Preparation and characterisation of high performing magnesite-halloysite nanocomposite and its application in the removal of methylene blue dye

Accepted Manuscript Preparation and characterisation of high performing magnesite-halloysite nanocomposite and its application in the removal of methylene blue dye T. Ngulube, J.R. Gumbo, V. Masindi, A. Maity PII:

S0022-2860(19)30173-5

DOI:

https://doi.org/10.1016/j.molstruc.2019.02.043

Reference:

MOLSTR 26200

To appear in:

Journal of Molecular Structure

Received Date: 1 October 2018 Revised Date:

9 February 2019

Accepted Date: 11 February 2019

Please cite this article as: T. Ngulube, J.R. Gumbo, V. Masindi, A. Maity, Preparation and characterisation of high performing magnesite-halloysite nanocomposite and its application in the removal of methylene blue dye, Journal of Molecular Structure (2019), doi: https://doi.org/10.1016/ j.molstruc.2019.02.043. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Graphical abstract

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Preparation and characterization of high performing magnesite-halloysite nanocomposite and its application in the removal of methylene blue dye ♣

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T. Ngulube a, J.R Gumbob, V Masindi c&d and A. Maity e&f.

Department of Ecology and Resources Management, School of Environmental Sciences, University of Venda, Private bag X5050,

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Department of Hydrology and Water Resources, School of Environmental Sciences, University of Venda, Private bag X5050, Thohoyandou, 09 50, Limpopo, South Africa.

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Thohoyandou, 0950, Limpopo, South Africa. Tel: +27159628563, Email: [email protected]

Council for Scientific and Industrial Research (CSIR), Built Environment, Hydraulic Infrastructure Engineering, P.O BOX 395, Pretoria,

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0001, South Africa.

Department of Environmental Sciences, School of Agriculture and Environmental Sciences, University of South Africa (UNISA), P. O. Box 392, Florida, 1710, South Africa 5

DST/CSIR National Centre for Nanostructured Materials, Council for Scientific and Industrial Research (CSIR), Pretoria, South Africa

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Department of Applied Chemistry, University of Johannesburg, Johannesburg, South Africa

Abstract

Nanoparticles have novel characteristics enabling them to efficiently decontaminate water

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hence the application of nanotechnology in wastewater treatment is being widely explored to tackle water pollution challenges. Industries are in a quest for decolouration and contaminant depollution technologies. In that regard, this study was designed with the aim of preparing a nanocomposite from calcined cryptocrystalline magnesite and halloysite nanoclay and then

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evaluating the influence of several parameters in the removal of Methylene Blue (MB) from aqueous solution by the prepared nanocomposite. Physicochemical characterization was

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carried out to get an insight of pre- and - post adsorption characteristics of the material. According to the results, the uptake of MB was rapid, and the maximum adsorption capacity and percentage removal were observed to be 0.65 mg/g and 99.66% respectively. Two adsorption isotherms and kinetic models were applied to describe the dye adsorption behaviour. Experimental results fitted the Langmuir (R2 = 0.98) and pseudo-second order models (R2 = 1) perfectly hence demonstrating that adsorption took place on a homogenous adsorbent layer via chemisorption. Furthermore, regeneration results showed that the nanocomposite can be used repeatedly recording a 35% removal at the 4th regeneration cycle. In overall, the results suggested that the nanocomposite is a suitable adsorbent for ♣

Corresponding author: Email; [email protected]

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ACCEPTED MANUSCRIPT decolourising industrial wastewater. Due to its local availability and non-toxic nature, calcined cryptocrystalline magnesite – halloysite nanocomposite can be considered a good alternative of conventional dye adsorbents commonly used in wastewater treatment especially in developing countries like South Africa. Keywords: adsorption; calcined cryptocrystalline magnesite; halloysite, methylene blue;

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nanocomposite; wastewater treatment Introduction

Raw earth materials like magnesite obtained directly from mines have large crystallite sizes

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with low surface-area-to-volume ratio (Mu and Wang, 2016), and such properties limit their application in nanotechnology especially in adsorption processes. For such a material to be

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applicable in adsorption processes, it must be modified to increase its surface area which leads to high adsorption capacities. Several studies have reported on the effect of mechanical modification on the morphological and structural characteristics of different adsorbents (Kumrić et al., 2013; Ramadan et al., 2010; Vdović et al., 2010). However there exists only a few research studies evaluating the use of a ball miller to enhance the adsorption characteristics of the developed adsorbent (Kumrić et al., 2013; Nenadović et al., 2009;

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Vdović et al., 2010). Disintegration, alteration of the crystalline network, particle arrangement, particle size reduction ultimately leads to a larger surface area which enhances contaminant removal efficiency of the adsorbent (Hrachová et al., 2008; Makó et al., 2001;

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Vdović et al., 2010).

