Hydrothermal synthesis of aragonite from acid mine drainage (AMD) of the Witwatersrand basin in Gauteng, South Africa

• Hydrothermal urea hydrolysis was used to treat acid mine drainage. • Aragonite CaCO 3 was recovered from all three Witwatersrand gold field basins. • Diverse aragonite super-structures were obtained without utilising organic additives. Hydrothermal urea hydrolysis has been extensively used for homogenous precipitation processes mainly because the resulting products are generally of high crystallinity, uniform particle size and shape, as well as not generating waste brine. In this study, acid mine drainage water samples from the three Witwatersrand goldfields basins (Eastern, Central and Western, Gauteng, South Africa) were subjected to hydrothermal urea hydrolysis to investigate if any mineral(s) could be recovered. In these experiments, three urea concentrations (3.3, 4.0 and 10.0 [urea]/[total metal] ratio) were used while the reaction time (3 h) and temperature (80 °C) were kept constant. The resulting materials were characterised to reveal their chemical compositions, crystalline phases and morphologies. The bulk properties as determined using the Fourier Transform Infrared Spectroscopy, Thermogravimetric Analysis and X-ray Diffraction Spectroscopy showed that the obtained products were predominantly calcium carbonate, the aragonite polymorph, for all three basins. The particles obtained from the polluted mine water samples displayed different morphologies, while mostly were characterised by needle and/or rod-like morphologies with varying lengths and diameters in nanometre range (average aspect ratios ranged from 3.1 to 13.2) as shown by the Scanning Electron Microscope images. Other morphologies, cauliflower-, bouquet- and urchin-like particles were obtained without the use of organic additives. The method was demonstrated to be effective in the removal of calcium (more than 98 % on average) and some evidence of heavy metals, manganese in particular, also being removed from the polluted water. The findings highlighted a possibility of a single method that can be adopted for the remediation of acid mine drainage of the three basins to recover aragonite calcium carbonate, an industrially valuable mineral.