Radionuclide and Metal Sorption on Cement and Concrete

Cementitious materials are being widely used as solidification/stabilisation and barrier materials for a variety of chemical and radioactive wastes, due to their favourable retention properties for metals and other inorganic contaminants. These result from mineral phases in hydrated cement that (i) possess a high density of sites for the fixation of contaminants through a variety of sorption and incorporation reactions and that (ii) buffer pH in the range 10–13, depending on composition, which tends to limit the solubility of many metal ions. The present book represents a state-of-the-art review and evaluation of the type and magnitude of the various sorption and incorporation processes in hydrated cement systems for twenty-five elements. It is aimed at describing the interaction of dissolved constituents with hydrated cementitious materials (i.e. already set or hardened). While the immobilisation of contaminants by mixing with dry cement clinker before hydration is not explicitly addressed, many underlying processes will be similar. On the basis of a consistent review and re-evaluation of the literature data, we established a quantitative database on the solid/liquid distribution behaviour (“sorption database”) for radionuclides and other elements in hydrated cement systems. This database is closely tied to the safety analysis of the near-surface disposal of radioactive waste in Belgium, which is reflected in the focus on radioelements. However, the book addresses also a number of stable elements and heavy metals, which makes it relevant for the interaction of conventional contaminants with cement-based barriers.

The cementitious materials considered here include hydrated cement grout, concrete, and mortar. These materials may be used for waste conditioning, as waste container, and for backfilling. Dry cement (clinker) is a hydraulic binder; concrete is a mixture of hydrated cement, water, and coarse and fine aggregates; mortar and grout are a mixture of water, hydrated cement, and fine aggregates. Clinker is a mixture of several anhydrous minerals, mainly calcium silicates. Ordinary Portland cement (OPC) consists of finely ground clinker plus a small amount of gypsum. Hydration results in the formation of portlandite Ca(OH)₂, largely amorphous calcium silicate hydrates (CSHs), and minor crystalline phases containing aluminium, iron, and sulphate (e.g. ettringite, monosulphate, hydrogarnet, and hydrotalcite). Solutions in equilibrium with fresh hydrated cement are hyperalkaline (pH ≥ 13.2) with correspondingly high Na- and K-concentrations. It follows that hydrated cement systems are not in equilibrium with infiltrating water, which leads to dissolution and re-precipitation reactions. It is well established that the degradation of hydrated cement follows a pattern of several different, more or less distinct states, characterised by progressively lower pore water pH and Ca/Si ratio. Hydrated cement solid phases provide a variety of potential sites for different uptake reactions for dissolved elements, ranging from surface adsorption to incorporation and solid-solution formation (these processes are termed sorption henceforth). At low sorbate concentration (below the respective solubility limit), use of a single distribution coefficient for quantifying uptake under a defined set of conditions (e.g. a specific degradation state) is a defensible approach.

This chapter addresses the behaviour of halside anions in cementitious environments. In any typical aqueous environmental setting, chlorine exists exclusively in the form of chloride (Cl⁻). Inorganic iodine may be present as iodate (IO₃ ⁻) and iodide (I⁻), with the latter being typical for cementitious environments. Radioactive isotopes of both elements (³⁶Cl, ¹²⁵I, ¹³¹I, ¹²⁹I) are relevant constituents of different types of radioactive waste. The most influential factor on sorption values for radioactive chloride is aqueous total chloride concentration, including stable chloride present in cement solid phases and infiltrating water. All available studies indicate a decrease of chloride sorption with increasing total chloride (stable chloride and ³⁶Cl) aqueous concentration. Close examination of available data indicates different chloride behaviour within two distinct concentration ranges, namely the predominance of surface versus incorporation processes. At aqueous chloride concentrations well below millimolal level, the magnitude of Cl sorption can be directly related to the surface charge of CSH phases. At higher total Cl concentrations, ³⁶Cl behaviour is probably controlled by the formation of Friedel’s salt (a chloride-containing calcium aluminate hydrate) or solid solution formation with alumino-sulphate phases. Isotopic dilution may also play a role in these cases. Data obtained for dilute suspensions show that the behaviour of iodide is analogous to that of chloride in qualitative terms. A comparison with data for intact hydrated cement paste suggests that the solid/liquid ratio has a significant effect in case of iodide, but not in case of chloride. Until this issue is resolved, data from dilute systems should not be used for quantifying iodide sorption.

