The ultraviolet light activation of persulfate (PS) was evaluated for the degradation of cobalt cyanocomplexes, which are considered as some of the most recalcitrant compounds present in mining wastewater. The influence of the solution pH (11 and 13), initial concentration of PS (0.1, 0.3, 0.5, 0.7 and 0.9 g/L), dissolved oxygen and initial concentration of contaminant were evaluated. Photolysis results showed that CoCN63-is photosensitive to UVC radiation, while the activation of PS by alkaline pH does not contribute to the degradation of the cyanocomplex. There was no presence of CN-at both solution pH values using UVC/PS. But at pH 13, the degradation of cobalt cyanocomplexes and the pseudo-first-order rate constant increased. This was attributed to the effective conversion of SO4•-to HO• and to the increase in the oxidative photolysis of PS at high pH. Additional tests demonstrated better performance of UVC/PS in the absence of oxygen which may be caused by the quenching effect of O2 to the higher energy excited state of the cyanocomplex that must be reached to initiate degradation reactions. Increasing the initial concentration of CoCN63-will increase the amount of Co removed but it represents the higher specific energy consumption.
The photocatalytic degradation of hexacyanocobaltate ion ([Co(CN)6]3−) by TiO2 supported on borosilicate sheets was studied in a novel photoreactor operated under UVA-LEDs irradiation. The presence and absence of O2 during the process, and the reuse of the TiO2-impregnated sheets were evaluated. The semiconductor was supported by the dip-coating method, and the influence of g TiO2/g Methanol (MetOH) ratio in the suspension and substrate roughness modification were evaluated in the detachment percentage and adherence of TiO2. The highest semiconductor detachment percentage was obtained at 0.36 g TiO2/g MetOH ratio. Besides, it was determined that for the ten layers supported, the substrate roughness modification does not influence the detachment of TiO2. In the first photocatalytic tests, the results show that a better degradation performance was achieved by direct photolysis than photocatalysis according to the higher concentration of CN− released from the cyanocomplex. However, a decrease in the concentration of cobalt in solution was not observed. When the sheets were reused, a decrease of 10% in the concentration of cobalt was achieved, and 14% of CN− was released from the cyanocomplex. This was attributed to the formation of microchannels, hollows amongst other imperfections that increase the surface area and active sites of the coating when TiO2 peels off. The simplified kinetics analysis shows that for UV + O2 (oxic environment) UV + N2 (anoxic environment) similar kinetic parameters were obtained, indicating that both processes follow the same homogeneous pathway in the degradation of [Co(CN)6]3−. However, for the UV + TiO2+O2 and UV + TiO2+N2, their different kinetic parameters suggest a nonhomogeneous degradation mechanism with different pathways induced by the presence or absence of O2.
The influence of the addition of cations on the adsorption of [Fe(CN)6] 3− on granular activated carbon (GAC) was evaluated. The tests were performed at three pH values (3, 8.2, and 13) to determine the repulsion or electrostatic affinity between the adsorbent and adsorbate. Afterward, the cations (K+, Ca2+, and Al3+) at three pollutant-cation molar ratios (1:1, 1:10, and 1:50) were added to the system, and the influence of those was identified by the changes in the adsorption efficiency. The results show that the higher removal (%) was obtained at pH 13 without neither the presence of iron precipitates nor the liberation of HCN. The adsorption of [Fe(CN)6] 3-was enhanced by the addition of K + at 1:10 and 1:50 mol ratio since higher removals were achieved (75.27 % and 76.81 % respectively) than those obtained in the absence of cations (64.18 %) or in the presence of Ca2+ (67.58 %) and Al3+ (65.13 %) at a 1:10 mol ratio. The behavior in adsorption in the presence of cations shows that the ion-pair adsorption mechanism can describe the physical phenomenon, showing an increase in the fraction removed and the rate of adsorption with increasing cation charge. The adsorption kinetics using K+ with a 1:10 pollutant-cation molar ratio was fitted to the Lagergren pseudo-first-order model. The GAC adsorption capacity describes the pollutant adsorption rate with the predominance of physical interactions. The experimental data were fitted by the Langmuir and Freundlich isotherms, indicating a monolayer adsorption phenomenon consistent with the previously proposed ion-pair adsorption mechanism.
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