Regular Article
Enhanced demulsification from aqueous media by using magnetic chitosan-based flocculant

https://doi.org/10.1016/j.jcis.2018.02.024Get rights and content

Abstract

A series of quaternized chitosan (QC)-grafted magnetic nanoparticles (MNPs) were successfully synthesized for demulsification from aqueous environments. Fe3O4 MNPs were synthesized by using a coprecipitation method, followed by surface coating with silica and aminopropyl to form a surface for further grafting of QC molecular chains. The synthetic magnetic flocculants were characterized by various technologies and their demulsification performances were evaluated in detail as a function of dosage, QC grafting ratio (Gq), pH and magnetic field. Results showed that pH did not significantly affect oil–water separation performance and MNPs with high Gq exhibited enhanced separation efficiency. The separation capacity was estimated to be >105 mg of diesel oil/mg of magnetic flocculant. Recycling experiment indicated the magnetic flocculant could be recycled up to at least 7 cycles at various pH levels. The grafted QC layer endowed the hybrid MNPs with permanent positive surface charges, thus allowing them to flocculate negatively charged oil droplets via electrostatic patching. The magnetic field could not only accelerate the separation of resulting flocs, but also remove the MNPs-coated dispersed oil droplets. In conclusion, QC-grafted MNPs provide a potentially new technique for developing environmentally friendly and highly efficient magnetic flocculant for practical demulsification applications.

Introduction

In recent years, with the increasing industrial oily wastewater and oil spill accidents, development of advanced materials or techniques that can efficiently separate oil–water mixture have attracted extensive research attention. Facile and efficient process for oil–water separation is greatly desired, especially for emulsified oily wastewater, in which the diameter of oil droplets (d) is usually less than 20 μm [1]. The most difficult step in separating emulsified oil is demulsification. Conventional techniques, such as gravity separators and air flotation, are not able to separate emulsified oil from oily wastewaters [2]. Although various filtration materials (metallic meshes, textiles/fabrics, and polymeric membranes) with special wetting properties have been developed for selective oil–water separation [3], [4], [5], [6], they are easily fouled in practical application; moreover, it usually requires elevated pressure to maintain the permeate flux due to the small pore channels in the filtration materials, which is energy intensive [7]. Recently, it was reported that magnetic superhydrophobic/superoleophilic sponges (foams) can efficiently absorb oil slick; however, they are incapable of separating the emulsified oil from aqueous media [7], [8], [9], [10], [11].

In addition to filtration and adsorption techniques, nanoparticles show considerable attracting features in oil–water demulsification and separation [12], [13], [14], [15]. Recently, the application of magnetic nanoparticles (MNPs) in oil–water separation has been paid more and more attentions. MNPs with desired surface properties are a kind of promising nanomaterials, due to their special magnetic response, easy separation and potential recyclability [16], [17], [18]. By using various surface modification techniques, MNPs wettability can be manipulated for achieving more effective assembly at oil–water interfaces and/or within emulsified droplets, thereby imparting magnetic properties to the emulsified oil droplets [19], [20]. Superhydrophobic nanoparticles are usually incapable of separating emulsified oil because of their poor dispersibility in water phase, although a breakthrough on using superhydrophobic particles to achieve demulsification was reported recently [21]. In previous studies, majority of the MNPs were designed and modified to be amphiphilic; particles with this special wettability can well disperse in aqueous phase, and then transfer from continuous phase to the surface of emulsified oil droplets. As a result, the magnetically tagged oil droplets can be easily isolated from the continuous phase with the help of an external magnetic field. For example, oleic acid-coated or polyvinylpyrrolidone-coated Fe3O4 MNPs were successfully used to remove the emulsified oil droplets in water [22], [23], [24], [25]; similarly, poly(N-isopropylacrylamide)-grafted or poly(2-dimethylaminoethyl methacrylate)-grafted MNPs also exhibited good demulsification performance at suitable temperature or pH conditions [26], [27]. For these MNPs, interfacial activity has been considered to be the main driving force for their sorption at the surface of oil droplets, while electrostatic interaction was rarely involved.

