The inclusion of multidentate adsorption reactions has improved the ability of surface complexation models (SCM) to predict adsorption to mineral surfaces, but variation in the mass action expression ...for these reactions has caused persistent ambiguity and occasional mishandling. The principal differences are the exponent (α) for the activity of available surface sites and the inclusion of surface site activity on a molar concentration versus fraction basis. Exemplified by bidentate surface complexation, setting α at two within the molar-based framework will cause critical errors in developing a self-consistent model. Despite the publication of several theoretical discussions regarding appropriate approaches, mishandling and confusion has persisted in the model applications involving multidentate surface complexes. This review synthesizes the theory of modeling multidentate surface complexes in a style designed to enable improvements in SCM practice. The implications of selecting an approach for multidentate SCM are illustrated with a previously published data set on U(VI) adsorption to goethite. To improve the translation of theory into improved practice, the review concludes with suggestions for handling multidentate reactions and publishing results that can avoid ambiguity or confusion. Although most discussion is exemplified by the generic bidentate case, the general issues discussed are relevant to higher denticity adsorption.
We consider the Cauchy problem for the complex valued semi-linear heat equation
∂
t
u
-
Δ
u
-
u
m
=
0
,
u
(
0
,
x
)
=
u
0
(
x
)
,
where
m
≥
2
is an integer and the initial data belong to ...super-critical spaces
E
σ
s
for which the norms are defined by
‖
f
‖
E
σ
s
=
‖
⟨
ξ
⟩
σ
2
s
|
ξ
|
f
^
(
ξ
)
‖
L
2
,
σ
∈
R
,
s
<
0
.
If
s
<
0
, then any Sobolev space
H
r
is a subspace of
E
σ
s
, i.e.,
∪
r
∈
R
H
r
⊂
E
σ
s
. We obtain the global existence and uniqueness of the solutions if the initial data belong to
E
σ
s
(
s
<
0
,
σ
≥
d
/
2
-
2
/
(
m
-
1
)
) and their Fourier transforms are supported in the first octant, the smallness conditions on the initial data in
E
σ
s
are not required for the global solutions. Moreover, we show that the error between the solution
u
and the iteration solution
u
(
j
)
is
C
j
/
(
j
!
)
2
. Similar results also hold if the nonlinearity
u
m
is replaced by an exponential function
e
u
-
1
.
The mobility of hexavalent uranium in soil and groundwater is strongly governed by adsorption to mineral surfaces. As strong naturally occurring adsorbents, manganese oxides may significantly ...influence the fate and transport of uranium. Models for U(VI) adsorption over a broad range of chemical conditions can improve predictive capabilities for uranium transport in the subsurface. This study integrated batch experiments of U(VI) adsorption to synthetic and biogenic MnO2, surface complexation modeling, ζ-potential analysis, and molecular-scale characterization of adsorbed U(VI) with extended X-ray absorption fine structure (EXAFS) spectroscopy. The surface complexation model included inner-sphere monodentate and bidentate surface complexes and a ternary uranyl-carbonato surface complex, which was consistent with the EXAFS analysis. The model could successfully simulate adsorption results over a broad range of pH and dissolved inorganic carbon concentrations. U(VI) adsorption to synthetic δ-MnO2 appears to be stronger than to biogenic MnO2, and the differences in adsorption affinity and capacity are not associated with any substantial difference in U(VI) coordination.
Sulfate adsorption on mineral surfaces is an important environmental chemical process, but the structures and respective contribution of different adsorption complexes under various environmental ...conditions are unclear. By combining sulfur K-edge XANES and EXAFS spectroscopy, quantum chemical calculations, and surface complexation modeling (SCM), we have shown that sulfate forms both outer-sphere complexes and bidentate–binuclear inner-sphere complexes on ferrihydrite surfaces. The relative fractions of the complexes vary with pH, ionic strength (I), and sample hydration degree (wet versus air-dried), but their structures remained the same. The inner-sphere complex adsorption loading decreases with increasing pH while remaining unchanged with I. At both I = 0.02 and 0.1 M, the outer-sphere complex loading reaches maximum at pH ∼5 and then decreases with pH, whereas it monotonically decreases with pH at I = 0.5 M. These observations result from a combination of the ionic-strength effect, the pH dependence of anion adsorption, and the competition between inner- and outer-sphere complexation. Air-drying drastically converts the outer-sphere complexes to the inner-sphere complexes. The respective contributions to the overall adsorption loading of the two complexes were directly modeled with the extended triple layer SCM by implementing the bidentate–binuclear inner-sphere complexation identified in the present study. These findings improve our understanding of sulfate adsorption and its effects on other environmental chemical processes and have important implications for generalizing the adsorption behavior of anions forming both inner- and outer-sphere complexes on mineral surfaces.
