Control of the Exciton Radiative Lifetime in van der Waals Heterostructures.

Optical properties of atomically thin transition metal dichalcogenides are controlled by robust excitons characterized by a very large oscillator strengths. Encapsulation of monolayers such as MoSe_{2} in hexagonal boron nitride (hBN) yields narrow optical transitions approaching the homogenous exciton linewidth. We demonstrate that the exciton radiative rate in these van der Waals heterostructures can be tailored by a simple change of the hBN encapsulation layer thickness as a consequence of the Purcell effect. The time-resolved photoluminescence measurements show that the neutral exciton spontaneous emission time can be tuned by one order of magnitude depending on the thickness of the surrounding hBN layers. The inhibition of the radiative recombination can yield spontaneous emission time up to 10 ps. These results are in very good agreement with the calculated recombination rate in the weak exciton-photon coupling regime. The analysis shows that we are also able to observe a sizable enhancement of the exciton radiative decay rate. Understanding the role of these electrodynamical effects allows us to elucidate the complex dynamics of relaxation and recombination for both neutral and charged excitons.

Microbial characterization and quantification of an anaerobic sludge degrading dimethyl phthalate.

AIMS: Characterization and quantification of microbial community in dimethyl phthalate (DMP)-degrading anaerobic sludge using molecular techniques. METHODS AND RESULTS: An enriched anaerobic sludge effectively degrading over 99% of dimethyl phthalate in an upflow anaerobic sludge blanket (UASB) reactor for 530 days was characterized and quantified by 16S rRNA-based molecular methods. A total of 78 Bacteria clones were classified into 22 operational taxonomic units (OTUs) in nine divisions, including Firmicutes, Proteobacteria, Chloroflexi, Thermotogae, Bacteroidetes/Chlorobi, Spirochaetes, Acidobacteria and two candidate divisions. The two most abundant OTUs were likely responsible, respectively, for the de-esterification of DMP and the subsequent phthalate degradation. The outer layer of the granule was dominated by Bacteria; whereas the interior was by Archaea, of which 89 +/- 5% were acetoclastic Methanosaetaceae and 11 +/- 5% hydrogenotrophic Methanomicrobiales. CONCLUSIONS: Twenty-two Bacteria OTUs in DMP-degrading anaerobic sludge distributed in nine divisions. The two most abundant OTUs were likely responsible respectively for the de-esterification of DMP and the subsequent phthalate degradation. Layered granular microstructure of DMP-degrading anaerobic sludge suggested that the rate of DMP de-esterification is faster than its inward diffusion rate. SIGNIFICANCE AND IMPACT OF THE STUDY: This work is the first study to characterize and quantify the microbial community in the anaerobic phthalic ester degrading sludge from the anaerobic reactor.

Occurrence of ammonia-oxidizing Archaea in activated sludges of a laboratory scale reactor and two wastewater treatment plants.

AIMS: Characterization of the ammonia-oxidizing archaea (AOA) community in activated sludge from a nitrogen removal bioreactor and wastewater treatment plants (WWTPs). METHODS AND RESULTS: Three primer sets specific for ammonia mono-oxygenase alpha-subunit (amoA) were used to construct clone libraries for activated sludge sample from a nitrogen removal bioreactor. One primer set resulted in strong nonspecific PCR products. The other two clone libraries retrieved both shared and unique AOA amoA sequences. One primer set was chosen to study the AOA communities of activated sludge samples from Shatin and Stanley WWTPs. In total, 18 AOA amoA sequences were recovered and compared to the previous reported sequences. A phylogenetic analysis indicated that sequences found in this study fell into three clusters. CONCLUSIONS: Different primers resulted in varied AOA communities from the same sample. The AOA found from Hong Kong WWTPs were closely similar to those from sediment and soil, but distinct from those from activated sludge in other places. A comparison of clone libraries between Shatin WWTP and bioreactor indicated the AOA community significantly shifted only after 30-day enrichment. SIGNIFICANCE AND IMPACT OF THE STUDY: This study confirmed the occurrence of AOA in a laboratory scale nitrogen removal bioreactor and Hong Kong WWTPs treating saline or freshwater wastewater. AOA communities found in this study were significantly different from those found in other places. To retrieve diverse AOA communities from environmental samples, a combination of different primers for the amoA gene is needed.

