Sol-Gel Materials for WSUD Water Treatment Applications - ACS

Chapter 2

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Sol-Gel Materials for WSUD Water Treatment Applications Stephen R. Clarke,* Elda Markovic, and Kim-Anh Thi Nguyen Mawson Institute, University of South Australia, Mawson Lakes, South Australia, 5095, Australia *E-mail: [email protected]

ARC Linkage LP0777033 funded a two-year WSUD water treatment project, titled “Modifying and Improving Porous Sol-Gel Materials for Water Purification”, supported by international research partners from the National University of Singapore, Public Utilities Board (Singapore), Deltares (Holland), and United Water (Australia). This research was undertaken by Dr Clarke’s research laboratories (now at the University of South Australia) who employed silicon polymer chemistry and nanotechnology to develop a sol-gel material with the potential to offer water treatment at a very low energy. This research was supported by international and Australian commercial partners, along with Australian Government funds from the Australian Research Council (ARC), which provided a majority of the research funds for this work.

Introduction Deltares (formerly Delft Hydraulic – Netherlands) joined forces with the National University of Singapore (NUS) to form the Singapore-Delft-WaterAlliance (SDWA) which has its operations based at NUS, in Singapore. SDWA subsequently linked with Dr Clarke’s research laboratories and United Water researchers to receive an Australian Government funded ARC Linkage grant to use organosilicon and silicone modified materials for the treatment and purification of run-off storm water. This international project linked Australian, Singaporean, Dutch and French researchers, wishing to improve quality of life through the development of new water treatment technologies. © 2013 American Chemical Society In Progress in Silicones and Silicone-Modified Materials; Clarson, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


Singapore and Australia, like most nations around the globe have considerable issues with unsightly concrete drains, as shown in Figure 1. These concrete drains are used to re-direct vast quantities of excess storm water and run-off water into our lakes, seas and oceans.

Figure 1. Concrete storm water drains used extensively in Australia, in Singapore and around the world

In times of growing water scarcity, generally attributed to climate change, it seems amazing that in many countries, including Singapore and Australia, which have major issues with water supply, that this waste storm and run-off water is rarely collected and treated for subsequent re-use. The purpose of the research in this project was to develop organosilicon and silicon modified materials, for the treatment and purification of run-off and storm water, allowing unsightly concrete drains shown in Figure 1 to be transformed into people friendly waterways as shown in Figure 2. The silicon polymers developed in this research allow treated waste water to be cleaned, by removing harmful dissolved inorganic and organic toxins. The organosilicon and silicone modified materials synthesised by Dr Clarke’s research team (formerly at Flinders University) but now at the University of South Australia in Australia were applied to water treatment technologies developed by SDWA in Singapore, and United Water in Australia.

16 In Progress in Silicones and Silicone-Modified Materials; Clarson, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


Figure 2. An impression of what stark concrete stormwater drains should look like using sol-gel water treatment materials.

Sol-Gel Chemistry It was proposed for this project, from the outset, to employ organosilicon sol-gel chemistry to functionalize silica filtration media that was being used by SDWA and United Water. It was believed these materials could enhance the removal of soluble inorganic and organic toxins.Clarke’s laboratories believed the science of choice to best achieve this outcome was sol-gel chemistry (1, 2). The sol-gel process involves the hydrolysis and condensation reactions of the starting alkoxides as shown in Figure 3.

