Kinetic and isotherm studies of Cu(II) adsorption onto H3PO4-activated rubber wood sawdust

doi:10.1016/j.jcis.2005.05.087

Journal of Colloid and Interface Science

Volume 292, Issue 2, 15 December 2005, Pages 354-362

https://doi.org/10.1016/j.jcis.2005.05.087 Get rights and content

Abstract

Adsorption of Cu(II) from aqueous solution onto H₃PO₄-activated carbon using rubber wood sawdust (RSAC) was investigated in a batch system. Kinetic and isotherm studies were carried out by considering the effects of various parameters, such as initial concentration, contact time, pH, and temperature. The optimal pH value for Cu(II) adsorption onto RSAC was found to be 6.0. Thermodynamic parameters such as standard Gibbs free energy ( $Δ G^{0}$ ), standard enthalpy ( $Δ H^{0}$ ), and standard entropy ( $Δ S^{0}$ ) were evaluated by applying the Van't Hoff equation. The thermodynamics of Cu(II) adsorption onto RSAC indicates its spontaneous and exothermic nature. Langmuir, Freundlich, and Temkin isotherms were used to analyze the equilibrium data at different temperatures. The Langmuir isotherm fits the experimental data significantly better than the other isotherms. Adsorption kinetics data were tested using pseudo-first-order, pseudo-second-order, and intraparticle diffusion models. Kinetic studies showed that the adsorption followed a pseudo-second-order reaction. The initial sorption rate, pseudo-first-order, pseudo-second-order, and intraparticle diffusion rate constants for different initial concentrations were evaluated and discussed. Adsorption mechanism studies revealed that the process was complex and followed both surface adsorption and particle diffusion. The rate-controlling parameter and effective diffusion coefficient were determined using the Reichenberg plot. It was found that the adsorption occurs through film diffusion at low concentrations and at higher concentration the particle diffusion becomes the rate-determining step.

Introduction

Copper is present in the wastewater of several industries, such as metal cleaning and plating baths, refineries, paper and pulp, fertilizer, and wood preservatives and it is highly toxic [1], [2]. The excessive intake of copper by man leads to severe mucosal irritation, widespread capillary damage, hepatic and renal damage, central nervous problems followed by depression, gastrointestinal irritation, and possible necrotic changes in the liver and kidney [2]. The World Health Organization (WHO) recommended a maximum acceptable concentration of Cu(II) in drinking water of 1.5 mg l⁻¹ [3]. It is essential that potable waters be given some treatment to remove copper before domestic supply.

In wastewater treatment technology, various techniques have been used for copper ion removal. Among these, physicochemical and biosorption methods have been successfully applied. An effective method for copper ion removal is chemical precipitation. However, disposal of large quantities of sludge is found to be difficult [4]. Conventional methods that have been used include ion exchange, reverse osmosis, electrochemical treatment, evaporative recovery, and adsorption. These conventional techniques can reduce metal ions, but they do not appear to be highly effective due to the limitations in the pH range as well as the high material and operational costs [5]. Among these various treatment techniques, activated carbon adsorption is one of the most commonly used due to its high efficiency and easy operation.

In recent years, considerable attention has been focused on the removal of copper from aqueous solution using adsorbents derived from low-cost materials. Several adsorbents, such as sawdust, silica, and iron oxide [6], sewage sludge ash [7], anatase-type titanium dioxide [8], olive mill residues [9], inorganic colloids [10], blast furnace sludge [11], functionalized silica, red mud, and fly ash [12], peat [13], paper mill sludge [14], and activated carbon [15], [16] have been used for the treatment of Cu(II)-rich effluents at the solid–solution interface. In this work, rubber wood sawdust, which has low economic value, was taken for the preparation of activated carbon and its applicability to the removal of copper ions present in wastewaters was studied.

The objective of this work is to carry out studies on the kinetics, isotherm, and mechanism of Cu(II) adsorption onto activated carbon indigenously prepared from rubber wood sawdust as the adsorbent. This study provides insight into metal ion adsorption from aqueous solutions in terms of equilibrium and kinetics. It also provides important information that could be used in the design and optimization of metal ion adsorption operations. Furthermore, the results of this study could be helpful in the prediction of the metal ion rate of transport. Therefore, work is aimed at evaluating the kinetics and diffusion parameters for the adsorption of Cu(II) onto RSAC and to find out the viability of RSAC for the adsorption process. The effects of various system parameters such as initial concentration, time, pH, and temperature are studied and the results obtained are discussed.

Section snippets

Materials and methods

All the reagents used were analytical grade chemicals. Copper ions were prepared by dissolving copper sulfate pentahydrate in double distilled water. The pH of the solutions was adjusted using sulfuric acid and sodium hydroxide.

Adsorbent

Rubber wood sawdust, a waste product, was collected and dried. The dried sawdust was mixed with the phosphoric acid at a weight ratio of 1:2 and soaked for 24 h so that the reagents were fully adsorbed onto the raw material. The mixture was dried at 110 °C for 1.5 h and

Effect of pH

The effect of pH on adsorption of Cu(II) on RSAC was studied at a temperature of 30 °C by varying the initial pH of 20 mg l⁻¹ of Cu(II) solution for a fixed adsorbent dosage of 0.5 g and the results are shown in Fig. 1. The amount adsorbed was found to increase with increase in pH and showed a maximum adsorption at pH 8.0. The optimum initial pH was, however, chosen to be 6.0 because precipitation of copper hydroxide was observed at pH greater than 6.0. Similar observations were previously

Conclusions

Kinetic and isotherm studies revealed that rubber wood sawdust activated carbon prepared using phosphoric acid as the activating agent can be effectively employed for the adsorption of copper ions. Copper adsorption was found to be pH-dependent and maximum removal was observed at pH 6.0. Adsorption follows the Langmuir isotherm and obeys a pseudo-second-order model. The adsorption process was found to be controlled by the film diffusion at lower concentrations of the adsorbate and shifted to

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Kinetic and isotherm studies of Cu(II) adsorption onto H3PO4-activated rubber wood sawdust

Abstract

Introduction

Section snippets

Materials and methods

Adsorbent

Effect of pH

Conclusions

Chemosphere

J. Colloid Interface Sci.

Water Res.

Resour. Conserv. Recycl.

Water Res.

Water Res.

Colloids Surf.

Water Res.

Colloids Surf. A Physicochem. Eng. Aspects

Water Res.

Colloids Surf.

Sep. Purif. Technol.

Water Res.

J. Colloid Interface Sci.

Resour. Conserv. Recycl.

Water Res.

J. Colloid Interface Sci.

Water Res.

Water Res.

Process Biochem.

Kinetic and isotherm studies of Cu(II) adsorption onto H₃PO₄-activated rubber wood sawdust