Yu Tian

Ubiquitous oxygen vacancies (Vo) existing in metallic compounds can activate peroxymonosulfate (PMS) for water treatment. However, under environmental conditions, especially oxygenated surroundings, the interactions between Vo and PMS as well as the organics degradation mechanism are still ambiguous. In this study, we provide a novel insight into the PMS activation mechanism over Vo-containing Fe–Co layered double hydroxide (LDH). Experimental results show that Vo/PMS is capable of selective degradation of organics via a single-electron-transfer nonradical pathway. Moreover, O2 is firstly demonstrated as the most critical trigger in this system. Mechanistic studies reveal that, with abundant electrons confined in the vacant electron orbitals of Vo, O2 is thermodynamically enabled to capture electrons from Vo to form O2•– under the imprinting effect and start the activation process. Simultaneously, Vo becomes electron-deficient and withdraws the electrons from organics to sustain the electrostatic balance and achieve organics degradation (32% for Bisphenol A without PMS). Different from conventional PMS activation, under the collaboration of kinetics and thermodynamics, PMS is endowed with the ability to donate electrons to Vo as a reductant other than an oxidant to form 1O2. In this case, 1O2 and O2•– act as the indispensable intermediate species to accelerate the circulation of O2 (as high as 14.3 mg/L) in the micro area around Vo, and promote this nano-confinement electron-recycling process with 67% improvement of Bisphenol A degradation. This study provides a brand-new perspective for the nonradical mechanism of PMS activation over Vo-containing metallic compounds in natural environments.

The nitrogen conversions in relation to NH3 and HCN were investigated during microwave pyrolysis of sewage sludge. The nitrogen distributions and evolution of nitrogen functionalities in the char, tar, and gas fractions were conducted. The results suggested that the thermal cracking of protein in sludge produced three important intermediate compounds, including the amine-N, heterocyclic-N, and nitrile-N compounds. The deamination of amine-N compounds resulted from labile proteins cracking led to the formation of NH3 (about 7.5% of SS-N) between 300 and 500 °C. The cracking of nitrile-N and heterocyclic-N compounds in the tars from the dehydrogenation and polymerization of amine-N generated HCN (6.6%) from 500 to 800 °C, respectively. Moreover, the ring-opening of heterocyclic-N in the char and tar contributed to the release of NH3 accounting for about 18.3% of SS-N with the temperature increasing from 500 to 800 °C. Specifically, the thermal cracking of amine-N, heterocyclic-N and nitrile-N compounds contributed to above 80% of the total (HCN+NH3) productions. Consequently, it might be able to reduce the HCN and NH3 emissions through controlling the three intermediates production at the temperature of 500-800 °C.

Graphitic carbon nitride (g-C3N4) can be used as a photocatalyst to reduce CO2. Doping is an efficient strategy for improving the photocatalytic activity and tuning the electronic structure of g-C3N4. The sulfur-doped g-C3N4 (S-doped g-C3N4) as a promising photocatalyst for CO2 reduction was investigated by density functional theory methods. The electronic and optical properties indicate that doping S enhances the catalytic performance of g-C3N4. From the reduction Gibbs free energies, the optimal path for CO2 reduction reaction to CH3OH production catalyzed by S-doped g-C3N4 is CO2 → COOH* → CO → HCO* → HCHO → CH3O* → CH3OH. In comparison with g-C3N4, doping S can alter the rate-determining step and reduce the Gibbs free energy from 1.43 to 1.15 eV. CO2 reduction activity of S-doped g-C3N4 is better than that of g-C3N4, which is in well agreement with the experimental results. Our work provides useful insights into designing nonmetal-doped g-C3N4 for photocatalytic CO2 reduction reactions.