Macroporous Microparticles Using Three

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Interface Components: Nanoparticles, Colloids, Emulsions, Surfactants, Proteins, Polymers

Synthesis of Mesoporous/Macroporous Microparticles Using ThreeDimensional Assembly of Chitosan-Functionalized Halloysite Nanotubes and Their Performance in the Adsorptive Removal of Oil Droplets from Water Asma Eskhan, Fawzi Banat, Mohammad Abu Haija, and Sameer Al-Asheh Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b04167 • Publication Date (Web): 09 Jan 2019 Downloaded from http://pubs.acs.org on January 10, 2019

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Synthesis of Mesoporous/Macroporous Microparticles Using ThreeDimensional Assembly of Chitosan-Functionalized Halloysite Nanotubes and Their Performance in the Adsorptive Removal of Oil Droplets from Water b c a, a, Asma Eskhan *, Fawzi Banat *, Mohammad Abu Haija and Sameer Al-Asheh a

b

Department of Chemical Engineering, Khalifa University of Science and Technology, SAN Campus, P. O. Box 2533, Abu Dhabi, United Arab Emirates

Department of Chemistry, Khalifa University of Science and Technology, SAN Campus, P. O. Box 2533, Abu Dhabi, United Arab Emirates c

Department of Chemical Engineering, American University of Sharjah, P.O. Box 26666, Sharjah, United Arab Emirates *Corresponding Authors: [email protected] [email protected]

ABSTRACT Halloysite nanotubes (HNTs) were assembled into mesoporous/macroporous microparticles (c-gHNTs MPs) using Pickering template-assisted approach. To unravel the stabilization mechanism in Pickering emulsion form, several emulsions and microparticles were prepared at various conditions and visualized using confocal laser scanning microscopy. The prepared c-g-HNTs MPs were used to treat emulsified oil solutions resulting in a maximum removal efficiency of 94.47%. The kinetics data of oil adsorption onto c-g-HNTs MPs was best fitted by the pseudo second order kinetic model (R2 = 0.9983). The maximum monolayer adsorption capacity of oil onto c-g-HNTs MPs as predicted by the multilayer BET model was found to be 788 mg/g. Compared with pristine HNTs, c-g-HNTs MPs exhibited higher self-settleability rates in aqueous solutions as well as in emulsified oil solutions demonstrating their candidacy for practical water treatment applications. The c-g-HNTs MPs were repeatedly used for five adsorption-desorption cycles with minimal losses noticed in their performance.

KEYWORDS: Halloysite, Chitosan, Pickering, Mesoporous, Microparticles, Adsorption, Oil.

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INTRODUCTION Environmental protection and sustainable use of natural resources have become a major global concern in the late decades. The governments and managing agencies in many countries have undertaken efforts to identify the hazards associated with the emerging pollutants in the environment and place adequate controls to mitigate the risks beyond these hazardous materials. Oily effluents are one of the emerging pollutants generated by industries such as refining crude oil, manufacturing lubricants and petrochemical intermediates.1 These effluents contain oil and grease compounds which are consisted of three main groups: paraffins, naphthenes and aromatics, along with other toxic materials such as heavy metals, naphthenic acids, sulfides and ammonia. Therefore, they are considered to be a major source of aquatic environmental pollution.2 In view of this, a number of legislations and laws have been issued to set limitations on discharging oily industrial effluents into sewer systems and waterbodies.3 To comply with the imposed laws and reduce oil concentrations below the environmentally acceptable discharge limits, various innovative technologies have been developed over the past years. Among these are for example, electrochemical treatment, membrane filtration, biological treatment, adsorption, and flotation using biosurfactants.4,5 As compared to other techniques used in the field of wastewater treatment, adsorption seems to be superior due to its low energy consumption, high efficiency and ease of scalability and operation. Numerous adsorption studies on the removal of oil from aqueous solutions can be found in the literature in which abundantly available, cheap and biocompatible adsorbents have been utilized, such as agricultural byproducts,6 biosorbents7 and natural minerals.8 Nowadays, efforts have been directed towards modifying the conventional adsorbents to enhance their adsorptive capacities. For instance in a previous study, magnetic iron oxide nanoparticles were grafted with chitosan and successfully used to treat emulsified oil wastewater9. In another study, barley straw modified with a cationic surfactant (hexadecylpyridinium chloride monohydrate) was reported to exhibit a good performance in adsorbing emulsified oil from aqueous solutions.10 In a third study, modified rice husk using a biocompatible cationic polymer, i.e. polyethylenimine, achieved a higher oil adsorption capacity compared with the unmodified one.11 However, most of the existing modified adsorbent materials are characterized with small particle sizes (almost powder form) which are highly dispersible in water, making their separation from aqueous phases a vexatious task. Some approaches have been proposed to enhance the separation efficiency of such adsorbents from water. One of these approaches is to embed them with magnetic nanoparticles and separate them by magnetic fields.12 Gravitational separation, on the other hand, uses the free naturally available gravitational force, unlike other techniques such as filtration and centrifugation. Driven by this, we postulate that assembling adsorbent colloidal particles into larger micromatrices would make them settle faster and in a more efficient manner. This will allow to separate the adsorbent from water by simple gravitational settling, while at the same time the voids formed in the adsorbent structure can serve as traps for the adsorbate molecules (oil droplets in this study) and promote their diffusion and/or capillary uptake. The use of Pickering emulsion templating for the synthesis of new nanostructured materials including microspheres, Janus colloidal particles, microcapsules and colloidosomes have lately received tremendous attention, owing to the following reasons: i) long-term emulsion stability against coalescence, ii) fabrication of new materials with distinctive features such as conductivity, responsiveness and porosity and iii) the ability to use various organic (e.g., polysaccharides, proteins) or inorganic (e.g., clay, silica) particles as Pickering emulsifiers.13,14 The Pickering 2 ACS Paragon Plus Environment

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emulsion templating method involves preparation of particle-stabilized emulsion followed by solidification of the coated droplets through solvent diffusion and evaporation. However, it is necessary to functionalize the particles’ surface with amphiphilic molecules, in order to meet the partial wetting condition and to increase their stability at the oil-water interface.15 In addition, the particles adsorbed at the interface need be held together by means of attractive interactions so that they can survive through the different steps of solvent diffusion, solvent extraction and drying. This can be fulfilled by one or more of the following methods: connecting the particles via adsorbed polymers,16 fusing the particles,17 adding surfactants of opposite charge to the particles,14 interfacial polymerization18 or crosslinking of the particles.19 The production of microparticles by Pickering emulsion technique is scalable and can be performed under mild processing conditions. Other techniques such as spray drying uses complex microfluidic devices and high temperatures and limit the productivity to a drop-by-drop basis.20 Halloysite nanotubes (HNTs), a clay mineral abundantly available in nature, are currently gaining increasing interest due to their outstanding merits. HNTs consist of positively charged alumina octahedron sheet and negatively charged silica tetrahedron sheet in 1:1 stoichiometric ratio, chemically similar to kaolinite. They possess a unique hollow nanotubular structure with an external diameter of ~50 nm, an inner lumen of ~15 nm and a length of ~500–1000 nm.21 The high specific surface area of HNTs and the combined properties of their inner and outer surfaces make them attractive for a variety of applications including drug delivery, tissue engineering, electronics, catalysis and as promising adsorbents for water purification.22,23 However, the direct application of raw HNTs as an adsorbent is limited by their high colloidal stability and slow sedimentation rates.24,25 In addition, previous studies have shown that pristine and hydrophobized HNTs can be used to stabilize oil in water emulsions for applications such as biphasic catalysis and oil spill bioremediation.15,26,27 Therefore, in this work it is hypothesized that assembling HNTs into porous microparticles having high self-settleability rates would produce a promising, efficient and low cost adsorbent. Chitosan is a cationic polysaccharide derived from chitin and it is the second abundant biopolymer in nature after cellulose.28 It is an excellent natural adsorbent for various dyes and heavy metal ions; this is due to the amine and hydroxyl functional groups in its structure that act as strong adsorption and complexation sites.25 Chitosan is a nontoxic, biodegradable, biocompatible and antibacterial biopolymer. However, it lacks stability under acidic mediums and it tends to agglomerate in aqueous solutions forming gels.29 Thus, its chemical and mechanical stabilities need to be reinforced through chemical crosslinking, immobilization on hard surfaces or immobilization in various matrices.30 In this work, modification of HNTs by chitosan is intended to combine the features of both materials. The main objective of the present study is to prepare halloysite micropowder from halloysite nanoclay in order to facilitate its separation from aqueous solutions and therefore exploit this natural resource as a potential adsorbent. Herein, pristine HNTs (p-HNTs) is functionalized with chitosan using two pathways: a physical pathway by simple coating and a chemical pathway by a grafting modification. Subsequently, the chitosan-functionalized HNTs are assembled into porous microstructures using surfactant mediated Pickering emulsion template and solvent diffusion method. The newly developed adsorbents are then characterized by various techniques. The performance of chitosan-grafted (c-g-HNTs) and chitosan-coated (c-c-HNTs) HNTs adsorbents are compared in terms of their crude oil adsorption capacities and physicochemical properties. Furthermore, the role of different factors on the adsorption of oil is studied including physical/chemical functionalization of adsorbent, assembly into porous microstructures, 3 ACS Paragon Plus Environment

