1 mL of supernatant was taken, filtered using cellulose acetate syringe filters with a pore size of 0.20 µm, and mixed with 9 mL of 0.05 N NaOH solution. The mixed solution was placed in a water bath at 50 °C for 15 min. Afterward, it was cooled with running water to room temperature and kept in room temperature water for 15 min to obtain the derivative artemisinin (artemisinin-alkali reaction) to be measured at an absorption peak of 291 nm . The exact amount of sample was taken for the adsorption process, but it was mixed with 5 mL of 0.05 N NaOH and 2 mL of ethanol . The same procedure described above was carried out for detection in the spectrophotometer.