Solar photodegradation of Rhodamine B dye by Cu2O/TiO2 heterostructure: experimental and computational studies of degradation and toxicity.

CONTEXT AND RESULTS: In this study, the heterojunction Cu2O/TiO2 is used for the degradation of a cationic dye, Rhodamine B, under solar light irradiation over a wide pH range. The physical and optical properties of both semiconductors Cu2O and TiO2 are correlated with the photo-electrochemical characterization to establish the energy diagram of the heterojunction Cu2O/TiO2. X-ray diffraction, UV-visible, SEM, EDX, and BET analyses are conducted for both photocatalysts. The band gap (Eg) of 3.26 eV is obtained for TiO2 with an indirect optical transition. In the case of Cu2O, the transition is directly allowed at 2.05 eV. According to the BET analysis, the specific surface area of TiO2 particles is higher (82.65 m2 g-1) than that of Cu2O (29.81 m2 g-1). The flat band potentials, determined from the Mott-Schottky plots, are 0.3 and - 0.32 VSCE for TiO2 and Cu2O, respectively. The photocatalytic activity is directly proportional to the mass ratio, and the best result is obtained for the mass ratio 1:1 of Cu2O/TiO2. COMPUTATIONAL AND THEORETICAL TECHNIQUES: Furthermore, a theoretical study is conducted by using density functional theory to optimize the structure, reactivity sites of the RhB molecule, and physical parameters like the energy of the frontier molecular orbitals and electronegativity and to predict the proposed mechanism of RhB degradation as well as its intermediates. Also, molecular dynamics simulation is used to determine the adsorption behavior of RhB on TiO2 (101) and Cu2O (111) surfaces. The ecotoxicity evaluation showed that degradation products have significantly lower acute toxicity than RhB.

Cephalexin degradation initiated by OH radicals: theoretical prediction of the mechanisms and the toxicity of byproducts.

In this work, the density functional theory is used to study the local reactivity of cephalexin (CLX) to radical attack and explain the mechanism of the reaction between CLX and hydroxyl radical attack leading to degradation byproducts. The reaction between •OH and CLX is supposed to lead to either an addition of a hydroxyl radical or an abstraction of a hydrogen. The results showed that the affinity of cephalexin for addition reactions increases as it passes from the gas to the aqueous phase and decreases as it passes from the neutral to the ionized form. Thermodynamic data confirmed that OH addition radicals (Radd) are thermodynamically favored over H abstraction radicals (Rabs). The ecotoxicity assessments of CLX and its byproducts are estimated from the acute toxicities toward green algae, Daphnia, and fish. The formation of byproducts is safe for aquatic organisms, and only one byproduct is harmful to Daphnia.