Adsorption of RhB dye on soy protein isolate-based double network spheres: Compromise between the removal efficiency and the mechanical strength

Abstract

Soy protein isolate-based double network spheres (SPIDNS) were designed and adopted for the first time for Rhodamine B (RhB) dye removal from wastewater. The SPIDNS were prepared through the aziridine crosslinker and the Ca2+ ionic double crosslinking method with major crosslinking agents of soy protein isolate and sodium alginate, in which aziridine crosslinker was also employed as a modifier to regulate the mechanical strength. The physicochemical properties of the materials were explored by numerous technologies, including SEM NMR, FT-IR, and XPS, which confirmed the chemical and physical crosslinked networks of SPIDNS being established after aziridine crosslinker and Ca2+ ionic crosslinking process. Adsorption performances for RhB removal were investigated under various conditions by using SPIDNS with different dosages of aziridine crosslinker for 0%, 1.8%, 3.6%, 5.3%, 6.9%, and 8.5%. Experimental results demonstrated that the mechanical stability of the adsorbent was enhanced with increasing the amount of aziridine crosslinker from 0% to 8.5%; whereas, the adsorption capacity decreased from 190 to 120 mg/g. Therefore, a compromise between the removal efficiency and the mechanical strength should be taken into account. Studies of adsorption kinetics, isotherms, and thermodynamics displayed that the spontaneous RhB adsorption on SPIDNS was well described by a pseudo-second-order kinetic model and Langmuir isotherm model with monolayer adsorption behavior, for which both the correlation coefficients R2 were higher than 0.99. Moreover, electrostatic interactions between the positively charged RhB molecules and the negatively charged SPIDNS were proposed as the principal adsorption mechanism, which was confirmed by Zeta potential analysis. The good mechanical stability and spherical porous structure make the SPIDNS with reliability and potential for practical application.