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internal/external crosslinking of adsorbent, initial oil concentration, adsorbent dosage, contact time, solution pH, temperature and ionic strength. The oil adsorption by the adsorbent developed in this work is analyzed using equilibrium, kinetic, regeneration and settleability studies.

MATERIALS AND METHODS Materials Halloysite nanoclay (true SG=2.53), chitosan (low molecular weight), sodium dodecyl sulfate (SDS), 3-Aminopropyltriethoxy silane (APTES), triethylamine (TEA), N-(3dimethylaminopropyl)-Nˈ-ethylcarbodiimide hydrochloride (EDC) and N-hydroxysuccinimide (NHS), fluorescein 5-isothiocyanate (FITC) and rhodamine B were purchased from Sigma-Aldrich (USA). Maleic anhydride (MA) was obtained from ACROS Organics (USA). Sodium hydroxide (NaOH), epichlorohydrin, acetone and ethanol were purchased from Merck KGaA (Germany). Toluene, n-hexane, acetic acid and ethyl acetate were obtained from Fisher Scientific (UK). Dimethyl sulfoxide (DMSO) was supplied by Fisher Scientific (UK). All the chemicals were used as received without further purification. The crude oil sample was collected from Asab oil field in Abu Dhabi, with the following properties: (density = 0.8392 g/cm3, at 25 °C; API gravity = 33.83 °API; and surface tension = 24.98 ± 0.025 mN/m). Synthesis of chitosan-grafted halloysite nanotubes (c-g- HNTs) Chitosan-grafted halloysite nanotubes (c-g-HNTs) were prepared as described elsewhere with minor modifications.31 Briefly, 4 g of HNTs were dispersed in 80 ml toluene and stirred at 600 rpm for one hour. Next, 8 ml of APTES and 4 ml of TEA were added to the suspension in a twonecked flask and stirred overnight at 80°C and 600 rpm under nitrogen gas atmosphere. The obtained product was separated by centrifugation, washed several times with ethanol/deionized (DI) water mixtures and lastly with acetone. The HNTs pellet was resuspended in 40 ml acetone. Next, 1.6 g of MA was dissolved in 40 ml acetone and the two mixtures were added to each other and stirred at 600 rpm overnight at room temperature. The obtained product was washed several times with acetone then washed with water. The HNTs pellet was resuspended in 60 ml DI water and then added to 60 ml of 1 wt% chitosan solution dissolved in 1% acetic acid. Next, 1.6 g of EDC and 1.6 g of NHS were added to the mixture and stirred at 600 rpm overnight at room temperature. Finally, the collected product was washed with DI water, then resuspended in DI water and stored for further use. Synthesis of chitosan-coated halloysite nanotubes (c-c-HNTs) Chitosan-coated halloysite nanotubes (c-c-HNTs) were prepared by dispersing 4 g of HNTs in 60 ml DI water and stirring the mixture at 600 rpm for 1 hr. The mixture was then added to 60 ml of 1 wt% chitosan solution dissolved in 1% acetic acid and stirred overnight at 600 rpm and room temperature. The obtained product was washed with DI water, then resuspended in DI water and stored for further use. Synthesis of chitosan-functionalized porous microparticles Chitosan-functionalized HNTs (c-g-HNTs or c-c-HNTs) were assembled into porous microstructures using Pickering emulsion templating and solvent diffusion method. Initially, 0.1 wt% SDS was added to 90 ml of DI water and stirred until dissolved. Next, 15 ml of ethyl acetate 4 ACS Paragon Plus Environment

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was added to the mixture while being stirred at 500 rpm. After 10 minutes, 10 ml of DI water containing 1 g of chitosan-functionalized HNTs (c-g-HNTs or c-c-HNTs) was added to the emulsion while being stirred at 500 rpm. After 10 minutes, 5 ml of 1 M NaOH was added to the mixture and stirred overnight at 500 rpm and room temperature. Next day, the precipitate was collected by centrifugation and washed several times with ethanol/DI water mixtures. The precipitate was then dried in a vacuum oven at 20°C overnight. After passing the product through a mesh sieve (100 µm) to break down any agglomerates, a free-flowing powder was obtained and stored for further use. Synthesis of crosslinked chitosan- grafted porous microparticles To externally crosslink the microparticles, 3 ml of epichlorohydrin was added to the aqueous phase following the NaOH addition step mentioned above. The mixture was stirred at 500 rpm and 50°C for 3 hours and left to stir overnight at room temperature. To internally crosslink the microparticles, 3 ml of epichlorohydrin was mixed with ethyl acetate before its addition to the aqueous phase as described above. The mixture was stirred at 500 rpm and 50°C for 3 hours and left to stir overnight at room temperature. Characterization Fourier transform infrared (FT-IR) spectra were recorded for the samples in the range of 4000− 400 cm−1 using a Bruker VERTEX 70 FTIR spectrophotometer. The morphological microstructures were observed using a FEI Quanta FEG 250 scanning electron microscope (SEM) at an accelerated voltage of 5-15 kV. Prior to imaging, the particles were dispersed in DI water then a drop of the dispersion was placed on a circular aluminum stub using a double sided sticky tape and left to dry overnight. The dried samples were then coated with carbon and/or gold. The transmission electron microscope (TEM) imaging was performed on FEI Tecnai G20. Leica SP8 confocal laser scanning microscope (CLSM) was used to visualize the Pickering emulsions and the synthesized microparticles. To stain the microparticles, 1 mg of FITC dye dissolved in DMSO (1 mg/mL) was mixed with 1 mL of Na2CO3-NaHCO3 buffer (0.226 M and 0.057 M, respectively) and then added to 11.5 mL of the microparticles. To stain c-g-HNTs, 4 mg of FITC dye dissolved in DMSO (4 mg/ mL) was mixed with 4 mL of Na2CO3-NaHCO3 buffer (0.226 M and 0.057 M, respectively) and then added to 0.8 g of c-g-HNTs suspended in 8 ml of DI water. To stain pHNTs, 50 ml of 0.5 µM rhodamine B solution was prepared and mixed with 0.2 g of p-HNTs. After the mixtures were mildly shaken in the dark overnight, the resulting samples were washed with DI water until no color was detected in the supernatant.32 Pickering emulsions were prepared freshly on-site prior to analysis. To do so, the stained clay was dispersed in ethyl acetate emulsion and shaken vigorously. A drop of each sample was placed between a glass slide and a cover slip for observation. The CHNS elemental analysis was performed on EuroVector Euro EA elemental analyzer. The potential measurements for particle dispersions and oil emulsions were carried out using Brookhaven NanoBrook 90Plus Zeta instrument which employs the electrophoretic light scattering for the surface charge evaluation of particles. Nitrogen adsorption-desorption isotherms were measured at 77.35 K using Quantachrome Autosorb-1 instrument. Prior to measurements, samples were degassed at 95°C under vacuum for twelve hours. The specific surface area of the samples was calculated using the multi-point Brunauer–Emmett–Teller (BET) method. The Barrett-Joyner-Halenda (BJH) mesopore analysis was employed to calculate the pore size distributions from the desorption branch of the isotherms. Thermal analysis was performed using SDT Q600 (simultaneous TGA/DSC instrument). Approximately, 6- 13 mg of each sample was placed in a ceramic crucible under nitrogen flow rate of 50 mL/min and then heated from room 5 ACS Paragon Plus Environment

