Tyrosinase reaction/chitosan adsorption for removing phenols from wastewater

A two‐step approach for removing phenols from aqueous solutions was investigated. In the first step, weakly adsorbable phenols are converted to quinones by the enzyme mushroom tyrosinase. The tyrosinase‐generated quinones are then chemisorbed onto chitosan, a readily available waste product of the shellfish industry. In the absence of enzyme, quinone was observed to be rapidly adsorbed onto chitosan. Also, the enthalpy for quinone adsorption onto chitosan was observed to be −24.7 kcal/mol, which compares to enthalpies of ‐7 kcal/mol for adsorption of phenols and quinone onto activated charcoal. With the monophenol reactant cresol, the tyrosinase enzyme was observed to be somewhat stabilized in the presence of chitosan. This stabilization of tyrosinase is presumably due to the rapid adsorption of the reactive quinones onto chitosan. In contrast, tyrosinase was not stabilized by chitosan when the o‐diphenol catechol was the reactant. The ability of chitosan to stabilize tyrosinase for monophenols but not for o‐diphenols is discussed in terms of the relative rates of phenol oxidation by tyrosinase and quinone chemisorption onto chitosan. When mushroom tyrosinase and chitosan were added simultaneously to dilute, phenol‐containing solutions, a nearly complete removal of UV‐absorbing material was observed. This observation demonstrates the feasibility of removing phenols from dilute solutions using the tyrosinase reaction/chitosan adsorption approach.