Removal of dyes by silica nanoparticles with immobilized laccase

Abstract

Dyes give a big impact on ecosystem; thus several methods have been developed for dye removal processes. This study investigated the dye removal by the combination of adsorption and biodegradation process. Silica nanoparticles (SN), modified silica nanoparticle (MSN), silica nanoparticle with laccase (SNL), and modified silica nanoparticle with laccase (MSNL) were synthesized. All samples were characterized with scanning electron microscope (SEM), nitrogen adsorption-desorption (NAD), Fourier transform infrared (FTIR), and energy dispersive X-ray (EDX). It was found that the introduction of cationic surfactant and laccase did not change the morphology but it affected the surface area, pore characteristics and chemical properties of the SN. The dye adsorption performance using SN and MSN was evaluated in batch adsorption experiment at various experimental conditions. The adsorption of methylene blue (MB) by SN showed a good adsorption performance (qe = 0.2291 mmol/g) as compared to the MSN (qe = 0.0430 mmol/g). In contrast, for methyl orange (MO), the MSN showed a very good adsorption performance (qe = 0.1849 mmol/g), while no adsorption was observed for the SN. The pH values did not give any significant effect on the dye adsorption and the cetyltrimethylammonium bromide (CTAB) concentration of 1mM was found to be the maximum value for SN modification. The adsorption equilibrium and kinetic data for both MO and MB fit the Temkin and Langmuir isotherm models well, respectively while the kinetic adsorption data follows the Elovich kinetic model with film diffusion found to be the rate-limiting step. The dye adsorption process was found to be exothermic, spontaneous and physisorption. The regeneration shows that SN and MSN are reusable for multiple cycles. For the laccase immobilization, MSN performed higher laccase adsorption (1.6696 µmol/g) as compared to SN (1.1047 µmol/g). The removal of dye by SNL and MSNL was analyzed in term of adsorption and degradation of both MO and MB dyes. Results show that the removal of MB by SNL was higher (qe = 0.2573 mmol/g) than SN (qe = 0.2291 mmol/g). Meanwhile, removal of MO by MSNL was higher (qe = 0.2454 mmol/g) as compared to MSN (qe = 0.1849 mmol/g). These results demonstrated that the surface modification of SN by cationic surfactant gave higher catalytic activity of laccase, hence giving higher removal performance of dye. The adsorption isotherm data analysis shows that the SNL and MSNL are well fitted to the Langmuir and Temkin model respectively. The Elovich kinetic model is however the best model to describe the dye adsorption kinetic data of both SNL and MSNL. The dye removal by degradation was analyzed using Michaelis-Menten enzymatic reaction equation which found that higher specific activity was observed for MSNL (88.5724 U/g) as compared to SNL (22.6360 U/g). This resulted in higher initial enzymatic reaction velocity, Vmax (58.0 µM/min) for MO (MSNL) and lower for MB (SNL) (58.0 µM/min).