Oxidative degradation of organic pollutants by activated peroxymonosulfate and high-valent metal complexes

Abstract

Providing access to clean water is one of the grand engineering challenges in the 21st century. The study in this thesis aimed to develop efficient oxidation processes for water/wastewater treatment via activating peroxymonosulfate (PMS), a novel oxidant for sulfate radical generation. To evaluate the effectiveness of the proposed novel oxidation, a series of emerging and conventional organic pollutants were selected as model compounds.

Iron is an ideal material for remediation applications, but the combination of either iron oxides or Fe2+ with PMS shows extremely low efficiencies. Firstly, experiments were carried out to improve the reactivity of structural iron. Copper-iron bimetallic oxides and iron-doped g-C3N4 were synthesized and used to activate PMS. The copper-iron bimetallic oxides had significantly greater reactivity than common activators. Strong synergy between structural Cu(I) and Fe(III) was identified. Approximately 100% degradation of phenol was achieved with Fe-g-C3N4, whereas less than 5% degradation was achieved with Fe2+ or pristine g-C3N4 under identical conditions. Mechanistic investigation showed that the O–O bond of the PMS under the activation of iron-doped g-C3N4 may undergo heterolysis, producing FeIV=O as the primary active species. Secondly, investigation was conducted to improve the efficiency of Fe2+-PMS via mitigating the scavenging effect of excess Fe2+. Structural Fe(II)-containing materials including siderite and pyrite were used as sources for Fe2+. Near-100% degradation of phenol was achieved by siderite-PMS. In contrast, only 34% and 25% of the phenol were degraded by Fe2+- and nanoscaled magnetite-persulfates, respectively. The release of Fe2+ and production of sulfate radicals were controllable. When pyrite was used as the activator, near-100% degradation of 1,4-dioxane was achieved with PMS; the degradation rates with peroxydisulfate and hydrogen peroxide were only around 50% and 15%, respectively.

Thirdly, effects were made to reduce the consumption of metals. Supported metallic palladium (Pd0) was found to have high reactivity for PMS activation and such a process did not reply on the electron donating from Pd0. The metal loading-normalized rate constant with Pd/Al2O3 was more than 16,800 times those with copper-iron bimetallic oxides. Surface-bound sulfate radicals were proposed as the dominant active species and silicon dioxide was found to be the most effective support for Pd0. Approximately 100% conversion of PMS to radicals could be achieved with the supported Pd0.

As trivalent copper was found to be involved during the activation of PMS by structural copper, the properties of two representative trivalent copper ions were characterized. The Cu3+ periodate ions had high reactivity and selectivity to degrade electron-rich antibiotics; the concentrations of sulfamethazine, sulfamethoxazole, and sulfadiazine (100 µg L−1) were reduced to 1.8, 7.5, and 42.5 ng L−1, respectively, after 2 min of reaction with 10 µM Cu3+.

The findings regarding the accelerated generation of radicals and their efficient use are expected to promote the application of advanced oxidation processes in water/wastewater treatment. Selective oxidation is important not only in water treatment but also in the other industrial processes. The high selectivity of FeIV=O and Cu3+ may provide vital insights into the development of selective oxidants.