Static Leach Tests Research Articles

Chemical Durability And Mechanical Properties Of Cementitious Matrices Containing Ashes Obtained After Combustion Of Waste From Moukondo Landfill In Brazzaville, Republic Of Congo.

The regular burning of solid waste in the open air and on the ground is commonplace in the Republic of Congo. Burning waste produces pollutants and ashes that can be hazardous to the soil and surrounding waters. Inadequate management of the ash generated by the open burning of household waste results in site pollution, particularly heavy metal contamination (Pb, Ni, Cr, Cu, Zn...). The aim of this study is to develop cementitious matrices containing burnt ashes that are sufficiently effective to retain the heavy metals contained in these ashes in their structure. To this end, in order to assess the chemical durability of these cementitious matrices, we carried out static leaching tests at pH=4 and 7 at 25°C in distilled water on the raw ashes firstly, and then on the 24 cementitious matrices synthesized with different ash/cement/lime ratios. The mechanical durability of the cementitious matrices was assessed using flexural and compressive strength tests. The leaching results obtained demonstrated the effective immobilization of heavy metals in cementitious matrices, whose leachates contain very low levels of heavy metals compared with those of raw ash. Flexural and compressive strength tests showed an improvement in strength from 25°C to 60°C. Several cementitious matrices doped with waste ashes show flexural strengths higher than the recommended standard for flexural strength EN1339 (5 MPa) and compressive strength X31-211 (1 MPa) after 28 days curing at 60°C

Chemical stability and leaching mechanism of YIG and HEG at different pH conditions

AbstractDesigning waste forms using the cocktail effect of high‐entropy ceramics can improve the efficiency of ceramic waste forms. In this work, traditional garnet (Y1.2Nd1.8Fe5O12, YIG) and high‐entropy (HE) garnet (Y0.6Gd0.6Sm0.6Eu0.6Dy0.6Fe5O12, HEG) are synthesized through ignition and plasma sintering to form dense ceramic waste forms. The chemical stability of YIG and HEG was studied by using static leaching tests at pH = 3, 5, 7, 9, 11. The results indicate that acidic environments can increase the leaching rate of ceramics. Leaching behavior leads to a decrease in lattice volume, but the elements remain uniformly distributed. The leaching mechanism indicates that YIG and HEG are controlled by dissolution and surface effect in the early stage (1–7 days) of leaching. The leaching mechanism in the later stage (14–42 days) is characterized by diffusion control. The diffusion coefficient of long‐term leaching indicates that HEG is more suitable as the immobilization substrate for high‐level radioactive waste (HLW) than YIG.

Phenomenological description of the effect of glass dissolution on cement paste evolution in an integrated glass dissolution experiment

The interaction between nuclear glass surrogate and ordinary Portland cement (OPC) was investigated through integrated static leach tests conducted at ambient temperature. Tests were performed in stainless-steel cells in which OPC hydrated cement pastes were separated from glass powders by a stainless-steel filter. This article gives a phenomenological description of the processes affecting the hydrated cement paste arising from glass dissolution. The results show that the ingress of Si from glass dissolution leads to the degradation of the cement paste over a distance of 100–200 μm within 900 days of interaction. The Hardened Cement Paste (HCP) is affected by two main processes, which are the pozzolanic reaction and the alkali-silica reaction (ASR). These processes lead to porosity changes of HCP in contact with the stainless-steel filter and significant changes in the diffusive properties of the HCP. Especially, the effect of such processes on boron and lithium retention was studied and showed that precipitation of ASR-like gel and low Ca/Si C–S–H (Calcium Silicate Hydrate) highly retain these two elements. In the presence of an Al-rich glass (SM539), ASR is unlikely due to the preferential formation of C-A-S-H (Calcium Aluminium Silicate Hydrate).