In practice, grinding has been widely used as a traditional method to process inorganic

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minerals before they are used in various industrial application due to the inexpensiveness of the method and the simplicity in operation (Mu and Wang, 2016). It is practical to process raw minerals preliminarily by crushing and grinding them to augment their adsorption properties before any application in water treatment purposes. In this study, halloysite nanoclay mineral and calcined cryptocrystalline magnesite were incorporated in the preparation of a nanocomposite adsorbent. The synthesis involved a simple physical method employing a vibratory ball mill to mix the two materials. Usually, milling is used for reducing particle size together with additional unforeseen changes, that have a direct effect on the physical and chemical properties of the inorganic clay minerals leading to the enhancement of their contaminant removal potential.

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ACCEPTED MANUSCRIPT Technologies applied for removal of dyes from wastewater can be categorised into physical, biological and chemical. Though these conventional methods have been extensively used, they do have some limitations (Kobya et al., 2007; Du et al., 2013). For instance, biological treatment methods consume time and tend to be ineffective in the removal of highly structured polymer dyes that are not easily biodegradable. Moreover, biological methods

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have a limitation in most wastewaters because most commercial dyes are toxic to the organisms used (Buthelezi, 2012). On the other hand, chemical coagulation causes additional contamination because of undesired reactions in treated water (Kobya et al., 2007). Moreover, these methods are generally expensive and most times there is inadequate

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treatment efficiency due to the high variability of textile wastewaters (Drouiche et al., 2012). There are however some methods like photodegradation which are gaining much attention because of their effectiveness, less toxic after effects and low cost in treating coloured water

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(Naushad et al., 2016; Pathania et al., 2015; Thakur et al., 2017; Gnanasekaran et al., 2018; Sharma et al., 2018). Nevertheless, adsorption tends to be amongst the most favoured waste water treatment techniques. In the adsorption method, raw effluent is contacted with the adsorbent material. The adsorbent material retains the dye molecules either by physical, chemical or ion exchange mechanisms. The adsorbent gets saturated after a certain period of

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time and requires regeneration. The adsorption process is extensively used and gives satisfactory results (Wang et al., 2008). For this reason, adsorption seems to be a more attractive method in terms of cost, simplicity of design and operation. Moreover, the adsorbent can be easily recovered and reused. Even though a wide variety of adsorbents have

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been applied in the process of wastewater treatment, naturally available earth materials have been the adsorbents of choice in most developed countries.

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The objective of this study was therefore to prepare a nanocomposite adsorbent from halloysite nanoclay and calcined cryptocrystalline magnesite and assess the performance of the calcined cryptocrystalline magnesite - halloysite nanocomposite (CCMHN) in MB removal from aqueous solution. MB is usually used to dye silk, wood and cotton (Acemioglu, 2005). MB is reported to be less hazardous, however, it has numerous harmful effects where severe exposure to humans may cause jaundice, cyanosis, shock, nausea and quadriplegia (Li et al., 2017). When compared to other studies that have used composite materials for dye removal purposes, this work is better because the composite used was prepared from naturally occurring earth materials that do not require chemical pre-treatment, the method of composite 3

ACCEPTED MANUSCRIPT preparation is simple and very quick. Moreover, the composite material can be regenerated and reused many times for the removal of MB. 2

Experimental procedure

2.1

Chemicals and Materials

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Calcined cryptocrystalline magnesite was obtained from Sterkfontein carbonate group Pty (Ltd). Cryptocrystalline magnesite was calcined at 900 K using a thermal oven. Halloysite nanoclay – kaolin clay was purchased from Sigma Aldrich, Germany. HCl (32%), NaOH pellets (AR grade) and Methylene Blue trihydrate (analytical reagent) with a molecular

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weight of 373.89 g/mol were bought from Rochelle chemicals, South Africa.

Preparation of calcined cryptocrystalline magnesite - halloysite nanocomposite

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(CCMHN)

To prepare CCMHN, the optimised parameters reported by Masindi et al. (2017) were used. To achieve that equal portions of calcined cryptocrystalline magnesite (100 g) and halloysite (100 g) were mixed together in a glass beaker and portions of 50 g of the mixture were milled using a stainless-steel vibratory ball mill (Retsch RS 200, Germany) for 30 min at 1000 rpm. After the nanocomposite was milled into a fine powder (< 50 nm) it was put in a plastic zip

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lock back, sealed and stored until utilisation.

Preparation of MB working solutions

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To prepare the stock solution (1000 mg/L), a specific amount of dye powder was dissolved in deionised water (ELGA Micra, Veolia Water Solution and Technologies, UK). All dye solutions were prepared using deionized water to prevent possible ion interferences.

