Detecting and analysing microplastics in local bottled drinking water

Abstract

Plastics play a pivotal role in various industries owing to their versatile properties such as strength, durability, light weight, and cost-effectiveness. Yet, their widespread use has led to the pollution of Earth's water systems. Over time, plastic waste degrades into microplastics, particles smaller than 5 mm. Recent studies have highlighted the growing concerns associated with microplastics, especially in bottled beverages, including bottled water. The global understanding of the extent of microplastic contamination remains limited, prompting this research. This has been particularly driven by the lack of information on microplastics in bottled drinking water produced and sold in Malta. The aim of this study was to detect, quantify, analyse, and characterise the microplastics found in local bottled water. A total of 50 samples of local bottled water from 5 different brands were tested. The microplastics were collected on glass fibre filters by employing a vacuum filtration system to filter the water. To detect and quantify the microplastics, a fluorescent dye, Nile red, was used to stain the polymeric particles. The dye was mixed with methanol to obtain the optimal solution, at a concentration of 10 µg/mL. The stained particles were then detected, analysed, and quantified using fluorescence microscopy, employing a herringbone pattern for sampling. After taking into account negative control samples, the overall average concentration of microplastics per Litre (ppL) of bottled water was calculated to be 35,877 ± 23,542 ppL. Notably, 84% of the 50 water samples exhibited significant microplastic contamination. The average microplastic diameter was measured to be 2.3696 ± 0.0035 µm. For chemical characterisation, 10 larger particles from each brand which totalled to 50 particles were manually extracted onto a silicon wafer and analysed using Raman spectroscopy. From the characterised particles, 17 were cellulose, 8 were polyurethane, 7 were polymethyl methacrylate, 6 were polyethylene, 5 were low-density polyethylene, 2 were polyvinyl chloride, 2 were polyphenylene terephthalamide, 1 was polyethylene terephthalate, 1 was polyvinyl ether, and 1 was polyacrylamide. The characterised particles revealed varying morphologies, with 36 identified as fragments and the remaining 14 as fibres. Neglecting the particles that were attributed to laboratory sources, the chemically characterised microplastics were noted to be primarily originating from the bottle caps.