The humic acid (HA) used is a commercial product provided by
Acros Company (USA). These compounds are in the form of a brown
powder. A stock solution of 1 g/L was prepared by dissolving 1 g of HA
in 62.5 mL of NaOH (2 N), then made up to 1 L with d-H2O. This solution
was stirred for 24 h and stored in the dark at 4 °C.