Cd(II) adsorption from aqueous solution by raw and modified kaolinite

Highlights

• The kaolinite was modified by manganese oxide.

• Adsorption capacity of raw and modified kaolinite was found to be 14.11 and 36.47 mg/g.

• Modified kaolinite is a good adsorbent for the removal of Cd(II) from aqueous solution.

Abstract

This study aimed to investigate the adsorption and desorption potential of raw kaolinite (Kaol) and manganese oxide (MnO₂)-modified kaolinite (MnO-Kaol) for the removal of cadmium (Cd(II)) ions from aqueous solution and waste water. The chemical and morphological characterizations of kaol and MnO-Kaol were carried out by using Scanning Electron Microscopy (SEM) and Fourier Transform Infrared (FT-IR) analysis methods. The BET surface analysis results showed that the specific surface area of kaol increased by about 68% after MnO₂ modification. The cadmium adsorption feasibility of MnO-Kaol was examined systemically by evaluating the effects of initial pH of solution, contact time, adsorbent concentration, and temperature of solution on the adsorption efficiency. From the Langmuir model, the adsorption capacities of kaol and MnO-Kaol were found to be 14.11 and 36.47 mg g^− 1. The Dubinin–Radushkevich (D–R) model showed that the Cd(II) adsorption onto MnO-Kaol was progressed essentially by chemical ion-exchange. The recovery test demonstrated that the MnO-Kaol has good reusability performance after repeated ten times adsorption–desorption cycles. The thermodynamic studies revealed that the adsorption process of Cd(II) onto MnO-Kaol has exothermic nature and more feasible with increasing temperature of solution. The kinetic investigations also denoted that the adsorption process better fits the pseudo-second-order kinetic model.

Introduction

Toxic heavy metals are important pollutants in water and wastewater, and they have become a public health care due to their non-biodegradable and continual nature. The toxicity risk of these metals is increased through accumulation in living organisms and following bioamplification in the food chain (An et al., 2001, Jiang et al., 2010). Among the toxic heavy metals, cadmium is highly toxic and about 80% of global Cd output is discharged to the environment as a byproduct of zinc smelting process. It is used in different industrial areas such as the production of alkaline batteries, pigments, coatings, and stabilizers for plastics (Wilburn, 1999). Cadmium compounds are often found as attached with small particles in air and their breathing with very high levels can seriously give harm to the lungs. USA Environmental Protection Agency has set a limit of only 5 ppb Cd(II) for drinking water (ATSDR, 1999) due to its carcinogenic effect on humans. Therefore, the removal of this ion from wastewaters has become a very important matter. Several methods such as electrodialysis (Mohammadi et al., 2005), ion exchange (Smara et al., 2007), reverse osmosis (Mohsen-Nia et al., 2007), ultrafiltration (Ennigrou et al., 2009), chemical precipitation (Ghosh et al., 2011), biosorption (Du et al., 2011) and adsorption (Luo et al., 2011) have been proposed for the removal of metal ions from wastewaters (Fu and Wang, 2011). Among these methods, adsorption is the most effective and simple method.

Adsorption method has also been employed generally for the elimination of Cd(II) due to its benefits such as easiness, rapidness, suitability, and design costs for the equipment and process (Volesky, 2001). The elimination of Cd(II) ions from aqueous solutions has been carried out by using different kinds of adsorbents such as polymer (Denizli et al., 2005) and modified-polymer (Pan et al., 2001), ion-exchange resin (Dong et al., 2010), modified carbon nanotubes (Gupta et al., 2011), and nano-membrane (Hajdu et al., 2012). However, such type of adsorbents is not ecofriendly and cost-effective for practical wastewater treatments. Thereby, a great attention has been paid to research on different types of low-cost unmodified and modified clay minerals for the removal of Cd(II) ions from aqueous solutions (Erdem and Ozverdi, 2005, Han et al., 2006a, Han et al., 2006b, Sarı et al., 2007a, Sarı et al., 2007b, Sari et al., 2007b, Zhang and Hou, 2008, Gupta and Bhattacharyya, 2008, Salem and Akbari, 2011).

On the other hand, unmodified and modified kaolinite (Kaol) have also been used for the removal of cadmium and other some heavy metal ions from aqueous samples (Jiang et al., 2009, Turan et al., 2007, Unuabonah et al., 2008Unuabonah and Adebowale, 2008). Lately, MnO₂-modified adsorbents have attracted more notice because of the following physical and chemical properties of MnO₂: (i) ionization ability at low pH, (ii) carrying ability of more negative charge in solution compared to SiO₂, TiO₂, Al₂O₃, and FeOOH (Allen et al., 1991), (iii) the loading ability at large amounts to the silica-based surfaces (Khraisheh et al., 2004), and (iv) the noticeably increasing ability of the specific surface areas of silica-based adsorbents (Eren et al., 2009). Therefore, in recent times, the interest in new type MnO₂-modified adsorbents is increasing with each passing day (Al-Deges et al., 2000; Han et al., 2006a, Zou et al., 2006 Al-Ghouti et al., 2009). In addition, in previous works (Sarı and Tuzen, 2013, Sarı et al., 2012), two different clay samples (expanded perlite and vermiculite) were also functionalized with MnO₂ to increase high adsorptive capacity and selectivity of these adsorbents for antimony and silver ions. In this context, the present study aims to explore the adsorption potential of MnO-Kaol in the removal of Cd(II) from aqueous solution and waste water samples. According to literature survey, there is no study in detail about the adsorption of Cd(II) from aqueous solution and waste water using MnO-Kaol. The effects of batch adsorption parameters were studied systemically. The prepared MnO-Kaol was subjected to the reusability test. The adsorption process was evaluated by examining some thermodynamics parameters, isotherm and kinetic models.