Membrane behavior and diffusion in unsaturated sodium bentonite

2015 Spring.Includes bibliographical references.Sodium-bentonite (Na-bentonite) is a highly active clay commonly used as a barrier or a component of a barrier for chemical containment applications (e.g., landfills, waste impoundments, vertical cutoff walls) due to the ability of Na-bentonite to limit solute (contaminant) transport resulting from high swell and low hydraulic conductivity. However, Na-bentonite also may exhibit semipermeable membrane behavior or solute restriction, which can result in enhanced performance of the barrier by reducing liquid and contaminant flux. Experimental studies to date have focused on the correlation between membrane behavior and diffusion of solutes almost exclusively under fully saturated conditions (i.e., degree of water saturation, S, of 1.0). However, clay barriers can exist at various degrees of water saturation (S < 1.0), and, based on our current, conceptual understanding of the mechanisms causing membrane behavior in saturated clays, the influence of membrane behavior on solute transport is likely to be even more significant in clays under unsaturated conditions. Based on these considerations, an innovative testing apparatus was developed to allow for the simultaneous measurement of membrane behavior and diffusion in unsaturated Na-bentonite. The test specimens were prepared using a dialysis method that allowed for control of the cation species on the exchange complex of the bentonite, removal of excess soluble salts, and estimation of diffusion properties. Membrane efficiencies (ω) and effective diffusion coefficients (D*) of bentonite specimens with S ranging from 0.79 to 1.0 were measured by performing multistage tests using solutions of potassium chloride (KCl). The source concentrations (Cot) of the KCl solutions were 20 mM, 30 mM, and 50 mM, which resulted in average concentrations in the specimen at steady-state diffusion (Cave) of approximately 10 mM, 15 mM, and 25 mM. For all values of S, a decrease in S correlated with an increase in ω and a decrease in D*. For example, for Cot of 50 mM, ω increased from 0.31 to 0.41 and D* for chloride decreased from 4.1 x 10-10 m2/s to 3.1 x 10-10 m2/s as S decreased from 1.0 to 0.84. The results of this study advance our fundamental understanding of solute transport mechanisms in Na-bentonite and contribute to the base of knowledge that must be established prior to incorporating membrane behavior effects in the design of barriers for chemical containment facilities

Analytical evaluation of steady-state solute distribution in through-diffusion and membrane behavior test under non-perfectly flushing boundary conditions

The through-diffusion and membrane behavior testing procedure using a closed-system apparatus has been widely used for concurrent measurement of diffusion and membrane efficiency coefficients of low-permeability clay-based barrier materials. However, the common assumption of perfectly flushing conditions at the specimen boundaries could induce errors in analyses of the diffusion coefficients and membrane efficiencies. In this study, an innovative pseudo three-dimensional (3D) analytical method was proposed to evaluate solute distribution along the boundary surfaces of the soil-porous disks system, considering the non-perfectly flushing conditions. The results were consistent with numerical models under two scenarios considering different inflow/outflow positions. The proposed model has been demonstrated to be an accurate and reliable method to estimate solute distributions along the boundaries. The calculated membrane efficiency coefficient and diffusion coefficient based on the proposed analytical method are more accurate, resulting in up to 50% less relative error than the traditional approach that adopts the arithmetic mean value of the influent and effluent concentrations. The retardation factor of the clay specimen also can be calculated with a revised cumulative mass approach. Finally, the simulated transient solute transport matched with experimental data from a multi-stage through-diffusion and membrane behavior test, validating the accuracy of the proposed method

Risk of COVID-19 after natural infection or vaccinationResearch in context

Background: While vaccines have established utility against COVID-19, phase 3 efficacy studies have generally not comprehensively evaluated protection provided by previous infection or hybrid immunity (previous infection plus vaccination). Individual patient data from US government-supported harmonized vaccine trials provide an unprecedented sample population to address this issue. We characterized the protective efficacy of previous SARS-CoV-2 infection and hybrid immunity against COVID-19 early in the pandemic over three-to six-month follow-up and compared with vaccine-associated protection. Methods: In this post-hoc cross-protocol analysis of the Moderna, AstraZeneca, Janssen, and Novavax COVID-19 vaccine clinical trials, we allocated participants into four groups based on previous-infection status at enrolment and treatment: no previous infection/placebo; previous infection/placebo; no previous infection/vaccine; and previous infection/vaccine. The main outcome was RT-PCR-confirmed COVID-19 >7–15 days (per original protocols) after final study injection. We calculated crude and adjusted efficacy measures. Findings: Previous infection/placebo participants had a 92% decreased risk of future COVID-19 compared to no previous infection/placebo participants (overall hazard ratio [HR] ratio: 0.08; 95% CI: 0.05–0.13). Among single-dose Janssen participants, hybrid immunity conferred greater protection than vaccine alone (HR: 0.03; 95% CI: 0.01–0.10). Too few infections were observed to draw statistical inferences comparing hybrid immunity to vaccine alone for other trials. Vaccination, previous infection, and hybrid immunity all provided near-complete protection against severe disease. Interpretation: Previous infection, any hybrid immunity, and two-dose vaccination all provided substantial protection against symptomatic and severe COVID-19 through the early Delta period. Thus, as a surrogate for natural infection, vaccination remains the safest approach to protection. Funding: National Institutes of Health

Risk of COVID-19 after natural infection or vaccinationResearch in context

Summary: Background: While vaccines have established utility against COVID-19, phase 3 efficacy studies have generally not comprehensively evaluated protection provided by previous infection or hybrid immunity (previous infection plus vaccination). Individual patient data from US government-supported harmonized vaccine trials provide an unprecedented sample population to address this issue. We characterized the protective efficacy of previous SARS-CoV-2 infection and hybrid immunity against COVID-19 early in the pandemic over three-to six-month follow-up and compared with vaccine-associated protection. Methods: In this post-hoc cross-protocol analysis of the Moderna, AstraZeneca, Janssen, and Novavax COVID-19 vaccine clinical trials, we allocated participants into four groups based on previous-infection status at enrolment and treatment: no previous infection/placebo; previous infection/placebo; no previous infection/vaccine; and previous infection/vaccine. The main outcome was RT-PCR-confirmed COVID-19 >7–15 days (per original protocols) after final study injection. We calculated crude and adjusted efficacy measures. Findings: Previous infection/placebo participants had a 92% decreased risk of future COVID-19 compared to no previous infection/placebo participants (overall hazard ratio [HR] ratio: 0.08; 95% CI: 0.05–0.13). Among single-dose Janssen participants, hybrid immunity conferred greater protection than vaccine alone (HR: 0.03; 95% CI: 0.01–0.10). Too few infections were observed to draw statistical inferences comparing hybrid immunity to vaccine alone for other trials. Vaccination, previous infection, and hybrid immunity all provided near-complete protection against severe disease. Interpretation: Previous infection, any hybrid immunity, and two-dose vaccination all provided substantial protection against symptomatic and severe COVID-19 through the early Delta period. Thus, as a surrogate for natural infection, vaccination remains the safest approach to protection. Funding: National Institutes of Health