This chapter discusses the sorption behaviour of several elements—caesium (Cs), strontium (Sr), radium (Ra), and silver (Ag)—that tend to form simple aqueous ions and typically interact with minerals mainly by ion exchange reactions. In contrast, these elements show only a very weak tendency towards the formation of coordinative bonds. Many types of radioactive waste contain a range of radioactive isotopes of these four elements. The main sorption process of Cs on hardened cement pastes (HCP) is an exchange-type process on the CSH phases. Therefore, Cs sorption is controlled by the concentration of competing ions (Na, K, and Ca) and by the charge of the interlayer CSH surface, which in turn are a function of the C/S ratio. Cs sorption increases with increasing HCP degradation and decreasing C/S ratio. This is directly related to the decreasing concentration of competing cations. Dissolved Na and K reach high concentrations in fresh hydrated cement (State I) but are largely leached in the following HCP degradation states. In degraded HCP (State II and following), aqueous Ca concentrations decrease with decreasing C/S ratio of the CSH phases. In case of Cs, aggregate material containing phyllosilicates (such as micas and clay minerals) used in mortar or concrete may influence Cs behaviour, as Cs sorbs particularly strong to these minerals. For radium and strontium, essentially a similar behaviour as for Cs is observed, with the difference that competition by Ca is more important in comparison with competition by Na and K. In case of silver, no reliable sorption values on cementitious materials are available to date. However, there are several independent indications that Ag sorption on cementitious materials is different from zero, and that Ag may show sorption behaviour analogous to alkali earth elements.

These elements are grouped together because, despite their differences in chemistry, their radioactive isotopes are taken up in cementitious materials by isotopic exchange, a physical process, rather than by some chemical sorption process. ⁴¹Ca, ¹⁴C, and ⁵⁹Ni plus ⁶³Ni are relevant constituents of radioactive wastes. For any radioactive isotope of a given element, isotopic exchange becomes important when the pore solution in the hydrated cement is already saturated with respect to some solubility-limiting phase of this element. In the case of Ca and C, this does not appear as a surprise, since both elements occur at high concentration levels in different HCP minerals as well as in the pore solution. In case of Ni, the formation of layered double hydroxides leads to a very low aqueous solubility in cementitious systems. Therefore, the comparatively low content of stable Ni in hydrated cement paste (stemming mainly from clinker production) is sufficient to reach the solubility limit for Ni in cementitious pore solutions.

The actinide elements—thorium, uranium, plutonium, neptunium, and protactinium—are important constituents of radioactive waste from nuclear fuel. Uranium also occurs in a variety of wastes, e.g. from mining. These elements all hydrolyse extensively in aqueous solutions and correspondingly strong sorption onto concrete, hydrated cement paste and individual CSH phases is observed. Sorption is also more or less constant over all states of cement evolution. The sorption mechanisms are, however, not well understood to date. In case of Th, relatively fast kinetics of uptake and reversibility are observed, indicating that uptake occurs through surface processes and probably does not involve incorporation into the structures of hydrated cement phases. In contrast to Th, which exists only in the tetravalent oxidation state in aqueous environments, uranium, plutonium, neptunium, and protactinium can exist also in lower and/or higher oxidation states. In analogy to their aqueous chemistry, very similar sorption behaviour is expected for all tetravalent forms. Sorption of U(VI) on cementitious materials is similar to Th(IV)/U(IV) in magnitude, but appears to involve different mechanisms (probably solid-solution formation). In lack of specific information, the same is assumed for Pu(VI). Relatively little is known for the pentavalent forms. Strong sorption is observed for Pa(V), but the underlying mechanism is not known. For Np(V), relatively weak sorption is assumed in lack of specific data.

Like other actinides, americium (Am) is an important constituent of radioactive waste from nuclear power production. In any typical aqueous environmental setting, americium exists exclusively in the trivalent oxidation state. Similar to tetravalent actinides, americium hydrolyses extensively in aqueous solutions and sorbs strongly onto concrete, hydrated cement paste and individual CSH phases. Due to the chemical similarity of trivalent actinides and lanthanides, information for americium can be supplemented with additional data for other well-researched elements, such as curium and europium. The combined evidence clearly demonstrates strong sorption under all conditions, with no clear trend across the different states of cement degradation. Spectroscopic information suggests that sorption occurs through surface adsorption as well as through incorporation into the CSH structure, replacing Ca. Similarly, incorporation into the calcite lattice is supported by spectroscopic evidence.

