Effect of polarity reversal on floc formation and rheological properties of a sludge formed by the electrocoagulation process

•Electrocoagulation polarity reversal improves sludge properties and reduces fouling.•Interfacial pH depends on electrode material, mode, and polarity reversal duration.•Hydrogen bubbles in cathodic cycles can enhance floc mechanical properties.•The optimal frequency for polarity reversal should be determined according to the solution chemistry. Anode fouling is one of the key limiting factors to the widespread application of electrocoagulation (EC) for treatment of different types of contaminated water. Promising mitigation strategy to fouling is to operate the process under polarity reversal (PR) instead of direct current (DC). However, the PR operation comes at the cost of process complexity due to the alternation of electrochemical and chemical reactions. In this study, we systematically investigated the link between evolving fouling layer during DC and PR close to iron and aluminum electrodes and morphological and rheological properties of the formed sludge. By operando visualization of EC process, we demonstrate that during PR operation, precipitation of the iron and aluminum species occurs close to the anode interface, resulting in flocs with higher porosity and lower density than those formed under DC conditions. However, rheological investigation revealed that the PR conditions resulted in a sludge with more pronounced solid-like signature, but this enhancement in its viscoelastic properties is closely related to a period of the current's polarity reversal. We attribute this unexpected result to higher shear rate and collision of particles during PR conditions. [Display omitted]