Jannie Maree

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.

Synthesis of high-purity precipitated calcium carbonate during the process of recovery of elemental sulphur from gypsum waste

We recently showed that the production of elemental sulphur and calcium carbonate (CaCO3) from gypsum waste by thermally reducing the waste into calcium sulphide (CaS) followed by its direct aqueous carbonation yielded low-grade carbonate products (i.e. <90 mass% as CaCO3). In this study, we used the insight gained from our previous work and developed an indirect aqueous CaS carbonation process for the production of high-grade CaCO3 (i.e. >99 mass% as CaCO3) or precipitated calcium carbonate (PCC). The process used an acid gas (H2S) to improve the aqueous dissolution of CaS, which is otherwise poorly soluble. The carbonate product was primarily calcite (99.5%) with traces of quartz (0.5%). Calcite was the only CaCO3 polymorph obtained; no vaterite or aragonite was detected. The product was made up of micron-size particles, which were further characterised by XRD, TGA, SEM, BET and true density. Results showed that about 0.37 ton of high-grade PCC can be produced from 1.0 ton of gypsum waste, and generates about 0.19 ton of residue, a reduction of 80% from original waste gypsum mass to mass of residue that needs to be discarded off. The use of gypsum waste as primary material in replacement of mined limestone for the production of PPC could alleviate waste disposal problems, along with converting significant volumes of waste materials into marketable commodities.

Recovery of Drinking Water and By-products from Gold Mine Effluents

South Africa is a water constrained country with a large mining industry. Effluents from the mining industry, which is rich in calcium sulphate, resulted in salination of the limited amount of surface water. South Africa is also a large importer of sulphur because it is required for the manufacture of sulphuric acid. It is argued that importation of sulphur can be replaced by recovering it as a by-product during treatment of sulphate-rich effluents. The removal of acid, metals and sulphate from mine water was assessed using the CSIR ABC (Alkali-Barium-Calcium) Desalination process. The CSIR ABC Desalination process was used for neutralization and removal of the total dissolved solids content from 7600 to 400 mg/l. Metals were removed effectively through precipitation with CaS or Ca(HS)2. The latter compound had a high solubility with higher metal removal rates compared to CaS. Sulphate remained in solution during metals precipitation with sulphide. The rate of sulphate removal during gypsum crystallization was influenced by the gypsum seed crystal content. The rate of sulphate removal during BaCO3 treatment was influenced by pH, CaCO3 solids and BaCO3 solid concentration. Ca(HS)2 was produced from CaS by passing CO2 through a CaS slurry. Further CO2 additions resulted in H2S-stripping. BaSO4 and CaCO3 were converted simultaneously to BaS and CaO, respectively. The optimum temperature was 1050°C. The cost of raw materials for the treatment of water with a TDS content of 7 600 mg/l amounted to R2.21/cubic metre (m3). The potential value of the water and by-products amounted to R11.10/m3 (US

Conversion of calcium sulphide to calcium carbonate during the process of recovery of elemental sulphur from gypsum waste

1.00 = ZAR7.60).

Process optimization of freeze desalination of brine using HybridICETM pilot plant

The production of elemental sulphur and calcium carbonate (CaCO3) from gypsum waste can be achieved by thermally reducing the waste into calcium sulphide (CaS), which is then subjected to a direct aqueous carbonation step for the generation of hydrogen sulphide (H2S) and CaCO3. H2S can subsequently be converted to elemental sulphur via the commercially available chemical catalytic Claus process. This study investigated the carbonation of CaS by examining both the solution chemistry of the process and the properties of the formed carbonated product. CaS was successfully converted into CaCO3; however, the reaction yielded low-grade carbonate products (i.e. <90 mass% as CaCO3) which comprised a mixture of two CaCO3 polymorphs (calcite and vaterite), as well as trace minerals originating from the starting material. These products could replace the Sappi Enstra CaCO3 (69 mass% CaCO3), a by-product from the paper industry which is used in many full-scale AMD neutralisation plants but is becoming insufficient. The insight gained is now also being used to develop and optimize an indirect aqueous CaS carbonation process for the production of high-grade CaCO3 (i.e. >99 mass% as CaCO3) or precipitated calcium carbonate (PCC).

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

Freeze desalination of synthetic brines prepared from sodium chloride was investigated. Brine solutions of levels between 2.3% to 10% sodium chloride were used as feed to the plant operated in a batch and continuous mode. The effects of pump, gear motor speed, and differential temperatures on ice formation and purity were investigated. The HybridICE process produced ice of quality levels between 80–96% purity. It was observed that the pump speed was directly proportional to the effluent flow rate coming out of the heat exchangers. Generally, it was observed that the quality of product ice was higher at low to medium flow rates of feed. The gear motor speed variation was inversely proportional to effluent flow rate from the heat exchangers. The HybridICE was found to be a viable desalination technology in terms of quality of water produced, energy consumption, and its easiness to be incorporated into existing refrigeration systems.

Theoretical performance of nanofiltration membranes for wastewater treatment

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.

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

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.

Deposition of toxic metal particles on rough nanofiltration membranes

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.

Recovery of Calcium Carbonate from Wastewater Treatment Sludge Using a Flotation Technique

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.