Towards circular and sustainable restoration of a deeply polluted river basin: The Odiel River catchment (SW Spain)

The Ría de Huelva estuary located in SW Spain is heavily polluted by acid mine drainage leachates and phosphate fertilizer industry effluents. This study assesses the effectiveness of a solid waste rich in calcite originated during the remediation of industrial effluents using the passive dispersed alkaline substrate (DAS) technology, for the treatment of highly acid and polluted mine drainages. The research consists of flowing the mine leachates through a column loaded with a combination of an alkaline reagent (i.e., calcite-rich waste resulting from the treatment of industrial leachates) scattered in a non-reactive matrix (i.e., wood chips) to increase the pH of acidic water while decreasing the solubility of dissolved pollutants. The alkaline treatment achieved average removal percentages of 100 % for Al and Fe, and close to 90 % for Cu. However, the treatment was not effective for other metals present in mine drainages such as Co and Ni. These results are comparable to those achieved in the DAS treatment with commercial limestone. The precipitation of oxyhydroxysulfates (i.e., schwertmannite and basaluminite) and carbonates (i.e., malachite and hydrozincite) minerals could be controlling the solubility of pollutants during the treatment. The solid wastes from the DAS treatment were subjected to two standardized leaching tests (EN 12457-2 from the European Union and toxicity characteristic leaching procedure (TCLP) from the United States) in order to ensure suitable management and avoid potential environmental impacts. According to European Union legislation, the high mobility of Cu, Zn and SO₄ confers some solid wastes from the DAS treatment the hazardous waste classification. However, according to United States regulation, these same solids are considered non-hazardous wastes. This research could help to improve the environmental sustainability of acid mine drainage treatment with DAS technology by replacing marketable limestone with a low-cost alkaline waste.