An in-depth identification and characterisation of microplastics in the Vaal River, South Africa

Abstract

Microplastics (MPs), polymer additives, and their associated contaminants have drawn significant attention in recent years as emerging pollutants. MPs pose a myriad of threats to the environment, freshwater biota, and terrestrial organisms. In South Africa, as is the trend internationally, the body of knowledge on the presence of MPs in freshwater is remarkably less than in marine systems. This study was dedicated to investigating the level of microplastic (MP) pollution in the surface waters of the Vaal River.

The occurrence, abundance, distribution, and chemical composition of MPs were assessed. MP abundances ranged from 0.13 - 2.5 particles/m3 (1.6×104 - 2.2×105 particles/km2) with an average abundance of 0.61 ± 0.57 particles/m3 (66×103 ± 54×103 particles/km2) (mean value ± stdev). MPs were observed in four shapes, predominantly, fibers (41.6%) and fragments (39.9%), with films (10.9%), and pellets (7.7%) seldomly observed. The sizes of MPs ranged from 0.06 - 5 mm, with an average of 0.96 ± 0.88 mm (mean ± stdev). Microplastics with sizes less than 2 mm were most prevalent representing 80% of the total particles. The average sizes (mean value ± stdev) of fibers, fragments, films, and pellets were, 1.2 ± 0.86 mm, 0.64 ± 0.59 mm, 1.7 ± 1.3 mm, and 0.32 ± 0.19 mm, respectively. There was a significantly higher abundance of coloured (82.7%) MPs compared to transparent MPs (17.3%). A variety of colours were observed including green (22.3%), black (19.1%), blue (18.3%), white (11.1%), and red (6.5%); in addition to other colours in minor percentages (5.4%, altogether).

Chemical characterisation of MPs revealed that 85.2% of identified polymers were polypropylene (PP) and polyethylene (PE) (Low-density polyethylene and high- density polyethylene). A minor proportion of poly(ethylene-co-vinyl acetate) (PEVA) (7.4%), poly(ethylene-co-acrylic acid) (PEAA) (5.6%), and polystyrene (1.9%) were also detected. Additionally, a few colourants were also identified including Pigment Blue 15, Pigment Yellow 83, Pigment Black 6, and Pigment White 6.

The high prevalence of fibers and fragments indicates that microplastic pollution is from secondary sources. These shapes are formed from the fragmentation of macroplastics. Their potential sources include domestic and industrial effluent, tributaries, and surface run-off from urban centres. This study has provided a benchmark for guiding future monitoring studies and identifying further research gaps.