The stability of UO2 is critical to the success of reductive bioremediation of uranium. When reducing conditions are no longer maintained, Mn redox cycling may catalytically mediate the oxidation of ...UO2 and remobilization of uranium. Ligand-stabilized soluble Mn(III) was recently recognized as an important redox-active intermediate in Mn biogeochemical cycling. This study evaluated the kinetics of oxidative UO2 dissolution by soluble Mn(III) stabilized by pyrophosphate (PP) and desferrioxamine B (DFOB). The Mn(III)–PP complex was a potent oxidant that induced rapid UO2 dissolution at a rate higher than that by a comparable concentration of dissolved O2. However, the Mn(III)–DFOB complex was not able to induce oxidative dissolution of UO2. The ability of Mn(III) complexes to oxidize UO2 was probably determined by whether the coordination of Mn(III) with ligands allowed the attachment of the complexes to the UO2 surface to facilitate electron transfer. Systematic investigation into the kinetics of UO2 oxidative dissolution by the Mn(III)–PP complex suggested that Mn(III) could directly oxidize UO2 without involving particulate Mn species (e.g., MnO2). The expected 2:1 reaction stoichiometry between Mn(III) and UO2 was observed. The reactivity of soluble Mn(III) in oxidizing UO2 was higher at lower ratios of pyrophosphate to Mn(III) and lower pH, which is probably related to differences in the ligand-to-metal ratio and/or protonation states of the Mn(III)–pyrophosphate complexes. Disproportionation of Mn(III)–PP occurred at pH 9.0, and the oxidation of UO2 was then driven by both MnO2 and soluble Mn(III). Kinetic models were derived that provided excellent fits of the experimental results.
In order to provide a simplified and low-cost solution of the terminal for a distributed actuation system, this paper proposes an electro-hydrostatic actuator (EHA) based on the linear drive ...principle. The proposed actuator is directly driven by a linear pump with a collaborative rectification mechanism, whose performance relies on the collaboration of the internal two units. A pair of linear oscillating motors are employed to drive the two pump units respectively. The control of the actuator is based on the modulation of the oscillating amplitude, frequency, and phase difference of the two motors. The advantage of this actuator is that no more valve control is needed to rectify the linear pump besides the high efficiency of the direct pump drive. In this paper, both schematic and detailed structure of the actuator is presented. The kinematic and dynamic characteristics are analyzed and modeled, based on which the control method is proposed. The experiments verify the validity of the actuator structure and control.
Algae organic matter (AOM), including intracellular organic matter (IOM) and extracellular organic matter (EOM), are major membrane foulants in the treatment of algae-polluted water. In this study, ...the effects of EOM and IOM (at dissolved organic concentrations of 8 mg/L) on the fouling of a poly(ether sulfone) ultrafiltration (UF) membrane were investigated using a dead-end down-flow UF unit. Changes in the membrane pore geometry and the interaction energy between the membrane and foulants were analyzed based on the extended Derjaguin–Landau–Verwey–Overbeek (XDLVO) theory. The data (relative standard deviation within 10%) showed that UF was able to retain 57% and 46% of IOM and EOM respectively, while the corresponding membrane fluxes rapidly reduced to 28% and 33% of their respective initial values after a specific filtration volume of only 3.75 mL/cm2. The fouling model implied that cake formation was the major mechanism. Specifically, IOM foulant had a much greater free energy of cohesion (−59.08 mJ/m2) than EOM foulant (3.2 mJ/m2), leading to the formation of a compacted cake layer on the membrane surface. In contrast, small molecules of hydrophobic EOM tended to be adsorbed into the membrane pores, leading to significant reduction of the pore size and membrane flux. Therefore, the overall fouling rates caused by EOM and IOM were comparable when both of the above-mentioned mechanisms were considered.
