Elsevier

Chemical Engineering Journal

Volume 374, 15 October 2019, Pages 1013-1024
Chemical Engineering Journal

Efficient entrapment and separation of anionic pollutants from aqueous solutions by sequential combination of cellulose nanofibrils and halloysite nanotubes

https://doi.org/10.1016/j.cej.2019.06.008Get rights and content

Highlights

  • The stepwise addition of HNTs and CNFs efficiently removes anionic dye from water.

  • Both HNTs and CNFs participate in the dye capture.

  • CNFs also aggregate the dye-loaded colloidal clay particles.

  • The size and morphology of the clay mineral determine the dye removal efficiency.

Abstract

The synergistic combination of different nanomaterials for improved performance in environmental applications such as the removal of aqueous micropollutants has attracted increasing interest in recent years. This study demonstrates a novel sequential adsorption–aggregation concept that harnesses tubular halloysite nanotubes (HNTs) and flexible cellulose nanofibrils (CNFs) for the removal of a small, anionic dye molecule, chrome azurol S, from water. Hollow HNTs were first allowed to interact with the aqueous dye solution, after which the dye-loaded colloidal nanotubes were aggregated and separated from the water phase with cationized CNFs. The combination of 25 mg CNFs with 1 g HNTs at pH 7 resulted in efficient removal of dye (80%) and turbidity (~100%) and the removal of dye was further promoted in more acidic conditions (within the pH range of 6–8.5) because of the attractive electrostatic interactions. Cationic CNFs not only enabled the separation of dye-loaded clay particles from the water phase through a rapid aggregation but also participated in dye removal through adsorption (~20%). In comparison with nano-sized HNTs, the dye removal performance of micro-sized and chemically similar kaolin was poor (43%). Given the good availability of both HNTs and CNFs and the low consumption of the more expensive component (i.e., CNFs) in the process, the concept is straightforward, readily applicable, environmentally benign, and potentially cost-effective.

Introduction

Clays are naturally occurring inorganics, composed primarily of phyllosilicate minerals such as kaolinite and montmorillonite [1]. The atoms in phyllosilicates are arranged in tetrahedral and octahedral sheets that typically form layered and bulky platelets. Tubules, fibers, and laths of various sizes also exist [1]. Halloysite nanotubes (HNTs) are two-layered clay mineral particles with a chemical composition [Al2Si2O5(OH)4·nH2O] similar to that of kaolinite [Al2Si2O5(OH)4] [2]. However, in contrast to platy kaolinite, HNTs typically have a hollow tubular morphology [2]. The tube walls of HNTs comprise ~15–20 aluminosilicate layers, with a spacing ranging from 1.0 nm (in hydrated form, n = 2) to 0.7 nm (in dehydrated form, n = 0) [3]. HNTs are polydisperse in size, typically having an outer diameter of 50–70 nm, lumen diameter of 10–20 nm, and length ranging from 0.5 to 1.5 µm [3]. The external surface and the inner lumen of HNTs carry different charges at pH 2–8 in water [4], the former comprising negatively charged silica (SiO2, ζ-potential ~ −50 mV) [5] and the latter comprising positively charged alumina (Al2O3, ζ-potential ~ +20 mV) [5]. The experimentally determined ζ-potential of HNTs in water at pH 4–8 is ~ −30 mV, which reflects the charge difference of these two layers [5]. Besides being abundant and inexpensive, HNTs have low in vitro [6] and in vivo [7] toxicity.

HNTs are versatile materials that can interact with and adsorb many different substances, thanks to the charge difference between the inner and outer surfaces. Driven by cationic electrostatic interactions, neutral and anionic substances can be loaded into the lumen of HNTs [3], [8], [9]. In addition, cationic substances can be adsorbed on the external surface of the tube [3], [9]. Sometimes substances can even be intercalated between the layers of the tubes’ walls [10]. Currently, HNTs are widely studied for biomedical purposes [11], [12], [13], but their potential for environmental applications has hitherto been underexplored. HNTs have been shown to adsorb e.g. anionic and cationic dyes [14], [15], [16], metal ions [17], [18], [19] and pharmaceuticals [20], [21]. Therefore, one potential environmental application of HNTs is the treatment of various kinds of polluted waters [22], [23]. The separation of nano-sized clay particles from water after the treatment and the hindered water flux in the packed column structures is often a challenge [24], although approaches, such as magnetic HNT adsorbents [25], [26], [27], have emerged.

To overcome these drawbacks, HNTs have often been combined with natural or synthetic polymers to form, for instance, beads [28], [29], [30], [31], sponges [32] or membranes [33], [34], [35], [36], [37]. However, these methods typically require a significant amount of time and the use of additional chemicals. In a more straightforward approach, clay minerals have simply been mixed with a polymer to yield clay-polymer nanocomposites, which are then directly used for wastewater purification [38], [39]. In another study, pristine HNTs were combined with a synthetic cationic polymer for the removal of a cationic dye, basic blue 7, from water [24]. The mixing of HNTs with the polymer resulted in aggregation [40] and the formation of a HNT–polymer hybrid material, which was used as a matrix in column filtration of a dye solution. Although the hybrid was easily separated from water and transferred to a column, its dye removal performance was not optimum, as the cationic polymer covered the anionic surface of the HNTs, suggesting that better performance may be obtained if the HNTs are added to the treated mixture before the polymer is added.