Figure 3. The general hydrolysis and condensation chemical reactions involved in the sol-gel process. 17 In Progress in Silicones and Silicone-Modified Materials; Clarson, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


However, a number of factors affect the rate of hydrolysis and condensation deemed responsible for the properties of a particular sol-gel inorganic network such as, pH, temperature and time of reaction, reagent concentrations, nature and concentration of catalyst, H2O/Si molar ratio (R), aging temperature and time, and drying (3, 4). The sol-gel technique is a two step process that involves the formation of inorganic networks through the evolution of a colloidal suspension (sol) and gelation of the sol to form a network in a continuous liquid phase (gel) (5). Metal alkoxides are mostly used because they react readily with water. The most widely used metal alkoxides are the alkoxysilanes, such as tetramethoxysilane (TMOS) and tetraethoxysilane (TEOS). Other alkoxides such as aluminates, titanates, and borates are also employed in the sol-gel process, very often in mixtures with TEOS. Using this process, it is possible to produce ceramic or glass materials in various forms; ultra-fine or spherical shaped powders, thin film coatings, ceramic fibbers, microporous inorganic membranes, monolithic ceramics and glasses, or extremely porous aerogel materials. In general, silanol formation occurs by an hydrolysis reaction, where a water molecule replaces the alkoxide group (OR) with hydroxyl groups (OH). This is followed by subsequent condensation reactions involving either the conversion of silanol groups (Si-OH) to siloxane bonds (Si-O-Si) with water being the leaving molecule, or the reaction of a silanol with an alkoxy group and alcohol being the leaving molecule. This results in the formation of water and alcohol as by-products of the sol-gel process. In most cases, condensation starts before hydrolysis reaction is completed. However, variations in conditions such as, pH, H2O/Si molar ratio (R), and catalyst can drive completion of hydrolysis before condensation begins (6). Hydrolysis can occur without addition of an external catalyst, but it is most rapid and complete with the addition of catalyst. Mineral acids (HCl) and alkalis, such as ammonia, are most commonly used, however, other catalysts are acetic acid, KOH, amines, KF, and HF (1). In addition, it has been observed that the rate and extent of the hydrolysis reaction is most influenced by the strength and concentration of the acid or base catalyst. All strong acids behave similarly, whereas weaker acids require longer reaction times to achieve the same extent of reaction. Compared to acidic conditions, base hydrolysis kinetics is more strongly affected by the nature of the solvent (7).

Acid Catalysed Hydrolysis Mechanism When acid is used as catalyst, protonation of an alkoxide group is a rapid first step (Figure 4). Electron density is withdrawn from the silicon atom, making it electrophilic and thus susceptible to water attack. This results in the formation of a penta-coordinate transition state with significant SN2-type character. 18 In Progress in Silicones and Silicone-Modified Materials; Clarson, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


Figure 4. Acid catalysed hydrolysis mechanism Base Catalysed Hydrolysis Mechanism Under basic conditions, the hydrolysis reaction is first-order in base concentration (Figure 5). However, if the silane concentration is increased, the reaction changes from a first-order to a more complex second-order reaction, related to silanolate cluster formation. If weaker bases (ammonium hydroxide and pyridine) are employed, higher speeds of reaction are possible only if large concentrations are used. Compared to acidic conditions, base hydrolysis kinetics is more strongly affected by the nature of the solvent (7).

Figure 5. Base catalysed hydrolysis mechanism Therefore, base-catalysed hydrolysis of silicon alkoxides proceeds much more slowly than acid-catalysed hydrolysis at an equivalent catalyst concentration (7). Under basic conditions, it is likely that water dissociates to produce hydroxyl anions in a rapid first step. The hydroxyl anion then attacks the silicon atom. Again, an SN2-type mechanism has been proposed in which the -OH displaces OR with inversion of the silicon tetrahedron. When the first hydrolysis reaction has occurred, the next alkoxide group is more easily removed from the monomer then the previous one and reaction proceeds stepwise (8). The hydrolysis reaction has been performed with R (H2O/Si molar ratio) ranging from 1 to over 50, depending on the desired polysilicate product, with an increase in R expected to promote the hydrolysis reaction, prior to condensation. It was found that the acid-catalysed hydrolysis of TEOS is firstorder in water concentration (7). However, it has also been observed that under basic conditions the rate has a zero-order dependence on the water concentration. Generally, with under stoichiometric additions of water (R

Sol-Gel Materials for WSUD Water Treatment Applications - ACS

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