 Since several lectures I have been discussing about the geomaterial characterization and in particular the chemical characterization. We have discussed lot of things under the realm of chemical characterization and in particular in the lecture we were discussing about the contaminant transport through porous media and this is where I had coined this concept of sorption, desorption mechanisms or the characteristics we had discussed about the one-dimensional advection diffusion equation and I had also informed you that this is the best way to quantify the geomaterial contaminant interaction. So today's discussion would be revolving around the mechanisms of sorption desorption and how would you determine the KD parameter because if you remember in the previous lecture when we were discussing about the one-dimensional adve there I had pinpointed that unless you know the KD parameter you cannot get the retardation coefficient R and you cannot solve the equation which would give you the concentration of contaminant with respect to distance and time. So this is where the KD becomes very, very important. In other words KD is also important to quantify the amount of sorption or the reverse process which is desorption is going to occur. Incidentally KD is known as the distribution coefficient and having done this I will go back to thermal characterization, electrical characterization and magnetic characterization. So to begin with the first question in mind is that what is sorption? Sorption is a mechanism which constitutes of two components that is absorption and adsorption. So as the name suggests absorption is something which is assimilation, inclusion or incorporation of something in a matrix while the adsorption process is surfacial. This is accumulation of substance on the surface and when we say surface this has to be a sort of a substrate like a clay particle alright. So imagine that on the clay particle the cations are getting packed. So this is a sort of a adsorption phenomena. However coming back to the absorption the molecules or the atoms they have a tendency in the bulk to occupy the porous space or the pores. So like absorption of water by the porous media particularly like sponges or stones which are having very high porosity. So this is the absorption process. The adsorption is something where the atoms and molecules move from the bulk space that is the solid liquid or gases and they get deposited onto solid or liquid surface. So this becomes the adsorption process. So as the name suggests the adsorption is going to be something which is related to adherence alright. We have used this term in conventional geomechanics also in the form of the hygroscopic moisture content where the thin layer of water gets adhered onto the particles alright. And that thin layer could also contain the contaminants and when this process happens we say there is addition of a thin layer of molecules of some substance which is surrounding the surface of the solids or the liquids. So this becomes adsorption. I hope you can realize from this comparison that until now in classical geomechanics we have been using the concepts of absorption. The simple answers to this type of discussions would be if you have to find out the porosity of a porous media. So suppose if I give you a rock sample and if I ask you what is the porosity. So the normal procedure is you soak the water. So soak the sample in the water for 3-4 days, take it out, find out the weight difference of the you know wet to dry and then compute the moisture content and it gives you a sort of indication of what the porous structure would be. Very gross way of finding out the porosity of the system. Because with the discussion which we had until now in this class you must have realized that water molecules might not be penetrating to the very fine pores which are present in the porous media. And hence this method is empirical method, it does not give you the complete porosity particularly the porosity which is going to contribute to most of the mechanisms which control the contaminant transport in the geomaterial. So another way of looking at this would be suppose there is a particle of activated charcoal which happens to be a substrate and this activated particle is attracting the gas molecules or the liquid molecules to get adhered on the surface and this is what the absorption phenomena would be. I can use this concept to create filters of different types alright. So if I create a situation where the particles or the substrates are extremely active chemically these particles are going to attract the cations which are present in the sludges alright or the gas molecules and they have a tendency to create a bonding with them and hence the purification process can be done. So by definition this type of situation can be defined as the purification by adsorption wherein impurities are filtered from the liquids or the gases by their adsorption onto the surface of a high surface area solid which is a substrate such as an activated charcoal. I am sure you must have realized that what we have done is from the liquid or the gaseous phases we have separated out some molecules or atoms which are adhering on the substrate. So we have created a sort of a partition we have created a partition between the system from the liquid phase it has got separated to a phase where these molecules or atoms come and sit on a solid surface and that is the reason we call this as a partitioning phenomena or distribution phenomena alright. So imagine there is some concentration of contaminants in the liquid or gaseous form and when I bring this system very close to an activated surface which could be clay naturally activated or chemically activated. So what is going to happen these atoms or you know molecules they will get parked on the surface of the system which is substrate and hence the partitioning has occurred. This is the concept of sorption phenomena is this okay. Now the terms which we use in sorption for defining the sorption mechanism are adsorbates and the second one is adsorbent. So substrate is also known as adsorbent substrate is a clay particle charcoal particle any surface which is participating in adsorption phenomena adsorbates are the ones which get sobbed onto the surface mostly molecules or the atoms alright cations let us say. So they will get palped on the substrates or adsorbents. So in case of the adsorbed cations they get tightly held on the surface of the negatively charged tri clay particles clay particles are the substrates and the cations are adsorbates I think I have discussed and you must be able to realize what it is. So in a very layman's language the sorption