Processing Municipal Waste for Phytostabilization of Heavy Metal Contaminated Soils

Background: Organic amendments are often used for the phytostabilization of heavy metal-contaminated soils. This study investigated the suitability of different municipal waste processing methods for phytostabilization. Methods: The two feedstocks, biowaste, and green waste, were tested without pretreatment after composting or after pyrolysis at different temperatures and atmosphere compositions. The suitability of the differently pretreated amendments for phytostabilization was tested on sewage farm soil contaminated with Cd, Cu, Ni, and Zn by measurement of maize growth, mineral content, and mobile heavy metal concentrations in the bulk and rhizosphere soil. Results: In contaminated soils, shoot and root growth increased markedly due to the soil amendments. Shoot concentrations of all four heavy metals were significantly higher in contaminated than non-contaminated soil, whereas Fe concentrations were lower in contaminated soil. The amendments increased shoot Cd concentrations and did not significantly affect the other elements. In contaminated soil, concentrations of heavy metals were generally lower in the rhizosphere than in the bulk soil. The soil amendments markedly lowered soil heavy metal concentrations, whereby the decrease was significantly influenced by feedstock and pretreatment. The differences in the amendments’ ability to reduce the soil mobile heavy metals were primarily, but not exclusively, determined by their effect on soil pH. Conclusion: The feedstock processing conditions significantly influenced the phytostabilization performance of green waste and biowaste. Optimal processing for phytostabilization depends on the heavy metal most important in the contamination of a specific site.