Metal-Air Fuel Cell Electrocoagulation for the Treatment of Arsenic Contaminated Groundwater

In this dissertation an alternative approach known as metal-air fuel cell electrocoagulation was proposed in order to resolve the well known shortcoming associated to the conventional electrocoagulation process. This combined concept of electrocoagulation and a fuel cell was applied for simultaneous pursuit of treating arsenic contained groundwater with electrical power production. In chapter 3, the single-chambered fuel cell with iron anode and air-cathode electrocoagulation (IAFCEC) was developed, The use of a single chamber fuel cell with an iron anode and air cathode is a new and innovative concept in electrocoagulation. In this study, we investigated the predominant reactions that contribute to the production of electricity and iron hydroxides in solution. Solutions composed of 0.06 M NaHCO3 and 0.05 M NaCl at an initial pH of 5 were determined to be optimal for producing the maximum power density of 1997 mW/m2 after 24 h. Increases in the bicarbonate concentration and ionic strength of the solution induced a corresponding decrease in the anode potential and increase in the cathode potential, which resulted in an increase in the cell potential and power density. Further, increasing the NaHCO3 concentration to 0.1 M and the ionic strength of the solution to 0.56 M induced an increase in the maximum power densities to 2436 and 4343 mW/m2, respectively. Initial pH values of 7.5 and 8.5 in solutions contained 0.06 M NaHCO3 and 0.05 M NaCl were employed to synthesize magnetic iron hydroxides including magnetite and maghemite. These results suggest that this fuel cell technology can be used not only for electrocoagulation with the removal of contaminants, but also for producing useful products such as electricity and magnetic iron hydroxides. Advances in waste water air-metal fuel cells will enable more efficient power generation and systems suitable for scale-up. In chapter 4, a dual-chambered metal-air fuel cell electrocoagulation was developed. The fuel cells with an iron anode and an air-carbon cathode separated by an ion exchange membranes have been used to treat arsenate during power production. To select an effective catholyte, the dual-chambered fuel cell consisted 90 mL of 0.1 M HCl or 0.5 M NaCl as the catholyte and 1 L of 0.1 M NaHCO3 as the anolyte at an initial pH 5. The 0.1 M HCl was an effective catholyte, with which 1 mg L-1 arsenate in 1 L of anolyte was reduced to 5 ??g L-1 in 1 h, and the maximum power density was about 6.3 W/m2 with an anion exchange membrane fuel cell (AEM_FC) and 4.4 W/m2 with a cation exchange membrane fuel cell (CEM_FC). Therefore, 90 mL of 0.1 M HCl was used as a catholyte to treat 20 L of 0.1 M NaHCO3 anolyte contained 1 mg L-1 arsenate at an initial pH of 5 or 7. The arsenate level at pH 5 decreased to less than 5 ??g L-1 in 4 h, and the maximum power density was 5.9 W/m2 and 4.7 W/m2 with AEM_FC and CEM_FC, respectively. When using 0.01 M NaHCO3 as the anolyte at pH 5, arsenate was reduced to less than 5 ??g L-1 in 8 and 24 h for AEC_FC and CEM_FC, respectively. However, when using an anolyte at pH 7, arsenate could not be effectively removed in 24 h. Therefore, when using carbonate as an anolyte, the solution should be adjusted to a weakly acidic pH in order to remove arsenate. In chapter 5, a single-chambered metal-air fuel cell electrocoagulation was developed. The effects of operational parameters (type and concentration of an individual supporting electrolyte as well as mixed supporting electrolytes and initial pH) on arsenate removal and power generation were investigated. The presence of low concentration (10 mM) of an individual NaCl electrolyte was very efficient for arsenate removal. The 1 mg L-1 arsenate in 1 L of anolyte with an initial pH of 7 was reduced to 1 ??g L-1 in 4 h, and power density produced was approximately 112 mW/m2. However, presence of individual Na2SO4 and NaHCO3 electrolytes had adverse effects on arsenate removal as well as power density. The use of 8 mM of Na2SO4 mixed with 10 mM of NaCl created optimal conditions as mixed supporting electrolyte. The power density increased to approximately 308 mW/m2 and arsenate was reduced up to 15 ??g L-1 after 24 h of treatment time. Variation of initial pH (5, 7, and 9) did not significantly affect arsenate removal rate or power density in either optimum supporting electrolyte condition. Complete arsenate removal using low concentration of NaCl electrolyte with neutral pH of 7 indicates that aluminum-air fuel cell electrocoagulation (AAFCEC) is a useful treatment process. In chapter 6, the performance of MAFCEC was critically evaluated with respect to As contained real groundwater treatment. In this research, we first identified and described the difference in As removal efficiencies of iron-air fuel cell electrocoagulation (IAFCEC) and aluminum-air fuel cell electrocoagulation (AAFCEC). Then, our discussion was extended to evaluate the reliability and reusability of IAFCEC and AAFCEC with long term operation (20 cycles) comprising same electrodes (anode and cathode electrode) for As contained real groundwater treatment. Most importantly, we provide a performance evaluation of both reactors (IAFCEC and AAFCEC) with respect to As removal efficiency, and current production in pursuit of the optimum treatment system for As. The maximum As removal efficiency measured from IAFCEC and AAFCEC over 24 h duration (cycle 1) was 96% and 80% respectively. Similarly, in comparison the As removal efficiency reached below the standard level (99%) by IAFCEC and AAFCEC within 4 h and 24 h, respectively, when tested over 20 cycle with continuous operation with As contained groundwater. The maximum power density measured from IAFCEC and AAFCEC over 24 h duration (cycle 1) was 184 mW/m2 and 128 mW/m2 respectively. Similarly, in comparison the power density reached to maximum level of 246 mW/m2 and 300 mW/m2 by IAFCEC and AAFCEC within 12th cycle, respectively, with continuous operation with As contained groundwater. However, the performance of both types fuel cell in terms of power density slightly decreased with passage of time (cycle 12 to cycle 20) with both types of reactor (IAFCEC and AAFCEC).Docto