Intensification of processes for the treatment of wastewater using activated persulfate

Resumen

The continued water quality deterioration because of increasingly intensive use has become a subject of great concern because of its economic and health implications. Despite the advances in the conventional methods for wastewater treatment, they still suffer several drawbacks such as high retention time, low efficiency for the removal of recalcitrant compounds or sensitivity to variation of flow rate and organic load. Therefore, it is necessary to use other technologies capable of overcoming these problems and fulfill the environmental legislation. Advanced Oxidation Process (AOPs) have provided a feasible alternative to conventional technologies, due to their great potential to oxidize, partially or totally, numerous organic compounds. These processes are based on the in situ generation of hydroxyl radicals (OH•), a highly powerful oxidizing agent (E: 2.8 V). Sulfate radicals (SO4 •−) have emerge as a suitable technology to oxidize pollutants due to their long lifetime and high oxidation potential (E: 2.5-3.1 V). Persulfate (PS) is the main choice to produce SO4 •− by activation using different methodologies (e.g, using thermal, UV and chemical activation). This compound shows several advantages such as an easy storage and transport due to its solid state at ambient temperature, a high water solubility and a relatively low cost. Besides, sulfate, an inoffensive inorganic compound, is the main resulting product from persulfate reduction. This work assesses the synergistic effect of combining several PS activation alternatives, in order to intensify this process, using phenolic compounds such as phenol and 4-chlorophenol (4-CP), and azo dye Disperse Blue 3 (DB3) as model pollutants. Following are described the four procedures investigated to activate PS: 1. Electro activation of persulfate using iron sheet as low cost sacrificed electrode. This iron source, commonly used in civil engineering, provides continuously Fe2+ during the electro-oxidation process, which increased the SO4•− generation compared with PS heat-activation alone (in the range 30-90 ºC). Nevertheless, the main drawback of this technology lays on the excessive generation of iron sludge. 2. Nanoscale Fe0 doped with Ag. The highly reactive nanoparticles were synthesized through reduction of FeCl2 with NaBH4. The PS decomposition is faster than that achieved by electro-activation but minimizing the sludge production. However, the pH control in this process must be considered. 3. Combination of ilmenite (FeTiO3) and UV-LED. Raw ilmenite (FeTiO3) was used as an inexpensive iron-based catalysts to mediate the decomposition of PS into SO4•- radicals, that is steeply increased by irradiating with UV-LED (λ: 405 nm) due to the photoreduction of Fe(III) to Fe(II) in the ilmenite surface. Thus, the combined treatment becomes a feasible “low-cost” alternative to activate PS. The effect of temperature, as an attempt to intensify the process, is also assessed. 4. Cathodic reduction of PS, using Ti anodes coated with IrO2 –Ta2O. This treatment promotes a faster PS decomposition avoiding the iron-sludge generation. The effects of current density, PS concentration and conductivity aqueous solution in combination with temperature is also studied.