Jiahai Ma

Optical Properties of Humic Substances and CDOM: Effects of Borohydride Reduction

Treatment of Suwanee River humic (SRHA) and fulvic (SRFA) acids, a commercial lignin (LAC), and a series of solid phase extracts (C18) from the Middle Atlantic Bight (MAB extracts) with sodium borohydride (NaBH(4)), a selective reductant of carbonyl-containing compounds including quinones and aromatic ketones, produces a preferential loss of visible absorption (> or = 50% for SRFA) and substantially enhanced, blue-shifted fluorescence emission (2- to 3-fold increase). Comparison of the results with those obtained from a series of model quinones and hydroquinones demonstrates that these spectral changes cannot be assigned directly to the absorption and emission of visible light by quinones/hydroquinones. Instead, these results are consistent with a charge transfer model in which the visible absorption is due primarily to charge transfer transitions arising among hydroxy- (methoxy-) aromatic donors and carbonyl-containing acceptors. Unlike most of the model hydroquinones, the changes in optical properties of the natural samples following NaBH(4) reduction were largely irreversible in the presence of air and following addition of a Cu(2+) catalyst, providing tentative evidence that aromatic ketones (or other similar carbonyl-containing structures) may play a more important role than quinones in the optical properties of these materials.

Strongly enhanced Fenton degradation of organic pollutants by cysteine: An aliphatic amino acid accelerator outweighs hydroquinone analogues.

Quinone-hydroquinone analogues have been proven to be efficient promoters of Fenton reactions by accelerating the Fe(III)/Fe(II) redox cycle along with self-destruction. However, so far there is little information on non-quinone-hydroquinone cocatalyst for Fenton reactions. This study found that cysteine, a common aliphatic amino acid, can strongly enhance Fenton degradation of organic pollutants by accelerating Fe(III)/Fe(II) redox cycle, as quinone-hydroquinone analogues do. Further, cysteine is superior to quinone-hydroquinone analogues in catalytic activity, H2O2 utilization and atmospheric limits. The cocatalysis mechanism based on the cycle of cysteine/cystine was proposed.

An Efficient Anthraquinone–Resin Hybrid Co-Catalyst for Fenton-Like Reactions: Acceleration of the Iron Cycle Using a Quinone Cycle under Visible-Light Irradiation

The Fenton reaction (H2O2+Fe /Fe) has been widely used to oxidize organic compounds for various purposes, such as DNA damage by free radicals, organic or inorganic synthesis, and environmental detoxication. As a versatile catalysis system, Fenton-like reactions have particular features: readily available and inexpensive reagents, extremely large rate constants (ca. 10 m 1 s ), and a facile procedure. The generally accepted free-radical-chain mechanism for the Fenton reactions is shown below, and the slow reaction [Eq. (2)] is the rate-determining step.

Reduction-induced molecular signature of humic substances: structural evidence for optical changes

The redox behavior of humic substances (HS) has drawn significant attention. It has been recently reported that aquatic HS after NaBH4 reduction exhibit a loss in UV absorption but enhanced and blue-shifted fluorescence emission. This unique property was proven to also apply to terrestrial HS in the current study. To further understand the underlying relationship between the molecular structural changes and the optical changes, multiple techniques were employed to obtain solid spectroscopic evidence for Aldrich humic acid. Attenuated total reflectance infrared spectroscopy, Raman spectroscopy and solid-state 13C nuclear magnetic resonance spectroscopy were used together to demonstrate that the alcohol-like moieties increased while the carbonyl moieties decreased in HS molecules during reduction, which is responsible for optical properties. In addition, the obtained results could explain the change in molecular size as evidenced by dynamic light scattering. The integrated results presented in this study unambiguously provide solid spectroscopic evidence for the charge transfer model, which is a relatively new concept framework for understanding HS.

Aqueous phototransformation of bisphenol S: the competitive radical-attack pathway to p-hydroxybenzenesulfonic acid

The kinetics, environmental influencing factors, products and reaction mechanism of aqueous phototransformation of bisphenol S (BPS), as an alternative to bisphenol A, which is of environmental concern, were investigated. p-Hydroxybenzenesulfonic acid, as the major transformation product was confirmed by gas chromatography - mass spectrometry, electrospray ionization, ¹H nuclear magnetic resonance and fluorescence spectrum analysis. A reaction pathway was proposed based on the reactive oxygen species related results by electron paramagnetic resonance and radical traps. The competition of the excited state of BPS between transferring electron to O₂ to •O₂(-) and directly oxidizing H₂O to •OH was revealed.

Fluorescence anisotropy as a means to determine extracellular polysaccharide hydrolase activity in environmental samples

Current approaches to measure the activities of microbial extracellular enzymes in aquatic environments are hampered by slow throughput or by differences between the structure of simple substrate proxies and macromolecules. Here we show that measurements of fluorescence anisotropy can be used to determine the hydrolysis rate of two fluorescently labeled polysaccharides, laminarin and xylan, in environmental samples. A simple analysis shows that the anisotropy of these fluorescently labeled polysaccharides can be approximated using a modification of the Perrin equation.

Contribution to the reduction-induced fluorescence enhancement of natural organic matter: Aromatic ketones outweigh quinones

This work involves the comparison of the fluorescence excitationxa0-xa0emission matrices of different low-molecular-weight carbonyl compounds and natural organic matter (NOM). The aim is to determine if quinone or aromatic ketone groups are more responsible for the reduction-induced fluorescence enhancement of NOM. After reduction, the aromatic ketones showed a significantly greater fluorescence change than the quinones, proving that the former play a more important role. Further analysis of the fluorescence of the NOM samples after re-oxidization by oxygen with a Cu2+ catalyst, provided additional reliable evidence in support of the dominant role of aromatic ketones in the fluorescence change. This work demonstrates that aromatic ketone moieties should be given more attention when considering the physicochemical properties of NOM and related environmental processes.