Co speciation in hardened cement paste: a macro- and micro-spectroscopic investigation.

Cement-based materials play an important role in multi-barrier concepts developed worldwide for the safe disposal of hazardous and radioactive wastes. Cement is used to condition and stabilize the waste materials and to construct the engineered barrier systems (container, backfill and liner materials) of repositories for radioactive waste. In this study, Co uptake by hardened cement paste (HCP) has been investigated with the aim of improving our understanding of the immobilization process of heavy metals in cement on the molecular level. X-ray-absorption spectroscopy (XAS) on powder material (bulk-XAS) was used to determine the local environment of Co in cement systems. Bulk-XAS investigations were complemented with micro-beam investigations to probe the inherent microscale heterogeneity of cement by using micro-X-ray-fluorescence (micro-XRF) and micro-XAS. Micro-XRF was used to gain information on the spatial heterogeneity of the Co distribution, whereas micro-XAS was employed to determine the speciation of Co on the microscale. The Co-doped HCP samples hydrated for time-scales from 1 hour up to 1 year were prepared under normal atmosphere, to simulate similar conditions as for waste packages. To investigate the role of oxygen, further samples were prepared in the absence of oxygen. The study showed that, for the samples prepared in air, Co(II) is oxidized to Co(III) after 1 hour of hydration time. Moreover, the relative amount of Co(III) increases with increasing hydration time. The study further revealed that Co(II) is predominately present as a Co-hydroxide-like phase and/or Co-phyllosilicates, whereas Co(III) tends to be incorporated into a CoOOH-like phase and/or Co-phyllomanganates. In contrast to samples prepared in air, XAS experiments with samples prepared in the absence of oxygen revealed solely the presence of Co(II). This finding indicates that oxygen plays an important role for Co oxidation in cement. Furthermore, the study suggests that Co(III) species or Co(III)-containing phases should be taken into account for an overall assessment of the Co release from Co-containing cement-stabilized waste under oxidizing conditions.

Spectroscopic investigation of Ni speciation in hardened cement paste.

Cement-based materials play an important role in multi-barrier concepts developed worldwide for the safe disposal of hazardous and radioactive wastes. Cement is used to condition and stabilize the waste materials and to construct the engineered barrier systems (container, backfill, and liner materials) of repositories for radioactive waste. In this study, Ni uptake by hardened cement paste has been investigated with the aim of improving our understanding of the immobilization process of heavy metals in cement on the molecular level. X-ray absorption spectroscopy (XAS) coupled with diffuse reflectance spectroscopy (DRS) techniques were used to determine the local environment of Ni in cement systems. The Ni-doped samples were prepared at two different water/cement ratios (0.4, 1.3) and different hydration times (1 hour to 1 year) using a sulfate-resisting Portland cement. The metal loadings and the metal salts added to the system were varied (50 up to 5000 mg/kg; NO3(-), SO4(2-), Cl-). The XAS study showed that for all investigated systems Ni(ll) is predominantly immobilized in a layered double hydroxide (LDH) phase, which was corroborated by DRS measurements. Only a minor extent of Ni(ll) precipitates as Ni-hydroxides (alpha-Ni(OH)2 and beta-Ni(OH)2). This finding suggests that Ni-Al LDH, rather than Ni-hydroxides, is the solubility-limiting phase in the Ni-doped cement system.

The use of (micro)-X-ray absorption spectroscopy in cement research.

Long-term predictions on the mobility and the fate of radionuclides and contaminants in cementitious waste repositories require a molecular-level understanding of the geochemical immobilization processes involved. In this study, the use of X-ray absorption spectroscopy (XAS) for chemical speciation of trace elements in cementitious materials will be outlined presenting two examples relevant for nuclear waste management. The first example addresses the use of XAS on powdered cementitious materials to determine the local coordination environment of Sn(IV) bound to calcium silicate hydrates (C-S-H). Sn K-edge XAS data of Sn(IV) doped C-S-H can be rationalized by corner sharing binding of Sn octahedra to Si tetrahedra of the C-S-H structure. XAS was further applied to determine the binding mechanism of Sn(IV) in the complex cement matrix. The second example illustrates the potential of emerging synchrotron-based X-ray micro-probe techniques for elucidating the spatial distribution and the speciation of contaminants in highly heterogeneous cementitious materials at the micro-scale. Micro X-ray fluorescence (XRF) and micro-XAS investigations were carried out on Co(II) doped hardened cement paste. These preliminary investigations reveal a highly heterogeneous spatial Co distribution. The presence of a Co(II)-hydroxide-like phase Co(OH)2 and/or Co-Al layered double hydroxide (Co-Al LDH) or Co-phyllosilicate was observed. Surprisingly, some of the initial Co(II) was partially oxidized and incorporated into a Co(III)O(OH)-like phase or a Co-phyllomanganate.

