Effect of Extracting-steam Explosion Modification on Lead Biosorption of Macadamia Nutshells

In this study, the effects of types and concentrations of presoaking agents, steam pressure, retention time, and post-extraction on the Pb(II) adsorption onto steam-exploded macadamia nutshells were systematically studied by a single-factor experiment combined with response surface methodology. The modification process was optimized, and the changes in the physical and chemical properties of the modified shells were characterized by scanning electron microscopy and infrared spectroscopy. The result proved that process parameters of steam explosion modification such as types and concentrations of presoaking agents, steam pressure, retention time and post-extraction had significant effects on the adsorption of macadamia nutshell for Pb(II) in water. HCl and NaOH solutions were optimized as the presoaking agent and the post-extractant, respectively. The optimum modification process was the HCl concentration of 0.49 mol/L, steam pressure of 2.29 MPa, and retention time of 8.4 min. The modification makes the surface of macadamia nutshells rough and porous, increased the specific surface area and the physical space for Pb(II) adsorption. The chemical bonds between lignin and carbohydrates were destroyed by the modification. The lignin was partly removed during the post-extraction. Some hydroxyl groups were combined with Na⁺, and ion exchange occurred between Pb²⁺ and Na⁺ during the adsorption process. The Pb(II) adsorption capacity of the modified macadamia nutshells was 4.6 times much as that of unmodified ones. A suitable SE process can greatly improve the biosorption performance of agricultural waste.