 We have already discussed about the point of zero charge how it has to be determined experimentally thereby potentiometric titration or by zeta size which gives the zeta potential determined as a function of the p-s values and from that we can find out point of zero charge of the p-zc values. Now, as I have already mentioned that depending on the p-s values and the p-zc of this judgment or the clay mineral whatever we are talking about. So, based on that the metal and uptake can be decided. For example, if the p-zc value of a particular clay mineral is around 7 then the p-s value lower than 7 that will exist in the propanated form so naturally the uptake of the propanated species will not be there onto this mineral surface. But if we increase the p-s value to higher than back of the p-zc then this hydrogen ions are dissociated or removed from the surface of this clay mineral and we have the O minus or negatively charged species at the surface of this clay mineral and then which will be now forming the complex with the cationic species it present in the aquatic system. If it is forming a complex or if it is forming anionic species the actinide ion then it will naturally will not form complex at the p-s value higher than that of the p-zc. Now, coming to the surface charged characteristics of this minerals the figure shows that how you have this negatively charged in the faces you have and also the positives are edges are there. So, they will be actually binding with the cationic as well as the anionic species that is the cationic species will be binding with the paces and anionic species will be binding with the paces. Then in this case when the presence of the variable charge surfaces as shown here the number of surface sites are constant but their individual charge and the total surface charge will be vary as a function of the solution composition. The surface charge depends on the absorption surface binding of the potential determining ions such as the hydrogen ion and the formation of the surface complex also affect the surface charge. A mix of anion, cation and neutral species can jor and modify the place. Now, what are the factors which are affecting the absorption process that is number one it is the surface area and the amount of absorption site we have a very large surface area that is the same mineral you are having finer particles it will have a very large surface area for a particular weight of the clay mineral. So, in that case the surface area will increase naturally that will be also having higher uptake of the anionic ions of the metal ions and also the amount of absorption sites that is the density of the surface absorption sites in the mineral surface as shown here you have this bulk mineral phase which is shown here. Now, how many sites are there which are binding in the metal ion that also is dependent on the amount of actinide or the metal ion which is taken up by the mineral phase or the bulk mineral phase. Then the relative attraction of the aqua species to the absorption sites there are some other species which are present in the aqua space which are also going to the absorption sites then permanent structural charge and also variable charge these are the factors which are deciding the absorption process. Now, the mechanism of this can be two way first is the ion exchange which is happening in case of the clay minerals like spectraites and zeolites. So, in this case we have this example is given here that you have the neptunium ions are there. So, the charge is not given, but you can consider suppose neptunium plus 5 state that is neptunil ion is there. So, then how it will be forming complexes with the bulk mineral phase or it will be transporting as such. So, as shown here some of this neptunium will be forming of course, directly it will be forming complex with the surface of this bulk mineral phase then it can also bind with some of the mineral colloids with this bigger size species which is shown here this is the mineral colloids which are there. Now, what is the colloid that I will be discussing subsequently in this lecture. So, is this by this neptunium is binding to this mineral colloid which is there and this also can move there is a transport of this can be there, neptunium can as such it can be transported, but then it will be less the magnitude of transport will be much much less here with the very small ions are moving, but if it is bound to a mineral colloid and it is shown here then it will be the transport range will be much much larger. So, it will go to a longer distance or it can bind get bound to bulk mineral phase which is actually shown here. So, in the bulk mineral phase that directly the neptunium may be binding or it may be binding as a function at a complex of this mineral colloid. So, either way you will find that which is the neptunium becomes now immobilized because it is bound to the bulk mineral phase and the mobile in a neptunium is which is going as such transported as the ion or it is getting transported as a complex form in the mineral colloid. Now, this binding is can be by ion exchange where the actinide ion can get exchanged with an ion present in the mineral surface. This is called the ion exchange or it can be by something called a surface complexation. So, that the mechanism of the surface complexation I will be discussing in some of the subsequent slides. So, this can surface complexation can take place on the surface of iron, manganese, aluminium, titanium or silicon oxides which have this hydroxide, carbonates or the sulphides at the clay edges. So, this type of things can be this surface complexation can take place. Now, the actinide mineral interaction. So, we have this mineral surface as shown here the schematic is given here. It can have depending on the pH value see here if the increasing pH value