 These are the references shown here for the actinide chemistry course, including the general actinide chemistry, actinides in the environment as well as the trans actinides. So, we have discussed about the actinide complexation and separation. Now, today we will be discussing about actinides in the environment. In some of the previous lectures, we have discussed about the factors which are responsible for this actinide in the environment. Today, we will be discussing about the factors in more detail about the actinides in the environment. Now, there are several actinides we know in the earth's crust, already we have discussed before, like uranium, thorium to some extent in actinium and tractinium. But, neptunium and plutonium are also found in nature at very, very minute concentrations. But we know that they are the activation products in the nuclear reactor. So, then how this neptunium and plutonium we can say that they may be possible in the nature, that is because of the natural reactor in oklo, that I have already mentioned in one of the previous lectures. That is certain observation that support the occurrence of the natural reactor at oklo in Gabon, West Africa is because of the low uranium 235 content. But we know that this natural uranium should have around 0.72 percent of uranium 235. But in the Gabon area, this natural reactor whatever we call there the uranium 235 content was found to be lower than this 0.72 percent which is present in the ores. Then, there is also this large concentration variation for this uranium 235 which is less than 0.5 percent to see and also this high content of neodymium 143 and low concentration of 142 neodymium that is a signature of some nuclear reaction was taking place at that point. Other factors which are responsible for the radio nucleates in the environment the sources are the nuclear test. So, due to the nuclear test nearly 4.5 times of plutonium are released into the environment and this various activities in the nuclear fuel cycle that also can have like reprocessing waste management that can also contribute to the presence of plutonium and other heavier actinides in the environment like neptunium. There can also be accidental release some of the accidents I have mentioned here this table below like the wind scale accident in 1957 in UK and there are some of these categories of these accidents which are mentioned the put out of this table that is number 5 category is the accidents with wider consequences. Number 6 is serious accident and number 7 is major accident. So, this the former Soviet Union also there was an accident in 1957 that is the peace team and it is falling in the category 6 that is a serious accident 3 mile island in USA in 1979 falls in category 5 and this Chernobyl reactor accident which is now in Ukraine it is 1986 the accident took place and it is falling under the category 7 and also 2011 the Fukushima Daiichi nuclear reactor accident that is in Japan it falls in the category number 7. Now there was an interesting observation very close to the Nevada test site where the large number of nuclear weapon testing has been done around 828 nuclear tests have been conducted at the Nevada test site between 1956 to 1992. The left hand side figure gives details of the test sites. We are aware the tests have been taken place like this Benham test site, Malbo test site, Typo test site, Belmont test site. So, there are different testing sites where the nuclear weapon testing has been carried out and there were some wells which are away from this nuclear test site. So, that is something called the ER-25 set of wells. So, this well complex which is there at the slightly away from this nuclear test sites like Benham, Typo, Belmont and Malbo. So, in this well actually this drilling has been done and the samples of plutonium has been detected and they found that this signature of this plutonium that is the 240 to 239 isotopic ratio is corresponds to that of the Benham test site. So, which is much away from this well ER-25 that well complex number one and number three well they were having this plutonium activities which matched with the 240 to 239 ratio of the surface and the cavity glass activity at the nuclear test site at the Nevada test site of the Benham test site. And this shows here for the Benham how this testing has been done and this has been modelled that probably there is some groundwater transport of this plutonium which is going here and very close to the well actually this is the well complex where these samples have been taken well number one and well number three and the sample has taken two meters deep from the earth surface at the well number one and at well number three at a deeper place in sampling has been done and both places they have found that even though the Typo test site is very close to the place where drilling was done but the signature was that of the Benham nuclear test site that suggests that the plutonium from Benham test site has migrated to the well of the well number one as well as well number three. This is due to the typical mechanism of plutonium migration which will be discussed in the next lecture. Now what are the factors actually which is deciding this actinides in the environment? First thing is of course the oxidation state which is also considered as the EH value, the hydrolysis which depends on the oxidation state as well as the pH value you know that this groundwater pH can be different different places same also with the sea water sea water and other different pH. Now other factors which are also relevant is the complex formation we know that if it is hydrolyzing then the complex formed is the hydroxocomplex but apart from that if there is any other complexing agent which is present in the groundwater system or in the environment and also their concentration, the concentration is large then the fraction of plutonium or the actinide which is present in the complex form will be relatively larger and also if the complexation constant is higher then also the plutonium or the actinide in the complex species will be large. Next of course is the solubility. Now what is the solubility of this complex which is formed whether this complex is soluble in the groundwater system or