Simultaneous organic/inorganic removal from water using a new nanocomposite adsorbent: A case study of p-nitrophenol and phosphate

Highlights

• A new nanocomposite was designed for co-removal of phosphate and p-nitrophenol from water.

• Nanosized HFOs was incorporated into hypercrosslinked polystyrene to obtain bifunctionality.

• The mechanism for co-adsorption of phosphate and PNP was discussed.

• The exhausted adsorbent could be readily regenerated for continuous runs.

Abstract

A new nanocomposite adsorbent, HFO-802, was fabricated by incorporating nanosized hydrated ferric oxides (HFOs) inside the hyper-cross-linked polymeric adsorbent NDA802. The co-removal of inorganic and organic pollutants was examined using p-nitrophenol (PNP) and phosphate as the model compounds. Three widely used adsorbents, including powdered activated carbon, macroporous polystyrene adsorbent (XAD-4) and the host adsorbent (NDA802), were tested for comparison. HFO-802 exhibited superior properties when compared to the other adsorbents during the simultaneous removal of phosphate and PNP. These better properties were attributed to the unique structure of HFO-802; i.e., the encapsulated HFO nanoparticles exhibit preferable removal of phosphate through inner-sphere complexation, whereas the host NDA802 captures PNP through micropore filling, π–π interactions, and acid–base interactions. More attractively, the exhausted HFO-802 was amenable to effective regeneration by using an alkaline solution, allowing for repeated use with a constant co-removal efficiency over 10 continuous cycles of operation. The effect of solution pH, contact time and ionic strength on HFO-802 co-removal was determined. The results highlighted a new method to fabricate bifunctional adsorbents for the co-removal of inorganic and organic pollutants by encapsulating metal oxide nanoparticles inside a microporous solid host.

Introduction

Currently, an increasing number of processes have been developed to remove pollutants, whether inorganic or organic, from water. As one of the most widely used processes, adsorption can efficiently remove inorganic or organic pollutants from water [1], [2], [3], [4]. A variety of adsorbent materials, such as activated carbon [3], [5], polymeric adsorbents [1], [2], biosorbents [6], [7], chitosan [8], and other low-cost materials [9], have been developed and applied in the laboratory or the field. As a classical adsorbent in water treatment, activated carbon possesses have a high capacity owing to the large surface area and varied surface functional groups. In spite of these properties, exhausted activated carbon is difficult and costly to regenerate to allow for repeated use [4]. Over the past few decades, synthetic polymeric adsorbents comprised of nanoporous structures, crosslinking matrices, sound skeleton strengths, and tunable surface chemistry have emerged as an alternative to activated carbon for water decontamination of diverse contaminants [1], [2]. Generally, the exhausted polymeric adsorbent could be effectively regenerated under mild conditions for long-term use without noticeable capacity loss [10], [11], [12].

Various inorganic and organic pollutants, such as mining nutrients and effluent organic matter (EfOM), commonly coexist in the bio-treated sewage and pose an adverse effect on the ecosystem and human health [13], [14]. In review of the available adsorbents, most cannot simultaneously remove inorganic and organic pollutants in a single reactor. Adsorption of both classes of pollutants usually occurs through different pathways, and thus requires different active sites. For example, the majority of inorganic pollutants are present as ionic species in water. Thus, ion exchange through electrostatic interactions and/or chemical complexation with the active sites generally plays a dominant role in their adsorption. For organic pollutants, van der Waals interactions usually serve as the basic interaction for various adsorbents. In addition, the micropore filling is vital for those with micropore structures like activated carbon [15]. Additionally, specific interactions, such as hydrogen bonding and acid–base interactions, always occur between the organic adsorbate and the functional groups of the adsorbent. In general, developing adsorbent materials capable of co-removing organic and inorganic pollutants is a compelling but still challenging topic.

In our recent report [16], we synthesized a recyclable, aminated, hyper-cross-linked polymeric adsorbent, NDA802. NDA802 featured amino groups, a large specific surface area, and sufficient micropore regions for the effective removal of effluent organic matter from wastewater. As expected, NDA802 showed a poor removal of inorganic contaminants. Over the past few decades, hydrated ferric oxides (HFO) have been shown to exhibit a high surface area and specific affinity for adsorbing inorganic contaminants (e.g., arsenic, phosphate, and toxic metals) from aqueous systems [17], [18], [19]. Our previous work also revealed that nanosized HFOs supported by polymeric anion exchangers are promising for the removal of inorganic pollutants from aqueous solutions. These HFOs featured a high preference towards the target pollutants, easy handling, and long-term reusability after regeneration [20], [21]. However, the HFOs could not remove organic contaminants effectively. From the foregoing works, one could expect that the immobilization of nanosized HFO inside NDA802 could possibly yield a promising adsorbent for the simultaneous removal of inorganic and organic contaminants.

Based on the above analysis, we fabricated a novel, bifunctional nanocomposite adsorbent, HFO-802, by immobilizing HFO nanoparticles into NDA802. To examine the performance of HFO-802 for the simultaneous removal of inorganic and organic pollutants, phosphate and p-nitrophenol (PNP) was employed as the model compounds because of their ubiquity in industrial effluents. Co-removal of phosphate and PNP by HFO-802 was investigated as a function of pH, competing anions, and contact time. Cyclic adsorption–regeneration experiments were also performed to evaluate the feasibility of HFO-802 for long-term application.