Hydrogeochemical and mineralogical study of the ecological treatment of phosphogypsum leachates

Abstract

The production of phosphoric acid in the phosphate fertilizer industry, via the wet chemical attack of phosphate rock with sulfuric acid, generates an unwanted by-product known as phosphogypsum. This waste is mainly composed of gypsum (CaSO4·2H2O) and contains high amounts of metal(loid)s, radionuclides and a residual fraction of free acids. Despite the high content of toxic impurities, most of phosphogypsum wastes generated around the world are stored in stacks close to coastal areas and exposed to continuous weathering processes. Thus, approximately 100 million tons of phosphogypsum wastes were stockpiled onto salt-marsh soils of the Tinto River estuary (Huelva, SW Spain). This phosphogypsum stack is a continuous source of pollution to the estuarine environment due to the existence of two types of highly acidic and polluted waters; on the one hand, the process water stored in surface ponds on the stack, which was used to transport the waste as a slurry from the industry, and on the other hand, the polluted groundwater contained in the stack that is drained outside through numerous sources of contamination, known as edge outflows. The main objective of this research work is to develop an effective treatment system for acidic and polluted effluents from the phosphogypsum stack, since it is assumed to be the only viable option to minimize its impact on the surrounding environment. Firstly, edge outflows were sampled around the whole perimeter of the phosphogypsum stack during four sampling campaigns under different hydrological conditions, in order to study their seasonal and spatial hydrochemical variability. In addition, water-table variations within the phosphogypsum stack were analyzed, in order to know the hydrological response of the system to the different weathering agents. For this purpose, a CTD-Diver was installed in a bore-hole within the profile of the phosphogypsum pile. In periods with absence of rains, the water-table of the stack remained almost constant most of the time (± 2 cm) and only oscillated with the tidal fluctuations. Nevertheless, during the rainy events, the water-table level of the pile increased up to 20 cm and subsequently decreased, defining peaks that coincided with rainfalls. The hydraulic response of the phosphogypsum stack, similar to that of a coastal karst aquifer, and the geochemical characteristics of the leachates pointed to an estuarine origin for the edge outflows. With respect to the hydrochemical behavior of edge leachates, the concentration of most contaminants (e.g., PO4, F, Al, Zn, U or Cd) suffered a slight decrease from the dry-warm to the rainy period, due to a dilution effect by rainwater recharge. These These leachates discharge high loads of pollutants to the estuary, e.g., PO4, F, As and U (average values of 5000 tons/yr, 300 tons/yr, 6.9 tons/yr and 3.0 tons/yr, respectively). Secondly, a titration experiment was performed to assess the feasibility of a potential alkaline treatment system for phosphogypsum leachates. The experiment was based on the addition of a Ca(OH)2 solution to different types of phosphogypsum-related acidic leachates. The alkaline addition provoked an efficient removal of most dissolved pollutants. In fact, removal values close to 100% were achieved for PO4, Fe, Al, Cr, Zn Cd, and U. However, it was not so effective for As (removal of 57–82%) due probably to competition between As (in the form of oxyanion) and anionic P species for the same binding sites in the newly-formed solids. The removal of elements in solution occurred by co-precipitation and/or adsorption with phosphate phases, as well as by fluoride precipitation. The newly-formed precipitates during the treatment were subjected to two standardized leaching tests (EN 12457-2 from the European Union and TCLP from the United States) for their classification and management according to their hazardousness. In this sense, some of the solids precipitated during the treatment should be classified as hazardous wastes due to the high concentrations of As released. Thirdly, column experiments were performed in the laboratory to simulate the passive treatment of edge outflows using the dispersed alkaline substrate (DAS) technology. Passive treatment was chosen given the orphan site condition of some zones of the phosphogypsum stack. The experiment consisted of flowing the acid leachates through columns loaded with a mixture of an alkaline reagent (i.e., limestone, barium carbonate (whiterite, BaCO3), biomass ash, fly ash, MgO, Mg(OH)2 or Ca(OH)2) and an inert matrix. The Ca(OH)2-DAS and MgO-DAS treatment systems were the most effective, achieving near total removal for PO4, F, Fe, Zn, Cu, Al, Cr, and U. However, the total removal of As was only reached in the Ca(OH)2-DAS treatment. The removal of contaminants was mainly due to the precipitation of phosphate minerals. Once the high effectiveness of the MgO-DAS and Ca(OH)2-DAS treatment systems was demonstrated, these treatments were replicated under equal conditions in order to collect solid samples for their mineralogical and geochemical characterization, as well as to evaluate their potential environmental risk. These DAS treatments generated significant amounts of metal-rich wastes. Therefore, the hazardousness of these solids must be evaluated prior to their disposal in a landfill, in order to avoid potential environmental impacts. To this end, the DAS wastes were subjected to the European Union EN 12457-2 and United State TCLP leaching tests. According to European Union legislation, some of these solids should be classified as hazardous wastes due to the release of high concentrations of SO4 or Sb. However, according to United States regulation, all DAS wastes are considered as non-hazardous material. In addition, the solid wastes from DAS treatment systems could be considered a secondary source of critical raw materials.Finally, batch experiments were carried out in the laboratory to simulate the treatment of phosphogypsum-related leachates by adding an alkaline industrial waste (biomass ash), in order to design a combined system of sustainable treatment and metal recovery. These experiments consisted of batch reactions between biomass ash and phosphogypsum leachates at different solid-liquid (S:L) ratios (i.e., 1:2.5, 1:5, and 1:10). The alkaline treatment at a S:L ratio of 1:2.5 showed a high success in the elimination of contaminants, reaching removal values close to 100% for F, Fe, Zn, Al, Cr, U, Cu and Cd. These results are comparable to those achieved during the treatment with a conventional reactive (i.e., Ca(OH)2). The precipitation of phosphate and fluoride phases was the main mechanism responsible for the depletion of pollutants during the treatments. In addition, the solids precipitated during the treatments contained high concentration of elements of high economic interest such as rare earth elements plus Y (REY), Sc, Be, V, Ga or U, with concentrations of up to 3992 mg/kg, 164 mg/kg, 7.0 mg/kg, 2974 mg/kg, 40 mg/kg and 2963 mg/kg, respectively. The potential recovery of these valuable elements contained in the newly-formed solids could help to offset the costs associated with the passive treatment