www.VadoseZoneJournal.org Role of Surface Roughness in Chemical Detachment of Colloids Deposited at Primary Energy Minima This study theoreƟcally and experimentally examined effects of surface roughness on detachment of colloids deposited under favorable chemical condiƟons on reducƟon of soluƟon ionic strength. A superposiƟon approach based on elemental geometric models was developed to esƟmate variaƟon of Derjaguin–Landau–Verwey–Overbeek (DLVO) inter- acƟon energies between a colloid and a rough surface under different soluƟon chemistries. TheoreƟcal analysis showed that most colloids aƩached at rough surfaces via primary- minimum associaƟon are irreversible on reducƟon of soluƟon ionic strength because primary minima are deeper and the detachment energy barriers are greater at lower ionic strength. A fracƟon of colloids iniƟally aƩached at Ɵps of nanoscale protruding asperiƟes, however, will detach from a rough surface at low ionic strength because the net force act- ing on the colloids can become repulsive (i.e., calculated DLVO interacƟon energy curves show monotonic decreases of interacƟon energies with separaƟon distance at low ionic strength). Column experiments were conducted with 1156-nm polystyrene latex par Ɵcles and rough sand (300–355- μ m diameter) to examine the detachment of colloids iniƟally deposited at primary minima. Experimental results confirmed that a fracƟon of colloids are released at low ionic strengths. Our theoreƟcal and experimental results are consistent with literature observaƟons, adding convincing evidence to challenge the usual belief that colloids aƩached at primary minima are irreversible on reducƟon of soluƟon ionic strength. Although the importance of surface heterogeneity on colloid deposiƟon has been widely recognized, our study implies that surface heterogeneity also plays a cri Ɵcal role in colloid detachment under both favorable and unfavorable condiƟons. AbbreviaƟons: AFM, atomic force microscopy; DLVO, Derjaguin–Landau–Verwey–Overbeek; SEI, surface element integraƟon. Knowledge of colloid transport behavior in porous media is of impor- tance in a variety of applications such as remediation of contaminated soil and groundwater, granular iltration in water and wastewater treatment, and natural iltration of pathogenic microorganisms (Ryan and Elimelech, 1996; Tufenkji and Elimelech, 2005; Shang et al., 2010). Deposition and detachment are two primary factors controlling the transport of colloids in porous media. Particle deposition in porous media has received considerable attention in the literature. In particular, a systematic framework, i.e., the colloid iltra- tion theory (CFT) has been developed to predict particle deposition in porous media (Yao et al., 1971; Rajagopalan and Tien, 1976; Tufenkji and Elimelech, 2004a; Ma et al., 2009). he CFT considers that the colloid deposition rate is controlled by three individual mechanisms: Brownian difusion, interception, and sedimentation. In CFT, the colloid deposition rate is characterized by single collector contact eiciency, a parameter that quantiies the frequency of colloid collisions with a collector grain. Despite successful pre- dictions by the CFT for colloid deposition under favorable chemical conditions (i.e., when Derjaguin–Landau–Verwey–Overbeek [DLVO] interaction energy barriers are absent), large discrepancies have been frequently reported between theoretical calculations and experimental observations under unfavorable conditions (Ryan and Elimelech, 1996). he CFT assumes that particle and collector surfaces are perfectly smooth; however, the surfaces of natural colloids and collectors all contain some degree of physical nonunifor- mity at various scales (Suresh and Walz, 1996). herefore, surface roughness has been frequently regarded as one of the primary factors (surface roughness, charge heterogene- ity, and secondary minimum) causing the discrepancies between theoretical predictions and experimental results (Elimelech and O’Melia, 1990a,b; Ryan and Elimelech, 1996; Effects of surface roughness on detachment of colloids deposited at primary minima were examined via experiments and theoreƟcal analysis. We provide theoreƟcal demonstra- Ɵon and experimental evidence that colloids ini Ɵally aƩached at Ɵps of nanoscale protruding asperiƟes will detach from a rough surface at low soluƟon ionic strengths. C. Shen, B. Li, and Y. Huang, Key Lab. of Plant-Soil InteracƟons, Ministry of Environ- ment, Key Lab. of Soil and Water, Ministry of Agriculture, and Dep. of Soil and Water Sciences, China Agricultural Univ., Beijing 100193, China; L.-P. Wang, Dep. of Mechanic Engineering, Univ. of Delaware, Newark, DE 19716; and Y. Jin, Dep. of Plant and Soil Sci- ences, Univ. of Delaware, Newark, DE 19716. *Corresponding authors (yĬuang@china. com; yjin@udel.edu). Supplemental material is available online. Vadose Zone J. doi:10.2136/vzj2011.0057 Received 7 June 2011. Special Section: Soil Architecture and Function Chongyang Shen Lian-Ping Wang Baoguo Li Yuanfang Huang* Yan Jin* © Soil Science Society of America 5585 Guilford Rd., Madison, WI 53711 USA. All rights reserved. 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