Selenium, molybdenum, and technetium belong to groups VIB, VIA, and VIIA, of the periodic table, respectively, and form highly soluble and mobile oxo-anions in aqueous solutions under oxidising and moderately reducing conditions (molybdenum only exists as the hexavalent MoO₄ ²⁻ oxo-anion under all conditions). As uranium fission products, isotopes of these elements are common in radioactive wastes. Based on good evidence for selenate, Se(VI), and on supplemental information and chemical analogies for molybdate, Mo(VI), and pertechnetate, Tc(VII), these elements are preferentially taken up by the sulphate–aluminate minerals (ettringite, monosulphate) in their highest oxidation states, where the oxo-anions can substitute for sulphate ions. Thus, sorption of these ions would be favoured by a high content of sulphate-aluminate minerals and a low aqueous concentration of competing sulphate ions. This is also relevant for selenite oxo-anions, Se(IV), but selenite sorbs additionally on all other mineral phases of importance in HCP. Under strongly reducing conditions, Se(−II) and Tc(IV) become relevant. No reliable information is available for Se(−II), and zero sorption must be assumed. On the other hand, technetium (IV) sorbs strongly onto hydrated cement paste and CSH phases, which is consistent with its hydrolysis behaviour.

The most prevailing oxidation state of palladium (Pd) in water is +II. It hydrolyses strongly, but forms also important complexes with soft ligands, such as chloride. Palladium is one of the most important transition metals among the fission products in waste from nuclear fuel. No data on the sorption of Pd (or other platinum groups elements) on cementitious materials are available to date. A reasonable estimate of Pd behaviour can be obtained by considering appropriate chemical analogies. Based on valency, hydrolysis behaviour, and availability of sorption data, lead (Pb) is selected as best choice. The main factors influencing Pb sorption on cementitious materials are the initial Pb concentration (an increase leads to a decrease of sorption) and especially pH or C/S ratio, where a decrease leads to an increase of lead sorption. Kinetic data for Pb sorption indicate a fast initial (chemical) sorption step, followed by a much slower process, possibly involving diffusion into the solid matrix.

Niobium and tin exist in the penta- and tetravalent state, respectively, over the entire redox span of aqueous solutions (>pH 3). Both Sn(IV) and Nb(V) hydrolyse extensively and exist under cementitious conditions as negatively charged hydroxo-complexes. Tin and niobium are contained in various wastes from nuclear power production. Both elements sorb strongly on cementitious materials under all conditions, in accordance with their hydrolysis behaviour and with the affinity of the hydrolytic species to calcium ions. While the significant extent of sorption is well established, the mechanisms of sorption on cement phases are not well understood. Spectroscopic information indicates that the structural environment of Sn sorbed to CSH and hydrated cement paste differs between the two solids, but corresponds to the formation of inner-sphere complexes in both cases. However, it has to be admitted that the available information is not conclusive with regard to (i) identifying the sorption-dominating mineral phase and (ii) the distinction between surface sorption or/and incorporation.

Tritium (³H or T) is a radioactive isotope of hydrogen occurring in radioactive waste in the form of gas or tritiated water (HTO). Tritium also has been and is being released into the environment as a result of military weapons testing and nuclear power generation. Radioactive waste may contain different beryllium isotopes. For both hydrogen and beryllium isotopes, there is some indication as well as chemical reasoning that sorption or isotope exchange may occur and retard transport. However, the lack of clear experimental data does not allow to assign any non-zero sorption value to either tritium or beryllium with sufficient confidence.

Zirconium (Zr) is used in nuclear reactors (in the form of zircaloys) because of its low thermal neutron capture cross section and high resistance to corrosion. In aqueous solutions, zirconium exists exclusively in the tetravalent oxidation state. Zr(IV) hydrolyses strongly with Zr(OH)₄(aq), and Zr(OH)₆ ^{2 −} being the only relevant species in cementitious environments (and low zirconium concentration). Very high sorption of zirconium has been observed in all cementitious materials investigated to date (hydrated cement pastes, CSH phases). Sorption appears to increase with the degree of cement degradation, in particular from a C/S ratio of 1.3 to 1.0. While the actual sorption mechanisms for zirconium are not known, the available evidence suggests that mainly CSH phases are responsible for zirconium sorption on cementitious materials.