New insight into hydrodynamic and cake erosion mechanism during rotating-disk dynamic microfiltration of concentrated bentonite suspensions at different salinity conditions

Effects of ionic strength and bentonite ratio on the migration of Cr(VI) in clayey soil-bentonite engineered barrier

Soil-bentonite (S-B) barriers have been widely used for heavy metal pollution containment. This study conducted batch adsorption tests and diffusion-through tests to evaluate how ionic strength and bentonite ratio influence the migration of Cr(VI) in natural clay-bentonite mixtures. The test results indicated that the adsorption of Cr(VI) exhibited an obvious anion adsorption effect, the pH of the soil mixture increased with the addition of bentonite, resulting in a decrease in the positive surface charge. This change led to a decrease in Cr(VI) adsorption capacity, from 775.19 mg/kg for pure clay to 378 mg/kg for mixture samples with excessive bentonite. Furthermore, as the ionic strength increases from 0 to 0.1 M, the Cr(VI) adsorption capacity increases slightly due to the weakening of electrostatic repulsion on the clay particle surface, but the effective diffusion coefficient (De) increases by 21.97%. The compression of the diffusion double layer (DDL) under high ionic strength conditions enlarges the diffusion path and enhances the migration of Cr(VI) through the pore flow paths. Moreover, hydrated bentonite effectively fills the interaggregate pores of natural clay, thus creating narrower and more tortuous flow paths. However, excessive bentonite increases the pH value and pore volume, resulting in changes to the soil microstructure and disrupting the continuous skeleton of natural clay, which is unfavorable for Cr(VI) containment. Based on the study of the Cr(VI) contaminated site, a bentonite ratio of 2:10 is recommended for optimal natural performance of the natural clay-bentonite barrier.

Fate and cotransport of Pb(II) and Cd(II) heavy ions with bentonite colloidal flow in saturated porous media: The role of filter cake, counter ions, colloid concentration, and fluid velocity

This study examines the transport and retention of colloidal particles and heavy ions in porous sand, focusing on the environmental risks associated with waste from oil and gas drilling. Experimental and numerical models assess the influence of flow rate, external filter cake layer, and ionic strength on bentonite clay particles and heavy ions, such as cadmium (Cd) and lead (Pb), in near-wellbore (high-flux) and far-field (low-flux) scenarios. Colloidal filtration theory and the one-dimensional convection-dispersion equation with two-site kinetic model for attachment and detachment were utilized to calibrate and predict the transport of colloidal suspension in porous media. The research investigates the role of internal and external filter cakes on sand column pressure distribution and heavy ion absorption. Results indicate that the mobility of colloids and heavy ions is influenced by the ionic strength and pH of the carrying fluid. Colloidal clay suspensions show a higher affinity for Pb (II) absorption, while Cd (II) exhibits increased mobility in both clean sand and colloidal environments. Notably, the formation of an external filter cake significantly delays the breakthrough of heavy ions, up to four times longer than in clean sand, and reduces Cd (II) and Pb (II) outlet concentrations by 86% and 93%, respectively. This cake also limits clay concentration and particle size passage. High clay concentrations or injections under high ionic conditions induce clay bridging in pore throats, enhancing internal filtration and heavy ion retention. Conversely, low clay fluxes allow freer particle passage, increasing heavy ion loads and outlet concentrations.

Hydrodynamic Analysis and Cake Erosion Properties of a Modified Water-Based Drilling Fluid by a Polyacrylamide/Silica Nanocomposite during Rotating-Disk Dynamic Filtration

In this study, the potential of using a polyacrylamide-silica nanocomposite (PAM-S) to control the filtration properties of bentonite water-based drilling muds under different salinity conditions was evaluated. Static filtration tests under low-pressure/low-temperature (LPLT) conditions accompanied by rheological measurements have been carried out to analyze the role of silica nanoparticles (NPs) and nanocomposites (NCs) in the base fluid properties. Moreover, high-pressure/high-temperature (HPHT) static filtration was also investigated to evaluate the thermal stability of PAM-S. Afterward, dynamic filtration has been conducted in a filtration cell equipped with an agitating system with a disk-type impeller to investigate the hydrodynamic and formation of a filter cake under shear flow conditions. Fluid flow velocity and wall shear stress (WSS) distribution over the filter cake were analyzed using an exact 3D computational fluid dynamic (CFD) simulation. A transparent filtration cell with a camera was used to accurately record the fluid flow field inside the filter press and validate the CFD results. The obtained results indicated that adding silica NPs at a concentration of less than 2 wt % increases the fluid loss due to reducing rheological properties such as yield point. While silica NPs could not significantly change the mud properties, the experimental results showed that, under both LPLT and HPHT conditions, the PAM-S NC could reduce the total filtration loss by 70% at a low concentration of 0.75 wt %. Moreover, during dynamic filtration, the results indicated that there is a linear relationship between the cake thickness and the inverse of WSS at different operating pressures. However, no correlation could be found between predeposited mud cake erosion and WSS. At a rotating disk speed of 1000 rpm, more than 60% of the predeposited mud cake was eroded after 30 min for a saline mud sample while for the NC-treated mud sample cake erosion is considerably reduced and reaches up to 20% at 1.5 wt % PAM-S.