Po-Heng Lee

Microbiology and potential applications of aerobic methane oxidation coupled to denitrification (AME-D) process: A review

Aerobic methane oxidation coupled to denitrification (AME-D) is an important link between the global methane and nitrogen cycles. This mini-review updates discoveries regarding aerobic methanotrophs and denitrifiers, as a prelude to spotlight the microbial mechanism and the potential applications of AME-D. Until recently, AME-D was thought to be accomplished by a microbial consortium where denitrifying bacteria utilize carbon intermediates, which are excreted by aerobic methanotrophs, as energy and carbon sources. Potential carbon intermediates include methanol, citrate and acetate. This mini-review presents microbial thermodynamic estimations and postulates that methanol is the ideal electron donor for denitrification, and may serve as a trophic link between methanotrophic bacteria and denitrifiers. More excitingly, new discoveries have revealed that AME-D is not only confined to the conventional synergism between methanotrophic bacteria and denitrifiers. Specifically, an obligate aerobic methanotrophic bacterium, Methylomonas denitrificans FJG1, has been demonstrated to couple partial denitrification with methane oxidation, under hypoxia conditions, releasing nitrous oxide as a terminal product. This finding not only substantially advances the understanding of AME-D mechanism, but also implies an important but unknown role of aerobic methanotrophs in global climate change through their influence on both the methane and nitrogen cycles in ecosystems. Hence, further investigation on AME-D microbiology and mechanism is essential to better understand global climate issues and to develop niche biotechnological solutions. This mini-review also presents traditional microbial techniques, such as pure cultivation and stable isotope probing, and powerful microbial techniques, such as (meta-) genomics and (meta-) transcriptomics, for deciphering linked methane oxidation and denitrification. Although AME-D has immense potential for nitrogen removal from wastewater, drinking water and groundwater, bottlenecks and potential issues are also discussed.

Rapid Selective Circumneutral Degradation of Phenolic Pollutants Using Peroxymonosulfate–Iodide Metal-Free Oxidation: Role of Iodine Atoms

The development of environmentally friendly, oxidation-selective advanced oxidation processes (AOPs) for water decontamination is important for resource recovery, carbon dioxide abatement, and cost savings. In this study, we developed an innovative AOP using a combination of peroxymonosulfate (PMS) and iodide ions (I-) for the selective removal of phenolic pollutants from aqueous solutions. The results showed that nearly 100% degradation of phenol, bisphenol A, and hydroquinone was achieved after reaction for 4 min in the presence of 65 μM PMS and 50 μM I-. PMS-I- oxidation had a wide effective pH range, with the best performance achieved under circumneutral conditions. The ratio between [PMS] and [I-] influenced the degradation, and the optimal ratio was approximately 1.00 for the degradation of the phenols. Neither sulfate nor hydroxyl radicals were found to be the active species in PMS-I- oxidation. Instead, we found evidence that iodide atoms were the dominant oxidants. In addition, both Cl- and Br- also promoted the degradation of phenol in PMS solution. The results of this work may promote the application of reactive halogen species in water treatment.

Surface-bound sulfate radical-dominated degradation of 1,4-dioxane by alumina-supported palladium (Pd/Al2O3) catalyzed peroxymonosulfate

Sulfate radicals have been demonstrated as an alternative to hydroxyl radicals in advanced oxidation processes. Unfortunately, the efficient activation of peroxymonosulfate (PMS), one of the most commonly used oxidants for the generation of sulfate radicals, still relies heavily on cobalt-bearing materials that are potential carcinogens. Although copper-iron bimetallic materials are promising activators, stoichiometric amounts of metals are required to achieve satisfactory performance. In this study, we propose a real catalytic process that is capable of degrading extremely recalcitrant 1,4-dioxane using a combination of alumina-supported metallic palladium (Pd/Al2O3) with PMS. The metal loading-normalized pseudo-first-order constant for 1,4-dioxane degradation with Pd/Al2O3 was more than 16,800 times that with copper-iron bimetallic materials. Complementary to Fenton reagents, Pd/Al2O3-PMS had a wide effective pH range from 4.0 to 8.5. In the absence of a substrate, PMS underwent more rapid decomposition under all conditions investigated, which suggests that its activation did not likely proceed via the previously proposed non-radical mechanism. On the basis of the strong inhibitory effects of common scavengers, we instead propose that surface-bound sulfate radicals were probably the dominant active species. A near-100% conversion rate of PMS to radicals was achieved with the Pd/Al2O3 catalyst.

