Feedstock and Pyrolysis Effects on the Ecotoxicological Quality of Leachates

Pyrolysis-derived products are increasingly being researched to improve soil and manage organic by-products from agroforestry and livestock operations. However, their chemical behavior and agronomic safety vary with feedstock and pyrolysis conditions. Eucalyptus bark contains phenolic and tannin compounds with allelopathic effects, while sheep wool contains nitrogen and sulfur compounds, which, at low pyrolysis temperatures, may produce ecotoxic substances. In soil solution, these compounds may reduce seed germination and plant growth. Therefore, assessing the ecotoxicology of leachates from pyrolysis-derived products is important to ensure the safe agricultural use of products from a circular economy.

This study assessed the ecotoxicology of leachates from pyrolysis-derived products using bioassays with two sensitive plant species. Pyrolysis-derived products included Eucalyptus bark (EB), sheep wool residues (SWR), and mixed EB/SWR at various ratios (100% EB, 100% SWR, 30/70, 40/60, 60/40, and 70/30). Feedstocks were pyrolyzed at various temperatures (150 to 500 °C) and residence times (10, 20, and 30 minutes). Simulated leachates (DIN extraction) were obtained from the raw eucalyptus biomass (EM) and pyrolysis-derived products and used in a bioassay (filter paper, n=5 replicates/treatment, n=25 seeds/replicate) with Lactuca sativa and Allium cepa..

The bioassays were carried out in a growth chamber maintained at a temperature of 25 ± 1 °C and a photoperiod of 16 h light and 8 h dark. The leachates were characterized by the elemental analyses of the macronutrients (K, Ca, Mg, P, and Na) and the ecotoxicity was determined by the measuring the percentage of germination, the time to 50% germination (T₅₀), the total dry biomass, as well as the total dry plant biomass and roots and shoots elongations. The statistics were conducted using ANOVA and the means were separated using Dunan’s multiple range test for the means which were significantly different at a 5% level (p ≤ 0.05).

The leachates presented differences in their chemical characteristics. Higher pyrolysis temperatures result in leachates with a higher pH and greater concentrations of Ca, Mg, and K. Conversely, leachates from unprocessed eucalyptus biomass and low-temperature pyrolysis products exhibited higher electrical conductivities and concentrations of elements. The final germination percentages were relatively similar across all treatments for both species, indicating a relatively robust germination response to a broad range of chemical characteristics of the leachates The remaining plant parameters were more sensitive to the treatment effects. Low-temperature pyrolysis treatments contributed to the reduction of root elongation and total dry biomass. On the other hand, leachates from pyrolysis-derived products obtained at temperatures greater than or equal to 400 °C positively influenced overall seedling performance, particularly the mixtures of eucalyptus bark and sheep wool. Pyrolysis temperature played a vital role in determining leachate quality and potential ecotoxicity. The products of high-temperature pyrolysis (≥400 °C) produced leachates with lower ecotoxicity and, in some instances, positive effects on early seedling development. Conversely, unprocessed biomass and low-temperature pyrolysis products contained leachate constituents capable of inducing unfavorable physiological responses in sensitive plants.

Keywords: Simulated Leachates, Bioassay; Pyrolysis; Lettuce; Onion; sheep wool wastes