Energy Level Statistics of Quantum Dots

We investigate the charging energy level statistics of disordered interacting electrons in quantum dots by numerical calculations using the Hartree approximation. The aim is to obtain a global picture of the statistics as a function of disorder and interaction strengths. We find Poisson statistics at very strong disorder, Wigner- Dyson statistics for weak disorder and interactions, and a Gaussian intermediate regime. These regimes are as expected from previous studies and fundamental considerations, but we also find interesting and rather broad crossover regimes. In particular, intermediate between the Gaussian and Poisson regimes we find a two-sided exponential distribution for the energy level spacings. In comparing with experiment, we find that this distribution may be realized in some quantum dots.Comment: 21 pages 10 figure

New insights into the relationship between draw solution chemistry and trace organic rejection by forward osmosis

© 2019 This study elucidates the impact of draw solution chemistry (in terms of pH and draw solute species) and membrane fouling on water flux and the rejection of trace organic contaminants by forward osmosis. The results show that draw solution chemistry could induce a notable impact on both water flux and TrOCs rejection. In addition, the impact was further influenced by membrane fouling. The reverse flux of proton (or hydroxyl) could alter the feed solution pH, which governed the separation of ionisable TrOCs. In addition, charged compounds generally exhibited higher rejections than neutral ones by the clean membrane. Electrostatic interaction, rather than size exclusion, was therefore the dominant rejection mechanism for most compounds. There was also a weak correlation between rejection and molecular sizes of the 43 TrOCs. Compared with Na+, Li+ with a larger hydrated radius showed a significantly lower reverse salt flux, resulting in a lower ionic strength and therefore a stronger electrostatic interaction. A fouling cake layer consisted of low molecular weight neutral organics could also affect TrOC rejection due to pore blockage and cake-enhanced concentration polarisation

Effects of shearing on biogas production and microbial community structure during anaerobic digestion with recuperative thickening

© 2017 Recuperative thickening can intensify anaerobic digestion to produce more biogas and potentially reduce biosolids odour. This study elucidates the effects of sludge shearing during the thickening process on the microbial community structure and its effect on biogas production. Medium shearing resulted in approximately 15% increase in biogas production. By contrast, excessive or high shearing led to a marked decrease in biogas production, possibly due to sludge disintegration and cell lysis. Microbial analysis using 16S rRNA gene amplicon sequencing showed that medium shearing increased the evenness and diversity of the microbial community in the anaerobic digester, which is consistent with the observed improved biogas production. By contrast, microbial diversity decreased under either excessive shearing or high shearing condition. In good agreement with the observed decrease in biogas production, the abundance of Bacteroidales and Syntrophobaterales (which are responsible for hydrolysis and acetogenesis) decreased due to high shearing during recuperative thickening

Synthesis, properties and water permeability of SWNT buckypapers

The ability of macrocyclic ligands to facilitate formation of dispersions of single-walled carbon nanotubes (SWNTs) was investigated using a combination of absorption spectrophotometry and optical microscopy. Vacuum filtration of aqueous dispersions containing SWNTs and various macrocyclic ligands (derivatised porphyrin, phthalocyanine, cyclodextrin and calixarene) afforded self-supporting membranes known as buckypapers. Microanalytical data and energy dispersive X-ray spectra were obtained for these buckypapers and provided evidence for retention of the macrocyclic ligands within the structure of the membranes. The electrical conductivities of the membranes varied between 30 ± 20 and 220 ± 60 S cm−1, while contact angle analysis revealed they all possessed hydrophilic surfaces. The mechanical properties of buckypapers prepared using macrocyclic ligands as dispersants were shown to be comparable to that of a benchmark material prepared using the surfactant Triton X-100 (Trix). Incorporation of the macrocyclic ligands into SWNT buckypapers was found to increase their permeability up to ten-fold compared to buckypapers prepared using Trix. No correlation was observed between the water permeability of the membranes and the average size of either their surface or internal pores. However, the water permeability of the membranes was found to be inversely dependent on their surface area

Reducing or increasing inequalities? The role of private water enterprises in rural Viet Nam

Extreme inequalities are recognised as being detrimental to human rights and economic development (Stiglitz 2012), and in response, the 2030 Agenda for Sustainable Development has explicitly included addressing inequalities as one of the 17 Global Goals. In order to reduce inequalities an integrated approach across multiple dimensions of human development is required, including access to safe water. This research investigated stakeholder perceptions of rural piped water services in Viet Nam to better understand issues of equality, access and affordability. It asked the question: can poor households access piped water services provided by small scale private enterprises in rural Viet Nam? This question is important because little is known about whether or not poor households access piped water services, related issues of affordability of connection fees and tariffs, and other potential barriers. It is also important because private enterprises are increasingly providing piped water services in Viet Nam, supported by incentives from Government and international donors including some civil society organisations (CSOs

