Aging and mitigation of microplastics during sewage sludge treatments: An overview

Decreased particle size enhances the aging behavior of microplastics during sewage sludge composting: Physicochemical properties and cadmium loading

Although aerobic composting is capable of aging microplastics (MPs), the influence of size on MPs aging during composting and loading of cadmium (Cd) remains unclear. Therefore, we investigated variations in the physicochemical properties of polyethylene terephthalate microplastics (PET-MPs) with different sizes (1.0 -5.0, 0.2 -1.0, and 0.05 -0.2 mm) during composting and the concentration of Cd accumulated on the surface of different-sized aged PET-MPs. The results indicated that PET-MPs exhibited size-dependent as they aged during composting, with smaller sizes aging faster. After composting, the 0.05 -0.2 mm PET-MPs had the greatest increase in specific surface area (205.5 %), compared with the 1.0 -5.0 mm (18.7 %) and 0.2 -1.0 mm (95.6 %) PET-MPs. The greatest increase in the carbonyl index/oxygen-to-carbon atom ratio was also observed for the 0.05 -0.2 mm PET-MPs, which were 2.25 / 3.27 and 0.02 / 2.11 times higher than those of the 1.0 -5.0 mm and 0.2-1.0 mm PET-MPs, respectively. Similarly, size-dependent accumulation of Cd on the aged PET-MPs was also observed: 0.05-0.2 mm (5.37 mg/kg Cd) > 0.2 -1.0 mm (2.90 mg/kg Cd) > 1.0-5.0 mm (0.78 mg/kg Cd). These findings demonstrate that the aging behavior of polymer is closely related to their size, emphasizing the role of size in the fate and pollutant loading of polymer.

Fate, characteristics, and potential threat of microplastics in sludge under various dewatering treatments

Microplastics (MPs), an emerging environmental pollutant, have been found in wastewater sludge with increasing frequency. Their occurrence, surface properties, and adsorption characteristics may be altered during various sludge dewatering processes. This study explored and compared the performance of four types of sludge dewatering processes (FeCl3 + CaO, Fe2+ + H2O2 + CaO, Fe2+ + peroxymonosulfate (PMS) + CaO, and Fe2+ + CaO2 + CaO) in improving sludge dewaterability, the fate and characteristics of MPs during treatments, and their effect on the adsorption of heavy metals by aged MPs. Results showed that iron-based advanced oxidation processes (Fe-AOPs) indicated superior performance in improving sludge dewaterability compared to conventional FeCl3 + CaO treatment, as evidenced by the water content of sludge cakes being reduced to below 54.0 % (w/t) in Fe-AOPs. Fe2+ + PMS + CaO and FeCl3 + CaO effectively reduced MPs concentrations in both dewatered sludge and filtrate, thereby mitigating potential environmental risks. The potential risk associated with heavy metal adsorption onto treated MPs was greater for Fe2+ + PMS + CaO than for FeCl3 + CaO. In summary, Fe2+ + PMS + CaO offered a feasible method for sludge dewatering and MPs removal, particularly suited to sludge with low heavy metal concentrations. FeCl3 + CaO treatment effectively mitigated co-toxicity between heavy metals and MPs, proving more suitable for sludge with high heavy metal content. This study offers new insights into the selection of appropriate sludge treatments regarding MPs.

Synergistic effect and mechanism analysis of biochar regulator on heavy metal passivation and microplastic degradation in sewage sludge compost

Heavy metal passivation and microplastic in sludge threaten environment and cause human health risks, and thus necessary to find effective remediation strategies. To solve this issue, the synergistic effect of biochar on pollutants remediation during sludge composting is still not well explored. In this study, different doses of cotton stalk biochar (0 %, 2.5 %, 5 %, 7.5 %, 10 % CSB; and labeled T1-T5) were applied to sludge composting to investigate the synergistic effect of CSB on pollutants (copper, zinc, and microplastics) and explore the influence mechanism. Results showed that CSB could effectively increase the yield of humic acid (15.85-22.08 g/kg) and reduce the content of extractable copper (59.37-81.10 %) and extractable zinc (27.07-51.45 %). Among them, T5 was superior in the passivation of heavy metals. In addition, CSB optimized the environmental factors to increase the degradation rate of microplastics by 16.23∼57.86 %, exhibiting dose-dependent improvement. The microbiological analysis showed that CSB could decrease the relative abundance of Firmicutes (29.16-59.20 %) and increase the relative abundance of Proteobacteria (14.10-33.48 %), Actinobacteriota (2.55-46.25 %) and Ascomycota (11.36-65.71 %) in the high temperature stage of compost. For correlation analysis, T4 and T5 could better enhance the positive correlation between environmental factors and microorganisms. In summary, T5 could minimize the content of heavy metals and microplastics in compost products, and it had the highest level of application value. Hence, this study is of great significance for reducing the pollutant risk of sludge composting.

