Removal and Recycling of Inherent Inorganic Nutrient Species in Mallee Biomass and Derived Biochars by Water Leaching

Abstract

Biomass growth extracts inorganic nutrients from soil as inherent nutrient species in the biomass. Unless at least some of these inherent inorganic nutrients are eventually recycled to the soil, biomass utilization during its full life cycle may not be sustainable. This study reports the removal and recycling of inherent inorganic species in mallee biomass and its derived biochars by water leaching. A series of biochars were produced from the pyrolysis of various mallee components including wood, leaf, and bark under various conditions. An increasing pyrolysis temperature leads to increases in biochar C content and aromaticity and decreases in biochar H and O contents as well as oxygen functional groups. Most of the alkali and alkaline earth metallic species (Na, K, Mg, and Ca) and P are retained in the biochars, while substantial amounts of S, N, and Cl are released during pyrolysis. For biomass samples, almost all of K, Na, and Cl and large proportions of S, P, and Mg can be recycled by water leaching, but limited Ca and little N can be recycled. However, nutrients recycling via water leaching of biochars results in substantial reductions in the overall recycling of most nutrient species originally present in biomass, due to either substantial release of nutrients (Cl, S, and N) during pyrolysis or the forms of nutrient species (Na, K, Mg, P) in biochars becoming increasingly water insoluble. The results also suggest that heat treatment may be employed to tune the biochars to facilitate the recycling of Ca which is the dominant inherent inorganic nutrient species of the samples investigated. It is noted that water leaching can also remove small amounts of organic matter, generally <2% (quantified as total organic carbon) of the total carbon in these biochars.