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This work studied the influence of several parameters on free cyanide (CN−) degradation (50 mg L−1) by the UVC-activated persulfate (PS) at alkaline conditions (UVC/PS). Firstly, photolysis and alkaline activation of PS were evaluated. Then, the effect of initial PS concentration (0.2, 0.4, and 0.6 g L−1) and dissolved oxygen in solution (absence/presence) were studied. Lastly, the influence of phosphate, carbonate, and nitrate presence at different concentrations (50, 150, 350, and 500 mg L−1) on CN− elimination was tested. Additionally, the electric energy per order (EEO), a measure of the energy consumption in the process was determined, and a mechanistic view of CN− degradation was proposed. The results show that photolysis and alkaline activation of PS degraded 8 and 11% of CN−, respectively, whereas their combination presented a synergistic effect on CN− pollutant elimination. While oxygen had a vital role in photolysis due to the formation of 1O2 to oxidize CN− to CNO−, HO• and SO4•− were primarily responsible for CN− degradation by UVC/PS. It was also found that cyanide removal followed a pseudo-first-order kinetics whose apparent reaction rate constant (k) increased from 0.0104 to 0.0297 min−1 as the initial concentration of PS increased from 0.2 to 0.6 g L−1, indicating a strong dependency of the removal efficiency on the PS amount. Remarkably, cyanide degradation by the combined UVC/PS showed a high CN− conversion and selectivity even in the presence of high concentrations of phosphate, carbonate, and nitrate ions (500 mg L−1), which resulted in CN− removals higher than 80% after 60 min of degradation treatment. Furthermore, the EEO values were similar in the presence and absence of phosphate or carbonate; however, they decreased slightly with nitrate presence. All these results suggest the feasibility of the combined UVC/PS process for the elimination of cyanide such as that found in mining wastewater.

The ultraviolet light activation of persulfate (PS) under alkaline conditions was evaluated for treating hexacyanoferrate (III) ion ([Fe(CN)6]3-). The effect of the wavelength type (i.e., UVA and UVC), initial PS concentration (0.3, 0.6, and 0.9 g L−1), and pH value (11 and 13) on the degradation of 50 mg L−1 of [Fe(CN)6]3- were studied. Finally, the role of the main degrading agents (SO4•-, O2•-, or HO•) involved in the degradation process was determined using scavengers and a degradation pathway for [Fe(CN)6]3- was proposed. Results show that [Fe(CN)6]3- can be decomposed by UVC light, while UVA is not effective neither in [Fe(CN)6]3- photolysis nor in the activation of PS. Alkaline activation alone at pH 13 can also not degrade the cyanocomplex. However, the combination of UVC with PS (0.3 g L−1) at pH 13 showed high efficiency in the elimination of [Fe(CN)6]3-, achieving 93.3% of removal after 125 min of treatment. The highest CN- release and Fe dissolved removal also occurs at pH 13 and 0.3 g L−1 PS. Further increases in initial PS concentrations may lead to an excess of radicals in solution, resulting in detrimental recombination reactions that affect the efficiency of the process. Quenching tests showed that the importance of radicals involved in the degradation of [Fe(CN)6]3- follows the order: HO• > O2•- >>> 1O2 or SO4•-, and allowed to demonstrate that singlet oxygen could participate in the UVC photolysis of [Fe(CN)6]3-. All these results suggest the feasibility of this technology to treat this type of industrial wastewater efficiently.

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 C o C N 6 3 - 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 C N - 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 S O 4 • - to H O • 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 O 2 to the higher energy excited state of the cyanocomplex that must be reached to initiate degradation reactions. Increasing the initial concentration of C o C N 6 3 - will increase the amount of Co removed but it represents the higher specific energy consumption.

The data collected in the present work correspond to percentages of phenol degradation by means of photoelectrochemical oxidation (PEC). Also, the information related to the energetic and kinetic performance of this advanced oxidation process (AOPs) is shown. The tests were divided into two stages: 1. Supporting electrolytes tests to determine the electrolyte that presents a better response to photocurrent and 2. Degradation of phenol to obtain the adequate conditions for the elimination of the contaminant. A central rotary composite design with uniform precision at two levels was used to analyze the influence of the initial pH, electrode potential and the initial concentration of substrate. Finally, with all the data obtained, calculation of degradation rates and the electrical energy per order EEO were made. Keywords: Photocurrent, Supporting electrolyte, Energy consumption, Kinetic performance, Chronoamperometry, Linear voltammetry

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.

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 non-homogeneous degradation mechanism with different pathways induced by the presence or absence of O2.