mineral surface can have a zero charge at the PZC and if it is lower than the PZC it will have a positive charge and at the higher than the PZC it will have a negative charge. So, this is how the mineral surface will be behaving as a function of the pH and there can be interaction between the actinide and the clay oxide surface which is a electro static or it can be a chemical interaction. Based on that the complexes will be formed. If you have the anionic species then they will be interacting at the lower pH value or at the positive species will be binding. The mineral surface at the positive part of the mineral surface will be binding to the negatively charged complexes or at the higher pH value where you have the mineral surface having a net negative charge it will be binding with the positively charged ions or the complexes. Now, for surf sun or retention they should have opposite charges that is mineral surface and the binding actinides ions should have the opposite charges and if you want to be released or the dessert from the mineral surface then you should have the same charged species. In that case it will be released from the mineral surface. Now, the mechanism and the nature of the complex formation as I have already mentioned this is a two types of interactions. One is a ionic sense where the interlayer cation in that place which is getting replaced by the cattants, the actinides and also there can be surface complexation with the surface hydroxyl group. In this case it is binding with the hydroxyl groups present at the mineral surface and the nature of the complex can be either inner sphere or outer sphere in case of inner sphere there will be either partial or no hydrogen sphere of this metal ion when it is forming a complex. For example, we have shown here some complex this bigger one is the metal ion and it is having some water molecules are present. Now, this will be moving actually now when it is going to bind with this clay mineral in that case this water molecules which are there around the metal ion those water molecules are to be removed and then it will be forming a complex. In case of the outer sphere complexation the water molecules need not be removed and there can be interaction between the water molecule which is binding to the metal ion with that of the clay mineral surface so that is how it is forming an outer sphere complex. In this case one we have shown an inner sphere complex where all the water molecules are removed from the inner coordination sphere of the metal ion and that is how it is forming complex with the clay mineral. If one wants to have the final details of the mechanism then the following can be considered. Now, we also have outer sphere substance inner sphere surface complexation. So, there are some of the examples I have shown here in case of the outer sphere substance we have this this is the the actinide complex of the actinide. Actinide is symbolized as anx plus and there are y number of water molecules forming a hydrate or the actinide complex of the actinide ion. Now, this entire thing is coming this entire species is coming very close to this surface and then it is forming outer sphere substance. So, that is how it is interacting with the negatively charged part and then it is binding in the surface and also there can be inner sphere surface complexation. In this case you have this actinide H2O y the same species whatever I have mentioned here and in this case the same species is now interacting with the surface species which is the which is so on here along with that it is forming some bonding that is how it is the inner sphere surface complexation that can be actinide colloid attachment in this case you have this colloid means this actinide is binding with the colloids where the colloid can be either intrinsic colloid or pseudo colloid. There can be surface precipitation in this case this actinide ion is interacting with hydroxide and it is forming a precipitate as you know this hydrolysis it can form the species like OH x species will be formed and this will be forming a precipitate and it will be lying on the surface. There can be surface induced redox reaction also where these oxidation state can go change from x plus to x minus 1 if the reduction is taking place and there also can be co-precipitation or solid solution formation where you have this actinide ion which is interacting with the mineral surface and it is forming a actinide mineral co-precipitation or the solid solution formation. So, these are the different types of interactions with the actinides are forming with the clay mineral at the mineral surface that as I have already mentioned it can form subson, it can form surface complexation, it can have precipitation, it can have co-precipitation that can be also reduction in the surface. Now, this can be monitored by spectroscopic techniques those are the exhausts or the TRLFS and also other techniques such as SIGAML, Agilite, Coriotransfer, Intraexpectroscope. Now, we will take some example Uranium-6 uptake we would like to study by Gethite and Kaolinite. So, in this case what we do is we first we should try to understand the PCSN diagram of that of the Uranium-ion. Now, as I have already mentioned with the actinide chemistry course this Uranium-ion also forms very strong complexes with carbonate and the last option with the hydroxides. So, in the case where you do not have any carbonate present in the system you have the PCS, Uranium-ion at the very low pH values and which actually goes on decreasing and as you form the mono hydroxides species and subsequently you have the dihydroxides species also and like that you will have different polymer species also are formed. This is how this the speciation of Uranium-ion is increasing pH values it shows, but if you have the carbonate present in the aquating