A qRT-PCR-based method for the measurement of rrn operon copy number.

AIM: To develop a convenient and accurate method for estimating the rrn operon copy number (Y(rrn)) in cells of pure prokaryotic cultures based on quantitative real-time polymerase chain reaction (qRT-PCR). METHODS & RESULTS: Using Escherichia coli, the Y(rrn) of which is known to be 7, as a reference, the rrn concentrations of target species and E. coli in sample solutions were measured based on their respective threshold cycle numbers (C(t)), whereas the cell concentrations of both species were measured by microscopic counting after staining. The Y(rrn) of the target species was then calculated from the initial cell concentrations and the rrn concentrations of the target species and E. coli. Using this method, the Y(rrn) values of four species, i.e. Xanthomonas campestris, Staphylococcus aureus, Aeromonas hydrophila and Pseudomonas fluorescens, were estimated as 1.80, 4.73, 8.58 and 5.13, respectively, comparable to their respective known values of 2, 5, 10, and 5, resulting in an average deviation of 8%. CONCLUSIONS: The whole cell qRT-PCR based methods were convenient, accurate and reproducible in quantification of rrn copy number of prokaryotic cells. SIGNIFICANCE AND IMPACT OF THE STUDY: qTR-PCR is a fast and reliable DNA quantification approach. Compared with previous qTR-PCR based methods measuring rrn copy number, the present method avoided the prerequisite for the information on genome size and GC content of target bacteria or a gene with known copy number, thus should be more widely applicable.

Distribution coefficients of dietary sugars in artificial Candida biofilms.

Candida species are the most important fungal pathogens in humans and cause a variety of superficial and systemic diseases. Biofilm formation is a major virulence attribute contributing to Candida pathogenicity. Although the concentration and distribution of nutrients as well as antifungals across the biofilm thickness play a pivotal role in the development and persistence of Candida biofilms, only limited information is available on the latter aspects of Candida biofilms. Therefore, we attempted to characterize the diffusion coefficient (De) of common dietary sugars such as glucose, galactose, and sucrose in Candida albicans biofilms using horizontal attenuated total reflection-Fourier transform infrared spectroscopy (HATR-FTIR). Artificial Candida biofilms were formed using agarose polymers. De of three sugars tested, glucose, galactose, and sucrose in this artificial Candida biofilm model was found to be 4.08E-06 +/- 3.63E-08, 4.08E-06 +/- 3.70E-08, and 5.38E-06 +/- 4.52E-08 cm(2) s(-1), respectively. We demonstrate here the utility of HATR-FTIR for the determination of diffusion of solutes such as dietary sugars across Candida biofilms.

Predictions of nitrate diffusion in sediment using horizontal attenuated total reflection (HATR) by Fourier transform infrared (FTIR) spectrometry.

Using a novel and simple method based on horizontal attenuated total reflection (HATR) by Fourier transform infrared (FTIR) spectrometry, the effective diffusion coefficient, De, of nitrate in a contaminated anthropogenic sediment was estimated as 7.34 x 10(-6)cm2 s(-1). This method, which requires as little as 1 mL of sediment sample, was able to measure the De of a chemical species with a reproducibility of +/-3% in about 5h. Based on this De and a pre-determined nitrate reduction rate, the profiles of nitrate concentration in two sediment columns were satisfactorily predicted from a mathematical model. Results showed that the profile in this aged sediment depended mainly on the diffusion of nitrate and, only to a much lesser degree, the rate of nitrate reduction. Measurements in 55 anthropogenic sediment samples collected from five locations and various depths of a contaminated site further showed that the De of nitrate increased linearly with the water content of the sediment, but decreased with the sediment density. The technique demonstrated in this study shall be applicable for the risk assessment of toxic pollutants in contaminated sediments, and for planning the spatial and time intervals of nitrate injection strategy in bioremediation.

