Beneficial Use of Steel Slag Fines to Immobilize Arsenite and Arsenate: Slag Characterization and Metal Thresholding Studies

This study presents the results of an extensive beneficial-use evaluation of 3/8-in. minus steel slag fines (SSF) to immobilize arsenic. Two primary sets of experiments were undertaken to assess (1) the ability of SSF to immobilize 100 mg/kg arsenite (As³⁺) and arsenate (As⁵⁺) in dredged material when blended with SSF, including slag cement doses (up to 2%) to determine if additional environmental polishing was necessary; and (2) the ability of SSF alone to immobilize each As species. Visually, the SSF materials resemble an AASHTO No. 9 (fine) aggregate, with a small fraction passing the No. 200 (0.075 mm) sieve. In order to establish the design parameters for deploying the slag media in geoenvironmental applications (soil blending, drainage, reactive trenches, and filters), the soil classification and grain-size distribution, specific gravity, loss on ignition (ash content), standard and modified Proctor compaction behavior, direct shear strength, and swell behavior of the SSF media were evaluated. Additionally, the following geochemical attributes of the SSF media were evaluated: bulk chemistry, mineralogy, pH, anion scan, total priority pollutant list (PPL) metals, toxicity characteristic leaching procedure (TCLP), and synthetic precipitation leaching procedure (SPLP) leaching behavior for PPL metals. Arsenic thresholding studies were performed, in which the uptake of each As source on the SSF materials was evaluated. The SSF materials immobilized approximately 7,900 mg/kgAs³⁺ and 8,800 mg/kgAs⁵⁺, producing TCLP and SPLP concentrations less than 0.010 mg/L in three of four cases. X-ray diffraction (XRD) and scanning electron microscopy (SEM) studies were used in combination with MINTEQ modeling to isolate the mechanisms responsible for the As immobilization in the SSF materials.