Microwave-assisted activated carbon from cocoa shell as adsorbent for removal of sodium diclofenac and nimesulide from aqueous effluents

Highlights

• Microwave-assisted cocoa shell activated carbon was prepared and characterized.

• The anti-inflammatories, diclofenac and nimesulide, were adsorbed onto MWCS-1.0.

• Adsorption maximum values are 63.47 (diclofenac) and 74.81 mg g⁻¹ (nimesulide).

• General order kinetic model suitably explained the adsorption process.

• MWCS-1.0 was effectively used for treatment of simulated hospital effluents.

Abstract

Microwave-induced chemical activation process was used to prepare an activated carbon from cocoa shell for efficient removal of two anti-inflammatories, sodium diclofenac (DFC) and nimesulide (NM), from aqueous solutions. A paste was obtained from a mixture of cocoa shell and inorganic components; with a ratio of inorganic: organic of 1 (CSC-1.0). The mixture was pyrolyzed in a microwave oven in less than 10 min. The CSC-1.0 was acidified with a 6 mol L⁻¹ HCl under reflux to produce MWCS-1.0. The CSC-1.0 and MWCS-1.0 were characterized using FTIR, SEM, N₂ adsorption/desorption curves, X-ray diffraction, and point of zero charge (pH_pzc). Experimental variables such as initial pH of the adsorbate solutions and contact time were optimized for adsorptive characteristics of MWCS-1.0. The optimum pH for removal of anti-inflammatories ranged between 7.0 and 8.0. The kinetic of adsorption was investigated using general order, pseudo first-order and pseu do-second order kinetic models. The maximum amounts of DCF and NM adsorbed onto MWCS-1.0 at 25 °C are 63.47 and 74.81 mg g⁻¹, respectively. The adsorbent was tested on two simulated hospital effluents. MWCS-1.0 is capable of efficient removal of DCF and NM from a medium that contains high sugar and salt concentrations.

Introduction

The concern about the occurrence and fate of pharmaceutical compounds in the aquatic environment is on the increase [1], [2]. The concern is heightened because there is no much information about the potential effects of pharmaceuticals on living organisms, though the pharmaceuticals are present at low concentrations (ng L⁻¹ or μg L⁻¹) in natural waters [3]. The major problems are: most of pharmaceuticals are polar substances with high solubility in water [3], and are not completely removed by conventional wastewater treatment plants [4]. Therefore, a large number of the highly used prescription drugs have been globally detected in the aquatic environment [3]. In this context, there is an increasing demand for efficient methods to remove pharmaceuticals from wastewater [5], [6], [7], [8], [9].

A useful and reliable method for removal of organic compounds from wastewater is an adsorption technology [10], [11], [12], [13]. It is a process in which pollutants are transferred from the effluent to a solid phase thereby alleviates exposure of living organisms to pollutants [14], [15]. After effluent treatment through adsorption, it can be released into the environment [15], [16] or used for some industrial processes. Adsorbents can also be regenerated after adsorption process and reutilized [15], [16].

Activated carbons are famous because they possess outstanding adsorption characteristics due to their improved pore structures [17], [18]. Therefore, activated carbon is one of the materials mostly used for the treatment of industrial wastewaters [19], [20], [21], [22], [23]. The ability of activated carbons to adsorb pollutants from aqueous solutions depends on two major factors: experimental conditions of the activation processes [17], [18] and the nature of organic material utilized for the preparation of activated carbon [19], [20]. Activated carbons can be prepared by physical and chemical activations [19]. Chemical activation utilizes H₃PO₄, ZnCl₂, KOH, NaOH, H₂SO₄, and K₂CO₃, among others [17], [18], [23], [24]. In the chemical activation process, inorganic activating agent(s) and the raw materials are thoroughly mixed in an aqueous medium. The mixture is oven-dried (100–120 °C) and subsequently carbonized (400–800 °C) [17], [18]. The inorganic components of activated carbon are removed using acid solutions or water [17], [18]. This method does not allow homogeneous distribution of the inorganics with the organic carbonaceous materials because it is difficult to dry impregnated organic components [17], [18]. To prevent nonhomogeneous distribution of the inorganic activating agent on the carbon precursor, the use of lime [1], [21] mixed with the inorganic components was recently proposed. The inclusion of lime improves distribution of the inorganic component into the organic matrix during the pyrolysis step to obtain desirable adsorbents [1], [21].

Conventional tubular furnace has been widely used for production of activated carbons via chemical and physical activation processes [17], [18], [19], [20], [21]. The application of microwave irradiation as a source of heating was recently reported [24], [25], [26], [27]. The microwave heating offers some advantages over conventional heating methods. Some of these advantages are: rapid temperature rise that leads to shorter pyrolysis time for producing activated carbons (<15 min) [27], and a remarkable decrease in energy consumption [27]. The major difference between microwave heating and conventional heating is the mode in which the heat is generated. Heat transfer takes place by conduction in conventional furnace, but in microwave heating energy is supplied directly to the organic precursor at molecular level, and converted the energy into heat by dipole rotation and friction within the organic matrix [26], [27].

In this work, powdered cocoa shell was mixed with inorganic components (20% lime + 40% FeCl₃ + 40% ZnCl₂) with inorganic: organic ratio of 0:1, 1:1, 1:1.5 and 1:2. Water was included in the preparation to form a paste [21]. The paste was dried at 100 °C for 6 h and subsequently placed in a quartz reactor, which was disposed in a microwave oven and was heated using a heating program under inert atmosphere. A 6.0 mol L⁻¹ HCl was used to acidified the carbonized materials (CSC-0, CSC-1.0, CSC-1.5, CSC-2.0) under reflux to obtain chemically activated cocoa-shell carbons induced by microwave (MWCS-0, MWCS-1.0, MWCS-1.5, MWCS-2.0). In this study, the preliminary experiments showed that MWCS-1.0 possessed the best adsorption capacity for removal of diclofenac (DCF) and nimesulide (NM) anti-inflammatories from aqueous solutions. Adsorption influencing factors such as initial pH of pharmaceutical solutions, time and temperature were investigated. The adsorbents were applied for treatment of simulated hospital wastewater effluents that are contaminated with pharmaceuticals.