Characterization of geosynthetic clay liner bentonite using micro-analytical methods

In barrier design, familiarity of the structure and composition of the soil material at the micron scale is necessary for delineating the retention mechanisms of introduced metals, such as the formation of new mineral phases. In this study, the mineralogical and chemical makeup of the bentonite from a geosynthetic clay liner (GCL) was extensively characterized using a combination of conventional benchtop X-ray diffraction (XRD) and micro X-ray diffraction (μXRD) with synchrotron-generated micro X-ray fluorescence (μXRF) elemental mapping and μXRD (S-μXRD). These methods allow for the non-destructive, in situ investigation of a sample, with μm spatial resolution. Synchrotron-based hard X-ray microprobes are specifically advantageous to the study of trace metals due to higher spatial resolution (<10 μm) and higher analytical sensitivity (femtogram detection) than is possible using normal laboratory-based instruments. Minerals comprising less than 5% of the total bentonite sample such as gypsum, goethite and pyrite were identified that were not accessible by other conventional methods for the same GCL bentonite. Two dimensional General Area Diffraction Detector System (GADDS) images proved to be particularly advantageous in differentiating between the microcrystalline clay, which appeared as homogeneous Debye rings, and the ‘spotty’ or ‘grainy’ appearance of primary, more-coarsely-crystalline, accessory minerals. For S-μXRD, the tunability of the synchrotron X-rays allowed for efficient distinction of both clay minerals at low scattering angles and in identifying varying Fe oxide minerals at higher angles. GCL samples permeated with metal-bearing mining solutions were also examined in order to consider how mechanisms of metal attenuation may be identified using the same techniques. In addition to the cation exchange capacity from the montmorillonite clay, tests showed how minerals comprising only 1–2% of the bentonite such as goethite could potentially play a significant role in sequestering a range of metals, specifically Ni, Zn and Cu.