The optimisation of microwave digestion procedures and application to an evaluation of potentially toxic element contamination on a former industrial site

The optimization of a microwave dissolution procedure for potentially toxic elements in a contaminated soil sample has been evaluated using a central composite design approach. By considering the operating parameters of temperature, digestion time, microwave power and acid volume it was possible to identify the following optimum conditions: temperature, 160 °C; power, 750 W; digestion time, 25 min; and, an aqua regia volume of 13 mL. These microwave digestion conditions were then applied to 19 samples obtained from a former industrial site in Newcastle upon Tyne. Of the range of potentially toxic elements present at the site as a consequence of former industrial activity (As, Cd, Cr, Cu, Mo, Ni, Pb and Zn), the majority of topsoil samples indicated elevated concentrations of one or more of these elements. In particular, data obtained using either inductively coupled plasma mass spectrometry or flame atomic absorption spectroscopy indicates the high and wide concentration of Pb on the site (188–60300 mg/kg). Comparing the resulting potentially toxic elements data with UK soil guidelines values (SGVs) suggests at least parts of the site represent areas of potential human health risk. It was found that lead soil values exceeded the SGV on 17 out of the 19 sampling sites; similarly for As 6 out of 19 sampling sites exceeded the SGV. While for Cd, Cr and Ni the soil levels were below the stated SGVs.