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Deionised water was used to dilute the stock solution to obtain the desired working solutions concentration range (1 to 30 mg/L). MB concentration in the experimental solution determination was done by measuring solution absorbance at λmax = 664 nm using an UV– VIS spectrophotometer (Thermo Scientific Orion Aqua matte 7000, China). 2.4

Characterisation of the adsorbent

Point of zero charge (PZC) was performed via the solid addition method as described by Izuagie et al. (2016). X ray Diffraction analysis was done using a PANalytical X’Pert Pro powder diffractometer in θ–θ configuration with an X’Celerator detector and variable divergence- and fixed receiving slits with Fe filtered Co-Kα radiation (λ=1.789Å). Fourier 4

ACCEPTED MANUSCRIPT Transform Infra-Red analysis was done using FTIR spectrometer (Bruker Alpha model, USA). TEM images were attained with a JEOL JEM-2100F Field Emission Transmission Electron Microscope (JEOL, Japan). SEM images were attained on a Zeiss Ultra Plus and Zeiss Cross Beam 540 Field Emission Scanning Electron microscopes (Carl Zeiss, Germany). Thermal stability of the nanocomposite was studied, using a Thermo Gravimetric Analyser

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(TGA Q500, TA instrument) under air atmosphere with a flow rate of 50 mL/min and a heating rate of 10 °C/min. Surface area was determined using Brunauer Emmett Teller (BET) analysis (Micromeritics Tristar II, Norcross, GA, USA). 2.5

Adsorption Studies

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The batch study was done using 250 mL glass Erlenmeyer flasks and a fixed volume of 50 mL dye solutions was used for all experiments. The flasks were shaken at a specified time at

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250 rpm in a waterbath shaker (Labotec, Model 207, South Africa). The effect of several variables of contact time (15 - 90 min), CCMHN dose (0.5 - 3 g), pH (2 - 12), temperature (25 – 55 oC) and initial dye concentration (1 - 30 mg/L) were studied. The initial pH of the solution was adjusted using 0.1 M NaOH or HCl as required. The pH of the solution was measured with a CRISON MM40+ multimeter probe (Hach Lange, Spain) for pH, Electrical Conductivity, Total Dissolved Solids and temperature. At the end of the adsorption

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experiments the concentration of dye was given by measuring the absorbance of the solution with λ = 664 nm using a UV-visible spectrophotometer. For quality control and assurance, all the experiments were carried out in triplicate and the mean values were presented. Adsorption kinetics

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Studying dye adsorption kinetics of adsorbent materials is necessary to choose the optimum

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operational settings for the complete batch system. Adsorption kinetics demonstrates the rate of dye uptake. In this study, two adsorption kinetic models were applied.

2.6.1

The Lagergren pseudo-first-order model

The pseudo first order kinetic model is usually applied for weak concentrated aqueous solutions. This model is given by the Lagergren relationship (Lagergren, 1898) based on adsorbed quantities. It is the first equation recognized to describe adsorption kinetics in a liquid/solid system. Its linear form is given as: 

log( −  ) = log ( ) − ( . ) … … … … … … … … … … … … … … … … … … … … … … . (3)

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ACCEPTED MANUSCRIPT where qe and qt are the amounts of dye adsorbed (mg/g) at equilibrium and at time t (min) respectively and K1 (min-1) is the pseudo-first-order rate constant. 2.6.2

The pseudo-second-order model on dye adsorption

McKay, 1999) generally known and described as: 





=   +  



 

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Adsorption kinetics may also be described by the pseudo-second order model (Ho and  … … … … … … … … … … … … … … … … … … … … … … … … … … … … … (4)

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Adsorption isotherms

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adsorbed (mg/g) at equilibrium and at time t (min).

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where K2 (g/mg/min) is the pseudo-second-order rate, qe and qt are the amounts of dye

Adsorption isotherms generally define the equilibrium adsorbate concentration in the bulk of the solution and the amount adsorbed on the adsorbent surface. Studying adsorption isotherms helps in predicting the ability of the adsorbent to take up an adsorbate and to design adsorption systems (Sivakumar et al., 2014). Data obtained in this study was analysed using

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the conventional Langmuir and Freundlich models.

The Langmuir model

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The Langmuir model (Langmuir, 1918) is based on these assumptions: formation of a single layer of adsorbate on the adsorbent surface, the presence of defined adsorption sites and a

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uniform surface short of interaction between the adsorbed molecules.

The Langmuir equation is written as follows: 





=   ! + 



"

………………………………………………………………………… (5)

Where Ce is the equilibrium concentration (mg/L), Qe is the amount adsorbed at equilibrium (mg/g), b represents the Langmuir isotherm constant and Qm is the maximum adsorption capacity for a complete monolayer coverage.

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The Freundlich model

The Freundlich model assumes that the distribution of interaction energies can be explained by heterogeneity of adsorption sites (Freundlich, 1906). Contrary to the Langmuir model, the Freundlich model does not give an upper limit for adsorption which restricts its application to dilute media. Nevertheless, the Freundlich model acknowledges that adsorbed molecules

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interact. The equation is written as follows:

#$%& = ' #$% ! + #$% () … … … … … … … … … … … … … … … … … … … … … … … … … . . (6)

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KF and 1/n are the Freundlich constants, describing the adsorption capacity and intensity respectively.

