Trace Cd, Co, and Pb elements distribution during Sulcis coal pyrolysis: GFAAS determination with slurry sampling technique

Abstract

A simple and fast method based on graphite furnace atomic absorption spectrometry (GF AAS) and slurry sampling technique (SlS) was developed to determine trace Cd, Co and Pb in high-sulphur coal (Sulcis, Italy) and coal chars derived at 600, 750 and 950°C under N2 atmosphere for developing a clean coal for electricity production. The proposed method was then coupled to a four-step sequential chemical extraction method for assessment of metals distribution in coaled samples. The slurries were prepared by varying sample mass (1–50mg), volume (1–3mL) and kind of dispersing medium (1% v/v Triton X-100 and 2N HNO3), and sonication time (5–30min). Pyrolysis/atomization temperatures as well carrier gas flow rate were optimised. Pd(NO3)2 and NH4H2PO4 were employed to stabilize Cd and Pb, respectively, in the pyrolysis stage of furnace program. The use of HNO3 as dispersing agent was found to be effective in lowering the high level of background absorption on the Cd analytical signal produced by raw coal matrix. Conversely, coal charred samples did not show significantly level of background interferences, so that Triton X-100 dispersing agent could be used for all analytes. Calibration curve against acid-matched standards was allowed for Cd, whereas the standard addition calibration was used for Co and Pb owing to chemical matrix interferences. The precision, expressed as relative standard deviation (% RSD, n=5), was better than 5% for Cd, Co, and Pb at 1, 10, and 15μgL−1 levels, respectively. The accuracy of the analytical method was checked by analyzing a BCR No. 182 steam coal certificated reference material and the results were in good agreement with certificated and informed values. The solid detection limits (3σblank) were as low as 0.001 Cd, 0.01 Co, and 0.01 Pb mgkg−1, using 30mg sample mass and slurry concentration of 30m v−1 for Cd, and 50mg sample mass and 50m v−1 slurry concentration for Co and Pb. The content of elements in Sulcis coal was found to be 0.33 Cd, 4.0 Co, and 3.8 Pb mgkg−1 and mostly associated to sulphates and di-sulphides as indicated by the leaching test. Under pyrolysis conditions Cd significantly volatilised (about 64%) at temperature higher than 600°C, whereas residue chars at 950°C are enriched in Co and Pb up to 20%. The proposed method is suitable for routine metals monitoring in coaled samples.