13-21: On the nature of hydroxide ion uptake during alkali processing of wheat straw

The present study is a contribution toward the characterization of the nature of hydroxide ion neutralization during alkali processing of herbaceous biomass. Alkali uptake experiments were done using the pH stat method under relatively mild processing conditions (pH 11.0, 30^oC). Analyses were based on 96-hour time courses for hydroxide ion neutralization/acid generation in a 2% (w/w) wheat straw suspension. Alkali uptake was modeled as occurring in three phases: (1) an initial pH adjustment phase where alkali uptake appears dominated by the ionization of solvent accessible Brønstad acids, (2) an early phase of pH maintenance where rates of alkali uptake are primarily attributable to solvent accessible ester hydrolysis and initiation of alkali-induced swelling, and (3) a late phase of pH maintenance where rates of alkali uptake appear largely dependent on rates of alkali penetration and the generation of acidic degradation products. Readily solubilized Brønstad acids accounted for ~ 60% of the buffering capacity associated with the pH adjustment phase. Esters of acetic, ferulic acid and p-coumaric acid together accounted for ~ 28% of the observed alkali neutralization during the pH maintenance phase with acetyl esters being predominant.  A minimum of 28% of alkali uptake during the pH maintenance phase could be attributed to low molecular weight acid-degradation products. The combined results provide a quantitative example of hydroxide loads required for prolonged processing of a prototypical herbaceous feedstock.