The paper discusses the efficiency of activated carbon used for removal of two dyes crystal violet and phenol red - in aqueous solution by oxidation with hydrogen peroxide. The tests were conducted using four different activated carbons: two fresh carbons (WDex and WG-12), one used carbon (F-200S) and one subjected to regeneration with Fenton reagent (F-200R). The first stage of the analysis involved assessing the removal efficiency of the two dyes using sorption or oxidation with hydrogen peroxide at different concentrations of the oxidant. Commercially available fresh activated carbon, WDex, showed the highest sorptive capacity, which was 34 mg/g and 63 mg/g for crystal violet and phenol red, respectively. Under similar conditions, the dye sorption on the WG-12 carbon was 12 mg/g for crystal violet and 40 mg/g for phenol red. The sorptive capacities of F-200R and F-200S carbons were definitely lower; they were less than 10 mg/g for both dyes. Attempts to remove the dyes from solution by oxidation with hydrogen peroxide at concentrations of 375 divided by 7500 mg H2O2/L were unsuccessful. However, the addition of activated carbon to the dye-H2O2 system led to an increase in the rate of solution decolourization. The experiments were conducted at a fixed concentration of crystal violet of approximately 19 mg/L and of phenol red of 21 mg/L, a constant amount of activated carbon in solution (0.5 g) and concentrations of hydrogen peroxide ranging from 1500 to 7500 mg/L. The fastest change in concentration of crystal violet solution, i.e. as early as within 20 minutes of reaction, was observed for the WDex carbon. It decreased by 40% at dose of oxidant 1500 mg H2O2/L, and by 70% at dose of oxidant 3750 mg H2O2/L. Increasing the time of reaction to 80 minutes resulted in total decolourization. Similarly, a higher rate of decolourization of crystal violet solution was observed when the reactive system contained the WG-12 carbon. Within 20 minutes of reaction, the concentration decreased by 30% at dose of oxidant 1500 mg H2O2/L. The concentration was approximately 40% lower when dose of oxidant increased to 3750 mg H2O2/L. The total decolourization was achieved within 160 minutes of reaction at a decrease in the concentration by approximately 90%. When used carbon (F-200S) was added to the crystal violet-hydrogen peroxide system, the rate of the dye decomposition was higher than during sorption. When the regenerated carbon (F-200R) was used, the total decolourization of the solution was observed as early as within 120 minutes at a concentration of hydrogen peroxide of 3750 mg H2O2/L. Similar experiments conducted for phenol red show that the presence of activated carbon in the dye-H2O2 system caused an increase in the dye removal rate. However, even under the most favourable conditions, in the presence of the fresh carbon (WDex), the decolourization was only slightly higher than 80%, within 160 minutes of reaction. The decomposition efficiency of the phenol red solution in the presence of the WG-12, F-200R and F-200S carbons was much lower, although in each case the use of the phenol red-activated carbon-oxidant reactive system was more favourable than sorption or oxidation only. The increase in the dye removal rate was attributable to the generation of hydroxyl radicals in the reactive system, as they were responsible for the oxidation of organic compounds. The rate at which the selected dyes are removed from solution in the dye-hydrogen peroxide-activated carbon system is well described by the kinetic equation of the second-order reaction (II). Carbons with higher dechlorination efficiency are more effective catalysts for generating hydroxyl radicals in hydrogen peroxide solution.
