Ultrasonic enhanced adsorption of methylene blue onto the optimized surface area of activated carbon: Adsorption isotherm, kinetics and thermodynamics

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Abstract

In this study, the influence of pyrolysis time and temperature on yield, fixed carbon and physicochemical properties of the biochar produced from Empty Fruit Bunch (EFB) were investigated. Response Surface Methodology was utilized to optimize the influence of process parameters on the surface area of activated carbon produced from the biochar in the absence and presence of ultrasonication and was coded EFB-AC and EFB-UAC respectively. The EFB-AC and EFB-UAC were characterized using BET, TGA, HRSEM, XRD, FTIR and XPS. The batch adsorption behavior of MB onto EFB-AC and EFB-UAC was evaluated. The results revealed that the optimum surface area (SBET) of 2114 m2/g was achieved for EFB-UAC at activation temperature (600 °C), activation time (45 min) and KOH concentration (1.5 M). It was found that EFB-UAC has high surface area, better thermal stability, better pores development and improved surface chemistry compared to the EFB-AC. The maximum adsorption of MB occurred at pH (10), adsorbent dosage (30 mg), contact time (40 min), Temperature (50 °C) and MB initial concentration (50 mg/L). The experimental data was best described by Langmuir isotherm model, having an adsorption capacity of 400 mg/g and 435 mg/g for EFB-AC and EFB-UAC, respectively. The kinetic model was best suited by pseudo-second order model for MB adsorption onto EFB-AC and EFB-UAC. The thermodynamics investigation showed that adsorption of MB was endothermic in nature due to strong electrostatic interaction and formation of hydrogen bonding. The study revealed that ultrasonic assisted variations of the adsorption parameters significantly improved the adsorption of MB onto EFB-AC and EFB-UAC.

Introduction

The global utilization of pigments and dyes in chemical industries for instance paper, textiles, plastic, printing, food, pharmaceutical, cosmetics and rubber industries have been widely recognized and documented (Fakhri et al., 2017; Lima et al., 2019). The effluents generated from these industries affects the environment and human body adversely (Ibupoto et al., 2018). About 10% of the hundred thousand tons of produced dyes in these industries are discharged into the environment as wastewater (Marrakchi et al., 2017). Basically, synthetic dyes unlike natural dyes exhibit molecular stability and are classified as cationic, anionic and non-ionic dyes because of their charge (Li et al., 2019). Methylene blue (MB), is a commonly used cationic dye and a major constituent of wastewater from these industries (Wang et al., 2018b; Yang et al., 2019). Previous studies have revealed that exposure to MB can cause eye injuries and skin damage, while high heart rate, digestive disorder, nausea, vomiting and tissue necrosis are due to direct ingestion of MB (Banerjee et al., 2017; Konicki et al., 2017; Wang et al., 2018a). Therefore, development of an efficient and advanced treatment method for the removal of MB from wastewater is considered important in order to maintain a safe and healthy environment.

Several attempts have been made to remove MB using physical treatment such as filtration (Yang et al., 2019), reverse osmosis (Qasim et al., 2019), flocculation (Suresh et al., 2018) and coagulation (Pinto et al., 2019), chemical treatments; which include photodegradation (Jiang et al., 2018) and photo-Fenton (Hodaifa et al., 2019), and biological treatments via anaerobic or aerobic biodegradation (Gaur et al., 2018). However, these methods possessed some environmental and economic drawbacks which include generation of secondary pollutants, high cost, operations delay, complex treatment techniques, inadequate removal efficiency and oxidizing agents dependence (Charola et al., 2019; Li et al., 2017a,b). On the contrary, adsorption technology has been identified as a superior alternative treatment method due to its low cost, efficiency, ease of operation conditions, adsorbents diversity, inertness to materials and environmentally friendly compared to other treatment techniques (Guo et al., 2019; Konicki et al., 2018; Tian et al., 2018; Zhang et al., 2016). Activated carbon remains one of the most investigated adsorbents by researchers due to its porous structure, high surface area, abundance of surface functional groups and high adsorption capacity (Altıntıg et al., 2017; Kluczka et al., 2019). However, the production cost of commercial activated carbon is high hence underutilized agricultural wastes have been identified as a viable feedstock substitute to produce cost effective activated carbon.

Researchers have exploited different renewable and low agricultural byproducts namely; rice husk (Menya et al., 2017), mustard husk (Charola et al., 2019), almond shell (Zbair et al., 2018), sunflower (Moralı et al., 2018), coconut shell (Islam et al., 2017), acorn shell (Altıntıg et al., 2017), pecan nut shell (Lima et al., 2019), peanut shell (Cai et al., 2018), corn bract (Lin et al., 2018) and walnut shell (Miyah et al., 2018) amongst others to produce activated carbon. In this study, Empty Fruits Bunches from Palm tree was used to produce activated carbon. It is interesting to note that the global production capacity of oil palm is 75.51 million MT and Indonesia (43.00 million MT), Malaysia (20.70 million MT), Thailand (3.00 million MT), Colombia (1.68 million MT) and Nigeria (1.02 million MT) are the leading producers (USDA, 2019).