phenomena can be you know simulated like designing a parking lot. So imagine in parking lot what happens most of the vehicles come and they get parked. So many times we use the term that molecules get parked on a parking lot. So this parking lot is nothing but the substrate a clay particle and the vehicles which are coming and getting parked are the molecules or the cations alright. Now I think you can visualize the whole scenario that several trucks and you know the small cars and the big cars competing together to get a place in the parking lot. Something of them of the same sort is happening in case of sorption phenomena also. So sorption would depend basically on the size of the cations as well and the valency of the cations as well and the chances are depending upon their valency they might get preference. But a bulky cation might have to wait because of its size clear. So when we were talking about cation exchange capacity of the geomaterials one of you had asked this question that you know what is the significance of washing several times and allowing the geomaterial to interact with sodium acetate and calcium chloride solution. So I think now you can realize what we have done in the process of determining the cation exchange capacity is we had allowed sorption and desorption to occur clear. So all the sodium ions which are present in the geomaterial under natural circumstances will get displaced by the cations like calcium which have higher preference clear. So this is the sorption process and then in the next step what we have done is we have washed this with the water. So we have done a desorption process. Ultimately what we have done is by repeating this cycle we have tried to quantify how many cations can still remain parked on the surface of the adsorbent. This is okay. So that is the quantification which you have already done. So this is the basic model. If you take the clay particle which is negatively charged the cations will come and get parked on the surface and this is a sort of a sorption process. So I just need a surface. This might happen in air also aerosol formation. So during winter, mornings and nights what you have observed you know the particulate matter which is present in the environment because of the temperature attracts dust particles and these dust particles get logged over there and they cannot precipitate because of their very low density clear. So this could happen in the gaseous phase also desorption phenomena. So desorption is a reverse process. So until now what we talked about is the sorption process. What is desorption? Desorption is the phenomena where the substance is released from or through a surface okay watching cleaning up process is a reverse process of the sorption phenomena. So this is opposite to the sorption process and this type of mechanism is going to occur once the sorption has got equilibrated. After eating sufficient food, what is our tendency we repel the food is it not. So after getting saturated, the tendency is that you say no I would not take anything it is a sort of a aversion, it is a sort of a repulsion. So that means the desorption process will always start after the sorption has occurred and it has achieved its equilibrium position. So equilibration is between the fluid phase that is the gaseous phase or the liquid phase and an absorbing surface. So this absorbing surface could be the boundary of the two fluids or this could be the solids like particles. So I am sure now you are realizing that these mechanisms would control the you know quantification of contaminant geomaterial interaction. Suppose if I ask you a question whether a geomaterial is going to interact with a contaminant or not, the best way to answer this question would be you allow the interaction to occur and see how much concentration of contaminants has got plugged onto the surface. If it is insignificant the you know analysis of the answer would be either your geomaterial is passive or the contaminants are also passive but if it happens to be substantial then obvious the indications would be that contaminants are also active and surface is also very active. So these are the two extremes. You can always create another two extremes by saying that the contaminants are less active, surface is more active, surface is less active and contaminants are more active. So when the concentration or the pressure of the substance in the bulk phase is lowered you know some of the soft substance make move out of the system and they may get again dissolved into the bulk phase. This is what is known as depressurization technique. So if you lower down the pressure what will is going to happen? There could be a process by which the bonding between the surface and the molecules gets reduced and hence molecules are free to move into the phase through which they came onto the surface. These are some basic concepts which I thought I will give you before we start discussing further. Yeah you can apply something correct so you can suck out the contaminants and that is the technique which is normally used in field also to decontaminate the soils. So what I should be doing first I will add some water into the soil so that the cations will leach out into the pore solution and then I may suck out the pore solution and in the process of sucking out the pore solution it might so happen that this pore solution will also accelerate the desorption process of the cations or the molecules. So you are inducing a pressure gradient which might be resulting into the desorption process. Is the mechanism clear? They are in sorption process how can we make sure that only the contaminants are getting adsorbed to the substrate. Like useful whether this will be a D from the geomaterial can also. No repeat your question please. How can we make sure that only the contaminants from the geomaterials are getting adsorbed to the surface. Adorbed or desorbed. Adorbed. So this is the process of sorption because only cations will get solved on a surface. Because of the clay particle as negative charge and cations are positively charged so what is going to happen only the positive will come and sit on the negatively charged particles. If the surface happens to be neutral then this is not going to happen. So you may extend this logic to the situation that sands will not sorb anything. So when you are designing the filters which are made up of sands their sorption desorption mechanism will not work because sands are neutral particles fine. But most of the time what do you see what do you see is that these filter beds are granular media are you realizing where the engineering would be. So if I take these sand particles and