Modeling the transport of sodium dodecyl benzene sulfonate in riverine sediment in the presence of multi-walled carbon nanotubes

Highlights

• Effects of MWCNTs on the transport of SDBS in riverine sediment were explored.

• Adsorption capacity of the sediment for SDBS increased with the content of MWCNTs.

• Retardation factor increased with incorporating MWCNTs in water or sediment.

• MWCNTs increased the accumulation of SDBS in the top sediment layer.

Abstract

The environmental risks of carbon nanotubes have received considerable attention. In this work, the effects of multi-walled carbon nanotubes (MWCNTs) on the adsorption of sodium dodecyl benzene sulfonate (SDBS) by riverine sediment and the transport of SDBS in sediment were studied. MWCNTs could significantly increase the adsorption capacity of the sediment for SDBS, thus affecting the transport of SDBS in sediment. Maximum adsorption capacity of the sediment for SDBS increases from 2.29 to 2.99 mg/g with the increasing content of MWCNTs from 0% to 1.5%. Breakthrough curves (BTCs) of SDBS obtained from the column experiments were described by the convection-dispersion equation, Thomas model, and Yan model. The estimated retardation factor R increases with the incorporation of MWCNTs, either in water or sediment. Additionally, the value of R is well correlated to the content of MWCNTs in sediment. Compared with Thomas model, Yan model is more suitable for fitting the BTCs with all the values of R² ≥ 0.951, but it tends to overestimate the maximum adsorption capacity when the content of MWCNTs in sediment is relatively higher. The results of SDBS retention in sediment indicate that MWCNTs can increase the accumulation of SDBS in the top sediment layer, while they can impede the transport of SDBS into deeper sediment layer when incorporated into the sediment. These effects should be taken into consideration for risk assessment of CNTs in the aquatic environment.

Introduction

Carbon nanotubes (CNTs), composed of carbon atoms in a periodic hexagonal arrangement, are hollow cylinders with a diameter in the nanometer range. Single-walled nanotubes (SWCNTs) and multi-walled nanotubes (MWCNTs) are two main types of CNTs. Since their observation was first reported by Iijima in 1991 (Iijima, 1991), CNTs have been attracting much attention of researchers because of their unique mechanical, thermal, optical, and electronic properties, as well as many potential applications (Popov, 2004, Zhang et al., 2007, Huang et al., 2008, Tang et al., 2008, De Volder et al., 2013). Current production capacity of CNTs worldwide has exceeded 5000 tonnes per year, and is increasing at an annual growth rate of 32.5% (Patel, 2011, De Volder et al., 2013). Increasing production and application of CNTs will inevitably result in the release of these nanomaterials into the environment. In a multimedia environment (atmosphere, soil, water, and sediment), mass accumulation of CNTs was mostly in soil and sediment (Yang et al., 2010, Liu and Cohen, 2014). Based on the research of Koelmans et al. (2009), the estimated concentrations of manufactured carbon-based nanoparticles in aquatic sediment are ranging from 1.2 to 2000 μg per kilogram of the dry sediment. And it is likely that the concentrations of CNTs in sediment will increase in the future.

CNTs have strong adsorption affinity for various organic and inorganic contaminants (Gong et al., 2009, Song et al., 2017a, Song et al., 2017b). As sediment is also the ultimate reservoir of various contaminants in aquatic ecosystem, the interaction between CNTs and contaminants may alter the fate and transport of these contaminants, significantly influencing their mobility, toxicity, and bioavailability (Xu et al., 2012a, Zeng et al., 2013a, Zeng et al., 2013b, Cheng et al., 2016). For example, Sun et al. (2015) found that CNTs released into sediment would increase the adsorption capacity of Cd(II) by sediment. Fang et al. (2013) demonstrated that TX100 suspended MWCNTs could facilitate the transport of phenanthrene in soil columns, while Li et al. (2013) reported that 5 mg/g CNTs could significantly retain polycyclic aromatic hydrocarbons in soil. Recent research by Liang et al. (2016) showed that CNTs could enhance the mobility of tetrabromobisphenol A in saturated porous media. Zhang et al. (2017) also observed facilitated transport of chlordecone and sulfadiazine in the presence of CNTs in soil. However, studies investigating the effect of CNTs on the transport of contaminants in real riverine sediment were insufficient.

Since sodium dodecyl benzene sulfonate (SDBS) is commonly used to increase the dispersity and stability of CNTs in aqueous solutions, most of the current studies focused on the effect of SDBS on the properties, transport, and fate of CNTs (Tian et al., 2011, Ju et al., 2012, Wusiman et al., 2013). However, few studies investigated the effect of CNTs on the transport and fate of SDBS. As an anionic surfactant, SDBS is usually present in detergent, soap, as well as cosmetic, and widely used as emulsifier, dispersant, lubricant, and preservative in industrial processes (Myers, 2005, Taffarel and Rubio, 2010). Because of its extensive applications, a large amount of SDBS is released into the aquatic environment, causing serious environmental problems. The adverse effects of the surfactant on the aquatic environment and human health have been studied and reported elsewhere. According to the available literature, SDBS exhibits toxic effects towards algae, benthic invertebrates, fishes, and human cells (Qv and Jiang, 2013, Mu et al., 2014, Zhang et al., 2015, Zhang et al., 2016). Considering the ecological and human health risks of SDBS, the environmental behavior of SDBS in the presence of CNTs in the aquatic environment should be studied.

In this study, research on the transport of SDBS in riverine sediment in the presence of MWCNTs was conducted. The objectives of the present study were (1) to investigate the effect of MWCNTs on the adsorption of SDBS by sediment, and (2) to study the transport of SDBS in the presence and absence of MWCNTs in riverine sediment by column experiments and numerical modeling.