Co-pyrolysis of sewage sludge and metal-free/metal-loaded polyvinyl chloride (PVC) microplastics improved biochar properties and reduced environmental risk of heavy metals

Co-pyrolysis of sewage sludge and plastics have been utilized for producing biochars as a strategy to fix plastic pollution. However, comparative studies on the characteristics and environmental risk of heavy metals in biochars obtained by the co-pyrolysis of sludge and microplastic with/without metal additives are seldom. Here we demonstrated the effects of simulated co-pyrolysis (at 400 degrees C) of sewage sludge and metal-free or metal-loaded polyvinyl chloride (PVC) microplastics at different mass ratios (1:0, 19:1, 3:1, 1:3, sewage sludge: PVC (w/w)) respectively. Results revealed that co-pyrolysis of metal-loaded PVC and sewage sludge resulted in higher electrical conductivity, ash content, and an acidic pH of biochars as compared to the co-pyrolysis of metal-free PVC and sewage sludge. Addition of metal-loaded PVC increased total concentrations of calcium (Ca), magnesium (Mg), cadmium (Cd), and lead (Pb) in biochars, but reduced the bioavailability of Cd, chromium (Cr), nickel (Ni), and zinc (Zn) in biochars. Analysis of chemical speciation showed that heavy metals (except Pb) in biochars derived from co-pyrolysis of sewage sludge and metal-loaded PVC had higher percentage of more stable fraction (residual fraction) and lower potential ecological risk index (RI) value. S1 AP3 (sludge: metal-loaded PVC = 1:3) biochar had the lowest environmental risk based on RI value (14.41). To sum up the present study suggests that the addition of metal-loaded PVC microplastic in sewage sludge had a positive impact on the immobilization of heavy metals during co-pyrolysis process.

Automated summary

The co-pyrolysis of sewage sludge and plastics, particularly polyvinyl chloride (PVC) microplastics with or without metal additives, has been investigated for the production of biochars as a solution to plastic pollution. The study compared the characteristics and environmental risks associated with heavy metals in biochars obtained from the co-pyrolysis of sludge and microplastic. The results showed that the addition of metal-loaded PVC increased the total concentrations of certain heavy metals in the biochars, but reduced their bioavailability. Furthermore, the heavy metals in biochars derived from the co-pyrolysis of sludge and metal-loaded PVC had a higher percentage of stable fraction and lower potential ecological risk index value. Overall, the study suggests that the co-pyrolysis of metal-loaded PVC microplastic and sewage sludge positively affects the immobilization of heavy metals during the process.