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Regeneration studies

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respectively. The constants n and KF are determined from the slope and the intercept

To determine if CCMHN could be regenerated and re used after adsorption, its regeneration potential was evaluated using 0.01 M NaOH. An MB solution of 10 mg/L and 2 g/ 50 mL

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solid/liquid ratio was used to perform the adsorption experiment. The adsorption capacity was calculated and then the remaining CCMHN was placed in the oven to dry for 12 h at 105 o

C. The dried CCMHN was poured into 100 mL, 0.01 M NaOH solution and then centrifuged

at 5 000 rpm for 15 min using an MRC Labs BLCEN-208 (LabGear, USA) centrifuge.

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Thereafter it was rinsed in 100 mL deionised water and then dried for 12 h at 105oC in an oven. As described above, the procedure of adsorption – desorption cycles were carried out

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three times.

Results and discussion

Optimization of the synthesis of the nanocomposite adsorbent

To determine the optimum conditions for fabricating the nanocomposite that would effectively remove dyes from aqueous solution, a dye adsorption experiment was carried out at each stage of optimization to determine which conditions gave the highest MB dye removal. Three ratios of the calcined cryptocrystalline magnesite and halloysite nanoclay respectively (1:1; 2:1; 1:2) were evaluated and the results of dye removal experiment were as follows: 1:1 = 96%, 1:2 = 95% and 2:1 = 92%. The highest MB dye removal was recorded 7

ACCEPTED MANUSCRIPT when ratio of calcined cryptocrystalline magnesite and halloysite nanoclay was 1:1. Based on that, 1:1 ratio was chosen as the best to prepare the nanocomposite to use for the dye adsorption experiments. Adsorbent physicochemical characteristics

3.2.1

Point of Zero Charge (PZC)

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3.2

Figure 1 shows PZC analysis of CCMHN. Figure 1 displays that the 3 curves (various KCl electrolyte concentrations) had no common intersection point at ∆pH = 0 thus a specific pH could not be considered as pHpzc. However, the curves almost intersected at pH 10 hence for

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the purposes of this adsorption study, it can be assumed that CCMHN has a PZC of 10. At pH < pHpzc the surface of the adsorbent is positive, at pH = pHpzc, the surface charges are neutral and at pH > pHpzc, the surface of the adsorbent is negative (Gatabi et al., 2016).

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Therefore, the adsorption of MB (a cationic dye) is expected to be high at pH greater than 10 because of electrostatic attraction between positively charged MB ions and the negatively charged surface of the adsorbent (Rao and Kashifuddin, 2016). 0,1 M KCl 0 -2 ∆pH

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10

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Initial pH

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0,001 M KCl

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0,01 M KCl

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Figure 1: Point of Zero Charge of CCMHN 3.2.2

X-ray Diffraction (XRD)

XRD analysis is displayed on Figure 2 and it shows that calcined cryptocrystalline magnesite consists of magnesite and periclase as the main mineral phases. Halloysite nanoclay was

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ACCEPTED MANUSCRIPT observed to contain halloysite, quartz and kaolinite minerals. The composite (CCMHN) is seen to contain halloysite, quartz, brucite, periclase and magnesite. Preparing the nanocomposite via ball milling evidently modified the material. From the diffractogram of CCMHN, the intensity and number of peaks that were observed on unmodified halloysite nanoclay and calcined cryptocrystalline magnesite were reduced. MB adsorbed CCMHN was

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observed to contain brucite, periclase, quartz and kaolinite. Similar observations were reported by Masindi et al. (2017) for their magnesite-bentonite composite used in the remediation of metalliferous mine drainage. Brucite is basically Mg(OH)2 and its high presence on MB adsorbed CCMHN could be a result of periclase (MgO) reacting in aqueous

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solution of dyes to form Mg(OH)2. Hence, the presence of brucite after adsorption of the dye could be evidence of a precipitation reaction taking place with hydroxides precipitating out of

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the solution (Bouyakoub et al., 2011). 50000

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Intensity (out of scale)

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30000 25000

Halloysite Composite Composite and MB

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Calcined magnesite

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15000 10000

10

20

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40 50 60 2 theta degree

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Figure 2: XRD diffraction patterns of calcined cryptocrystalline magnesite, halloysite, CCMHN and MB adsorbed CCMHN

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Fourier Transform InfraRed (FTIR)

The FTIR spectra of calcined cryptocrystalline magnesite, halloysite, CCMHN and MB adsorbed CCMHN are shown in Figure 3. On the CCMHN, the presence of an absorption band at 505.92 cm-1 on the spectrum corresponds to the Si-O-Si bending hence showing that the material contains silicate which makes up halloysite (Ballav et al., 2014). The absorbance