this complex is precipitating. This also matters if it is soluble complex form then it can go in the flow of this groundwater and the migration can be where relatively longer distance and also there can be another thing which is called the complex formation and the and also colloid formation. There can be colloid formation of this plutonium or other actinides which are present in the environment and this colloid formation this depends on the pH hydrolysis constant as well as the amount of the actinide present and there can be also these colloids which are formed they can act as carriers and they can also serve other actinides. So, this is called this colloids which I will be discussing subsequently and then another factor is the surption. This depends on the actinide species which is present in the groundwater system also it depends on the ion species and what are the complexing ions present and also the colloids which are present in the groundwater system. So, finally there is a filtration effect it depends on the size and particulate matter which is carrying the actinides and in the rock surface it can get filtered that is how the actinides in the particulate matter it can be immobilized on the rock surface and all these factors are part of the speciation of the actinides which is depending on the environmental conditions. Now, what is the speciation? The speciation may be defined by the determination of various chemical and physical forms the element in this case the actinides and the investigation such as its oxidation state all possible inorganic or organic complexion and also the precipitation etc. There are methods in which the speciation can be understood and also you can find out the nature as well as the amount of the species which are formed involving the actinides and the techniques are same exhausts, genes, prlfs and also xps. So, these are the techniques which are used. These techniques they get actually a better understanding of the surface as well as aqueous speciation of the actinides. Now, why study this speciation? As we know the presence of actinides is dangerous if it goes to our food chain because its radio toxicity is high also the actinides and the heavier elements they have very high chemical toxicity and both these factors this radio toxicity as well as the chemical toxicity going to be harmful for the living beings and also this chemical properties of these actinides and their complexes which decides the migration of actinides. So, what are important factors here is first of course is the complexion which also includes the hydrolysis of the actinides. Then whatever these complexes are formed whether they are soluble in the groundwater system or not. Then they are mobility now the mobility can be as such also but if it is not forming a colloid then naturally the mobility is going to be much less. So, that is why the colloid formation is going to be important factor in this migration of the actinides. We will be discussing about these colloids in detail subsequently in this lecture. Finally, the bioavailability now this whatever radio nucleates are for example, plutonium. If it is migrating in the groundwater system, it is also available in the aquatic system like the fish and other marine living species which are there then it enters the food chain and that is how this becomes toxic to the human being. So, basically we have to evaluate all these factors solubility, mobility and bioavailability but in this lecture we will be mostly talking about solubility and mobility this bioavailability and toxicity we are not going to cover in this lecture. Now, the factors which are deciding species are pH which is existing the nature that is the particularly the groundwater system the marine conditions. Then oxidizing deducing conditions termed as the pH and also the presence of inorganic and organic complexing agents. So, these are the main factors that is the pH, pH and the complexing agents which beside the speciation of the actinides. In this particular case, we have taken example of neptanium-5. Neptanium-5 as you know it is existing as the NPO2 plus as the ionic species and this is considered to be most mobile of the other neptanium species, ionic species like neptanium-4 plus, neptanium-3 plus or neptanium-4, 2, 2 plus this is the neptanil, hexavalent neptanilion. So, compared to all these species the neptanium-5 species is more nearby and that is why we are considering here the neptanium-5 speciation. We give the distribution of the neptanium-5 species as a function of the pH and the concentration of neptanium taken here is 10 to the power minus 7 molar. You can see here that at low pH values you get this neptanium-5 species which is the black line which is showing here up to pH around 9 or so. You can find this neptanium-5 that is NPO2 plus species is predominant. Beyond that it is going to undergo hydrolysis and there is a possibility of Hermitian of NPO2 OH which is a neutral species or NPO2 OH twice minus that is an anionic species. So, you find that first this NPO2 OH species which is forming and then subsequently it is forming the NPO2 2 minus species. So, that is how at pH greater than 12 you get the anionic species of neptanium. Now, this EHPS diagram is also called as the pore band diagram which is already discussed in one of the previous lectures and this shows actually the stability of the particular species under a given pH and also atmospheric conditions. So, this diagram indicate actually which species they predominate under any given environmental condition of PE and pH. So, this side figure is showing for the neptanium again and you can see here this area in this pore band diagram these marks the region where a single species is stable very clearly shown here. This is the area which is demarcated in this and this is where the NPO2 plus species is stable and that is how for the subsequently also for the other areas also this is marked how these species are stable in which area. Also these lines indicate two species are coexisting together these vertical lines where they mention this is a purely acid based reaction and this does not depend on the EH value and the horizontal lines on the other hand they show that these are PF independent