Vitrification of lead–bismuth alloy nuclear waste into a glass waste form

AbstractLead–bismuth eutectic (LBE), a promising coolant in advanced nuclear systems, can be activated by neutrons during nuclear reactor operations. The decommissioning of nuclear facilities would generate lead–bismuth (Pb–Bi) alloy‐contaminated nuclear waste. The current metallic nuclear waste treatment approach involves remelting followed by cementitious solidification. This increases the waste volume and the risk of radionuclide migration in groundwater. Therefore, this study developed a method for vitrification of Pb–Bi alloy waste. Different amounts of SiO2 were added at 750–1100°C in the air to turn the simulated LBE waste into glass waste form. The values of the normalized elemental leaching rates (Pb, Bi, Si, Te, and Ni) determined using the 28‐day static leaching test were less than .2 g m−2 d−1 and varied with SiO2 addition. Furthermore, a three‐stage evolution of the glass structure with SiO2 addition was proposed according to the structural analysis performed using Raman and X‐ray photoelectron spectroscopies. The evolution stages were as follows: (i) the stage of heavy metal transition from covalent to ionic heavy metals (7.5 wt% < SiO2 < 15 wt%), (ii) the stage of increase in bridging oxygen (15 wt% < SiO2 < 20 wt%), and (iii) the stage of domination of the Si–O network (20 wt% < SiO2 < 25 wt%). The evolution of the glass structure resulted in varying glass chemical durability. Finally, the glass‐forming region of (20–48)PbO–(35–70)Bi2O3–(7.5–25)SiO2 (wt%) and the temperature needed to melt those glasses were determined through the melting test, where radionuclides and toxic heavy metals showed undetectable volatilization during vitrification. Hence, turning LBE waste into glass waste form will be a potential approach for treating Pb–Bi alloy nuclear waste.

Dissolution behavior and aging of iron–uranium oxide

Understanding the dissolution behavior of the fuel debris is necessary for the safe decommissioning of Fukushima Daiichi Nuclear Power Plants. The dissolution behavior of FeUO4 compounds formed by a high-temperature reaction of UO2 with iron, a stainless-steel component of reactor structural materials, was investigated under atmospheric conditions. The compounds were prepared in an electric furnace using U3O8 and Fe3O4 as starting materials, and their solid states were analyzed using X-ray diffraction, scanning electron microscopy–energy-dispersive X-ray spectroscopy, and X-ray absorption fine-structure spectroscopy. The fission products were produced via thermal-neutron irradiation. The concentration of nuclides dissolved in water was examined by performing static leaching tests of FeUO4 compounds for up to three months. A redox reaction was proposed to occur between trivalent Fe and pentavalent U ions in the early stage of FeUO4 dissolution. It was thermodynamically deduced that the reduced divalent Fe ion was finally oxidized into a trivalent ion in the presence of dissolved oxygen, and iron hydroxide limited the solubility of Fe. Meanwhile, the concentration of hexavalent U (i.e., uranyl ion) was limited owing to the presence of secondary minerals such as metaschoepite and sodium uranate and subsequently decreased, possibly owing to sorption on Fe oxides, for example. The concentrations of multivalent ions of fission products, such as Ru and Ce, also decreased, likely for the reason above. By contrast, the concentration of soluble Cs ions did not decrease. The validity of this interpretation was supported by comparing the results with the dissolution behavior of a reference sample (Fe-free U3O8).

Assessing the influence of geological stress induced damage and initial pH levels on the metal leaching behaviors of cemented paste backfill

Cemented paste backfill (CPB) is generally considered environmentally friendly when used in mining operations. However, the combination of geological stresses and groundwater seepage in an underground environment may cause the metallic elements contained in CPB to escape confined areas and potentially pollute the external environment. In this paper, geological stress-induced damage to CPB and the initial pH levels of the surrounding environment are assessed to determine their influences on the metal leaching behavior of CPB. Research methods used in the analysis include component characterization, static leaching tests, and kinetic theory analysis. The research results show that: as damage to CPB increased, both the pH and conductivity of the leachate increased, while the concentration of metallic elements in the leachate also increased except for aluminum (Al). Different metallic elements showed various leaching behaviors as the initial pH of the leaching solution increased. The release process of metallic elements from CPB can be segmented into three separate stages: the surface release stage, the internal release stage, and the adsorption and precipitation stage. Kinetic equations are obtained to describe the release rate of metallic elements. The leaching process is a dynamic equilibrium system formed by the release, absorption, and precipitation of metallic elements in the leachate. This research provides data for the evaluation of environmental effects on CPB.

