Microfluidics isolate the rapid electrostatic regime of Pb(II) adsorption onto carboxylated nanoparticles

Metal adsorption in porous environments occurs under flowing conditions, where transport, diffusion, and reaction act on comparable timescales and thermodynamic equilibrium is rarely achieved. To address this non-equilibrium regime at the pore scale, we use a microfluidic-polarographic approach that provides controlled access to short residence times under laminar flow. This study investigates Pb(II) adsorption onto carboxylated polystyrene latex nanoparticles (PSL-COOH) under dynamic flow conditions. Batch experiments show a biphasic behaviour, with rapid electrostatic adsorption followed by slower site-specific binding. Under laminar flow conditions, Pb(II) adsorption reaches ∼97-98% over a wide range of flow rates, corresponding to residence times between 0.9 and 35 s. Reactive transport modelling shows that, although outer-sphere adsorption is intrinsically fast, Pb(II) adsorption within the microfluidic channel remains diffusion-limited under laminar co-flow. Accounting for downstream droplet formation demonstrates that efficient mixing provides a short additional interaction time sufficient to complete Pb(II) adsorption and explain the nearly flow-independent adsorption observed experimentally. These results show that microfluidic-polarographic systems enable controlled investigation of short residence-time adsorption regimes and provide a framework to characterise adsorption under transport-limited, non-equilibrium flow conditions relevant to porous environmental systems.