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at wavelength number 858 cm-1 corresponds to OH deformations linked to Al3+ found in halloysite material. The band at 1058 cm-1 is given to perpendicular Si–O–Si stretching vibrations (Szczepanik et al., 2015; Cheng et al., 2010). The band at 1550 cm-1 corresponds to asymmetric stretching vibrations of carbonate. The band at 3620 cm−1 is given to the

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kaolinite internal hydroxyl group stretching vibration while that at 3697 cm−1 is given to the interlayer hydroxyl stretching mode (Adebowale et al., 2014). The spectrum of MB adsorbed

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CCMHN and raw CCMHN exhibited minor differences in some bands but generally, the bands were almost the same before and after MB adsorption. The intensity of the peak for MB adsorbed CCMHN increased at 515, 911 and 1028 cm-1. These bands are characteristic of Si-O-Si bending and OH groups and the change in their intensities shows that these functional groups took part in MB adsorption from aqueous solution. The mechanisms of

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adsorption involving these functional groups is shown illustrated under section 3.7.

550

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Halloysite Magnesite Their composite Composite and MB

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2050 2550 Wave number (cm-1)

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3550

Figure 3: FTIR spectra of calcined cryptocrystalline magnesite, halloysite, CCMHN and MB adsorbed CCMHN 10

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Transmission Electron Microscopy (TEM)

Figure 4 shows TEM images of calcined cryptocrystalline magnesite, halloysite, CCMHN and MB adsorbed CCMHN. From image (A) and (B), it can be observed that calcined cryptocrystalline magnesite particles have spheroidal and hexagonal morphology. The dominant morphology of halloysite (C) and (D), is tubular. Some tubules are long and thin,

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and others are short and stubby or emerging from other tubes. A closer look at the TEM images of CCMHN, images (E) and (F) shows a large rod particle which is hollow, characteristic of the tubular morphology of halloysite. Other particles have different hexagonal and circular shapes while most of the particles in MB adsorbed CCMHN - (G) and

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(H) have undefined similar morphology but conspicuous hollow tubular rods can be seen on image (H). The slight change in the particle shapes suggests possible interaction between MB

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and CCMHN. MB adsorption could further be evidenced by images (G) and (H) showing some particles being attached on the inside wall of nanotubes. The particle diameter of the

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materials was in the range of 20 to 50 nm.

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Figure 4: TEM images of calcined cryptocrystalline magnesite - (A) and (B), halloysite - (C) and (D), CCMHN - (E) and (F) and MB adsorbed CCMHN - (G) and (H) 12

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Scanning Electron Microscopy (SEM)

Figure 5 shows the SEM images of calcined cryptocrystalline magnesite - (A) and (B), halloysite - (C) and (D), CCMHN - (E) and (F) and MB adsorbed CCMHN - (G) and (H). Some spherical and hexagonal aggregate particles (largely crystallized) are seen on all the images. No notable difference in the morphology of particles is shown before and after MB

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adsorption.

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Figure 5: SEM images of calcined cryptocrystalline magnesite- (A) and (B), halloysite - (C) and (D), CCMHN - (E) and (F) and MB adsorbed CCMHN - (G) and (H)

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Thermogravimetric Analysis (TGA) study

The thermogravimetric analysis of CCMHN and MB adsorbed CCMHN is presented in Figure 6. Both thermograms showed a similar trend of multistage decomposition corresponding to the loss of moisture and the exit of water adsorbed on the surface of clay colloids. The CCMHN and MB adsorbed CCMHN thermograms showed weight losses of

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15.39% and 21.99% respectively. This means that MB adsorbed CCMHN decomposed more than the raw CCMHN. Since MB is an organic compound, it might have decomposed at about 350 °C where most organic residues start to decompose. Since the profiles were almost similar, the thermal stability will be highlighted using the MB adsorbed CCMHN

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thermogram. At temperatures less than 150 °C, a total mass of 1.75% was lost, which corresponds to adsorbed water phase change (Azarkan et al., 2016). Between 150 °C and 330

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°C, a 1.92% mass loss was observed. This mass loss tallies with phase change of interfoliar water and thereafter there was a high mass loss (7.94%) from 330 to 550 °C consistent with kaolinite dehydroxylation. Lastly, the mass loss in the range 550 °C to 726 °C (3.42%) corresponds to a combination of illite dihydroxylation, crystalline water phase change and decomposition of calcite (Bentahar et al., 2016). Above 600 °C, the observed mass loss can be ascribed to the decomposition of dolomite and calcite found in the magnesite material.

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Furthermore, the loss mass at this final stage can be ascribed to volatization of all the impurities attached onto the nanocomposite surfaces. Similar trends of thermogravimetric profiles were also reported by Bentahar et al. (2016.). A further increase in temperature above 726

o

C shows no further decomposition, meaning that above that temperature the

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nanocomposite material is thermally stable.