Geochemical evidence of fluoride behavior in loess and its influence on seepage characteristics: An experimental study

High background levels of fluorine in groundwater and soil in arid and semi-arid loess regions pose a severe threat to socio-economic development and human health, necessitating the evaluation of fluorine migration in loess. In this study, static leaching and dynamic seepage tests as well as scanning electron microscopy, mercury intrusion porosimetry, and X-ray fluorescence analyses were conducted using loess as the porous medium. Additionally, simulations using PHREEQC software were performed. The results indicated that the studied loess had a high background level of fluorine. Geochemical processes closely related to fluorine include dissolution of gypsum and dolomite, precipitation/dissolution of calcite and fluorite, and ion exchange between CaX and NaX. Under seepage of water with high fluorine levels, soil particles flocculated and formed aggregates; furthermore, the contact area between soil particles reduced, resulting in point-to-point contact between particles. Consequently, pores changed from small (intra- and inter-particle pores) to large (intra- and inter-granular pores) scale pores. Permeability initially decreased rapidly, then remained relatively stable, and subsequently rapidly increased with the passage of time. This was attributed to the dissolution of calcium minerals in loess, yielding Ca2+, which induced the precipitation of fluorite and promoted the dissolution of carbonates. Cation exchange and dissolution of other components were also important factors influencing permeability. The findings of this study can elucidate the coupling between loess microstructure, seepage behavior, and geochemical actions under the influence of high‑fluorine water, and are of great significance for in-situ regional research in loess areas.

Evaluation on leachability of heavy metals from tailings: risk factor identification and cumulative influence

The leachability of heavy metals (HMs) in tailings is significantly affected by multivariate factors associated with environmental conditions. However, the leaching patterns of HMs in molybdenum (Mo) tailings due to environmental change and cumulative influences of multi-leaching factors remain unclear. The leaching behaviors of HMs in Mo tailings were studied through static leaching tests. The key leaching factors were discussed via simulating acid rain leaching scenario in terms of global and local environmental conditions. The potential risk factors were identified, and their cumulative influences on the leachability of HMs were evaluated with boosted regression trees (BRT) and generalized additive model (GAM) analyses. Environmental factors showed interactive effects on the leachability of HMs in tailings. The leachability of HMs in tailings decreased significantly with the interaction of increasing liquid/solid (L/S) ratio and pH. Rebound of leachability was observed with high L/S ratio (> 60) and long-time leaching (> 30h). L/S ratio and pH were the most sensitive factors to the leachability of HMs with the corresponding contribution of 40.8% and 27.1%, respectively, followed by leaching time and temperature (~ 16%). The total contribution of global climate-associated factors, i.e., L/S ratio, leaching time, and temperature to the leachability of HMs was up to 70%, while leachate pH shared the other 30%. With the increase of persistent heavy rain in summer globally, As and Cd were found to having higher leaching risks than the other HMs in tailings, although an obvious decrease in their leachability was obtained due to the improvement of acid rain pollution in China. The study provides a valuable method for the identification of potential risk factors and their associations with the leaching behaviors of HMs in tailings under the background of obvious improvement on acid rain pollution in China and global climate change.

Characteristics of iodine-bearing silver-impregnated alumina sorbents and their direct solidification via hot isostatic pressing.

Waste management for radioiodine is a key issue for the sustainable nuclear fuel cycle. The iodine adsorption behavior on a bed column of a silver-impregnated alumina sorbent (AgA) under conditions designed to match those of the Rokkasho reprocessing facility dissolver off-gas (DOG) system was investigated using different volatilized iodine concentrations. Cross-sectional observations of iodine-bearing AgA grains revealed that iodine was adsorbed as silver iodide and silver iodate, and gradually distributed from the surface to the inside of the AgA. The iodine distribution throughout the AgA beds allowed us to estimate the length of the mass-transfer zone. This suggests that the iodine load fraction in AgA (adsorbed iodine/total impregnated silver) will be averaged to 50% in the expected facility equipment design. This study also describes the waste form durability after disposal. To reproduce the average iodine loading in the waste form, 100%-loaded AgA grains were mixed with an equal amount of commercially available alumina reagents and consolidated through hot isostatic pressing at 175MPa and 1,325°C for 3h. The resultant 50%-loaded solid was used for the static leaching test over 4.5years, where the leached iodine was less than 0.2% under simple reducing conditions. This suggested that the HIPed solid of AgA from Rokkasho DOG showed preferable water resistance for after disposal safety.

