A method was developed to enhance metal ion adsorption of soybean hulls for wastewater treatment using copper ion (Cu2+) as a typical metal ion. Hulls, extracted with 0.1 N NaOH, were modified with different citric acid (CA) concentrations (0.1-1.2 M) at 120 degrees C for 90 min. CA-modified hulls had adsorption capacities for Cu2+ from 0.68 to 2.44 mmoles/g, which was much higher than for unmodified hulls (0.39 mmoles/g). The total negative charge for these hulls also increased with increasing CA concentration and was about twice the copper ion adsorption capacity at all CA concentrations. The need for NaOH (base) extraction (BE) before CA modification was examined. CA-modified, non-extracted (NE) and CA-modified, BE hulls were compared for adsorption kinetics and adsorption capacity. Base extraction resulted in modified hulls with faster adsorption kinetics and slightly lower adsorption capacity for copper ion than NE hulls. For BE, CA-modified hulls, increasing the temperature from 25 degrees C to 60 degrees C appeared to have no effect on the rate of copper ion removal from solution. CA modification of soybean hulls greatly enhanced metal ion removal and resulted in a product with possible commercial potential for metal ion remediation.