Occurrence and behaviour of colloidal silica and silica-rich nanoparticles through stages of reverse osmosis treating coal seam gas associated water.

Reverse Osmosis (RO) membrane filtration is a very common process for treating a wide range of groundwater types including produced water from coal seam gas (coalbed methane) wells. Mineral scaling limits water recovery for RO membranes and costs money in terms of treatment and downtime. Silica scaling can be particularly troublesome as it is often irreversible. Mitigating silica scaling requires an understanding of its occurrence, speciation mechanism and its interdependency with other operation factors. This study uses a range of techniques to show that silica colloids form during later stages of an RO process with very high recovery. This happens at silica concentrations above the solubility that would normally indicate high risk of silica scale. However, instead of scale, colloids preferentially formed which means the process can operate at high recoveries with RO performance maintained by regular cleaning cycles. The concentration of the colloidal silica through the RO stages was measured through the difference in total and dissolved silica. Once the existence was established with this technique, the particles were trapped and their size, morphology and composition were investigated with Scanning Electron Microscopy (SEM) in conjunction with Energy Dispersive X-Ray Spectroscopy (EDS). This revealed the particles to be predominantly silica with limited other elements involved.

Silica fouling during groundwater RO treatment: The effect of colloids' radius of curvature on dissolution and polymerisation.

Silica fouling during groundwater reverse osmosis (RO) treatment can have a significant impact on filtration performance. To better understand this phenomenon, the equilibrium kinetics of amorphous colloidal silica were studied at conditions relevant to RO of silica-rich alkaline groundwater. The impact of particle size was investigated using synthetic monodisperse silica nanoparticles. Bench scale experiments were conducted by monitoring dissolved silica concentration of aqueous suspensions of colloids of 100 and 300 nm diameter and pH 8.5 to 9.5. The equilibrium data was determined from existing established rate law equations. This study concluded that surface energy has a major impact on silica dissolution rate constant, particularly for colloidal silica. Observations of Ostwald ripening in bidisperse silica dispersions further confirmed these results, which indicate that dissolution and redeposition is responsible for the problematic silica fouling behaviour during RO treatment. 2D modelling based on inferred equilibrium data allows visualization of scale layer growth in agreement with cross-sectional scanning electron micrographs of autopsied membranes.

Capacitance of Nanoporous Carbon-Based Supercapacitors Is a Trade-Off between the Concentration and the Separability of the Ions.

Nanoporous carbon-based supercapacitors store electricity through adsorption of ions from the electrolyte at the surface of the electrodes. Room temperature ionic liquids, which show the largest ion concentrations among organic liquid electrolytes, should in principle yield larger capacitances. Here, we show by using electrochemical measurements that the capacitance is not significantly affected when switching from a pure ionic liquid to a conventional organic electrolyte using the same ionic species. By performing additional molecular dynamics simulations, we interpret this result as an increasing difficulty of separating ions of opposite charges when they are more concentrated, that is, in the absence of a solvent that screens the Coulombic interactions. The charging mechanism consistently changes with ion concentration, switching from counterion adsorption in the diluted organic electrolyte to ion exchange in the pure ionic liquid. Contrarily to the capacitance, in-pore diffusion coefficients largely depend on the composition, with a noticeable slowing of the dynamics in the pure ionic liquid.

Mixed Matrix Carbon Molecular Sieve and Alumina (CMS-Al2O3) Membranes.

This work shows mixed matrix inorganic membranes prepared by the vacuum-assisted impregnation method, where phenolic resin precursors filled the pore of α-alumina substrates. Upon carbonisation, the phenolic resin decomposed into several fragments derived from the backbone of the resin matrix. The final stages of decomposition (>650 °C) led to a formation of carbon molecular sieve (CMS) structures, reaching the lowest average pore sizes of ~5 Å at carbonisation temperatures of 700 °C. The combination of vacuum-assisted impregnation and carbonisation led to the formation of mixed matrix of CMS and α-alumina particles (CMS-Al2O3) in a single membrane. These membranes were tested for pervaporative desalination and gave very high water fluxes of up to 25 kg m(-2) h(-1) for seawater (NaCl 3.5 wt%) at 75 °C. Salt rejection was also very high varying between 93-99% depending on temperature and feed salt concentration. Interestingly, the water fluxes remained almost constant and were not affected as feed salt concentration increased from 0.3, 1 and 3.5 wt%.

Downstream processing of reverse osmosis brine: Characterisation of potential scaling compounds.

Reverse osmosis (RO) brine produced at a full-scale coal seam gas (CSG) water treatment facility was characterized with spectroscopic and other analytical techniques. A number of potential scalants including silica, calcium, magnesium, sulphates and carbonates, all of which were present in dissolved and non-dissolved forms, were characterized. The presence of spherical particles with a size range of 10-1000 nm and aggregates of 1-10 microns was confirmed by transmission electron microscopy (TEM). Those particulates contained the following metals in decreasing order: K, Si, Sr, Ca, B, Ba, Mg, P, and S. Characterization showed that nearly one-third of the total silicon in the brine was present in the particulates. Further, analysis of the RO brine suggested supersaturation and precipitation of metal carbonates and sulphates during the RO process should take place and could be responsible for subsequently capturing silica in the solid phase. However, the precipitation of crystalline carbonates and sulphates are complex. X-ray diffraction analysis did not confirm the presence of common calcium carbonates or sulphates but instead showed the presence of a suite of complex minerals, to which amorphous silica and/or silica rich compounds could have adhered. A filtration study showed that majority of the siliceous particles were less than 220 nm in size, but could still be potentially captured using a low molecular weight ultrafiltration membrane.

