In this work, we present a catalytic method to convert toxic phenols into polymers with low solubility using only sunlight as an energy source. Wastewater pretreatment is necessary to reduce fouling of and backpressure on desalination membranes, improving both maintenance and replacement costs. Flocculation of contaminants would present a cost-effective means to pretreat aqueous media, but these methods generally require high energy input, such as through electrochemical oxidation with applied voltage and a sacrificial oxidant. In addition, utilization of enzymes such as horseradish peroxidase (HRP) require a steady supply of high energy oxidant such as hydrogen peroxide. Here, we show photochemical polymerization of phenolic contaminants into flocculation-ready polymers using only sunlight and air. For this, we utilized hybrid nanoscale catalysts containing both a semiconductor with a bandgap matching visible light and a noble metal nanoparticle to facilitate the reduction process. In initial studies, platinum-capped cadmium sulfide nanorods (Pt@CdS) were shown to polymerize guaiacol, a phenolic model compound derived from processed lignin. In the absence of oxygen, this process also produced hydrogen gas as a potential fuel. Next, running the same catalytic reaction in the presence of air resulted in an increased polymerization rate with a guaiacol conversion of 26% over 24 hours. Addition of HRP to form a tandem process improved the yield to 48%, highlighting the ability of the photocatalyst to produce high energy oxidative species. Analysis of the flocculated phenolic compounds by Gel Permeation Chromatography also showed that the product polymers possessed larger molecular weight than enzyme-only tests, with number average molecular weights increasing from 540 to &gt;1200 Daltons. Finally, we showed that that metal oxide catalysts such as iridium oxide could not only substitute for the more toxic and less photostable CdS, but also showed superior yield of 78% guaiacol flocculation over 24 hours in tandem with HRP. Overall, this project demonstrates the potential of utilizing photocatalysts in tandem with an enzymatic catalyst to flocculate phenolic materials from aqueous solution using only sunlight as a driving force. Future work will focus on expanding the range of substrates, including phenol, vanillin, dopamine, fluoroguaiacol, gallic acid, and 2,6-dihydroxybenzoic acid.

Multicatalytic, light-driven polymerization of phenols for flocculation of wastewater impurities

256th National Meeting (2018)

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