Growth and structure of flocs following electrocoagulation

Abstract

Abstract The growth and structure of iron precipitate flocs produced in salt water from a bench-scale electrocoagulation (EC) system was investigated. During floc growth, changes in the scattering exponent, an indicator of a flocs’ degree of compaction, were reflected by changes in the particle size (diameter of a volume equivalent sphere). Flocs initially formed loose, open structures that spanned broad size ranges, suggesting low repulsion between particles. The initial aggregates then broke and reformed into more compact structures. Comparing plots of scattering exponent against time, it was found that operating at higher currents caused this process to occur more quickly; however, the plots were all similar in shape, suggesting that the structural progression of the EC flocs was not affected. The final floc structures had an average scattering exponent of 2.34 (standard deviation 0.02), which is consistent with literature for flocs produced from iron-based chemical coagulants despite differences in electrolyte ionic strength. Analysis via transmission electron microscopy (TEM) suggested that EC flocs also exhibited amorphous, fractal structures. Operating at higher currents (providing larger iron concentrations) resulted in larger flocs. The average steady-state floc sizes when operating at 0.087, 0.174, and 0.261 A were 93, 147, and 191 μm, respectively. Floc size distributions also reached steady-state more quickly due to the higher frequency of particle collisions. Using a stainless steel (SS) cathode rather than an aluminum one resulted in 15% larger flocs (154 μm and 134 μm, respectively). However, given that experimental conditions (mixing and current) were otherwise equivalent, more research is required to determine the cause of this difference. The results of this work were consistent with literature regarding chemical coagulation (CC) and may have implications for the design of downstream clarification processes.