In
this study, the previously overlooked effects of contaminants’
molecular structure on their degradation efficiencies and dominant
reactive oxygen species (ROS) in advanced oxidation processes (AOPs)
are investigated with a peroxymonosulfate (PMS) activation system
selected as the typical AOP system. Averagely, degradation efficiencies
of 19 contaminants are discrepant in the CoCaAl-LDO/PMS system with
production of SO4
•–, •OH, and 1O2. Density functional theory calculations
indicated that compounds with high E
HOMO, low-energy gap (ΔE = E
LUMO – E
HOMO), and low vertical
ionization potential are more vulnerable to be attacked. Further analysis
disclosed that the dominant ROS was the same one when treating similar
types of contaminants, namely SO4
•–, 1O2, 1O2, and •OH for the degradation of CBZ-like compounds, SAs,
bisphenol, and triazine compounds, respectively. This phenomenon may
be caused by the contaminants’ structures especially the commonly
shared or basic parent structures which can affect their effective
reaction time and second-order rate constants with ROS, thus influencing
the contribution of each ROS during its degradation. Overall, the
new insights gained in this study provide a basis for designing more
effective AOPs to improve their practical application in wastewater
treatment.
Pyrogenic carbonaceous matter (PCM) catalyzes the transformation of a range of organic pollutants by sulfide in water; however, the mediation mechanisms are not fully understood. In this study, we observed for the first time that the degradation of azo dyes by sulfide initially underwent a lag phase followed by a fast degradation phase. Interestingly, the presence of PCM only reduced the lag phase length of the azo dye decolorization but did not significantly enhance the reaction rate in the fast degradation phase. An analysis of the azo dye reduction and polysulfide formation indicated that PCM facilitated the transformation of sulfide into polysulfides, including disulfide and trisulfide, resulting in fast azo dye reduction. Moreover, the oxygen functional groups of the PCM, especially the quinones, may play an important role in the transformation of sulfide into polysulfides by accelerating the electron transfer. The results of this study provide a better understanding of the PCM-mediated abiotic transformation of organic pollutants by sulfide in anaerobic aqueous environments.
Promotion
of iron solubility using ligands is the preliminary step in the homogeneous
electro-Fenton (EF) process at a mild pH, but the chelate efficiencies
of most organic ligands are unsatisfactory, resulting in insufficient
Fe(II) availability. In this study, atomic H* was, for the first time,
introduced to the EF process to accelerate the regeneration of the
Fe(II)-complex at a mild pH using a Ni-deposited carbon felt (Ni-CF)
cathode. The introduction of atomic H* significantly elevated total
organic carbon (TOC) abatement of ciprofloxacin (CIP) from 42% (CF)
to 81% (Ni-CF) at a natural pH. In the presence of humic acids (HAs),
atomic H* introduced via Ni-CF enhanced the CIP degradation rate to
10 times that of the CF at a mild pH. The electron spin resonance
(ESR), density functional theory (DFT) calculations, electrochemical
characterization, and in situ electrochemical Raman
study clearly demonstrated that the atomic H* generated from the Ni-CF
cathode was highly efficient at reducing Fe(III)-complexes at a natural
pH. Additionally, the Ni-CF could generate atomic H* without significant
nickel leaching. Thus, the atomic H* could continuously facilitate
iron cycling and, consequently, enhance pollutant mineralization via
the homogeneous EF process at a mild pH in an environmentally friendly
manner.
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