Leachability Index Research Articles

Laboratory stabilization/solidification of surrogate and actual mixed-waste sludge in glass and grout

Grout and glass formulations were developed for the stabilization of highly radioactive tank sludges. These formulations were tested in the laboratory with a surrogate and with a sample of an actual mixed waste tank sludge. The grout formulation was tested at wet-sludge loadings of 50–60 wt%, giving a volume increase of about 40–50 vol%. Dried sludge was vitrified into glass at waste oxide loadings of 40–50 wt%, giving a volume decrease of about 50–60 vol%. The Resource Conservation and Recovery Act (RCRA) metals included in surrogate testing were Ag, Ba, Cd, Cr, Ni, Pb, Se, Tl and Hg. Since vitrification would volatilize, not stabilize mercury, it was not included in the surrogates vitrified. The actual sludge sample was only characteristically hazardous for mercury by the toxic characteristic leaching procedure (TCLP) but exceeded the Universal Treatment Standard (UTS) limit for chromium. The grout and glass formulations stabilized these RCRA metals within UTS limits. In addition, a grout leachability index of about 9–10 was measured for both 85Sr and 137Cs, meeting the recommended requirement of >6. The glass leachability index was estimated to be >18 for cold cesium and strontium.

Cemented materials in the LLW and MLW Spanish disposal

BWR and PWR cemented matrices to confine low and medium simulated liquid radioactive wastes have been submitted to the leaching process in de-ionized water at 20oC and 40oC, to obtain the medium leachability index (L) and the effective diffusion coefficient (D e ) of different ions. Otherwise, it has been studied the associated expansion of the backfilling mortar of the concrete containers of the Spanish repository of these wastes, due to a possible attack of the sulfate ions coming from the cemented matrices.

Improvement of Cs leaching resistance of solidified radwastes with copper ferrocyanide (CFC)-vermiculite

Cesium removal from de-ionized water, seawater, and limewater using copper ferrocyanide (CFC) and porous media including silica gel, bentonite, vermiculite, and zeolite as adsorbents were investigated; CFC was incorporated with vermiculite to prepare a compound adsorbent for improving the Cs-leaching resistance of solidified borate radwastes. It was shown that the Cs-removal efficiency by CFC, defined as the percentage of cesium removed or adsorpted from solution, was largely affected by pHs of the solutions. Good removal efficiency occurred at pHs ranging from 3 to 12 with the best from 7 to 10. Vermiculite and zeolite were shown to have better removal power than silica gel and bentonite, and vermiculite was chosen to incorporate with CFC to make compound adsorbents because of its good compatibility with CFC floc. Compound adsorbents with different CFC contents were used as additives in the solidification of radioactive borate wastes for improving the cesium leaching resistance of the solidified products. Experimental results showed that the cesium leachability index measured following the method described in ANSI/ANS 16.1 increased from 7.96 to 9.76 by adding 0.25% of a compound adsorbent containing 20% CFC and 80% vermiculite. It indicated that the compound adsorbent is very useful for improving cesium-leaching resistance of the solidified borate wastes.

95/05405 Solidification/stabilisation of arsenic-containing waste: Leach tests and behaviour of arsenic in the leachate

In this study, metallurgical-slag-based binder (MSB) with different dosages of calcium hydroxide (CH) was mixed with high-arsenic-containing mine tailings (HAMT) to form green mining fill samples (GMFs) for As solidification/stabilisation (S/S). The As leaching characteristics of the GMFs were evaluated using pH-dependent leaching tests, semi-dynamic leaching tests and toxicity leaching tests. The effective diffusion coefficient (De) decreased from 6.98 × 10−14 to 5.90 × 10−15 cm2/s and the leachability index (LI) increased from 13.53 to 14.73 after 3 wt.% CH was added to the GMFs. The GMFs containing 0 wt.% CH (GMF-0C) and those containing 3 wt.% CH (GMF-3C) reached pH = 2 with acid addition amounts of 9.0 meq/g-dry and 9.3 meq/g-dry at 90 d curing time, and the maximum As leaching concentrations of GMF-0C and GMF-3C reached 10.47 mg/L and 7.47 mg/L, respectively, indicating that GMF-3C exhibited better acid neutralisation and As retention capacities than GMF-0C. Further, a Tescan Integrated Mineral Analyser (TIMA) was used to analyse the dominant hydration products of GMF-3C, which revealed that calcium silicate hydrate, CASH, ettringite and zeolite phases represented approximately 22.5 wt.% of the products. These results provide an understanding regarding the safe large-scale utilisation of GMFs.

