U. Eranka Illangakoon,Deng-Guang Yu,Bilal S. Ahmad,Nicholas P. Chatterton,Gareth R. Williams
A series of 5-fluorouracil (5-FU) loaded core/shell electrospun fibers is reported. The fibers have shells made of Eudragit S100 (ES-100), and drug-loaded cores comprising poly(vinylpyrrolidone), ethyl cellulose , ES-100, or drug alone. Monolithic 5-FU loaded ES-100 fibers were also prepared for comparison. Electron microscopy showed all the fibers to have smooth cylindrical shapes, and clear core–shell structures were visible for all samples except the monolithic fibers. 5-FU was present in the amorphous physical form in all the materials prepared. Dissolution studies showed that the ES-100 shell was not able to prevent drug release at pH 1.0, even though the polymer is completely insoluble at this pH: around 30–80% of the maximum drug release was reached after 2 h immersion at pH 1.0. These observations are ascribed to the low molecular weight of 5-FU permitting it to diffuse through pores in the ES-100 coating, and the relatively high acid solubility of the drug providing a thermodynamic impetus for this to happen. In addition, the fibers were observed to be broken or merged following 2 h at pH 1.0, giving additional escape routes for the 5-FU.
The transport of H 2 SO 4 (at pH = 1.0, −1.0 and −3.0) through two mineralogically different compacted clays (Kc and Km) was examined using single-reservoir diffusion cells with constant source concentrations. At the end of the 216 day test period, geochemical analyses indicated increased depth of acid diffusion with increased reservoir acidity for both Kc and Km cells. Elevated Ca, Al, Fe and Si concentrations were associated with decreased pH values in all cells. XRD results showed that these elevated concentrations corresponded to the loss of carbonate and montmorillonite peaks and decreased peak intensities for illite and kaolinite in the Kc and Km pH −1.0 and −3.0 Km cells. Moreover, Si X-ray absorption near-edge structure (XANES) indicated dissolution of the phyllosilicate phases, a relative increase in the amount of quartz, and the potential formation of an amorphous silica phase. The results of this study showed that, despite the extreme pH values considered, movement of H 2 SO 4 solutions with pH < 1.0 may be greatly retarded in the presence of a strongly neutralizing mineral phase, such as dolomite, within the clay.
Although widely investigated in relation to acid mine drainage systems at pH > 1.0, we know little about the impact of sulfuric acid (H 2 SO 4 ) on the geochemistry and mineralogy of clays at pH < 1.0 (including negative pH values). Thus, laboratory batch experiments were conducted on three mixed clay samples with different mass ratios of phyllosilicates (smectite, illite , and kaolinite) to investigate the impact of H 2 SO 4 from pH 1.0 to −3.0 for exposure periods of 14, 90, 180, and 365 days. Si and Al K- and L 2,3 -edge X-ray absorption near edge structure (XANES) spectroscopy were employed on these samples to determine the chemical and structural changes that occur during acidic dissolution of phyllosilicates that cannot be distinguished using X-ray diffraction analyses. A series of silicate, phyllosilicate, and Al-bearing standard compounds were also studied to provide an explanation for the observed changes in the clay samples. The Si XANES results indicated the preferential dissolution of the phyllosilicates (pH ⩽ 1.0, t ⩾ 14 d), the persistence of quartz even at pH ⩾ −3.0 and t ⩾ 365 d, and the formation of an amorphous silica-like phase that was confined to the surface layer of the altered clay samples at pH ⩽ 0.0 and t ⩾ 90 d). Al XANES results demonstrated dissolution of Al-octahedral layers (pH ⩽ 1.0, t ⩾ 14 d), the persistence of four-fold relative to six-fold coordinated Al, and the precipitation of an Al-SO 4 -rich phase (pH ⩽ −1.0, t ⩾ 90 d). An existing conceptual model of phyllosilicate dissolution under extremely acidic conditions was modified to include the results of this study.
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