Moisture and Heavy Metal Stabilization for Coal Combustion Residuals Using Sodium Polyacrylate

摘要

Due to the recent (2015) federal regulations pertaining to disposal of coal combustion residuals (CCR) from electric utilities, methods to dewater the CCR impoundments, establish structural stability, and mitigate the leaching of heavy metals from CCR leachate into ground water has become a pressing issue in the industry. In order to contribute towards easing the challenges in CCR excavation and groundwater contamination risk, this research aimed at evaluating the use of a super absorbent polymer (SAP) sodium polyacrylate, as an additive in moisture stabilization of saturated coal ash, to increase its mechanical strength, and to evaluate uptake of heavy metals from coal ash leachate. Undrained shear strength of coal ash samples containing 1% to 4% weight fractions of SAPs was measured by Pocket Penetrometer, Torvane, and Unconfined Compression Strength device. During the two phases of this project, up to five ponded ash samples collected from coal ash ponds located in the southeastern region of the U.S. were evaluated. The samples were compacted at standard Proctor effort at water contents ranging from 58% to 70%. The addition of the SAPs improved the undrained cohesion from 0 to 3 psi for selective SAP mixes at weight fractions as little as 1%. In order to evaluate the stabilization or uptake of heavy metals by the SAP, aqueous solutions of arsenic, barium, cobalt, selenium and vanadium at 0.5 mg/L initial concentration were mixed with SAP formulations containing sodium polyacrylate, bentonite clay, and wood flour. After a contact time of 24 hours or more, the leachate was analyzed. The SAPs were able to uptake 50% to 80% of cobalt and barium. The SAPs did not uptake arsenic, selenium and vanadium. The state or valency of the heavy metal ion or complex it forms was the key factor that influenced the effectiveness of the SAP uptake of heavy metals. The weight fractions of the SAPs had no influence on the uptake efficiency for the weight fractions assessed in this research.