(685e) Design and Manufacture of a Torus Microreactor for the Removal of Azo Dyes By Laccase Immobilized on Magnetite Nanoparticles

Water pollution is one of the main environmental problems that mankind is facing over the coming few decades. Oceans, aquifers, rivers, lakes, and groundwater are being polluted at an unprecedented rate. This directly affects not only the health of an increasingly higher number of human communities but ecosystems of other living organisms [1]. Industrial and agricultural processes are responsible due to the continuous discharge of large volumes of wastewaters to these aquatic environments [2][3]. Some of the most common discharged pollutants include pathogens, excess nutrients, suspended solids and sediments, pesticides, plastics, fertilizers, acids, detergents, phenols, minerals, and heavy metals [4][5].

The annual production of synthetic dyes or azo dyes approaches 70 million tons [6]. These xenobiotic chemicals stand as one of the major sources of water pollution, they are relatively simple to synthesize, soluble in water and their natural occurrence is rare [7][8][9]. Azo dyes find applications in various industries including textiles, leather goods, paper, plastics, foodstuffs, cosmetics and candles [10][11]. From them, the vivid colors develop, most notably yellow, orange and red [11]. In recent years, the growing demand of these products has in turn increased the production of dyes and consequently the amount of waste [9].

Despite the industry's efforts to couple wastewater treatment processes to their manufacturing plants, 90% of reactive textile dyes entering activated sludge sewage treatment plants will pass through unchanged and will be eventually discharged to rivers [12][8]. As a result, between 30 to 150 thousand tons of dyes are discharged into water bodies, soil and aquatic ecosystems annually [13][6]. The continuous exposure to azo dyes is suspected to be harmful for living organisms [6]. Water sources contaminated by these compounds have low penetration of sunlight and oxygen, which is essential for the survival of various aquatic organisms [13]. Moreover, the anaerobic degradation entails sub-products of high biological toxicity that may end up in the food chain. An important number of dyes are made of known carcinogens and toxics, such as benzidine and other aromatic compounds, which main effects to humans include damage to ADN and proteins [8][13][11]. Current regulations in Europe, China, Taiwan, Korea and Japan limit the discharge of synthetic dyes to approximately 30 parts per million when used in textiles and leather items that could have prolonged contact with the skin [11]. Accordingly, there has been a recent effort to implement more stringent effluent treatment regulations that enforce industry to find methods to lower the color level in their wastewater prior to discharging into surface waters [11][13].

The primary methods to remove azo compounds are coagulation/flocculation, adsorption, precipitation, flotation, membrane filtration, bioflocculants treatments, ion pair extraction, ultrasonic mineralization, electrolysis, ion exchange, advanced oxidation processes, sonication, photocatalysis, and ozonation [6][7][13]. A major limitation of this processes is however, their simultaneous dependence on both anaerobic and aerobic conditions. This leads to costly processes and/or extremely toxic byproducts [10][20]. Additionally, the biocatalytic degradation of azo dyes and wastewater treatment via laccase has been previously described in different contributions [9][12][7]. Laccase is an oxidoreductase enzyme, capable of oxidizing phenolic compounds into phenoxyl radicals, with the aid of 4 copper electrons in its structure [9][7]. The enzyme promotes decolorization in azo dyes, due to the presence of aromatic polyphenolic moieties in their structures [7].

Here we explore the design and manufacture of a microreactor to enable the enzyme-based degradation of Azo dyes. Laccase molecules for the biodegradation of azo compounds were covalently immobilized on amino-terminated silanized magnetite (Fe₃O₄). Silanization was conducted with 3-Aminopropyltriethoxysilane (APTES) while laccase molecules were conjugated via imine bonds with the aid of glutaraldehyde as crosslinker. We hypothesize that a torus microreactor is suitable for maximizing biodegradation processes due to the absence of dead volume, efficient mixture of reagents and the continuous reaction loop allowed by the toroidal shape [21]. Additionally, the strong magnetic response of magnetite allows the application of magnetic fields to maintain the nanoparticles suspended during the treatment process and maximize contact between the components. A first attempt for find an optimal configuration for the micro-torus reactor was explored in silico with the aid of Comsol Multiphysics® by analyzing mixing patterns and fluid dynamics. This was accomplished by coupling the CFD and particle tracing modules. Device prototyping was conducted in polymethyl methacrylate using a laser cutter system and commercially available fittings for the assemblage and subsequent testing. Currently, a spectrophotometry assay is performed before and after to evaluate the degradation achieved with the process. Finally, to monitor the extent degradation of azo molecules in an in-line and real-time manner, the device was instrumented with an in situ spectrophotometry system. Functionalization of nanoparticles surfaces in conjunction with torus microreactors appear well suited for degradation and/or capture of other pollutants of concern.

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