In one of our recent studies [28], (3-aminopropyl)triethoxysilane (APTES)-coated MNPs were successfully synthesized for emulsified oil–water separation. It was found that, under neutral condition, interfacial activity indeed played an important role; however, under acidic condition, the accumulation of MNPs onto oil droplet surfaces is mainly driven by electrostatic attraction, thereby enhancing its separation efficiency. Nevertheless, the cationic charge intensity of APTES-coated MNPs was still not high enough and decreased significantly with pH rising. Therefore, the demulsification performance of APTES-coated MNPs was still less-than-desirable, and even very poor under alkaline condition due to the charge repulsion between negatively charged MNPs and oil droplets. Most recently, we grafted a highly cationic polyelectrolyte (quaternized chitosan, QC) onto APTES-coated MNPs and preliminarily reported its demulsification performance in the form of a brief letter [29]. It was found that the demulsification effect was significantly improved after QC grafting. However, the study was very preliminary and not systematic. For example, some key influence parameters, such as QC grafting ratio (Gq), were not examined and optimized; more importantly, the demulsification process and mechanism were also not investigated.

Therefore, the main objective of this study was: (1) to develop a series of magnetic flocculants, namely QC-grafted MNPs with different Gq, and optimize the demulsification efficiency; (2) to explore and clarify the demulsification mechanism, as well as the function of magnetic separation. Accordingly, in the current study, a series of magnetic flocculants were carefully synthesized and applied to separate emulsified oil droplets in aqueous media. Influence of Gq, dosage, pH value and reusing, as well as various demulsification procedures, on the separation efficiency of emulsified oil were investigated in detail; meanwhile, the oil–water separation process was also examined. On this basis, the demulsification mechanism of synthesized magnetic flocculants was illustrated.

Section snippets

Materials

Iron chloride hexahydrate (FeCl3·6H2O), Iron(II) chloride tetrahydrate (FeCl2·4H2O), sodium hydroxide (NaOH), sodium metasilicate nonahydrate (Na2SiO3·9H2O), polyacrylic acid (PAA), (3-aminopropyl)triethoxysilane (APTES, 97 wt%) and glutaraldehyde (aqueous solution, 50 wt%) were purchased from Aladdin Chemistry (Shanghai, China). Quaternized chitosan (QC) was purchased from Shanghai Macklin biochemical Co. Ltd. and its substituting degree of quaternary ammonium was above 90%. Hydrochloric acid

Characterization of magnetic flocculant

The morphology, structure and composition of Fe3O4, Fe3O4@SiO2 and Fe3O4@SiO2-APTES had been systematically analyzed and reported in one of our earlier study [28], accordingly, this work mainly focused on the characterization of newly synthesized magnetic flocculant Fe3O4@SiO2-QC. Fig. 2 shows the XRD patterns of Fe3O4@SiO2-QC. It was found that the XRD patterns were similar to that of Fe3O4 nanoparticles. Diffraction peaks with 2θ at 30.3, 35.6, 43.4, 53.8, 57.4 and 62.8 were clearly observed,

Conclusions

Previously, many amphiphilic MNPs were successfully synthesized and applied in emulsified oil–water separation; results showed that interfacial activity of the MNPs was the main driving force for their sorption of at the oil droplet surface, while electrostatic interaction was rarely involved [22], [23], [24], [25], [26], [27]. This work demonstrated that electrostatic interaction played an extremely important role during emulsified oil–water separation process. The QC grafting on Fe3O4@SiO2

Acknowledgements

The authors wish to thank the financial support from the National Natural Science Foundation of China (NNSFC) project (#21506045 and #20714037), and the Zhejiang Provincial Public Technology Application Research Project (#2017C33101).

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