Enzyme-enhanced adsorption of laccase immobilized graphene oxide for micro-pollutant removal

Abstract

Laccase immobilization has been proven to improve the adsorption efficiency for promoting the micro-pollutant removal performance. However, the enzyme-enhanced adsorption mechanism remains poorly investigated. The main objective of this work is to examine the water purification performance of laccase immobilized graphene oxide (Lac-GO), to reveal the enzyme-enhanced adsorption mechanism and to explain the synergy of adsorption and enzymatic degradation, from the views of theory and experiment. Above all, by the appropriate laccase immobilization, the enzyme-enhanced adsorption process formed, and enzymatic degradation was introduced by the change of micromorphology. Then, the laccase immobilization did not affect the GO adsorption behavior, which was dominated by monolayer and homogeneous adsorption, based on the analysis of adsorption thermodynamics and adsorption kinetics theories. Nevertheless, the enzyme-enhanced adsorption showed higher adsorption capacity and greater adsorption rate, because the enzyme is able to degrade the micro-pollutant adsorbed on GO surface, releasing the occupied active sites and delaying the adsorption saturation. Moreover, the calculation of particle diffusion dynamic model clarified that the micro-pollutant diffusion process accelerated after laccase immobilization, since the enzymatic degradation eliminated the diffusion resistance. Afterwards, the GO adsorption and enzymatic degradation exhibited the clear synergy mechanism for micro-pollutant removal, because of the enzymatic degradation enhancement, diffusion resistance elimination and adsorption saturation reduction. In addition, at the optimal operating conditions (laccase concentration of 333.3 mg L−1, Malachite Green concentration of 100 mg L−1, adsorbent concentration of 200 mg L−1, rotation rate of 150 rpm, and temperature of 40 °C), the enzyme-enhanced adsorption process displayed a significant equilibrium adsorption capacity and high micro-pollutant diffusion rate. In final, this study provides an in-depth understanding to improve the adsorption processes for water purification by the unique advantages of enzyme-enhanced adsorption process.