Microscale investigations of Ni uptake by cement using a combination of scanning electron microscopy and synchrotron-based techniques.

Cement is used to condition waste materials and for the construction and backfilling of repositories for low-and intermediate-level radioactive waste. In this study, Ni uptake by hardened cement paste has been investigated with the aim of improving our understanding of the immobilization process of Ni(ll) in cement on the microscale. Information on the cement microstructure, Ni distribution, Ni concentration, and speciation of the Ni phases formed in the cement system and their association with specific cement minerals has been gained by using scanning electron microscopy (SEM) and synchrotron-based micro-X-ray fluorescence (micro-XRF) and micro-X-ray absorption spectroscopy (micro-XAS). The Ni-doped samples were prepared at a water/cement ratio of 0.4 using a sulfate-resisting Portland cement and were hydrated for 30 days. The metal loadings of the system were varied from 50 up to 5000 mg/kg. SEM investigations show that for all metal loadings the Ni phases form rims around inner-calcium silicate hydrates, suggesting a direct association with this cement phase. The micro-XAS measurements further reveal that a mixture of Ni phases form at Ni-enriched regions. Data analysis indicates that Ni(ll) is predominantly immobilized in a layered double hydroxide-type phase (Ni-Al LDH) and only to a minor extent precipitates as Ni-hydroxides (alpha-Ni(OH)2 and beta-Ni(OH)2). At 50 mg/kg Ni loading, however, the p-XAS measurements suggest the presence of an additional Ni species. In the latter system Ni-Al LDH is found in Ni-rich regions, whereas at Ni-poor regions an unknown species is formed.

Ni phases formed in cement and cement systems under highly alkaline conditions: an XAFS study.

X-ray absorption fine structure (XAFS) spectroscopy was applied to assess the solubility-limiting phase of Ni in cement and cement minerals. The study reveals the formation Ni and Al containing hydrotalcite-like layered double hydroxides (Ni-Al LDHs) when cement material (a complex mixture of CaO, SiO2, Al2O3, Fe2O3 and SO3) was treated with Ni in artificial cement pore water under highly alkaline conditions (pH = 13.3). This finding indicates that Ni-Al LDHs and not Ni-hydroxides determine the solubility of Ni in cement materials.

Sorption mechanisms of zinc to calcium silicate hydrate: X-ray absorption fine structure (XAFS) investigation.

In this study, X-ray absorption fine structure (XAFS) spectroscopy has been used to further elucidate the binding mechanisms of Zn(II) to calcium silicate hydrate (C-S-H), the quantitatively most important cement mineral. Such knowledge is essential for the assessment of the longterm behavior of cement-stabilized waste materials. XAFS spectra of the Zn(II) equilibrated with C-S-H(I) for up to 28 days are best modeled by tetrahedral coordination of Zn(II) by four O atoms in the first atomic shell. Beyond the first coordination shell, data analysis of more highly concentrated samples suggests the presence of two distinct Zn distances and possibly the presence of an Si shell. On the basis of the comparison with a set of reference compounds, this coordination environment can be reasonably related to the structure of hemimorphite, a naturally occurring zinc silicate, and/or the presence of gamma-Zn(OH)2. At the lowest Zn uptake, the above fitting approach failed and data could be described best with a Zn-Si and a Zn-Ca shell. Previous work has been able to show that Zn(II) diffuses into the C-S-H(I) particles and does not form discrete precipitates, so the findings appear to confirm the incorporation of Zn(II) in the interlayer of C-S-H(I).

Toxin-specific ultrastructural alterations of the mouse liver after burn injuries and the possibility of a specific antitoxic therapy.

