Continuous flow system for combined chemical and enzyme-catalyzed removal of nitroaromatics from synthetic wastewater

Nitroaromatic compounds (NACs) are major environmental pollutants and their degradation is difficult to achieve. Using zero-valent iron reduction of the NACs coupled with peroxidase-catalyzed capture of the resulting anilines, a two-step strategy for removal of NACs from waste- and process-water, is investigated here. The concentration range of NACs studied was that which would be present in industrial wastewater streams (millimolar, 123 ppm), a concentration range considerably higher than those studied previously with groundwater by other researchers. Zero-valent iron (Fe0) has been successfully employed to reduce nitrobenzene, o-, m- and p-nitrotoluenes to corresponding anilines in synthetic wastewater in both batch and continuous flow reactors. Anaerobic conditions were maintained in the reactors by including Na₂SO ₃ as an oxygen scavenger in the presence of CoCl₂.6H ₂O, which acted as a catalyst. Batch reactors exhibited adsorption of aniline on the Fe0, which could be described by a Langmuir isotherm. A 200 g Fe0 (particle size: 1–2 mm) bed completely converted 1 mM of NAC flowing upward for about 600 pore volumes before experiencing flow reduction due to clogging by corrosion products. A green-black precipitate was formed at the influent end of the column, an Fe0 corrosion product identified as maghemite.; The enzymatic treatment following the zero-valent iron reduction was done in a plug-flow reactor (PFR) using a crude preparation of the enzyme soybean peroxidase extracted from soybean hulls. The complete reaction time for the two steps was 5–5½h. Parameters like pH, peroxide to substrate ratio, enzyme concentration and alum concentration were optimized. At pH 7–7.2, the optimum H₂O₂ to substrate ratio was found to be 1.5 for the aryl amines, aniline, o-, m- and p-toluidines, investigated in this study. Alum concentrations between 50–100 mg/L were useful in removing the end-color from the treated water. NACs were quantitatively reduced to their corresponding amines, which were completely removed from the wastewater in the enzymatic treatment step.