Adsorption of Metal Ions onto the Chemically Modified Agricultural Waste

Abstract

This paper discusses about the adsorption of metal ions such as Cu(II), Cd(II), Zn(II), and Ni(II) from aqueous solution by sulfuric acid treated cashew nut shell (STCNS). The adsorption process depends on the solution pH, adsorbent dose, contact time, initial metal ions concentration, and temperature. The adsorption kinetics was relatively fast and equilibrium was reached at 30 min. The adsorption equilibrium follows Langmuir adsorption isotherm model. The maximum adsorption capacity values of the modified cashew nut shell (CNS) for metal ions were 406.6 mg/g for Cu(II), 436.7 mg/g for Cd(II), 455.7 mg/g for Zn(II), and 456.3 mg/g for Ni(II). The thermodynamic study shows the adsorption of metal ions onto the STCNS was spontaneous and exothermic in nature. The kinetics of metal ions adsorption onto the STCNS followed a pseudo-second-order kinetic model. The external mass transfer controlled metal ions removal at the earlier stages and intraparticle diffusion at the later stages of adsorption. A Boyd kinetic plot confirms that the external mass transfer was the slowest step involved in the adsorption of metal ions onto the STCNS. A single-stage batch adsorber was designed using the Langmuir adsorption isotherm equation. IntroductionThe presence of heavy metals in wastewater is always a serious environmental problem because the heavy metals are not biodegradable and it can be accumulated in living tissues. The presence of these heavy metals in water even at a very low level concentration, it would be harmful to human health and also aquatic organisms in terms of toxicity and carcinogenicity. The heavy metals such as copper, cadmium, zinc, and nickel are among the most common pollutants available in industrial wastewater because it finds many industrial applications. But its removal and recovery from the wastewater is important for the living environment [1]. Different methods were investigated and applied to remove excessive heavy metals from wastewater includes chemical precipitation, electrolysis, evaporation, ion exchange, and membrane processes. However, these methods are either expensive or inefficient when heavy metals exist in low concentrations [2]. So it is important to find new treatment methods for removing heavy metals from wastewater. In the present days, adsorption is proved to be one of the simplest and most effective treatment methods for the treatment of heavy metals containing wastewater. Now-a-days activated carbon is widely used as an adsorbent in water and wastewater treatments because of its high porosity, large internal surface area, and relatively high mechanical strength.Despite its widespread application in industries, activated carbon remains an expensive material [3]. Therefore, it is important to investigate and develop a new low-cost effective adsorbent from the low-cost materials that can be applied to the wastewater treatment. Some of the effective adsorbent from low-cost materials that have been exploited to remove the metal ions from the aqueous solutions such as vegetable biomas...

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