Arsenic Crystallization Technology (ACT) is a potentially eco-friendly, effective technology for stabilization of arsenic-bearing solid residuals (ABSRs). The strategy is to convert ABSRs generated by water treatment facilities into minerals with a high arsenic capacity and long-term stability in mature, municipal solid waste landfills. Candidate minerals considered in this study include scorodite, arsenate hydroxyapatites, ferrous arsenates (symplesite-type minerals), tooeleite, and arsenated-schwertmannite. These minerals were evaluated as to ease of synthesis, applicability to use of iron-based ABSRs as a starting material, and arsenic leachability. The Toxicity Characteristic Leaching Procedure (TCLP) was used for preliminary assessment of candidate mineral leaching. Minerals that passed the TCLP and whose synthesis route was promising were subjected to a more aggressive leaching test using a simulated landfill leachate (SLL) solution. Scorodite and arsenate hydroxyapatites were not considered further because their synthesis conditions were not found to be favorable for general application. Tooeleite and silica-amended tooeleite showed high TCLP arsenic leaching and were also not investigated further. The synthesis process and leaching of ferrous arsenate and arsenated-schwertmannite were promising and of these, arsenated-schwertmannite was most stable during SLL testing. The latter two candidate minerals warrant synthesis optimization and more extensive testing.
In this work, we study the abiotic re-mineralization of ferrihydrite under reducing conditions, obtained by adding zero-valent iron (ZVI) to a suspension of ferrihydrite particles. Under similar conditions, the system (ferrihydrite and ZVI) proceeded along two different transformation pathways differentiated by whether a magnetic stirrer or an overhead stirrer was used for mixing. X-ray diffraction characterization of the solid products showed that magnetite was the sole product of ferrihydrite transformation when a magnetic stirrer was used, whereas both goethite and magnetite were formed when an overhead stirrer was used. The system also behaved differently in terms of transformation kinetics and amount of magnetite formed. The quantification of magnetite generated was performed using a procedure developed in this study. The role of four mechanisms was investigated to explain these observed differences, namely-(1) presence/absence of high local Fe 2? concentrations, (2) mechanical abrasion, (3) presence/absence of a magnetic field, and (4) presence/ absence of a crystalline ZVI surface. Ferrous ions are expected to be concentrated near the magnetic bead on the magnetic stirrer as opposed to a more dispersed distribution with the overhead stirrer. This mechanistic study concluded that the presence of high local Fe 2? concentrations in the system leads to magnetite formation and the absence of the same leads to mixed goethite/magnetite or magnetite-free systems. These findings have significant implications for the mobilization of arsenic from iron (III) hydroxides as the conditions move from oxidizing to reducing, such as often occurs in engineered landfills and natural carbon-rich sediments.
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