About 2.0 g portion of dried sediment was taken in 100 mL beaker and 15 mL of concentrated HNO3 was added. The content was heated at 130 °C for 5 h until 2–3 mL remaining in the beaker. After digestion materials were passed through Whatman no. 41 filter paper, washed with 0.1 M HNO3 solution and made up to 100 mL volume with deionized water.