Copper Sludge from Printed Circuit Board Production/Recycling for Ceramic Materials: A Quantitative Analysis of Copper Transformation and Immobilization

The fast development of electronic industries and stringent requirement of recycling waste electronics have produced a large amount of metal-containing waste sludge. This study developed a waste-to-resource strategy to beneficially use such metal-containing sludge from the production and recycling processes of printed circuit board (PCBs). To observe the metal incorporation mechanisms and phase transformation processes, mixtures of copper industrial waste sludge and kaolinite-based materials (kaolinite and mullite) were fired between 650 and 1250 °C for 3 h. The different copper-hosting phases were identified by powder X-ray diffraction (XRD) in the sintered products, and CuAl2O4 was found to be the predominant hosting phase throughout the reactions, regardless of the strong reduction potential of copper expected at high temperatures. The experimental results indicated that CuAl2O4 was generated more easily and in larger quantities at low-temperature processing when using the kaolinite precursor. Maximum copper transformations reached 86% and 97% for kaolinite and mullite systems, respectively, when sintering at 1000 °C. To monitor the stabilization effect after thermal process, prolonged leaching tests were carried out using acetic acid with an initial pH value of 2.9 to leach the sintered products for 20 days. The results demonstrated the decrease of copper leachability with the formation of CuAl2O4, despite different sintering behavior in kaolinite and mullite systems. This study clearly indicates spinel formation as the most crucial metal stabilization mechanism when sintering copper sludge with aluminosilicate materials, and suggests a promising and reliable technique for reusing metal-containing sludge as ceramic materials.

Detoxification and immobilization of chromite ore processing residue in spinel-based glass-ceramic

A promising strategy for the detoxification and immobilization of chromite ore processing residue (COPR) in a spinel-based glass-ceramic matrix is reported in this study. In the search for a more chemically durable matrix for COPR, the most critical crystalline phase for Cr immobilization was found to be a spinel solid solution with a chemical composition of MgCr1.32Fe0.19Al0.49O4. Using Rietveld quantitative X-ray diffraction analysis, we identified this final product is with the phases of spinel (3.5wt.%), diopside (5.2wt.%), and some amorphous contents (91.2wt.%). The partitioning ratio of Cr reveals that about 77% of the Cr was incorporated into the more chemically durable spinel phase. The results of Cr K-edge X-ray absorption near-edge spectroscopy show that no Cr(VI) was observed after conversion of COPR into a glass-ceramic, which indicates successful detoxification of Cr(VI) into Cr(III) in the COPR-incorporated glass-ceramic. The leaching performances of Cr2O3 and COPR-incorporated glass-ceramic were compared with a prolonged acid-leaching test, and the results demonstrate the superiority of the COPR-incorporated glass-ceramic matrix in the immobilization of Cr. The overall results suggest that the use of affordable additives has potential in more reliably immobilizing COPR with a spinel-based glass-ceramic for safer disposal of this hazardous waste.

The mechanism study of efficient degradation of hydrophobic nonylphenol in solution by a chemical-free technology of sonophotolysis

Nonylphenol is a hydrophobic endocrine disrupting compound, which can inhibit the growth of sewage bacteria in biological processes. This study investigated the degradation of 4-n-nonylphenol (NP) in water by a chemical-free technology of sonophotolysis with emphasis on the impacts of several important parameters, including light intensity, solution pH, two commonly seen inorganic ions (i.e. NO3(-) and HCO3(-)), and principally on the examination of degradation mechanisms. It was found that, solution pH could significantly influence both NP degradation efficiency and the synergistic effect of sonophotolytic process, where higher synergistic effect was obtained at more acidic condition. In addition, the presence of NO3(-) accelerated NP degradation by both acting as a photosensitizer and providing NO2˙ radicals, while HCO3(-) had little effect on NP degradation. Identification of intermediates of NP degradation indicated that NP sonophotolysis was mainly initiated by the formation of hydroxy-NP, and a new intermediate di-hydroxy-NP was identified for the first time ever in this study. Through thermodynamic analysis, results indicated that both ortho- and meta-hydroxy-NP species can coexist in the solution but the ortho-4-NBZQ (4-nonyl-benzoquinone) is dominant. In addition, the mechanism of ortho-hydroxy-NP formation was suggested by the addition of HO˙ and H˙ radicals.

Degradation of contaminants by Cu+-activated molecular oxygen in aqueous solutions: Evidence for cupryl species (Cu3+)

Copper ions (Cu2+ and Cu+) have shown potential as Fenton-like activators for the circumneutral removal of organic contaminants from aqueous solutions. However, the major active species (cupryl species (Cu3+) versus hydroxyl radical (OH)) produced during the activation of hydrogen peroxide by Cu+ remain unclear. In this study, Cu+-O2 oxidation, in which hydrogen peroxide is produced via the activated decomposition of dissolved molecular oxygen, was used to degrade sulfadiazine, methylene blue, and benzoic acid. The results showed that both sulfadiazine and methylene blue could be efficiently degraded by Cu+-O2 oxidation in a wide effective pH range from 2.0 to 10.0. Quenching experiments with different alcohols and the effect of Br- suggested that Cu3+ rather than OH was the major active species. Electron paramagnetic resonance detected 5,5-dimethyl-2-hydroxypyrrolidine-N-oxyl (DMPO-OH), which was probably produced by the oxidation of DMPO by Cu3+ or OH formed as a product of Cu3+ decomposition. 4-hydroxybenzoic acid was produced during the degradation of benzoic acid by Cu3+. The findings of this study may help to explain the inconsistency regarding the dominant active species produced by the interaction of Cu+ and hydrogen peroxide.