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100 Composite Composite and MB

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% weight loss

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80 75 200

300

400 500 600 700 Temperature (°C)

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800

900 1000

Figure 6: Thermogravimetric analysis of raw CCMHN and MB adsorbed CCMHN 3.2.7

Brunauer–Emmett–Teller (BET) analysis

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One of the reasons why clay-based materials are favourite candidates used in adsorption systems is because of their high surface areas which ultimately leads to high adsorption capacities (Ngulube et al. 2017). Table 1 shows the surface areas of raw and MB reacted CCMHN and the general observation is that after MB adsorption, there was a reduction in The surface area decrease designates the entry of MB molecules into

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surface area.

nanocomposite interlayers leading to the obstruction of layer channels and consequent

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reduction of the overall surface area of the material (Yan et al., 2015). Table 1: Surface areas of raw (CCMHN) and MB reacted CCMHN (MB-CCMHN) Single

point BET Surface Area

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surface area (m²/g) (m²/g)

Area(m²/g)

CCMHN

17.9525

18.1753

25.3153

MB-CCMHN

16.9215

17.0475

23.6503

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Surface

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Study of the influence of various parameters on the adsorption of MB

The effect of contact time, adsorbent dosage, initial MB concentration, pH and temperature

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on the adsorption of MB is shown in Figure 7.

Figure 7: Removal of MB by CCMHN as a function of (A) - contact time [1 g, 10 mg/L, 50 mL , 250 rpm, room temperature] (B) – dosage [60 minutes, 10 mg/L, 50 mL , 250 rpm, room temperature] (C) - initial MB concentration [60 minutes, 2 g, 50 mL , 250 rpm, room temperature] (D) - pH [60 minutes, 2 g, 10 mg/L, 50 mL , 250 rpm, room temperature] (E) temperature [60 minutes, 2g, 10 mg/L, pH4, 50 mL , 250 rpm] 17

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The influence of time of contact on the adsorption of MB

The study of the kinetics of MB removal by CCMHN is presented in Figure 7(A). The study of the progressive development of the MB percent removal, displayed that it rose rapidly within the first 60 minutes and then reached equilibrium after. The observed percentage increase during the first 60 minutes is possibly due to adsorption sites availability on the

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surface of the adsorbent (Li et al., 2017). According to the curve on Figure 7(A), the contact time required to reach equilibrium condition was 60 min and thus it was taken as the optimum time for subsequent experiments.

The influence of adsorbent dosage on the adsorption of MB

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3.3.2

The trend of dye removal percentage with adsorbent dose is given in Figure 7(B). Dosage is among the chief parameters that influence the adsorption process by affecting the adsorbent’s

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adsorption capacity. The dye percentage removal increased as the adsorbent dose was increased from 0.5 to 1.5 g and then gradually increased until 3 g. The rapid increase at the initial stages is associated with an increased surface area of the adsorbent and abundance of vacant adsorbent sites (Kumari et al., 2017). The further continual constant adsorption is probably due to the binding of almost all MB molecules on the surface of the adsorbent

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surface and equilibrium attainment in solution between the adsorbate and the adsorbent (Liu et al., 2015; Cho et al., 2015). Consequently, in all the subsequent experiments, 2 g of adsorbent dose was used. A similar trend was described for MB adsorption by modified

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lignocellulosic materials (Manna et al., 2017).

The influence of initial dye concentration on adsorption of MB

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Figure 7(C) shows the trend of CCMHN adsorption capacity and MB percentage removal vs initial MB concentration. The results show a general trend of decreasing percentage removal as the initial MB concentration is increased. Adsorption capacity however showed an opposite trend wherein adsorption capacity increased as initial MB concentration was increased.

The results of the decrease in percentage removal with increasing dye

concentration are similar to those reported for MB adsorption onto mesoporous birnessite where an increase of adsorption capacity from 33.3 to 106.1 mg/g was observed (Pang et al., 2017). Such a trend could be credited to the concentration gradient driving force increasing with increasing the initial dye concentration.

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The influence of pH on the on adsorption of MB

The initial solution pH usually has a notable influence on adsorption processes by affecting the molecular structure of dyes in aqueous solution and affecting the surface charges of the adsorbent material too. In this study, the pH of the solution was maintained between 2 and 12. Figure 7(D) shows that percentage dye removal decreased from 93.80% - 65.47% from

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pH 2 – pH 4. The overall observation is that as the pH increased, the percent removal of MB by CCMHN decreased. From the PZC results, it is expected that, the removal of MB, a cationic dye is expected to be high when the adsorbent is negatively charged at pH above 10, however MB percent removal was lower, possibly implying that the solution pH had a

capacity of the material increased with increase in pH.