reactions and also they are not dependent on the EH values or the potential whatever is prevailing under the condition. Now, the slope of the lines suggests that both the acid base or the redox dependent reactions are considered in this case or both are prevailing. Now, we also see this effect of pH, EH and complexing agents on the speciation of the actinides. Now, what are the complexing agent mostly under the groundwater condition we have this humic acid and the pulvic acid. So, humic acid actually it is the plant origin acid which is after many years it is transformed into the humic substances. Now, this humic substances they have a very complex structure and this varies the source age and temperature realize other conditions. This composed of different factions that the human faction is insoluble under all pH conditions and the humic acid is insoluble at pH value less than 2 and the pulvic acids are soluble under all pH conditions. So, mostly because of the humic acids are very complex structure. So, the studies with the actinides are with the model compounds which mimic part of the humic substances. Here I have shown a schematic of this how this humus is first you can extract with the alkali and which is insoluble that is termed as the human and which is soluble that if it is treated with acid and precipitated that is termed as the humic acid fraction and which is not precipitated that is the pulvic acid and then you can re dissolve the humic acid and you can again add electrolyte and you get precipitation which is something called a gray humic acid and which is not precipitated it is called the brown humic acid. This is the repetitive structure of the humic acid is given here. Now, this humic complexation is affected by controlling the pH. You know that by increasing the pH there is increase in the strength of this humic acid complexation that is because some of the functional groups like it has the carboxylic acid as well as phenolic type of groups are there this gets deionized and you get COO minus or O minus type of functional groups which are taking part the complexation with the metal ion or in this case the actinide ions and that is only possible if we increase the pH to very high values. So basically humic acid will have a number of PK values if it is whether it is a carboxylic acid functional group or a phenolic functional group the phenolic group will be at a higher pH value it will be dissociating and there is no single structure which is able to satisfy all properties of the humic acid and its properties may change with the place of origin of the humic acid. Now, while considering the stability of this humic acid complexes as already mentioned before the tetravalent actinides will be forming much stronger complexes than the trivalent than the hexavalent and in the set of tetravalent actinide ions you find that which is having higher ionic potential that forms a stronger complex. So, plutonium form forms a stronger complex with the humic acid compared to uranium-4 which in turn forms a stronger complex than thorium-4 and thorium-4 forms stronger complex than americium-3 or other trivalent actinide ions which in turn forms stronger complex than the uranium-6 ion that is the uranilla. That is actinide humic acid complexation this can modify the radio nucleic oxidation state that is the reduction of neptinium-6 can go on to neptinium-4 and plutonium-6 to plutonium-4. This has been reported as has been mentioned in the previous nevada test side report also even though we are expecting plutonium-5 to put that has been reduced to plutonium-4 that is how the plutonium concentration has been much less. Now why study the actinide substance and migration behavior? This is actually an application part and whatever we have already studied in the actinide species as well as the complexation in any unfortunate scenario if there is some leak of the radio nucleide through the environment as we have already mentioned by accidents or from the mythified waste blocks which are there under the deep geological repository from there also if there is some leakage of the radio nucleide then or finally of course if from the nuclear weapon testing there is a lot of actinides are there so because of that we try to understand this actinide substance and migration behavior and this will help us understanding this migration and movement of the radio nucleide from one place to the other and this information is essential due to to avoid the radio nucleide becoming from a part of our food chain and also choosing a better buffer backfill materials in case of the deep geological repository and also the remediation of any radioactive contaminated site. Now before going to see the interaction between the radio nucleide and the clay or oxide surface let us see some of the properties or oxide clay surface in the aqueous media now how does the oxide at the clay surface look like in the aqueous solution the surface oxide and clay has a surface waste group as shown here you have some of these waste groups mentioned here which is forming a bond with this water molecule here so many places you have their surface waste group which is forming a bond with the water molecule here and that is how it is responsible for forming the complexes with the actinides and we also have these clay minerals which have the surface hydroxyl groups I will be showing in the next few slides how these are acting and the oxides also are present like we have the silica and alumina but they also have replaceable hydrogen ions some sort of a hydroxyl groups are there and that is how this complexion is taking place these clay minerals also have a silicate like structure and are composed of mainly the tetrahedral e arrange the silicates and the octahedral e arrange the aluminate groups and depending upon the arrangement of the silicates and aluminates the clay can be divided into several groups such as the smectite, galvanites etc. it comes in the next few slides so this is the tetrahedral structure of the silicate structure and this is the octahedral aluminate structure or aluminum oxide structure and you have these layers structures where you have this tetrahedral layers and the octahedral layers and another tetrahedral