Progress on the Surface Nanobubble Story: What is in the bubble? Why does it exist?

Interfaces between aqueous solutions and hydrophobic solid surfaces are important in various areas of science and technology. Many researchers have found that forces between hydrophobic surfaces in aqueous solution are significantly different from the classical DLVO theory. Long-range attractive forces (non-DLVO forces) are thought to be affected by nanoscopic gaseous domains at the interfaces. This is a review of the latest research on nanobubbles at hydrophobic surfaces from experimental and simulation studies. The review focusses on non-intrusive optical view of surface nanobubbles and gas enrichment on solid surfaces by imaging and force mapping. By use of these recent experimental data in conjunction with molecular simulation work, all major theories on surface nanobubble formation and stability are critically reviewed. Even though the current body of research cannot comprehensively explain all properties of surface nanobubbles observed, the fundamental understanding has been significantly improved. Line tension has been shown to be incapable of explaining the contact angle of nanobubbles. Dense gas layer theory provides a new explanation on both large contact angle and long-time stability. The high density of gas in these domains may significantly affect the gas-water interface which is in line with some observation made on bulk nanobubbles. Along this line of inquiry, experimental and simulation effort should be focussed on measuring the density within surface nanobubbles and the properties of the gas water interface which may be the key to explaining the stability of these nanobubbles.

A review of molecular modelling of electric double layer capacitors.

Electric double-layer capacitors are a family of electrochemical energy storage devices that offer a number of advantages, such as high power density and long cyclability. In recent years, research and development of electric double-layer capacitor technology has been growing rapidly, in response to the increasing demand for energy storage devices from emerging industries, such as hybrid and electric vehicles, renewable energy, and smart grid management. The past few years have witnessed a number of significant research breakthroughs in terms of novel electrodes, new electrolytes, and fabrication of devices, thanks to the discovery of innovative materials (e.g. graphene, carbide-derived carbon, and templated carbon) and the availability of advanced experimental and computational tools. However, some experimental observations could not be clearly understood and interpreted due to limitations of traditional theories, some of which were developed more than one hundred years ago. This has led to significant research efforts in computational simulation and modelling, aimed at developing new theories, or improving the existing ones to help interpret experimental results. This review article provides a summary of research progress in molecular modelling of the physical phenomena taking place in electric double-layer capacitors. An introduction to electric double-layer capacitors and their applications, alongside a brief description of electric double layer theories, is presented first. Second, molecular modelling of ion behaviours of various electrolytes interacting with electrodes under different conditions is reviewed. Finally, key conclusions and outlooks are given. Simulations on comparing electric double-layer structure at planar and porous electrode surfaces under equilibrium conditions have revealed significant structural differences between the two electrode types, and porous electrodes have been shown to store charge more efficiently. Accurate electrolyte and electrode models which account for polarisation effects are critical for future simulations which will consider more complex electrode geometries, particularly for the study of dynamics of electrolyte transport, where the exclusion of electrode polarisation leads to significant artefacts.

Origin of interfacial nanoscopic gaseous domains and formation of dense gas layer at hydrophobic solid-water interface.

Interfacial gas enrichment (IGE) covering the entire area of hydrophobic solid-water interface has recently been detected by atomic force microscopy (AFM) and hypothesized to be responsible for the unexpected stability and anomalous contact angle of gaseous nanobubbles and the significant change from DLVO to non-DLVO forces. In this paper, we provide further proof of the existence of IGE in the form of a dense gas layer (DGL) by molecular dynamic simulation. Nitrogen gas adsorption at the water-graphite interface is investigated using molecular dynamic simulation at 300 K and 1 atm normal pressure. The results show that a DGL with a density equivalent to a gas at pressure of 500 atm is formed and equilibrated with a normal pressure of 1 atm. By varying the number of gas molecules in the system, we observe several types of dense gas domains: aggregates, cylindrical caps, and DGLs. Spherical cap gas domains form during the simulation but are unstable and always revert to another type of gas domain. Furthermore, the calculated surface potential of the DGL-water interface, -17.5 mV, is significantly closer to 0 than the surface potential, -65 mV, of normal gas bubble-water interface. This result supports our previously stated hypothesis that the change in surface potential causes the switch from repulsion to attraction for an AFM tip when the graphite surface is covered by an IGE layer. The change in surface potential comes from the structure change of water molecules at the DGL-water interface as compared with the normal gas-water interface. In addition, the contact angle of the cylindrical cap high density nitrogen gas domains is 141°. This contact angle is far greater than 85° observed for water on graphite at ambient conditions and much closer to the 150° contact angle observed for nanobubbles in experiments.