if I activate them by using some bioactivity Now what I have done I have done the coating on the surface of the sand particles in the form of a microbial coating and these microbes are themselves negatively charged. So practically there is no difference between the clay surface and the sand surface as far as the charge carrying capacity is concerned. Now this is the technology which you have done you have converted a material which was inert into a electromagnetically charged particle by putting a layer of bacteria got it. Now this is fit for acting as a environmental filter you are talking about whether we can create barriers by using this concept or not yes it is a barrier system. So when we talked about permeable reactive barriers yeah so these are the good examples your permeable reactive barrier has to sort everything at the same time it allows permission through it also. So that means absorption is going on as well as adsorption is also going on imagine you close your eyes and imagine that unless the fluid enters into the pores alright adsorption is not going to occur. So for adsorption to occur first thing is absorption, just a minute, just a minute. So that means the fluid phase has to enter the porous media imagine all the pores are filled up with the fluids when there is sufficient time now what is going to happen now there is exchange phenomena which is going to get actuated between the fluids which are present in the pores and the solid matrix. So absorption has to happen first followed by adsorption clear once everything gets saturated if you take out this system and put it in fresh water what is going to happen concentration gradients are going to develop and higher concentration which is present in the system porous media would move out and this becomes a desorption process. So when you design PRBs you have to talk about desorption-sorption mechanisms yes please. How good they are in like the activator clay particle how good they are in like arresting the heavy metals. These all heavy metals no cations are heavy metals like yes. So when you design environmental filters basically you wanted to get rid of heavy metals which are cations, strontium, chromium, mercury, nickel, cadmium whatever. These are mostly like we assume that in a landfill each of these are the primary. Yes correct, correct. So there are lot of studies where people have done where they have utilized you know different type of contaminants, synthetic as well as naturally occurring lot of studies have been done where the synthetic groundwater has been created and particular this type of studies are done in the atomic industry where you try to simulate the groundwater with different species of cations and then you try to see the competitive sorption taking place and what is the net result of you know sorption phenomena. So these studies are quite prevalent fine. I will try to explain it more and more when as we move on anything else. So this is a simple depiction of sorption desorption process. If you consider the clay particles you know there is a adsorbed cation which is adhering to the surface of the clay particles in the dry condition there is no fluid. So cations are strongly sorbed on clay particles fine. A good example of this would be most of the time you use talcum powder. Why? Have you ever questioned? Talcum powder is in the dry form. So why do you use it on your skin? Question this. That is a beautiful example and talcum powder is nothing but a sort of a clay particle very fine particle surface area is very high mineralogical it is very active. So what is going to do is going to soak all your sweet, agreed? So basically this is a sorption process which is taking place on talcum powder and of the sweet. This is negatively charged fluid having lot of bacteria in it. So this is the mechanism. Now add a little bit of water to this. So the moment you add little bit of water to this what happens the water molecule goes and sits inside the system where the cations and the clay platelets are already present. So the water molecules enters into the interlayers we call them as interlayers once the water is added. A stage comes where this system becomes saturated like this. And once the system becomes saturated you must be noticing that clay layers are getting separated out. Why? This is diffuse double layer concept clear? So diffuse double layer once it gets saturated completely the desorption process starts is a simple mechanism. So once the cations get fully hydrated so water acts as a source of you know the mechanism which causes sorption and desorption to occur because fluid phase particularly gaseous phase sorption is going to be a difficult process to simulate why? It is not easy to park gas particles on a substrate though people are trying to do this carbon sequestration is a good example of this. Cold bed methane is another good example of this CBM. So if you type on net you will get lot of information you know CO2 sequestration and cold bed methane CBM. So what they are trying to do they are capturing carbon dioxide from the atmosphere injecting it into the porous media. So they are recharging the reservoirs of hydrocarbons which we are otherwise consuming and this becomes a sustainable solution fine yes. Water has a strong adherence as compared to the negative the cations like from the picture it depicts like this. It can come in way and replace that cation that were earlier adhered. Is this possible? I mean there is something known as solubility of the cationic suspensions yes it is possible. And sir one question is like slightly different from this at what really that way that day we were discussing like double layer is fully formed at very high water content. So in the simple Atterberg limits we simply say it is around liquid limit it is formed but do you have any idea like around what water content this double layer is fully formed or there is like ambiguity in that. See double layer normally is not ascribed to the moisture content first of all. Double layer is ascribed to the thickness of the system the cationic layer which is getting adhered onto the substrate. So double layer is always at you know associated with the size of the clay particles after the cations have stabilized on it. They have adhered on this we do not normally attribute it to the moisture content. Like compaction theory and everywhere we always talk about this phenomena. So there we are not talking about these mechanisms double layer and all we do not talk about there. We should have in fact sufficient amount of free water for double layer to get formed. So that means all these concepts are valid beyond the liquid limit or at liquid limit.