Real-time quantifications of dominant anaerobes in an upflow reactor by polymerase chain reaction using a TaqMan probe.

This study was conducted to demonstrate the application of quantitative real-time polymerase chain reaction (qRT-PCR) for the quantification of dominant bacteria in an anaerobic reactor using a designed TaqMan probe. A novel group of uncultured thermophilic bacteria affiliated with Thermotogales was first found in a phenol-degrading sludge from a 55 degrees C upflow anaerobic sludge blanket (UASB) reactor, which effectively removed 99% of phenol at loading of 0.51 g-phenol l(-1) d(-1) h of hydraulic retention. A TaqMan probe was then designed targeting this group of Thermotogales affiliated bacteria (TAB), and used to monitor its concentration in the reactors. Results showed that the TAB population in the 55 degrees C reactor increased proportional to the phenol degrading rate. Results also showed that the TAB population ranged 3.5-9.9% in the 55 degrees C phenol-degrading sludge, but only 0.0044% in the 37 degrees C sludge and 0.000086% in the 26 degrees C sludge.

Anaerobic treatment of phenol in wastewater under thermophilic condition.

Over 99% of phenol was effectively degraded in an upflow anaerobic sludge blanket (UASB) reactor at 55 degrees C with 40 h of hydraulic retention time (HRT) for a wastewater containing 630 mg/L of phenol, corresponding to 1500 mg/L of chemical oxygen demand (COD) and a loading rate of 0.9 g-COD/L/d. The maximum specific methanogenic activity (SMA) of the phenol-degrading sludge was 0.09 g-CH4-COD/g-volatile suspended solids (VSS)/d. Based on 16S rDNA analysis, a total of 21 operational taxonomy units (OTUs) were found in the sludge, of which eight (42.6% of the total population) were related to the sequences in the GenBank with similarity of over 97%, and 13 (79.6%) were affiliated with the known thermophilic species. Additional SMA data and phylogenetic analysis suggest that the degradation pathway of phenol for thermophilic sludge was likely via caproate, instead of benzoate as for the mesophilic sludge.

Fermentative hydrogen production in packed-bed and packing-free upflow reactors.

Fermentative hydrogen production from a synthetic wastewater containing 10 g/L of sucrose was studied in two upflow reactors at 26 degrees C for 400 days. One reactor was filled with packing rings (RP) and the other was packing free (RF). The effect of hydraulic retention time (HRT) from 2 h to 24 h was investigated. Results showed that, under steady state, the hydrogen production rate significantly increased from 0.63 L/L/d to 5.35 L/L/d in the RF when HRT decreased from 24 h to 2 h; the corresponding rates were 0.56 L/L/d to 6.17 L/L/d for the RP. In the RF, the hydrogen yield increased from 0.96 mol/mol-sucrose at 24 h of HRT to the maximum of 1.10 mol/mol-sucrose at 8 h of HRT, and then decreased to 0.68 mol/mol-sucrose at 2 h. In the RP, the yield increased from 0.86 mol/mol-sucrose at 24 h of HRT to the maximum of 1.22 mol/mol-sucrose at 14 h of HRT, and then decreased to 0.78 mol/mol-sucrose at 2 h. Overall, the reactor with packing was more effective than the one free of packing. In both reactors, sludge agglutinated into granules. The microbial community of granular sludge in RP was investigated using 16S rDNA based techniques. The distribution of bacterial cells and extracellular polysaccharides in hydrogen-producing granules was investigated by fluorescence-based techniques. Results indicated that most of the N-acetyl-galactosamine/galactose-containing extracellular polysaccharides were distributed on the outer layer of the granules with a filamentous structure.

Anaerobic treatment of Hong Kong leachate followed by chemical oxidation.

