Enhanced Performance of the Composite of FeMgAl-Layered Double Hydroxide and Rice Husk Biochar to Adsorb Heavy Metal Ions of Lead and Cadmium in Aqueous Solutions

Abstract

The layered double hydroxide (LDH) of FeMgAl and its composite with the rice husk biochar (FeMgAl/RHB) was tried for the first time for removing heavy metal ions of lead (Pb2+) and cadmium (Cd2+) in batch tests. Scanning electron microscopy and Fourier transform infrared spectroscopy demonstrated that FeMgAl/LDH was successfully synthesized and integrated into RHB. Furthermore, the presence of Cd2+ and Pb2+ was confirmed by energy-dispersive X-ray spectroscopy, exhibiting an effective adsorption process. The equilibrium contact time was estimated at 15 and 60 min, for Pb2+ and Cd2+, respectively, with high removal efficiencies and adsorption capacities upon immediate contact. Within the observed range (0–100 mg·L−1) of initial metal concentrations, the removal efficiencies decreased constantly using both adsorbents, while 50 or 60 mg·L−1 of Pb2+ and Cd2+, respectively, was estimated as the optimum values due to a steady increase in the adsorption capacities with composite adsorbent presenting a better efficiency, twice of the metal uptake capacity compared with FeMgAl/LDH. The lowest used amount of each adsorbent (0.05 g) exhibited highest adsorption capacity which decreased with increasing amount of adsorbent (up to 0.8 g), whereas the removal efficiencies increased for both heavy metal ions. A solution of pH value 0.7–0.8 was estimated as an optimum value with a uniform increasing pattern of adsorption capacities and percentage removal of both heavy metal ions using the FeMgAl/LDH as well as the composite adsorbent. A perfect fitting of the adsorption data was seen using the pseudo-second-order kinetic model, suggesting the chemical sorption to be the control mechanism of adsorption. The fitting of the Langmuir and Temkin isotherm models proposed a monolayer as well as heterogeneous adsorption system. The same was further confirmed based on the reasonable fit of both (Sips and Redlich–Peterson) three-parameter isotherms in addition to a physical adsorption system, as suggested by the Dubinin–Radushkevich isotherm model.