Fractal-like concepts for evaluation of toxic metals adsorption efficiency of feldspar-biomass composites

Abstract

Effective treatment of metals polluted water may be achieved using low-cost materials synergistically modified to obtain composite adsorbents. Thus, feldspar–biomass composites have been prepared via calcination of feldspar (FLC) and Carica papaya seeds or pine cone seeds to yield Carica papaya-FLC (CPF) and pine-cone-FLC (PCF) composite adsorbents, respectively. The composites physicochemical properties and effectiveness for Pb(II), Cd(II) and Cu(II) removal in aqueous solutions were examined. Results showed that calcination did not promote any structural modification in the basic FLC lattice. Though CPF and PCF exhibited lower specific surface area when compared with the FLC, their cation exchange capacities were enhanced. Unlike on the FLC, the metals adsorption on CPF and PCF were higher (>40%) for Cu(II) and Cd(II), pH independent and faster. The study showed that adsorption process on a system with varying types of surface sites and with varying affinity for an adsorbent can best be described by the fractal-like kinetic and equilibrium adsorption isotherm models (fractal pseudo-second order kinetic model and fractal Brouers–Sotolongo adsorption model) compared with the classical models (pseudo-first order and pseudo-second order kinetic models, Langmuir and Freundlich adsorption isotherm models). These adsorption processes were thermodynamically spontaneous, feasible and endothermic; however the modification significantly reduced their energy requirements. The Brouers–Sotolongo model showed that modification of FLC increased the width of the sorption energy distribution on the composites indicating the heterogeneity of the surface adsorption sites. Though no single classical theory of sorption can presently be put forward to explain the overall adsorptive process, metals adsorption on these feldspar-biomass composites are heterogeneous and complex processes with fractal architecture.