Leaching Characteristics of Organic and Inorganic Matter from Biomass by Water: Differences between Batch and Semi-continuous Operations

Over 30% and ∼2% (on a carbon basis) organic matter can be leached from mallee leaf and wood by water, respectively, producing acidic leachates containing organic acids. As a result, there are significant differences in the leaching characteristics of both organic and inorganic species in biomass between batch and semi-continuous leaching operations. Under conventional batch leaching, the acidic leachate continuously contacts the biomass for a prolonged period, resulting in the leaching of at least some water-insoluble inorganic species (e.g., organically bound) from biomass. Therefore, the batch leaching method clearly overestimates the amount of water-soluble inorganic species in biomass and exhibits two-step leaching kinetics, i.e. a rapid leaching step for an initial short period followed by a slow leaching step for a relatively long period. This study further develops a semi-continuous leaching method to address this issue via minimizing the contact between the leachate and the biomass sample. The semi-continuous leaching quantifies the true water-soluble inorganic species in biomass. Its leaching kinetics include only the first rapid leaching step, with the disappearance of the second slow-leaching step due to the absence of the interaction between acidic leachate and biomass. The results suggest that in the sequential extraction scheme used in chemical fractionation, semi-continuous (instead of batch) water leaching method should be used for quantifying water-soluble inorganic species in biomass. Attention should also be paid to the potentially substantial loss of fuel materials when utilizing water leaching as a pretreatment method to remove inherent inorganic species in biomass for fuel quality improvement. As result of overestimating water-soluble inorganic species and loss of organic matter, care must be taken when using water batch washing as a method for studying the effect of the inherent water-soluble inorganic species on thermochemical reactions of biomass.