Pyrolysis Treatment Enables Safe Application of Sewage Sludge in Horticulture: Tracking Potentially Toxic Elements Through the Biochar-Soil–Plant System in Tomato

Biochar production via pyrolysis is a promising option for treating sewage sludge (SS). Here we investigated the uptake of potentially toxic elements (PTEs) from SS and its biochar (450 °C) into cherry tomato (Lycopersicon esculentum L.) plants and their fruits in pot experiments (2, 5 and 10%), to assess the health risks associated with fruit consumption. We further studied the ability of the PTE-rich amendments to immobilize PTEs already present in soil through spiking of soil (pH 7.4) with Cd, Cu and Zn. Our findings suggest that tomato consumption is not a human health risk when grown in soil amended with SS and its biochar; in most treatments PTE uptake into fruits was similar to the control. Vegetative plant growth was highest in SS-amended soils. Yet 5 and 10% biochar-amended soils significantly increased tomato fruit yield relative to untreated SS application (400–500% higher than 10% SS-amended soil) and significantly decreased fruit uptake of Cu and Zn. Importantly, biochar also significantly immobilized PTEs present in contaminated (spiked) soil. As expected, available (DTPA-extractable) PTE contents were typically lower in biochar-amended soil and sequential extraction indicated that the PTEs were predominantly bound in more recalcitrant soil fractions (organic matter associated and Fe–Mn oxides-bound). We conclude that pyrolysis can be a key technology for treatment of PTE-rich SS and subsequent use of the produced biochar in urban, and potentially even PTE-contaminated, horticultural sites.