Thermal modification of activated carbon surface chemistry improves its capacity as redox mediator for azo dye reduction

The surface chemistry of a commercial AC (AC 0) was selectively modified, without changing significantly its textural properties, by chemical oxidation with HNO 3 ( AC HNO 3 ) and O 2 ( AC O 2 ) , and thermal treatments under H 2 ( AC H 2 ) or N 2 ( AC N 2 ) flow. The effect of modified AC on anaerobic chemical dye reduction was assayed with sulphide at different pH values 5, 7 and 9. Four dyes were tested: Acid Orange 7, Reactive Red 2, Mordant Yellow 10 and Direct Blue 71. Batch experiments with low amounts of AC (0.1 g L −1) demonstrated an increase of the first-order reduction rate constants, up to 9-fold, as compared with assays without AC. Optimum rates were obtained at pH 5 except for MY10, higher at pH 7. In general, rates increased with increasing the pH of point zero charge (pH pzc), following the trend AC HNO 3 < AC O 2 < AC 0 < AC N 2 < AC H 2 . The highest reduction rate was obtained for MY10 with AC H 2 at pH 7, which corresponded to the double, as compared with non-modified AC. In a biological system using granular biomass, AC H 2 also duplicated and increase 4.5-fold the decolourisation rates of MY10 and RR2, respectively. In this last experiment, reaction rate was independent of AC concentration in the tested range 0.1–0.6 g L −1.