The influence of temperature on the adsorption of MB

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3.3.5

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negligible influence on the charges of the adsorbent’s surface. Nevertheless, the adsorption

Assessment of the effect of solution temperature on the adsorption process was done to evaluate the capability of CCMHN in dye adsorption in cases of a variety of wastewaters, considering different conditions that dye wastewater exists in. The influence of solution temperature on MB adsorption was done at four temperatures ranging from 25 to 55ºC. The

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quantity of MB adsorbed on CCMHN at different temperatures is given in Figure 7(E). It is seen that, increasing the temperature of the solution decreased the amount of MB adsorbed. The drop of adsorption capacity with rising temperature shows that adsorption was an exothermic process (Yagub et al., 2014). This may probably be because high temperatures

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reduce forces of adsorption between the available adsorbent sites and the dye molecules as a result, decreasing the rate of adsorption (Salleh et al., 2011). For this reason, it can be said

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that there is no need to increase the solution temperature since the maximum adsorption capacity of CCMHN is recorded at room temperature.

Like results were recorded by

Elmoubarki et al. (2015) where they noted that the MB adsorption was less affected by temperature, the adsorbed amounts slightly decreased with the increase of solution temperature from 10 to 50 oC. 3.4

MB adsorption kinetic studies

3.4.1

Application of Lagergren pseudo-first-order model on dye adsorption

For the Lagergren pseudo-first-order model to be deemed fit, the plot of t vs log (qe-qt) must be linear and the calculated results from this study did not give any linear relationship. This 19

ACCEPTED MANUSCRIPT indicates that the kinetics of adsorption of MB on CCMHN does not correspond to the kinetics of the pseudo-first order.

3.4.2

Application of pseudo-second-order model on dye adsorption

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Modelling of MB adsorption kinetics onto CCMHN by the pseudo-second order model is presented on Figure 8. The kinetic parameters attained by fitting MB adsorption kinetics data are displayed on Table 2. The kinetic models’ cogency is derived from the correlation coefficients values (R2). From Figure 8 is noted that the data fitted well to the pseudo-second

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order kinetic model. Therefore, the pseudo-second order is the appropriate adsorption model to describe MB dye adsorption onto CCMHN hence indicating that the rate limiting step during the adsorption process could be chemisorption. However, the experimental and

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10

20

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400 350 300 250 200 150 100 50 0

30

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t/qt

calculated adsorption capacities shown on Table 2 are not very close to each other.

40

R² = 1

50

t

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Figure 8: Pseudo second order kinetic model

20

60

70

80

90

ACCEPTED MANUSCRIPT Table 2: Adsorption kinetics and isotherm parameters for MB removal by CCMHN Pseudo-second-order Parameter values

Qe (exp) mg/g

Qe (cal) mg/g

K2 (g/mg/min) R2

0.6547

0.0006

0.001

1

Qm (mg/g)

Ka (L/mg)

RL

R2

0.7079

1.3570

0.0445

0.9766

KF (mg/g)

1/n

n

R2

2349

0.5245

1.9066

Parameter values

3.5

Isotherms of MB adsorption

0.8812

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Parameter values

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Freundlich model

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Langmuir model

The data obtained in this study was analysed using the common Langmuir and Freundlich adsorption isotherm models. When the correlation coefficients values (R2) of the applied isotherm models where compared, the best fit was given by the Langmuir model (Figure 9(a))

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than the Freundlich model (Figure 9(b)) for MB adsorption onto CCMHN with a high R2 value of 0.98. Data fitting onto the Langmuir model shows that adsorption of MB took place on homogeneous surfaces (Ngulube et al., 2017). RL (the separation factor) values indicate the nature of adsorption to be unfavorable if RL > 1, linear if RL = 1, favourable if 0 < RL < 1

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and irreversible if RL = 0. From the data in Table 2, the RL value is greater than 0 but less than 1 indicating that Langmuir isotherm is favourable. The maximum Langmuir monolayer

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coverage capacity (Qm) was determined to be 0.7079 mg/g. Accordingly, it was close to the experimental adsorption capacity (0.6547 mg/g) confirming the fit. Similar results were also obtained by Yan et al. (2015).

21

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3.6

Regeneration study

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Figure 9: (a) - Langmuir (b) - Freundlich isotherm model for MB adsorption onto CCMHN

It is very vital to regenerate spent adsorbents to determine their reusability to mechanise a cost-effective adsorption medium in wastewater treatment plants. Chemical regeneration by 0.01 M NaOH was done on the nanocomposite adsorbent and the regeneration restored 71.55 and 35% MB removal from cycle 2 to 4 respectively (Figure 10). From the recovery

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efficiency, it was evident that as the spent adsorbent was regenerated more, its capacity to adsorb MB was decreasing but it still managed to remove an appreciable amount of MB from aqueous solution after 4 cycles. The outcomes evidently demonstrated the stability and good

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reusability of the nanocomposite prepared in this work.

