In addition to the particles and deionized water, an acid is required for leaching. Hydrochloric acid 30% technical (HCl) from VWR International GmbH (Darmstadt, Germany), sulphuric acid 96% technical (H2SO4) from Carl Roth GmbH + Co. KG (Karlsruhe, Germany) and citric acid (C6H8O7) from Carl Roth GmbH + Co. KG (Karlsruhe, Germany) are used. The three acids are added in different stoichiometric ratios (𝑆𝑡). The ratio is defined by the following equation with the amount of used acid in the experiments (3):