After 4 h of electrolysis COD removal was only 45% when no ferrous ions were added to the
solutions, while the presence of ferrous ion greatly improved COD removal up to more than
90%. In particular 1.0 mM was the optimal concentration of ferrous ions and the single step
or the stepwise addition of Fe2þ ions enables the same COD removal. The oxidation rate
increased with increasing of current density and temperature and with decreasing of pH.
The UV–vis analysis indicated that the discoloration of the solution occurs simultaneous
with the destruction of aromatic rings and alizarin red is oxidised firstly to colourless
intermediates (mainly phthalic acid, small carbonyl species) and then to carbon dioxide.