62ª Reunião Anual da SBPC

A. Ciências Exatas e da Terra - 4. Química - 1. Físico-Química

COMPARISON OF ELECTROCHEMICAL PROCESSES FOR DEGRADATION OF ORGANIC COMPOUNDS: DIRECT AND INDIRECT OXIDATION

Aline Maria Sales Solano 1 Christian Urgeghe 2 Djalma Ribeiro da Silva 1 Nedja Suely Fernandes 1 Carlos Alberto Martínez-Huitle 1

1. Departamento de Química-Universidade Federal do Rio Grande do Norte-UFRN, Brasil 2. Industrie De Nora S.p.A - Grupo De Nora Departamento de Eletrodos-Milan, Itália

INTRODUÇÃO:

The textile industry produces large quantities of wastewater during the washing and dyeing process that contain large quantities of dye that are not fixed on the fiber surface, and as a result are disposed together with the textile effluent. For the removal of dyes from wastewater a wide range of techniques, such as chemical precipitation, chemical oxidation or adsorption with subsequent biological treatment has been developed. Currently, the electrochemical processes for treating wastewater containing dyes have been proposed as an innovative alternative. Therefore, the feasibility of electrochemical conversion/destruction of organic pollutants in wastewater has attracted much attention from several research groups. For that reason, the principal objective of this study is to investigate the direct and indirect electrochemical treatment of synthetic textile wastewaters containing Methyl Red (MR) dye, analyzing the influence of the main operating parameters on the color removal. In addition, the influence of active chlorine (electrogenerated oxidant that participates in the indirect oxidation) on decolourization efficiency of dye solutions was studied. Finally, energy consumption and electrical energy cost for removing color was estimated to evaluate the viability of this process.

METODOLOGIA:

The oxidation experiments of MR were performed in undivided electrochemical cell (400 mL), under galvanostatic conditions using a power supply. Ti-supported Pt anode was supplied by Industrie De Nora SpA. (Milan, Italy) and it was used as the anode, while a titanium plate as the cathode (both electrodes with 35 cm2). Experiments were performed at 25 °C for studying the role of applied current density (20, 40 and 60 mAcm-2); furthermore, the effects of the agitation were investigated in the range from 200 to 600 rpm, by applying a current density of 40 mAcm-2. In the case of indirect oxidation, experiments were carried out varying the NaCl concentration in solution (0.2, 0.4 and 0.8 g) containing MR dye, under 400 rpm of agitation rate and 40 mAcm-2. At the same time, quasi-steady polarization curves were carried out in solutions of NaCl at different concentrations, using Na2SO4 in order to understand the mechanisms: direct and indirect oxidation. Potential values were referred against a double-walled, saturated calomel electrode (SCE). In addition, polarization curves were carried out in order to confirm the production of species strongly oxidants in solution. Color removal was monitored by measuring absorbance decrease; using a spectrophotometer.

RESULTADOS:

The complete removal of color (MR solution containing 100 mgL-1) was achieved, independently of the applied current density. It indicates the complete dye elimination by means of its reaction with electrogenerated OH. radicals. However, the decolourisation time depends mainly on the applied current density. On the other hand, the agitation rate influences on the color removal as a function of the time. Higher magnetic stirrer rates led to more rapid and high efficient electrochemical process, which suggests that the process is controlled by mass transport. Mediated decolourization removal was studied by applying 40 mAcm−2 of current density, in the presence of 0.2, 0.4 and 0.8 g of NaCl in 400 mL of synthetic dye solution; obtaining that the process was accelerated significantly by the presence of the halide salt in solution; interestingly, the mediated process did not depend on applied current density, but depends on NaCl concentration. Polarization curves showed that the whole polarization curve is shifted to more positive potentials, with increasing the chloride concentration; confirming the production of oxidant species (active chlorine). Therefore, the electrooxidation is accelerated by the presence of NaCl, decreasing the energy consumption and process cost until a 40%.

CONCLUSÃO:

These results point out the high performance Ti/Pt anodic oxidation for treating synthetic dyeing wastewaters compared to other advanced oxidation processes. At this moment, this electrochemical process (direct or mediated) can be a feasible process for decolorizing textile wastewaters polluted with dyes because it can be used considering the specific operating conditions (current density and agitation rate). The influence of current density and agitation rate as functions of time, during galvanostatic electrolyses of MR synthetic wastewaters, showed that the oxidation by the electro-generated reagents from the anodic oxidation of supporting electrolyte (e. g.: OH) plays important role in the efficiency of the electrochemical process. On the other hand, the effect of NaCl in solution showed that an increase of NaCl concentration in synthetic dye solution enhances the color rate elimination. Therefore, the indirect electrochemical oxidation seems to be an efficient process than that directs oxidation.

Instituição de Fomento: Universidade Federal do Rio Grande do Norte - UFRN and Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES.

Palavras-chave: Anodic oxidation, Electrochemical generation of oxidants, Direct and Indirect electrochemical processes .