Industrial wastewater treatment using electrochemically generated ozone

Abstract

The remediation of industrial wastewater is highly challenging, difficult task, and demands highly efficient technologies. Electrochemical and ozonation technologies are among the most efficient methods in treating the industrial wastewater. The electrochemical generation of ozone can provide very high concentrations of the reagent in both the gas phase and solution. The aim of the research reported in this thesis was to develop durable and highly efficient Ni/Sb – SnO2 anodes to generate ozone and to investigate their efficiency in treating industrial wastewater. Different anode sizes were studied: 0.64 cm2, 6.25 cm2 and 24 cm2 using Ti mesh as substrate. With respect to the 0.64 cm2 anodes, replacing Sb and Ni chlorides with their respective oxides and adding Au or Pb had little or no effect upon the anodes electrochemical properties. The research showed that all 0.64 cm2 anodes were porous with dimensionalities < 2. However, the presence of the Au in the precursors reduced the ozone current efficiency. The 0.64 cm2 anodes achieved ozone current efficiencies of ca. 30% at cell voltages of 2.7 V routinely. Using 6.25 cm2 anodes prepared with the Sb and Ni oxides in the precursor solution and annealed at 550 oC gave electrodes which were durable for more than 200 h operation at a current density of 100 mA cm-2 (corresponding to cell voltages of ca. 3 V) in 1 M HClO4. These current densities and service life are the highest reported for Ni/Sb – SnO2 anodes. A service life of more than 600 h was achieved in a later investigation. The 6.25 cm2 anodes achieved current efficiencies up to 38%, with 25 -30% routinely achievable. The presence of Ni is crucial for ozone generation with optimum Ni content (in the precursor solution) of ca. 1.04 at % Ni. The optimum annealing temperature was 460 oC. In terms of the 24 cm2 anodes, they were employed to prepare membrane electrode assemblies (MEA’s) for ozone generation from deionised (Millipore) water. MEA’s with air breathing cathodes suffered from flooding of the cathode pores, resulting in limited current densities. MEA’s with hydrogen – evolving cathodes did not suffer from flooding or low current densities. Overall, current efficiency of ca. 36 % at cell voltage of 1.6 V (40 mA cm-2) with Millipore water as anolyte was obtained using MEA’s with air breathing Preface vii cathodes; corresponding to a power consumption of 16.7 kWh (kg O3)-1 which is the lowest reported for electrochemical ozone generation of any description, MEA’s with H2 cathodes achieved a current efficiency of 33% at ca. 25 mA cm-2 and a cell voltage of 2.5 V, corresponding to ca. 25 kWh (kg O3)-1. The 0.64 cm2 anodes were used to decolourise solutions containing : Reactive Blue 50 (RB50), Naphthol Green B (NGB) and Congo Red (CR) dyes. The operational conditions of the decolourisation process were investigated and the optimum conditions were: 3 g dm-3 Na2CO3 as electrolyte, 50 mA cm-2 and 200 mg dm-3 dye in Millipore water. RB50 solutions could be decolourised completely within 20 min, with 90% of the COD removal after 60 min, NGB and CR proved more refractory. Indirect oxidation mediated by OH radicals was the main decolourisation mechanism at the Ni/Sb – SnO2 anodes. Ozonation, UV254 irradiation and O3/UV were used to decolourise the dye solutions for comparison with electrochemical decolourisation at the Ni/Sb – SnO2 anodes. Ozone was generated by MEA – based electrochemical cells and ozonation occurred in a bubble column reactor (BCR). The O3/UV combination was the most efficient, achieving 100 % decolourisation of RB50 and NGB solutions within 20 and 35 min, respectively, with 33% and 64% COD removal after 60 min.