Adsorption characteristics of anionic surfactant onto laterite soil with differently charged surfaces and application for cationic dye removal

In this paper, we report the adsorption characteristics of the anionic surfactant, sodium dodecyl sulfate (SDS) onto laterite soil with positively and negatively charged surfaces. The laterite soil was characterized using X-ray fluorescence (XRF), scanning electron microscopy (SEM), as well as total organic carbon (TOC), Brunauer-Emmett-Teller (BET) and zeta potential measurements. The adsorption of SDS onto a positively charged laterite surface at pH 4 decreased with an increase in ionic strength, indicating that the electrostatic attraction was the main driving force for controlling the adsorption. On the other hand, the adsorption of SDS onto a negatively charged laterite surface at pH 10 was induced by both electrostatic and hydrophobic interactions as the adsorption increased with increasing NaCl concentrations. The adsorption isotherms of SDS onto laterite at different pH values and NaCl concentrations were fitted well by the two-step adsorption model. The adsorption mechanism of SDS onto laterite soil with differently charged surfaces was discussed in detail based on the changes in functional surface groups determined by Fourier transform infrared spectroscopy (FT-IR), variation in the change in surface charge as evaluated by zeta potential measurements, and adsorption isotherms. The application of SDS adsorption to the modification of the laterite surface for the removal of the cationic dye, rhodamine B (RhB) was also investigated. Optimum conditions for RhB removal were found to be pH 4, adsorption time 60 min, and adsorbent mass 0.25 g. The removal efficiency of RhB was >94% after five recycles. Our results indicate that the surface modification of laterite soil with SDS is valuable for the removal of cationic dyes from aqueous solutions.