Activated carbon derived from the date palm leaflets as multifunctional electrodes in capacitive deionization system
Graphical abstract
Introduction
The shortage of fresh water and contamination if water body on earth have become a severe problem nowadays [1]. Many desalination technologies such as distillation, filtration (ultra and nano), reverse osmosis, coagulation and multi-stage flash have been widely used to produce fresh water from sea water and brackish water [[2], [3], [4], [5], [6], [7], [8]]. Though, these techniques suffer from different concerns such as high energy consumption, expensive, fouling and production of secondary pollution etc. [9,10]. An alternate way to produce fresh water from brackish water which can improve such drawbacks is low cost and less energy consumption based capacitive deionization (CDI) approach. CDI is an electrochemical control method in which cations and anions from saline water are electrosorbed onto the porous structure electrodes by applying the potential. The electrosorbed ions are stored in the form of electrical double layer at the interface of the electrode surface and ionic solution. By reversing or shorting the two electrodes can regenerate the electrode surfaces by releasing the adsorbed ions back into the solution. Electrode materials which possess highly porous structure with high surface area, high ions adsorption capacity, hydrophilic nature, good electrical conductivity, excellent chemical stability are the important factors to be considered to enhance the performance of CDI [[11], [12], [13]].
Carbon based material is the most suitable to apply as an electrode in CDI process and activated carbon, activated carbon cloth, carbon aerogel, mesoporous carbon, graphene, carbon nanotube and carbon nanofibers are extensively used nowadays [[14], [15], [16], [17], [18], [19], [20]]. Activated carbon prepared from renewable biomass waste has attracted great attention due to their vast availability, inexpensive and environmentally friendly [14,[21], [22], [23]]. Some of the selected biomass waste based activated carbon materials reported previously is as follow. S. Gaikwad et al. reported mesoporous activated carbon electrodes produced from tea waste biomass by chemical and thermal modification. The electrode was applied in CDI system for simultaneous removal of hexavalent chromium and fluoride and the electrode shows ions adsorption capacity of 2.49 mg/g in 100 ppm mixed solution [24]. Rangaraj et al. fabricated 2 dimensional nitrogen doped porous carbon nanosheets from tamarind shell by pyrolysis method using potassium hydroxide activation and urea as a precursor for nitrogen source. The nitrogen doped porous carbon sheet showed improvement in electrical conductivity and wettability with maximum salt adsorption of 18.8 mg/g in 600 ppm NaCl solution [25]. Hai and his colleagues reported the utilization of date seed biomass to produce porous activated carbon by pyrolysis technique. Their electrode achieved salt adsorption capacity of 22.5 mg/g with desalination efficiency of 86.4 % in 250 ppm NaCl solution [26]. Liu et al. proposed porous carbon nanosheet fabricated from lotus leaf and applied as electrodes in supercapacitors, CDI system and methylene blue adsorption [21]. Wang et al. used plant fruits as carbon source to fabricate nitrogen doped slit-like micropores carbon by simple hydrothermal reaction method. The plant-fruits-derived carbon showed a selectivity towards Cl− and NO3− with ions adsorption capacity of 31.87 mg g−1 and 72.65 mg g−1, respectively [27].
Although many efforts have been set to fabricate activated carbon with high salt adsorption capacity, tuneable pore size distribution with excellent electrical conductivity is still require exploring. In order to enhance the transport of ions within the pore structure, high mesoporosity to microporosity ratio is required. In addition, un-overlapping electrical double layer formation at the electrode surface is the key aspect to consider in the development of activated carbon for CDI electrode [28]. Date palm is one of the main crops in Oman and ∼180,000 tons of date palm leaflets are produced annually as agricultural waste with little or no use [29]. One of the advantages of the biomass produced from agricultural by-products are low ash with high carbon contents [30]. Numerous studies have been reported as activated carbon derived from biomass as adsorbent for the removal of various ions and dye molecules [31,32]. However activated carbon utilized as multifunctional electrodes for removal of ions and dye molecules using CDI technique has not been explored yet.
In this study, activated carbon powder was derived from agricultural biomass of date palm leaflets by sodium hydroxide (NaOH) activation and used as electrode material for CDI process. Prepared activated carbon powder was characterized by various tools to investigate the surface nature and electrochemical properties. The surface properties analysis was carried out to study the surface morphology, structural defects, the chemical structure, surface composition and the chemical state of activated carbon electrodes. The electrochemical analysis was conducted to investigate the specific capacitance and electrical properties of activated carbon electrodes. The fabricated activated carbon powder was assembled as the electrodes in CDI cell and conducted desalination and organic dye removal and electro degradation processes. The results indicated that an ions adsorption capacity of 5.38 mg/g with maximum ions adsorption rate of 0.046 mg/g-s in 100 ppm NaCl solution at 1.8 V (DC). Moreover, the removal and electro degradation of 10 ppm methylene blue molecule from aqueous solution with and without applying potential was also investigated and 85% of dye removal/degradation was achieved under the influence of applied potential. To the best of our knowledge, this is the first time to report activated carbon derived from the date palm leaf utilized as multifunctional electrodes for removal of salt ions and electro degradation of MB using CDI technique.
Section snippets
Materials
Carbon black (Super P Conductive) and methylene blue were purchased from Alfa Aesar, Germany. Polyvinylpyrrolidone (PVDF) was procured from Sigma-Aldrich, USA. 1-methyl-2 pyrrolidone (NMP) solution was obtained from Merck, Germany. Sodium hydroxide and sodium chloride (analytical grade) was obtained from BDH, UK. Dry date palm leaflets were collected from a local farm in Muscat and thoroughly washed with deionized water in order to remove dust and superficial impurities then left to dry at room
Surface properties of activated carbon
The surface morphology and elemental composition of AC powder was observed by FE-SEM attached with EDXS. The surface of AC powder comprises with nanoscale highly porous structures mixed with cylindrical-like cavities called vascular bundle (Fig. 1a: indicated with red arrows). Fig. 1b shows the detail structure of vascular bundle which is still intact during carbonization and activation process. The natural existence of this structure helps to increase the surface area which further enhance the
Conclusions
Activated carbon powder was produced from date palm leaflets using sodium hydroxide activation method and used as electrodes in CDI system for removal of salt ions and degradation and removal of dye molecules. A detail analysis of AC surface properties and electrochemical properties were conducted. The surface properties revealed that fabricated AC showed highly porous structure with specific functional groups on its surface. The electrochemical properties showed a characteristic feature of
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
CRediT authorship contribution statement
Htet Htet Kyaw: Conceptualization, Methodology, Formal analysis, Writing - original draft. Said M. Al-Mashaikhi: Conceptualization, Investigation, Methodology, Project administration. Myo Tay Zar Myint: Conceptualization, Methodology, Formal analysis, Writing - review & editing. Salim Al-Harthi: Supervision, Writing - review & editing. El-Said I. El-Shafey: Data curation, Formal analysis, Supervision, Writing - review & editing. Mohammed Al-Abri: Supervision, Writing - review & editing.
Acknowledgement
The authors would like to acknowledge the partial support of Nanotechnology Research Center, Department of Physics (Surface Science Lab) and Department of Chemistry, College of Science, Sultan Qaboos University. The authors also would like to thank financial support from Sultan Qaboos University internal grant projectIG/DVC/NRC/20/01.
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