Specifically, Nigeria is known to be one of the leading producers of oil palm. Unfortunately, the processing of oil palm to produce palm oil generates enormous quantity of wastes such as palm kernel shells (6%), mesocarp fibres (15%) and empty fruit bunches (EFB) (23%) in a ton fresh fruit bunch of oil palm. At the moment, there is a growing concern over the indiscriminate disposal of EFB into the environment, which constitute a great danger to public health (Liew et al., 2017). Not only that the indiscriminate burning of oil palm wastes in boiler for steam production also causes air pollution and release of harmful gases into the atmosphere. In addition, the utilization of oil palm wastes as a combustion material for electricity also produces undesirable ash. This necessitates the conversion of lost cost EFB into useful product like activated carbon that can remove undesirable pollutants from industrial wastewater that would be of benefit to the environment and the scientific community in the search for cheaper adsorbent material.

Furthermore, different techniques among which is pyrolysis have been identified as one of the most reliable thermochemical process for the conversion of solid waste feedstock to high value biochar (Morin et al., 2018). Pyrolysis is usually carried out in the absence of oxygen depending on the applied temperature and heating rate (Chen et al., 2018). However, slow pyrolysis often leads to high yield of biochar that is rich in carbon content with an aromatic structure containing oxygen. The large quantity of oxygen rich surface functional groups of biochars enhances their chemical activation for different applications (Lee et al., 2017). Several authors have reported the modification of the properties of biochar by acids, bases, steam, metal oxides, clay minerals, carbonaceous materials and organic compounds to enhance its surface area and porosity (Ait Ahsaaine et al., 2018; Zbair et al., 2018).

Response surface method (RSM) has been recognized as a tool that revealed the interaction among two or more of the process parameters. Different researchers have employed RSM to optimize the experimental variables in the preparation of activated carbon from agricultural feed stocks such as mustard husk (Charola et al., 2019), peanut shells (Xu et al., 2017), tobacco stem (Yu et al., 2019), rice husk (Zhang et al., 2017), almond shell (Zbair et al., 2018), coconut shell (Dissanayake Herath et al., 2019), Africana seed hulls (Garba and Rahim, 2014), cassava stem (Sulaiman et al. 2018). However, most of these studies did not investigate the influence of ultrasonication on the optimization of the experimental conditions. Ultrasonication is a process that enhances mass transfer through utilization of sound waves to generate fluctuations in pressure within aqueous medium which resulted to the growth, formation and eventual collapse of microbubbles in microseconds. The sonication effect also lead to micro-explosions under high pressure and temperature (Sodeifian and Ali, 2018). To the best of our knowledge, the optimization of the influence of process parameters on the surface area of developed activated carbon from EFB via RSM technique under ultrasonic effect has not been investigated. Thus, the present study focused on the preparation of activated carbon from EFB via ultrasonic assisted optimization of temperature, time and KOH concentration to produce material with high surface area and adsorption capacity for MB molecules. Furthermore, statistical approach of Design of Experiment (DOE) using RSM was adapted for Empty Fruits Bunch-Ultrasonicated Activated Carbon (EFB-UAC) preparation to determine the parameters with the most impact on the surface area of the activated carbon. The prepared EFB-AC and EFB-UAC were characterized using several analytical techniques. This was followed by the investigation of the ultrasonic assisted adsorption behavior of EFB-AC and EFB-UAC for the removal of MB from aqueous solution via batch process.

Section snippets

Materials

Analytical grade sodium hydroxide (NaOH), potassium hydroxide (KOH), hydrochloric acid (HCl) and MB (C16H18CIN3S*H20, MW = 319.86 G/mol) with percentage purity in the range of 95–99.9% used were obtained from Sigma Aldrich. The chemicals/reagents were used as received without further purification. The nitrogen gas (95–99.9%) used in this study was purchased from Vinee Gas Ltd. Abuja, Nigeria.

Samples collection and pre-treatment

The EFB feed stocks were randomly collected from different locations within Neni, Anambra State. The EFB

Characterization of EFB

The results of the proximate and ultimate analysis of EFB revealed that 14.70% moisture content was obtained. Specifically, the volatile matter content of 64.20 wt% indicates that the material is rich in organic matter, and may be suitable for biochar production. The low ash content (3.00 wt%) suggests that the feedstock is good for biochar production. The fixed carbon composition of 18.10 wt% signifies the potential to be transformed into carbonaceous materials during pyrolysis (Liew et al.,

Conclusion

The prepared EFB-AC and EFB-UAC from EFB were successfully applied to adsorb MB from aqueous solution under ultrasonic influence. The study showed that EFB-AC has a surface area of 1036 m2/g, while EFB-UAC has a higher surface area of 2114 m2/g. The characterization of EFB-AC and EFB-UAC using BET, TGA, HRSEM, XRD, FTIR and XPS showed that EFB-UAC has high surface area, better thermal stability, better pores development and improved surface chemistry compared to the EFB-AC due to the influence

Conflicting interests declaration

The author(s) declared no potential conflicts of interest.

Acknowledgements

This work was financially supported by the Petroleum Technology Development Fund of Nigeria (grant number PTDF/ED/LSS/PhD/TCE/123/17). The authors also appreciate the management of Centre for Genetic Engineering and Biotechnology (CGEB), Federal University of Technology, Minna, Nigeria for making the necessary facilities available for this research. The contributions of Prof. W.D. Roos of Department of Physics, University of the Free State, Bloemfontein, South Africa in the XPS analysis of the

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