Facile synthesis of highly reactive and stable Fe-doped g-C 3 N 4 composites for peroxymonosulfate activation: A novel nonradical oxidation process

Ferrous ions (Fe2+) are environmentally friendly materials but show extremely inefficient persulfate activation. Polymeric graphitic carbon nitride (g-C3N4) has recently shown potential to activate persulfates, but the process requires intense light irradiation. To overcome these drawbacks, we designed an innovative heterogeneous iron catalyst by doping Fe into g-C3N4 (Fe-g-C3N4) and used it to activate peroxymonosulfate (PMS) for degradation of pollutant phenol. The catalysts synthesized were fully characterized with various techniques, such as X-ray diffraction, Mössbauer spectroscopy, and X-ray photoelectron spectroscopy. Fe was found to be coordinated with the framework of g-C3N4. Approximately 100% degradation of phenol was achieved with Fe-g-C3N4 after 20 min of reaction, whereas less than 5% degradation of phenol was achieved with Fe2+. Fe-g-C3N4-PMS had a wide effective pH range, and its reactivity was nearly independent of natural illumination. In contrast to the previously proposed radical mechanisms, quenching experiments revealed that nonradical oxidation contributed to the observed degradation. The OO bond in the activated PMS likely underwent heterolysis, producing high-valence iron species (FeIVO) as the primary active species. These findings have important implications for the development of a selective heterogeneous nonradical-oxidation process.

Transformation of hazardous lead into lead ferrite ceramics: Crystal structures and their role in lead leaching

This study quantitatively determined the transformation of lead into lead ferrite ceramics and examined the influence of structural defects in lead ferrites (i.e. Pb2Fe2O5, PbFe4O7 and PbFe12O19) on lead leaching. Mechanisms of metal incorporation were examined from quantifying the phase compositions of lead ferrites in the products of sintering lead oxide with hematite. At low-temperature of 700°C, Pb was preferentially incorporated into the Pb2Fe2O5 crystals, and the incorporation efficiency ranged from 25.7 to 97.5% depending on different Pb/Fe molar ratios. By increasing temperatures to 750-850°C, Pb2Fe2O5 was subsequently reacted with hematite for the formation of PbFe4O7 and PbFe12O19 in Pb/Fe of 1/4 and 1/12 systems. PbFe12O19 was found to be the high-temperature (1000°C) stable phase for incorporating lead, and the incorporation efficiency ranged from 28.6 to 92.1% by different Pb/Fe molar ratios. Leaching tests demonstrated that PbFe12O19 was more resistant to acid attack than Pb2Fe2O5 and PbFe4O7. The crystal structural defects in Pb2Fe2O5 and PbFe4O7 were determined to be the factors influencing their intrinsic phase durability. On the other hand, PbFe12O19 was relatively free of structural defects and was found to be the preferred stabilization product to reduce the environmental hazard posed by lead.

Mineralization of perfluorooctanesulfonate (PFOS) and perfluorodecanoate (PFDA) from aqueous solution by porous hexagonal boron nitride: adsorption followed by simultaneous thermal decomposition and regeneration

Poly- and perfluoroalkyl substances (PFASs) are of global concern due to their toxicity, high persistency, bioaccumulation, and worldwide occurrence. Boron nitride (BN), consisting of light elements and bearing excellent thermal stability, has shown great potential in wastewater purification as a readily-recyclable sorbent. In this study, porous hexagonal BN nanosheets (h-BNs) were synthesized and for the first time their sorption capacities toward perfluorooctanesulfonate (PFOS) and perfluorodecanoate (PFDA) (two representative PFASs) were evaluated under various solution compositions. The h-BNs used after sorption were regenerated by calcining at 600 °C in air for 20 min. The h-BNs synthesized were found to have fast sorption kinetics for both PFOS and PFDA, and the sorption processes fitted well with the Freundlich model and pseudo-second-order kinetics. Under the conditions of 50 mg L−1 PFDA or PFOS, 0.2 g L−1 h-BNs, and pH 6.0, sorption capacities of ∼0.72 mg m−2 and ∼0.45 mg m−2 were achieved for PFDA and PFOS, respectively. The effects of H+ and Ca2+ showed that electrostatic interactions were responsible for the sorption. The reutilization experiments revealed that the h-BNs had a persistent sorption capacity after three cycles. To reduce the production of fluorine-containing gases, calcium hydroxide was used as a calcination additive and the fluorine-fixing product calcium fluoride was successfully detected. The results suggest that h-BN sorption may be a promising approach for the removal of PFASs from an aqueous solution.