Preparation of activated carbons from walnut shells via vacuum chemical activation and their application for methylene blue removal

Abstract

Activated carbons were prepared from walnut shells by vacuum chemical activation with zinc chloride as the activation agent. To optimize the preparation method, the effects of the main process parameters (such as system pressure, activation temperature, and impregnation ratio) on the properties (expressed in terms of specific surface area and pore volume) of the obtained activated carbons were studied. It was found that the optimum activated carbon obtained with system pressure of 30 kPa, activation temperature of 450 °C, and impregnation ratio of 2.0 has a BET surface area of 1800 m²/g and total pore volume of 1.176 cm³/g. The prepared activated carbons were characterized by SEM, TEM, and FTIR. Then they were used as an adsorbent for the removal of methylene blue from aqueous solutions. The results showed that the methylene blue adsorption capacity was positively correlated to the BET surface area. The highest methylene blue adsorption capacity was 315 mg/g for the optimum activated carbon. The effect of the dose of the activated carbon was evaluated and the adsorption isotherm of methylene blue was determined. Redlich–Peterson and Langmuir–Freundlich models were found to best represent the equilibrium data, suggesting heterogeneous surface adsorption of methylene blue on the activated carbon.

Introduction

The wide use of dyes in textile industries has lead to a variety of environmental problems, especially water pollution. Methylene blue (MB) is the most commonly used substance for coloring among all other dyes of its category and is generally used for dying cotton and silk [1]. Due to the harmful impacts of such dye on water, it is environmentally important to remove them from waste streams before discharge to public water sources.

Sorption is generally regarded as an effective technique for the treatment of dye-containing wastewater. Activated carbons, because of their large surface area and relatively high sorption capacity for a wide variety of dyes, have become the most promising and effective adsorbent [2], [3]. Nevertheless, their applications are restricted, because the most widely used carbonaceous materials for the production of commercially activated carbon are derived from natural materials such as wood or coal, which are expensive and are often imported [4]. Therefore, in the recent years, there is growing interest in the production of activated carbons from agricultural wastes because of their abundant resources and cheap prices. Several suitable agricultural wastes including coffee husks [5], rice husks [6], pistachio-nut shells [7], cotton stalks [3], coconut husks [8], cherry stones [9], corn cobs [10], and plum kernels [11] have been investigated in the last years as activated carbon precursors and are receiving renewed attention. Furthermore, converting the agricultural wastes into value-added activated carbons provides a new way for agricultural waste treatment.

Typically, the preparation of activated carbon can be divided into physical activation and chemical activation. Physical activation consists of the pyrolysis of the precursor material and activation of the resulting char in steam or carbon dioxide. Chemical activation is a single step process and is held in presence of some chemical reagents, such as KOH, NaOH, K₂CO₃, ZnCl₂, FeCl₃, H₃PO₄, and H₂SO₄. Chemical activation normally takes place at a lower temperature than that used in physical activation. In addition, the carbon yield in chemical activation is usually higher than in physical activation because the chemical agents possess dehydrogenation properties which can inhibit the formation of tar and reduce the production of other volatile substances. Among the chemical activation agents, ZnCl₂ is the most widely used since it resulted in high surface areas and high yields [12], [13], [14].

There are many studies in the literature relating to the preparation of activated carbons from agricultural wastes via chemical activation. However, most of the studies at present are carried out under atmospheric conditions. Lua and Yang have reported that activated carbons obtained under vacuum have better properties (e.g., higher specific surface area) than that prepared under atmospheric conditions [7]. Our previous studies also show that the morphology, pore size distribution, Brunauer–Emmett–Teller (BET) surface area, and adsorption properties of activated carbons are closely related to the system pressure [15]. In order to distinguish the traditional chemical activation (i.e., under atmospheric condition) from the chemical activation under vacuum condition, we call the latter vacuum chemical activation. Though the vacuum chemical activation has obvious advantages over traditional chemical activation, as far as we know, there are still very few reports on the preparation of activated carbons by vacuum chemical activation, let alone the applications of activated carbons prepared by this method.

Walnut shell, a good precursor for activated carbon production, is a major agricultural waste in China. According to statistics, there is more than 100,000 tonnes of walnut shells are produced in China annually. To make better use of the cheap and abundant agricultural waste, it is proposed to use it for activated carbon production. Therefore, the objective of this study was to prepare relatively well developed porosity activated carbons from walnut shells on a laboratory scale via vacuum chemical activation utilizing ZnCl₂ as the activation agent. To optimize the preparation method, the effects of the main process parameters (system pressure, impregnation ratio, activation temperature) on the characteristics of the prepared activated carbons were studied. The low-cost walnut shell-based activated carbons were fully characterized and subsequently used as an adsorbent for methylene blue removal. Isotherms for the adsorption of methylene blue on the obtained activated carbons were measured and fit to five different isotherm equations to determine the best isotherm model to represent the experimental adsorption data.