Appropriate Liquid-to-Solid Ratio for Sorption Studies of Bentonite

This study demonstrates the importance of selecting an appropriate liquid-to-solid ratio ($\mathrm{L}:\mathrm{S}$) for batch sorption study (BSS) of bentonite (BE). The current recommended $\mathrm{L}:\mathrm{S}$ is 20 for BSS of soils and sediments, which works well for most of the soils/sediments. However, for expansive soil like BE, $\mathrm{L}:\mathrm{S}=20$ was found to be inadequate due to the formation of thick gel like consistency preventing proper interaction of metal ion with solid surface. This resulted in considerable underestimation of sorption capacity of BE. Effort was made to identify appropriate $\mathrm{L}:\mathrm{S}$ for BE for which there is no specific guideline available in the literature. Under controlled pH condition and ${\mathrm{Pb}}^{2+}$ as the model heavy metal ion, it was noted that the sorption capacity of BE increases with $\mathrm{L}:\mathrm{S}$ beyond 20 and with an increase in initial metal ion concentration. The highest percentage removal of ${\mathrm{Pb}}^{2+}$ was observed at $\mathrm{L}:\mathrm{S}=50$ and lowest at $\mathrm{L}:\mathrm{S}=100$ for initial concentration greater than $300\mathrm{mg}/\mathrm{L}$. The minimum $\mathrm{L}:\mathrm{S}$ possible for BE is 20, below which the interaction between the solution and soil solids becomes difficult due to formation of a thick gel like consistency. The interaction of BE with ${\mathrm{Pb}}^{2+}$ was further investigated from the results of field emission scanning electron microscopy (FESEM) integrated with energy dispersive X-ray spectroscopy (EDX). Fourier transform infrared (FTIR) spectrometer was used to identify the variations in interaction between the ${\mathrm{Pb}}^{2+}$ ions and functional group present on BE for various $\mathrm{L}:\mathrm{S}$. It was concluded that for all practical purposes, $\mathrm{L}:\mathrm{S}=50$ is a suitable ratio for determining sorption capacity of expansive soil like BE.