For heavy metal analysis, 0.05g of sieved soil was weighed and 3 acids were added for digestion: 0.5ml nitric acid, 2.5 ml hydrofluoric acid and 0.5 ml perchloric acid. Digestion was carried out in an ultrasonic bath at 50°C for 2 hours. The samples were then dried on a hotplate at 150°C.