Changling Fang

Novel Photo-Sulfite System: Toward Simultaneous Transformations of Inorganic and Organic Pollutants

An efficient and green advanced oxidation process (i.e., photo-sulfite reaction) for the simultaneous oxidation of sulfite and organic pollutants in water is reported. The photo-sulfite system (UV-Fe(III)-sulfite) is based on the Fe-catalyzed sulfite oxidation and photochemistry of Fe(III) species. SO4(•-) and (•)OH radicals were identified in the photo-sulfite system with radical scavenging experiments using specific alcohols. This novel technology was consistently proven to be more favorable than the alternative Fe(III)-sulfite systems for the degradation of 2,4,6-trichlorophenol (2,4,6-TCP) and other organic pollutants at all conditions tested. The reactivity of photo-sulfite system was sustained due to the spontaneous switch of photoactive species from Fe(III)-sulfito to Fe(III)-hydroxo complexes with the depletion of sulfite and the decrease in pH. In contrast, in the absence of light the performance of the Fe(III)-sulfite system was greatly diminished after the consumption of sulfite. The formation of the Fe(III)-sulfito complex is a necessary step for initiating the photo-sulfite reaction. Inhibition of the oxidation of 2,4,6-TCP and methyl orange (MO) was observed in the presence of ligands that can stabilize one or more of the reactants: Fe(III), Fe(II), or sulfite. Our study provides a new facile route for the generation of SO4(•-) and simultaneous removal of organic and inorganic pollutants.

Peroxymonosulfate activation by phosphate anion for organics degradation in water.

Activation of peroxygens is a critical method to generate oxidative species, but often consumes additional chemical reagents and/or energy. Here we report a novel and efficient activation reaction for peroxymonosulfate (PMS) by phosphate anions (PBS). The PBS/PMS coupled system, at neutral pH, is able to decompose efficiently even mineralize a variety of organic pollutants, such as Acid Orange 7, Rhodamine B and 2,4,6-trichlorophenol. In contrast, no measurable degradation was observed when the PMS was replaced by other peroxygens (i.e. hydrogen peroxide and peroxydisulfate). Both PMS and PBS are indispensable for the oxidative degradation of pollutants. Increasing pH and concentrations of PMS and PBS significantly accelerate the degradation of organics. It is proposed that OH would be the major radical for contamination degradation at pH 7.0 through the radical quenching experiments. This work provides a new way of PMS activation for decontamination at neutral pH, in particular for phosphate-rich wastewater treatment.

Transformations of chloro and nitro groups during the peroxymonosulfate-based oxidation of 4-chloro-2-nitrophenol

Dechlorination and denitration are known to occur during the oxidative degradation of chloronitroaromatic compounds, but the possibility of re-chlorination and re-nitration of chloro and nitro groups is not assessed despite of its importance in evaluating the applicability of advanced oxidation processes (AOPs). In this study, transformation of chloro and nitro groups in degradation of 4-chloro-2-nitrophenol (4C2NP) by sulfate radical generated via Co-mediated peroxymonosulfate activation was investigated. Both chloride and nitrate ions were found as the main inorganic products of chloro and nitro groups in 4C2NP, but their levels were much lower than that of degraded parent 4C2NP. A typical dual effect of chloride on the 4C2NP degradation kinetics was observed, whereas no measurable influence was found for addition of low level nitrate. Re-chlorination took place, but re-nitration was not verified because several polychlorophenols but none of polynitrophenols were detected. The specific degradation mechanism involved in the transformation of nitro group and chloro group was proposed.

Enhanced AOX accumulation and aquatic toxicity during 2,4,6-trichlorophenol degradation in a Co(II)/peroxymonosulfate/Cl⁻ system.

Chloride ion is known to affect on degradation kinetics in different ways during HO· and SO4(·-)-based advanced oxidation processes (AOPs). However, its effect on absorbable organic halogen (AOX) evolution and acute toxicity of treated water remains unknown, despite the importance of the two parameters in evaluating the applicability of AOPs. In the present study, Co/peroxymonosulfate (Co/PMS) and UV/hydrogen peroxide (UV/H2O2) treatment of 2,4,6-trichlorophenol was compared in terms of AOX formation, chlorinated byproducts and acute toxicity. Both Co/PMS and UV/H2O2 systems were more reactive under acidic conditions, resulting in elevated AOX levels when compared with those at neutral pH. The presence of high levels of chloride led to an accumulation and increase of AOX in the Co/PMS system. The toxicity of chlorinated byproducts was evaluated using Photobacterium phosphoreum, and the results revealed a sharp increase in acute toxicity of Co/PMS reaction solutions on addition of chloride ion. However, addition of Cl(-) had no apparent impact on AOX and toxicity of UV/H2O2 reaction solutions. These findings may have significant technical implications for selecting feasible technologies to treat high salinity wastewater.

Distinct effects of oxalate versus malonate on the iron redox chemistry: Implications for the photo-Fenton reaction

The dicarboxylic acids oxalate (Oxal) and malonate (Mal) are frequently detected as the final low-molecular-weight organic acids during oxidative degradation of aromatic compounds. Here a distinct effect of Oxal versus Mal on iron-based photocatalytic technologies was reported by testing the degradation efficiency of the dye rhodamine B. The rates of dye degradation in irradiated Fe(III) solutions depended on Fe(III/II) speciation, photoreactivities of Fe complexes and reactivities of Fe(II) complexes with H2O2. Photolysis of the Fe(III)-oxalato complex was favorable due to the formation of O2-, HO2 and OH for oxidizing the dye; however, an excess of H2O2 could quench the excited state of ferrioxalate, decreasing the degradation efficiency. In contrast, activities of UV/Fe(III) in the presence of Mal were significantly diminished because Fe(III)-Mal complexes, with much lower quantum yield of Fe(II) from photoreduction, dominated Fe(III) speciation. The results provide data for an understanding of the mechanism of iron redox (photo)chemistry mediated by diacids, which will aid in selecting appropriate Fe ligands, screening photo-Fenton conditions and designing UV/Fe(III) treatability.

