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Aggregation and Deposition Kinetics of Carboxymethyl Cellulose-Modified

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Aggregation and Deposition Kinetics of Carboxymethyl Cellulose-Modified water research 46 (2012) 1735e1744 Available online at www.sciencedirect.com journal homepage: www.elsevier.com/locate/watres Aggregation and deposition kinetics of carboxymethyl cellulose-modified zero-valent iron nanoparticles in porous media Trishikhi Raychoudhury a, Nathalie Tufenkji b, Subhasis Ghoshal a,* a Department of Civil Engineering, McGill University, Montreal, Quebec H3A 2K6, Canada b Department of Chemical Engineering, McGill University, Montreal, Quebec H3A 2B2, Canada article info abstract Article history: Transport and deposition of carboxymethyl cellulose (CMC)-modified nanoparticles of Received 6 August 2011 zero-valent iron (NZVI) were investigated in laboratory-scale sand packed columns. Received in revised form Aggregation resulted in a change in the particle size distribution (PSD) with time, and the 18 December 2011 changes in average particle size were determined by nanoparticle tracking analysis (NTA). Accepted 19 December 2011 The change in PSD over time was influenced by the CMC-NZVI concentration in suspen- Available online 30 December 2011 sion. A particleeparticle attachment efficiency was evaluated by fitting an aggregation model with NTA data and subsequently used to predict changes in PSD over time. Changes Keywords: in particle sizes over time led to corresponding changes in single-collector contact effi- Nanoparticle transport ciencies, resulting in altered particle deposition rates over time. A coupled aggregation- Groundwater remediation colloid transport model was used to demonstrate how changes in PSD can reduce the Colloid aggregation transport of CMC-NZVI in column experiments. The effects of particle concentrations in À À Colloid deposition the range of 0.07 g L 1 to 0.725 g L 1 on the transport in porous media were evaluated by Ionic strength comparing the elution profiles of CMC-NZVI from packed sand columns. Changes in PSD Chlorinated solvents over time could reasonably account for a gradual increase in effluent concentration between 1 and 5 pore volumes (PVs). Processes such as detachment of deposited particles also likely contributed to the gradual increase in effluent concentrations. The parti- cleecollector attachment efficiency increased with CMC-NZVI particle concentration due þ to a rise in dissolved Na concentration with increased addition of Na-CMC. This inad- vertent change in ionic strength led to decreased effluent concentrations at higher CMC- NZVI concentrations. ª 2011 Elsevier Ltd. All rights reserved. 1. Introduction eliminate or transform certain pollutants rapidly, its trans- port to target locations and its reactivity may be limited by its An emerging remediation technique for in situ groundwater rapid aggregation (Phenrat et al., 2007). Several studies have remediation of chlorinated solvent and heavy metal- demonstrated improved colloidal stabilization of NZVI by contaminated aquifers is the injection of reactive nano- coating them with polymers or polyelectrolytes (Kanel et al., particles of zero-valent iron (Tratnyek and Johnson, 2006; 2007; Phenrat et al., 2008; Saleh et al., 2007; Schrick et al., Zhang, 2003). Although at appropriate doses, NZVI can 2004). Polyelectrolyte or polymer coatings can inhibit NZVI * Corresponding author. Tel.: þ1 514 398 6867; fax: þ1 514 398 7361. E-mail address: [email protected] (S. Ghoshal). 0043-1354/$ e see front matter ª 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2011.12.045 1736 water research 46 (2012) 1735e1744 À1 Nomenclature kdep particle deposition rate coefficient [T ] À3 kdep,i deposition rate coefficient of the ith sized particles C effluent particle mass concentration [M L ] À1 À3 [T ] C0 influent particle mass concentration [M L ] À1 À3 kdet particle detachment rate coefficient [T ] Ci mass concentration of ith size aggregate [M L ] 2 À1 massi mass of ith particles [M] D hydrodynamic dispersion coefficient [L T ] À n particle number concentration [L 3] Df fractal dimension of aggregates ni particle number concentration of ith aggregates Ni number of single particles forming the aggregates À3 À [L ] S solid phase deposited mass density [M M 1] dep t time [T] T temperature À1 2 À2 v Darcy’s velocity [L T ] Tadhesive adhesive torque [M L T ] À x distance along column length [L] T torque applied [M L2 T 2] applied a particleecollector attachment efficiency Z maximum number of size categories pc a particleeparticle attachment efficiency d number-averaged mean diameter measured by pp Navg b particleeparticle collision frequency function NTA [L] À [L3 T 1] dà number-averaged mean diameter calculated from Navg 3 porosity model [L] h single-collector contact efficiency d mean collector diameter [L] 0 c h single-collector contact efficiency of ith sized d particle diameter of ith aggregates [L] 0,i p(i) aggregate d ¼ diameter of the smallest particle size [L] À p(i 1) r bulk density of sand [M L 3] aggregation by a combination of electrostatic and steric particles. The observed deposition