Sc 144

The <75 μm fractions in triplicate of each sample were homogenized, lyophilized and grounded in porcelain crucibles. Aliquots of ca. 0.26 g were weighed (0.1 mg precision) in porous, previously weighed, crucibles. In order to eliminate the carbonate fraction, HCl volumes (1:1 v/v) sufficient to cover the sample were added to the crucibles for 24 h. Before the acid reaction, the sediment residues were rinsed with distilled water until complete elimination of HCl (to pH ~ 6). Sample residues were dried at 65°C for 3 h and weighed in order to calculate the percentage of insoluble residue. TOC and TS measurements were performed with a carbon and sulphur analyzer (LECO SC 144) according to the procedure described by Mendonça-Filho et al. [34 ] and following the ASTM D 4239 [35 ]. The TOC/TS ratio, commonly referred to as C/S, was determined for each station.

A total elemental composition analysis was conducted by acid digestion according to ASTM D5198-09, for which 5 g of the sample was digested at 90–95 °C for 2 hours with a 1:1 nitric acid digestion. The elemental compositions after digestion were measured via inductively coupled plasma-optical emission spectrometry (ICP-OES, Vista-MPX CCD Simultaneous ICP-OES, Varian Inc., CA, USA) and inductively coupled plasma-mass spectrometry (ICP-MS, Agilent 7700x ICP-MS, Agilent Technologies Inc., CA, USA). A carbon analyzer (SC144 DR LECO Inc., St. Joseph, MO, USA) was used to determine the total carbon (TC), total inorganic carbon (TIC), and total organic carbon (TOC).
Table 3 summarizes the total elemental compositions and carbon content in the red mud samples. Concentrations of major elements (mass ratio >1% wt./wt.) are reported in mass percentage (%wt./wt.), while concentrations of trace elements (<1%) are reported in µg/g. As (28.7–203 µg/g), Pb (43.7–132 µg/g), and Cr (480–1370 µg/g) were the most abundant trace elements in the samples. In addition, the total carbon contents in the combined process red mud (i.e., HN-A-C, TC = 1.6%) were higher than the red mud from the Bayer process (TC = 0.7–1.3%), which may result from the addition of the lime in limestone during the combined process [1 (link)].