The influence of pH on resin acid solubility related to biodegradation kinetics of resin acid in pulp mill effluent

Resin acids are hydrophobic diterpene carboxylic organic acids that change in solubility with dissociation as a function of pH. Due to their importance with respect to pulp mill effluent toxicity and contradictory literature data surrounding the influence of pH on biodegradation, batch growth experiments were performed to consider whether changes in resin acid solubility, controlled by pH, would alter the kinetics of their biodegradation. Preliminary findings suggest that while there are appreciable changes in solubility in the pH range of 6 to 8, degradation kinetics with well acclimated mixed cultures are reasonably constant with a local maximum in the neighbourhood of pH 8. Above pH 8, a decrease in microbial yield and growth rate was observed, and is thought to be due to stress associated with elevated pH. Below pH 7, a significant residual resin acid concentration remained at the end of the maximum exponential growth phase. This residuum was essentially colloidal and significant with respect to resin acid aquatic toxicity. The present study also found microbial membrane fatty acid to be a promising measure for changes in total biomass.