In traditional coagulation, the pH value decrease as result of the reactions of hydrolysis of aluminum (Eq. 27 -Eq. 30), and an augmentation of pH is necessary in many cases of the wastewater treatment. However, in EC there is a buffer effect in water since there is also hydroxyl being generated as well on the cathode (Eq. 18). Thus, buffer behavior is certainly an advantage over the traditional chemical coagulation and it is even more advantageous for oilfield, since no handling and transportation of alkaline and dangerous substance is necessary. Harif et al. (2012) concluded EC is able to produce flocs in a wider range of pH at a faster apparent rate compared with the chemical coagulation. This would suggest EC is more suitable for smaller time of flocculation required.
For the reactions above, their equilibrium constants allow the calculation of each species concentration over the pH value. However, in wastewater treatment operations, the destabilization of the compounds can happen before the equilibrium of those mentioned reactions is achieved. Furthermore, these theoretical calculated concentrations have a simplification, which does not account for the polymeric species and those have an important influence on coagulation.
The abatement of the impurities, especially the negatively charged ones, is closely related with those positive surface charged specimens created by the coagulant added in the EC. The impurities found in the produced water can be grouped as:
45 soluble organic and inorganic compounds; dispersed organic and inorganic matter; and microorganisms.
Dispersed matter abatement
The electric repulsion force, described at the Colloidal Stability section, prevents particles from being approximate to one another. Nevertheless, a natural attraction force between particles (van der Waals) will be always acting. This balance of repulsive and attractive force is the basis of the DLVO (Derjaguin-Landua-Verwey- Overbeek) theory for understanding the stability of dispersed oil droplets or solids in the produced water.
The removal of the hydrophobic dispersed compounds must first pass through the destabilization to further agglomerate (flocculate) within a practicable time for processes and easily removed from water. The coagulants are compounds responsible to reduce the repulsive force between two charged particles and in EC, those are generated in-situ. This destabilization promoted by the coagulant can occur by four different mechanisms, as the theory is briefly described below:
COMPRESSION OF THE ELECTRICAL DOUBLE LAYER- the thickness of a double layer can be reduced introducing conter-ions on it with high valence (like Al3+)
and this will reduce the repulsive force by the distance. This allows particles come closer, increasing the probability of encountering and conglomeration. In the practice, this mechanism occurs in a very limited range of coagulant concentration and after this critical point restabilization occurs.
ADSORPTION/CHARGE NEUTRALIZATION- once the adsorption of counter-ions with high valence number in the surface of the charged negative surface occurs, the electrical repulsive energy is diminished with the distance. Summing the energy, the net interaction will turn into attractive since the van der Waals become dominate. In the practice, this mechanism occurs in a coagulant concentration higher than the critical concentration for the compression of electrical double layer and can be dominant for a larger range of coagulant concentration. Although, restabilization of dispersive state is possible if the coagulant concentration is much higher to the compared the pollutant concentration, since the net charge near of the surface will turn positive.
ADSORPTION/BRIDGING- the coagulant is polymerized and different parts of the polymer chain are adsorbed in other colloidal particles. This polymer bridges different pollutant particle and allow then to come closer, resulting in a bigger particle. If an overdose of coagulant is practice, the polymeric chain will be mostly adsorbed in only one particle surface creating steric resistance for approximation of another particle. In this case, a restabilization will occur. These both mechanisms involving adsorption will result in restabilization, as it is shown in Figure 4 at the point CSC (critical stabilization point). The adsorptive mechanism are dependent on the pollutant concentration and until enough coagulant concentration is not reached, no destabilization will be observed and this is illustrate at the point CCC1 (Critical coagulant concentration) in Figure 4.
ENTRAPMENT IN THE FLOCS (SWEEPING)- higher coagulant salt dosage in
water will form an insoluble precipitate in a broader pH range. These flocs migrating for the extreme of the vessel and conglomerating will entrap the dispersed particles on it, like sweeping the water phase. In this mechanism, no restabilization will occur. The higher dosage for this mechanism is demonstrate as the point C2 or CCC2 in Figure 4.
These mechanisms can happen simultaneously and the prevailing mechanism(s) can be dictated by the pH value, coagulant dosage, as mentioned in the previous subsection, and colloid concentration. For high colloid concentration, like at C1
illustrate in Figure 4, restabilization of the suspended state will not happen and only two regions are possible: insufficient coagulant or destabilization.
Although at very low colloid concentration until a critical point, point A, in Figure 4, little opportunity for two particles to make contact is seen and the coagulant dosage must be considerable high to destabilize. The only mechanism possible for this case is sweep coagulation. For intermediate colloid concentration, from point A to point C1 in Figure 4, four regions are possible with function of the augmentation of
coagulant dosage. Those regions are from lower to higher coagulant dosage: insufficient coagulant; enough coagulant for adsorption/destabilization; restabilization; and destabilization for enmeshment (Bratby, 2006).
Figure 4. scheme of destabilization and restabilization zones for colloid concentration and coagulant dosage at a given pH (adaptation of Bratby, 2006)
Soluble compounds abatement
The abatement of dissolved organic compounds can be made possible by the following mechanisms: enmeshment on the flocs generated, electrochemical oxidation, complexation of organic compound with hydrolyzed aluminum, and electrostatic attraction to the aluminum hydroxide (Hakizimana et al., 2017, Kabdaşlı et al., 2012).
For the ions, or functional groups in molecules easier to reduce or oxidize, oxidation- reduction reactions may happen on the electrode surface. This is the case for the reduction of Cr(VI) to Cr(III), and the oxidation of As(III) into As(VI). Additionally, some contaminants become adsorbed on the electrodes forming a passivation layer on the surface, as is the case for the fluoride. Abatement of hardness can occur with EC by the precipitation of carbonate and sulfate on the alkaline conditions at the cathodic region through the reactions listed in Eq. 31 to Eq. 37. Furthermore, carbonate salts can also be enmeshed into the flocs generated. The precipitation is
ZONE 1 Insufficient Coagulant