Simultaneous persulfate activation by electrogenerated H2O2 and anodic oxidation at a boron-doped diamond anode for the treatment of dye solutions.

The development of new or upgraded electrochemical water treatment technologies is considered a topic of great interest. Here, Tartrazine azo dye solutions were treated by means of a quite innovative dual electrochemical persulfate (S2O82-, PS) activation that combines H2O2 generation at an air-diffusion cathode and anodic oxidation (AO) at a boron-doped diamond (BDD) anode using a stirred tank reactor. This so-called AO-H2O2/PS process was compared to AO with stainless steel cathode, both in 50 mM Na2SO4 medium, finding the oxidation power increasing as: AO < AO-H2O2 < AO/PS < AO-H2O2/PS. In the latter, the dye and its products were mainly destroyed by: (i) hydroxyl radicals, formed either from water oxidation at BDD surface or via reaction between H2O2 and S2O82-, and (ii) sulfate radical anion, formed from the latter reaction, thermal PS activation and cathodic S2O82- reduction. Hydroxyl radicals prevailed as oxidizing agents, as deduced from trials with tert-butanol and methanol. The reaction between S2O82- and accumulated H2O2 was favored as temperature increased from 25 to 45 °C. The effect of PS content up to 36 mM, dye concentration within the range 0.22-0.88 mM, current density (j) between 8.3 and 33.3 mA cm-2 and pH between 3.0 and 9.0 on the process performance was examined. All decolorization profiles agreed with a pseudo-first-order kinetics. The best results for treating 0.44 mM dye were attained with 36 mM PS at pH 3.0, j = 16.7 mA cm-2 and 45 °C, yielding total loss of color, 62% TOC removal and 50% mineralization current efficiency after 360 min. The slow mineralization was attributed to the persistence of recalcitrant byproducts like maleic, acetic, oxalic, formic and oxamic acids. It is concluded that the novel AO-H2O2/PS process is more effective than AO/PS to treat Tartrazine solutions, being advisable to extend the study to other organic pollutants.

Vermiculite as heterogeneous catalyst in electrochemical Fenton-based processes: Application to the oxidation of Ponceau SS dye.

Modified sodium vermiculite, an iron-rich clay mineral, has been used in novel heterogeneous electrochemical Fenton-based treatments, so-called electro-Fenton (EF)-vermiculite, UVA photoelectro-Fenton (PEF)-vermiculite and solar photoelectro-Fenton (SPEF)-vermiculite. Tests were made with 130 mL of 0.150 mM Ponceau SS diazo dye in 0.050 M Na2SO4 at pH 3.0, in the presence of 1.0 g L-1 catalyst microparticles. The electrolyses were performed in an undivided cell with a boron-doped diamond anode (BDD) and air-diffusion cathode for H2O2 production, at 33.3 mA cm-2. Decolorization and mineralization were upgraded in the sequence: EF-vermiculite < PEF-vermiculite < SPEF-vermiculite. The removal of organics occurred by the combined action of OH oxidant formed at the BDD surface and homogeneous and heterogeneous Fenton's reactions, along with the photolysis caused by UVA light or sunlight. The homogeneous Fenton's reaction resulted from iron ions leaching, but the heterogeneous mechanism was prevalent. Comparative treatments by anodic oxidation in the presence of H2O2 and homogeneous EF were less effective than EF-vermiculite. The diazo dye absorbance decays agreed with a pseudo-first-order kinetics. SPEF-vermiculite was the most powerful process, yielding total decolorization and 84.1% mineralization after 300 and 360 min, respectively. The influence of catalyst concentration, current density and diazo dye content on PEF-vermiculite performance was examined. Oxalic, oxamic, malic, tartronic and acetic acids were detected as final short-linear carboxylic acids.

Total mineralization of mixtures of Tartrazine, Ponceau SS and Direct Blue 71 azo dyes by solar photoelectro-Fenton in pre-pilot plant.

Mixtures of monoazo Tartrazine, diazo Ponceau SS and triazo Direct Blue 71 dyes with 105 mg L-1 of total organic carbon (TOC) in 0.050 M Na2SO4 at pH 3.0 have been treated by solar photoelectro-Fenton (SPEF). Experiments were carried out in a 2.5 L pre-pilot plant with a Pt/air-diffusion cell coupled to a solar planar photoreactor. Comparative trials were made by anodic oxidation with electrogenerated H2O2 (AO-H2O2) and electro-Fenton (EF) to better understand the role of oxidizing agents. AO-H2O2 gave poor degradation due to the low oxidation ability of OH formed at the Pt anode and H2O2 produced at the cathode. Similar color removal was achieved in EF and SPEF because the main oxidant was OH formed in the bulk from Fenton's reaction. EF yielded partial mineralization by formation of molecules with high stability against OH. In contrast, these by-products were rapidly photolyzed under sunlight irradiation in SPEF, which was the most powerful treatment. Up to 8 linear final carboxylic acids were detected, along with the release of sulfate and ammonium ions. The effect of Fe2+ and azo dye concentrations, and current density over the SPEF performance was assessed. Total mineralization of azo dyes mixtures occurred when operating up to 105 mg L-1 TOC with 0.50 mM Fe2+ at 100 mA cm-2.

Electrochemical advanced oxidation processes as decentralized water treatment technologies to remediate domestic washing machine effluents.

Water scarcity is one of the major concerns worldwide. In order to secure this appreciated natural resource, management and development of water treatment technologies are mandatory. One feasible alternative is the consideration of water recycling/reuse at the household scale. Here, the treatment of actual washing machine effluent by electrochemical advanced oxidation processes was considered. Electrochemical oxidation and electro-Fenton technologies can be applied as decentralized small-scale water treatment devices. Therefore, efficient decolorization and total organic abatement have been followed. The results demonstrate the promising performance of solar photoelectro-Fenton process, where complete color and organic removal was attained after 240 min of treatment under optimum conditions by applying a current density of 66.6 mA cm-2. Thus, electrochemical technologies emerge as promising water-sustainable approaches.

Solar photocatalytic application of NbO2OH as alternative photocatalyst for water treatment.

Water recycling and industrial effluents remediation are a hot topic of research to reduce the environmental impact of the human activity. Persistent organic pollutants are highly recalcitrant compounds with hazardous effects associated to their fate in water bodies. Several novel technologies have been developed during the last decades to deal with this novel contamination. However, the natural sources and idiosyncrasy of each country lead to the potential application of different technologies. In this context, we have focused on the development of phocotalytic treatment of solutions containing dyes using a novel photocatalytic material, the NbO2OH. The NbO2OH was synthesized and characterized with different techniques. Several assays demonstrated the solar photoactivity of this novel oxyhydroxide catalyst, achieving complete decolorizations after 10min of treatment under optimal conditions of 1.0gL-1 NbO2OH photocatalyst loading, 0.1M of H2O2 as electron scavenger, pH4.0 and methyl orange concentrations up to 15mgL-1. Also, the catalyst recuperation demonstrated the potential reuse of this photocatalyst without losing catalytic response after five cycles. This work is of significant importance because niobium is a natural resource, mainly extracted in Brazil and the annual global sunlight irradiation in the near-equatorial region of northeast Brazil is over the average solar irradiation of the planet. Thus, the solar photocatalytic treatment using NbO2OH in northeast Brazil appears as a highly potential environmental-friendly nanotechnology to mitigate the water pollution.