Coalescence turbidity

Water and Waste Water Treatment. PAG products are used in water treatment for removal of suspended soHds (turbidity) and other contaminants such as natural organic matter from surface waters. Microorganisms and colloidal particles of silt and clay are stabilized by surface electrostatic charges preventing the particles from coalescing. Historically, alum (aluminum sulfate hydrate) was used to neutralize these charges by surface adsorption of Al cations formed upon hydrolysis of the alum. Since 1983 PAG has been sold as an alum replacement in the treatment of natural water for U.S. municipal and industrial use. 

It has been known for some time that on the addition of a very small amount of suitable electrolytes to a sol, the corresponding substances are thrown out of the solution and form a precipitate. If a suitable quantity of sodium chloride is added to a ferric hydroxide sol, the sol becomes turbid and ultimately precipitates. The electrolyte thus causes the colloid particles to coalesce. So, the process by means of which the particles of the dispersed phase in a sol are precipitated is known as coagulation or flocculation. 

Table II. Turbidity Tq of a Suspension of Coalescing Aggregates as a Function of the Turbidity Tq of a Suspension of Uncoalesced Singlets...

Although the coalescence assumption may lead to small errors in the scattering cross section, it does enable the rapid correlation of turbidity data (n) with electronic particle counter data (n and fi), provided the relative refractive index and size regime are known. Because the electronic particle counter measures the aggregate volume, and therefore determines only an equivalent spherical radius, the data from the electronic particle counter relate only to the coalesced-sphere approach. 

In summary, the diflBculties in determining aggregate form factors, particle form factors, phase shifts, and distribution functions combine to make the Rayleigh-Debye approach too complicated for practical application. On the other hand, the coalesced-sphere approach using the Jobst approximations to the Mie scattering eflBciencies allows rapid correlation of turbidity with particle size distributions. Consequently, a coalesced-sphere approach was adopted for experimentation in the E. coli-PEI system. 

Typical coacervation occurs with the combination serum albumin — gum arabic on acidification. With the microscope one sees beautiful thoroughly liquid coacervate drops, which readily coalesce with each other (see p. 233, Fig. 2). Immediately after the production of the coacervation the latter is completely reversible added KCl completely suppresses the coacervation. If however one adds KCl some time after the coacervation has been brought about, the system no longer clears completely but remains turbid to an increasing extent the longer one has waited before adding KCl. 

Fig. 12 gives similar results with the so-called coacervate volume method. In contrast to the viscosimetric or turbidity method this is a direct method. It is serviceable in sufficiently concentrated colloid mixtures. In it one reads off directly the volume of the separated coacervate in a graduated tube (after sedimentation and coalescence to a homogeneous liquid column). Here also we see that added salts suppress coacerv-ation and that the double valency rule holds. 

The Carboxyfluorescein concentration of the vesicle suspension Cv and after destroying the vesicles with the surfactant Triton X 100 C/ were calculated from the fluorescence intensity of the diluted aqueous solutions. The turbidity of the vesicle solution declined within seconds after detergent addition (vesicle busting), and we obtained then a clear, aqueous solution. Typical results of these measurements are summarized in Fig. 12. Due to Ihe self-quenching properties the destruction of the vesicles with a high inner Carboxyfluorescein concentration (0.05-0.2 mol/1) led to an increase of the Fluorophore concentration in the outer phase. This occurred if a large amount of emulsion droplets coalesced with the lower water phase, thus releasing their Carboxyfluorescein content. 

---