Epsilon 4

The concentrations of Pb, Cu, Cd, As, Hg, Cr⁶⁺, and Cl in the eluted solution of the original sample and in the treated samples were investigated in regard to the recycling possibility of the incineration ash. The concentrations of Pb, Cu, Cd, and As were measured via ICP–OES analysis. The concentrations of Hg and Cr⁶⁺ were measured using a UV–visible light spectroscopy system. The concentration of Cl was also investigated even though it is not a heavy metal, because the Cl concentration must be controlled as per the waste management laws of Korea. The concentration of Cl was calculated using ion chromatography (ICS-2000, Dionex, USA). In addition, to assess the change in the composition of the incineration ash sample before and after heavy metal removal, measurements were performed using an X-ray fluorescence spectrometer (XRF, EPSILON 4, PANalytical, NLD). Before the XRF analysis, the incineration ash samples were calcined at 700 °C to remove organic matter and achieve thermal stability.

Samples were air-dried at room temperature, then roots and impurities were removed and ground to <200 mesh using a vibrating grinding method. A total of 5 g of ground sample was weighed, and a boric acid fixed concentric cake specimen was made using a press with a pressure of 25 t and a holding time of 30 s, and the sample number was marked on the back. The compressed samples were measured using a PANalytical Epsilon 4 benchtop energy dispersive X-ray fluorescence spectrometer from PANalytical, The Netherlands. The major elements are given as percent oxides, and the trace elements are in ppm. The chemical index of alteration (CIA) value calculation formula [30 (link)] and the correction formula of CaO* [30 (link),31 (link)]) are defined as: $$\mathrm{CIA}=\frac{{\mathrm{Al}}_2{\mathrm{O}}_3}{{\mathrm{Al}}_2{\mathrm{O}}_3+{\mathrm{Na}}_2\mathrm{O}+{\mathrm{CaO}}^{*}+{\mathrm{K}}_2\mathrm{O}}\times 100$$
$${\mathrm{CaO}}^{*}=\mathrm{CaO}-\left(\frac{10}{3}\times {\mathrm{P}}_2{\mathrm{O}}_3\right)$$
In these two equations, CaO* is the content of CaO stored in silicate minerals, excluding the content of CaO in carbonate and phosphate [30 (link)]. Due to CaO and Na₂O in silicates usually existing in a 1:1 ratio, McLennan concluded that mCaO* = mNa₂O when the molarity of CaO was greater than Na₂O, and mCaO* = mCaO when it was less than or equal to Na₂O. All mCaO* values in the paper are obtained according to this method [31 (link)].

An Epsilon 4 tabletop XRF spectrometer (PANalytical, Malvern Instruments, Spectris Plc., London, UK), equipped with an Ag anode X-ray tube source and a silicon drift detector with an energy resolution of ~135 eV (Mn Kα), was used to perform the XRF measurements. As in our previous study, copper with a thickness of 300 µm was selected as the primary X-ray filter [17 (link),25 (link)], and incident X-rays with diameter ranges of ~15 mm were used for irradiation. As the GO collection membrane filter was folded into a square with a side length of 13 mm, the majority of the membrane, with the exception of its four corners, was irradiated by the incident X-rays. A cylindrical stainless-steel weight was placed on the thin sample to prevent the sample from floating. The output power of the X-ray tube was 15 W, and the tube voltage and current were set to 50 kV and 0.3 mA, respectively. The X-ray accumulation time for each spectral measurement was 300 s. However, to obtain statistically relevant results and, therefore, to determine the optimal solid/liquid ratio and obtain the fitting parameters for the individual elements, an accumulation time of 3600 s was employed.