Comparison of UV/hydrogen peroxide and UV/peroxydisulfate processes for the degradation of humic acid in the presence of halide ions

This study compared the behaviors of two classic advanced oxidation processes (AOPs), hydroxyl radical-based AOPs (•OH-based AOPs) and sulfate radical-based AOPs (SO4•−-based AOPs), represented by UV/ hydrogen peroxide (H2O2) and UV/peroxydisulfate (PDS) systems, respectively, to degrade humic acid (HA) in the presence of halide ions (Cl− and Br−). The effects of different operational parameters, such as oxidant dosages, halide ions concentration, and pH on HA degradation were investigated in UV/H2O2/Cl−, UV/PDS/Cl−, UV/H2O2/Br−, and UV/PDS/Br− processes. It was found that the oxidation capacity of H2O2 and PDS to HA degradation in the presence of halides was nearly in the same order. High dosage of peroxides would lead to an increase in HA removal while excess dosage would slightly inhibit the efficiency. Both Cl− and Br− would have depressing impact on the two AOPs, but the inhibiting effect of Br− was more obvious than that of Cl−, even the concentration of Cl− was far above that of Br−. The increasing pH would have an adverse effect on HA decomposition in UV/H2O2 system, whereas there was no significant impact of pH in UV/PDS process. Furthermore, infrared spectrometer was used to provide the information of degraded HA in UV/H2O2/Cl−, UV/PDS/Cl−, UV/H2O2/Br−, and UV/PDS/Br− processes, and halogenated byproducts were identified in using GC-MS analysis in the four processes. The present research might have significant technical implications on water treatment using advanced oxidation technologies.

Significantly enhanced base activation of peroxymonosulfate by polyphosphates: Kinetics and mechanism

Base activation of peroxydisulfate (PDS) is a common process aiming for water treatment, but requires high doses of PDS and strongly basic solutions. Peroxymonosulfate (PMS), another peroxygen of sulfurate derived from PDS, may also be activated by a less basic solution. However, enhancing the base-PMS reactivity is still challenging. Here it is reported that pyrophosphate (PA) and tripolyphosphate (PB) can efficiently enhance PMS activation under weakly alkaline conditions (pH 9.5) via the formation of superoxide anion radical (O2•-) and singlet oxygen (1O2). The rate constant of Acid Orange 7 (AO7) degradation in PA/PMS system (kPA/PMS) was nearly 4.4-15.9 fold higher than that in PMS/base system (kPMS/base) without any polyphosphates. Increases in PA (or PB) concentration, PMS dose and pH favored the rapid dye degradation. Gas chromatograph-mass spectrometer (GC-MS) data confirmed AO7 and 2,4,6-trichlorophenol (2,4,6-TCP) were decomposed to a series of organic intermediates. The radical quenching and probe oxidation experiments indicate the degradation of organic compounds in the PA/PMS and PB/PMS processes was not reliant on sulfate radical (SO4•-) and hydroxyl radical (OH) species but on O2- and 1O2 reactive species. Comparison experiments show that the polyphosphate/PMS process was much more favorable than PDS/base process. The present work provides a novel way to activate PMS for contaminant removal using industrial polyphosphate wastewaters.

Importance of reagent addition order in contaminant degradation in an Fe(II)/PMS system

Addition order of reagents in the Acid Orange 7 (AO7) degradation process was investigated by varying the concentration of Fe(II), the Fe(II)/peroxymonosulfate (PMS) molar ratio and stepwise addition of Fe(II) and PMS. The importance of addition order of reagents was confirmed and an order of Fe(II)–PMS to improve the oxidation efficiency was recommended.

Both degradation and AOX accumulation are significantly enhanced in UV/peroxymonosulfate/4-chlorophenol/Cl− system: two sides of the same coin?

The presence of external chloride can lead to a 47-fold increment in degradation rates of 4-chlorophenol than those in the absence of chloride in UV/peroxymonosulfate process. The other side of the same coin is an undesirable accumulation and increase in absorbable organic halogen (AOX) was observed in the presence of chloride, with formation of some more toxic tetrachlorinated byproducts.

Fe-catalyzed photoreduction of Cr(VI) with dicarboxylic acid (C2–C5): divergent reaction pathways

AbstractReduction of carcinogenic hexavalent chromium (Cr(VI)) by ferrous iron (Fe(II)) to essentially nontoxic trivalent chromium (Cr(III)) is a simple and effective method for Cr(VI) decontamination. In this study, the photoreduction efficacy of Cr(VI) in acidic Fe(III)/Fe(II) solutions was examined in the presence of dicarboxylic acids (DCA) (i.e. oxalic acid (Oxal), malonic acid (Mal), succinic acid (Suc), and glutaric acid (Glu)). Rates of Cr(VI) reduction under UV irradiation depend on the photochemistry of Fe(III)/Fe(II) complexes with DCA, following the order of Oxal > Suc ≈ Glu > Mal. Fe(III)-oxalato complexes are favorable for transforming Cr(VI) due to the formation of relative abundance of Fe(II) and reductive radicals such as , after photoreaction, whereas Fe(III)-Mal complexes upon UV irradiation generate the lowest amount of Fe(II) among all the Fe-DCA complexes, and thus have poor capacity for Cr(VI) reduction. The effects of Suc and Glu on the Cr(VI) reduction are insignificant since the ...