Long-Term Stabilization of Arsenic-Bearing Solid Residuals under Landfill Conditions

Abstract

The maximum contaminant level (MCL) for arsenic in drinking water was reduced to 10 parts per billion in 2006 by the USEPA. As a result, approximately 10,000 tons of arsenic-bearing residuals (ABSRs) are estimated to be generated every year from water treatment processes. It has also been established that the standard Toxicity Characteristic Leaching Procedure (TCLP), underestimates arsenic leaching from ABSRs, particularly under mature, mixed solid waste landfill conditions. This makes it critical to investigate stabilization technologies that would ensure long-term stability of arsenic residuals after disposal. Arsenic is ubiquitously associated with iron oxides in natural environments as well as water treatment residuals. Hence, knowledge of iron oxide transformations under landfill conditions is critical to understanding the fate and mobility of the associated arsenic. In this work, the effect of high local Fe(II) concentrations on ferrihydrite transformation pathways was studied. Magnetite was the sole transformation product in the presence of high local Fe(II) concentrations. In the absence of high Fe(II) concentrations, goethite was the major transformation product along with minor quantities of magnetite. These results have implications for arsenic mobility from ABSRs since goethite and magnetite have different arsenic sorption capacities and mechanisms. Two technologies were investigated for the stabilization of ABSRs - Arsenic Crystallization Technology (ACT) and Microencapsulation. The strategy for ACT was to convert ABSRs into minerals with a high arsenic capacity and long-term stability under landfill conditions. Scorodite, arsenate hydroxyapatites, ferrous arsenate, arsenated schwertmannite, tooeleite and silica-amended tooeleite, were synthesized and evaluated for their potential to serve as arsenic sinks using TCLP and a simulated landfill leachate test. Ferrous arsenate type solids and arsenated schwertmannite showed most promise in terms of low arsenic leachability and favorable synthesis conditions. Microencapsulation involved coating arsenic-loaded ferrihydrite with a mineral having high stability under landfill conditions. Based on results from a previous study, vivianite was investigated as a potential encapsulant for ABSRs. A modified version of the TCLP was used to evaluate the effectiveness of microencapsulation. Although vivianite did not prove to be a promising encapsulant, our efforts offer useful insights for the development of a successful microencapsulation technology for arsenic stabilization.