Long-term behaviour of toxic metals in stabilized steel foundry dusts

A study of the long-term behaviour of stabilized steel foundry dust (SFD) wastes has been performed using a dynamic leaching test (DLT). Two stabilized/solidified forms were produced by solidifying the SFD (containing Pb, Cr, Cd and Zn) using either cement or cement and anhydrite (waste material) as binders. The results of the dynamic leaching test were fitted to a semi-empirical mathematical model based on simple leaching rate mechanisms, which permitted the evaluation of an apparent diffusion coefficient and a leachability index, thus providing a measure of the contaminants' mobility in the solidified waste. In the case of Pb and Zn, the rate of leaching was controlled by either an initial resistance or an initial washoff, followed by diffusion of the metallic contaminants. The leaching indexes obtained in both cases were higher than 12, suggesting that both solidification/stabilization processes are acceptable.

Effect of neutralizing agent content on137Cs leaching from solidified boric acid waste products

For the preservation of environment from radioactive contamination, one of the properties necessary for solidified radioactive wastes is resistance to the release of radionuclides due to leaching by ground water after land disposal. In order to seek the optimum solidifying formulation for cement solidified boric acid concentrate from pressurized water reactor, a variety of specimens were prepared, varying the content of neutralizing agents and neutralization methods. For the leaching measurements, the ANS 16.1 Standard Leach Test was carried out for the specimens which had been mixed with small concentration of137Cs to estimate the leachability index. According to our investigation, the optimum contents of neutralizing agents for neutralization of boric acid radioactive waste were determined in view of resistance ability to leaching. Eventually the leachability indices have turned out to be 5.80∼7.91 depending on the formulations.

Determination of the leachability index of137Cs from cement solidified radioactive wastes

The radioactive boric acid wastes generated from the nuclear power plants have been solidified with cement. One of the properties required for solidified radioactive wastes is resistance to leaching. The leachability of137Cs from solidified waste specimens prepared by various formulation has been studied according to the proposed ANS 16.1 Standard Leach Test. For the evaluation of radiological safety in the solidified radioactive waste disposal, the leachability indices have been determined from the leaching test data. The results have turned out to be 5.97–7.60, depending on formulation and solidifying matrix.

A Theoretical Study of the Effect of the Leach Interval on a Semidynamic Leach Test (Ansi/Ans-16.1-1986)

ABSTRACTNumerical solution of Fick's second law of diffusion demonstrated that the error introduced in the ANSI/ANS-16.1-1986 procedure, by assuming a zero surface concentration, varied from about 30% for a leachability index of 5 to about 0.1% for an index of 10. Waste forms typically have indexes of 7 or greater, implying errors of less than 5% with this assumption. The estimated leachability index differs from the actual value by less than 0.2 when the numerical solution is analyzed as input experimental data, except for a leachability index as low as 5 (essentially the value for water).

Leachability of Cement Encapsulated West Valley Radwaste Streams

AbstractLow-level radwaste streams to be generated during the cleanup of the nuclear fuel reprocessing plant at West Valley, New York will require disposal in an environmentally acceptable manner. A study was conducted to determine if presently identified waste streams could be encapsulated in cement to produce a waste form which complies with present NRC guidelines for the disposal of.low-level radwaste streams. One waste stability guideline is leachability which was examined in this study. The leach testing was performed in accordance with the ANS 16.1 procedure. This static, deionized water test examined the leach rate of contaminants from radioactively spiked (Cs-137) samples of each waste type. The data obtained from this method were used to generate from diffusion calculations a parameter called a leachability index (L) which is related to the leaching characteristics of the solidified waste. All waste forms exceeded an L value of 6, which is the minimum suggested by the NRC “Branch Technical Position on Waste Form” for the disposal of Class B and Class C radioactive wastes.

The chemistry and flux of throughfall and stemflow in subalpine balsam fir forests

The fluxes of water and chemicals in throughfall and stemflow of three subalpine balsam fir forests of New Hampshire, USA were measured. The three fir stands were highly dissimilar in structural characteristics, serving as a test of the importance of stand structure on these fluxes. The sum of throughfall and stemflow water exceeded incident precipitation by more than 18 to 29%. This difference was attributed to the unmeasured input of cloud droplets impacted on canopy surfaces. The principal effect of stand structure was a positive relationship between stand density and stemflow. The flux of NH+4, Na+, SO=4 and Cl− in stemflow was also significantly higher in the most dense stand. The fir canopy effects on capture and chemical alterations of elements were compared with analogous behavior of a nearby deciduous forest. There were several major contrasts, the most striking of which was a flux/canopy pool ratio (leachability index) in the hardwood canopy ranging from two to 32 times higher than in the fir canopy.