Into each triplicate centrifuge tubes containing 25 ml of the spinach and mustard green, collard green, and bitter leaf supernatants, respectively was added 25 ml of the lead nitrate solution (1300 ppm) prepared above. The twelve tubes and their contents along with triplicate tubes containing the 1300 ppm lead solution and its two-fold diluted solutions were vortexed, tightly secured on the rack of a heavy duty Eberbach 6000 shaker, and agitated for 24 hours at room temperature.