Adsorption Behavior of Heavy Metal Ions from Aqueous Solution by Soy Protein Hollow Microspheres

Heavy metals have become ecotoxicological hazards owing to their tendency to not degrade but accumulate in the vital organs of biological bodies. Biosorption is now an efficient method to purify industrial wastewater containing toxic heavy metal ions by using biomass as sorbents. In this work, mimicking the fabricating process of “Tofu”, soy protein was heat-denatured and transformed into soy protein hollow microspheres (SPMs) with a diameter of about 4–45 μm, which were then used as biosorbents to remove heavy metal ions in the water system. The trace amount of remaining metal ions was tested by atomic absorption spectroscopy, and the sorption kinetic and isotherm models were calculated and set up to describe the adsorption behavior. The results showed adsorption capacities of Zn(II), Cr(III), Cd(II), Cu(II), Pb(II), and Ni(II) by SPMs at 70 °C of 254.95, 52.94, 120.83, 115.01, 235.56, and 177.11 mg/g, respectively, which are much higher than that of many other natural polymeric sorbents. The pseudofirst-order kinetic model and Freundlich isotherm model were well correlated to the experimental data. Overall, SPMs were efficient sorbents for binding heavy metal ions, and their sorption capacities were dependent on factors like denaturation content, temperature, time, pH, and initial ionic concentration.