1、Chapter 1The Electrokinetic ConnectionParticle Charge Prevents CoagulationThe key to effective coagulation and floccu-lation is an understanding of how individ-ual colloids interact with each other. Tur-bidity particles range from about .01 to 100microns in size. The larger fraction isrelatively eas
2、y to settle or filter. Thesmaller, colloidal fraction, (from .01 to 5microns), presents the real challenge. Theirsettling times are intolerably slow and theyeasily escape filtration.The behavior of colloids in water is stronglyinfluenced by their electrokinetic charge.Each colloidal particle carries
3、 a like charge,which in nature is usually negative. Thislike charge causes adjacent particles torepel each other and prevents effectiveagglomeration and flocculation. As aresult, charged colloids tend to remaindiscrete, dispersed, and in suspension.On the other hand, if the charge is signifi-cantly
4、reduced or eliminated, then thecolloids will gather together. First formingsmall groups, then larger aggregates andfinally into visible floc particles which settlerapidly and filter easily.Uncharged Particles are free to collide and agregate.Charged Particles repel each other1Chapter 1The Electrokin
5、etic ConnectionMicroscopic Electrical ForcesThe Double LayerThe double layer model is used to visualizethe ionic environment in the vicinity of acharged colloid and explains how electricalrepulsive forces occur. It is easier to under-stand this model as a sequence of stepsthat would take place aroun
6、d a singlenegative colloid if the ions surrounding itwere suddenly stripped away.We first look at the effect of the colloid onthe positive ions, which are often calledcounter-ions. Initially, attraction from thenegative colloid causes some of the positiveions to form a firmly attached layer aroundth
7、e surface of the colloid. This layer ofcounter-ions is known as the Stern layer.Additional positive ions are still attracted bythe negative colloid but now they are re-pelled by the positive Stern layer as well asby other nearby positive ions that are alsotrying to approach the colloid. A dynamicequ
8、ilibrium results, forming a diffuse layerof counter-ions. The diffuse positive ionlayer has a high concentration near thecolloid which gradually decreases withdistance until it reaches equilibrium withthe normal counter-ion concentration insolution.In a similar but opposite fashion, there is alack o
9、f negative ions in the neighborhood ofthe surface, because they are repelled bythe negative colloid. Negative ions arecalled co-ions because they have the samecharge as the colloid. Their concentrationwill gradually increase as the repulsiveforces of the colloid are screened out by thepositive ions,
10、 until equilibrium is againreached with the co-ion concentration insolution.Positive Counter-IonNegative Co-IonTwo Ways to Visualize theDouble LayerThe left view shows thechange in charge densityaround the colloid. Theright shows the distributionof positive and negativeions around the chargedcolloid
11、.Highly NegativeColloidStern LayerDiffuse LayerIons In EquilibriumWith Solution2Double Layer ThicknessThe diffuse layer can be visualized as acharged atmosphere surrounding thecolloid. At any distance from the surface,its charge density is equal to the differencein concentration of positive and nega
12、tiveions at that point. Charge density is great-est near the colloid and rapidly diminishestowards zero as the concentration of posi-tive and negative ions merge together.The attached counter-ions in the Sternlayer and the charged atmosphere in thediffuse layer are what we refer to as thedouble laye
13、r.The thickness of the double layer dependsupon the concentration of ions in solution.A higher level of ions means more positiveions are available to neutralize the colloid.The result is a thinner double layer.Decreasing the ionic concentration (bydilution, for example) reduces the numberof positive
14、 ions and a thicker double layerresults.The type of counter-ion will also influencedouble layer thickness. Type refers to thevalence of the positive counter-ion. Forinstance, an equal concentration of alumi-num (Al+3) ions will be much more effectivethan sodium (Na+) ions in neutralizing thecolloida
15、l charge and will result in a thinnerdouble layer.Increasing the concentration of ions or theirvalence are both referred to as double layercompression.Diffuse LayerIon ConcentrationIon ConcentrationLevel of ions in solutionDiffuse LayerDistance From ColloidDistance From ColloidLower Level of Ions in SolutionVariation of Ion Density in the Diffuse LayerIncreasing the level of ions in solution reduces thethickness of the diffuse layer. The shaded arearepresents the net charge density.Higher Level of Ions in Sol