Unveiling the impacts of microplastic pollution on soil health: A comprehensive review

Soil contamination by microplastics (MPs) has emerged as a significant global concern. Although traditionally associated with crop production, contemporary understanding of soil health has expanded to include a broader range of factors, including animal safety, microbial diversity, ecological functions, and human health protection. This paradigm shifts underscores the imperative need for a comprehensive assessment of the effects of MPs on soil health. Through an investigation of various soil health indicators, this review endeavors to fill existing knowledge gaps, drawing insights from recent studies conducted between 2021 and 2024, to elucidate how MPs may disrupt soil ecosystems and compromise their crucial functions. This review provides a thorough analysis of the processes leading to MP contamination in soil environments and highlights film residues as major contributors to agricultural soils. MPs entering the soil detrimentally affect crop productivity by hindering growth and other physiological processes. Moreover, MPs hinder the survival, growth, and reproductive rates of the soil fauna, posing potential health risks. Additionally, a systematic evaluation of the impact of MPs on soil microbes and nutrient cycling highlights the diverse repercussions of MP contamination. Moreover, within soil-plant systems, MPs interact with other pollutants, resulting in combined pollution. For example, MPs contain oxygen-containing functional groups on their surfaces that form high-affinity hydrogen bonds with other pollutants, leading to prolonged persistence in the soil environment thereby increasing the risk to soil health. In conclusion, we succinctly summarize the current research challenges related to the mediating effects of MPs on soil health and suggest promising directions for future studies. Addressing these challenges and adopting interdisciplinary approaches will advance our understanding of the intricate interplay between MPs and soil ecosystems, thereby providing evidence-based strategies for mitigating their adverse effects.

Leaching behavior of microplastics during sludge mechanical dewatering and its effect on activated sludge

Dewatering is an indispensable link in sludge treatment, but its effect on the microplastics (MPs) remains inadequately understood. This study investigated the physicochemical changes and leaching behavior of MPs during the mechanical dewatering of sludge, as well as the impact of MP leachates on activated sludge (AS). After sludge dewatering, MPs exhibit rougher surfaces, decreased sizes and altered functional groups due to the addition of dewatering agents and the application of mechanical force. Meanwhile, plastic additives, depolymerization products, and derivatives of their interactions are leached from MPs during sludge dewatering process. The concentration of MP-based leachates in sludge is 2-25 times higher than that in water. The enhancement of pH and ionic strength caused by dewatering agents induces the release of MP leachates enriched with protein-like, fulvic acid-like, and soluble microbial by-product-like substances. The reflux of MP leachates in sludge dewatering liquor to the wastewater treatment system negatively impacts AS, leading to a decrease in COD removal rate and inhibition of the extracellular polymeric substances secretion. More importantly, MP leachates cause oxidative stress to microbial cells and alter the microbial community structure of AS at the phylum and genus levels. These findings confirm that MPs undergo aging and leaching during sludge dewatering process, and MP leachates may negatively affect the wastewater treatment system.

A comprehensive risk assessment of microplastics in soil, water, and atmosphere: Implications for human health and environmental safety

Microplastics (MPs) are pervasive across ecosystems, likely posing significant environmental and health risks based on more and more evidence. In this study, we searched through the Web of Science Core Collection and obtained 1039 papers for visualization and analysis. In order to discuss the chemical composition, migration, transformation and potential risk of MPs, 135 sets of relevant data in soil, water, and atmosphere were collected in China as a typical region, which is a hotspot region for investigation of MPs. The results showed that the primary polymer categories of MPs in the environment to be polypropylene, polyethylene, and polystyrene. The soil contains a significant quantity of MPs, averaging at 12,107.42 items·kgdw-1, while water contains averaging at 97,271.18 items m-3. The total pollution load indexes for all three environments are at risk level I. Based on current risk assessment methods, the potential ecological risk of MPs is low. However, based on the polymer components, migration and transformation patterns, and especially the complexes with other pollutants, it indicates an increasing indirect risk. Interactions with some other pollutants are likely amplify the ecological and health risks associated with MPs. Aggregative results showed that the present risk assessment models could not assess the risks of MPs well. Thus, we suggested develop a risk assessment methodology for MPs based on relevant research progress. Some factors such as the size and form of MPs, sources and distribution, bioaccumulation, social acceptance and economic costs could be considered adding in the present risk assessment models. Finally, promotion of development and application of green chemically synthesized bioplastics such as using synthetic biology to help degrade plastics would be an alternative and sustainable option to relieve the adverse environmental and health concerns of MPs.