Nanocomposite microparticle (nCmP) systems exhibit promising potential in the application of therapeutics for pulmonary drug delivery. This work aimed at identifying the optimal spray-drying ...condition(s) to prepare nCmP with specific drug delivery properties including small aerodynamic diameter, effective nanoparticle (NP) redispersion upon nCmP exposure to an aqueous solution, high drug loading, and low water content. Acetalated dextran (Ac-Dex) was used to form NPs, curcumin was used as a model drug, and mannitol was the excipient in the nCmP formulation. Box-Behnken design was applied using Design-Expert software for nCmP parameter optimization. NP ratio (NP%) and feed concentration (Fc) are significant parameters that affect the aerodynamic diameters of nCmP systems. NP% is also a significant parameter that affects the drug loading. Fc is the only parameter that influenced the water content of the particles significantly. All nCmP systems could be completely redispersed into the parent NPs, indicating that none of the factors have an influence on this property within the design range. The optimal spray-drying condition to prepare nCmP with a small aerodynamic diameter, redispersion of the NPs, low water content, and high drug loading is 80% NP%, 0.5% Fc, and an inlet temperature lower than 130°C.
The interactions between soil organic matter (SOM) and minerals are key processes controlling the organic carbon (OC) stability in the environment, but the underlying mechanisms responsible for OC ...distribution on minerals are still not clear. In this study, a common crystalline iron oxide, hematite, and a typical SOM component, humic acid (HA), were chosen to explore, at the nano and even sub-nano scales, HA adsorption and distribution on hematite under the impact of Pb(II) ions. Adsorption experiments and spherical aberration corrected scanning transmission electron microscopy (Cs-STEM) coupled with energy dispersive X-ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS) demonstrated that Pb(II) enhanced the adsorption of HA on hematite, but the nano scale distributions of HA varied between experimental conditions. The Cs-STEM, EDS, and the EELS analyses clearly unveiled that, with the presence of Pb(II), the hematite aggregates became more dispersed, and HA was not only adsorbed on the edges hematite nanoparticles, but also was localized in the extended areas away from the hematite particles/aggregates, which resulted in a heterogeneous distribution of OC in hematite-HA systems. Our imaging methods provided direct visualization of the mineral-SOM-metal interactions at nano scales, which shed lights on the underlying mechanisms for OC stabilization in natural environments and can enable more robust constraints in the models of organic matter-mineral interactions.
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•Adsorption and distribution of HA on hematite was visualized at nano and sub-nano scales by Cs-STEM.•Pb(II) enhanced HA adsorption on hematite and hematite aggregates became more dispersed.•Nano scale distributions of HA were different with different sequences of HA and Pb addition.•The imaging methods help to elucidate the mechanisms of organic matter-mineral interactions at nano scales.
A thorough understanding of the complex redox coupling among manganese, arsenic, sulfur and oxygen in subsurface environments is still obscured by their metastable intermediate valances and ...speciation. Arsenic sulfide minerals may be disturbed by natural or anthropogenic activities, and encounter oxidants such as oxygen and reactive trivalent Mn species, and how these abiotic interactions impact the mineral dissolution and transformation of arsenic and sulfur species, remains unknown. In this study, we investigated the effects of dissolved Mn(III) and manganite (γ-MnIIIOOH) on the dissolution behaviors of orpiment (As2S3) and realgar (AsS) under anoxic and oxic conditions. Complementary control experiments were also performed with dissolved arsenite without reduced sulfur. Oxygen, dissolved Mn(III) or manganite did not induce the oxidation of dissolved arsenite within several weeks. Orpiment’s initial dissolution is a non-redox process releasing of arsenite and sulfide, and the three above oxidants promoted the dissolution of orpiment by rapid oxidation of dissolved sulfide. However, only when both dissolved Mn(III) and dissolved oxygen were present, substantial accumulation of arsenate and sulfate were observed. These results suggested the critical role of sulfur species in abiotic arsenic transformation and a synergetic effect of Mn and oxygen on sulfur oxidation. In contrast to orpiment, the dissolution of realgar was a redox reaction that involved the oxidation of As(II) to As(III) and the direct releasing of sulfide, which could be promoted by both dissolved oxygen and manganite. The effect of dissolved Mn(III) and oxygen on the formation of arsenate and sulfate was also clearly observed during the dissolution of realgar. Despite of the slow abiotic oxidation of dissolved arsenite to arsenate in the presence dissolved Mn(III) and oxygen, the coexistence of sulfide could enable rapid accumulation of arsenate, accompanied by substantial transformation to sulfate. The evidence of thioarsenic species in these experiments provided a plausible explanation as an alternative pathway for the oxidation of the two elements by dissolved Mn(III). These results provide new insights for the Mn-As-S cycling in redox transition environments.
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