Electrocoagulation versus chemical coagulation: Coagulation/ flocculation mechanisms and resulting floc characteristics Tali Harif*, Moti Khai, Avner Adin Soil & Water Sciences Department, Robert H. Smith Faculty of Agricultural, Food and Environmental Quality Sciences, The Hebrew University of Jerusalem, POB 12, Rehovot 71600, Israel article info Article history: Received 1 December 2011 Received in revised form 29 February 2012 Accepted 19 March 2012 Available online 29 March 2012 Keywords: Electrocoagulation Aluminum Floc Growth Zeta potential Scattering exponent abstract Electrocoagulation (EC) and chemical coagulation (CC) are employed in water treatment for particle removal. Although both are used for similar purposes, they differ in their dosing method e in EC the coagulant is added by electrolytic oxidation of an appropriate anode material, while in CC dissolution of a chemical coagulant is used. These different methods in fact induce different chemical environments, which should impact coagulation/floccu- lation mechanisms and subsequent floc formation. Hence, the process implications when choosing which to apply should be significant. This study elucidates differences in coag- ulation/flocculation mechanisms in EC versus CC and their subsequent effect on floc growth kinetics and structural evolution. A buffered kaolin suspension served as a repre- sentative solution that underwent EC and CC by applying aluminum via additive dosing regime in batch mode. In EC an aluminum anode generated the active species while in CC, commercial alum was used. Aluminum equivalent doses were applied, at initial pH values of 5, 6.5 and 8, while samples were taken over pre-determined time intervals, and analyzed for pH, particle size distribution, z potential, and structural properties. EC generated fragile flocs, compared to CC, over a wider pH range, at a substantially higher growth rate, that were prone to restructuring and compaction. The results suggest that the flocculation mechanism governing EC in sweep floc conditions is of Diffusion Limited Cluster Aggre- gation (DCLA) nature, versus a Reaction Limited Cluster Aggregation (RLCA) type in CC. The implications of these differences are discussed. ª 2012 Elsevier Ltd. All rights reserved. 1. Introduction 1.1. Coagulationeflocculation and factors determining floc evolution CoagulationeFlocculation in general is a two phase process aimed at removing stable particles by forming larger aggre- gates that can be separated from the aqueous phase by a subsequent separation step. The preliminary phase is the coagulation phase in which destabilization is induced, either by the reduction of repulsive forces between particles or by the enmeshment in precipitates (Hogg, 2005). For insoluble particles, such as many minerals (e.g. Kaolin), inter-particle repulsion is usually due to electrical double layer interaction. The addition of soluble ionic species will affect the surface potential (electrical potential difference between the particle surface and the bulk solution) of colloidal particles by either adsorption to the particle surface or by double layer compression. Ionic species that are specifically adsorbed at the surface can include multivalent cations and anions, ionic * Corresponding author. E-mail addresses: talih@atlantium.com (T. Harif), moty@ely.org.il (M. Khai), adin@vms.huji.ac.il (A. Adin). Available online at www.sciencedirect.com journal homepage: www.elsevier.com/locate/watres water research 46 (2012) 3177 e3188 0043-1354/$ e see front matter ª 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2012.03.034