Visualisation of Multi-Scale Desorption Dynamics in Clay-Coated Microfluidic Channels: Optimising Recovery Strategies for Valuable Contaminants

Recovering valuable water contaminants is a cornerstone of sustainable water management, addressing environmental challenges, and resource scarcity, and promoting sustainable resource management and circular economy principles. Among various techniques for contaminant sequestration and recovery, sorption-desorption methods stand out for their operational simplicity, cost-effectiveness, and high efficiency, while minimising harmful by-products during both removal and recovery processes. While sorption processes have been extensively studied, desorption dynamics remain underexplored despite their importance in recovering and recycling commercially valuable substances. Traditionally, dynamic sorption-desorption processes are studied using column experiments with effluent and solid surface analysis, nevertheless, these methods fail to spot pore-scale solid-fluid interactions. Moreover, studying pore-scale interfacial processes in soil is challenging due to the opacity and heterogeneity of soil environments. Overcoming these challenges demands innovative, multidisciplinary approaches to visualize and analyse these processes.

To advance the understanding of pore-scale desorption dynamics, this study introduces an innovative microfluidic approach for investigating contaminant desorption in clay-rich porous media. Polydimethylsiloxane (PDMS) microfluidic channels were functionalised with transparent clay coatings to replicate the physicochemical properties of natural clay-rich soil environments. These clay-coated channels represent the complex, multi-scale, tortuous pore networks characteristic of heterogeneous clay-rich systems. However, creating stable coatings that endure flow conditions, replicate geomaterial properties, and enable pore-scale visualisation remains challenging. To address this, we proposed a solvent-free powder coating method combined with plasma and heat treatments, followed by the injection of a water-based solution to form a porous network of clay aggregates. This coating strategy supports the direct visualisation of fluid-solid interactions at pore scale under varying flow conditions, providing unique insights into contaminant recovery dynamics.

The proposed coating protocol effectively creates stable clay coatings on PDMS substrates under various flow conditions, ensuring reliable and reproducible observations for dynamic flow experiments. Flow and tracer experiments were conducted across a range of pore geometries and flow rates, which reveal the influence of the microscale flow attributes on desorption processes across various flow dynamics and porous geometries. The results demonstrate that desorption behaviour is intricately influenced by the interplay of flow dynamics and pore geometry. Higher flow rates were found to accelerate contaminant desorption, significantly reducing the time required for recovery, but often leaving higher residual contaminant concentrations. Therefore, increasing the flow rates does not always enhance recovery efficiency, as residual contaminant concentrations often remain higher under high flow rate conditions. Conversely, lower flow rates, though slower in achieving complete desorption, were found to result in a lower residual contaminant mass. These findings highlight a critical trade-off between recovery speed and total contaminant removal, thus indicating the importance of optimising flow conditions to balance recovery process efficiency and environmental footprint.

The insights gained hold significant potential for designing reactive porous filters with precise flow control, enabling more effective and sustainable remediation strategies, particularly for emerging contaminants like pharmaceuticals, heavy metals, and persistent pollutants. By optimising flow conditions and understanding the role of porous media characteristics, this research advances efficient contaminant recovery systems aligned with sustainable management and circular economy goals, promoting resource recovery and reuse from contaminated water.