2.4. Digestion procedure
The soil samples were dried at 110 C for 3 h, ground to pass
through a 2 mm mesh sieve and homogenized for analysis. A
procedure recommended by environmental protection agency
(EPA 3050B) was used as a conventional extraction method. 1 g of
soil sample was heated to 95 C with 10 mL of 50% HNO3 without
boiling. After cooling the sample, it was refluxed with repeated
addition of 65% HNO3 until no brown fumes were given off by the
sample. Then the solution was allowed to evaporate until the
volume was reduced to 5 mL. After cooling, 10 mL of 30% H2O2 was
added, slowly without allowing any losses. The mixture was
refluxed with 37% HCl at 95 C for 15 min. Distilled water was
added and filtered. A clear solution was used for AAS measurement
after dilution to 50 mL. The total extraction procedure was lasted
for 100–200 min.