[1] Straining may influence the mobility of colloid-sized particles within groundwater aquifers as well as within granular filters that are used in wastewater treatment. We conducted column transport experiments using latex microspheres as the colloids and quartz sand as the porous medium to investigate the response of colloid straining to changes in colloid diameter (d p ) and sand grain diameter (d g ). For these experiments the negatively charged microspheres were suspended in deionized water, and the quartz sand was thoroughly cleaned to minimize physicochemical deposition (attachment), which permitted the determination of straining in an unambiguous way. The measurements of strained (immobile phase) and effluent (aqueous phase) colloid concentrations could be described with a transport model that accounted for an exponential decline in straining rates with increasing concentrations of strained colloids. Best fit values of the model coefficient that quantified clean bed straining rates (k o ) were negligibly small for d p /d g < 0.008 and, above this threshold, varied linearly with d p /d g . Our findings suggest that accurate inferences on the mobility of colloid-sized particles will require consideration of the effects of straining when d p /d g exceeds 0.008.

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We explore the effects of colloid shape on straining kinetics by measuring the filtration of spherical and nonspherical colloids within saturated columns packed with quartz sand. Our observations demonstrate that the transport of peanut-shaped colloids matches the transport of spherical colloids with diameters equal to the minor-axis length of the peanut-shaped colloids. The straining rates of the spherical colloids vary linearly with the ratio of colloid diameter (d(p)) to sand-grain diameter (d(g)) for 0.0083 < d(p)/d(g) < 0.06. This linear relationship also quantifies the straining rates of the peanut-shaped particles provided that the particle's minor axis length is used for d(p). Results of pore-scale simulations reveal that a peanut-shaped particle adopts a preferred orientation as it approaches a pore-space constriction such that its major axis tends to align with the local flow direction. The extent of this reorientation increases with the particle's aspect ratio. Findings from this research suggest that straining is sensitive to changes in colloid shape and thatthe kinetics of this process can be approximated on the basis of measurable properties of the nonspherical colloids and porous media.

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