Transition Metal Dissolution in Lithium-Ion Cells: A Piece of the Puzzle

Static leaching tests were performed using the chemically delithiated positive electrode materials, LiFePO4, LiCoO2, LiMn2O4, LiNiO2, and LiNi0.8Mn0.1Co0.1O2. Instead of the common electrolyte, which contains LiPF6, the solvent consisted of only ethylene carbonate (EC) and ethyl methyl carbonate (EMC), limiting the possible reactions to only those that depend on the solvent. The product liquids from these experiments showed that there were lithium-bearing species in common, such as Li(EC)+ and Li(EMC)+. Interestingly, we found evidence of electrolyte degradation products in both the positive- and negative-ion mass spectral results. The positive-ion results showed that the products tended to coordinate to lithium. The negative-ion results showed that most of the products tended to complex with transition metals. It was difficult to discern which positive ion was associated with which negative ion.

Release characteristics of heavy metals in high-sulfur coal gangue: Influencing factors and kinetic behavior

High-sulfur coal gangue (HS-CG) is extremely unstable in the environment, releasing acid mine drainage with high concentrations of harmful heavy metals (HMs). The effects of HS-CG particle size, leaching solution pH, Fe3+ and acidophilic microorganisms on the release of HMs from the HS-CG and their kinetic behavior were studied using static leaching tests. The results showed that the smaller the particle size of HS-CG and the more acidic the leaching solution, the greater the release of HMs. As the chemical catalyst, the external addition of 300 mg/L Fe3+ can make the leaching amount of Fe, Mn, Cu, Zn, Ni, Cr reached 10,224.93, 93.88, 52.25, 11.56, 7.55, 2.97 mg/kg respectively, and the release of HMs was 1.36–2.60 times of the tests without the addition of iron. However, the concentration of Fe3+ above 800 mg/L promoted the production of jarosite on the surface of HS-CG, which led to decrease in the release of HMs. The HMs forms in HS-CG were different, while the effect of microorganisms on the leaching of Zn (54.99%) and Mn (52.35%) in the higher acid soluble fraction was more obvious, their leaching amount reached 87.21 and 107.58 mg/kg respectively. The kinetic analysis indicated that the rate-controlling step was mainly redox reaction at first, and then gradually controlled by the diffusion of ash layer. So, the kinetic equation controlled jointly by two rate-controlling stages has been proposed to describe the dissolution of HS-CG. This work help develop pertinent strategies for mine area remediation via controlling the HMs generation path.

Leaching behavior of multiphase solidified melt prepared from stainless steel and Zircaloy

ABSTRACT Formation of metallic fuel debris is highly probable in the damaged core of the Fukushima Daiichi Nuclear Power Station. Estimating the number of radionuclides released from the fuel debris to an aqueous solution is indispensable for proper handling of fuel debris and waste management. In the present study, a simple approach is introduced to roughly evaluate the mass of elements leached from complex multiphase surfaces considering the surface-area-weighted average of the contributions of individual phases. Static leaching tests were performed under the acidic and the alkaline conditions to investigate the gap between the simplified assumption and the actual dissolution behavior. Alloy samples of a stainless steel–Zircaloy (SUS–Zry) solidified melt and two single-phase samples ((Fe,Cr)2Zr- and Zr2(Fe,Ni)-type phases, which comprised the surface of the SUS–Zry alloy) were used in the static leaching tests. The masses of the Fe, Cr, Ni, and Zr leached from the SUS–Zry alloy fitted with those evaluated by the surface-area-weighted approach by 1–2 orders of magnitude of precision.

Chemical durability and degradation mechanisms of CsPbI3 as a potential host phase for cesium and iodine sequestration.