Landfill leachate of Hong Kong was first treated by the upflow anaerobic sludge blanket (UASB) process. At 37 degrees C, pH 7.1-8.5 and a HRT of 5.1-6.6 days, the process removed 66-90% of COD in the leachate for loading rates of 1-2.4 g-COD/l day depending on the strength of landfill leachate. The final effluent contained 1440-1910 mg-COD/l and 70-91 mg-BOD/l. About 92.5% of the total COD removed was converted to methane and the rest was converted to biomass with an average net growth yield of 0.053 g-VSS/g-COD-removed. The granules developed in the UASB reactor were 0.5-1.5 mm in size and exhibited good settleability. The UASB effluent was then further polished by two oxidation processes. The UASB-ozonation process removed 93.0% of the 12900 mg/l of COD from the raw leachate. The UASB-Fenton-ozonation process improved the COD removal efficiency to 99.3%. The final effluent had only 85 mg/l of COD and 10 mg/l of BOD5. Ozonation was most effectively conducted at pH 7-8 with the addition of 300 mg/l of H2O2 and for the duration of 30 min. Ozonation also significantly improved the biodegradability of the organic residues. Nearly 50% of these residues could be used as carbon source in denitrification.

Effective diffusion coefficients of glucose in artificial biofilms.

A technique using horizontal attenuated total reflection (HATR) by Fourier transform infrared (FTIR) spectrometry was demonstrated for the determination of effective diffusion coefficient (De) of an inert solute in biofilm. Glucose was the selected solute, and agarose (1%) hydrogel films containing various concentrations of activated sludge biomass were used to simulate the biofilm. The agarose films were formed on the surface of an internal reflection crystal in contact with a bulk solution containing 0.5M glucose. Glucose molecules diffused through the film by concentration gradient, and the glucose concentration at the biofilm-crystal interface was measured over time by HATR-FTIR. Based on the glucose concentration and film thickness, the De of glucose was calculated according to the Fick's Law. Results showed that De of glucose in the biomass-free agarose films averaged 6.46 +/- 0.21 x 10(6) cm(2) s(-1), which is about 5% lower than the average reported De of glucose in water. The De of glucose decreased with increasing biomass concentration in the artificial biofilm. For the agarose films containing 0.45%, 0.90% and 1.80% of biomass, the De of glucose were lowered to 6.38 +/- 0.22 x 10(-6), 6.08 +/- 0.23 x 10(-6) and 5.62 +/- 0.17 x 10(4) cm(2) s(-1), respectively.

Quantification of bacterial adhesion forces using atomic force microscopy (AFM).

This study demonstrated that atomic force microscopy (AFM) can be used to obtain high-resolution topographical images of bacteria, and to quantify the tip-cell interaction force and the surface elasticity. Results show that the adhesion force between the Si3N4 tip and the bacteria surface was in the range from -3.9 to -4.3 nN. On the other hand, the adhesion forces at the periphery of the cell-substratum contact surface ranged from -5.1 to -5.9 nN and those at the cell-cell interface ranged from -6.5 to -6.8 nN. The two latter forces were considerably greater than the former one, most likely due to the accumulation of extracellular polymer substance (EPS). Results also show that the elasticity varied on the cell surface.

Acidification of mid- and high-strength dairy wastewaters.

Batch and continuous experiments were conducted to study the influence of dairy wastewater strength (2-30g-COD/L) on acidification at pH 5.5 and 37 degrees C. Results of batch experiments showed that carbohydrate was preferentially acidified as compared to protein and lipid. Production of VFAs (mainly acetate, propionate and butyrate) and hydrogen corresponded to carbohydrate acidification, whereas production of alcohols (mainly ethanol, propanol and butanol), plus i-butyrate and higher molecular-weight VFAs, corresponded to protein acidification. In treating high-strength wastewaters (8-30 g-COD/ L), acetate, butyrate and P(H2) decreased after reaching their peak levels before leveling off. Results of continuous experiments with 12h of hydraulic retention showed that acidification decreased with the increase of wastewater COD, from 57.1% at 2 g-COD/L to 28.8% at 30 g-COD/L; among the constituents in dairy wastewater, 92-99% of carbohydrates, 59-85% of protein and 12-42% of lipid were acidified. High-strength wastewater favored production of hydrogen and alcohols, especially propanol and butanol. The biomass yield was 0.258g-VSS/g-COD.

