Abstract
The adsorption characteristics of sodium dodecylbenzene sulfonate (SDBS) on the surface of montmorillonite can lay a foundation for obtaining the optimum concentration of the anionic surfactant. The best absorption wavelength of SDBS was determined using an ultraviolet spectrophotometer. The standard curves of concentration and absorbance of SDBS were established. The amount of SDBS adsorbed on the surface of montmorillonite at various concentrations was calculated by stirring adsorption method. Scanning electron microscopy–energy dispersive X-ray spectrometry (SEM-EDS), X-ray diffraction (XRD), zeta potentiometer, and Fourier transform infrared (FTIR) spectroscopy were used to observe the changes of the structure, main ions, interlayer spacing, potential, and main functional groups on the montmorillonite surface before, and after, adsorption. The test results of SEM with EDS (SEM–EDS) showed that the surface of the montmorillonite after SDBS adsorption was rougher, and the adsorption capacity of the surface was enhanced as the SDBS concentration increased. The XRD results indicated that SDBS adsorbed on the interlayer of montmorillonite repulsed interlayer water and reduced the interlayer water content. With the increase of SDBS concentration, the interlayer spacing of the montmorillonite available for adsorbing SDBS decreased further. Additionally, interlayer adsorption and surface adsorption exist simultaneously in montmorillonite in SDBS solution. The distribution of total adsorption capacity of SDBS in the layers and on the surface of montmorillonite accords with the adsorption result simulated by a pseudo-second-order kinetic model. The increase in concentration of SDBS adsorbed by montmorillonite is the main reason for the decreased initial adsorption rate. The zeta potential test showed that the addition of H+ to the SDBS solution could reduce electrostatic repulsion and promote the adsorption of SDBS on montmorillonite. The results of this study provide an experimental basis for the study of the mechanism of SDBS adsorption on montmorillonite.
Highlights
Montmorillonite, as a clay mineral carrying negative charges on the surface, is used as an adsorbent for most organic compounds (Prost and Yaron, 2001; Bhattacharyya and Gupta, 2008; Xi et al, 2010; Zhu et al, 2014)
The test methods for studying adsorption capacity of surfactant on the surface of clay minerals mainly include nuclear magnetic resonance (NMR), spectroscopic ellipsometry, and the static adsorption method combined with the use of an ultraviolet–visible spectrophotometer or a fluorophotometer
Spectroscopic ellipsometry can be employed to calculate the thickness of the adsorbed layers on the surface of the samples by testing the differences between polarization states of the incident and reflected beams for the samples before and after adsorption, and deducing the amount of adsorption by integrating other relevant parameters: the large test errors in the method for nonuniform surfaces result in inaccurate results (Luciani and Denoyel, 1997; Denoyel, 2002)
Summary
Montmorillonite, as a clay mineral carrying negative charges on the surface, is used as an adsorbent for most organic compounds (Prost and Yaron, 2001; Bhattacharyya and Gupta, 2008; Xi et al, 2010; Zhu et al, 2014). Finding out the changes in adsorption capacity and adsorption kinetic characteristics of a surfactant on the surface of clay minerals under different time, temperature, and concentration conditions can lay a foundation for studying adsorption mechanisms. The test methods for studying adsorption capacity of surfactant on the surface of clay minerals mainly include nuclear magnetic resonance (NMR), spectroscopic ellipsometry, and the static adsorption method combined with the use of an ultraviolet–visible spectrophotometer or a fluorophotometer. The NMR determination method is mainly used to test the T1 and T2 spectra of the samples before, and after clay minerals adsorb surfactant, so as to calculate their surface adsorption capacity. The static adsorption method combining with test instruments, such as an ultraviolet–visible spectrophotometer, a fluorophotometer, a total organic carbon analyzer, or a liquid chromatography analyzer, is used to test the adsorption capacity of a surfactant on the surface of clay minerals under different conditions (Liu et al, 2016). This research attempts to provide an experimental basis for investigating the adsorption mechanisms of anionic surfactants on the surface of montmorillonite
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