Richard Mbaya

Characterization and performance of nanofiltration membranes

The availability of clean water has become a critical problems facing the society due to pollution by human activities. Most regions in the world have high demands for clean water. Supplies for freshwater are under pressure. Water reuse is a potential solution for clean water scarcity. A pressure-driven membrane process such as nanofiltration has become the main component of advanced water reuse and desalination systems. High rejection and water permeability of solutes are the major characteristics that make nanofiltration membranes economically feasible for water purification. Recent advances include the prediction of membrane performances under different operating conditions. Here, we review the characterization of nanofiltration membranes by methods such as scanning electron microscopy, thermal gravimetric analysis, attenuated total reflection Fourier transform infrared spectroscopy, and atomic force microscopy. Advances show that the solute rejection and permeation performance of nanofiltration membranes are controlled by the composition of the casting solution of the active layer, cross-linking agent concentration, preparation method, and operating conditions. The solute rejection depends strongly on the solute type, which includes charge valency, diffusion coefficient, and hydration energy. We also review the analysis of the surface roughness, the nodule size, and the pore size of nanofiltration membranes. We also present a new concept for membrane characterization by quantitative analysis of phase images to elucidate the macro-molecular packing at the membrane surface.

Effect of annealing treatment on the crystallisation and leaching of dumped base metal smelter slags

Leaching tests of base metals contained in two smelter slags were undertaken in ammonia and nitric acid solutions aiming to recover Co, Cu and Zn. Leaching tests were conducted at 25 and 60°C at pH=0 and 3 in HNO(3) and pH=12 in NH(4)OH media. XRD analysis revealed that the dumped slags were amorphous. Annealing these slags at 1180°C produced crystalline phases comprising diopside, magnetite and fayalite. SEM and EDS analysis revealed that Cu and Pb compounds have concentrated in the magnetite phase, whereas another phase rich in Zn and Cu was located in the diopside matrix. ICP-OES analysis of the pregnant leaching solutions (PLS) showed that 30-60% of Co, Cu and Zn were released from the amorphous slags treated in HNO(3) at pH=0, and lesser in ammonia. However, the contamination by Fe and Pb was higher at pH=0. The contamination of the PLS obtained by leaching of the crystallised slags remained low. The low Fe and Pb contamination was attributed in this case to the chemical stability of the crystalline phases formed upon annealing treatment. The higher solubilisation of metals contained in amorphous slags was attributed to the collapse of silicate structures during nitric acid leaching at pH∼0.

Microscopical characterizations of nanofiltration membranes for the removal of nickel ions from aqueous solution

The nanofiltration (NF) process is electrostatically governed and the surface free energy plays a key role in the separation of particulates, macromolecules, and dissolved ionic species. Streaming potential measurement and the surface charge mapping by Kelvin probe atomic force mircoscopy (AFM) have been carried out. Forces of interaction near the surface of nanofiltration membranes were further studied by a force spectroscopy using atomic force microscopy. The two membranes used are more negatively charged at high pH values; hence the higher the solution chemistry, the higher and faster will be adhesion of ions on the surface of the nanofiltration membranes. It was observed that the three acquired signals from non-contact AFM (contact potential difference, amplitude and phase) were rigorously connected to the surface structure of the nanofiltration membranes. In addition to the surface structure (roughness), electrostatic interactions can also enhance initial particle adhesion to surfaces of nanofiltration membranes. The performance of the NF membranes was further investigated for the removal of nickel ions from aqueous solution, and the results were correlated to the mechanical responses of the nanofiltration membranes obtained from AFM and the streaming potential measurement.

Theoretical performance of nanofiltration membranes for wastewater treatment

Abstract Mechanisms of ionic transport in nanofiltration are poorly known. Modelling can be used to predict membrane performance, to reveal separation mechanisms, to select appropriate membranes, and to design processes. Several models have been proposed to describe nanofiltration membranes. Some models rely on simple concepts, while other models are more complex and require sophisticated solution techniques. Here, we review predictive models used for characterizing nanofiltration membranes for the separation of wastewater. The most popular model uses the extended Nernst–Planck equation, which describes the ionic transport mechanisms in details. Results obtained by using the extended Nernst–Planck equation show that the performance of nanofiltration membranes is strongly dependent on charge, steric, and dielectric effects.

HybridICE® filter: ice separation in freeze desalination of mine waste waters.

Freeze desalination is an alternative method for the treatment of mine waste waters. HybridICE(®) technology is a freeze desalination process which generates ice slurry in surface scraper heat exchangers that use R404a as the primary refrigerant. Ice separation from the slurry takes place in the HybridICE filter, a cylindrical unit with a centrally mounted filter element. Principally, the filter module achieves separation of the ice through buoyancy force in a continuous process. The HybridICE filter is a new and economical means of separating ice from the slurry and requires no washing of ice with water. The performance of the filter at a flow-rate of 25 L/min was evaluated over time and with varied evaporating temperature of the refrigerant. Behaviours of the ice fraction and residence time were also investigated. The objective was to find ways to improve the performance of the filter. Results showed that filter performance can be improved by controlling the refrigerant evaporating temperature and eliminating overflow.

