RESUMEN
Mill scale is the metallurgical waste produced by the rolling mill in the steel hot rolling process. This hazardous waste is mainly composed of oxide iron, such as hematite, magnetite and wustite. It may have a different and alternative final destination by becoming a catalyst for wastewater treatment. In this work, the catalytic potential of mill scale (MS) from a steel plant was evaluated for hexavalent chromium reduction from synthetic and real matrices under slurry conditions (MS particles dispersed in the solution) or immobilized in Raschig rings. Experiments were conducted in an annular photoreactor irradiated by UVA light. Raschig rings were coated with MS by electrostatic link with polyethylene-grafted-maleic anhydride copolymer (PEGMA) film, and further packed in the annular zone of the UV photoreactor. SEM, XRD and FTIR analysis showed a homogeneous film of MS firmly attached on Raschig rings surface. In this way, the iron-rich industrial steel waste acted as both source of iron and photocatalyst, allowing the reduction of Cr(VI) to Cr(III) in the bulk solution and MS surface, respectively, in the presence of tartaric acid as hole and hydroxyl scavenger and Fe-complexing agent. The Raschig rings (248 g) coated with MS (23 g) achieved total Cr(VI) reduction (below detection limit) after 45 min of reaction (k = 2.0 × 10-2 mg L-1 min- 1) under UVA radiation, considering the following initial conditions: [Cr(VI)]0 = 10 mg L-1, [tartaric acid]0/[Cr(VI)]0 molar ratio = 6:1, pH = 3.0, T = 25 °C. The same system was tested for the treatment of a real effluent from a galvanic industry containing 6 mg L-1 of Cr(VI). Using the same tartaric acid/Cr(VI) molar ratio (6:1) and pH 3.0, the Cr(VI) present in the effluent was totally reduced (below detection limit) in 360 min (k = 1.93 × 10-2 mg L- 1 min- 1), showing similar kinetic behavior as the process with the synthetic matrix. In all experiments, the concentrations of dissolved iron (Fe(II) and Fe(total)) were below the disposal limit established by Brazilian legislation, and total chromium removal was achieved by Cr(III) precipitation after the photocatalytic reaction.
Asunto(s)
Residuos Industriales , Contaminantes Químicos del Agua , Cromo , Concentración de Iones de Hidrógeno , Hierro , Oxidación-Reducción , AceroRESUMEN
This paper presents the synthesis of a hybrid material through the use of natural pozzolan and titanium(IV) isopropoxide using the sol-gel method and its application in the photocatalytic hexavalent chromium reduction. The characterization data indicated a mesoporous material possessing a surface area of 271.7 m2 g-1. The morphology studies (SEM and TEM) showed nanosheet hybrid structures. The analysis of DRUV, FTIR, XRD, and Mössbauer spectroscopy provides a different electronic structure of the synthetized material when compared with the originals, proving the hybridization process between pozzolan and titanium(IV) isopropoxide. The photocatalytic reduction of Cr(VI) to Cr(III) using the hybrid material showed a better performance than conventional photocatalysts (precursor and TiO2-P25). Operational conditions such as chromium initial concentration (0.02-0.20 mM), solution pH (3-6), and type of scavenger (citric or tartaric acid) were evaluated in order to determine the best experimental conditions for the Cr(VI) photoreduction. At their optimum (catalyst load of 15 mg L-1, tartaric acid as scavenger, [scavenger]0/[Cr(VI)]0 M ratio = 3:1, pH 3, and 25 °C), the total photoreduction of 0.20 mM Cr(VI) was achieved in 180 min. The novel hybrid materials synthesized from pozzolan and titanium(IV) isopropoxide showed to be a potential catalyst for the Cr(VI) reduction in aqueous solution. Graphical abstract.