Abstract
Calcium carbonate (CaCO3)-enriched biomaterial derived from freshwater mussel shells (FMS) was used as a non-porous biosorbent to explore the characteristics and mechanisms of cadmium adsorption in aqueous solution. The adsorption mechanism was proposed by comparing the FMS properties before and after adsorption alongside various adsorption studies. The FMS biosorbent was characterized using nitrogen adsorption/desorption isotherm, X-ray diffraction, scanning electron microscopy with energy dispersive spectroscopy, Fourier-transform infrared spectroscopy, and point of zero charge. The results of batch experiments indicated that FMS possessed an excellent affinity to Cd(II) ions within solutions pH higher than 4.0. An increase in ionic strength resulted in a significant decrease in the amount of Cd(II) adsorbed onto FMS. Kinetic study demonstrated that the adsorption process quickly reached equilibrium at approximately 60 min. The FMS biosorbent exhibited the Langmuir maximum adsorption capacity as follows: 18.2 mg/g at 10 °C < 26.0 mg/g at 30 °C < 28.6 mg/g at 50 °C. The Cd(II) adsorption process was irreversible, spontaneous (−ΔG°), endothermic (+ΔH°), and more random (+ΔS°). Selective order (mmol/g) of metal cations followed as Pb2+ > Cd2+ > Cu2+ > Cr3+ > Zn2+. For column experiments, the highest Thomas adsorption capacity (7.86 mg/g) was achieved at a flow rate (9 mL/min), initial Cd(II) concentration (10 mg/L), and bed height (5 cm). The Cd(II) removal by FMS was regarded as non-activated chemisorption that occurred very rapidly (even at a low temperature) with a low magnitude of activation energy. Primary adsorption mechanism was surface precipitation. Cadmium precipitated in the primary (Cd,Ca)CO3 form with a calcite-type structure on the FMS surface. A crust of rhombohedral crystals on the substrate was observed by SEM. Freshwater mussel shells have the potential as a renewable adsorbent to remove cadmium from water.
Highlights
Toxic metals present in either discharged water from industrial activities and sent downstream or originating from environmental geochemistry, inevitably cause some potential health risks for humans and pose a great threat to the environment (Pourret and Bollinger, 2018)
Calcium carbonate (CaCO3)-enriched biomaterial derived from freshwater mussel shells (FMS) was used as a non-porous biosorbent to explore the characteristics and mechanisms of cadmium adsorption in aqueous solution
The results suggested that cadmium can readily be incorporated in the CaCO3 structure to form the (Cd, Ca)CO3 precipitates on the FMS surface
Summary
Toxic metals present in either discharged water from industrial activities and sent downstream or originating from environmental geochemistry, inevitably cause some potential health risks for humans and pose a great threat to the environment (Pourret and Bollinger, 2018). Numerous conventional treatment techniques have been applied to remove cadmium from water environments, such as coagulation/flocculation, flotation, ion exchange, membrane, adsorption, electrochemical treatment, and chemical precipitation (Carolin et al, 2017). Among such existing methods, adsorption has been acknowledged to be the most economically favorable method; it requires low energy consumption for removing cadmium from water environments, even at a very low cadmium concentration. Such composite biomaterials demonstrated a high affinity to many potentially harmful metals (i.e., Cd, Cu, Pb, Ni, and Zn) (Flores-Cano et al, 2013; Ahmad et al, 2012; Du et al, 2011; Hsu 2009; Shaheen et al, 2013) and rare earth elements (Zhou et al, 2018) in water media
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