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temperature to 800 °C at a rate of 10 °C /min. Images of the emulsions were taken using a polarized light microscope (Eclipse LV100N POL, Nicon, USA). The average size of the emulsion droplets was measured using a camera control software (ACT-2U, Ver 1.70). Batch adsorption experiments Crude oil emulsions with different concentrations (100-1000 mg/L) were prepared by mixing oil and water using a commercial homogenizer for 10 minutes. Then, 50 mg of the adsorbent was added into multiple flasks, each containing 30 ml of the prepared emulsions. The flasks were then placed in a water shaking bath at room temperature and 150 rpm. Next, aliquots were collected from the flasks at predetermined time intervals. After allowing the adsorbent to settle by gravity for 15 minutes, the supernatant solutions were analyzed using Shimadzu, TOC-L total organic carbon instrument. All experiments were carried out in triplicates. The oil uptake (qt) at time t was calculated according to the following equation: qt =

(Co - Ct ) V m

(1)

where Co is the initial concentration of oil (mg/L), Ct is the concentration of oil at time t (mg/L), m is mass of adsorbent (g) and V is volume of solution (L). At equilibrium, the oil uptake (qe) was calculated as follows: qe =

(Co - Ce ) V m

(2)

where Ce is the equilibrium concentration of oil (mg/L). Adsorption kinetics To explore the contributions of various mechanisms to the kinetics of oil adsorption, three adsorption kinetic models were applied to the data. The Lagergren’s pseudo first order reaction model, the Ho’s pseudo second order reaction model and the intraparticle diffusion model. The pseudo first order model can be expressed as:33 ln (qe - qt ) = ln qe - k1 t

(3)

where qt (mg/g) and qe (mg/g) represent the oil uptakes at time t (min) and at equilibrium, respectively, and k1 (min-1) is the pseudo first order rate constant of adsorption. According to equation (3), a plot of ln (qe - qt) versus time (t) yields a straight line with a slope of k1 and an intercept of ln qe. The pseudo second order model can be written as:34 t 1 t = + qt k2 q2e qe

(4)


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where qt (mg/g) and qe (mg/g) represent the oil uptakes at time t (min) and at equilibrium, respectively, and k2 (g mg-1 min-1) is the pseudo second order rate constant of adsorption. The values of k2 and qe can be determined from the slope and intercept of the linear plot of t/qt versus time (t). The intraparticle diffusion model, proposed by Weber et al.,35 is the most widely used kinetic model to predict the rate controlling step, which can be expressed as: 𝑞𝑡 = 𝑘𝑑 𝑡1/2 + 𝐶

(5)

where kd (mg g-1 min-1/2) is the intraparticle diffusion rate constant and C is a constant related to the thickness of the boundary layer; the larger the value of C, the greater is the effect of the boundary layer. According to Weber- Morris model (equation 5), the intraparticle diffusion is involved in the adsorption process if a plot of qt versus t1/2 yields a straight line and it is the sole rate limiting step if the straight line passes through the origin. Adsorption isotherms The Brunauer-Emmet-Teller (BET) model36 is a theoretical equation derived for multilayer adsorption systems. It is the most widely applied isotherm model in gas-solid equilibrium systems. The extinction of the classical BET model to liquid-solid interfaces can be written as:37 qe = q

(6)

K1 Ce ) mono, max ( (1- K2 Ce ) (1- K2 Ce + K1 Ce )

where qmono, max is the maximum monolayer adsorption capacity (mg/g), K1 (L/mg) and K2 (L/mg) are the equilibrium constants of adsorption for the first layer and the subsequent multilayers, respectively. Regeneration experiments The reusability of the adsorbent was tested over subsequent adsorption-desorption cycles. After each adsorption cycle, the adsorbent was washed overnight with a mixture of ethanol and hexane (1:1), then washed with DI water for two hours. The regenerated adsorbent was dried in a vacuum oven at 20°C and then reused in the next cycle. Self-settleability experiments The self-settleability of the samples were investigated in DI water and in 300 ppm crude oil emulsion. The three powdered samples were first stirred overnight in DI water at 500 rpm to separate any agglomerated particles. Thereafter, the dispersions were poured into vials containing DI water or crude oil emulsions (450 ppm). Each vial contained the optimal adsorbent dosage used in this study and a total volume of 15 ml. The dispersions were then mixed for two minutes using a vortex stirrer. The turbidities of the dispersions were quantified using HACH 2100Q portable turbidimeter. Statistical description of data The average values of three independent experiments were reported in this study. The standard errors of the mean were indicated as error bars added to the plots. The applicability of kinetic and isotherm models to fit the adsorption data were evaluated based on R2 values estimated using Microsoft Excel 2013. 7 ACS Paragon Plus Environment

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RESULTS AND DISCUSSION Chitosan-functionalization of HNTs and synthesis of porous microparticles HNTs were initially functionalized with chitosan to enhance the hydrophobicity of the tubes and increase their stability against shear stresses at the oil water interface.31 The FTIR spectra of pHNTs, c-g-HNTs and c-c-HNTs are presented in Figure 1a. The typical peaks detected in the spectrum of p-HNTs at 3695, 3620, 1642, 1003 and 907 cm-1 can be assigned to O-OH stretching vibration of water molecules adsorbed to the solid surface, O-H stretching vibration of Si-OH groups, bending of H-OH bond of water molecules retained in the matrix, Si-O-Si groups stretching in the tetrahedral sheet, and O–H deformation vibration of inner Al-OH groups, respectively.38 The peak at 1642 detected in the spectrum of c-g-HNTs and c-c-HNTs corresponds to the C=O stretching vibration of primary amide. The new peak in the spectrum of c-g-HNTs centered at 1560 cm-1 corresponds to N-H bending vibration in the grafted (secondary) amide groups.39 The less intense peak observed in c-c-HNTs at 1541 cm-1 can be attributed to N-H bending of secondary amide of chitin residues in the chitosan coated on HNTs surface. The peak at 1420 cm-1 in the spectrum of c-g-HNTs and c-c-HNTs can be assigned to deformation vibration of CH2 bending of chitosan.31 The intense peaks detected in the FTIR spectrum of c-g-HNTs compared with the one detected for c-c-HNTs demonstrate the fact that the covalent bonds formed between chitosan and HNTs surface result in a more effective and more stable functionalization of HNTs with chitosan. The TGA curves of p-HNTs, c-g-HNTs and c-c-HNTs are shown in Figure 1b. The inset shows the DTG curves. The initial weight loss in the three samples at temperatures below 100 °C can be related to the evaporation of adsorbed water. The weight loss in the three samples at around 120 °C (point 1 in the figure and in the inset) is due to the loss of residual interlayer water.40 The steeper weight losses in the chitosan-functionalized samples between 240 °C and 400 °C (point 2) correspond to the decomposition of chitosan loaded on HNTs due to dehydration of saccharide rings, depolymerization and decomposition of side groups.41 The substantial loss in the three samples at 480 °C (point 3) is due to dihydroxylation of AlOH groups.42 The weight loss of c-gHNTs at point 2 is 2.8 %, while it is 2.1 % in c-c-HNTs indicating that covalent chitosanfunctionalization is more effective than physical coating of HNTs surfaces. The -potential values of p-HNTs, c-g-HNTs and c-g-HNTs MPs are presented in Figure 1c. Chitosan-functionalization of p-HNTs endowed them with a positive charge under acidic and neutral conditions. This is due to the protonation of the amine groups in chitosan chains. In addition, a higher isoelectric point (IP) was observed for c-g-HNTs MPs (pH 8.2) compared with that of c-g-HNTs (pH 7.4) which indicates that a higher OH- concentration is required to neutralize the surface charge of the assembled particle. This could result from the different topography of the assembled c-g-HNTs MPs. The 3-D assembly of chitosan-functionalized HNTs into porous microstructures using Pickering ethyl acetate in water emulsion template and solvent diffusion method is schematically illustrated in Figure 2. The assembly process can be described by the following steps: 1) modification of HNTs’ charge and hydrophobicity by chitosan, 2) assembly of chitosan-functionalized HNTs on the SDS-stabilized emulsion droplets via electrostatic interaction and Pickering effect, 3) in situ 8 ACS Paragon Plus Environment

