Synergistic photocatalysis of a hydrochar/CeO2 composite for dye degradation under visible light.

The synthesis and characterization of a hydrochar/CeO2 composite along with its evaluation in methylene blue degradation under visible light are presented. The methodology consisted of a single-pass hydrothermal method, having as synthesis conditions 9 h of reaction time, 210 °C, autogenous pressure, and a biomass/CeO2 ratio of 100:1. The composite characterization revealed good dispersion of CeO2 in the carbonaceous matrix and significant synergy in the composite activation using visible irradiation. The photodegradation experiments showed an efficiency of 98% for white LED light, 91% for UV light, 96% for solar irradiation, and 85% for blue LED light using as conditions pH 7.0, 50 mg of composite, 50 mL of solution, 10 mg/L of dye initial concentration, and 120 min of contact time. Meanwhile, the reusability experiments evidenced a reuse capacity of up to five times with a constant photodegradation efficiency (99%); moreover, it was determined that the presence of electrolytes at pH below 7.0 during degradation negatively affected methylene blue degradation. Finally, the results of this work demonstrate that the hydrochar/CeO2 composite can be synthesized by a green method and used for the efficient treatment of water contaminated with methylene blue.

Synthesis of bifunctional nanostructured adsorbents based on anionic/cationic clays: effect of arrangement on simultaneous adsorption of cadmium and arsenate.

The development of bifunctional hybrid materials based on natural clays and layered double hydroxide (LDH) and their application on the simultaneous adsorption of Cd(II) and As(V) was investigated in this work. Two different synthesis routes, in situ and assembly, were employed to obtain the hybrid materials. Three types of natural clays, namely bentonite (B), halloysite (H), and sepiolite (S), were used in the study. These clays are characterized by a laminar, tubular, and fibrous structural arrangement, respectively. The physicochemical characterization results indicate that the hybrid materials were formed through interactions between the Al-OH and Si-OH groups present in the natural clays, and the Mg-OH and Al-OH groups present in the LDH for both synthesis routes. However, the "in situ" route yields a more homogenous material because the LDH formation is performed on the natural clay surface. The hybrid materials showed an anion and cation exchange capacity up to 200.7 meq/100 g and an isoelectric point near 7. The arrangement of natural clay has no impact on the properties of hybrid material but influences the adsorption capacity. The adsorption of Cd(II) onto hybrid materials was enhanced in comparison with natural clays, obtaining adsorption capacities of 80, 74, 65, and 30 mg/g for 15:1 (LDH:H)INSITU, 1:1 (LDH:S)INSITU, 1:1 (LDH:B)INSITU, and 1:1 (LDH:H)INSITU, respectively. The adsorption capacities of hybrid materials to adsorb As(V) were between 20 and 60 µg/g. The 15:1 (LDH:H)INSITU sample showed the best adsorption capacity being ten folds greater than halloysite and LDH. In all cases, the hybrid materials showed a synergistic effect for Cd(II) and As(V) adsorption. The adsorption study of Cd(II) onto hybrid materials showed that the primary adsorption mechanism is cation exchange between the interlayer cations in natural clay and Cd(II) in the aqueous solution. The adsorption of As(V) showed that the adsorption mechanism is attributed to anion exchange between CO23- in the interlayer space of LDH and H2ASO4- in the solution. The simultaneous adsorption of As (V) and Cd (II) shows that, during the As(V) adsorption, there is no competition by the adsorption sites. Still, the adsorption capacity towards Cd(II) was enhanced 1.2-folds. This study ultimately revealed that the arrangement of clay has a significant influence on the adsorption capacity of the hybrid material. This can be attributed to the similar morphology between the hybrid material and natural clays, as well as the important diffusion effects observed in the system.

Removal of sulfamethoxazole, sulfadiazine, and sulfamethazine by UV radiation and HO• and SO4•- radicals using a response surface model and DFT calculations.

In this work, the degradation of sulfamethazine (SMT), sulfadiazine (SMD), and sulfamethoxazole (SMX) by using UV light, UV/H2O2, and UV/S2O8-2 was analyzed. Direct photolysis was studied by varying the lamp power and the solution pH. DFT calculations were carried out to corroborate the efficiency of the degradation as a function of the solution pH. The variation of the apparent rate constant, kap, was determined in the indirect photolysis by employing an experimental Box-Behnken-type response surface design. The results evidenced that SMX can be efficiently degraded by applying UV radiation independent of the operating conditions. Nevertheless, the quantum yields for SMT and SMD were close to zero, indicating a low energy efficiency for their photochemical transformation. The effect of the solution pH showed that the photodegradation of sulfonamides depends both on the amount of radiation absorbed as the electronic density. Calculations based on density functional theory and supported by the quantum theory of atoms in molecules allowed to describe fragmentation patterns in the systems under study, proving the lability of S14-C2, N17-C18, and N22-O22 bonds, for SMT, SMD, and SMX, respectively. From response surface methodology, four statistically reliable equations were obtained to determine the kap value as a function of the system operating conditions. Finally, SO4•- radicals proved to have a higher reactivity to degrade SMT and SMD compared with HO• radicals regardless of the operating conditions of the system.

Comparative study of the photodegradation of bisphenol A by HO(•), SO4(•-) and CO3(•-)/HCO3 radicals in aqueous phase.

The aim of this study was to determine the effectiveness of oxidation processes based on UV radiation (UV, UV/H2O2, UV/K2S2O8, and UV/Na2CO3) to remove bisphenol A (BPA) from aqueous solution. Results showed that UV radiation was not effective to remove BPA from the medium. The addition of radical promoters such as H2O2, K2S2O8, or Na2CO3 markedly increased the effectiveness of UV radiation through the generation of HO(•), SO4(•-), or CO3(•-)/HCO3(•) radicals, respectively. The reaction rate constants between BPA and HO(•), SO4(•-), and CO3(•-)/HCO3(•) radicals were k(HO(•)BPA)=1.70±0.21×10(10)M(-1)s(-1), k(SO4(•-)BPA)=1.37±0.15×10(9)M(-1)s(-1) and k(CO3(•-)/HCO3(•)BPA)=3.89±0.09×10(6)M(-1)s(-1), respectively. The solution pH had a major effect on BPA degradation with the UV/H2O2 system, followed by UV/K2S2O8, and UV/Na2CO3 systems. All oxidation systems in this study showed 100% effectiveness to remove BPA from wastewater, due to its large content of natural organic matter (NOM), which can absorb UV radiation and generate excited triplet states ((3)NOM*) and various reactive oxygen species. With all three systems, the total organic carbon in the medium was markedly decreased after 5 min of treatment. The toxicity of byproducts was higher than that of BPA when using UV/H2O2, similar to that of BPA with the UV/Na2CO3 system, and lower than that of BPA after 40 min of treatment with the UV/K2S2O8 system.