Enhanced PAHs adsorption using iron-modified coal-based activated carbon via microwave radiation

https://doi.org/10.1016/j.jtice.2016.03.050Get rights and content

Highlights

  • An effective adsorbent with iron through microwave radiation was prepared.

  • The iron ion can be effectively loaded on 0.05Fe-MCAC surface.

  • The altered texture and chemical properties play an important role in PAHs adsorption.

  • The PAHs adsorption of 0.05Fe-MCAC own lower activation energy and greater stability.

Abstract

As an effective adsorbent, coal-based activated carbon (CAC) was modified with iron through microwave radiation to adsorb polycyclic aromatic hydrocarbons (PAHs) from aqueous solutions. The effects of iron nitrate concentration, microwave power, and microwave radiation time on CAC were investigated. The optimum modification conditions were as follows: iron nitrate concentration, 0.05 M; microwave power, 300 W; and microwave radiation time, 5 min (designated as 0.05Fe-MCAC). CAC and 0.05Fe-MCAC were characterized using scanning electron microscopy, nitrogen adsorption–desorption, surface acidity and basicity tests, and Fourier transform infrared spectroscopy. The adsorption capacities of PAHs were also determined. After modification the following phenomena occurred: pore structures developed; BET surface area increased from 686.06 to 1079.67 m2 g−1; the number of oxygen-containing groups decreased; and basicity was enhanced. The PAHs adsorption capacity of 0.05Fe-MCAC was higher than that of CAC. The altered texture and chemical properties of 0.05Fe-MCAC played an important role in PAHs adsorption. The pseudo-second-order models of CAC and 0.05Fe-MCAC fitted well. The PAHs adsorption of 0.05Fe-MCAC owned lower activation energy and greater stability than that of CAC. Results indicated that iron-modified CAC via microwave radiation is a promising and efficient adsorbent of PAHs from aqueous solutions.

Introduction

Polycyclic aromatic hydrocarbons (PAHs) commonly refer to a large class of dangerous and harmful ecosystem pollutants. They are generated during the incomplete combustion of hydrocarbons and other organic compounds, such as coal, petroleum, and biomass [1], [2]. PAHs can be transported in long distances in water but cannot be easily biodegraded because of their chemical persistence and semi-volatile nature [3]. In addition their toxic, mutagenic, and carcinogenic properties, they are considered as priority pollutants [4]. PAHs are effectively removed from aqueous solutions through adsorption [5], [6]. Nevertheless, economical and applicable absorbents should be developed to remove PAHs efficiently.

Activated carbon (AC) is characterized by a wide surface area and developed porosity; as such, it is considered a highly promising absorbent for a wide range of technical applications [6], [7], [8]. AC is also a commonly used adsorbent in wastewater treatments worldwide. AC is modified to regulate its pore structure and chemical properties. This process is an effective route to enhance the removal efficiency of AC as an adsorbent of various pollutants. Liu et al. [9] modified bamboo-based AC and found that the modified AC can effectively adsorb methylene blue. Shaarani et al. [10] demonstrated that ammonia-modified AC for adsorption of 2, 4-dichlorophenol was superior to the AC. Yin et al. [11] also modified AC-enhanced contaminant uptake from aqueous solutions. However, coal-based AC (CAC) has been rarely developed as an absorbent of PAHs from aqueous solutions [12], [13], [14]. Other materials are also more commonly used than CAC. Organic compound adsorption by AC is generally influenced by various properties, such as pore size, pore diameter distribution, surface properties, and surface functional groups [15], [16]. Therefore, structure and surface functional groups should be appropriately controlled to improve the adsorption of PAHs from aqueous solutions.

Metal-modified AC may provide an enhanced porosity structure and can vary in terms of pore structure and surface functional groups; thus, adsorption properties are altered [17]. Various metal oxides, such as manganese, mercury, and iron, have been added to AC to improve their adsorption performance [18], [19], [20]. However, iron-modified CAC used for the adsorption of PAHs from aqueous solutions has yet to be reported. Microwave is currently used in various fields to heat dielectric materials because this technique requires a considerably short treatment time and consumes low energy. In our previous study, microwave radiation can change the physical properties of AC [14]. Thus, iron-modified CAC via microwave radiation should be investigated to enhance the adsorption of PAHs.

This study aimed to modify CAC with iron via microwave radiation and to investigate the properties of the modified CAC and its adsorption capacities for PAHs from aqueous solutions. CAC and 0.05Fe-MCAC were characterized through scanning electron microscopy (SEM), nitrogen adsorption-desorption, surface acidity and basicity, and Fourier transform infrared spectroscopy (FTIR) methods. The adsorption capacity of naphthalene on 0.05Fe-MCAC was also examined, and the adsorption kinetics of PAHs on CAC and 0.05Fe-MCAC were systematically analyzed. PAHs adsorption on CAC and 0.05Fe-MCAC were compared, and the correlation of this property with their structures was determined. This paper could evaluate the effectiveness of 0.05Fe-MCAC in the removal of PAHs from aqueous solutions and could enhance our understanding of the adsorption behavior of PAHs onto 0.05Fe-MCAC.

Section snippets

Preparation of CAC

Coal-based activated carbon (CAC) was prepared according to our previous study [21], where the used coal (Xinjiang Tebian Electric Apparatus Stock Co., Ltd, China) was activated using potassium hydroxide under microwave radiation. Microwave radiation was conducted in a 2.45 GHz microwave oven (MM823LA6-NS, Midea). The preparation conditions of CAC were as follows: hydroxide potassium-to-coal ratio (w/w), 1:1; microwave power, 700 W; and radiation time, 10 min. The obtained products were in an

Effects of iron nitrate solution concentrations

We investigated the effects of the concentration of iron nitrate solution on the preparation of Fe-MCAC for naphthalene adsorption under the same microwave radiation conditions (Fig. 1). As the concentration of iron nitrate solution increased, the amount of naphthalene adsorbed on Fe-MCAC gradually decreased. The 0.05Fe-MCAC sample modified with 0.05 M iron nitrate solution yielded a maximum adsorption capacity of 159.00 mg/g, which was significantly higher than the adsorption capacity of the CAC

Conclusions

CAC was modified with iron via microwave radiation. The obtained 0.05Fe-MCAC sample was characterized through SEM, FTIR, surface acidity and basicity, and nitrogen adsorption–desorption methods. The surface chemistry and textural properties of CAC changed when it was modified by iron nitrate via microwave radiation. The developed pore structure, increased surface area, improved basic properties, and reduced oxygen-containing groups favored PAHs adsorption from aqueous solutions. The modified

Acknowledgments

This work was supported financially by funding from the National Natural Science Foundation of China (51,262,025) and International scientific and technological cooperation project of Xinjiang Bingtuan (2013BC002).

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