Application of Zn/Al layered double hydroxides for the removal of nano-scale plastic debris from aqueous systems

Highlights

• Removal of nano-scale plastic debris (NPDs) from aqueous solution using adsorption technique.

• The adsorption of NPDs on layered double hydroxide (LDH) was favorable and spontaneous.

• Ionic strength and pH of solution plays an important role in sorption of NPDs on Zn-Al LDH.

• The electrostatic attraction was the major phenomenon, causing the removal of NPDs.

Abstract

Nano-scale plastic debris (NPDs) are emerging as potential contaminants as they can be easily ingested by aquatic organisms and carry many pollutants in the environment. This study is aimed to remove NPDs from aqueous environment for the first time by using eco-friendly adsorption techniques. Initially, the interaction between NPDs and synthesized Zn-Al layered double hydroxide (LDH) was confirmed by pH titration of Zn-Al LDH against NPDs at varying mass ratio (50:1 to 50:7) and FTIR analysis for both before and after 2 h of contact time. Fast removal was observed in deionized water and synthetic freshwater with maximum sorption capacity (Q_max) of 164.49 mg/g,162.62 mg/g, respectively, according to Sips isotherm. Whereas, removal was least in synthetic hard water having a Q_max value of 53 mg/g. For 2 mM concentration of SO₄²⁻ and PO₄^3-, the adsorption capacity significantly decreased to 2%. The removal efficiency was found 100 % at pH 4, while at pH 9, it reached 37 % due to increased competitive binding and destabilization of LDH under alkaline conditions. The process of sorption was spontaneous in different types of water studied. The study reveals that Zn-Al LDH can be used as potential adsorbent for the removal of NPDs from freshwater systems.

Introduction

Pollution caused by the exponential use of plastic items in our daily life is contributing to the accumulation of wreckage in aquatic ecosystems and a matter of serious concern all across the world (Tallec et al., 2019; Nguyen et al., 2019; Lehner et al., 2019; Besseling et al., 2014). An enormous amount of plastic debris released from the household, industrial, and agricultural wastes are the primary source of plastic pollution (Alimi et al., 2018). Once they reach the environment, its physical and chemical weathering leads to the generation of the microscale (< 5 mm) and nanoscale plastic debris (NPDs) (< 1 μm) in the environment (Koelmans et al., 2015). Direct release of microplastics and NPDs in the environment from personal care, cosmetics, and thermal cutting of polystyrene also has been reported (Hernandez et al., 2017; Zhang et al., 2012). Wastewater treatment plants (WWTPs) are considered as the main source for the release of micron-sized plastic particles in the freshwater system. For example, Mason et al. reported a discharge of ∼50,000 up to nearly 15 million micron-sized plastic particles from the wastewater treatment plant (Mason et al., 2016).

Once these microplastic and NPDs enter the aquatic environment, their interaction differs with aquatic organisms and other components of the environment as compared to their pristine form. This difference can be attributed due to the change in their chemical structure and physical properties (Yu et al., 2019a). The cleavage of the bonds present on the surface of polymers makes them more vulnerable to degradation (Lehner et al., 2019). Weathered forms of microplastic, as well as NPDs, were found to have toxic effects on aquatic organisms. However, NPDs are of particular concern because of their smaller size and high surface area to volume ratio. Due to their small size, they can easily penetrate the biological barriers as compared to their microplastic counterparts (Lehner et al., 2019; Booth et al., 2016; Bhattacharya et al., 2010; Rothen-Rutishauser et al., 2007; Gopinath et al., 2019). Further, it has been widely reported that the availability and toxicity of NPDs to aquatic organisms depend on their stability in the environment. Various stability and aggregation studies of NPDs under varying environmental conditions had been found that ionic strength, dissolved organic matter, and suspended solids play an important role in controlling their fate in the aquatic environment (Singh et al., 2019; Yu et al., 2019b; Liu et al., 2019). Due to increased bioavailability, these particles subsequently get transferred from one trophic level to others via the food chain (Cedervall et al., 2012; Mattsson et al., 2015; Chae et al., 2018). Recent studies documented the uptake, accumulation, and toxicity of polystyrene NPDs in mussels, zooplankton, algae, and daphnia (Wegner et al., 2012; Pitt et al., 2018; Nolte et al., 2017; Nasser and Lynch, 2016). However, an increased generation of NPDs may have a possible detrimental effect on human health being at the top of the food chain (Revel et al., 2018).

The toxicity induced by individual NPDs in aquatic organisms could be marginal and may not have any chronic effects. Nevertheless, the synergistic toxic effects of other contaminants in the presence of NPDs are well studied (Lee et al., 2019; Ma et al., 2016). Several studies have shown that the hydrophobic surface of NPDs has a high tendency to adsorb persistent organic pollutants (POPs) like polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs) and thereby increasing its toxicity towards various organisms (Zhang et al., 2018; Jeong et al., 2018). It has been found that they are also involved in the association and transport of heavy metals in the environment (Alimi et al., 2018; Yu et al., 2019a; Darbha et al., 2012).

Different techniques such as membrane bioreactor, coagulation, and ultrafiltration were used at WWTPs for the removal of microplastics (Talvitie et al., 2017; Lares et al., 2018; Arossa et al., 2019; Ma et al., 2019; Long et al., 2019). However, the removal efficiency is still poor. Moreover, the mentioned techniques are expensive and have limitations in the removal of nano-sized particles. Therefore, this underlines an urgent need to find a possible solution for the removal of NPDs from the aqueous environment.

Limited work on the removal of microplastics and no reported studies on the removal of NPDs from aqueous solutions allowed us to form the major applied scientific objective. The focus of the current work is to explore the adsorption as an eco-friendly and cost-effective technique for the removal of NPDs from aqueous systems. Based on the preliminary experiments on the interaction between layered double hydroxides (LDH) and NPDs, batch sorption studies were carried out. LDH, which is also called anionic clay, has been used with or without modification for the removal of heavy metals and organic contaminants such as Cr, Cu, dyes, pesticides, etc. (Lazaridis et al., 2004; Li et al., 2016; Cardoso and Valim, 2006; Chaara et al., 2011). LDH can be considered as the most appropriate adsorbent for the anionic contaminants without any modification as they have high anion exchange capacity (AEC≈3 meq g⁻¹) (Legrouri et al., 2005). The positive charge present on the surface of LDH due to partial substitution of trivalent ion for the divalent metal ion is another mechanism for the sorption of anionic species on the LDH.

There is no globally accepted definition for NPDs; thus, for this study, we have considered NPDs as plastic items with at least one dimension smaller than < 1 μm. In this study, we have used negatively charged polystyrene (PS) nanoparticles as a model for NPDs because it has been widely reported that commonly used forms of plastics, on erosion possess negative charge on their surface (Fotopoulou and Karapanagioti, 2015; Hossain et al., 2019). Nanostructured Zn-Al layered double hydroxide (Zn-Al LDH) was synthesized and used for the adsorption of NPDs from aqueous solution. The effect of coexisting anionic species and varying solution pH on the adsorption was also investigated. To mimic the environmental conditions, the adsorption behavior of NPDs on Zn-Al LDH in synthetic freshwater (SF) and synthetic hard water (SH) was also studied.