Quantitative evaluation of non-active land input ofmicroplastics: A case of PET polymer

Abstract

Microplastics (MPs) are plastics with size less than 5 mm. In recent years, microplastics have attracted more attention due to their ubiquitous environmental distribution, especially in water environment. Little is known about the pollution of microplastics in the terrestrial environment, especially in areas where there is no active input of MPs. It has been documented that MPs are widely detected in atmospheric dustfall and sewage sludge. However, there are few studies on the input fluxes of MPs to terrestrial environment, such as dust deposition and land-application of sludge. Polyethylene terephthalate (PET) is a thermally stable and chemically inert polyester and can be used to produce polyester fabrics, beverage bottles, plastic films, etc. PET accounts for one-third of total plastics in China, with an annual consumption of 37.5 million tons. In this study, we optimized an alkali-assisted chemical depolymerization method coupled with liquid chromatography-mass spectrometry to detect PET polymers in the environmental samples. The limits of quantification for PET MPs in atmospheric dustfall and sewage sludge were 195 ng (per 50 mg dustfall) and 180 ng (per 100 mg sludge), respectively. The method was applied to quantify PET polymers in atmospheric dustfall and sewage sludge samples collected in Tianjin, China. The results showed that PET could be detected in all dustfall samples with concentrations ranging from 776 to 5399 μg/g. The input fluxes of PET MPs via dust deposition were calculated as 100–586 μg m–2 d–1 in Tianjin. Additionally, region- and season-related fluxes were estimated. High deposition fluxes were observed in the upper and lower directions of the prevailing wind in Tianjin. There was a significant difference in the deposition flux of PET polymers between dry season and rainy season. The annual settlement of PET MPs in Tianjin was estimated to be 4.8−28 t. The concentrations of PET polymers in sewage sludge were 296−519 μg/g. The laundry water of polyester fabric clothing may be the main source of PET polymer in sewage treatment system. The total amounts of PET MPs discharged with sludge were calculated to be 186−326 t/a in Tianjin and 2370−4150 t/a in China. This accounts for roughly 0.006%− 0.011% of its total annual consumption in China. Assuming that 15% of the sewage sludge is applied as fertilizer, the estimated annual input of PET MPs to farmland is 28−49 t in Tianjin and 356−622 t in China. Besides, concentrations of PET polymers were found as high as 73−944 μg/g in the mineralized garbage soil from a landfill. This indicates an estimated input of ~20000 tons of PET MPs to terrestrial environment, through the utilization of mineralized garbage soil for municipal application. Atmospheric deposition is an important way for microplastics to enter the terrestrial environment, and the spatial and temporal distribution of the input flux is relatively uniform. The utilization of sludge and mineralized garbage soil may lead to the intensive investment of microplastics in farmland and urban environment, and its ecological risk is worthy of attention.