsorption ability
toward AsÂ(V) at environmentally relevant concentrations. It was revealed
that the variation of corrosion products of nZVI in the presence of
MgÂ(OH)<sub>2</sub> and Mg<sup>2+</sup> is an important factor for
increase in the adsorption ability toward AsÂ(V). X-ray diffraction
(XRD) analysis indicated that the weakly basic condition induced by
MgÂ(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 AsÂ(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 AsÂ(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