Electrokinetic characterisation of particle deposits from streaming potential coupled with permeate flux measurements during dead-end filtration

Although the electrokinetic characterisation of particle deposits during filtration is of a great importance for better understanding of the mechanisms involved in the deposit build-up, few studies have been devoted to this characterisation so far. The present study focuses on the description and validation of a method for in situ and real time electrokinetic characterisation of cakes. This method is based upon simultaneous measurements of transversal streaming potential and permeate flux before and during the cake formation. The measurements of the streaming potential at a constant pressure have been preferred to those obtained by pressure steps in order not to modify the cake structure. Indeed, a densification of the cake structure with pressure steps might lead to pressure-dependent streaming potential coefficients. Mono-disperse suspensions of (negatively charged) polystyrene latex and (positively charged) melamine at various particle concentrations were filtered in a dead-end mode through a negative polyethersulfone membrane. It was found that the global streaming potential coefficient increased in absolute value with the deposited latex quantity, whereas it decreased and changed of sign during the filtration of melamine suspensions. The streaming potential coefficient of the deposit was deduced from the electrokinetic and hydraulic measurements performed through the membrane and “membrane + deposit”. The method was validated by (i) the right charge sign obtained for latex and melamine deposits, (ii) the independence of the streaming potential coefficient of deposits on both deposit thickness and growth kinetics. The main advantage of the method is its ability to evaluate the streaming potential coefficient of the single deposit without disturbing its build-up. Nevertheless, it is limited to the characterisation of sufficiently resistive deposits.