An assessment of the efficiency of adsorbents produced from a mix of lemon and orange skins in the removal of metals from wastewater

Abstract

Waste materials generated from various agricultural products, processes, and activities have been widely investigated as potential biomass to produce adsorbents with the capacity to remove heavy metals from effluents of various sources because they are cheap. The mechanisms associated with using agricultural waste as adsorbents include ion exchange, chemisorption, complexation, adsorption, and diffusion through pores. These pores have characteristics that may vary with the type of agricultural waste used to prepare the adsorbent. The use of agricultural-waste-derived activated carbon to recover potentially toxic metals including aluminium (Al), cadmium (Cd), iron (Fe), lead (Pb), nickel (Ni), and zinc (Zn) from aqueous solution could play an imperative role in the economy of industries since it could reduce the financial and environmental cost associated with disposal of waste containing these metals. It also encourages recycling through re-utilization of the agricultural waste. This study looked at the effectiveness and selectivity of raw and phosphoric acid-activated adsorbents made from a mixture of orange and lemon skins in the recovery of Al, Cd, Fe, Pb, Ni, and Zn from Acid Mine Drainage (AMD). The morphology of the mixed orange and lemon skins-derived adsorbent was characterized using a scanning electron microscope (SEM) whereas functional groups present on the surface of the prepared adsorbent and their crystallinity were identified using Fourier transform infrared (FTIR) spectroscopy and X-ray diffraction (XRD) respectively. Batch experiments to find out how time, pH, adsorbent dosage, and temperature affected Al, Cd, Fe, Pb, Ni, and Zn removal from aqueous solutions using the activated lemon and orange skins were also carried out. The Freundlich and Langmuir isotherm models were used to study the equilibrium kinetics, adsorption isotherms, and the nature of the adsorption process associated with the use of the prepared adsorbent. The optimized parameters were used with the prepared adsorbent to remove Al, Cd, Fe, Pb, Ni, and Zn from AMD samples. The activated lemon and orange skins had mean pore size and BET surface area of 5.180 nm and 169.28 m2/g, respectively. It had visible pores on the surface which may indicate high surface area, high efficiency and good adsorption capacity and could be described as mesoporous in nature. Using the optimised conditions with the prepared vi adsorbent, percentage removal of Al, Cd, Fe, Pb, Ni, and Zn from the AMD were 90.0%, 95.5 %, 99.9 %, 66.0 %, 72.5 %, and 93.0 %, respectively. The data acquired corresponded well with the Langmuir isotherm model with R2 varying from 0.976 to 0.991, indicating a homogeneous uniform adsorption process. The kinetic investigations revealed that the process of adsorption of metals on the prepared adsorbent was most accurately described by a pseudo-second-order model. Additionally, the adsorbent was capable of being reused up to five times indicating a high potential of recycling them after use. The findings from this study indicate that adsorbents made from a mixture of orange and lemon skins have the potential to be cost-effective and can be used in the removal of HM’s from industrial wastewater, as was demonstrated with AMD. To find out how much metal can be retrieved under optimal conditions, desorption of the metals from the prepared adsorbent needs to be investigated. Further research also needs to be carried out to determine the elemental content, proximal characteristics, and the yield percentage of the adsorbent to understand how to improve Its characteristics as an adsorbent.