The commercially available humic acid powder with molecular
weights ranging from 2000 to 50,000, extracted from brown
coal using NaOH (Aldrich Chemical Co., Inc., USA), was used.
A stock solution was prepared by dissolving 1.4 g humic acid
to 1 L 0.01 mol/L NaClO4 solution. After adjusting pH to 8.5,
the solution was continuously mixed for 6 h until all solids disappeared.
The final concentration of the prepared stock solution
was 440 mg/L based on total organic carbon (DOC) analysis
with a Tekmar-Dohrmann Apollo 9000 TOC analyzer