First, 100 mg of the macroalgae or biochar was placed in a Teflon digestion vessel with 3.0 ml double distilled HNO₃ and 1.0 ml analytical grade H₂O₂. The solution was digested for 2 h at room temperature then heated in a microwave to 180 °C for 10 min, then diluted with Milli-Q water for analysis.