Co-transport of multi-walled carbon nanotubes and sodium dodecylbenzenesulfonate in chemically heterogeneous porous media☆
Graphical abstract
Introduction
Carbon nanotubes (CNTs) consist of rolled-up graphene sheets (Iijima, 1991) that have been used in many commercial applications such as electrical cables and wires (Janas et al., 2014), hydrogen storage (Dillon et al., 1997), solar cells (Guldi et al., 2005), radar absorption (Lin et al., 2008), and may potentially be employed in environmental remediation (Mauter and Elimelech, 2008; Pan and Xing, 2012) and water treatment (Camilli et al., 2014; Li et al., 2012; Zhang et al., 2010). The widespread use of CNTs will undoubtedly result in their release into the environment (Gottschalk et al., 2009). Published studies have investigated the transport behavior of CNTs in porous media under various physicochemical conditions, such as solution ionic strength (IS), water content, grain size, input concentration, dissolved organic matter, and surfactants (Jaisi and Elimelech, 2009; Kasel et al., 2013a; Kasel et al., 2013b; Liu et al., 2009; Lu et al., 2013; Lu et al., 2014; Yang et al., 2013; Yuan et al., 2012).
Surfactants are often used to stabilize CNT suspensions (Lu et al., 2013; Lu et al., 2014; Yu et al., 2007), and the mobility of CNTs in porous media was strongly influenced by the presence of various stabilizing agents such as sodium dodecylbenzenesulfonate (SDBS), octyl-phenolethoxylate, and cetylpyridinium chloride. However, these studies were conducted over a limited ranged in CNTs (high) and surfactant (low) input concentrations. The total concentration of surfactants that discharges in municipal sewer is about 20–70 mg L−1 (Matthijs et al., 1999). These surfactant concentrations are much greater than the predicted discharge of CNTs (Gottschalk et al., 2009), and may be high enough to stabilize CNT suspensions. However, little research has investigated the influence of surfactants on CNT transport when the surfactant concentration was greater than the CNT concentration. Furthermore, previous literature considered the simultaneous transport of a mixture of surfactants and CNTs, whereas the more environmentally relevant scenario of sequential release of surfactants and CNTs due to waste discharge has not yet been investigated.
Previous research with CNTs and surfactants focused on determination of breakthrough curves (BTCs), but did not measure the influence of surfactants on CNT retention profiles (RPs) (Lu et al., 2013; Lu et al., 2014). Liang et al. (2013) demonstrated that the presence of surfactants had a large influence on the shape of the RPs for silver nanoparticles in quartz sand, but little influence on their BTCs. This was attributed to competitive blocking (e.g., filling) of silver nanoparticle retention sites by surfactant. Similarly, Becker et al. (2015) showed that the stabilizing agent can compete for the same retention sites as quantum dot nanocrystals. Natural porous media often exhibit surface charge heterogeneity due to Fe and Al oxyhydroxides with a net positive surface charge (Parks, 1965) and common silica minerals with a net negative surface charge (Alvarezsilva et al., 2010) at ambient pH. Nanoparticle attachment onto positively charged sites can be inhibited by surfactant sorption which can neutralize or reverse the surface charge (Lin et al., 2012; Wang et al., 2012c). The influence of competitive blocking on CNT BTCs and RPs is therefore expected to be a function of the chemical heterogeneity of the porous medium. In the absence of surfactant, Zhang et al. (2016b) demonstrated that increasing the goethite-coated fraction of quartz sand increased the retention of CNTs due to the combined influence of surface roughness and positively charged sites. To the best of our knowledge, no research studies have examined the influence of surface roughness and controlled soil chemical heterogeneities on the transport and fate of functionalized CNTs in the presence of surfactants. Additional research is needed to assess the potentially significant influence of surfactants on competitive blocking and CNT RPs, especially in porous media with chemical heterogeneity and roughness.
The objective of this study is to better understand and quantify the role of anionic surfactant SDBS concentrations (10–50 mg L−1) on the transport, retention, and remobilization behavior of functionalized multi-walled carbon nanotubes (MWCNTs, 5 mg L−1) in chemically heterogeneous porous media. The sorption affinity for SDBS to quartz sand (QS), goethite-coated quartz sand (GQS), and MWCNTs was determined in batch experiments. Column experiments were employed to determine BTCs for both MWCNT and SDBS, and RPs for MWCNTs in chemically heterogeneous mixtures of QS and GQS. Kinetic retention, release, and competitive blocking parameters for MWCNTs and SDBS were determined by inverse optimization of the collected column data. Furthermore, this research sheds novel insight on the roles of competitive blocking, chemical heterogeneity and nanoscale roughness, and injection sequence on MWCNT retention and release, and the develop of MWCNT retention profiles. This knowledge can be useful for environmental applications and risk management of MWCNTs in the presence of surfactant and various amounts of soil chemical heterogeneity.
Section snippets
Porous media and MWCNTs
Quartz sand (QS, 240 μm), which was used in experiments (Quarzwerke GmbH, 50226 Frechen, Germany) was purified following a protocol in the literature. The preparation of goethite-coated quartz sand (GQS) has been described in a former study (Zhang et al., 2016b). The chemically heterogeneous porous medium was prepared by combining various amounts of QS with a known mass fraction of GQS (λ, the mass ratio of GQS in the mixed porous medium; i.e., λ = 0 and 1 for a porous medium with only QS and
Adsorption of SDBS on QS, GQS and MWCNTs
The functionalization of MWCNTs produced an increased amount of groups containing oxygen (e.g. carboxylic groups) in comparison to the pristine MWCNTs (Kasel et al., 2013a; Wei et al., 2007), resulting in its negative zeta potential in 1 mM KCl solution (Fig. S1). Zeta potentials of MWCNTs, QS, and goethite decreased with increasing SDBS concentration, suggesting that the increased electrostatic repulsion from adsorbed SDBS can enhance the stability of functionalized MWCNTs. The pH of the
Conclusions
The presence of the anionic surfactant SDBS in sewer or wastewater treatment effluent will greatly influence the co-transport of MWCNTs in chemically heterogeneous sand. Modeling results indicated that competitive blocking only played a secondary role in enhancing the transport of MWCNTs by SDBS. Rather, SDBS adsorption onto the surfaces of quartz and goethite minerals, and especially MWCNTs decreased their zeta potentials and/or even reversed the charge of positively charged minerals. This
Acknowledgements
Funding was provided by the National Key R&D Program of China (Project No. SQ2018YFD080023), the 111 Project (Project No. B18060), the National Natural Science Foundation of China (Project No. 41701547), and China Postdoctoral Science Foundation (Project No. 2017M612806 and 2018T110909). The authors would like to acknowledge Stephan Köppchen for bromide measurement. Thanks to Herbert Philipp and Claudia Walraf for their technical assistance.
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