Synthesis, Characterization, and Application of Ettringite for Sequestration of 14C from Simulated Waste Solution

Abstract

Carbon -14 (14C) with a half-life of 5730 years is a great environmental concern for deep geological disposal of irradiated graphite because it is readily mobile in groundwater and atmospheric systems. Herein, we present the synthesis and characterization of Ettringite (a hydrous calcium aluminum sulfate naturally occurring mineral found in cementitious matrices) and its use for effective sequestration of 14C from simulated waste solutions as HCO3- and CO32- at intermediate pH (~ 8.5) and high pH (~11.5) conditions, respectively. Ettringite (powder form) was synthesized via hydrothermal route and characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), Brunauer – Emmett – Teller (BET) surface area, and Fourier transform infrared (FTIR) analyses. The characterization results confirmed the highly crystalline/uniform/homogeneous ettringite material. In this study, we tested the removal of stable carbon isotope (12C) derived from 100 ppm NaHCO3 aqueous solution (as a surrogate of 14C). A series of tests were performed for effective removal of 14C from simulated waste solution at various experimental conditions (e.g., pH, effect of time, sorbent dose, sorbate dose, etc.). The inorganic carbon concentrations were analyzed using total carbon analyzer prior and after the removal tests. The obtained results revealed ~ 92 % sequestration of 12C (either HCO3- or CO32- species) from simulated waste solution using ettringite in 48-hours (h) and 8h contact times at intermediate pH (~8.5) and high pH (~11.50), respectively. We observed a considerable phase alteration of ettringite into calcite at intermediate pH (~8.5) 12C removal tests, which shows the instability of ettringite structure at low or intermediate pH condition. However, the ettringite structure was quite stable at high pH (~ 11.50) 12C removal experiments, as the XRD analysis data of ettringite powder confirmed the presence of all major ettringite peaks after the sequestration experiments. Additionally, the applicability of pseudo second order kinetics was determined for the removal of 12C onto ettringite. Some additional experiments and further characterizations of ettringite powder after 12C removal experiments are underway to interpret the interaction mechanism of HCO3- and CO32- with ettringite at the defined experimental conditions. Moreover, our work will further explore the application of synthesized ettringite for sequestration of other hazardous anions, including, IO3-, ReO4-/TcO4-, and PO43- under different physicochemical conditions. We believe that this study would facilitate us to predict the affinity of ettringite mineral towards various hazardous anions and prevent their mobility in environment.