Verification of Contaminant Sorption by Soil–Bentonite Barrier Materials Using Scanning Electron Microscopy/Energy Dispersive X-Ray Spectrometry

Abstract

The performance of a soil–bentonite barrier material as a sorbent for heavy metals was investigated in the laboratory using an influent containing 20 mg/L of ${\mathrm{Pb}}^{2+}$ at a pH of 5. The target parameters were the hydraulic conductivity of the soil–bentonite mix and the difference between ${\mathrm{Pb}}^{+2}$ concentrations in the influent and effluent. A hydraulic conductivity of $1.0\times {10}^{-8}\mathrm{cm}/s$ was achieved, the mixture was found to meet common regulatory specifications for hazardous waste containment. After four pore volumes of flow through specimens placed in a column, no ${\mathrm{Pb}}^{2+}$ was detected in the effluent. Sorption was verified through acid extraction and identification of ${\mathrm{Pb}}^{+2}$ on barrier sample cross sections using microanalysis of specimen slices with a scanning electron microscope and the associated energy dispersive x-ray spectrometry. Energy dispersive x-ray spectrometry spectra indicated that the ${\mathrm{Pb}}^{2+}$ partitioned selectively to the bentonite fraction of the mix. The results confirm the ability of this mixture of soil–bentonite to function as an effective barrier for aqueous ${\mathrm{Pb}}^{2+}$ solution. This method of microanalysis appears to have promise as an effective tool for assessing relative affinity of contaminants for specific mineralogical constituents of a barrier mixture and may have applications in sorption performance assessments of other multicomponent barrier systems. If each barrier material is tested separately, the effects of the texture of the mix on sorption and hydraulic characteristics of the mix cannot be effectively assessed.