behavior was quantita- repulsion forces when these interactions are large enough to tively explained by particle-size dependent deposition rate overcome inter-particle magnetic and van der Waals attrac- coefficients. Although those studies have demonstrated that tive forces. The colloidal stabilization of NZVI by polymers changes in particle size resulting from aggregation influence has been shown to reduce the deposition of NZVI on granular transport, the role of aggregation kinetics or time-varying media and thus increase its mobility, relative to that of bare PSD on nanoparticle transport behavior has not been NZVI (He et al., 2009; Kanel et al., 2007; Phenrat et al., 2008). examined. Polymeric coatings of NZVI are unlikely to provide The objectives of this study were to assess the rates of complete stabilization because of incomplete or insufficient aggregation of CMC-NZVI particles, as well as the effects of surface coverage (Cirtiu et al., 2011). The extent of colloidal aggregation kinetics on CMC-NZVI transport and deposition in stabilization is also dependent on the polymer characteristics, granular porous media at different nanoparticle concentra- the thickness of the polymer layer on the particle surface, and tions. Changes in the equivalent mean diameter of a CMC- on the solution chemistry (Phenrat et al., 2008; Saleh et al., NZVI suspension due to aggregation were measured over À 2008, 2007). Even at g L 1 doses of polymers and poly- a time period equivalent to the particle residence time during electrolytes there is evidence of incomplete stabilization a transport experiment. An aggregation kinetics model was within minutes (Phenrat et al., 2008, 2007; Raychoudhury fitted to this data to obtain the particleeparticle attachment a et al., 2010; Saleh et al., 2007). Thus, it is important to assess efficiency ( pp) and calculations were performed to determine the changes in PSD of polymer-stabilized NZVI due to the change in the PSD with time during the transport experi- aggregation. ments. In an effort to better understand the role of nano- The influence of aggregation of polyelectrolyte-modified particle aggregation kinetics on nanoparticle transport, NZVI on the deposition and transport of these particles in a modified colloid transport model which couples the gov- granular media has been discussed in recent studies (Petosa erning transport equation to the aggregation kinetics equation et al., 2010; Phenrat et al., 2010, 2009; Raychoudhury et al., was developed. A schematic of the multiple concurrent 2010). Those studies suggested that aggregation of the nanoparticle aggregation, deposition and transport processes nanoparticles will influence their deposition in granular occurring in granular porous media is shown in Fig. 1. Column media and that the particle size determines the magnitude of transport experiments were performed for different nano- h the single-collector contact efficiency ( 0), which is defined particle concentrations, and the effluent CMC-NZVI concen- as the ratio of the number of collisions between the particle trations over time were fitted to the modified colloid transport and the collector and the total number of particles model to assess whether particle-size dependent deposition approaching the collector. The effect of colloid aggregate size rates could account for the observed shape of the experi- on colloid transport in porous media has been examined by mental breakthrough curves. The potential role of other Chatterjee and Gupta (2009). In their study, injection of nanoparticle deposition mechanisms such as blocking of a polydisperse suspension of inert micron-sized particles deposition sites, detachment of nanoparticles and the composed of silica and metal oxides in a column packed with changes in electrolyte composition with CMC-NZVI concen- borosilicate glass beads resulted in larger particles being tration, were also considered for explaining the observed deposited at shallower bed depths compared to the smaller breakthrough curves. water research 46 (2012) 1735e1744 1737 The particle sizes of CMC-NZVI over time in aqueous 2. Materials and methods suspensions were determined by nanoparticle tracking anal- ysis (NanoSight LM10). Prior to CMC-NZVI size measurement, 2.1. Synthesis of CMC-NZVI the NTA accuracy was verified by checking the size of stan- dard NanoSight polystyrene latex particles of 100 nm and CMC-NZVI was prepared according to the methods described 200 nm diameter. For CMC-NZVI size measurement, suspen- À by He and Zhao (2007). Briefly, an aqueous solution of 0.065 M sions of 0.07, 0.2 and 0.725 g L 1 CMC-NZVI (as total Fe) were $ FeSO4 7H2O (AlfaAesar, purity greater than 99%) was added to m À prepared in N2 atmosphere with filtered (0.22 m) degassed a5gL 1 of Na-CMC (90 K, Aldrich) solution and mixed thor- 0.1 mM NaHCO3 and sonicated by a 40 kHz ultrasonic cleaner oughly for 30 min. The solution was then reduced by the drop- (Cole-Palmer 8891) for 10 min to ensure homogeneity of the e wise addition of a solution of NaBH4 (Sigma Aldrich) at a rate suspensions, and the suspension was filled in a vial to zero À1 À of 55.5 mg min under an N2 atmosphere.
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