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pH change for aggregation of Pickering emulsion droplets and phase separation, 4) diffusion and evaporation of ethyl acetate, and 5) resuspension in ethanol, washing and drying of precipitate. The electrostatic interactions between the SDS-charged interface and chitosan-functionalized HNTs are expected to facilitate their interfacial adsorption onto emulsion droplets and stabilize them against desorption. To gain more insights into the stabilizing effects of chitosan modification, SDS addition and pH change, confocal images of Pickering emulsions and microparticles prepared at different conditions were compared with one another (Figure 3). When p-HNTs was used to prepare the Pickering emulsion, a thin film of HNTs adsorbed at the SDS-stabilized oil droplets, as can be detected in image 3a. This can arise from the vigorous mixing action in the preparation process. No aggregates of p-HNTs were observed in the continuous phase as the nanotubes are charge-stabilized due to the addition of SDS. The large particles, appeared in the image, are possibly due to HNTs agglomeration that may have occurred during staining with rhodamine B. In comparison, chitosan modified HNTs (c-g-HNTs) formed thicker layers of agglomerates around SDS-stabilized oil droplets (image 3b). This can be justified by the less surface charge of chitosanmodified HNTs (c-g-HNTs) at the preparation conditions ( potential= +5.36 mV for non-stained and +10.66 mV for stained, at neutral pH) compared with p-HNTs (Figure 1c). In addition, the less repulsion between chitosan-modified HNTs causes their agglomeration in the bulk phase before they adsorb at the oil-water interface. This is due to van der Waals and hydrophobic attractions between chitosan molecules.43 Similar agglomerates were noticed in the aqueous phase alone without the addition of oil (less than 3 µm in size). This is also demonstrated by the clusters existing in the structure form of the microparticles. In comparison, the electrostatic repulsion between the strongly charged HNTs ( potential= -33.6 mV for non-stained and -21.29 mV for stained, at neutral pH) prevents their agglomeration and dense accumulation at the interface. This reflects the significant role of chitosan modification in minimizing the energy barrier between the chitosan-modified HNTs allowing them to approach each other by van der Waals and hydrophobic interactions. When the emulsion was prepared without the addition of SDS surfactant, p-HNTs were found to fully cover the oil droplets due to the less negatively charged interface and the emulsification shear forces. This full coverage will probably shield the repulsion between oil droplets ( potential of the initially prepared ethyl acetate in water emulsion before the addition of HNTs was found to be -61.22 mV) and force them to aggregate forming a perforating networklike structure (image 3c), through bridging flocculation mechanism.44 To understand the effect of SDS in stabilizing the interfacial adsorption of chitosan-modified HNTs, images 3b and 3d were compared. In image 3d, the agglomerates of chitosan-modified HNTs can be clearly seen in the continuous phase. However, unlike image 3b, the agglomerates aren’t irreversibly adsorbed to the interface and they tend to detach as the droplet moves. This confirms the role of electrostatic attractions between the SDS-charged interface and the chitosan-modified HNTs in stabilizing the Pickering emulsion. This can be evidenced as well from the images taken for the prepared microparticles (3e and 3f). The disintegration of the microparticle formed in the absence of SDS (image 3e) reflects its weak structure relative to the one formed by the SDS-stabilized template (image 3f) which survived through multiple washing cycles. Finally, the effect of pH change (step 3) on the assembly of chitosan-modified HNTs (images 3g and 3h) was investigated. The pH of the emulsion was adjusted to pH 9 through the addition of NaOH. The microparticle shown in image 3g was prepared without pH alteration. It is obvious that the change in the suspension pH 9 ACS Paragon Plus Environment

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caused aggregation and deposition of more agglomerates on the particle surface. This can also be emphasized by the irregular shapes of the formed microparticles. The in situ pH change results in the aggregation of Pickering emulsion droplets which in turn tend to float on the top of the solution encapsulating ethyl acetate droplets. A similar phase separation observation was reported when the pH of a Pickering emulsion prepared using chitosan-modified silica particles was adjusted to pH 9.43 The encapsulated ethyl acetate diffuses from the aggregated assemblies producing porous structures. The morphologies of p-HNTs, c-c-HNTs, c-g-HNTs and c-g-HNTs MPs were visualized using scanning electron microscopy (SEM), as displayed in Figure 4 (a-g). The morphological structure of HNTs didn’t markedly change after chitosan modification (images 4b and 4c). The assembly of chitosan-functionalized HNTs resulted in a nest-like porous structure as depicted in images 4d and 4e. The cross-section image of the microparticle (image 4g) reveals the existence of small cavities in its internal porous structure. These cavities could be due to ethyl acetate diffusion during solidification of the microparticle. Image 4f, shows the different sizes of the microparticles which lie in the range between 3 µm and 20 µm. Image 4h, demonstrates the solid core structure of the microparticle which is consistent with its internal structure (image 4g). To measure the mean droplet size, fifty droplets of ethyl acetate emulsion (image 4i) were analyzed and found to be in the range between 8.8 µm and 40 µm, resulting in a mean droplet size of 21 µm. The smaller size range quantified for the dried microparticles compared with the emulsion droplets demonstrates the collapse of the aggregated assemblies due to the shear stresses arising during diffusion and drying processes. Adsorption of emulsified oil from water A preliminary conducted experiment revealed that c-g-HNTs MPs had the highest percentage of oil removal from water. Therefore, c-g-HNTs MPs was selected as the adsorbent to be further investigated. As presented in Figure 5a, c-g-HNTs MPs could successfully remove 94.5 % of the emulsified oil, while p-HNTs removed only 38.3 %. A higher TOC reading was obtained when pure chitosan powder was tested as adsorbent. This indicates that lots of chitosan fine particles stayed suspended in the solution and thus a higher carbon content was obtained. On the contrary, the non-settled HNTs have no carbon in their structure as indicated by the elemental analysis results (Table 1) and therefore have no effect on the TOC reading. The internally and externally crosslinked c-g-HNTs MPs achieved lower removal percentages (68.1 % and 70.8 %, respectively) compared with those obtained for the non-crosslinked ones (94.5 %). The above results can be interpreted based on the textural parameters and the surface charges of the tested adsorbents. The potential of the crude oil emulsion prepared at 300 rpm was found to be -58.04 mV. Hence, the lowest performance noticed for p-HNTs can be attributed to the repulsion interactions between them and the oil droplets. In contrast, chitosan-functionalized HNTs have a positive surface charge and therefore can be attracted to the droplets’ surface. The N2 adsorption-desorption isotherms (Figure 5b) obtained for the samples revealed type II isotherm and type 3 hysteresis loop according to the IUPAC classification.45 This type of isotherm associated with hysteresis can be regarded as a composite type IV and type II isotherm, which are characteristics of mesoporours and macroporous structures, respectively.46 The pore size 10 ACS Paragon Plus Environment

