Unusual yield stress behaviour of silica suspensions in the presence of maleic and fumaric acids: Composite precipitate bridging

There are many instances of unusual colloidal suspension behaviour being reported. This study provided a new approach to finding the cause of one such behaviour helping to alleviate biofouling and scaling problems. The yield stress-pH behaviour of silica suspensions in the presence of maleate and fumarate is different depending upon the direction of the step change in the pH employed. In the direction from high to low pH, i.e. 11–2, the suspension displayed a continuous yield stress behaviour down to pH 6. In the opposite direction, the yield stress was small at pH 4–5, absent at pH 8.5 and increased sharply from pH ∼ 9.5. Precipitate bridging is responsible for the yield stress behaviour. At high pH, the precipitates were already well-formed to bridge particles. This was not the case in the low to high pH direction. The precipitates were composite formed by soluble silica, organic acid anion and positively charged metal ions hydrolysis products. Yield stress was observed under the conditions of i) high silica solubility, i.e. at pH > 9 and ii) fumaric or maleic acids being present in significant concentration. The Al(III), Fe(III) and Fe(II) impurities already present in relatively important quantity formed the hydrolysis products upon leaching or solubilisation of the silica. The composite precipitates bridged the silica particles via heterogeneous charge and van der Waals attraction. Their formation is limited by the amount of Al(III), Fe(III) and Fe(II) present and they must be small enough to be in sufficient number to have all the particles in the network bridged. Fumarate played a role in forming and stabilising the small precipitate particles, and enhancing the yield stress. Biofouling and scaling in processes involving this type of composite materials can be mitigated by making sure that a high pH operating environment is avoided.