Yaoguang Guo

Effects of chloride ions on bleaching of azo dyes by Co2+/Oxone regent: kinetic analysis

Orange II (Org II), one of the most common used azo dyes, was taken as a model to investigate the effects of chloride ion on dye decoloration in cobalt/peroxymonosulfate (Co/PMS) system. A significant decrease in the rate of Org II decoloration was observed upon addition of Cl(-) (0.05-10mM), but further addition of Cl(-) (>50mM) apparently accelerated dyes degradation. This dual effect of chloride on dyes bleaching was also observed as other halide ions (e.g. Br(-), I(-)) or other azo dyes were present in Co/PMS system. In the Co-free PMS solutions, the observed first-order rate constant always exponentially increased with the chloride content. The reactive chlorine species generated from chloride oxidation by PMS should be responsible for this non-radical mechanism for dye decoloration, however, these rapid decoloration of Org II as chloride ion was present, did not readily lead to much mineralization. Therefore, this finding may have significant technical implications for utilizing Co/PMS regent to detoxify chloride-rich azo dyes wastewater.

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.

Sequential reduction-oxidation for photocatalytic degradation of tetrabromobisphenol A: kinetics and intermediates.

C-Br bond cleavage is considered as a key step to reduce their toxicities and increase degradation rates for most brominated organic pollutants. Here a sequential reduction/oxidation strategy (i.e. debromination followed by photocatalytic oxidation) for photocatalytic degradation of tetrabromobisphenol A (TBBPA), one of the most frequently used brominated flame retardants, was proposed on the basis of kinetic analysis and intermediates identification. The results demonstrated that the rates of debromination and even photodegradation of TBBPA strongly depended on the atmospheres, initial TBBPA concentrations, pH of the reaction solution, hydrogen donors, and electron acceptors. These kinetic data and byproducts identification obtained by GC-MS measurement indicated that reductive debromination reaction by photo-induced electrons dominated under N(2)-saturated condition, while oxidation reaction by photoexcited holes or hydroxyl radicals played a leading role when air was saturated. It also suggested that the reaction might be further optimized for pretreatment of TBBPA-contaminated wastewater by a two-stage reductive debromination/subsequent oxidative decomposition process in the UV-TiO(2) system by changing the reaction atmospheres.

Leaching of heavy metals from Dexing copper mine tailings pond

The wastewater source of 4# tailing pond in Dexing copper mine consists of alkaline flotation pulp and acid mine drainage (AMD) from the nearby opencast mine. Therefore, the heavy metals in tailing ore are very likely to be released due to acidification from AMD. The leaching behaviors of Zn, Cu, Fe and Mn in mine tailings from Dexing copper mine were investigated by a series of laboratory batch experiments. The effectcs of pH, temperature, particle size and contact time on the leachability of such heavy metals were examined. It was evident that Zn, Cu, Fe and Mn were major heavy metals in the tailings while gangue minerals like quartz were major constituents in examined tailings. The tailing dissolution reaction was controlled by the acid, whose kinetics could be expressed according to the heterogeneous reaction models and explained by a shrinking core model with the surface chemical reaction as the rate-controlling step. The leachability of all metals examined depended on pH and contact time. The batch studies indicated that the maximum leaching ratios of Zn, Cu, Fe and Mn at pH 2.0 were 5.4%, 5.8%, 11.1% and 34.1%, respectively. The dissolubility of all metals examined was positively correlated to the temperatures. The particle size would not change dissolution tendency of those heavy metals, but decrease the concentrations of leached heavy metals.

Iron species in layered clay: efficient electron shuttles for simultaneous conversion of dyes and Cr(VI).

The simultaneous reduction of Chromium(VI) (Cr(VI)) and oxidation of cationic dyes in dispersions of Montmorillonite K10 (MK10) were examined under visible irradiation (λ>420 nm). The iron species (i.e. iron oxides, structural iron and exchangeable interlayer iron) in layered clays are active for catalytically reducing Cr(VI) by using Malachite green (MG) and Rhodamine B (RhB) as the electron donors. Molecular oxygen does not have a significant effect on clay-catalyzed Cr(VI) reduction, but is important for oxidative degradation of dye pollutants. MK10 catalysts are stable and reusable, and are therefore considered as a promising naturally-abundant material for decontamination of dye and heavy metals.

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.

A novel photosensitized Fenton reaction catalyzed by sandwiched iron in synthetic nontronite

The conventional photo-Fenton reaction often suffers from the constraints of operation pH, low iron loading, ultraviolet availability in solar light and instability of iron-based catalysts. Here we report a novel heterogeneous Fenton reaction which works with a dye-photosensitized structural Fe(III)/Fe(II) redox cycling mechanism. The synthesized nontronite catalyst (NAU) was characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared spectra (FTIR), X-ray photoelectron spectroscopy (XPS) analysis, and thermal gravimetric analysis (TG). NAU exhibited excellent catalytic activity over a wide pH range (3.0–8.0) for highly efficient degradation of Rhodamine B by hydrogen peroxide (H2O2) under visible light irradiation (λ > 420 nm). The excited dye molecule donates electrons to structural iron sandwiched in NAU which further catalyzes H2O2 to generate highly reactive ˙OH radicals. This iron-rich clay mineral (total Fe, 24.4 wt%) is chemically and mechanically stable. There are no measurable iron leaching, nor any noticeable loss of activity and damage to the clay structure observed after 6 recycles. Therefore, NAU clay has outstanding merits for the practical treatment of organic dye pollutants at large scale.

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.