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distribution of p-HNTs (Figure 5c) reveals three distinct peaks centered at 3.7 nm, 5 nm and 11 nm. The peak at 3.7 nm can be attributed to the newly formed mesopores between the rolled layers of HNTs as a result of dehydration.47,48 The peak at 11 nm, can be identified as the lumen of HNTs. The decreased intensity of the two primary peaks can be attributed to the grafted layer on HNTs surface resulting in a less available space for nitrogen adsorption.49 A larger pore size emerged at around 60 nm which can be assigned to the spaces between the spontaneously formed agglomerates of c-g-HNTs. Thus, the decreased BET surface area (Table 2) of c-g-HNTs (41.759 m2/g) as compared with p-HNTs (49.942 m2/g) can arise from their aggregation as well as from the grafted layers. The broader pore size range of microparticles distributed from mesoporous to larger pores can be attributed to the interparticle distances within the assembled microparticles.42 The large macropores are most likely formed due to solvent diffusion during the synthesis process. This can cause rearrangement of the tubes as the particle size keeps decreasing. Smaller pore populations in the mesopore range were observed for the crosslinked microparticles compared with the non-crosslinked ones. Crosslinking can be thought of as occurring at the contact points between the connected tubes. In addition to crosslinking between chitosan chains in the grafted layer on the surface, the formed joints between the tubes will keep them in close proximity resulting in a narrower pore distribution. The crosslinked microparticles achieved lower oil removal percentages compared with the non-crosslinked ones as can be predicted from their smaller BET surface area and smaller pores. Moreover, c-c-HNTs MPs were characterized with relatively smaller surface area and narrower pore size distribution compared with c-g-HNTs MPs. This could be due to desorption of the coated chitosan which may have then precipitated inside the pores decreasing their porosity. The adsorption characteristics of c-g-HNTs MPs towards emulsified oil were investigated under variant operational parameters, as displayed in Figure 6. The effect of initial crude oil concentration on the removal percentage and adsorption capacity of c-g-HNTs MPs is presented in Figure 6a. The increase in oil concentration from 100 ppm to 1000 ppm, caused an increase in the oil uptake from 54.93 mg/g to 576.6 mg/g. The average size of crude oil droplets was found to be 10 µm, while the particle size was found to be in the range between 3 µm and 20 µm. At higher initial oil concentrations, the probability of collisions between oil droplets and adsorbent particles is expected to increase and consequently enhance the oil uptake of the adsorbent. However, above a certain concentration (not shown in figure), the adsorbent will become saturated and therefore can no longer take oil from the bulk solution. Although saturation was not attained in the oil concentration range (100-1000 ppm) investigated in this study, we did not go beyond that; as preparing stable emulsions at higher oil concentrations using the commercial homogenizer was not possible. In addition to that, the typical oil concentration found in refinery’s wastewater, for example, is around 300 ppm.50 The removal percentage of oil, on the other hand, increased at 400 ppm to a maximum value of 94.94 %. The increase in the removal percentage observed at the beginning is due to the excess adsorbent. At higher concentrations, the ratio of oil droplets to the available adsorption sites increases leading to a decrease in the removal percentage. The effect of adsorbent dosage on the adsorption of oil is depicted in Figure 6b. As can be seen, the adsorption capacity decreased with the addition of more adsorbent. This is because of the fact that less adsorption sites will be occupied at the end of the adsorption process due to the lack of the existing oil droplets. In contrast, the removal percentage of oil gradually increased to a maximum value of 11 ACS Paragon Plus Environment

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93.12 % obtained at a dosage of 50 mg. The increase in the removal percentage at the beginning is attributed to the increase in the adsorbent surface area and adsorption sites. The maximum removal percentage of oil was found at 50 mg, therefore it was selected as the optimum dosage for the remaining experiments. Figure 6c, presents the effect of contact time on the adsorption capacity of oil. The adsorption capacity sharply increased with time at the beginning, then gradually increased until equilibrium was attained within 30 minutes. The sharp increase in the adsorption uptake at the beginning is due to the higher surface area available for adsorption. It is well known that solution pH has a significant effect on the charge of the adsorbent and the degree of ionization of adsorbate molecules as well. Therefore, the effect of pH on the adsorption of oil by c-g-HNTs was investigated, as demonstrated in Figure 6d. The adsorbent was found to exhibit a maximum uptake toward oil droplets in the range between pH 5 and pH 6.86 (original pH of emulsion). This can be attributed to the attraction forces between positively charged c-gHNTs MPs (Figure 1c) and negatively charged oil droplets. It is well understood in the literature that oil droplets acquire a negative surface charge in aqueous solutions due to the adsorption of hydroxide ions on their interfaces.51 This negative charge decreases in acidic solutions due to the less available hydroxide ions. In other words, the excess hydronium ions under acidic conditions are expected to shield the interactions between oppositely charged adsorbent and oil droplets resulting in lower adsorption capacity. Similarly, the less adsorption performance under alkaline conditions can be attributed to the repulsion forces between the like-charged adsorbent and oil droplets (Figure 1c). The findings of this work are in good agreement with the previously published ones.52,53 Finally, the effect of the solution ionic strength on the adsorption of oil is shown in Figure 6e. The oil droplets uptake decreased from 163.73 mg/g to 138.54 mg/g when the ionic strength increased from 0 to 0.05 M, respectively. This can be attributed to the screening effect of the surface charges.54 The higher the ionic strength of the solution, the more counter ions are available to neutralize the adsorbent and oil droplets’ surface charges and minimize the electrostatic attractions between them. Nevertheless, a further increase in the ionic strength resulted in an increase in the oil uptake. This can be explained by the increased coalescence rate between oil droplets.55 The higher ionic strength is expected to shield the repulsive forces between oil droplets resulting in the formation of large flocculated clumps. The three kinetic models applied to the adsorption data of oil onto c-g-HNTs MPs are shown in Figure 7 and the related parameters are listed in Table 3. The fitness of the data was evaluated based on the calculated linear regression correlation coefficient (R2). The pseudo-second-order kinetic model showed the best fit to the data, as indicated by its higher R2 value (0.9983). However, by assuming an average particle size of 11.5 µm (average of the size range of 3-20 µm) and calculating the number of adsorbent particles added to a single batch size, at the optimal dosage used in this study, the number of particles can be found to be 2.48 x107. In a similar manner, by calculating the number of oil droplets based on the average droplet size (10 µm), the number of oil droplets can be estimated to be 2.05 x107. The comparable sizes and quantities of oil droplets and adsorbent particles may suggest that adsorption occurs in a one to one ratio where they both contribute to the overall rate. This means that the adsorption kinetics are not based on a pseudo 12 ACS Paragon Plus Environment