22

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100 90 80 70 60 50 40 30 20 10 0 Cycle 1

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Percentage dye removal

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Cycle 2

Cycle 3

Cycle 4

Figure 10: Regeneration of CCMHN

Proposed mechanism

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3.7

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Regeneration cycles

From the characterisation studies of the adsorbent, before and after dye removal, mechanisms of dye uptake can be established. FTIR analysis study showed bands at 3600 – 3700 cm-1 which are associated with OH groups adsorbed water shifted after dye adsorption indicating

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that the hydroxyl groups played an important role in the adsorption process. MB is a basic dye and its interactions with the surface of the adsorbent and the dyes are chemical. It seems

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that there is a formation of covalent bonds between oxides (CaO, MgO and SiO2,) available on the adsorbent surface with hydroxyl groups and the positively charged MB ions. Moreover, many studies report that organic dye adsorption oxide materials usually occur via electrostatic attraction (Ciesielczyk et al., 2017). A schematic representation of hydrogen bonding and electrostatic interaction between the adsorbent’s surface and MB dye molecules is presented in Figure 11.

23

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Figure 11: Hydrogen bonding and electrostatic interaction between the surface of the adsorbent and MB ions.

Comparison of adsorption capacity of CCMHN for MB with other adsorbents.

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3.8

Table 3 is showing several adsorbents for MB and their adsorption capacities. In contrast with some of the reported adsorbents, the adsorption capacity for CCMHN is fairly low compared to many adsorbents especially the chemically modified ones. However, a lot of

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factors affect the variances in adsorption capacities for example operational conditions

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including dosage, pH and time.

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ACCEPTED MANUSCRIPT Table 3: Comparison of the maximum adsorption of MB from this study with previous studies. Material

Maximum adsorption

References

capacity qm (mg/g) CCMHN

0.71 iron

oxide 0.19

Mak and Chen 2004

Fly ash

1.10

Woolard et al., 2002

Fly ash

1.47

Janos et al., 2003

Red mud

2.49

Wang et al., 2005

Fly ash

5.57

Fly ash

7.99

Aminofunctionalized

13.698

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Acid-bound

Present study

Coal fly ash

16.6

Natural illitic clay mineral

24.87

lignin-chitosan 36.25

extruded blends

Weng and Pan, 2006

Amrhar et al., 2015

Wang et al., 2008 Ozdes et al., 2014

Albadarin et., 2017

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Activated

Kumar et al., 2005

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attapulgite clay nanoparticles

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magnetic nanoparticles

Heat-treated palygorskite clay 78.11

Chen et al., 2011

Brown macroalga

95.45

Daneshvar et al., 2017

97.75

Ai et al., 2011

composite

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Montmorillonite/CoFe2O4 Aminofunctionalized

215

Zhou et al., 2015

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attapulgite clay nanoparticles

Şahin et al., 2015

Plasma-surface modification 303 on bentonite clay

MOF nanocomposite

4

325.62

Alqadami et al., 2018

Conclusion

This study demonstrated that calcined cryptocrystalline magnesite - halloysite nanocomposite can be utilised as an effective adsorbent for the removal of methylene blue from aqueous solutions. The characterisation results showed that the nanocomposite has particles size 25

ACCEPTED MANUSCRIPT ranging between 20 nm and 50 nm. It was further shown that the nanocomposite contained halloysite, quartz, brucite, periclase and magnesite and some of the compounds found in these minerals assisted in the uptake of MB dye by the nanocomposite via electrostatic interactions. XRD analysis showed good variations of mineralogy before and after adsorption with reduction in peak intensities after MB adsorption especially for periclase with a 54.67%

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reduction. The results showed that adsorption was also largely affected by parameters like adsorbent concentration and adsorbent dosage. pH however had a negligible influence in the adsorption of MB by CCMHN. The Langmuir isotherm was the most suitable model for MB adsorption with an R2 of 0.9766 compared to the Freundlich which had an R2 value of 0.8812.

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The kinetics data was described well by the pseudo second order implying that the process of adsorption was dependant on both time and concentration. The batch study showed that optimum conditions to remove MB by CCMHN are: 60 min contact time, dosage of 2 g,

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temperature at 25oC and initial MB concentration of 5 mg/L of MB. Conflict of interest

The authors declare no conflict of interest. Acknowledgments

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This research was funded by the University of Venda Research Directorate (grant number S839), National Research Foundation (NRF) of South Africa (grant number 102221) and Water Research Commission (WRC) (grant number K5/2756/3). The authors are also thankful to the Council for Scientific and Industrial Research (CSIR), Department of Ecology

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and Resource Management & Hydrology and Water Resources labs (University of Venda) for

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extending all facilities required for this research.

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Highlights •

The preparation of calcined cryptocrystalline magnesite-halloysite nanocomposite via mechanochemical modification was successfully done by a vibratory ball miller. Physicochemical characterization showed good variations of mineralogy, morphology and particle size before and after dye treatment.

•

Decolorization of dye in aqueous solution was successfully achieved with a recorded percentage removal of 99.66 %.

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The nanocomposite exhibited high stability during regeneration studies.

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•

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•