Solidification/Stabilization of High Nitrate and Biodenitrified Heavy Metal Sludges with a Portland Cement/Flyash System

Abstract

Pond 207C at Rocky Flats Environmental Technology Site (RFETS) contains process wastewaters characterized by high levels of nitrates and other salts, heavy metal contamination, and low level alpha activity. The purpose of this research was to investigate the feasibility of treating a high-nitrate waste, contaminated with heavy metals, with a coupled dewateriug and S/S process, as well as to investigate the effects of biodenitrification pretreatment on the S/S process. Pond 207C residuals served as the target waste. A bench-scale treatability study was conducted to demonstrate an S/S process that would minimize final product volume without a significant decrease in contaminant stabilization or loss of desirable physical characteristics. The process formulation recommended as a result a previous S/S treatability study conducted on Pond 207C residuals was used as the baseline formulation for this research. Because the actual waste was unavailable due to difficulties associated with radioactive waste handling and storage, a surrogate waste, of known composition and representative of Pond 207C residuals, was used throughout this research. The contaminants of regulatory concern added to the surrogate were cadmium, chromium, nickel, and silver. Product volume reduction was achieved by dewatering the waste prior to S/S treatment. The surrogate was dewatered by evaporation at 60 to 80 C to total solids contents from 43% to 78% by weight, and treated with Portland cement and fly ash. Two cement to flyash ratios were tested, 2:1 and 1:2, by weight. Contaminant leachability testing was conducted with a 0.5 water to pozzolan (the cement/flyash mixture) ratio and both cement to flyash ratios. Each product was tested for unconfined compressive strength (UCS) and for contaminant leachability by the Toxicity Characteristics Leaching Procedure (TCLP). At the highest solids content achieved by dewatering, 78% solids by weight, the predicted final waste form volume f or Pond 207C residuals after S/S processing was reduced by over 60 A when compared to the baseline process. All tested process formulations produced final waste forms with an average UCS of 100 psi or greater. Percent fixation of Chrome (VI) increased at higher solids contents. Fixation of nickel varied from over 87% to 69%, and cadmium fixation was greater than 99% at every solids content tested. Silver TCLP extract concentrations were below detection limits in all cases except for one anomalous measurement. Final product volume reduction was not achieved with coupled dewatering and S/S processing after biodenitrification pretreatment. The waste slurry became too viscous to mix with reagents after dewatering to approximately 55% solids. Fixation of contaminant constituents and final product UCSs were similar to the results of S/S processing without biodenitrification. Due to the lack of volume reduction, biodenitrification was not successful as a pretreatment for S/S processing under the test conditions of this research.