system then you have this carbonate complexes are formed. So, at the low pH value you have the Uranium species as already mentioned and you also have the carbonate species form at around pH 5.5 or so. So, you have this Uranium-carbonate species which at a higher concentration of the OH- are at higher pH values you get the Uranium-carbonate species which have more number of carbonates are attached or you also can have the polymeric or in this case you have the dimeric species where you have the hydroxo as well as carbonate complexes are formed with the two pointies of Uranium-ion. So, now let us see whether we have the studies carried out at different pH values containing carbonate as well as without the carbonate. Now, this example we come here for this Uranium-ion uptake by geothite as well as carlinite. At pH 7 we have both geothite and carlinite have positive charge which is determined for their from their PZC value. So, before pH 7 and negative charge it can be after pH 7. So, now we will see mark here at the pH 7 is the value we mark here we will see before pH 7 and after pH 7. Now, carbonate free geothite if you take that is the red line which are the experimental values you see that. So, there will be initially there will be no absorption of Uranium-ion but subsequently which increasing pH values then there will be uptake of this Uranium-ion because of the hydroxy species and definitely it will increase and it will be going up even after pH 7 will get nearly 100 percent uptake of the Uranium-ion. Now, you go to geothite where the carbon dioxide is there shown by the blue line in the figure. In that case initially of course we have this uptake is there and but beyond pH 7 because of the carbonate complexation. So, it is forming now the negatively charged species. So, it starts dropping. So, it is not forming a complex in that case because after the PZC values there will be repulsive interactions. So, it is not forming any complex. Now, if you have the kaolinite in air in that case also similar behavior is observed like with height initially you have uptake starting from a pH 4 values and it is going up to pH 7 and then there is a relatively more steep decrease in the uptake of Uranium that means Uranium is coming out of kaolinite phase. So, now what we see the this is a very simplistic case we have seen in this case we had only carbonate present, but actual conditions ground water conditions you have the humic acid also present in the aquatic system that is the humic acid will be present and it will form complexes with the trivalent lanthanides or actinide ions. Here for the convenience we have taken the trivalent lanthanides which is behaving more or less similar way as that of the trivalent actinides. So, we have taken the Europium as the case study and we are trying to see how it is substance is there onto the alumina surface or what is the aluminum hydroxide surface, how this uptake of this? Europium-3 ion is there. So, we have three case studies, number one is just aluminum hydroxide suspension we have and we try to study the Europium substance onto that and for the case B we have the aluminum hydroxide suspension with dissolved humic acid and case C where you have the suspension of aluminum hydroxide and humic acid hybrid. So, both humic acid also in the suspension as well as aluminum hydroxide also in the suspension case of C. Now, we come to the first case A where you have only aluminum hydroxide suspension as shown in this figure with Europium-3 will be forming a complex with aluminum hydroxide at the pH value which is greater than the PZC value. Now, this is the black points that different with this case A. You see here that initially this uptake is not there or the substance is not there, but beyond pH 5 there is a slow increase in this uptake and then you find this some sort of a saturation is reaching beyond around pH 7.5 or so it is because of the complexion of Europium-3 ion with the deprotonated aluminum hydroxide which is there beyond pH 7.5. So, this is what is our case A. Now, coming to the case B, here we have the humic acid in the solution phase as shown in this case of the figure you have the humic acid also in the solution phase that is how now it is forming a complex. It can form complex in the two ways. First, Europium-humic acid complex is formed and that whole complex is present in the solution phase. Then, Europium without forming a complex with the humic acid, the chance is very very less, but definitely some fraction of the Europium-3 is not forming a complex with the humic acid and that particular fraction of the Europium-3 plus ion is binding to the aluminum hydroxide surface. The third case is the Europium-3 is forming a complex with the humic acid and the humic acid is binding to the aluminum hydroxide surface. So, in this case, in the last two cases that is the Europium-3 which is binding to aluminum hydroxide or Europium-3 humic acid complex which is binding to the aluminum hydroxide in this case, Europium-3 is getting immobilized on the aluminum surface. On the other hand, the Europium-3 humic acid surface, it is actually mobile. It is present in the solution phase and it is mobile. Now, in case of this case C, we have this aluminum and humic acid both are fixed. That means the humic acid is now interacting with the aluminum surface and it is a part of the alumina. Now, in this case, Europium-3 is forming a complex directly with alumina as shown here or it is binding to the humic acid which is in turn is already fixed with the alumina surface. This case is represented by the profile given by the red balls as the data points. Now, we also come to the thermodynamic modeling of the functional data that I was mentioning. It is something called the surface complexation modeling. So, here we have the different surface reactions. For