Adsorption of reactive dyes to granulated iron hydroxide and its oxidative regeneration

Granulated iron hydroxide (beta-FeOOH) is used as a regenerable sorbent and catalyst for reactive dye removal in textile wastewater treatment. In oxidative regeneration the previously sorbed reactive dye is catalytically oxidized after activation of hydrogen peroxide to hydroxyl radicals on the FeOOH surfaces to regain adsorption capacity and reuse the sorbents. A high initial H2O2 concentration is needed for an efficient mass transfer water/solid but its consumption per oxidized dye results only in 7-25 mg mg(-1). At room temperature a regeneration time of 3.5 h is necessary for decolorization and 6 h for further decomposition of the formed oxidation products to enable a reloading without any loss in adsorption capacity. Compared to Fenton's reagent, this oxidation process takes advantage of pH-independence between 4 and 8 and additionally no sludge is produced. Salts are commonly used in textile dying processes. While chloride improves the dye sorption, the presence of sulfate results in deteriorating sorption. Carbonate reacts as a scavenger of hydroxyl radicals resulting in a higher hydrogen peroxide consumption, but the efficiency of dye oxidation is only slightly affected. Nearly 20% of DOC of the sorbed dye can be attributed to short chain organic acids (formate, acetate, oxalate) indicating the far-reaching catalytic oxidation and enabling biological post-treatment.