Relative contribution of chemico-osmosis and electro-osmosis to the experimental determination of the reflection coefficient in semipermeable clay soils

The containment performance of bentonite-based barriers is known to be influenced by the semipermeable membrane behaviour of the bentonite, which arises from the electrical interactions between the clay particles and the ionic species dissolved in the pore solution. Most of the experimental research conducted to date has provided evidence of the clay membrane behaviour, the extent of which is typically quantified through the reflection coefficient, , when the permeant (electrolyte) solution contains a monovalent or divalent salt. Under such conditions, the osmotic counter-flow of solution is controlled to a great extent by the solute exclusion, which is also referred to as the chemico-osmotic effect. However, theoretical simulations of coupled solute transport and solvent flow suggest that, when two or more cations with different diffusivities are contained in the permeant solution, the electro-osmotic effect, which stems from the condition of null electric current density, can be comparable to or even greater than the chemico-osmotic effect. The relative importance of the aforementioned contributions to multi-electrolyte systems is examined herein through the interpretation of laboratory test results from the literature pertaining to a bentonite-amended clay soil permeated with aqueous mixtures of potassium chloride (KCl) and hydrochloric acid (HCl)

Strain-controlled oedometer test for the measurement of the chemico-osmotic properties of bentonites

The possibility of relating the macroscopic transport properties and the swelling behaviour of bentonites to a limited number of physico-chemical and fabric parameters has been investigated by means of a new laboratory apparatus, which allows the reflection coefficient, which is also known as the membrane efficiency coefficient, and the swell coefficient to be simultaneously determined on the same clay specimen. The results of two multi-stage tests, which were performed on a natural sodium bentonite under fully saturated conditions, while varying both the specimen porosity and the salt concentration of the equilibrium solutions, have been interpreted through a physically-based model in which the pore-scale electro-chemical interactions be-tween the solid skeleton, the aqueous phase and the ion species are explicitly taken into account. The efficiency of the tested bentonite in acting as a semi-permeable membrane and its swelling behaviour have been found to be accurately simulated when a single fabric parameter, referred to as the solid charge coefficient, is calibrated on the available experimental dataset, thus suggesting that the containment performance of bentonite-based barriers, used for a number of geoenvironmental applications, may be predicted on the basis of the results of a strain-controlled oedometer test

Proprietà osmotiche delle bentoniti in equilibrio con miscele di elettroliti

Il comportamento a membrana semipermeabile delle bentoniti, impiegate nei sistemi di contenimento degli inquinanti, è stato finora oggetto di ricerca sperimentale impiegando soluzioni acquose di un singolo sale, non contemplando l’effetto della compresenza di due o più elettroliti nella soluzione interstiziale. A motivo della rilevanza di queste ultime condizioni operative nel momento in cui le bentoniti vengono a contatto con liquidi contaminati, è stata eseguita una prova osmotica multi-stage su una bentonite sodica naturale in equilibrio con miscele acquose di cloruro di sodio e cloruro di potassio, e i risultati sono stati interpretati tramite un modello teorico che permette di decomporre il controflusso osmotico di soluzione in una componente chimico-osmotica, che trae origine dalla parziale esclusione degli anioni dai pori della bentonite e si manifesta anche in presenza di un singolo sale, e una componente elettro-osmotica, che trae origine dalla condizione di assenza di corrente elettrica attraverso il mezzo poroso in presenza di cationi caratterizzati da diversa diffusività e determina l’insorgenza di valori del coefficiente di riflessione, w, sia superiori all’unità (w = 1.064) sia inferiori a zero (w = - 1.168)

Assessing the influence of chemico-osmosis on solute transport in bentonite membranes based on combined phenomenological and physical modeling