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mechanism where the concentration of adsorbate is considered to be constant (in excess).56 Thereby, the system seems to follow a normal kinetics involving both reactants; the oil droplets and the adsorbent particles.57 The intraparticle model was also applied to the kinetics data, as shown in Figure 7c. A bilinear behavior was observed for the adsorption of oil droplets. Generally adsorption kinetics are controlled by diffusion of the adsorbate across the boundary layer and the interior pores of the adsorbent. However, in this case, the dimensions of oil droplets and adsorbent particles are of the same order of magnitude (Figure 7d). The oil droplet is too large to diffuse entirely into the macropores of the particle (particle pore size >1, this indicates that the gravitational forces are dominant and thus high potentials will not be sufficient to stabilize the colloidal dispersion. When Pe >1) and thus the particles settle with a terminal velocity which can be approximated from the Stoke’s law. Both c-g-HNTs and c-g-HNTs MPs have very similar potentials (Figure 1c). However, the c-g-HNTs MPs exhibited better settleability than the c-g-HNTs, in spite of their slightly reduced overall density. This is mainly due to the larger particle size of c-g-HNTs MPs compared with that of c-g-HNTs agglomerates which results in a higher sedimentation rate, as related by Equation 7. It is important to study the self-settleability behavior of colloidal adsorbents, particularly, in aqueous solutions where they are normally employed to remove a wide spectrum of soluble contaminants including dyes, heavy metals, acids, pesticides and pharmaceuticals. This applies as well for regeneration processes which are commonly performed in chemical solvents, being most of the time aqueous solutions or continuous mediums of organic solvents. However, the adsorption system investigated in this study consists of a mixture of colloidal particles and emulsion droplets all dispersed in water and therefore is more complex. Hence, we also measured the settleability behavior of p-HNTs, c-g-HNTs and c-g-HNTs MPs in oil emulsions (Figure 9b). The stability of the prepared crude oil emulsion was confirmed by its constant turbidity over the period of measurement. The settleability performance of the three samples was better in the oil emulsion than in DI water. The best settleability was observed for c-g-HNTs MPs. In thirty minutes, the recovery percentages of the adsorbents were found to be 71.4%, 83.7% and 95.5%, for p-HNTs, c-g-HNTs and c-g-HNTs MPs, respectively. After 90 minutes, the recovery percentage increased to 93.4%, 94.4% and 97.3%, for p-HNTs, c-g-HNTs and c-g-HNTs MPs, respectively. Interestingly, a drastic decrease in the colloidal stability of p-HNTs was noticed in the oil emulsion compared with that in DI water. Unlike what we noticed in DI water, the high zeta potential of pHNTs in this case was not sufficient to stabilize the system against aggregation. This implies that another mechanism involving the emulsion droplets came into play to the system. The higher settleability observed for the three samples in the crude oil emulsions compared with those in DI water can be attributed to a mechanism called depletion flocculation.66 When the particles present in excess with respect to the oil droplets, many of these particles remain non-adsorbed in the bulk phase between the emulsion droplets. This in turn induces droplet aggregation as a result of the depletion effects of particles in the narrow region between two approaching droplets compared to the bulk phase. The adsorbent/oil ratio used in the settleability measurements is much higher than the one used to prepare the Pickering emulsions using ethyl acetate. In addition, the emulsions in the settleability measurements were not sufficiently mixed to have the particles adsorbed onto the 15 ACS Paragon Plus Environment

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droplets. Hence, it is expected that many particles remained free in the bulk aqueous phase and induced depletion flocculation enhancing the settleabilities. This suggests that if a colloidal material is intended to be used for removing oil droplets from water, it should be added in an amount sufficient to saturate oil droplets (if under mixing conditions) and/or induce their depletion flocculation. Otherwise, the particles may adsorb onto the droplets and confer extra stability to the emulsion. However, more extensive investigations on the rheological behavior of this type of systems in relation to the oil/water and adsorbent/oil volume fractions and their surface charges are required to fully understand the underlying mechanisms behind the overlapping processes of adsorption and flocculation. Figure 9d, shows the high dispersibility and fast settleability characteristics of c-g-HNTs MPs. Regeneration tests The adsorption-desorption cycles of c-g-HNTs MPs – oil droplets system are displayed in Figure 10. One advantage of using mesoporous and/or macroporous materials is the ease of their regeneration. If the conventional chemical regeneration is employed to recover the adsorbent, the relatively large eluent molecules will not be able to penetrate into the micropores and/or submicropores and wash the contaminants out leading to incomplete regeneration. On the other hand, in mesoporous and macropores materials any eluent regardless of its molecular size can be used to directly interact with the adsorbate molecules and feasibly regenerate the adsorbent. In this work, regeneration of c-g-HNTs MPs was carried out using a miscible mixture of polar and nonpolar solvents, i.e., ethanol and hexane (1:1). These solvents can wash out the adsorbed oil and regenerate the adsorbent. In the third cycle, only 0.58% of the initial adsorption performance was not recovered. This may indicate that the adsorbent maintained its assembled structure during the regeneration cycles. In the fifth cycle, 6.7% of the initial adsorption performance was lost. The decay in the removal percentage noticed in the fifth cycle (88.13 %) is consistent with the increase in the carbon content of the adsorbent (Table 1) indicating that some oil has remained in the adsorbent after regeneration

CONCLUSIONS Halloysite nanotubes were assembled without compromising the surface area using a soft oil template into mesoporous/macroporous microparticles (c-g-HNTs MPs). The assembled structure was characterized with broader BJH pore distributions and larger cumulative pore volumes compared with p-HNTs. In addition, c-g-HNTs MPs exhibited an enhanced self-settleability behavior in aqueous solutions (83.53 %, in 30 min) as well as in emulsified solutions (96.48 %, in 30 min). Furthermore, c-g-HNTs MPs were found to be a potential adsorbent and achieved a maximum oil removal percentage of 94.74% in 30 minutes. On the other hand, crosslinking of cg-HNTs MPs resulted in narrower pore distributions and lower oil removal percentages. Collectively, results revealed that adsorption of oil onto c-g-HNTs MPs was favorable between pH 5 and pH 6.87, at an ionic strength of 0.05 M and under room temperature. The kinetics data was best fitted by the pseudo second order kinetic model and the equilibrium results were well fitted to the multilayer BET model. The maximum monolayer adsorption capacity of oil onto c-gHNTs MPs was found to be 788 mg/g. The adsorption was found to be endothermic in nature 16 ACS Paragon Plus Environment

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demonstrating a multilayer physisorption mechanism. Chitosan modification of p-HNTs was found to play a major role in stabilizing the adsorption of halloysites on the emulsion templates as well as in enhancing their self-settleability. Halloysite nanotubes were found to adsorb on oil interfaces under vigorous mixing conditions as revealed by confocal microscopy. However, when using a higher concentration of p-HNTs and under mild stirring condition they formed flocs with an enhanced settleability compared with their settleability in deionized water. The c-g-HNTs MPs was reused for five adsorption-desorption cycles with minimal loss in their performance.

ACKNOWLEDGEMENTS This research was supported by Khalifa University grant number LTR14013. The authors would like to thank Eng. Abeer Alyafeai from Khalifa University-SAN campus for the SEM analysis, Eng. Samuel Stephen for the TEM analysis and Dr. Rachid Rezgui from New York UniversityAbu Dhabi (NYUAD) for the confocal microscopy analysis included in this study.

