Impact of Barium and Cadmium on Defluoridation by Calcite: Batch Reactor and Column Tests

摘要

This study focuses on the investigation regarding impact of Ba2+ and Cd2+ on defluoridation by calcite because Spent Pot Lining (SPL, a byproduct generated during the aluminum production) leachate is a complex chemical cocktail. To better understand defluoridation processes and replicate the situation in the field, column tests were conducted. Results indicate that presence of neither Ba2+ nor Cd2+ has significant impact on fluoride retention when compared with the blank (no metals) because the relative concentration values (F/F-o) as well as the transport parameters like retardation factor (R-c) and distribution coefficient (K-d) are almost the same. PHREEQC, a geochemical software, was then applied to simulate the column test processes (i.e., 1D reactive/transport processes) based on the mechanism of CaF2 precipitation followed after calcium dissolution. As indicated in the batch reactor test, addition of Cd2+ may inhibit calcite dissolution; therefore, a new phase CalciteSupp (a phase representing the suppressed solubility of calcite) was defined. Modeling results exhibit good predictions of observed data, and the log_k solubility of the hypothetical CalciteSupp equilibrium phase implies the suppression of calcite dissolution in the metal samples. Moreover, saturation indices obtained by modeling indicate the precipitation of CdCO3 and BaCO3, revealing that addition of metals may influence fluoride removal at some point during the process, but has no big impact on overall defluoridation. The initial development of the PHREEQC geochemical model presented shows potential in being able to be used as a predictive tool for the design of fluoride remediation strategies, such as permeable reactive barriers, and transport parameters obtained on the basis of the column tests, and CXTFIT (a computer program for evaluating and estimating transport parameters) modeling can be applied in practical design.