Abstract
This work describes a greener and cost-effective microwave-assisted hydrogen peroxide digestion (MA-HPD) with the addition of 1 mL of HNO3 (70% v/v) to enhance extraction of selected metals (Al, Ba, Cd, Co, Cr, Cu, Mg, Na, Ni, Pb, Sb, Ti and V) in crude-oil, diesel, gasoline and kerosene samples prior to inductively coupled plasma-optical emission spectroscopic (ICP-OES) analysis. The most influential parameters of the MA-HPD method were investigated by using multivariate optimization tools (two-level full factorial and central composite designs) and fuel oil certified reference material (NIST1634c). The optimum conditions were observed to be 245 ℃ microwave temperature, 25 min digestion time, 0.1 g sample mass and 5 M H2O2 were the optimum digestion conditions with accepted accuracy (104.8-117.7%) and precision (≤ 4.1%). In overall, the metals that reported high concentrations in the crude oil, diesel, gasoline, and kerosene samples were Na (51.94-58.86 mg/kg) and Mg (36.08-47.4 mg/kg), while Cu was the lowest (0.55-2.89 mg/kg). When comparing the obtained concentration levels with other literature reports, a conclusion can be drawn that South Africa is importing oils of reasonable quality.