Abstract
In this work, the mechanisms of cadmium (Cd2+) adsorption on residual biomasses from husks of yam (Dioscorea rotundata), cassava (Manihor esculenta), cocoa (Theobroma cacao), corn (Zea mays) and oil palm bagasse (Elaeis guineensis) were studied in order to evaluate the effect of temperature, adsorbent dose and particle size in a batch system. Isotherms and adsorption kinetics were determined and adjusted to different models. The biomaterials were characterized using the techniques of Scanning Electron Microscopy (SEM) and Energy-Dispersive X-ray Spectroscopy (EDS). Results reveal that the possible mechanisms of Cd2+ adsorption in bioadsorbents were ion exchange and complexation with -COOH and -OH groups. From the experimentation, it was found that best conditions were presented at 55 °C, particle size 0.5 mm and 0.03 g adsorbent. The following biomass performance was obtained in terms of adsorption capacities: cocoa husk (CH) > corn cob residues (CCR) > cassava peel (CP) > palm bagasse (OPB) > yam peel (YP), according to the Langmuir and Dubinin- Radushkevich (D-R) models. The equilibrium of Cd2+ adsorption over YP and OPB was well described by Langmuir’s isothermal model, while for CH, CCR and CP the model that best fit experimental data was Freundlich’s model. The results of D-R model suggested that the process is controlled by physisorption mechanism with strong interactions among active sites and Cd2+ ions. The kinetics for all systems studied fit the pseudo-second order model. The values of the thermodynamic parameters established that cadmium removal is of endothermic nature and not spontaneous using YP and CP, and exothermic, spontaneous and irreversible when using OPB, CH and CCR. The results suggest the use of YP, OPB, CH, CP and CCR residues for the removal of aqueous Cd2+.
Highlights
The discharge of industrial effluents with toxic and hazardous materials is an alarming problem because of the serious water pollution that may cause [1]
Are potentially good for adsorbing Cd2+, at the optimal experimental conditions found: temperature of 55 ◦ C, particle size 0.5 mm and quantity of adsorbent 0.03 g, and that the variable with the greatest influence on the process was the dose of adsorbent, achieving adsorption capacities between 76 and 197 mg/g. (ii) The Scanning Electron Microscopy (SEM)-Energy-Dispersive X-ray Spectroscopy (EDS) analysis suggests that the Cd2 + adsorption mechanisms were ion exchange and complexation with the -COOH
-OH groups present in the structure of the cellulose, lignin, pectin and hemicellulose molecules of the biomasses studied. (iii) The adsorption kinetics of Cd2 + was fast in the initial minutes due to the availability of active centers, reaching equilibrium at 60 min, being the pseudo-second order (PSO) and Elovich models the ones that best fit the experimental data, suggesting that the process occurs by chemical reaction. (iv) The adsorption isotherms using yam peel (YP) and OPB were adjusted using the Langmuir model, indicating that the metal is adsorbed on the metal surface in a monolayer; while the balance of adsorption in cocoa husk (CH), corn cob residues (CCR) and cassava peel (CP)
Summary
The discharge of industrial effluents with toxic and hazardous materials is an alarming problem because of the serious water pollution that may cause [1] Among these pollutants, heavy metals are responsible for environmental and public health issues [2]. Cadmium is a heavy transition metal that exists in the aquatic environment through geochemical processes and is increasingly exposed to the air through anthropogenic industrial activities [3], that pollute natural water sources. Principal responsible of this environmental issue are effluents from metal plating industries, Cd-Ni batteries, pesticides, paints, pigments, plastics, metallurgy, fertilizers, alloys, stabilizers and metal plating [4,5]. The World Health Organization recommends that drinking water should have a maximum concentration of 0.003 mg/L of
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