Low-cost modified adsorbents derived from the solid residue of Rosmarinus officinalis L. for heavy metal uptake

Abstract

The current study investigates for the first time the potential of using solid residues collected from the hydrodistillation of R. officinalis, after their modification with Fe, as low-cost adsorbents to alleviate the heavy metals in aqueous solutions. Physicochemical properties and adsorptive performances of R. officinalis solid residue (RO-SR) and Fe-modified R. officinalis solid residue (Fe-RO-SR) were investigated. The Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy (EDS) revealed that the iron was successfully fixed to the surface of RO-SR. Furthermore, after iron modification, the specific surface area of ROSR was increased from 36.2 to 50.4 m2/g. Kinetics and isotherms of adsorption of Co onto RO-SR and Fe-RO-SR were fitted by applying different models, though the pseudo-second-order (Adj. R2 > 0.9985) and Langmuir (Adj. R2 > 0.9532) models fitted the experimental results better, respectively. The adsorption performances of RO-SR and Fe-RO-SR for Co were 37.31 and 50.25 mg.g−1, respectively. The equilibrium adsorption capacity was reduced from 22.56 g/L to 10.67 g/L when the temperature was raised from 25 °C to 45 °C. Furthermore, after four successful cycles of adsorption-desorption, the adsorbent's adsorptive performance remained above 83.04%, and the adsorbent selectivity revealed interesting heavy metal uptake. Selectivity tests revealed that the adsorption capacities are different for the heavy metal species at the same concentrations, and the series is Fe2+ > Co2+> Cu2+ > Cr3+. Fe-RO-SR could be an interesting, inexpensive ($4.56/Kg), and attractive potential adsorbent for heavy metal removal from aqueous solutions.