Fouling mechanisms of submerged ultrafiltration membranes in greywater recycling

This study examined the influence of greywater constituents on the fouling behaviour of submerged hollow fibre UF membranes during greywater treatment for recycling purposes. Experiments were carried out on a bench-scale equipment using a Zenon ZW1 module. The membrane was operated under constant flux where an increase in transmembrane pressure was used to determine the extent of fouling. Wastewater constituent variables used in this study were kaolin, cellulose, humic acid, surfactant, and calcium concentration. Results indicate that during filtration of synthetic greywater multivalent ions like calcium played an important role. Depending on concentration agglomerates of different structure and size were formed and the structure and size determined the extent of fouling and retention. It was also shown that the surfactant sodium dodecyl sulphate (SDS) may cause fouling through particle stabilisation and strongly interacts with calcium enhancing calcium retention especially in the critical micelle concentration (cmc) region. It is hypothesized that SDS competes with humic acids (HAs) for adsorption sites resulting in lower UV254nm retention. Solution chemistry and cake deposition also influence the retention and hence product water quality

Sorptive removal of dissolved organic matter in biologically-treated effluent by functionalized biochar and carbon nanotubes: Importance of sorbent functionality

© 2018 Elsevier Ltd The sorptive removal of dissolved organic matter (DOM) in biologically-treated effluent was studied by using multi-walled carbon nanotube (MWCNT), carboxylic functionalised MWCNT (MWCNT-COOH), hydroxyl functionalized MWCNT (MWCNT-OH) and functionalized biochar (fBC). DOM was dominated by hydrophilic fraction (79.6%) with a significantly lower hydrophobic fraction (20.4%). The sorption of hydrophobic DOM was not significantly affected by the sorbent functionality (∼10.4% variation) and sorption capacity followed the order of MWCNT > MWCNT-COOH > MWCNT-OH > fBC. In comparison, the sorption of hydrophilic fraction of DOM changed significantly (∼37.35% variation) with the change of sorbent functionality with adsorption capacity decreasing as MWCNT-OH > MWCNT-COOH > MWCNT > fBC. Furthermore, the affinity of adsorbents toward a hydrophilic compound (dinitrobenzene), a hydrophobic compound (pyrene) and humic acid was also evaluated to validate the proposed mechanisms. The results provided important insights on the type of sorbents which are most effective to remove different DOM fractions

Adsorptive interactions between membranes and trace contaminants

Widespread occurrences of endocrine-disrupting chemicals (EDCs) in waterways have attracted a great attention of the scientific community. While scientific evidence associated with human health is restricted due to the long-term effects, impacts of EDCs on trout at the common concentration encountered in sewage effluent have been confirmed by both in vitro and in vivo studies. The impacts of steroid estrogens such as estrone, estradiol (natural hormones) and ethinylestradiol (a synthetic hormone) are often more serious than other synthetic EDCs as they have far higher endocrine-disrupting potency, despite of their low concentration. This paper investigates retention and adsorptive behavior of the natural hormones estrone by two commercial reverse osmosis membranes TFC-S and X-20, using dead end stirred cell systems. While an adsorptive process that reaches a breakthrough governs the retention of estrone by the TFC-S membrane; a sieving mechanism is responsible for the high removal of estrone using the X-20 membrane

Removal of the Natural Hormone Estrone from Aqueous Solutions using Nanofiltration and Reverse Osmosis

The ability of a variety of nanofiltration and reverse osmosis membranes to retain the natural hormone estrone are examined here as a function of solution conditions. While size exclusion dominates retention with the tighter membranes, both size exclusion and adsorptive effects appear to be instrumental in maintaining high retention on nanofiltration membranes that otherwise exhibit relatively low ion retentions. These adsorptive effects may be driven by hydrogen bonding between estrone and the membrane. Electrostatic attraction appears to aid retention with an apparent slight decrease in retention at high NaCl concentrations. Deprotonation of estrone leads to a significant decrease in retention, most likely as a result of the effect of strong electrostatic repulsive forces decreasing the proximity of the negatively charged estrone to the negatively charged membrane surface and thus lowering the potential for adsorptive retention. This deprotonation effect is absent for tight RO membranes. The results reported here indicate that while open nanofiltration membranes may be effective in retaining estrone under some conditions, the extent of retention may be very susceptible to maintenance of adsorptive capacity at the membrane surface and depend on solution chemistry