Research | Is burning plastic the end of the matter? Microplastics laugh: You are too naive!

Microplastics (maximum diameter ≤ 5mm) are everywhere, and their potential toxicity is attracting more and more attention worldwide. There are currently two main approaches to reducing microplastic pollution: one is to reduce plastic consumption and plastic waste generation at the source, and the other is to strengthen the recycling and treatment of plastic waste. However, both approaches are fraught with difficulties. As a disposal method, waste incineration and energy recovery currently account for a large proportion of solid waste treatment systems in both developed and developing countries.

It is generally believed that incineration can permanently eliminate plastic waste, eventually converting polymers into carbon dioxide and inorganic fragments. However, the latest research shows that unburned materials still exist in the slag at the end of incineration, and may flow back into the environment through reuse (making bricks, paving roads, etc.) or dumping.

Garbage incineration map丨Pixabay

In order to verify whether incineration can completely eliminate microplastics and evaluate the amount of microplastics transported into the environment by slag, in the study "Is incineration the terminator of plastics and microplastics?" published in the Journal of Hazardous Materials in 2021, several Chinese scholars extracted and identified microplastics in waste incineration slag for the first time.

Journal of Hazardous Materials Journal image | sciencedirect

The study collected slag from 16 waste incineration plants and one slag treatment center in eight different cities in China. Of the 16 waste incineration plants, 12 used grate furnaces and four used fluidized beds. Of the 17 sampling points, seven served areas where source waste sorting had been implemented, while the other 10 had not yet been implemented. The waste entering the incineration plants was mainly domestic waste, with some industrial waste, commercial waste, construction and demolition waste such as packaging, plastic pipes, waterproof materials and wallpaper mixed with domestic waste. The daily processing capacity of the grate furnace was 330 to 867 tons/day, and that of the fluidized bed was 350 to 400 tons/day. The slag generation rate of the grate furnace was 15 to 25%, which was higher than that of the fluidized bed furnace (8 to 10%). The incineration plants selected for the study were newly built or upgraded in the past decade, with advanced technology, and can be used as representatives of typical incineration plants in China.

During the stable operation of the incinerator, a total of 31 piles of slag were sampled, and the mass of each sample was equal. Through a series of processes (including drying, screening, NaCl solution treatment, ultrasound, deionized water rinsing, etc.), particles suspected of microplastics were extracted from the slag samples. All particles suspected of microplastics were photographed, recorded, and identified by μ-FTIR technology. Among them, particles with uniform texture, no cell structure, no cracking when pressure was applied with tweezers, and obvious artificial coloring characteristics were further identified as microplastics. The study also conducted a combustion loss test on the slag samples. After the sample was removed of metal and ground, it was heated for combustion loss. The final weight loss of the cooled residue reflected the content of unburned substances. The conditions were strictly controlled throughout the experiment to avoid contamination from external plastics.

Incineration slag image | Ad Rem

In the slag samples, the study found a total of 892 suspected microplastic particles, of which 276 were identified as microplastics. The average abundance of microplastics in the slag was 116±172 particles/kg. The abundance of microplastics at the 13 grate furnace sites was 125±180 n/kg. Microplastics were identified in 2 of the 4 fluidized bed furnace sites, with an abundance of 84±167 n/kg. The abundance of microplastics in incinerators in areas where waste source sorting was implemented (33±79 particles/kg) was lower than that in areas where it was not implemented (228±264 particles/kg), and the difference was statistically significant (p

A total of 9 types of plastics were found in the slag: polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), polyester fiber (PES), polyamide (PA), polystyrene (PS), polyvinyl chloride (PVC), acrylonitrile butadiene styrene (ABS), polyurethane (PU) and epoxy resin. All 12 samples from the grate furnace contained PE or PP, proving that PE and PP are the two most widely produced and used polymers. Packaging is the main source of plastic waste. In incineration plants with high construction waste input, PS accounts for a high proportion because it is mainly used for packaging and construction materials. In the grate furnace samples without source sorting, the main categories of microplastics are PP and PS. For the grate furnace ash samples with source sorting, the highest proportion of microplastics is PE, followed by PET and PES; the main source of PET and PES microplastics is discarded textiles (fiber products).

Microplastics are divided into four categories according to their shape: granules (43%), fragments (34%), films (18%), and fibers. Granules refer to irregular, thick plastics with relatively uniform length, width, and thickness; the thickness of fragments is significantly lower than the other two dimensions; films are flat objects, translucent, and more flexible than fragments. Granular microplastics are mainly composed of PP, while fragment microplastics are mainly composed of PE and PS, which may be caused by the large-scale use of packaging. PET and PES account for 70% of the world's fiber plastics, which makes them the main microplastic fibers in slag. The granular and fragmented microplastics in the slag are irregular in shape and have rough edges, which also means that the plastic fragments remaining in the slag will gradually break into microplastics.

Microplastics image provided by Cole Brookson of Rochman Laboratory

The gas distribution in the fluidized bed enables better mixing of waste with bed materials (such as silica sand, limestone, alumina or ceramic materials), which improves heat transfer efficiency and contact probability. Therefore, the heat loss of fluidized bed furnace slag is lower than that of grate furnace slag, indicating that fluidized bed furnaces are more efficient in burning waste. In addition, the density of fluidized bed bed materials is higher than that of most commercial plastics. Plastic waste with a lower density than the bed material circulates and burns in the furnace, resulting in less microplastic content in fluidized bed furnace slag. Therefore, the abundance of microplastics in fluidized bed slag is lower than that in grate furnaces.

In slag, granular microplastics account for the majority. Previous studies have reported that the main shapes of microplastics in leachate and compost are flakes and fragments, respectively, suggesting that microplastics in slag may be significantly different from those in landfills and composts, possibly because flat flakes and fragments are more easily destroyed by combustion than particles during incineration. In addition, plastic items of shapes other than flakes and fragments will partially melt into lumps, hindering internal heat transfer, which may cause them to remain in the ash. Small particles have a larger surface area, better contact with the air, and more efficient heat transfer than large particles. Therefore, the abundance of microplastics in slag increases with decreasing particle size, which is consistent with previous summaries of landfills, composts, and natural solid matrices.

This study is the first to report the amount of microplastics extracted from the slag of municipal solid waste incineration plants. Microplastics extracted from the slag of 12 grate furnaces, one slag treatment center, and four fluidized bed incinerators in China were identified. The results provide experimental evidence that incineration cannot end plastic waste and slag is still a potential source of microplastics. The main form of microplastics in slag is particles, and the main type is PP. The abundance of microplastics increases with the decrease of particle size, indicating that plastic fragments may be crushed into microplastics throughout the incineration process. The abundance of microplastics in sites that perform waste source sorting is significantly lower than that in sites that do not perform waste sorting, indicating that waste source sorting has a positive impact on microplastic management. The abundance of microplastics is negatively correlated with per capita GDP, indicating that the abundance of microplastics is closely related to the composition of waste. It is estimated that each ton of waste put into the incinerator will produce 3.6 to 102,000 microplastic particles, so it is necessary to revise the emission of microplastics in waste management. Our findings provide experimental evidence to prove that incineration cannot end plastic waste and prove that slag is a potential source of microplastics released into the environment.

References

1.Yang, Z., et al. "Is Incineration the Terminator of Plastics and Microplastics?." Journal of Hazardous Materials (2020):123429.

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