Fabrication Of Polymer-Laccase Composites For Water Decolourisation From Dye Contaminants

The elimination of dyes from water still remains a challenging problem.Laccase is a unique,environmentally benign and efficient biocatalyst that can degrade pollutants.However,the application of laccase is greatly limited by its vulnerability to environmental changes and poor reusability.To address the bottlenecks of laccase-based decolorization process in water treatment,we fabricated a new biocatalyst(N-PS-Lac)through the adsorption of Trametes versicolor laccase onto three porous polystyrene beads carrying quaternary ammonium groups.Transmission electron micrographs and Fourier transform infrared spectra of the resultant N-PS-Lac proved that nanosized laccase clusters were embedded into the inner nano-pores/channels of the carriers(N-PS).The developed N-PS-Lac overcame the bottlenecks of traditional immobilized laccase,i.e.,considerable laccase leaching during their period of use,based on simple adsorption process.Negligible laccase was leached out from the host polymer N-PS in solution of pH 3-7 and NaCl concentration up to 0.5 M,which might be attributed to the electrostatic attraction and the possible twining between long-chain laccase and cross-linked host resin.After 2-year storage,the residual activity of N-PS-Lac increased from 39.3%to 101.7%when the ammonium groups increased from 2.09 to 3.24 mmol·g-1,while that for the free laccase was only 14.6%.Also,N-PS-Lac exhibited improved durability against the variation in pH or temperature as well as a thermal treatment at 60?.Gaussian curve fitting of FT-IR spectra indicated that the conformation of laccase in N-PS-Lac was rigidified,possibly because of the geometric restriction imposed by the host and the electrostatic attraction between the host and the laccase.Continuous seven-cycle batch decoloration of malachite green demonstrated that decolorization efficiency of N-PS-Lac kept constant for more than 320-h operation.Usually,synthetic mediator was essential for the degradation of dye by laccase.To avoid the potential secondary pollution caused by the introduced synthetic mediator,a two-step method,namely,adsorption of an azo dye AO7 followed by ectopic laccase degradation,was proposed to reuse the mediator HBT.During seven cyclic runs,the decolorization efficiency of AO7 by N-PS-Lac kept constant.A phytotoxicity assay confirmed that the AO7 solution was almost detoxified completely after being decolorized using the two-step method.Despite the remarkable improvement in the stability and reusability of laccase by immbilization,the immobilized laccase suffered from huge activity loss because of the significantly increased diffusion limitations between the substrates and the active sites of laccase.To address this problem,we fabricated a novel biocatalyst(LacPG)by coating polyethylenimine(PEI)onto the native laccase(Lac)enzyme followed by crosslinking by glutaraldehyde.The stability of enzymatic activity of the resulting LacPG against pH variation,thermal treatment,and long-term storage was notably enhanced.Compared to free Lac,LacPG exhibited significantly higher decolourisation efficiency in the degradation of a representative azo dye AO7,which was attributed to the electrostatic attraction between the PEI coating and AO7.LacPG was separated from AO7 solution using an ultrafiltration unit.The increased size and modified surface chemistry of LacPG(relative to Lac)facilitated ultrafiltration separation and reduced membrane fouling.Collectively,LacPG exhibited enhanced stability,high catalytic activity,and favourable attributes for membrane separation.In addition,LacPG was readily reused in the enzymatic membrane reactor at a high efficiency for decolourising water containing AO7.The developed strategy appeared to be very promising for enhancing the applicability of laccase in practical water treatment.