Date of Award
Doctor of Philosophy
Mining activities, as essential as they are for our economy and our society, bring pollutants such as acid mine drainage (AMD) which contains dissolved metal(loid)s into the environment. There are different technologies currently being practiced to treat AMD, but many of these methods are prohibitive in industry due to high energy, material and labor requirements. This study investigated two emerging technologies to treat AMD with high removal rates of some metals. In addition, as AMD contains strategic metals such as rare earth elements (REEs), hydrometallurgical and biosorptive approaches were used to recover REEs from AMD, hydrometallurgical recovery method was also applied for coal by-products for the method developed. A two-chamber bioelectrochemical system (BES) was used to remove different types of metals from AMD. After 7 days, the pH of the cathode solution increased from 2.5 to 7.3. More than 99% of Al, Fe and Pb were removed, and removal rates of 93%, 91%, 89% and 69% were achieved for Cd, Zn, Mn, and Co, respectively, at the biocathode. Energy-dispersive X-ray spectroscopy (EDS) studies revealed the deposition of the various metals on the cathode surface, and some metals were detected in precipitates from the cathode chamber. During the BES operation, ~30-50 mV of closed circuit voltage was obtained for different conditions. A single-chambered BES study was conducted for the removal of Cd, Ni, and Mn in mine drainage. Compared to a double chamber, a single chamber BES is easier to design and operate. The removal process was studied with activated sludge from a local wastewater treatment plant. The effect of applied voltage, time, and initial concertation of these metals on their removal rate was studied. For Cd initial concentrations of 625 and 165 µg/L, 1.0 V showed the highest removal efficiency, and ~93 and 95% of Cd were removed, respectively. For a Ni initial concentration of 2,440 µg/L, 72% was removed under 1.0 V compared to the control of 77%. However, for a lower initial Ni concentration of 190 µg/L, 1.0 V was better compared than other conditions, and it removed 92% of Ni. For a Mn initial concentration of 1,800 µg/L, 1.0 V had a better result, however, only ~19% of the Mn was removed. For a lower Mn initial concentration of 390 µg/L, 1.0 V was favorable only at 24 h and the removal rate was ~37%. Nanoscale zerovalent iron (nZVI) was used to remove contaminants from AMD. These contaminants include transition metals (Co, Ni, Cu, Mn, and Zn), alkali and alkaline earth metals (Li, Mg, and Ca), metalloid (As), nonmetals (Se and S), and active metal (Al). Purchased nZVI in concentrations of 10-6500 mg/L was used for a reaction duration of up to 480 min. The pH of the AMD increased linearly with increasing concentrations of nZVI, with a maximum of 6.0±0.1 at 6500 mg/L of nZVI. Cu and Al had the highest removal rate among all other elements. With 10 mg/L of nZVI, ~100% of Cu was removed within 120 min. Up to ~98% of Al was removed with 5000 mg/L of nZVI in 480 min. Reuse of the purchased nZVI was studied for the first time for AMD treatment; however, after reuse in the second cycle, the nZVI was no longer effective. Lab-made nZVI by the precipitation method was tested for a longer time of 48 h. Removal rates for different elements did not change after ~8 h (e.g., 480 min), and in general, the lab-made nZVI had better removal efficiency compared to the purchased nZVI, with removal rate of ~28-79% when using 80 mg/L of the lab-made nZVI. Besides Cu, Al, Ni, and Co, successful removal of Mg and Ca, as well as S, Co, Li, As, and Se from AMD was reported for the first time by using nZVI. Different coal ranks were examined for REE concentration from coal ash. Maximum REE content of more than 700 mg/kg was observed for the highest-rank coal (anthracite) sample, and that was used for leaching and recovery studies. Hydrometallurgical processes including leaching, solvent extraction, stripping, and precipitation were performed to recover REEs from coal ash. Nitric acid leaching tests were conducted at 95 ℃ using a 4×2×2 factorial design. The results indicated that the highest rate of light REEs (LREEs) recovery was achieved at the highest molarity of the acid solution, lowest solids content and longest retention time. However, the highest rate of heavy REEs (HREEs) recovery needed only an intermediate level of acid molarity. The highest recovery rates of 90% for LREEs and 94% for HREEs were obtained. Recirculation of the leachate was conducted to prepare the REE-concentrated solution for the solvent extraction. After two stages of leaching, a 33 mg/L of TREE concentration was obtained in the leachate. Solvent extraction (SX) tests conducted using three different extractants, namely, TBP, D2EHPA and Cyanex 572, and their combinations showed that D2EHPA was the best extractant for recovering REEs from the nitric acid leachate solution with an extraction efficiency of 99%. Nitric acid and sulfuric acid and their mixture were used in the stripping tests. The effect of solvent concentration (in the SX process) was also studied in the stripping stage. When 50% solvent concentration was used, a maximum of 58% stripping recovery was obtained. Oxalic acid helped precipitate ~94% of total REEs (TREEs) from the above aqueous solution. Calcination of the product was performed to reach a final product of 0.8% rear earth oxides (REOs). The same process flowsheet was also successfully tested for another coal ash sample. To recover REEs from AMD, two different approaches were carried out including hydrometallurgical technique and more environmentally friendly approach- biosorptive recovery. A complete process flowsheet including either solvent extraction or biosorption, followed by stripping, and precipitation was developed to recover REEs from an unconventional source of AMD for the first time. At the natural pH of 2.5 almost all REEs were extracted from the solution. Metal-loaded organic solution was reused for three cycles, and it was shown that after three cycles, there was no major reduction in the capacity of the extractant. Striping with 6.0 M HNO3 recovered 23.9±0.7, 74.7±2.1, and 53.1±1.4% of LREEs, HREEs, and TREEs from the organic phase accordingly. Using oxalic acid, and for pH of 2.0, 92.9±2.8% of LREEs, 10±1.5% of HREEs, and 56.2±1.8% of TREEs were precipitated. In the biosorptive extraction, >99% of TREEs were extracted from the solution. The REE-bearing bacteria was also stripped with 6.0 M HNO3, 2871.3±114.8 µg/L (45.0±1.8%) LREEs, 3851.0±154.0 µg/L (65.0±2.6%) HREEs, and 6722.0±268.9 µg/L (50.0±2.0%) TREEs were obtained. Both hydrometallurgical and biosorptive methods extracted almost all of the REEs in the AMD, though pH was adjusted to 4.0 for the biosorptive method. After stripping, comparable amounts of TREEs were obtained by both methods.
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