Deposition of toxic metal particles on rough nanofiltration membranes

Two nanofiltration (NF90 and Nano-Pro-3012) membranes were investigated for their capacity to remove metal ions. This study presents the effect of membrane roughness on the removal of toxic metal ions during dead end membrane filtration. Atomic force microscopy, scanning electron microscopy, WSXM software and ImageJ were used to characterize the roughness of the membranes. Gradual decrease in filtration permeate flux was observed as foulants accumulated at the interface of the membranes; filtration permeate flux varied from 20 L/m2/h to 14 L/m2/h and 11 L/m2/h to 6 L/m2/h for NF90 and Nano-Pro-3012, respectively. NF90 membrane was more prone to fouling than the Nano-Pro-3012 membrane: the percentage flux reduction was higher for NF90 (3.6%) than Nano-Pro-3012 (0.98%). The bearing ratio of the fouled NF90 exhibited a high peak of 7.09 nm than the fouled Nano-Pro-3012 with the peak of 6.8 nm.

HybridICE® HIF Filter:Principle And Operation

The HybridICE technology operates on the principle that growing ice crystals reject impurities during freezing and is a “zero liquid discharge” process, whereby the water is completely isolated from the dissolved waste species. The technology recovers water from waste waters for re uses for all purposes. The process allows the utilisation of both surplus process heat and cooling energy. The waste heat from the refrigeration cycle is, moreover, utilised for vacuum evaporation to recover a fraction of the water as condensate. The predominant water fraction is recovered by isolating the ice from a concentrated process brine stream. The process takes place in a static concentrator, known as the HybridICE Filter module (HIF) that separates the suspended ice crystals from the concentrated brine slurry to recover ice crystals as pure water. The recovered ice from the freeze crystallisation does not require rinsing with fresh water. Basic factors influence the quality and yield of recovered water. These include but are not limited to: TDS of the waste water stream; first ice point; ice content of the process waste water; mass-flow. The slurry ice feed stream was generated using the HybridICE freeze crystallisation plant. The objective was to establish the comparative behaviour of a low and high sodium chloride feed using 2% and 8% (m/m) NaCl feed brine streams.

Performance Of An Acid Stable NanofiltrationMembrane For Nickel Removal FromAqueous Solutions: Effects Of Concentration,Solution PHAnd Ionic Strength

The performance of a nanofiltration membrane for the removal of the nickel ion was studied as a function of the nickel concentration, solution pH, and the background ionic strength of the solution. Nanofiltration is investigated as a means to determine to what extent the nickel ions could be removed from acid mine drainage; thus the effect of solution chemistry on nanofiltration performance is investigated. Higher fluxes (47.6l/m²/h) were experienced at the lower nickel concentration (10mg/l) than at the higher (28.9l/m²/h) nickel concentration (100mg/l). Higher nickel ion rejections (97.3%) were obtained at the higher nickel concentration (100mg/l) than at the lower nickel concentration (93.6%). Higher flux was obtained at the higher pH (pH 4) with a 0.01M NaCl background solution than at lower pH (pH 3) when a 0.05M NaCl was used as background solution. Higher nickel ion rejections were obtained at higher pH (pH 4) for the two ionic strength background solutions. Higher fluxes were also obtained with the lower NaCl background solution. Slightly higher ion rejections were obtained with the lower NaCl background concentration. It therefore appears that this nanofiltration membrane should be successfully applied for the removal of nickel ions from acid mine drainage.

Flux Decline and Rejection Characteristics During Nanofiltration of Iron and Deionized Water

This work was studied to determine the flux decline during nanofiltration of iron and deionised water. The rejection characteristic of iron was also studied. A stirred-cell was used for the experiment and Inductively Coupled plasma optical emission was used for iron analysis at various pH and pressure. The significant increased in flux declined at pH 3.01 and 3.44 is possibly caused by crystallized solids formed at the surface of the membrane and thus lead to the reduction of iron rejection at pH 3.01 and 3.44. At higher pressure more water passes through the membrane, thereby increasing the iron rejection. Experiment of clean water flux was done using the deionised water after the different pH experiment to see if the membrane is not fouling. The rejection characteristic of iron was also studied.

Effects of laser bending on the microstructural constituents

This article will illustrate the correlation between microstructural and microhardness changes in high-strength-low-alloy steel that occur as a result of laser-bending. Laser bending is a process of bending metal shapes using the laser beam to uniformly distribute thermal energy across that target material. The correct thermal distribution in laser bending is achieved by optimum combination of the laser power intensity, scan speed and beam diameter for a specific metal thickness. The effects of the thermal distribution on the stability of metal were elucidated in terms of microstructure and microhardness. Experiments revealed a decrease in properties as a result of laser-bending. The applied heat exceeds the transformation temperature of the fine ferrite grain to austenite. Microstructural constituents on the affected heat treated zones were primarily allotriomorphic ferrite and thus weakened the ferrite grains. This was due to the heat effect on grain growth of the steel that brought about softening.