Critical role of surface roughness on colloid retention and release in porous media

• Published in: Water research (Oxford) Vol. 88; pp. 274 - 284
• Main Authors: Torkzaban, Saeed, Bradford, Scott A.
• Format: Journal Article
• Language: English
• Published: England Elsevier Ltd 01-01-2016
• Subjects: Colloid; Colloids; Colloids - chemistry; DLVO theory; Fluid dynamics; Fluid flow; Hydrodynamics; Hydrogen-Ion Concentration; Hydrology - methods; Nanostructure; Osmolar Concentration; Porosity; Porous media; Release; Retention; Sand; Surface Properties; Surface roughness; Porous media; Retention; DLVO theory; Colloid; Release
• ISSN: 0043-1354; 1879-2448
• DOI: 10.1016/j.watres.2015.10.022

This paper examines the critical role of surface roughness (both nano- and micro-scale) on the processes of colloid retention and release in porous media under steady-state and transient chemical conditions. Nanoscale surface roughness (NSR) in the order of a few nanometers, which is common on natural solid surfaces, was incorporated into extended-DLVO calculations to quantify the magnitudes of interaction energy parameters (e.g. the energy barrier to attachment, ΔΦa , and detachment, ΔΦd , from a primary minimum). This information was subsequently used to explain the behavior of colloid retention and release in column and batch experiments under different ionic strength (IS) and pH conditions. Results demonstrated that the density and height of NSR significantly influenced the interaction energy parameters and consequently the extent and kinetics of colloid retention and release. In particular, values of ΔΦa and ΔΦd significantly decreased in the presence of NSR. Therefore, consistent with findings of column experiments, colloid retention in the primary minimum was predicted to occur at some specific locations on the sand surface, even at low IS conditions. However, NSR yielded a much weaker primary minimum interaction compared with that of smooth surfaces. Colloid release from primary minima upon decreasing IS and increasing pH was attributed to the impact of NSR on the values of ΔΦd . Pronounced differences in the amount of colloid retention in batch and column experiments indicated that primary minimum interactions were weak even at high IS conditions. Negligible colloid retention in batch experiments was attributed to hydrodynamic torques overcoming adhesive torques, whereas significant colloid retention in column experiments was attributed to nano- and micro-scale roughness which would dramatically alter the lever arms associated with hydrodynamic and adhesive torques.
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•Density and height of nanoscale roughness significantly influenced colloid retention and release processes.•Colloid retention in the primary energy minimum occurred at low solution ionic strength.•Fractional colloid release from primary minima upon decreasing ionic strength and increasing pH was observed.•Microscopic roughness caused significant colloid retention in column experiments.