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11. Lin, K.-Y. A.; Chen, S.-Y., Enhanced Removal of Oil Droplets from Oil-in-Water Emulsions Using Polyethylenimine-Modified Rice Husk. Waste Biomass Valorization 2015, 6 (4), 495-505. 12. Wang, P.; Zhou, X.; Zhang, Y.; Wang, L.; Zhi, K.; Jiang, Y., Synthesis and Application of Magnetic Reduced Graphene Oxide Composites for the Removal of Bisphenol A in aqueous Solution–A Mechanistic Study. RSC Adv. 2016, 6 (104), 102348-102358. 13. Yang, Y.; Fang, Z.; Chen, X.; Zhang, W.; Xie, Y.; Chen, Y.; Liu, Z.; Yuan, W., An Overview of Pickering Emulsions: Solid-Particle Materials, Classification, Morphology, and Applications. Front. Pharmacol. 2017, 8, 287. 14. Simovic, S.; Heard, P.; Prestidge, C. A., Hybrid Lipid-Silica Microcapsules Engineered by Phase Coacervation of Pickering Emulsions to Enhance Lipid Hydrolysis. PCCP 2010, 12 (26), 7162-7170. 15. Chevalier, Y.; Bolzinger, M.-A., Emulsions Stabilized with Solid Nanoparticles: Pickering Emulsions. Colloids Surf. Physicochem. Eng. Aspects 2013, 439, 23-34. 16. Chao, C.; Zhang, B.; Zhai, R.; Xiang, X.; Liu, J.; Chen, R., Natural Nanotube-Based Biomimetic Porous Microspheres for Significantly Enhanced Biomolecule Immobilization. ACS Sustainable Chem. Eng. 2014, 2 (3), 396-403. 17. Hsu, M. F.; Nikolaides, M. G.; Dinsmore, A. D.; Bausch, A. R.; Gordon, V. D.; Chen, X.; Hutchinson, J. W.; Weitz, D. A.; Marquez, M., Self-Assembled Shells Composed of Colloidal Particles: Fabrication and Characterization. Langmuir 2005, 21 (7), 2963-2970. Guan, Y.; Meng, X.; Qiu, D., Hollow Microsphere with Mesoporous Shell by Pickering 18. Emulsion Polymerization as a Potential Colloidal Collector for Organic Contaminants in Water. Langmuir 2014, 30 (13), 3681-3686. 19. Cui, Y.; van Duijneveldt, J. S., Microcapsules Composed of Cross-Linked Organoclay. Langmuir 2012, 28 (3), 1753-7. Mwangi, W. W.; Ho, K.-W.; Ooi, C.-W.; Tey, B.-T.; Chan, E.-S., Facile Method for 20. Forming Ionically Cross-Linked Chitosan Microcapsules from Pickering Emulsion Templates. Food Hydrocolloids 2016, 55, 26-33. 21. Li, W.; Liu, D.; Zhang, H.; Correia, A.; Mäkilä, E.; Salonen, J.; Hirvonen, J.; Santos, H. A., Microfluidic Assembly of a Nano-in-Micro Dual Drug Delivery Platform Composed of Halloysite Nanotubes and a pH-Responsive Polymer for Colon Cancer Therapy. Acta Biomater. 2017, 48, 238-246. 22. Luo, P.; Zhao, Y.; Zhang, B.; Liu, J.; Yang, Y.; Liu, J., Study on the Adsorption of Neutral Red from Aqueous Solution onto Halloysite Nanotubes. Water Res. 2010, 44 (5), 1489-1497. Zhao, Y.; Abdullayev, E.; Vasiliev, A.; Lvov, Y., Halloysite Nanotubule Clay for Efficient 23. Water Purification. J. Colloid Interface Sci. 2013, 406, 121-129. Yuri, L.; Wencai, W.; Liqun, Z.; Rawil, F., Halloysite Clay Nanotubes for Loading and 24. Sustained Release of Functional Compounds. Adv. Mater. 2016, 28 (6), 1227-1250. 25. Peng, Q.; Liu, M.; Zheng, J.; Zhou, C., Adsorption of Dyes in Aqueous Solutions by Chitosan–Halloysite Nanotubes Composite Hydrogel Beads. Microporous Mesoporous Mater. 2015, 201, 190-201. 26. Regine, v. K.; Dimitrij, S.; Tobias, P.; Reinhard, S.; Renata, M.; Abhishek, P.; Svetlana, K.; Rawil, F.; Joachim, K.; Helmuth, M.; Yuri, L., Halloysites Stabilized Emulsions for Hydroformylation of Long Chain Olefins. Adv. Mater. Interfaces 2017, 4 (1), 1600435.


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45. Sing, K. S., Reporting Physisorption Data for Gas/Solid Systems with Special Reference to the Determination of Surface Area and Porosity (Recommendations 1984). Pure Appl. Chem. 1985, 57 (4), 603-619. 46. Thommes, M.; Kaneko, K.; Neimark Alexander, V.; Olivier James, P.; Rodriguez-Reinoso, F.; Rouquerol, J.; Sing Kenneth, S. W., Physisorption of Gases, with Special Reference to the Evaluation of Surface Area and Pore Size Distribution (IUPAC Technical Report). Pure Appl. Chem. 2015, 87 (9-10), 1051–1069. 47. Tan, D.; Yuan, P.; Annabi-Bergaya, F.; Yu, H.; Liu, D.; Liu, H.; He, H., Natural Halloysite Nanotubes as Mesoporous Carriers for the Loading of Ibuprofen. Microporous Mesoporous Mater. 2013, 179, 89-98. 48. Kohyama, N.; Fukushima, K.; Fukami, A., Observation of the Hydrated Form of Tubular Halloysite by an Electron Microscope Equipped with an Environmental Cell. Clays Clay Miner. 1978, 26 (1), 25-40. 49. Yuan, P.; Southon, P. D.; Liu, Z.; Kepert, C. J., Organosilane Functionalization of Halloysite Nanotubes for Enhanced Loading and Controlled Release. Nanotechnology 2012, 23 (37), 375705. Bennett, G. F.; Shammas, N. K., Separation of Oil from Wastewater by Air Flotation. In 50. Flotation Technology, Wang, L. K.; Shammas, N. K.; Selke, W. A.; Aulenbach, D. B., Eds. Humana Press: Totowa, NJ, 2010; Vol. 12, pp 85-119. 51. Marinova, K. G.; Alargova, R. G.; Denkov, N. D.; Velev, O. D.; Petsev, D. N.; Ivanov, I. B.; Borwankar, R. P., Charging of Oil-Water Interfaces Due to Spontaneous Adsorption of Hydroxyl Ions. Langmuir 1996, 12 (8), 2045-2051. 52. Ahmad, A. L.; Sumathi, S.; Hameed, B. H., Residual Oil and Suspended Solid Removal Using Natural Adsorbents Chitosan, Bentonite and Activated Carbon: A Comparative Study. Chem. Eng. J. 2005, 108 (1–2), 179-185. Cherukupally, P.; Acosta, E. J.; Hinestroza, J. P.; Bilton, A. M.; Park, C. B., Acid–Base 53. Polymeric Foams for the Adsorption of Micro-Oil Droplets from Industrial Effluents. Environ. Sci. Technol. 2017, 51 (15), 8552-8560. 54. Xu, J.; Wang, L.; Zhu, Y., Decontamination of Bisphenol A from Aqueous Solution by Graphene Adsorption. Langmuir 2012, 28 (22), 8418-8425. Lin, K.-Y. A.; Yang, H.; Petit, C.; Hsu, F.-K., Removing Oil Droplets from Water Using a 55. Copper-Based Metal Organic Frameworks. Chem. Eng. J. 2014, 249, 293-301. 56. Largitte, L.; Pasquier, R., A Review of the Kinetics Adsorption Models and Their Application to the Adsorption of Lead by an Activated Carbon. Chem. Eng. Res. Des. 2016, 109, 495-504. 57. Fogler, H. S., Elements of Chemical Reaction Engineering. Prentice Hall PTR: 2006. Wu, F.-C.; Tseng, R.-L.; Juang, R.-S., Comparisons of Porous and Adsorption Properties 58. of Carbons Activated by Steam and KOH. J. Colloid Interface Sci. 2005, 283 (1), 49-56. 59. Huang, J.; Yan, Z., Adsorption Mechanism of Oil by Resilient Graphene Aerogels from Oil-Water Emulsion. Langmuir 2018. 60. G. Stokes, G., On the Effect of Internal Friction of Fluids on the Motion of Pendulums. Transactions of the Cambridge Philosophical Society: 1850; Vol. 9. 61. Federico, M., The Settling Velocity of Mineral, Biomineral, and Biological Particles and Aggregates in Water. J. Geophys. Res.: Oceans 2013, 118 (4), 2118-2132. 62. Larson, R. G., The Structure and Rheology of Complex Fluids. Oxford University Press: New York, 1999. 20 ACS Paragon Plus Environment

