Pyrolysis of Spent Biomass from Mallee Leaf Steam Distillation: Biochar Properties and Recycling of Inherent Inorganic Nutrients

Abstract

A set of 24 biochars were produced from the slow and fast pyrolysis (at 400-700 °C) of a mallee leaf and two spent leaves collected after distilling the leaf in steam for 30 and 60 min. Under the same experimental conditions, the pyrolysis of the raw and spent leaves produced biochars of similar proximate and elemental analyses at similar char yields. However, the biochars from the raw leaf generally have higher contents of Na, K, Mg, Ca, and Cl than those from the spent leaves. The biochars were then leached via water to evaluate the recycling of inherent inorganic nutrients (i.e., Na, K, Mg, Ca, and Cl) and the leaching of organic carbon (C) in these biochars. On the basis of those in the mallee leaf, the quantities of Na, K, and Cl leached out by water from the biochars of the mallee leaf are considerably higher than those from the biochars of the spent leaves.However, the leaching of Mg in the slow-pyrolysis biochars follows an opposite trend. At pyrolysis temperatures higher than 400 °C, there are considerable decreases in the quantities of Na and Mg leached out from all of the biochars as well as in those of K and C from the fast-pyrolysis biochars. The results suggest that fast pyrolysis (at 500 °C) of the spent leaf collected after 60 min of steam distillation appears to produce favorable biochar for recycling inherent inorganic nutrients to soil.