Fast and efficient adsorption of bisphenols pollutants from water by using Hydroxypropyl-β-cyclodextrin polymer

Highlights

• An efficient way for removing bisphenols from water is proposed with HP-β-CD polymer.

• The adsorption rates of HP-β-CD polymer toward bisphenols are rapid.

• HP-β-CDP can be easily regenerated by mild washing step and has excellent reusability.

• The removal efficiencies can be still up to 100% through flow-through experiments.

Abstract

The presence of bisphenols in water systems is potentially dangerous on the environment and public health. In this study, a multiaperture polymer of hydroxypropyl-β-cyclodextrin (HP-β-CD) was prepared with tetrafluoroterephthalonitrile as crosslinker, and was applied to remove bisphenols (BPA, BPS and BPF) from water. Influence factors affecting the adsorption efficiency of the HP-β-CD polymer were investigated separately, including adsorption time, adsorbent doses, initial concentration of bisphenols and temperature. HP-β-CD polymer presents fast adsorption kinetics toward bisphenols, attaining 95% of adsorption equilibrium uptake for BPA within the first min and 90% of equilibrium uptake for BPS and BPF in 10 min. The kinetics data fitted well with pseudo-second-order kinetics model. The adsorption isotherms of HP-β-CD polymer toward bisphenols are well described by Freundlich adsorption model, suggesting heterogeneous active sites for bisphenols on the adsorbent. In addition, the HP-β-CD polymer can be easily recycled through a simple washing process with no change in performance. The adsorption ratio of bisphenols can still achieve 100% within a certain flow rate in flow-through experiments, also proving its fast adsorption ability toward bisphenols. In a word, HP-β-CD polymer exhibits effective performance in the removal of the three bisphenols with fast adsorption kinetics, easy regeneration, and excellent reusability.

Introduction

The discharge of industrial waste water into the natural water system will be harmful to the human body and aquatic life [1]. The organic pollutants in water have received more and more attention due to their mutagenicity, carcinogenicity and hereditability [2]. Usually, the treatment of high concentration of industrial waste water is simple before it is discharged. However, treatment methods for the low concentration of organic pollutant are generally complex and have defects such as high cost and difficult operation [3]. Bisphenols have been widely used as key modification monomers in the production of engineering and composite materials with excellent properties. For example, BPA as a key monomer has been widely applied in the plastics manufacture to produce epoxy resins and polycarbonate plastics [4]. Animal studies show that BPA can mimic the effects of estrogen, and even the use of it in very low doses can lead to early maturation in females, decrease in sperm count, and prostate growth [5]. In addition, BPA has certain embryo toxicity and teratogenicity, which can significantly increase the incidence of ovarian cancer, prostate cancer, leukemia and other cancers in animals [6]. Meanwhile, studies have revealed that BPA is related to asthma in mice, and preliminary human trials indicate that exposure to BPA early in pregnancy may lead to asthma in infants [7]. Many countries, such as Canada, the United States and China, have banned the use of BPA as a synthetic material. Therefore, BPS and BPF are widely used as substitutes for BPA in the synthesis of plastics industry and epoxy resin [8]. BPF has been used in the raw material of polycarbonate, epoxy resin, polyester and phenolic resin, as well as intermediate of synthetic pesticides, dyes and auxiliaries, which are widely applied in lacquer, varnish, dental repair equipment, dental sealants, adhesives, plastics, water pipes, food canisters and agriculture [9,10]. Recently, Moreman and coworkers have reported that BPS and BPF show similar toxic and estrogenic effects to BPA, highlighting health concerns regarding the use of BPA alternatives [11]. Nowadays, a large volume of wastewater with bisphenols is continuously delivered to sewage treatment plants, but they are usually not treated to the satisfactory extent [12]. As a result, kinds of approaches have been exploited for efficient removal of bisphenols from wastewaters, such as activated carbon (AC) adsorption [13], biodegradation [14], advanced oxidation processes (AOPs) [15], and membrane filtration [16]. For the organic micropollutants, researchers have paid more attention on AOPs and AC adsorption due to their scalability in technology and economy [17]. AOPs can effectively reduce the bisphenols from waste water, but generally consume lots of energy, chemical reagents, time, and even may generate oxidized byproducts with toxic activity [18]. AC adsorption process also removes BPA effectively without forming oxidation products and has become favored for water and wastewater treatment. However, the regeneration of AC is hard and energy consuming, and the adsorption kinetics of AC toward BPA is relatively slow [19]. In order to improve the efficiency of adsorption process designed for bisphenols, many other novel adsorption materials were developed, such as inherent microporous polymers [20], metal organic frameworks [21], covalent organic frameworks [22], and super cross-linked polymers [23]. However, the new sorbents with excellent adsorption capacity and rapid adsorption rate are still needed for the removal of bisphenols. In addition, the most studies focused on BPA removal and only few attempts were made to eliminate BPF and BPS from aqueous solutions.

Cyclodextrins (CDs) have remarkable encapsulation property due to the hydrophobic cavity and hydrophilic outer surface. The hydrophobic cavity enables them to encapsulate many organic compounds to form inclusion complexes. In addition, the physical and chemical characteristics of CDs can be modified depending on the guests, which can enable relatively facile synthesis of innovative materials for pollution removal from water [24,25]. Water-insoluble cyclodextrin polymers as novel adsorbents for wastewater treatment have become research hotspots due to their advantages such as easy preparation, easy modification, easy recycling, and better adsorption performance than cyclodextrin monomers. [[26], [27], [28], [29]]

Crini et al. [26] gave a detailed introduction to the synthesis and application of cyclodextrin polymers. In the field of pollutant treatment, the research scope includes phenols, benzenes, pesticides, pharmaceuticals, polycyclic aromatic hydrocarbons, dyes, heavy metals and so on. Up to now, the most studies still use epichlorohydrin as a cross-linking agent to form cross-linked polymer, and most of this type of polymer is a non-porous material. To synthesize organic polymers with permanent microporosity, a certain degree of rigidity is required for the crosslinking agent. Mhlanga et al. [30] used toluene-2,4-diisocyanate (TDI) with multifunctional group as a cross-linking agent to synthesize nanoporous cyclodextrin polymer, which was used for removing anilines at 20 μg/L. Alsbaiee et al. [31] cross-linked cyclodextrin with rigid tetrafluoroterephthalonitrile to synthesize a porous β-cyclodextrin polymer, which was used to treat multiple polar aromatic organic micro-pollutants in wastewater.

In this study, a crosslinked HP-β-CD polymer was synthesized with tetrafluoroterephthalonitrile as crosslinking agent and applied to removal of bisphenols containing BPA, BPS and BPF from water. The synthesized materials were characterized by the Fourier transform infrared (FT-IR) spectroscopy to comprehend the groups of the HP-β-CD polymer, by the thermal gravimetric analyzer to test the thermal stability of the material, and by scanning electron microscope (SEM) to understand the morphology. The factors influencing the adsorption ratio and adsorption capacity were investigated separately, including contact time, adsorbent doses, initial concentration of bisphenols, and temperature. The study of adsorption kinetics and adsorption thermodynamics were carried out to comprehend the adsorption mechanism. The application of the polymer was tested, including reusability of the material and flow-through experiments.