Research paperCyclic carbonate–sodium smectite intercalates
Graphical abstract
Introduction
A high swell capacity and low saturated hydraulic conductivity make sodium bentonite useful as a component in geosynthetic clay liners (GCL) (Eglofgstein, 2001, Bouazza, 2002 Gates et al., 2009). However, bentonites undergo significant increases in saturated hydraulic conductivity when exposed to saline leachates arising from osmotically induced desiccation and associated changes in texture (Quirk and Schoffield, 1955, Guyonnet et al., 2005), thereby enabling these leachates to potentially contaminate underlying sediments and groundwater. GCL performance is thus often limited by the ability of the bentonite component to swell and form and maintain strong gels (Bouazza and Gates, 2014).
Various types of modifications have been developed to improve the chemical and functional capabilities of bentonite, ranging from doping with soda-ash (Harvey and Lagaly, 2006), the addition of polymers (Theng, 2012) to modifying with organo-surfactants (Yang and Lo, 2004) or with solvents (Onikata, 1999), and some of these methodologies have been applied to GCLs (Eglofgstein, 2001, Katsumi et al., 2008, Scalia et al., 2014).
In particular, five-membered cyclic organic carbonate (COC), a class of polar aprotic solvents, have been shown to have unique properties such as high polarity and a propensity for coordinating with metal cations via cation-dipole interactions (Yamanaka et al., 1974, Chernyak, 2006). In addition to high solvency, COC molecules also possess low flammability, high boiling and flash points, low odour levels, low vapour pressure (Verevkin et al., 2008) and generally low toxicities (Anonymous, 1987). Propylene carbonate (PC) forms intercalation complexes with montmorillonite (Mt) that have been found to enhance the swelling of Mt in brines (Onikata and Kamon, 1996, Onikata, 1999), a phenomenon exploited in the ‘multiswellable’ bentonite products (Onikata et al., 1999, Katsumi et al., 2008). Unlike sodium bentonite, PC-modified bentonite retains good swelling in up to 0.3 M CaCl2 (Onikata, 2002) and is currently proposed as an alternative material for liner systems (Katsumi et al., 2008).
Glycerol carbonate (GC) and several of its COC derivatives, were catalytically synthesized from urea and 1,2-diols employing zinc monoglycerolate as a catalyst (Turney et al., 2013). Preparations of cyclic carbonate intercalates of Na+-Mt were made (see Supplementary information) to study the mechanisms by which the composite materials maintained a swollen state in high ionic strength leachates, with the aim to develop new materials for application in clay barriers against high levels of salinity. This study details the results from X-ray diffraction and infrared spectroscopic study of the structures and properties of the intercalates.
Section snippets
Materials & methods
Sodium-bentonite, supplied from Sibelco Pty. Ltd. (Melbourne) is mined near Miles, Queensland and has well known and consistent mineralogy. The < 0.2 μm fraction of 25 g of bentonite (containing > 98% smectite, Table 1) was separated by sedimentation and centrifugation following general procedures (Gates et al., 2002). The fine fraction was rinsed three times with ~ 2 M NaCl, washed and dialyzed to remove excess salts and freeze-dried to a powder. Approximately 13 g of < 0.2 μm Na+-Mt was prepared from
Interaction of GC with Na+-Mt
To determine the appropriate load rates of GC and its derivatives for modifying Na+-smectite, the XRD traces of GC/Na+-Mt intercalates were measured as a function of percent mass coverage (Fig. 1A). A d001 value of 1.29 nm was observed for the 001 reflection of the air-dried Na+-Mt, indicating mostly a 1-layer hydrate. On loading with GC, the d001 value generally increased from ≈ 1.40 nm (9% coverage) to 3.7 nm (900% coverage). For the coverage from 35 to 67%, two d001 values were apparent, one near
Conclusions
Various wt% of glycerol carbonate were mixed with Na+-Mt to probe the resulting GC/Na+-Mt intercalates using XRD and FTIR. An ordered complex was obtained with a 100 wt% GC load rate, whereas greater GC loadings resulted in considerable interlayer disorder. The observed interlayer space for intercalates of the more complex COC derivatives with Na+-Mt increased logically with molecular dimensions of the COC, but the functional moiety also impacted on the resulting d-value.
The main driving force
Author contributions
WPG, TWT and AFP conceptualized and developed the research project and secured Australian Research Council financial support (ARC DP1095129). US conducted the research presented herein as part of the requirements leading to a PhD.
Author information
The authors declare no financial interests.
Acknowledgements
This work was financially supported by Australian Research Council's Discovery Projects Scheme (grant no.: DP1095129). U. Shaheen gratefully acknowledges receipt of a fees remission scholarship and Postgraduate Publication Award from the Faculty of Science. The authors also thank Sanji Kulasegaram, Sally Duck and Craig Forsyth from Monash University for laboratory and instrumental support.
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