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The soil samples were stored in plastic bags for transportation and storage, and were air dried at a constant temperature of 50 degrees C for 3 days in a thermostatically controlled oven. They were subsequently gently disaggregated in a porcelain mortar and sieved to < 2 mm fraction, using a nylon screen to remove coarse material. Each soil sample was further sieved through a nylon 100-μm sieve in order to focus on geochemically reactive particles and stored at room temperature in a dark storeroom. The < 100 μm fraction was used for chemical analyses. All utensils were thoroughly cleaned between the samples to avoid cross contamination.
The total pool of Cu, Zn, Pb, Sn, Sb, Cd, Ni, Cr, Co, As and Mn in soil was determined by a 4-acid (HNO3, HClO4, HF, HCl) digestion followed by inductively coupled plasma mass spectrometry (ICP-MS) at the accredited ACME analytical laboratories in Vancouver, Canada. Geochemical results for the major elements Fe, Al, K, and Ca were also measured by ICP-MS and included to assist in the source apportionment of metals. The pH of each soil sample was measured in a soil to deionized water suspension of 1:2.5 (w/v).
In order to assess the environmental availability of potentially harmful elements in urban soil the ‘pseudototal’ content was determined in 45 selected samples using ICP-MS after digestion by aqua regia, and was used to establish the availability and reactivity ratio, i.e. per cent availability and reactivity. The availability of PHEs was assessed by applying the widely adopted extraction methods of ethylenediamine tetraacetic acid (EDTA), acetic acid (HAc) and physiologically based extraction test (PBET). The metal fractions obtained by these analytical procedures were operationally defined as potentially phytoavailable, mobilizable and human bioaccessible, respectively. The soil reactive content of PHEs was determined after extraction with dilute nitric acid (0.43 N).
