Occurrence characteristics of microplastics and heavy metals in pyrolysis incineration residues of small towns in Southern China

Closed oxygen-limited pyrolysis is becoming a widely used waste treatment method in small towns. In the present study, we selected some typical municipal solid waste incineration plants in Jiangxi Province in Southern China as the research areas, and collected the ash samples which from the stacking ground after pyrolysis of refuse and its surrounding environment soil with ash slag and surface soil as the research materials. The flotation separation method, scanning electron microscopy method, ICP-MS and BCR three-step continuous extraction methods were used to study the abundances of microplastics and the occurrence characteristics of different forms of heavy metals such as Cu, Cd, Pb, Zn and Cr. The results showed that there were some differences in the abundance of microplastics in samples from different sources. The abundance range of microplastics in ash from different garbage factories was from 131.00 n·kg-1 dw to 176.00 n·kg-1 dw (the amount of microplastics in dry soil samples per kg), which was significantly higher than that in the soil with ash slag and surface soil in the surrounding environment. The morphological types of microplastics were mainly debris (38.5%) followed by film (17.6%), fiber (26.7%) and foam (17.2%). The main colors of microplastics were black (29.4%), white (12.2%), transparent (20.8%), blue (12.4%) and red (25.3%). The proportions of the microplastics abundances with different particle sizes were > 5 mm (11%), 2.5—5 mm (13%), 1—2.5 mm (19%), 0.5—1 mm (24.2%) and Cd (73.48%) > Zn (55.69%) > Cu (38.8%) > Cr (38.02%). The results of correlation analysis showed that the abundance of microplastics and the occurrence of heavy metals in pyrolysis residue of the garbage were corelated and the correlation between the content of heavy metals and microplastics with different characteristics was different. There was a significant correlation (P<0.05) between black microplastics and most heavy metal elements, and also between microplastics with particle size less than 0.5 mm and most heavy metal elements.