The roles of calcium in sludge granulation during UASB reactor start-up.

Six upflow anaerobic sludge blanket (UASB) reactors were concurrently operated for 146 d to examine the effects of calcium on the sludge granulation process during start-up. Introduction of Ca2+ at concentrations from 150 to 300 mg/l enhanced the biomass accumulation and granulation process. The calcium concentration in the granules was nearly proportional to the calcium concentration in the feed, and calcium carbonate was the main calcium precipitate in the granules. The specific activity of granules decreased with increasing influent calcium concentration. The optimum calcium concentration for the granulation was from 150 to 300 mg/l. The addition of low-concentration calcium to the UASB reactors appeared to enhance the three steps of sludge granulation: adsorption, adhesion and multiplication, but it did not lead to a different proliferation of predominant microorganisms in the granules.

Enhanced sludge granulation in upflow anaerobic sludge blanket (UASB) reactors by aluminum chloride.

Two upflow anaerobic sludge blanket (UASB) reactors were concurrently operated for 146 days to examine the effects of aluminum chloride (AlCl3) on the sludge granulation process during start-up. Sludge granulation (defined as that over 10% of granules were larger than 2.0 mm) was achieved in the control reactor (R1) in approximate three months. Introduction of Al3+ at a concentration of 300 mg/l reduced the sludge granulation time by approximate one month. Throughout the experiment the AlCl3-added reactor (R2) had a higher biomass concentration, e.g., 13.8 g-MLVSS/l versus 12.8 g-MLVSS/l on Day 146. Granules became visible earlier in R2 compared with R1 (35 days versus 65 days). The average size of granules from R2 was larger than that from R1. The results demonstrated that AlCl3 enhanced the sludge granulation process in the UASB reactors.

Hydrogen production from wastewater by acidogenic granular sludge.

Sludge was granulated in a hydrogen-producing acidogenic reactor when operated at 26 degrees C, pH 5.5 treating a sucrose-rich wastewater. The influence of hydraulic retention time (HRT) and sucrose concentration on hydrogen production by the acidogenic granular sludge was investigated at a constant loading rate of 25 g-sucrose/(l x day). Results show that the gas composition was not greatly influenced by HRT or sucrose concentration. The hydrogen accounted for 57% to 68% of the biogas at HRT ranging 4.6-28.6 h and sucrose concentration ranging 4,800-29,800 mg/l. However, the hydrogen yield was more dependent on HRT and sucrose concentration. It ranged from 0.19 to 0.27 l/g-sucrose with the maximum yield occurring at HRT 13.7 h and sucrose concentration 14,300 mg/l in the wastewater. The acidified effluent was composed of volatile fatty acids and alcohols. The predominant products were butyrate (59-68%) and acetate (10-25%), plus smaller amounts of i-butyrate, valerate, i-valerate, caproate, methanol, ethanol, propanol, and butanol. The sludge yield averaged 0.2 g-VSS/g-sucrose. The carbon balance was 98-107% throughout the study.

Inhibition by chromium and cadmium of anaerobic acidogenesis.

The effects of chromium (III) and cadmium on the anaerobic acidogenesis of a simulated dairy waste were examined using serum vials. At Cd dosages less than 20 mg/l, the acidogenesis process was enhanced by the dosage, resulting in a higher degree of acidification, protein conversion, and hydrogen production than the control. At dosages over 20-mg/l, Cd inhibited the acidogenesis. The Cr (III) dosage of 5 mg/l reduced overall volatile fatty acid and alcohol generation, degree of acidification, conversions of lactose, lipid and protein, and total biogas production, with the exception of accumulation of hydrogen and propionate. At dosages exceeding 5 mg/l, Cr (III) had a severe inhibition on the acidogenesis. The Cd concentrations which caused a 50% reduction in total volatile fatty acid and alcohol production, degree of acidification and cumulative gas production were higher than the corresponding values caused by Cr (III), suggesting that Cr (III) was more toxic to acidogenic bacteria than Cd.