Clays in general have been shown to adsorb, for instance, natural organic matter (NOM), thus converting it to an insoluble form, which can then be removed by coagulation–flocculation processes using a cationic polymer [41]. The polymer itself also participates in the purification process by partly adsorbing NOM and forming precipitates [41]. However, there is a lack of studies expanding this concept for the removal of dissolved molecules (e.g., micropollutants) from water, possibly because the conventional coagulation–flocculation treatment has previously been declared inefficient for simultaneous micropollutant removal [42]. In addition, new and effective bio-based materials are needed to replace older synthetic polymers. One promising candidate is cellulose nanofibrils (CNFs), which are elongated and flexible nanomaterials originating from, for example, the mechanical refining of wood pulp or a combination of chemical/enzymatic treatments and mechanical force [43]. Depending on the raw material and the production method, the width of CNFs is typically 5–60 nm, and the length is a few micrometers [44]. In addition, the CNFs be functionalized with e.g. anionic carboxylic groups [45], [46], [47], [48] or cationic groups [49], [50], [51], [52], [53], [54], [55]. These functionalized CNFs can be designed using green solvent systems, such as recyclable deep eutectic solvents (DESs) [49].

In this study, we present a novel sequential approach that combines nanostructured clay and cellulose materials for the removal of small, anionically charged dye molecules from water. HNTs were first loaded with dye molecules and then aggregated using CNFs to enable their efficient separation through sedimentation. CNFs also played a role in the removal of dye molecules through adsorption. The cationic CNFs were produced through sequential periodate oxidation and DES-mediated functionalization from cellulose and combined with HNTs for the enhanced removal of aqueous anionic dye chrome azurol S (CAS). Kaolin clay was used as a reference material in adsorption experiments. The effects of CNF dose, pH and mixing time on the dye and turbidity removal were investigated. The chemical characteristics, sizes, and morphologies of the HNTs, CNFs, and kaolin were analyzed using polyelectrolyte titration, diffuse reflectance infrared transform spectroscopy (DRIFT), wide-angle X-ray diffraction (WAXD), laser diffraction (LS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM).

Section snippets

Raw materials and chemicals

Commercial softwood dissolving pulp (cellulose 96.2%, hemicelluloses 3.5%, total lignin < 0.5%, acetone soluble extractives 0.17%; Domsjö Fabriker AB, Sweden) was used as the cellulose raw material. The dry sheets were disintegrated in deionized water before use. LiCl (Reag. Ph. Eur.; VWR, Belgium), sodium metaperiodate (<99.8%; Honeywell/Fluka, USA), glycerol (Reag. Ph. Eur.; VWR, Belgium), aminoguanidine hydrochloride (>98%; TCI, Japan), and ethanol (96%; VWR, France) were used in the

Effect of CNF dose on the combined dye removal process

A significant difference was observed between HNTs and kaolin in CAS removal at pH 7 (Fig. 1). Combining 25 mg CNFs with 1 g HNTs (i.e., CNF dose 25 mg/g) resulted in maximum CAS removal of 80% (qe 19 mg/g), whereas with kaolin only 43% (qe 11 mg/g) removal was achieved. Interestingly, HNTs alone removed 42% of CAS (qe 10 mg/g), but kaolin alone removed only 7% of CAS (qe 2 mg/g). As the CNF dose increased from 2.5 to 25 mg/g, the trend of removal was almost linear, indicating that complete

Conclusions

The sequential combination HNTs and CNFs resulted in significantly improved removal of anionic dye compared with using either of them alone. The size and morphology of the clay mineral had a clear impact on the outcome, as the nano-sized HNTs performed better than the micro-sized kaolin. The demonstrated concept is simple and requires no chemical pretreatments of the clay mineral (e.g., tube etching) or heavy energy usage (e.g., drying, vacuum). Moreover, both nanomaterials are abundantly

Acknowledgements

Ms. Eveliina Kuorikoski is acknowledged for her assistance in the laboratory experiments and Mr. Panpan Li is acknowledged for his advice in the preparation of CNFs. We would also like to thank Mr. Jarno Karvonen for performing the LS measurements, Mrs. Kaisu Ainassaari for performing the BET analysis, Mr. Marcin Selent from the Center of Microscopy and Nanotechnology at the University of Oulu for performing the XRD measurements, and Mr. Jens Kling for performing the high-resolution TEM imaging

Funding

This work was supported by the Advanced Materials Doctoral Program of the University of Oulu Graduate School and the Bionanochemicals project of the Academy of Finland [grant number 298295].

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