Amplifiers of environmental risk of microplastics in sewage sludge: Thermal drying treatment.

Sewage sludge was already identified as an important source of microplastics (MPs) in the environment. Therefore, investigating the effects of sludge treatment processes on sludge-based MPs is essential for understanding the environmental risks and controlling their release. This study investigated the occurrence characteristics and elucidated the fragmentation mechanism of sludge-based MPs before and after the thermal drying treatment of sludge. The results showed that this treatment increased the abundance of sludge-based MPs by about 10-fold, with enhanced fragmentation and fracture parameters, and increased the abundance of <100 µm MPs to >60 %. Remarkably, both polypropylene-microplastics (PP-MPs) and polyethylene terephthalate-microplastics (PET-MPs) did not show significant chemical aging. The structural analysis showed that the molecular chain disorientation and secondary crystallization of PP-MPs and PET-MPs occurred. These transformations caused the contraction of the polymer molecular chains and the generation of micro-mechanical stresses, leading to the formation of warpage structures and stress cracking on the MPs' surface. These phenomena also contributed to the further fragmentation of the MPs and the development of finer MPs particles. The findings of the present investigations emphasize that the thermal drying of sewage sludge amplifies the environmental risk of sludge-based MPs.

New insights into the decrease in Cd2+ bioavailability in sediments by microplastics: Role of geochemical properties.

Microplastics (MPs) are considered to influence the bioavailability of heavy metals through direct adsorption, but this neglects the distribution of heavy metal chemical speciation indirectly influenced by MPs by altering geochemical properties. The present study aims to explore the link between the fractionation distribution of cadmium (Cd2+) and changes in geochemical properties in sediments induced by polyethylene terephthalate microplastics (PET-MPs). The PET-MPs reduced the acid-soluble fraction of Cd2+ in sediments and increased its organically bound fraction. In addition, the concentration of bioavailable Cd2+ in the sediment decreased by 4.09-25.96 % with increasing PET-MPs doses and aging. Whereas the thermal aging of PET-MPs led to a decrease in the molar ratio of oxygen-containing functional groups and the BET surface area, which is not favorable for improving the adsorption capacity of PET-MPs. On the other hand, the correlation analysis demonstrated the key role of PET-MPs in increasing the content of sediment organic matter and its humification level, which indirectly led to a decrease in Cd2+ bioavailability. The microbial analysis demonstrated that PET-MPs increase the relative abundances of Chloroflexi, hexokinase, and 6-phosphofructose kinase in sediments, thereby increasing the humification level of sediment organic matter. The present study provides a new perspective for understanding the environmental risks of MPs-altered heavy metals.

Microplastic Degradation in Sewage Sludge by Hydrothermal Carbonization: Efficiency and Mechanisms.

Sewage sludge is an important vehicle for the diffusion of microplastics (MPs) into the environment, and thus, efficient removal of MPs from sludge is in urgent need. In this study, hydrothermal carbonization (HTC) is proposed and its potential for the removal of MPs from sewage sludge is assessed. Optical microscopy and micro-FTIR analysis showed that the concentrations of MPs in sewage sludge decreased significantly, exhibiting a 79% reduction with a HTC temperature of 260 °C. The potential decomposition mechanism of condensation polymers and addition polymers were investigated through HTC experiments, using polyethylene terephthalate-microplastics (PET-MPs) and polypropylene-microplastics (PP-MPs). During the HTC process, the disintegration efficiency of PET-MPs was significantly higher than PP-MPs, due to the PET ester bond being easily monomerized by hydrolysis. Furthermore, analyses of physicochemical properties of the residual PP-MPs indicated that exposure to heat cause PP to undergo pyrolysis reaction, resulting in the random rupture of polymer molecular chains. Overall, these results provide the first insight into the critical role of HTC in the removal of MPs from sewage sludge, providing a novel solution for reducing the risk posed by MPs in sewage sludge in the future.

Characterization of heavy metals in textile sludge with hydrothermal carbonization treatment.

Presence of heavy metals in sludge can severely limit land application due to their bioavailability. The current work studied distribution and risk as well as leaching toxicity of heavy metals in textile sludge treated with hydrothermal carbonization (HTC) at different conditions. Treatment temperature and time can significantly affect characteristics of heavy metals in sludge. For the treatment at 220 °C and 3 h, the content of Cu, Cr, Mn, and Zn existed in form F1 + F2 (water soluble and bound to carbonate and Fe-Mn oxides) dropped by 4.7, 7.1, 8.8, and 7.3%, while the content of Cu, Cr, and Mn in form F4 (bound to quartz, feldspars, etc) increased by 12.9, 19.1, and 10.6%, respectively. This effectively lowered the bioavailability and leaching rate of heavy metals in sludge. Addition of weak alkaline Al(OH)3 could efficiently force the transformation of F1 to F4, possibly because of the increased pH value of sludge. HTC processing might be an effective way of fixing heavy metals in textile sludge.

Effect of hydrothermal carbonization on dewatering performance of dyeing sludge.

The effects of hydrothermal carbonization (hydrothermal carbonization temperature, hydrothermal carbonization time, pH) on the dehydration performance of dyeing sludge were studied. The specific resistance, viscosity and floccular morphology of sludge before and after hydrothermal carbonization were analyzed. The physical and chemical properties of the liquid were also determined. The results showed that the dehydration performance of sludge was optimum, when the reaction temperature was 180 °C, the reaction time 4 h and the pH was 5.0. Here the specific resistance to filtration and viscosity were 93.69% and 96.78% lower, respectively, than the control group. When the sludge was hydrothermally carbonized, the sludge flocs were broken due to extreme conditions of high temperature and high pressure, which formed a porous mesh structure with better water permeability. The cohesion of the sludge colloidal structure was reduced, the capillary suction time was reduced by 88.89%, and the sludge dewatering performance was improved. This study shows the feasibility of the use of hydrothermal carbonization in sludge reduction.