Electron-microscopic studies in mice revealed similar and comparable mitochondrial alterations of the liver cells 5-7 days after either a sublethal controlled burn injury or an i.p. injection of an equivalent dose of a burn toxin. Electron microscopy 5 days after i.p. application of different amounts of the burn toxin in rats showed that the extent of the liver alterations correlates directly to the applied dose (occurrence of: cristolysis--intramitochondrial vacuolization--total vacuolar changes of mitochondria(. Controls with the non-toxic/"native" compound isolated from normal skin or excision of a skin piece identical in size to the sublethal burn showed no ultrastructural changes in the liver of mice or rats. In a 2nd series of experiments the therapeutic effect of an antitoxic IgG raised in sheep was tested. The first 3 days after a standard burn or an i.p. injection of 15 mg burn toxin mice obtained 10 mg of the antitoxic IgG (2x/day). Controls injected with the "native" compound or excised as described were treated in the same way. The results showed a specific complete immunological protection from mitochondrial alterations by either the toxin or the burn injury. These results suggest the possibility of an antitoxic IgG-therapy in severe burns.

Experimental approach to the correlation of hemodynamic changes with increases in urinary lactate dehydrogenase as a new parameter reflecting serious renal tissue damages.

From previous investigations with nephroptotic patients increased urinary LDH was assumed to be a reliable marker indicating a renal tissue defect due to the organs descent in erect position. Animal experiments now allowed correlation of this enzymatic activity with controlled changes of anatomical and physiological parameters. Changes of the renal hemodynamics or urinary flow induced in acute experiments in dogs simulated kidney displacement in nephroptotic patients. Both ureters were cannulated for separate urine collection and one kidney was manipulated. The renal arterial or venous flow was reduced or the ureter was occluded under electromagnetic blood-flow control. Arterial constriction alone (30%/15 min) selectively caused a drastic decrease (approximately 80%) of Xenon wash-out (= nutrient-flow) in the renal cortex. Under the same conditions radio-labeled microspheres injected intracardially showed a centralization of the renal capillary blood flow from the outer cortex to the juxtamedullary zone. Urinary LDH activities increased up to 800% immediately after arterial constriction. In accordance with total LDH activity the percentage distribution of isoenzymes changed: LDH-I increased and the LDH-V decreased. Neither constriction of the renal vein nor ureteral occlusion had similar effects. In long-term experiments backward fixation of one kidney in rats would reflect the effects of kidney displacement over years in nephroptotic patients: animals were unilaterally nephrectomized and the remaining kidney was dislocated backwards (approximately 2,5 vertebrae) and fixed to the lateral pelvic wall. "Ptotic" rats showed during the following examinations a constant increase of urinary LDH up to 50% by 26 weeks postoperatively. In accordance with increased LDH the isotope nephrogram was pathological and arteriographies showed a stretched and narrowed renal artery. In a number of rats "ptotic" fixation was not effective enough. All these animals showed normal LDH, isotope nephrograms and arteriographies. Both animal experiments documented that reduced flow/hypoxia is essentially responsible for the tissue damage in the kidney manifested by increased release of urinary LDH.

Diagnostic relevance of urinary lactate dehydrogenase determination in nephroptosis and for the indication to nephropexy.

Previously reported experiments with animals suggested that reduced renal arterial flow might be the actual cause for the pathogenicity of nephroptosis. Clinical studies now give evidence that measurements of urinary LDH may be a criterion equal to the isotope nephrogram (ING) in considering this disease. Patients with a "mobile" kidney verified by i.v. pyelography were examined by an ING and a 1-day test for urinary LDH. In accordance with periodic kidney displacement total urinary LDH activities were measured in a 8-h urine volume in the supine position and a 8-h urine volume in the erect position of the patients. Evaluations were all expressed as percentage increase of LDH activity of the patient in the erect versus supine position and correlated with his ING-pattern. Among 45 nephroptotic individuals 34 showed, in accordance with a pathological ING, a mean LDH increase of more than a 100%. Eleven individuals had normal INGs and less than 20% increase equal to a group of 16 normal controls. We postulated a 30% increase as the upper limit between normal and pathological urinary LDH. The percentage distribution of isoenzymes was also altered within the pathological LDH range: LDH-I, which increases in normal controls, now decreased in nephroptotic patients. LDH-IV and V, which decrease in controls, now increased. Homomeric isoenzymes obviously show reciprocal behavior. The degree of kidney descent in cm does not correlate with percentage increase of urinary LDH, i.e. it is not a criterion for pathogenicity. Biopsies taken during nephropexy revealed that from an anamnestic duration of 50 weeks onwards the kidney is significantly affected and tissue damages become evident. If patients were re-investigated after nephropexy they showed normal i.v. pyelograms and normal LDH and no longer had clinical symptoms.