Sorption of heavy metals onto polyethylene microbeads and its effect on seabass

Abstract

Microbeads are one of the causes of microplastic pollution that is currently polluting ocean environment. It enters food chain via ingestion of marine vertebrates and invertebrates. This study aims to elucidate the interactions between polyethylene microbeads and heavy metals as well as to determine the possibility of polyethylene microbeads as heavy metal vectors for juvenile seabass. Two parts of experiments performed in this study, i.e. adsorption and ingestion. For the adsorption study, 10 g of virgin polyethylene microbeads (300 ^m) were submerged into 0.2, 0.4, 0.6, and 1.0 p,g/mL solutions of cadmium and chromium, and 0.4, 0.8, 1.2, 1.6, and 2.0 ^g/mL concentrations of lead in a batch of sorption experiments for 96 hours. In the ingestion experiment, seabass was exposed in control, single, co-exposure, and preloaded experiments. All exposure conditions were similar to the adsorption experiment with 10 g and 5 g of microbeads. Acid digestion and Atomic Absorption Spectroscopy were used to quantify the amount of heavy metal adsorbed on microbeads and accumulated in fish tissues. Maximum adsorption capacity of microbeads were 11 ^g/g for Cd, 1.7 ^g/g for Cr, and 9.0 ^g/g for Pb. The kinetic study concluded that the adsorption of polyethylene microbeads occurred at a pseudo-first-order reaction, which involves physical attraction. Adsorption isotherm fitted the Freundlich model signifying adsorption occurs rapidly and has the tendency to desorb due to weak binding. The rates of heavy metal adsorption onto microbeads were 11, 4.5, and 1.7 mL/g for Cd, Pb, and Cr, respectively, suggesting that Cd had a higher affinity towards microbeads polyethylene than Pb and Cr. In the ingestion study, most of the heavy metal were detected at the skin layer. Control experiment validated that, seabass uptake exposed microbeads via ingestion. The single experiment concluded that uptake of heavy metals in seabass tissues increased with concentration and time. The higher the amount of exposed microbeads, the higher the uptake of heavy metals in the gastrointestinal tract after 48 hours of co-exposure. This indicates that heavy metals were first adsorbed on the microbeads followed by their ingestion by the seabass. In the preloaded experiment, the concentration of heavy metal ions detected in the gastrointestinal tract was higher than the direct exposure in single and co-exposure. The uptake values in the preloaded experiment increased steadily with concentration, time, and quantity of microbeads. Preloaded exposure in this study verified that microbeads-loaded heavy metals were incidentally ingested by seabass during foraging. The incorporation of the three heavy metals in the ingestion study was performed using Minitab 16.0 multi analysis of variance (MANOVA). This study proved that polyethylene microbeads possess the potential to accumulate, transport, and transfer heavy metals from water to intestinal organ, thus increasing risk, threatening the marine food web, and possibly harming other consumers.