layer so they form a structure like this and finally you have a structure like this which is given here so you have the silica to alumina ratio if it is two is one then it can be light or a three layer two is one it can be smectite as it is given in the next slide so the structure of the clay minerals are given here you have this tetrahedral sheet or the octahedral sheet and you can have another tetrahedral another octahedral sheet this is how the stacking can be done for example we take this iliate so in this case we have a three layer structure having a tetrahedral octahedral and a tetrahedral sheet and this dimension is around one nanometer and this iliate actually is a non-expensive type of structure so where this part is actually having distance is very very less where you have the potassium ions embedded between these two types of this sheet structures whatever are there you for vermiculite or which is moderately expansive and you have this distance is around one to one point five nanometer for smectite it is highly expansive you have around one to two nanometer this distance on water molecules and contents can be there between the two layered structures given here and similarly for the chloride structure you don't have again a non-expansive system and you do not have anything in between these layered structures kaolinite which is at the top as a non-expensive one is to one structure now the type of charge in the clay so there are some permanent charges they're due to the isomer first substitution in the replacement of one ion for the another one so this can be like we have a one particular set of ions are there and which is replaced by another set of ions which are having similar size within the crystalline structure for example we have this aluminium 3 plus which is replaced by magnesium 2 plus in the octahedral sheet or we have this silicon 4 plus by aluminium 3 plus in the tetrahedral sheet in these cases you have this replacement of the charge and that is how you can have also permanent charge is there in the structure of the clay minerals as I have shown it also can have isomer first substitution and it is shown here the scheme is given here all this aluminium is substituted by magnesium and that is how you have a permanent charges present in this structure so this you have the neutral octahedral sheet which where you have the aluminium ions now there is a net negatively charged octahedral sheet where you have this magnesium in replacing the aluminium so this is due to the isomer first substitution not a type of charge in the clays there can be also variable charge so this is due to the adsorbed ion also this is p s dependent charge you can show these examples here adsorbed ion means some ion is adsorbent to this clay mineral and there will be different charge will be there also depending on the pH as shown in this scheme below you can see that you have this hydrated aluminium where at a lower pH value you get this type of structure where these hydrogen ions are there but if you have a higher pH values then you find that these hydrogen ions are now removed so you have the O minus ions are there so that is how you have the developed the surface charge that is O minus charges present in the surface and that can form the complex now one important parameter which is considered in case of the clay minerals is the point of zero charge commonly known as the PZC will be discussed in this slide so what is point of zero charge that is a pH at which the surface charge is zero it is neither positive charge nor negative charge so that charge that point is called as the point of zero charge and it varies from one clay or oxide to the another one I have shown a table here and see this point of zero charge of silica it is 2 to 2.5 kaolinite 3 to 4.6 we thought it is around 7 alpha alumina is 8.3 to 9.4 delta manganese dioxide is 2.8 beta manganese directed 7.2 to 8.7 and albite it is 2 hematite 7.0 to 8.8 so like that you can for different minerals or the clay minerals we can have the different PZC values now for any surface if the pH of the suspension is the less than that of the PZC then the surface is net positively charged and if the PZC is greater than the PZC then naturally the hydrogen ions are removed and the surface is net negatively charged so some of the important terms which are necessary for this understanding the clay mineral complexation with actinides they are defined in this transparency is the specific surface area this is the unit is meter square per gram clay concentration it can be grams per liter surface concentration can be moles per gram surface density moles per meter square and cation excess capacity can be milli equivalent of the CEC is considered the milli equivalent per gram surface charge can be coulombs per meter square and faraday constant is 96490 coulombs per mole so that is how these are defined and some convergence the surface concentration the moles per meter square is the surface concentration moles per gram divided by the specific surface area meter square per gram surface concentration in moles per liter can be surface concentration divided by the clay concentration surface charge is defined as the surface concentration multiplied by faraday constant divided by the specific surface area now how to measure the surface charge of the the potential can be done by the potentiometric titration. So, you see here how this at a different pH value you can get the surface charge density and that is how it will be varying initially and then when it comes down you get this pH value where you have this value is 0. So, that is called the charge density is 0 that is called the PZC. So, PZC can be measured by potentiometric titration. It can also be done by using instrument for the well measure the zeta potential and the different pH values. So, this is the photograph of this zeta potential meter which is used in the laboratories. So, in this case actually you automatically you get at as a function of pH the zeta potential values and you can find out for the substances like A, B and C you get these are the point of zero charge where the profile is like this for A, this is for B and this is for C. So, this is how you can get the zeta potential. Thank you.