Adsorption of Cd2+ and Pb2+ by biofuel ash-based geopolymer synthesized by one-step hydrothermal method

Abstract

A hydrothermal alkali modification method was used to produce geopolymer (BFA-GP) from biofuel ash (BFA). The product was characterized by scanning electron microscopy with energy dispersive spectrometry (SEM-EDS), transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared reflection (FTIR), and X-ray photoelectron spectroscopy (XPS). Significant amount of geopolymer and gismondine was produced by this modification. The surface area increased from 20.41 m2 g−1 to 56.63 m2 g−1. The adsorption capacity of BFA-GP can reach 29.92 and 137.49 mg g−1 for Cd2+ and Pb2+, respectively, in a pure solution at 298 K, which is about triple of that by the original BFA. Competitive adsorption of Cd2+ and Pb2+ showed that the binding affinity is Pb2+> Cd2+. Chemical sorption including electrostatic attraction, chelate reaction, and ion-exchange is the dominant mechanism of heavy metal adsorption on BFA-GP. And the adsorption thermodynamics indicates that adsorption reaction of heavy metal ions by BFA-GP is spontaneous and endothermic. This modification provided a cost-effective and environmentally friendly way to change BFA to adsorbent for heavy metals with promising application prospects.