Acidogenesis of dairy wastewater at various pH levels.

Continuous experiments were conducted to study the influence of pH in the range 4.0-6.5 on the acidification of dairy wastewater at 37 degrees C with 12 hours of hydraulic retention in an upflow reactor. Results showed that degradation of dairy pollutants increased with pH from pH 4.0 to 5.5. At pH 5.5, 95% of carbohydrate, 82% of protein and 41% of lipid were degraded. Based on chemical oxygen demand (COD), 48.4% of dairy pollutants were converted into volatile fatty acids and alcohols in the mixed liquor, 6.1% into hydrogen and methane in biogas, and the remaining 4.9% into biomass. The biomass yield at pH 5.5 was estimated as 0.32 mg-VSS/mg-COD. Further increase of pH, up to 6.5, increased degradation of carbohydrate, protein and lipid only slightly, but resulted in the lowering of overall acid and alcohol production due to their increased conversion into methane. Acetate, propionate, butyrate and ethanol are the main products of acidogenesis. Productions of propionate and ethanol were favored at pH 4.0-4.5, whereas productions of acetate and butyrate were favored at pH 6.0-6.5.

Anaerobic acidification of a synthetic wastewater in batch reactors at 55 degrees C.

Experiments were conducted to study the acidogenesis of a dairy wastewater in batch reactors at pH 5.5 and 55 degrees C. There was a biased fermentation sequence for carbohydrate and protein, and the protein fermentation was delayed by carbohydrate. The production of hydrogen was exclusively from the fermentation of carbohydrate. Acetate and butyrate concentrations both increased rapidly at the beginning and peaked at some points, then declined in the reactors fed with 8 g-COD (chemical oxygen demand)/l, or higher concentrations. Butanol and propanol fractions increased with the substrate concentration. The metabolism shifted from the volatile fatty acid-producing pathways to the alcohol-producing pathways when the substrate concentration increased beyond 8 g-COD/l. The acidogenic biomass yield was in the range 0.19-0.25 mg-VSS/mg-COD.

Adsorption of heavy metals by EPS of activated sludge.

Extracellular polymeric substances (EPSs) were extracted by high-speed centrifugation at 20,000 G for 30 min from an activated sludge treating municipal wastewater. Each gram of sludge, as measured in volatile suspended solids (VSS), contained 7.3 mg of EPS, including 6.5 mg of protein (EPSp) and 0.8 mg of carbohydrate (EPSc). The EPSp had a mean MW of 2.0 x 105; about 18% of EPSp had MW over 5 x 104 and 16% below 5 x 103. For heavy metal concentrations ranging 10-100 mg/l, EPS on average removed 99% of Zn2+, 98% of Cu2+, 97% of Cr3+, 85% of Cd2+, 69% of Co2+, 37% of Ni2+, and 26% of CrO4(2-). The relative degrees of metal removals were inconsistent with those reported for the activated sludge process. Each mg of ESP had the capacity to remove up to 1.48 mg of Zn2+, 1.12 mg of Cu2+, 0.83 mg of Cr3+, 0.90 mg of Cd2+, 1.10 mg of Co2+, 0.25 mg each of Ni2+ and CrO4(2-). Results suggest the feasibility of recovering ESP from waste sludge for use as adsorbent. Freundlich isotherm correlated satisfactorily with the adsorption data of Ni2+, Cu2+, Cd2+, and CrO4(2-) (R2 ranging 0.89-0.97), whereas Langmuir isotherm correlated satisfactorily with those of Zn2+, Cr3+ and Ni2+ (R2 ranging 0.93-0.96). Both correlated poorly for those of Co2+.