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63. Kovalchuk, N. M.; Starov, V. M., Aggregation in Colloidal Suspensions: Effect of Colloidal Forces and Hydrodynamic Interactions. Adv. Colloid Interface Sci. 2012, 179-182, 99106. 64. Derjaguin B.V.; Churaev N.V.; V.M., M., The Derjaguin-Landau-Verwey-Overbeek (DLVO) Theory of Stability of Lyophobic Colloids. In Surface Forces, Springer: Boston, MA 1987. 65. Duffy, J.; Larsson, M.; Hill, A., Suspension Stability; Why Particle Size, Zeta Potential and Rheology Are Important. 2012; Vol. 20. Mun, S.; Decker, E. A.; McClements, D. J., Effect of Molecular Weight And Degree of 66. Deacetylation of Chitosan on the Formation of Oil-in-Water Emulsions Stabilized by Surfactant– Chitosan Membranes. J. Colloid Interface Sci. 2006, 296 (2), 581-590.


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Figure 1. (a) FT-IR spectra of p-HNTs, c-g-HNTs and c-c-HNTs; the inset shows an enlarged spectrum in the range from 1100 to 2000 cm-1, (b) TGA curves of p-HNTs, c-g-HNTs and c-cHNTs; the inset shows DTG curves; numbers show the corresponding points in the plot and the inset, (c)  potential of p-HNTs, c-g-HNTs and c-g-HNTs MPs versus solution pH.

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Figure 2. A schematics illustration for the proposed assembly of chitosan-functionalized HNTs into a mesoporous-macroporous microparticle.


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Figure 3. (a-d) Confocal images of Pickering emulsions prepared using stained clay at the following conditions: (a) p-HNTs with SDS addition, (b) c-g-HNTs with SDS addition, (c) pHNTs without SDS addition, (d) c-g-HNTs without SDS addition; scale bar = 50 µm. (e-h) Confocal images of microparticles of c-g-HNTs stained after preparation at the following conditions: (e) without SDS addition, (f) with SDS addition, (g) without pH change (without NaOH addition), (h) with pH change (with NaOH addition); scale bar = 10 µm.


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1 m

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5 m

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Figure 4. (a-g) SEM images of: (a) p-HNTs, (b) c-c-g HNTs, (c) c-g-HNTs, (d) a microparticle prepared from c-g-HNTs, (e) the porous structure of the microparticle in (d), (f) microparticles prepared from c-g-HNTs with different sizes, (g) cross-sectional image of a microparticle prepared from c-g-HNTs, (h) TEM image of a microparticle prepared from c-g-HNTs, and (i) an optical microscope image of 0.1 wt% SDS and 15% (v/v) ethyl acetate in water emulsion.


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Figure 5. (a) Comparison of adsorptive removal percentages of oil (% Re) for different samples (m = 1.67 g/L, T = 18 ºC, time= 30 min, and no pH adjustment), (b) nitrogen adsorption-desorption isotherms at 77 K, and (c) pore size distribution of mesopores.


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Figure 6. Effect of (a) initial oil concentration, (b) adsorbent dosage, (c) time, (d) pH, and (e) ionic strength on the adsorption characteristics of oil onto c-g-HNTs MPs; (fixed parameters are: Co = 300 ppm, m = 1.67 g/L, T= 18 ºC, time= 30 min, and no pH adjustment).


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Figure 7. Linear plots of oil adsorption onto c-g-HNTs MPs (a) pseudo first order kinetic model, (b) pseudo second order kinetic model, (c) intraparticle diffusion model; (Co = 300 ppm, m = 1.67 g/L, T= 18 ºC, and no pH adjustment), and (d) schematic scale representation of the relative sizes of oil droplets and c-g-HNTs MPs adsorbent particles.


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Figure 8. Equilibrium isotherms of oil adsorption onto c-g-HNTs MPs at three different temperatures, the solid lines represent the BET model fits to the data; (Co = 0-1000 ppm, m = 1.67 g/L, time= 30 min, and no pH adjustment).


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Figure 9. (a) Settleability/decay of turbidity with time for p-HNTs, c-g-HNTS and c-g-HNTS MPs in DI water, (b) settleability/decay of turbidity with time for p-HNTs, c-g-HNTS and c-g-HNTS MPs in 300 rpm crude oil emulsion, (c) effect of particle size on the colloidal stability of clay (for simplicity, parameters were calculated assuming spherical geometries and insignificant variations in the assembled particle density), and (d) dispersibility of p-HNTs (on the right) and c-g-HNTS MPs (on the left) in 300 rpm crude oil emulsions.


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Figure 10. Adsorption-desorption cycles for c-g-HNTs MPs – oil droplets system; (Co= 300 mg/L, m= 1.67 g/L, T= 18 °C, and no pH adjustment).


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Table 1. Elemental analysis data of the prepared samples Sample p-HNTs c-c-HNTs c-c-HNTs MPs c-g-HNTs c-g-HNTs MPs Internally crosslinked c-g-HNTs MPs Externally crosslinked c-g-HNTs MPs c-g-HNTs MPs after the fifth regeneration cycle

N (wt %) 0.27 0.635 0.589 0.58 0.598 1.520

C (wt %) H (wt %) S (wt %) 2.017 0.389 2.037 0.708 2.062 2.04 2.179 1.854 2.243 2.255 2.226 2.177 2.229 28.950 3.730 -

Table 2. Textural properties of the prepared samples and the corresponding removal percentages of oil droplets from water Sample p-HNTs c-g-HNTs c-c-HNTs MPs c-g-HNTs MPs Internally crosslinked c-g-HNTs MPs Externally crosslinked c-g-HNTs MPs

BET Surface area (m2/g) 49.942 41.759 50.384 52.962

BJH cumulative pore volume (cc/g) 0.359 0.417 0.328 0.507

BJH mean pore diameter (nm) 3.711 10.012 11.315 11.221

Re (%) 38 88 82 94

46.378

0.292

8.964

68

47.818

0.470

8.972

71


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Table 3. Kinetic parameters for the adsorption of oil onto c-g-HNTs MPs Kinetic model

Pseudo first order

Pseudo second order

Intraparticle diffusion

Parameter

Calculated value

k1 (min-1) qe (mg/g) qe, exp (mg/g) R2 k2 (g/mg. min) qe (mg/g) qe, exp (mg/g) R2 kd,1 (mg/(g. min1/2)) C1 (mg/g) R21 kd,2 (mg/(g. min1/2)) C2 (mg/g) R22

0.38 157 166.25 0.9617 0.00288 175.44 166.25 0.9983 26.04 64.70 0.9872 7.66 122.8 0.9385

Table 4. BET isotherm parameters for the adsorption of oil onto c-g-HNTs MPs Parameter qmono, max (mg/g) K1 (L/mg) K2 (L/mg) R2

18 788 0.008 0.011 0.9408

Temperature (°C) 38 610 0.01 0.02 0.9668


58 450 0.0055 0.009 0.9411

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Macroporous Microparticles Using Three

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