simplicity, surface hydroxides are represented as SOH. So, in case of the silicates, we have this type of functional groups are there at the surface. The silaxial groups which are having triple bond SOH. Now, this can form, take up a proton and give this type of group where it forms a cationic surface site or it can also release a proton and giving an anionic surface site. So, that means the silica is behaving either way. It can have a cationic site also it can have an anionic site and these metal complexation reactions can be either with the cationic site also it can use the anionic sites depending on the nature of the complex the metal ion has formed. The complexation reaction also can be there directly binding to the SOH functional group which is there present in the silica surface where you have this MN plus metal ion which is binding to the SOH. Again, where this sort of a replacement reaction this hydrogen ion is coming out of the silica surface and similar way you also can have the hydroxy complex of the hydrolyzed species also interacting with the SOH surface and there also you have one hydrogen ion is coming out. So, now this is how the surface complexation is taking place in the silica surface. Now, when we have different type of clay minerals present in the aquatic system they will be forming similar type of complexes and the surface complexation model SCM which is actually a chemical model, but it takes into account this complexation which is taking part with the metal ion or in this case the actinide ion which is different clay minerals. It takes care of this molecular description of the substance it also has a equilibrium approach and it is analogous to the complex formation in the solution, but it has to take care of the mass balance, charge balance as well as the equilibrium cost. So, how to go about it? We have the equilibrium constants the K values as mentioned in the above reaction with the metal ion this KM or the KM OH values are already reported for a particular clay mineral or a particular metal ion in the literature. So, we can try to have our experiments done and after getting the experimental data we can try to fit into the different type of complexation the clay minerals and then from that we can get an idea what is the mechanism of this complex formation or the substance with the clay minerals. I have given one case study here in the right hand side figure where actually the neptunium substance and two different clay minerals has been done here in this case we have taken pentonite clay and this uptake has been seen at two different ionic strength conditions that is 0.01 molar as well as one molar conditions given by the black and the red data points. And this modeling has been reported in the literature this blue line is actually that of the montmorillonite which is reported by Tachy et al and this purple one is the again the uptake value of neptunium which you can calculate from the equilibrium constants which is reported by Bradbury and Byance this is for again montmorillonite and this black line which is given here this is reported by polar et al and this is for gate height. Now gate height apparently is playing a more important role in case of the bentonite clay whatever has been taken this particular study and it is as you can see this fitting curve whatever is obtained considering the gate height which is matching very well with the experimental data points suggesting that gate height is probably playing a more important role in case of the option of this neptunium ion and to this bentonite clay. This is how the surface complexation model actually this is helping and there are two approaches in this surface complexation model that is the component additive or the CA model which I have already discussed when you have the different type of clay mineral and you take the log k value from the literature and then you try to do it give the weightage to those fraction of the clay mineral and try to generate model the data and match with the experimental data and the second approach is the generalized composite or the GC where you have you have the subs data and keep on doing the iterative fitting and that is how you can find out what are the composition what are the mechanism of the option of the kinetics under the clay mineral. Now the colloidal transport of the actinides there are two types of actinide colloids which are identified in the groundwater and already we know that actinides when you go to very high pH values they undergo hydrolysis and they also form the polymers under the existing conditions. So those cases you get actually more than one number of actinide ions they come together and they form colloids so that is called as the increasing colloid or the real colloid some people call it also igne or primary colloid or even two colloids. So these are basically the actinide hydrolysis products that through the oxo or hydroxybrase formation that is another one is called a pseudo colloid then actually the colloid is formed by another metal ion and in case in that case this actinide is absorbed onto the colloid formed by the second metal ion that can be the other metal ion like the transition element metal ion which is forming a colloid under the groundwater condition or the experimental conditions and there actinide is getting absorbed onto the colloid of the other metal ion and this is called a associative or pseudo colloid. Now what are the factors which are determining the stability of the colloidal system? Now the factors of this colloidal system mainly depends on the electric charge on the surface of the colloid naturally the depending on the charge the charge is too much the colloid will break then because of the repulsion between the colloid particles and it will prevent them from coagulating and settling out of the solution