Hybrid sorbent-ultrafiltration systems for the removal of hormones and fluoride from water

The presence of trace contaminants in drinking water resources has been related to adverse health effects in living organisms and humans. Current technologies do not adequately remove these contaminants from water and /or require high energy supply. Exploring low cost, low energy processes in order to eliminate trace contaminants is essential considering that access to clean drinking water and energy is becoming more challenging in many parts of the world. Hormones and fluoride are the two contaminants studied in this research and hybrid systems which combine sorption with low pressure ultrafiltration are proposed for their removal. Sorption is a promising removal mechanism if efficient sorbents and operational conditions are selected, however, the introduction of sorbent materials can cause fouling in ultrafiltration. Fouling reduces the membrane permeability and increases the energy requirement of the system. The overall aim is to study the proposed hybrid sorbent- ultrafiltration systems in terms of contaminant sorption capacity and membrane performance. The systems are tested under varying sorbent size (52 -3000 nm for hormone, <38 -500 μm for fluoride removal) sorbent concentration (1.7 -84 mg /L for hormone, 1 -50 g/L for fluoride removal), sorbate concentration (100 ng /L hormone and 5 -500 mg /L fluoride) and solution pH (3 -12). The thesis can be split into two parts: one part for hormones and the other for fluoride.In the first part, a hybrid polystyrene nanoparticle -ultrafiltration system is investigated for hormone removal. Polystyrene nanoparticles are employed as they provide a large active surface area for the sorption and they can easily be manufactured in different sizes and with various functional groups. The results show that the system can only compete with the existing nanofiltration/reverse osmosis membrane systems if the sorption capacity of the polystyrene nanoparticles is increased. For this reason, carboxyl functionalized polystyrene nanoparticles were also tested. Contrary to expectations, even less hormone sorption is achieved with the functionalized particles. Further investigation of other functional groups such as amine /amidine for their hormone sorption capacity is recommended.In the second part, laterite and bone char are selected as two sorbents for the hybrid sorbent -UF system for fluoride removal as they are locally sourced, low cost materials in parts of Ghana and Tanzania, respectively, where fluoride contamination is a major problem. The sorption capacity and the membrane fouling of the hybrid system with the two selected sorbents are compared. Fluoride sorption capacity of the bone char system is higher than the laterite system and this is attributed to the difference in the available surface area. The fouling of the membranes operated with laterite at high initial fluoride concentrations and alkaline solutions is linked to the precipitation of iron and aluminium complexes. With further system optimization, both hybrid laterite and bone char systems show the potential to be viable solutions for fluoride removal, noting that the bone char system is more feasible for high fluoride concentrations above 10 mg /L. Based on lab scale experimental results, two hybrid laterite -ultrafiltration systems are designed to be tested in Ghana. The two systems, one with submerged hollow fibre and the other with direct dead end tubular ultrafiltration membrane modules, are operated with real surface and ground waters. The findings indicate that the amount of sorption obtained in the field is lower than that which is obtained with laboratory experiments due to the presence of interfering co -ions in the real waters and differences in membrane systems. The systems also show the potential to remove arsenic, uranium and lead. The system with hollow fibre membranes can be suggested as an appropriate system for ground water applications as it did not experience any fouling and the investment cost could be lower compared to the tubular membranes. However, if the surface waters are to be treated with the proposed hybrid system, the tubular membranes offers a system with no fouling. The hybrid laterite -UF system shows to be a promising treatment technology for fluoride contaminated waters in Ghana

Hybrid Sorbent-Ultrafiltration Systems for Fluoride Removal from Water

<p>Fluoride contaminated water sources are found in many parts of the world and the consumption of such water is causing dental and skeletal fluorosis in humans, especially in developing countries. Hybrid sorbent-ultrafiltration (UF) systems are proposed for the removal of fluoride from water for the first time in this study. Laterite and bone char were selected as they are low cost, accessible sorbents in developing countries. The performances of the laterite-UF and bone char-UF systems were compared in terms of fluoride removal and membrane permeability under varying fluoride concentration, solution pH, and sorbent load. For equilibrium fluoride concentration of 1.5 mg/L, the World Health Organization guideline for safe drinking water, the sorption capacity of bone char (1.1 mg/g) was larger than that of laterite (0.40 mg/g) and this was attributed to the larger surface area of bone char. For the laterite-UF system, increase in fluoride concentration resulted in a decline in UF permeability whereas for the bone char-UF system there was no influence of fluoride concentration on membrane permeability. The optimal solution pH at which the systems are operated at maximum sorption capacity while avoiding membrane fouling was determined as pH 5-6 for the laterite-UF and pH 7 for the bone char-UF system. For both systems, the permeability declined in a similar manner as the sorbent load increased. Although both systems require further optimization, they showed to be viable defluoridation technologies.</p

Micropollutant Sorption to Membrane Polymers: A Review of Mechanisms for Estrogens

Organic micropollutants such as estrogens occur in water in increasing quantities from predominantly anthropogenic sources. In water such micropollutants partition to surfaces such as membrane polymers but also any other natural or treatment related surfaces. Such interactions are often observed as sorption in treatment processes and this phenomenon is exploited in activated carbon filtration, for example. Sorption is important for polymeric materials and this is used for the concentration of such micropollutants for analytical purposes in solid phase extraction. In membrane filtration the mechanism of micropollutant sorption is a relative new discovery that was facilitated through new analytical techniques. This sorption plays an important role in micropollutant retention by membranes although mechanisms of interaction are to date not understood. This review is focused on sorption of estrogens on polymeric surfaces, specifically membrane polymers. Such sorption has been observed to a large extent with values of up to 1.2 ng/cm2 measured. Sorption is dependent on the type of polymer, micropollutant characteristics, solution chemistry, membrane operating conditions as well as membrane morphology. Likely contributors to sorption are the surface roughness as well as the microporosity of such polymers. While retention – or and reflection coefficient as well as solute to effective pore size ratio – control the access of such micropollutants to the inner surface, pore size, porosity and thickness as well as morphology or shape of inner voids determines the available area for sorption. The interaction mechanisms are governed, most likely, by hydrophobic as well as solvation effects and interplay of molecular and supramolecular interactions such as hydrogen bonding, π-cation/anion interactions, π-π stacking, ion-dipole and dipole-dipole interactions, the extent of which is naturally dependent on micropollutant and polymer characteristics. Systematic investigations are required to identify and quantify both relative contributions and strength of such interactions and develop suitable surface characterisation tools. This is a difficult endeavour given the complexity of systems, the possibility of several interactions taking place simultaneously and the generally weaker forces involved