Because small particles suspended in water have a higher interfacial energy than large particles, small particles have a tendency to form larger aggregates or flocs. Spontaneous flocculation of particles, however, is often prevented by electrostatic repulsion. This electrostatic repulsion occurs when particles carry a surface charge. Microalgae usually have a negative surface charge, the result of carboxylic, phos — phoryl, and amine/hydroxyl groups on the microalgal cell surface. Most important are the carboxylic groups, which are deprotonated at pH above 3-4, and thus have a negative charge. Phosphoryl groups become anionic above pH 7-8. Amine and hydroxyl groups lose a proton below pH * 10 and become, respectively, uncharged and negatively charged. The overall effect is net negative microalgal surface charge in most natural waters (Hadjoudja et al. 2010). Few studies, however, have inves­tigated the contribution of active groups on the microalgal cell surface to the surface charge, and more fundamental research is needed to better understand the linkage between microalgal surface charge and flocculation (Brady et al. 2014).

The electrical double layer consists of the negatively charged surface of the cell and the positively charged cloud of counterions close to the cell surface. The zeta potential, or Z potential, is the potential difference between the bulk solution and the slip plane in the electrical double layer (Fig. 12.2). The counterions that are between the surface and the slip plane remain associated with the cell when the cell is moving through the solution. The sign of the Z potential and the surface charge are the same. The Z potential is relatively easily quantified by measuring the mobility of the charged particles in an electric field (Ozkan and Berberoglu 2013) and is a useful measure of the stability of microalgal suspensions. A Z potential of -25 mV or less generally indicates a stable suspension, while a Z potential between -10 mV and 0 mV points to a low stability and will generally coincide with flocculation.

The negative surface charge of microalgal cells is largely controlled by depro­tonation of carboxylic functional groups. When pH is decreased in a microalgal suspension to levels below 3-4 where carboxylic groups are uncharged, the Z potential approaches 0 mV (Hadjoudja et al. 2010). Indeed, flocculation can be induced by reducing the pH to below 4 (e. g., Liu et al. 2013). Chemical modifi­cation of functional groups on the cell surface through oxidation by ozone, chlorine, potassium permanganate, or potassium ferrate addition may also facilitate floccu­lation (Sukenik and Shelef 1984; Henderson et al. 2008b).

The ionic strength of the medium has an important influence on the Z potential. At high ionic strength (e. g., seawater), the electrical double-layer surface is com­pressed and the Z potential becomes less negative, allowing particles to approach it other and to flocculate. For example, a suspension of clay particles will flocculate when the ionic strength of the medium is increased. High concentrations of divalent cations (e. g., Ca2+ or Mg2+) can also compress the double layer and, if present in sufficient concentrations, reverse the otherwise negative charge of surfaces. Despite the strong influence of ionic strength on the electrical double layer, there are no indications that suspensions of microalgae that live in seawater are less stable than

those of freshwater microalgae. This suggests that factors other than electrostatic repulsion also contribute to the stability of microalgal suspensions.

One factor that might play a role is steric stabilization by polymers that are associated with the microalgal cell surface (Fig. 12.3). Large polymers such as polysaccharides that are attached to the microalgal cell surface can extend into the

surrounding medium. These polymers may prevent cells from approaching each other and can therefore stabilize microalgal suspensions. More research is needed to better understand what factors contribute to the observed stability of microalgal suspensions, particularly in seawater.