Effective nuclear waste management of radioactive cesium and off-gas iodine from complex waste streams of used fuels is essential for the sustainable development of advanced nuclear fuel cycles. Once cesium and iodine are separated from their respective waste streams, host phases are required to immobilize them into a durable waste form matrix for long-term disposition. The inorganic metal halide perovskite, CsPbI3, has a unique crystal structure capable of incorporating both cesium and iodine simultaneously. Exposure to groundwater in geological repositories is a long-term concern for waste forms, as this may cause corrosion and decrease the waste form's ability to retain radionuclides. In this study, we explore the potential of CsPbI3 perovskite as a promising host phase to incorporate Cs and I, and investigate its chemical durability and degradation mechanisms in an aqueous environment. CsPbI3 was synthesized through a solution-based method and was consolidated into dense pellets by spark plasma sintering. The chemical durability of the CsPbI3 pellets was evaluated by static leaching tests in deionized water at different temperatures of 25, 58, and 90 °C. The elemental release mechanisms and surface alteration of the monolithic CsPbI3 pellets were investigated. Both I and Cs displayed a non-congruent leaching behavior and faster release rates as compared to Pb, particularly at longer leaching durations and higher temperatures. At the initial leaching stage, a PbI2 alteration layer formed on the surface of the pellet due to the rapid release of Cs and I, followed by the formation of a PbI(OH) alteration layer. The activation energies for both dissolution and diffusion controlled mechanisms were determined to be 44.90 kJ mol−1 and 45.40 kJ mol−1 for Pb, 27.10 kJ mol−1 and 40.82 kJ mol−1 for I and 24.27 kJ mol−1 and 23.86 kJ mol−1 for Cs, respectively. These results show a clear decrease in activation energies from Pb to I and Cs, suggesting a preferential release of I and Cs. The solution-based synthesis of CsPbI3 as a host phase for Cs and I and the fundamental understanding of the chemical durability and degradation behavior will be useful for further exploring its application for immobilizing iodine and cesium into final durable waste forms for long-term geological disposition.

Effect of cement kiln dust, lime and fly ash on metal leaching characteristics of oil sands tailings from Alberta, Canada

The oil sands industry of Alberta generates huge amounts of tailings in a slurry form that typically require up to 40 years to consolidate in very large tailings ponds which are up to 150 m in height. Cement kiln dust (CKD), a byproduct of the ordinary Portland cement manufacturing process, as well as lime and fly ash, collectively referred to as geopolymers, may have the potential to reduce the tailings slurry consolidation period from 40 years, thus affecting the sustainability of such tailings facilities. However, first, it must be demonstrated that these geopolymers will also decrease the metal leaching from thickened tailings (TT) and mature fine tailings (MFT) from the oil sands industry. This study was focused on the use of geopolymers to reduce the environmental impact of TT- and MFT-tailings in the Alberta oil sands industry. Toxicity characteristics leaching procedure (TCLP) and static leaching test (SLT) was used to examine the leaching of metals from tailings, with the SLT test effectively mimicking the leaching process in the tailings ponds environment. Under non-acidic conditions corresponding to the SLT test results, iron concentrations with values of about 530-705 ppm were found to be lower than previous studies on oil sand tailing ponds (2400 ppm). Results showed that geopolymer amendment of TT and MFT significantly reduced the leaching of heavy metals. SLT tests showed that amendment of MFT with 7% CKD decreased Pb, Mn, and Fe leaching, whereas TT-amended with 4% CKD decreased Cu, Pb, Zn, Mn, and Fe. Overall, the CKD amendment of TT showed more than 95% efficiency in the reduction of leaching of all heavy metals. In TCLP tests, TT-amended with 2% FA decreased the leaching of Pb and Ni to acceptable levels with substantial efficiency in reducing the leaching of Fe, Cu, and Zn. TCLP tests also showed that among different amendments, TT-amended with 4% CKD or 2% FA were the most effective proportions for controlling metal leaching from TT, while MFT-amended with 7% lime/FA or 3% CKD were the effective proportions for reducing metal leaching from MFT. Thus, it is deduced that CKD at 3%-4% w/w amendment would work best for reducing leachate levels of both TT and MFT. While amendment of tailings by means of a combination of fly ash and lime also were effective in reducing the leaching of metals, these two geopolymers were not as effective as the CKD amendment.

Eluviation and Leaching of Elements from Broken Fly-Ash-Based Porous Geopolymer.

