Search

The charged surfaces of micaceous minerals, especially illite, regulate the mobility of the major radioisotopes of Cs (134Cs, 135Cs, 137Cs) in the geosphere. Despite the long history of Cs adsorption studies, the nature of the illite surface sites remains incompletely understood. To address this problem, we present atomistic simulations of Cs competition with Na for three candidate illite adsorption sites - edge, basal plane, and interlayer. Our simulation results are broadly consistent with affinities and selectivities that have been inferred from surface complexation models. Cation exchange on the basal planes is thermodynamically ideal, but exchange on edge surfaces and within interlayers shows complex, thermodynamically non-ideal behavior. The basal planes are weakly Cs-selective, while edges and interlayers have much higher affinity for Cs. The dynamics of NaCs exchange are rapid for both cations on the basal planes, but considerably slower for Cs localized on edge surfaces. In addition to new insights into Cs adsorption and exchange with Na on illite, we report the development of a methodology capable of simulating fully-flexible clay mineral nanoparticles with stable edge surfaces using a well-tested interatomic potential model.

The workability of fresh Portland cement (PC) concrete critically depends on the reaction of the cubic tricalcium aluminate (C3A) phase in Ca- and S-rich pH >12 aqueous solution, yet its rate-controlling mechanism is poorly understood. In this article, the role of adsorption phenomena in C3A dissolution in aqueous Ca-, S-, and polynaphthalene sulfonate (PNS)-containing solutions is analyzed. The zeta potential and pH results are consistent with the isoelectric point of C3A occurring at pH ∼12 and do not show an inversion of its electric double layer potential as a function of S or Ca concentration, and PNS adsorbs onto C3A, reducing its zeta potential to negative values at pH >12. The S and Ca K-edge X-ray absorption spectroscopy (XAS) data obtained do not indicate the structural incorporation or specific adsorption of SO42- on the partially dissolved C3A solids analyzed. Together with supporting X-ray ptychography and scanning electron microscopy results, a model for C3A dissolution inhibition in hydrated PC systems is proposed whereby the formation of an Al-rich leached layer and the complexation of Ca-S ion pairs onto this leached layer provide the key inhibiting effect(s). This model reconciles the results obtained here with the existing literature, including the inhibiting action of macromolecules such as PNS and polyphosphonic acids upon C3A dissolution. Therefore, this article advances the understanding of the rate-controlling mechanism in hydrated C3A and thus PC systems, which is important to better controlling the workability of fresh PC concrete.

The aqueous chemistry of water films confined between clay mineral surfaces remains an important unknown in predictions of radioelement migration from radioactive waste repositories. This issue is particularly important in the case of long-lived anionic radioisotopes (129I‑, +TcO4‑,36Cl‑) which interact with clay minerals primarily by anion exclusion. For example, models of ion migration in clayey media do not agree as to whether anions are completely or partially excluded from clay interlayer nanopores. In the present study, this key issue was addressed for Cl‑ using MD simulations for a range of nanopore widths (6 to 15 Å) overlapping the range of average pore widths that exists in engineered clay barriers. The MD simulation results were compared with the predictions of a thermodynamic model (Donnan Equilibrium model) and two pore-scale models based on the Poisson-Boltzmann equation under the assumption that interlayer water behaves as bulk liquid water. The simulations confirmed that anion exclusion from clay interlayers is greater than predicted by the pore-scale models, particularly at the smallest pore size examined. This greater anion exclusion stems from Cl‑ being more weakly solvated in nano-confined water than it is in bulk liquid water. Anion exclusion predictions based on the Poisson-Boltzmann equation were consistent with the MD simulation results, however, if the predictions included an ion closest approach distance to the clay mineral surface on the order of 2.0 - 0.8 Å. These findings suggest that clay interlayers approach a state of complete anion exclusion (hence, ideal semi-permeable membrane properties) at a pore width of 4.2 - 1.5 Å.

Irrigation with treated wastewater is expected to increase significantly in California during the coming decade as a way to reduce the impact of drought and mitigate water transfer issues. To ensure that such wastewater reuse does not result in unacceptable impacts on soil permeability, water quality guidelines must effectively address sodicity hazard. However, current guidelines are based on the sodium adsorption ratio (SAR) and thus assume that potassium (K) and magnesium (Mg), which often are at elevated concentrations in recycled wastewaters, pose no hazard, despite many past studies to the contrary. Recent research has established that the negative effects of high K and Mg concentrations on soil permeability are substantial and that they can be accounted for by a new irrigation water quality parameter, the cation ratio of structural stability (CROSS), a generalization of SAR. We show that CROSS, when suitably optimized, correlates strongly with a standard measure of soil permeability reduction for an agricultural soil leached with winery wastewater, and that it can be incorporated directly into existing irrigation water quality guidelines by replacing SAR.

Recent microcosm studies have revealed that fluoroquinolone (FQ) antibiotics can have ecotoxicological impacts on photosynthetic organisms, but little is known about the mechanisms of toxicity. We employed a combination of modeling and experimental techniques to explore how FQs may have these unintended secondary toxic effects. Structure-activity analysis revealed that the quinolone ring and secondary amino group typically present in FQ antibiotics may mediate their action as quinone site inhibitors in photosystem II (PS-II), a key enzyme in photosynthetic electron transport. Follow-up molecular simulations involving nalidixic acid (Naldx), a nonfluorinated quinolone with a demonstrated adverse impact on photosynthesis, and ciprofloxacin (Cipro), the most commonly used FQ antibiotic, showed that both may interfere stereochemically with the catalytic activity of reaction center II (RC-II), the pheophytin-quinone-type center present in PS-II. Naldx can occupy the same binding site as the secondary quinone acceptor (Q(B)) in RC-II and interact with amino acid residues required for the enzymatic reduction of Q(B). Cipro binds in a somewhat different manner, suggesting a different mechanism of interference. Fluorescence induction kinetics, a common method of screening for PS-II inhibition, recorded for photoexcited thylakoid membranes isolated from Cipro-exposed spinach chloroplasts, indicated that Cipro interferes with the transfer of energy from excited antenna chlorophyll molecules to the reaction center in RC-II ([Cipro] >or= 5 microM in vitro and >or=10 microM in vivo) and thus delays the kinetics of photoreduction of the primary quinone acceptor (Q(A); [Cipro] >or= 0.6 microM in vitro). Spinach plants exposed to Cipro exhibited severe growth inhibition characterized by a decrease in both the synthesis of leaves and growth of the roots ([Cipro] >or= 0.5 microM in vivo). Our results thus demonstrate that Cipro and related FQ antibiotics may interfere with photosynthetic pathways, in addition to causing morphological deformities in higher plants.