Application of Zn–Fe layered double hydroxide and its composites with biochar and carbon nanotubes to the adsorption of lead in a batch system: kinetics and isotherms

Abstract

The adsorption of Pb2+ onto zinc–iron layered double hydroxide (ZnFe–LDH) and its composites coupled with single-walled carbon nanotubes (ZnFe–CNTs) and biochar, which had been obtained from orange peel waste biomass (ZnFe–OPb), was studied in detail. The successful formations of Zn2+- and Fe3+-LDH were investigated by scanning electron microscopy. Moreover, energy-dispersive X-ray spectroscopy revealed that Pb2+ was adsorbed successfully on ZnFe-LDH. Further, both ZnFe-LDH and ZnFe–CNTs exhibited ~ 60% removal efficiencies for Pb2+ at 20 mgL–1 with rapid adsorption at an initial contact time of 30 min, while the equilibrium was achieved at ~ 60 min with a 95% removal efficiency with ZnFe–OPb. Furthermore, the pseudo-second-order kinetics demonstrated the best fit, thus supporting the chemisorption essence of the adsorption mechanism. ZnFe–OPb demonstrated a twofold increase in the adsorption efficiency compared with simple LDH or ZnFe–CNTs, as the initial Pb2+ concentration was enhanced from 10 to 100 mgL–1. The percentage removal and adsorption capacity increased almost linearly as the pH of the solution changed from 2 to 5, suggesting that the optimum pH value was approximately 5 or 6 for all adsorbents. The percentage removal of Pb2+ also increased on increasing the dose. Additionally, the optimum removal efficiency of 99% was obtained with 0.7 g of ZnFe–OPb. The Langmuir model corresponds best with the adsorption data, while the Sips isotherm indicated that ZnFe-LDH exhibited the highest degree of heterogeneity compared with the other adsorbents in this study.