The compositions of the solution and rock provide a clear indication of the geochemical processes involved in the reaction between natural phosphate rock and synthetic RO concentrate. Initially, CaCO3 minerals dissolved rapidly with visible CO2 bubbling, leading to a rapid pH increase (Figure ) and an increase in Ca concentration with low solid loadings. This was supported by XRD analysis of the rock before and after leaching (Figure ), with a marked decrease in calcite diffraction peak areas.