The fly ash from powerplants used for coal mine end backfilling can effectively reduce the impact of ground fly ash accumulation on the environment. However, due to the long-term action of the overlying strata and groundwater, when the backfilling body is broken, heavy metals will also be leached, thus having an impact on the groundwater. Therefore, in this paper, the eluviation and leaching of elements from a broken fly-ash-based porous geopolymer is studied. The fly-ash-based geopolymer material was prepared to perform a dynamic eluviation and static leaching test, and it was found that the amount of Cu and Zn in the leachate was less abundant, whereas Pb was more abundant, but far less than the limit of the Class III groundwater quality standard. An acidic environment and a smaller solid–liquid ratio can promote the leaching of Cu and Zn, while the leaching of Pb is basically unaffected by the pH value. Moreover, the amount of Cu, Zn, and Pb in the lixivium increased with the increase in leaching time, and the amount of Cu and Zn in the lixivium was still low after 150 h of leaching, whereas the amount of Pb was high, approaching the limit value of the Class III groundwater quality standard, showing a tendency to increase after 100 h of leaching. A leaching orthogonal experiment was designed, and the results showed that the main order of each factor affecting the leaching of heavy metals from the fly-ash-based geopolymer was grain size > pH > solid–liquid ratio; thus, the leaching of heavy metals from fly-ash-based geopolymer can be controlled, which is significant with respect to the extensive use of fly-ash materials underground.

Langbeinite phosphosilicates K2-xCsxZr2P2SiO12 (x = 0, 0.5, 1.0, 1.5, 2.0) for cesium encapsulation; synthesis, chemical durability and thermal expansion study

The volatization of cesium at high temperatures makes its immobilization challenging to the nuclear scientists. Present work reports the feasibility of cesium incorporation into an orthorhombic langbeinite phosphosilicate. The structural flexibility, leachability and thermal expansion of K2-xCsxZr2P2SiO12 (x = 0, 0.5, 1.0, 1.5, 2.0) have been investigated. Powder X-ray results authenticates the accommodation of cesium into a single phasic orthorhombic langbeinite structure up to 29.17 wt %. Morphological study reveals the particle agglomeration and poor crystallinity, and vibrational spectral analysis shows peak broadening and peak shift upon increasing the cesium concentration. The static leach test has been performed on the pellet sample and the normalized leaching of Cs is found to be in the order of 101 g/m2. The average coefficient of thermal expansion for K0.5Cs1.5Zr2P2SiO12 is registered as 7.45 × 10−6/K between 303 and 873 K. Cesium insertion into K2Zr2P2SiO12 displays a large deviation in thermal expansion due to the size effect as observed by several researchers for other phosphate and phosphosilicate members.

Sewage sludge ash contaminated with radiocesium: Solidification with alkaline-reacted metakaolinite (geopolymer) and Portland cement

This study contributes toward developing measures for the disposal of radiocesium-contaminated sewage sludge ash (SSA). Here, we prepared two types of solidified bodies containing 30 wt% radiocesium-bearing SSA. The material used for the two solidified bodies were alkaline-reacted metakaolinite (geopolymer) and ordinary Portland cement (OPC). Cement has been used for solidification of low-level radioactive wastes, and geopolymer is a candidate of cement alternative materials. The characteristics of these solidified bodies were investigated by various aspects including mechanical strength, transformation of SSA components during solidification, and radiocesium confinement ability by leaching test. The compressive strength of geopolymer- and OPC-solidified bodies at 30 wt% SSA content was more than 40 MPa. After static leaching test at 60 °C, 137Cs was hardly leached out from the geopolymer-solidified bodies containing SSA at 30 wt% to ultrapure water (<0.1%), whereas more than 30% 137Cs was leached from the OPC-solidified bodies containing SSA at 30 wt% even though only ~9% of 137Cs in the SSA is soluble. These results strongly indicate that geopolymer is far superior to OPC for solidifying radiocesium-bearing SSA.

