Presentation on theme: "Introduction: The US Air Force uses a number of abrasive blast media (ABM) in its routine maintenance operations. The facilities at the Warner Robins Air."— Presentation transcript:
Introduction: The US Air Force uses a number of abrasive blast media (ABM) in its routine maintenance operations. The facilities at the Warner Robins Air Logistics Center generate approximately 100,000 pounds of acrylic ABM and 200,000 pounds of walnut hull, alumina and glass bead ABM per year. At the end of their lifecycle, the spent ABM materials have to be disposed of as hazardous waste due to their high metal content. Table 1 shows the TCLP (Toxic Characteristic Leaching Procedure) test result of the spent ABM materials and regulatory limits of chromium (Cr), cadmium (Cd) and lead (Pb). A treatment process is under development to convert the spent ABM materials into nonhazardous waste in order to reduce disposal cost. The technological scheme for removing toxic metals from the ABM includes leaching, sluicing, drying and treatment of the leachate. In order to be sure that the treated ABM pass TCLP test, more exhaustive aqueous conditions are applied to these samples. Strong acids such as nitric (HNO 3 ), sulfuric (H 2 SO 4 ) and hydrochloric (HCl) acids are used to enhance the leach rate and efficiency. These operations generate leachate with toxic heavy metals and a variety of anions including nitrate (NO 3 - ), sulfate (SO 4 2- ) and chloride (Cl - ) to be treated in a following step. Electrochemical coagulation was selected to treat cadmium-containing leachate because of its simplicity and effectiveness. The objective of this study is to understand the effect of anions on electrochemical processes, specifically on electrochemical coagulation. The existence of chloride, nitrate, sulfate and their combinations contribute to every step of the process: ionic conductivity, current density, current efficiency, pH change, coagulant generation, and cadmium removal. These anions also dictate the quantity and quality of coagulant generated in the reactor, and ultimately the sludge to be removed from the solution. Chemical Analysis: Turbidity was measured by using a HACH DR/4000 UV-vis Spectrophotometer and following the procedure of Method 10047 from the HACH Water Analysis Handbook. Turbidity was used as an indicator to evaluate the amount of coagulant generated in the solution. After 20 minutes of settling, the supernatant of each sample was analyzed for Cd content. Cadmium content was determined by atomic absorption spectroscopy with a Perkin-Elmer AAnalyst 300 atomic absorption spectrophotometer (AA). Calibration curves were prepared by sequential dilution of a 1,000 g/cm 3 standard solution purchased from Perkin- Elmer. Five percent nitric acid was used as the blank. Experimental details are described in the Standard Methods for the Examination of Water and Wastewater. Electrochemical System: A bench-scale electrochemical system, as shown in Fig. 1 was constructed to study the effect of anions on electrochemical processes. The reactor is made of PVC with a piece of aluminum sheet as the anode and graphite as the cathode. The surface area of the anode and cathode is 85 cm 2 each. The two electrodes are situated 1 cm apart from each other and are submerged in the solution. The volume of the reactor is 250 cm 3, which includes a 50-cm 3 connection tubing and circulation pump head. A model PPS 1002F/CE/MT DC power supply from Motech Industries, Inc. was used as the power source. The system was operated in a simple batch recirculation mode, with a circulation pump to prevent sedimentation of the coagulant. Fig. 1
Effect of Anions on Electrochemical Coagulation for Cadmium Removal Chien-Hung Huang and Luke Chen Department of Water Resources and Environmental Engineering Tamkang University (Taiwan) Chen-Lu Yang Advanced Technology and Manufacturing Center University of Massachusetts – Dartmouth Results and Discussion: Conductivity - Fig. 2 shows the relationship between conductivity and electrolyte concentration. The total conductivity of a solution is the summation of contributions from both cations and anions. Since the solutions of NaNO 3, Na 2 SO 4 and NaCl have the same amount of Na + ions in the solutions, the difference among the conductivities are from the contributions of anions. Coagulant Formation - Since pollutants are adsorbed onto coagulant before their removal, the formation of coagulant in the process is a key factor for the process to be effective. In this study turbidity was used as an indicator to evaluate the amount of coagulant generated in the solution. The differences in coagulant formation among the solutions are illustrated in Fig. 6. A tremendous amount of coagulant was observed in Cl - solution, while no coagulant was found in either NO 3 - or SO 4 2- solutions. Since there was no aluminum dissolved in the sulfate solution, no coagulant was expected. The Al anode dissolved in Cl - or NO 3 - solutions; hence, an abundance of Al 3+ ions were released into the solution. However, the solubility of aluminum species is highly pH dependent, as shown in Fig. 5. Because the final pH is about 11 in nitrate solution, all of the dissolved aluminum is in Al(OH) 4 - form. Therefore, no coagulant was observed. Aluminum Hydrolysis - The complete chemistry of aluminum hydrolysis reactions and products is not well understood. Recent reports cited by Peterson identified Al(OH) 2+, Al(OH) 2 +, Al(OH) 4 -, and Al(OH) 3 as major products of aluminum hydrolysis. These reactions along with equilibrium constants are listed in the Table and depicted in Fig. 5. Reaction Log (K) Al 3+ + 3(OH) - → Al(OH) 3(s) 31.2 Al 3+ + H 2 O → Al(OH) 2+ + H + -5 Al 3+ + 2H 2 O → Al(OH) 2 + + 2H + 1.5 Al(OH) 3 + H 2 O → Al(OH) 4 - + H + -12.2 Current - Fig. 3 shows that for each solution, current is proportional to concentration. However, for different anions, even with similar conductivity and the same applied voltage, the currents are different. To explain this phenomenon, Mollah and colleagues proposed that the voltage required to operate an electrocoagulation process, U o, can be expressed as the following equation. where E eq is the equilibrium potential, and are the activated overpotentials of the cathode and anode respectively, and are the concentration overpotentials, is the passive overpotential, is the I-R overpotential, is the distance between electrode plates, is the current density, and κ is the conductivity. Fig. 2 Fig. 4 Fig. 3 Fig. 5 Fig. 6 pH - Fig 4 shows the pH change during the course of operation. The pH of the 0.05 N NO 3 - solution rose from 6 to 11 in two minutes and stayed at 11 to the end of the experiment. Since there was no current in the cell, no reaction (not even water electrolysis) was observed. The pH change of the SO 4 2- solutions during a 10-minute electrochemical treatment fluctuated a bit but remained about the same through out the experiment. In Cl - solution, water was electrolyzed to generate hydrogen gas and OH - which raised the solution pH during the operation.
