Transport, co-transport, and retention of functionalized multi-walled carbon nanotubes in porous media

The information on transport and retention behavior of functionalized multi-walled carbon nanotubes (MWCNTs) in porous media is essential for environmental protection and remediation due to the wide applications of MWCNTs and lack of disposal regulations. The aim of this study is therefore to investigate: i) the attachment, transport, retention and remobilization of 14C-labeled functionalized MWCNTs in different porous media (quartz sand (QS), goethite-coated quartz sand (GQS), and soil), ii) co-transport of pollutants (chlordecone (CLD) and sulfadiazine (SDZ)) by MWCNTs, and iii) the role of surfactant on MWCNTs transport as a modeled soil remediation process, based on column and batch experiments at the environmentally relevant concentrations under various physiochemical conditions. The breakthrough curves (BTCs) and retention profiles (RPs) were determined and simulated based on advective- dispersive equation by using different numerical models that considered both time- and depth- dependent blocking functions. The effect of goethite coating on MWCNTs transport was conducted in mixtures of negatively charged QS and positively charged GQS. The linear equilibrium sorption model provided a good description of batch results, and the distribution coefficients (KD) drastically increased with the GQS fraction that was electrostatically favorable for retention. Similarly, retention of MWCNTs increased with the GQS fraction in packed column experiments. However, calculated values of KD on GQS were around two orders of magnitude smaller in batch than packed column experiments due to differences in lever arms associated with hydrodynamic and adhesive torques at microscopic roughness locations. Furthermore, the fraction of the chemically heterogeneous sand surface area that was favorable for retention was much smaller than the GQS fraction, presumably because nanoscale roughness produced shallow interactions that were susceptible to removal. These observations indicated that only a minor fraction of the GQS was favorable for MWCNT retention. These same observations held for several different sand sizes. In the saturated soil column experiments, BTCs for MWCNTs exhibited greater amounts of retardation and retention with increasing solution ionic strength (IS) or in the presence of Ca2+ in comparison to K+, and RPs for MWCNTs were hyper-exponential in shape. Fitted values of the retention rate coefficient and the maximum retained concentration of MWCNTs were higher with increasing IS and in the presence of Ca2+ in comparison to K+. Significant amounts of MWCNTs and soil colloids release was observed with a reduction of IS due to expansion of the electrical double layer, especially following cation exchange (when K+ displaced Ca2+) that reduced the zeta potential of MWCNTs and the soil. Analysis of MWCNTs concentrations in different soil size fractions revealed that >23% of the retained MWCNTs mass was associated with water-dispersible colloids (WDCs), even though this fraction was only a minor portion of the total soil mass (around 2.4%). More MWCNTs were retained on the WDC fraction in the presence of Ca2+ than K+. These findings indicated that some of the released MWCNTs by IS reduction and cation exchange were associated with the released clay fraction, suggesting the potential for facilitated transport of MWCNTs by WDCs. The sorption and mobility of two 14C-labeled contaminants, the hydrophobic CLD and the readily water-soluble SDZ, in the absence or presence of MWCNTs were investigated by applying in the column studies at different injected times. The experimental results indicated that the presence of mobile MWCNTs facilitated remobilization of previously deposited CLD and its co-transport into deeper soil layers, while retained MWCNTs enhanced SDZ deposition in the topsoil layers due to the increased adsorption capacity of the soil. The modeling results then demonstrated that the mobility of MWCNTs in the soil and the high affinity and entrapment of contaminants to MWCNTs were the main reasons for MWCNT-facilitated contaminant transport. On the other hand, immobile MWCNTs had a less significant impact on the contaminant transport, even though they were still able to enhance the adsorption capacity of the soil.An outlook study about the effect of surfactant on the transport of MWCNTs in QS and GQS was conducted in the presence of the sodium dodecylbenzenesulfonate (SDBS, anionic surfactant) or Triton® X-100 (TX100, nonionic surfactant). The adsorption of TX100 and SDBS on QS and GQS followed the order GQS > QS due to the higher surface area and surface charge of GQS. High-affinity type adsorption isotherms of TX100 and SDBS on MWCNTs were found. Transport results indicated that the mobility of MWCNTs was highly sensitive to the type of surfactant, the input concentration of surfactant, and the properties of porous media. In conclusion, all results mentioned above provided important insight into MWCNTs mobility in the subsurface environment