I would like to optimize the immobilization of laccase on rice husk biochar. Prior the assay, I mix the rice husk biochar, glutaraldehyde and laccase in citrate phosphate buffer (pH 4.0) for 4 hours. The ratio of biochar to volume of solution is 1:10. Laccase concentration used is 8 - 32mg/mL while the glutaraldehyde concentration is 5 - 20%(v/v). After the immobilization, I used ABTS solution to measure the enzymatic activity (420nm). The ABTS solution turns green once added to the laccase solution, but it also decolorizes to yellow after 3 minutes.

Any ideas on why the decolorization occurs?