Investigating hollandite–perovskite composite ceramics as a potential waste form for immobilization of radioactive cesium and strontium

Ceramic matrix containing zirconolite, hollandite, and perovskite phases is proposed as a potential host for HLW immobilization. Hollandite phase principally immobilizes Cs, while perovskite phase mainly immobilizes Sr. In this study, hollandite–perovskite composite ceramics are considered as a specialized waste form for immobilizing the separated Cs and Sr from HLW streams and synthesized by a solid-state reaction method at 1300 °C for 5 h. The phase compositions of the synthesized composites were characterized by XRD and BSE. The XRD results indicated that the as-prepared ceramics are composed of tetragonal hollandite Ba0.8Cs0.4Al2Ti6O16, cubic perovskite SrTiO3, alongside a lesser amount of TiO2. The BSE—EDX results confirm that Cs partitions into the hollandite matrix, while Sr incorporates into perovskite host with homogenous distribution. In addition, aqueous durability testing was carried out using the MCC-1 static leach test method. The normalized release rates of Cs and Sr in HP-3 sample (i.e., 75 wt% Ba0.8Cs0.4Al2Ti6O16 + 25 wt% SrTiO3) were < 10−2 g·m−2·d−1 after 42 days, exhibiting excellent chemical durability. These results indicate that the hollandite–perovskite ceramic matrix could be considered as a customized host matrix for immobilization of the separated Cs and Sr from HLW streams.

Investigating the Leaching Rate of TiTe&lt;sub&gt;3&lt;/sub&gt;O&lt;sub&gt;8&lt;/sub&gt; Towards a Potential Ceramic Solid Waste Form

An important property of glass and ceramic solid waste forms is processability. Tellurite materials with low melting temperatures and high halite solubilities have potential as solid waste forms. Crystalline TiTe3O8 was synthesized through a solid-state reaction between stoichiometric amounts of TiO2 and TeO2 powder. The resultant TiTe3O8 crystal had a three-dimensional (3D) structure consisting of TiO6 octahedra and asymmetric TeO4 seesaw moiety groups. The melting temperature of the TiTe3O8 powder was 820℃, and the constituent TeO2 began to evaporate selectively from TiTe3O8 above around 840℃. The leaching rate, as determined using the modified American Society of Testing and Materials static leach test method, of Ti in the TiTe3O8 crystal was less than the order of 10-4 g·m-2·d-1 at 90℃ for durations of 14 d over a pH range of 2-12. The chemical durability of the TiTe3O8 crystal, even under highly acidic and alkaline conditions, was comparable to that of other well-known Ti-based solid waste forms.

Chelating agent-introduced unconventional compound acid for enhancing coal permeability

The low permeability of coalbed methane (CBM) reservoirs can be attributed to cementing minerals, which make it difficult for water/gas to flow in the pores and fractures. Acid stimulation is considered a promising method to enhance coal permeability by dissolving these inorganic minerals. Laboratory experiments were carried out to investigate the interactions between acids and coal samples from the Qinshui Basin. XRD and SEM-EDS analyses showed that these coal samples contained a large amount of clay minerals, and some carbonates, pyrite, etc. Considering the complexity of the mineral concentration, three kinds of acids (organic acids, chelating agents, and fluoroboric acid) were optimized via static leaching tests, ICP-OES, etc. Based on the results, out of the four chelating agents, HEDTA showed the highest chelating amount for Ca2+ (380.38 mg/g) and Fe3+ (476.4 mg/g), and 1 wt% HEDTA (pH = 2) yielded the highest dissolution rate. For the organic acids, the dissolution rate decreased with an increase in acid concentration because of the common ion effect. The dissolution rate with 2.5 wt% OA-1 was 8.02% while that with 20w% HCl was only 7%. In addition, 639 mg/L Ca was detected in the 5 wt% OA-1 leachate, which was higher than that for the 5 wt% HCl (575 mg/L). Afterward, orthogonal methods were used to optimize the compound acid system which consisted of 1.5 wt% HEDTA +4 wt% OA-1 + 5 wt% HBF₄. According to the core-flooding test results, after acid injection for approximately 100 PV (pore volume), the porosity of coal core sample increased from 3.36% to 4.95%, and the permeability increased 3.29 times. In general, previous studies have been limited to the conventional acids (HCl and mud acid), yet the drawbacks such as clay breakage, migration, and precipitation cannot be neglected. Accordingly, this paper proposed a set of chelating agent-introduced compound acid and it shows great promise in the application of enhanced CBM recovery.