The ability of bentonite-based barriers to act as semipermeable membranes that inhibit the passage of solutes (ions) is well documented. This behavior induces chemico-osmotic liquid flux that can improve the performance of such barriers by reducing solute mass flux. This paper explores the potential significance of chemico-osmosis on solute transport through bentonite membranes using a phenomenological transport framework combined with a physical model relating the macroscale transport properties (membrane efficiency coefficient, w, and hydraulic conductivity, kh) to the microscale physicochemical and fabric properties of the bentonite. The model was used to simulate the coupled transport of monovalent salt (KCl) through a geosynthetic clay liner. The results indicate that the influence of chemico-osmosis is dependent upon the void ratio of the bentonite and the extent to which clay platelets are aggregated to form tactoids. Chemico-osmosis is predicted to have an increasingly more significant impact on solute transport with increasing source concentration (Cs0), despite decreasing w with increasing Cs0

Municipal solid waste management under Covid-19: Challenges and recommendations

Covid-19 is proving to be an unprecedented disaster for human health, social contacts and the economy worldwide. It is evident that SARS-CoV-2 may spread through municipal solid waste (MSW), if collected, bagged, handled, transported or disposed of inappropriately. Under the stress placed by the current pandemic on the sanitary performance across all MSW management (MSWM) chains, this industry needs to re-examine its infrastructure resilience with respect to all processes, from waste identification, classification, collection, separation, storage, transportation, recycling, treatment and disposal. The current paper provides an overview of the severe challenges placed by Covid-19 onto MSW systems, highlighting the essential role of waste management in public health protection during the ongoing pandemic. It also discusses the measures issued by various international organisations and countries for the protection of MSWM employees (MSWEs), identifying gaps, especially for developing countries, where personal protection equipment and clear guidelines to MSWEs may not have been provided, and the general public may not be well informed. In countries with high recycling rates of MSW, the need to protect MSWEs' health has affected the supply stream of the recycling industry. The article concludes with recommendations for the MSW industry operating under public health crisis conditions

Risk assessment procedure for the performance-based design of landfill lining systems and cutoff walls

The performance-based design of pollutant containment systems, such as landfill bottom liners and cutoff walls, requires the impact of pollutant migration on groundwater quality to be assessed. The effectiveness of pollutant containment systems is indeed demonstrated through the verification that the risk for human health and the environment due to the pollutant migration is limited to an acceptable level. This risk is quantified through the calculation of the pollutant concentration in the groundwater, which is expected to remain less than some prescribed level at a compliance point. The paper describes analytical and numerical solutions to pollutant transport, which allow the pollutant concentration in the groundwater to be calculated under different boundary conditions. Based on the results obtained from these solutions, the role played, not only by the hydraulic and diffusive properties of the containment barriers, but also by the hydrogeological features of the site (e.g. the groundwater velocity and the mechanical dispersion within the aquifer) is pointed out

Assessing the leakage rate through semipermeable bentonite-based barriers permeated with multi-electrolyte solutions

Most of the experimental research conducted to date on the semipermeable membrane behaviour of bentonites, which are used as part of pollutant containment systems, has involved permeant (electrolyte) solutions containing a single salt, thus not accounting for the simultaneous presence of two or more electrolytes. In view of the significance of the latter issue when bentonites are permeated with real contaminated liquids, the results of a multi-stage membrane test carried out on a powdered sodium bentonite in equilibrium with aqueous mixtures of sodium chloride (NaCl) and potassium chloride (KCl) are interpreted via a theoretical model, whereby the overall osmotic counter-flow of water in multi-electrolyte systems is decomposed into a chemico-osmotic effect, which arises from the partial exclusion of anions from the bentonite pores and also occurs in single-electrolyte systems, and an electro-osmotic effect, which stems from the condition of null electric current density in the presence of cationic species with different diffusivities and causes the measured reflection coefficient, w, to assume values both larger than unity (w = 1.064) and lower than zero (w = - 1.168). While positive anomalous osmosis (w > 1) correlates with an enhanced osmotic counter-advection of contaminants and, hence, with an improved containment performance of bentonite-based barriers, the occurrence of negative anomalous osmosis (w < 0) causes such barriers to behave worse than non-semipermeable clay liners

