Investigations in Sonoenzymatic Treatment of Industrial Wastewater Using Fe₃O₄@Laccase Nanocomposites

This paper reports studies in the treatment of industrial wastewater (WW) (for chemical oxygen demand (COD), total organic carbon, and toxicity reduction) using an Fe3O4@Laccase nanocomposite in the presence of sonication. The nanocomposite was synthesized by immobilization of laccase on the surface of amino-functionalized magnetic Fe3O4 nanoparticles. Statistical optimization of physical parameters of WW treatment was done using mechanical agitation of 300 rpm. The optimization resulted in 61.7% COD removal at pH = 4.5, temperature = 66.5 °C, and Fe3O4@Laccase loading of 1.46 g/L. The COD removal was enhanced by ∼46% with the application of 35 kHz sonication (at 10% duty cycle) in addition to mechanical agitation. Analysis of the deconvolution of FTIR spectra of the Fe3O4@Laccase nanocomposite revealed a reduction in α-helix and β-turn content and a rise in random coil content of the immobilized laccase after sonication. These changes in the secondary structure of laccase enhanced the activity of the enzyme, which resulted in faster kinetics of COD removal. The toxicity of treated WW was reduced by ∼70%. LC–MS analysis of the original and treated WW helped in the identification of more than 15 contaminants that underwent >75% degradation. The Fe3O4@Laccase biocatalyst also showed excellent recoverability and 72% retention of enzymatic activity (with just a 15% reduction in COD removal) for six consecutive treatment cycles.