ArticleMesoporous activated carbon-zeolite composite prepared from waste macadamia nut shell and synthetic faujasite
Graphical abstract
Introduction
Activated carbons (ACs) are the most widely used adsorbents for many pollutants, owing to their low cost, sustainable production, and high performance. Activated carbons can be produced from coals, wood, and many other materials having high carbon content. Macadamia is a plant native to Australia but also cultivated in Hawaii, Brazil, California, South Africa, Israel, Bolivia, Guatemala, Colombia, Kenya, Costa Rica, Malawi, Mexico, New Zealand and China. Macadamia nut shell (MNS) is one of the leading biomass wastes in Australia. According to Australian the Macadamia Society, macadamia nut production was around 50000 tons in-shell in 2016 [1] representing about 39800 tons of nutshell waste. Only around 5000 tons of nutshell waste was utilised as solid fuel and garden mulch, and the disposal of the rest is a concern. MNS is a suitable precursor for the production of activated carbons due to its hardness and durability. Activated carbons produced from macadamia nut shell have higher surface area, surface reactivity, and percent yields compared to carbons produced from pistachio, hazelnut, pecan, almond, black walnut and English walnut [2] Macadamia nut shell-based activated carbons (MACs) were successfully used as adsorbents for removing aurocyanide [3], phenol [4], and methylene blue [5].
Zeolites (ZEOs) are crystalline microporous aluminosilicates that are built up of a 3-dimensional framework of SiO4 and AlO4 tetrahedra, linked by sharing oxygen atoms, weakly bonded cations and water molecules within the pores and voids of the structure [6]. Isomorphous replacement of Si (IV) by Al (III) produces a negative charge in the lattice, and the balancing Na, K or Ca cations are readily exchangeable with cations present in solutions. Zeolites are widely used as molecular sieves, catalysts, ion-exchangers, and adsorbents. Various zeolite types/frameworks show high affinity toward specific pollutants, and this has been exploited in water in wastewater treatment over decades. Zeolites were recommended for the removal of ammonia [7], heavy metal ions [8], dyes [9], arsenic [10], radionuclides [11], and emerging pollutants [12].
Zeolites and activated carbons may be used in combination to take advantage of their different sorption characteristics. To achieve this aim, the simplest solution is to use ordinary mixtures of activated carbon-zeolite granules in packed columns as macrocomposites to enhance removal efficiencies for targeted contaminants [13]. Unfortunately, this method is impractical for powdered materials due to the substantially different material densities and the resulting dosing and separation problems. Employing hybrid and composite materials can resolve the problems of particle separation and different settling rates in water. Suitable hybrids can be produced by combining activated carbon and natural zeolite particles with an inorganic binder, such as Portland cement [14]. This approach allows the manufacture of desirable granular shapes and sizes by extrusion; however, it also diminishes the specific surface area.
The development of zeolite-carbon composites mainly stems from the utilisation of fly ash wastes, rich in aluminium and silica with significant unburnt coal content [15], [16]. Composites prepared from coal ash are suitable for various environmental protection applications [17], [18]. Such materials are produced with simple methods to ensure low production cost, hence their sorption performance can be compromised by the composition (including impurities) of the precursor, coupled with limited coal content [19]. Improved composites with higher carbon content can be produced by separating and purifying the precursors. In a recent publication, Purnomo [20] demonstrated the advantages of this approach by preparing a composite from bagasse fly ash that showed good performance in phenol removal.
Similar outcomes can be achieved using suitable activated carbons and synthetic zeolites prepared from readily available raw materials. Using verified recipes and methods from the International Zeolite Association Synthesis Commission and other sources, appropriate hydrophilic or hydrophobic (high Si to Al ratio, organophilic) zeolites can be selected to create functional composites. This line of research has been a major direction in related material research [21] leading to the introduction of many advanced materials [22] but so far with no reports on the development of functional composite adsorbents engineered for water purification.
In this study, therefore, we investigate a functional mesoporous composite adsorbent made from MAC and hydrophilic synthetic faujasite. The material characteristics of the novel composite were determined using established laboratory methods. Methylene blue dye model adsorbate was employed in batch experiments at natural pH for activity characterisation, also covering desorption and adsorbent reuse. Experimental data were analysed with nonlinear fitting to kinetic and equilibrium models and thermodynamic study to identify the controlling mechanism and parameters.
Section snippets
Activated carbon-zeolite composite (ACZ) preparation
Waste macadamia nut shell of the Integrifolia species (Fig. 1) was obtained from a factory in Southeast Queensland, Australia. The crushed shells were washed with distilled water and dried at 105 °C for 24 h. In a typical batch, 100 g of the dried shell was charred at 400 °C for 2 h at a heating rate of 5 °C·min− 1 in an iron reactor furnace (F62730, Thermolyne) with the exclusion of air. About 50 g of char was placed in a crucible boat and loaded into a quartz tube reactor having the dimensions
Material characteristics of ACZ composite
Table 1 presents the results of proximate analyses of MAC and ACZ obtained with a CHONS analyser. ACZ has 47% fixed carbon with 33.4% ash content and 7% volatiles, in good agreement with the 55% to 45% carbon to zeolite mass proportion used in the preparation. Compared with the MAC, the carbon content in the ACZ decreased by 6.95%, while the oxygen and hydrogen contents increased by 6.21% and 0.75% due to the presence of alumino silicates.
Semi-quantitative (surface spot) chemical compositions
Conclusions
A novel mesoporous activated carbon-zeolite composite was produced from macadamia nut shell-based activated carbon and synthetic zeolite by the hydrothermal treatment process. The extensive qualitative and quantitative characterisation studies conducted on the composite confirmed the impregnation of the MAC carrier by mesoporous faujasite nanocrystalline clusters. The product contained mainly meso- and macropores with a specific surface area of 418 m2·g− 1, nearly the average of the parent
Nomenclature
- aH
Harkins–Jura isotherm parameter
- bH
Harkins-Jura isotherm constant
- bt
Temkin isotherm constant, J·mol− 1
- Ce
Concentration of MB at equilibrium, mg·L− 1
- C0
Initial concentration of MB, mg·L− 1
- Ke
Distribution coefficient
- kav
Avrami kinetic constant, min− 1
- kb
Bangham rate constant, g·mg− 1·min− ϑ
- kDC
Diffusion-chemisorption constant, mg·g− 1·min− n
- kf
Freundlich isotherm constant, (mg·g− 1)(L·g− 1)n
- kj
Jovanovic constant
- kl
Langmuir isotherm constant, L·MB− 1·mg− 1
- kr
Rate constant of general order model, h− 1·(g·mg− 1)n − 1
- kt
Acknowledgement
The first author thanks the University of Southern Queensland and Australian Government Research Training Program Scholarship for providing a scholarship to pursue this work.
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