Use of geosynthetic clay liners for pollutant control

The osmotic, hydraulic and self-healing efficiency of bentonite based barriers (e.g. geosynthetic clay liners) for containment of polluting solutes are governed both by the chemico-physical intrinsic parameters of the bentonite, i.e. the solid density, the total specific surface,, and the total fixed negative electric surface charge, and by the chemico-mechanical state parameters able to quantify the solid skeleton density and fabric, i.e. the total and nano void ratio, the average number of platelets per tactoid, the effective electric fixed-charge concentration, and the Stern fraction (fStern). In turn, looking at saturated active clays only, the state parameters seem to be controlled by the effective stress history (SH), ionic valence (νi) and related exposure sequence of salt concentrations in the pore solution (cs). A theoretical framework, able to describe chemical, hydraulic and mechanical behaviours of bentonites in the case of one-dimensional strain and flow fields, has been set up. In particular, the relationships, linking the aforementioned state and intrinsic parameters of a given bentonite with its hydraulic conductivity, effective diffusion coefficient, osmotic coefficient and swelling pressure under different stress-histories and solute concentration sequences, are presented. The proposed theoretical hydro-chemico-mechanical framework has been validated by comparison of its predictions with some of the available experimental results on bentonites (i.e. hydraulic conductivity tests, swelling pressure tests and osmotic efficiency tests)

STABILITÀ DI UN DEPOSITO DI MATERIALE GRANULARE IN CONDIZIONI NON SATURE: CARATTERIZZAZIONE IDRAULICA DI LABORATORIO E MODELLAZIONE NUMERICA

A motivo di specifiche condizioni meteorologiche, approfondite analisi di stabilità delle opere in terra possono necessitare di una caratterizzazione sperimentale dei materiali in condizioni non sature allo scopo di riprodurne più fedelmente il comportamento idro-meccanico. A tal fine, il presente articolo riporta la caratterizzazione idraulica della frazione sabbioso-limosa di un campione prelevato da un cumulo di stoccaggio temporaneo di materiale granulare, proveniente dalla frantumazione di rocce quarzitiche. Il comportamento idraulico in condizioni non sature è stato studiato mediante prove in edometro a controllo di suzione, che prevedono la misura del contenuto d’acqua del provino a diversi valori imposti di suzione, ottenendo dunque la risposta in termini di ritenzione idrica. Previa valutazione del contributo delle condizioni di non saturazione alla resistenza al taglio, sono state messe a punto analisi numeriche di stabilità agli elementi finiti, finalizzate a simulare le condizioni di innesco di un movimento gravitativo che si è manifestato nello stesso deposito

Modelling Hydro-Chemo-Mechanical Behaviour of Active Clays Through the Fabric Boundary Surface

The osmotic, hydraulic and self-healing efficiency of bentonite based barriers (e.g. geosynthetic clay liners) for containment of polluting solutes are governed both by the physico-chemical intrinsic parameters of the bentonite, i.e. the solid density, the total specific surface, and the total fixed negative electric surface charge, and by the state and fabric parameters able to quantify the soil density and microstructure, i.e. the total and nano void ratio, the average number of platelets per tactoid, the effective electric fixed-charge concentration, and the Stern fraction. In turn, the fabric parameters seem to be controlled by the effective stress history, ionic valence and related exposure sequence of salt concentrations in the pore solution. A theoretical framework able to describe chemical, hydraulic and mechanical behaviours of bentonites has been set up. In particular, the relationships, linking the aforementioned intrinsic, state and fabric parameters of a given bentonite with its hydraulic conductivity, effective diffusion coefficient, osmotic coefficient and swelling pressure under different stress histories and solute concentration sequences, are presented. The proposed theoretical framework has been validated by comparison of its predictions with some of the available experimental results on bentonites