Conclusions: When dissolved in water, all three anions have a similar contribution to the solution’s electrolytic conductivity. A linear relationship between conductivity and concentration holds up to 0.1 N. With a concentration of 0.01 N in the solutions, nitrate and chloride were able to sustain a reasonable current density at 4.2 mA/cm 2 under an applied potential of 6 volts. The current dissolved aluminum and dissociated water molecules to produce aluminum hydroxide in the solution. Turbidity of chloride solution increased dramatically due to the formation Al(OH) 3 coagulant in the electrochemical process. Because of the excessive OH - produced in nitrate solution, Al(OH) 3 is dissolved to form Al(OH) 4 - ; therefore, no coagulant is observed. In sulfate solution, the aluminum electrode quickly forms an inert film that ultimately terminates the current. Due to the lack of current, no pH change, aluminum dissolution or coagulant was observed. When a small amount of nitrate is added to a sulfate solution, nitrate ions dominate the performance of the electrochemical process. The nitrate ions sustain a current that raises the pH of the solution. With a small amount of chloride in a nitrate solution, the solution behaves like there is no chloride present in the solution. On the other hand, trace amounts of chloride ions in a sulfate solution is able to penetrate the inert film on the anode, sustain a reasonable current density, raise the pH, generate coagulant and ultimately remove cadmium from the solution. In the presence of chloride ions, cadmium was initially removed by cathodic reduction followed by coagulation. Overall, this mechanism was very effective for cadmium removal. A 10-minute treatment removes more than 99.5% of the cadmium, even with a substantial amount of sulfate in the solution. In the absence of chloride ions, the electrochemical process is not able to produce coagulant. Without coagulant in the solution, cadmium can only be removed to certain extent through cathodic reduction. University of Massachusetts Dartmouth Chapter of Sigma Xi, the Scientific Research Society 15 th Annual UMass Dartmouth Research Exhibition April 28-29, 2009 Multi-component System - A solution of all three ions was prepared for testing the electrochemical process. Fig. 9 shows the pH change, coagulant formation and cadmium removal throughout the course of the treatment. Initially cadmium was removed through cathodic reduction. The removal rate picked up when coagulant was observed at the end of the first minute. At the end of the 10-minute treatment, cadmium concentration in the solution was less than 0.1 mg/L, which is more than 99.5% removal. Cadmium Removal - In the solution of sulfate and nitrate, the applied voltage drew a current of 0.4 amp through the solution. The reaction increased the pH from 6.3 to 10 in two minutes. However, no coagulant was observed. Although nitrate concentration is only about one fourth that of sulfate, the solution is dominated by the presence of nitrate. In every aspect (current density, pH change, coagulant generation) the mixture behaved exactly like the solution of nitrate alone. Fig. 7 shows the change of pH, formation of coagulant and removal of cadmium in a 10-minute electrochemical coagulation process. The test solution is a mixture of 0.01 N Na 2 SO 4 and 0.0024 N NaNO 3 with 20 mg/L Cd as target compound. Fig. 8 shows the coagulant formation and cadmium removal in a 10- minute electrochemical coagulation process. The test solutions are mixtures of 0.0024 N NaNO 3 and 0.01 N or 0.05 N NaCl with 20 mg/L Cd as target compound. In terms of Cd removal, a plateau was observed at both Cl- concentrations. For the solution of 0.01 N Cl-, the cadmium concentration dropped from 20 mg/L to 10.4 mg/L in one minute. After that the cadmium concentration remained unchanged for another minute and then further decreased to less than 1 mg/L. When compared with Fig. 7, it is believed that in the first three minutes cadmium was removed by cathodic reduction. The removal reached a maximum at about 50% as in the previous experiment. After three minutes of treatment, turbidity increased significantly, indicating that more cadmium was removed by coagulation. Fig. 8 Fig. 7 Fig. 9