Heavy metals are a common groundwater pollutant, often due to natural processes and polluting human activity. Many important aquifers are dominated bylimestone, a type of rock that comprises primarily of the mineral calcite (CaCO3). Calcite may undergo a chemical reaction with oxalic acid (C2H2O4) that results in its replacement to calcium oxalate (Ca-oxalate, CaC2O4·(H2O)x, where x ranges between 0 and 3). It is known that a variety of metals such as zinc, lead, and cadmium may precipitate from solutions that are saturated with respect to Ca-oxalate. Furthermore, these poisonous metals may be incorporated into the mineral lattice during the replacement reaction of calcite into Ca-oxalate, which unlike calcite, is very insoluble. Thus, the replacement reaction of calcite into Ca-oxalate in the presence of heavy metals may trap these pollutants into the mineral lattice of Ca-oxalate, and thus remove them from the aquifer groundwater. This replacement reaction may contribute to the development of innovative methods for removing metals from aquifers dominated by carbonate rocks. However, the level of efficiency of the abovementioned replacement reaction in fixing heavy metals into the mineral lattice is not fully understood To evaluate the efficiency of this reaction in removing pollutants, I carried out an experiment where I examinedthe reaction of carbonate rocks with oxalic acid in the presence of contaminants, where the extent to which the pollutants were incorporated into the solid minerals during the replacement reaction was determined. Through the characterization of the mineral composition of the solid phases using X-ray diffraction (XRD), I found that the replacement of limestone in the presence of oxalic acid does result in the desired replacement reaction. In addition, contaminant concentrationsmeasurements using an inductively coupled plasma mass spectrometer (ICP-MS) demonstrated a decrease in the concentrations of several metals due to the replacement reaction. These results indicate that the replacement reaction of calcite into Ca-oxalate may be utilized for the removal of heavy metals from carbonate-dominated aquifers. Although the replacement reaction lowered theconcentrations of certain heavy metals, the concentrations of other metals also diminished in the control group, in which no replacement reaction occurred. Thus, it appears that the contaminants were most likely removed from the solution through adsorption onto calcite, rather than by incorporation into the Ca-oxalate lattice during replacement. Thus, further experiments are needed to fully understand the conditions in which metals are affected by mineralogical changes.
This is how I came up with the idea for this project: