Laccase-zein interactions at the air-water interface: Reactors on an air bubble and naphthalene removal from water

Abstract

Zein and laccase were used to remove naphthalene from water using two approaches. The first approach sorbed naphthalene onto laccase-containing solid zein sorbents coagulated with either CaCl2 at pH = 13 or with Fe2Cl3 at pH = 10. These sorbents were never used before for this purpose. Fluorescence spectroscopy showed that laccase degraded naphthalene in water at neutral pH. Laccase activity was pH-dependent: laccase retained its activity after exposure to pH values as high as 10, and had maximum activity at pH = 5.5. Laccase immobilized in zein sorbents coagulated with Fe2Cl3 at pH = 10 had a relative activity of approximately 8%, whereas activity significantly decreased in zein sorbents coagulated with CaCl2 at pH = 13. Attenuated total reflection Fourier-Transform Infra-Red (ATR-FTIR) spectroscopy showed naphtalene sorption onto zein sorbents through hydrophobic interactions and CH/π bonding between naphthalene (the π base) and the aliphatic groups (hydrogen donors) of non-polar residues of zein. Gas Chromatography Mass Spectroscopy (GCMS) demonstrated naphthalene removal from water. The sorption capacity of zein sorbents was ≈ 14 mg/g at 21⁰C, and followed first-order kinetics with a rate constant of 0.0066 min−1. The second approach used zein and laccase to stabilize air bubbles in water, for air sparging applications. Air sparging injects air in contaminated aquifers to strip volatile contaminants from groundwater. Small, stable air bubbles with large surface area enhance contaminant removal. Previous studies used surfactants to stabilize air bubbles. However, they did not use foaming agents that were also able to adsorb and degrade contaminants. In this study, we produce “reactors on an air bubble” able to strip and degrade naphthalene in groundwater. Interfacial tension measurements showed that laccase and zein adsorbed at the air-water interface at either acidic or alkaline pH. At acidic pH, the compression isotherms of zein-laccase films at the air-water interface differed from those of the individual components, suggesting zein-laccase co-adsorption. At alkaline pH, iron promoted co-adsorption of zein-laccase at the air-water interface, likely by modulating their electrostatic interactions. Foams were stable at alkaline, with either zein alone or zein-laccase complexes, and iron enhanced foam stability. These results indicate that zein and laccase can facilitate the remediation of naphthalene by promoting air bubble stability and naphthalene sorption at the air-water interface, as well as its enzymatic degradation.