The main parameters analysed during the experiments were iron (total and soluble) and cyanide (total, soluble, total free and soluble free). For soluble iron and cyanide measurements the samples were filtered immediately after collection to prevent any possible re-dissolution of solids. Filtration of samples was performed using a 0.45 μm cellulose acetate filter membrane as indicated in [6] and [27] for soluble iron determination. Results have an analytical accuracy of 0.05 mg/l for iron and 0.1 mg/l for cyanide determinations.
Iron concentration was determined by flame atomic absorption spectrometry using a Hitachi Z-5000 spectrometer. To analyse soluble iron, samples were collected, filtered, and acidified with nitric acid (HNO3) to ensure a pH < 2. For total iron determination, the procedure was similar but the samples were not filtered. In both cases, the acidic digestion suggested in [6] and [27] was omitted because the samples only contained simple iron compounds, available for re-dissolution at pH < 2. This fact was experimentally confirmed.
To analyse iron by atomic absorption spectrometry in samples containing cyanide, they must be adjusted to a very low pH, which involves the release of dangerous hydrogen cyanide gas. In those samples, cyanide was previously removed from the samples to prevent that risk by a quantitative distillation procedure [6]. Iron was measured from the remaining solution that was obtained with the distillation used to quantify cyanide concentration. All samples were measured using the standard addition method to reduce matrix interferences. For soluble iron determinations, the samples were filtered before distillation.
Total free cyanide was measured directly in the alkaline sample using an ion selective electrode (ISE, Thermo Orion 9606BN) according to the method suggested in [6]. Soluble free cyanide was measured by the ISE method after filtration. Additional preservation was not required because the pH was >11 [6]. The lack of ISE response to complexed cyanide forms was experimentally verified [25]. Total cyanide was measured by the ISE method after the quantitative distillation procedure, already mentioned [6]. When measuring soluble cyanide, samples were filtered prior to distillation.
Fixed bed material was characterized by XRD (Shimadzu XD-D1).