phase also your factor which is determining the pH value that means the colloids have the zero surface charge that is the pH PZC then further the pH of the colloidal system from the PZC the larger is the surface charge and the more stable is the colloidal system that each colloidal system is characterized by its pH PZC so that the pH can be adjusted to stabilize or destabilize it also the factors which are important is the solid effect or the ionic strength on the surface charge so depending on that if the electrolyte is present in a colloidal solution the cations tend to adhere on a negative charge color to compensate the excess local charge of opposite sign and vice versa for the anions this will reduce the double layer potential of the colloid and enhance the coagulation process in that colloidal system. Now formation of the colloids and their studies they are already concerned this formation of the eigen colloids this is the eigen colloid of the true colloid of the intrinsic colloid in this case the hydrolysis is the primary state which is leading to the polymerization and that generation of the actinide colloids and the tendency of this formation of the actinide colloid of course it depends on the charge the ionic potential of the actinide that is the tetravalent actinide ion has the higher chance of forming an intrinsic colloid and also the actinide hydroxide and oxides usually are very small solubility in the natural water so making them the common source of the intrinsic colloid in the case of the pseudo colloids their attachment of the actinides of the ground water colloid which are already present in the ground water system is result from various geochemical processes and also it can also from the weathering of the rocks and minerals you have some colloids that present in the ground water system which will be basically the platform where these actinides will be for getting job and for subsequently they will be migrating in the ground water system. Now the formation of the colloids and their studies methods for the study of actinide colloids the properties that are of interest for the environmental point of view and the colloid size distribution can be done by studies like our DLS or so then also this charge and the colloid can be found out by the BZC and the actinide uptake by the colloid can be done by the normal uptake studies. By ultrasonitification by filtration or ultrafiltration this can be done to separate the colloids from the dispersed media. Electroporesis or zeta potential can be done to determine the colloidal charge and adsorption studies can be done in a normal way by adding a radio tracer into the solution phase and then to find out how much a radio tracer is going onto the colloids or the exchanger or in this case the clay mineral and that is how from the adsorption studies can be found out. The particle size of colloids are determined using DLS or the dynamical light scattering method also scanning electron microscopy or energy dispersive exerspectroscopy or transmission electron microscopy these are the techniques which can be used to get the size and as mentioned here you can find that these job contaminants this is the phase is the water phase is there and this is the sediment phase which is shown here and you see that this rock or the sediment phase is shown here now these are the contaminants the diesel contaminants which are the actinides so they are normally they are mobile in the water surface but they can get jobbed also onto this rock mineral surface rock surface which is the immobile surface this is how they can get immobilized onto the sediment or the rock they can also form internal intrinsic colloid or they can form a complex with the pseudo colloid and then also it will be in the mobile in the water surface and they can mobile get moving in this way and that is how this actinide can get transported. So, this is how this is a two phase method I have shown and this is the contaminant transport in a three phase system where the mainly the transportation is done by either by an intrinsic colloid formation or absorption onto a pseudo colloid. Now, to summarize actinide clay interaction you have a solid like here you have iron oxide or iron hydroxide which is present here taken for example it is true for any other oxide rock or clay materials and which can have the radio-nuclide which is having the water molecules here which is coming here and it is binding to the iron oxide this is called a subsum and can be desorption can be there where you have the radio-nuclide is forming a complex you can have a complexation with the ligand that is how it forms a complex that is coming out of the jobbed clay mineral which is a desorption but it determined on the pH value and the complexation constants once it is forming it can have the dissolution also then it will come again to the aquatic medium there can be co-precipitation and there can be dissolution that is how this complex also in the presence of ligand there can be dissolution from the clay mineral that is how this becomes mobile and finally it comes to the colloidal surface this is a colloidal surface is given so this is vertically it says a pseudo colloid where you have the radio-nuclide it's forming one two three four of them coming together and forming a complex block of species which can facilitate the transport of actinides this is the pseudo colloid where you have this another petal is forming the colloid and the radio-nuclide is binding with this and this is how it is embedded onto the pseudo colloid so this is how the colloidal system it is migrating in the groundwater system that is the reason for this actinide migration behavior in the environment so we have completed the actinide chemistry and the actinides in the environment the next two lectures will be on the transactinides