Encapsulation of cesium with a solid waste-derived sulfoaluminate matrix: A circular economy approach of treating nuclear wastes with solid wastes

Emission characteristics of condensable particulate matter during the production of solid waste-based sulfoaluminate cement: Compositions, heavy metals, and preparation impacts

Recycling solid waste for preparing sulfoaluminate cementitious materials (SACM) represents a promising approach for low-carbon development. There are drastic physical-chemical reactions during SACM calcination. However, there is a lack of research on the flue gas pollutants emissions from this process. Condensable particulate matter (CPM) has been found to constitute the majority of the primary PM emitted from various fuel combustion. In this study, the emission characteristics of CPM during the calcination of SACM were determined using tests in both a real-operated kiln and laboratory experiments. The mass concentration of CPM reached 96.6 mg/Nm3 and occupied 87% of total PM emission from the SACM kiln. Additionally, the mass proportion of SO42- in the CPM reached 93.8%, thus indicating that large quantities of sulfuric acid mist or SO3 were emitted. CaSO4 was one key component for the formation of main mineral ye'elimite (3CaO·3Al2O3·CaSO4), and its decomposition probably led to the high SO42- emission. Furthermore, the use of CaSO4 as a calcium source led to SO42- emission factor much higher than conventional calcium sources. Higher calcination temperature and more residence time also increased SO42- emission. The most abundant heavy metal in kiln flue gas and CPM was Zn. However, the total condensation ratio of heavy metals detected was only 40.5%. CPM particles with diameters below 2.5 μm and 4-20 μm were both clearly observed, and components such as Na2SO4 and NaCl were conformed. This work contributes to the understanding of CPM emissions and the establishment of pollutant reduction strategies for waste collaborative disposal in cement industry.

Calcination of sewage sludge-based sulphoaluminate cement clinker: Mineral formation mechanism and heavy metal transition behaviors

Utilizing sewage sludge (SS) to calcinate sulphoaluminate cement (SAC) is a promising technology for low-carbon transition of cement industry, but the unclear effects of SS-contained heavy metals limit the application of this technology. In this study, the effects of SS addition on the calcination of SAC clinker and the transformation of heavy metals were studied from the aspects of mineral phase change, microstructure evolution and heavy metal speciation respectively, covering the mineral formation temperature 900-1250 °C. The results show that the added SS will reduce the formation temperature and change the reaction pathways of mineral phases. When the content of SS increases from 10% to 25%, the compositions of mesophases CaO·Al2O3 and 4CaO·2SiO2·CaSO4 increase by 6.33% and 9.73%, respectively. Meanwhile, the formation of minerals will solidify Zn, Ni, Mn, Cu, Cr, and convert them into a more stable fraction (residual fraction), indicating a lower probability to harm the environment. Moreover, heavy metals present different migration behaviors. After calcination, Mn migrates from SS to 4CaO·Al2O3·Fe2O3 (52.48%), while Zn prefers to enter 3CaO·3Al2O3·CaSO4 (43.74%) and 4CaO·Al2O3·Fe2O3 (38.06%). This study offers new insights into the mineral formation mechanism and heavy metal transition behaviors of sewage sludge-based SAC.

Thermal co-treatment of aluminum dross and municipal solid waste incineration fly ash: Mineral transformation, crusting prevention, detoxification, and low-carbon cementitious material preparation

Harmless disposal and resource utilization of hazardous industrial wastes has become an important issue with the green development of human society. However, resource utilization of hazardous solid wastes, such as the production of cementitious materials, is usually accompanied by a pretreatment process to remove adverse impurities that contaminate the final product. In this study, aluminum dross (AD) was thermally co-treated with another hazardous waste, municipal solid incineration fly ash (MSWI-FA), to synergistically solidify F and Na, control leaching of heavy metals, and remove chloride impurities. Significant crusting was observed when AD was thermally treated by itself, but not when AD and MSWI-FA were thermally co-treated. In the process of co-thermal treatment, the remaining Cl, Na, and K contents were reduced to as low as 0.3%, 1.8%, and 0.6%, respectively. CaO and SiO₂ in MSWI-FA reacted with Na₃AlF₆ and Al₂O₃ in AD, and formed CaF₂ and Na₆(AlSiO₄)₆, which contributed to the prevention of crusting and limited the leaching concentrations of F and Na to below detection thresholds and 270.6 mg/L, respectively. In addition, heavy metals were well solidified, and dioxins were fully decomposed during thermal treatment. Finally, a sulfoaluminate cementitious material (SACM) with high early- and later-age strengths was successfully created via synergetic complementarity using thermally co-treated AD and MSWI-FA together with other solid wastes. Collectively, this study outlines a promising method for the efficient and sustainable utilization of AD and MSWI-FA.

Elaboration of geopolymer package derived from uncalcined phosphate sludge and its solidification performance on nuclear grade resins loaded with 134Cs

Nuclear-grade Spent Organic Resin (SOR) contains high concentrations of radioactive nuclides and metal contaminants, while phosphate sludge contains high amount of fine clayey particles and CO₃^2-, both posing a major threat to the biosphere. In this study, a novel geopolymer package (GP) was proposed to directly solidify SOR loaded with ¹³⁴Cs by incorporating uncalcined phosphate sludge (UPS) as feedstocks, activated by NaOH/KOH. The results showed that alkali-mixed reagents-activated GP is more advantageous in terms of chemical stability and mechanical properties than NaOH-activated GP, recording compressive strength values greater than the waste acceptance criteria and OPC. The 28-day compressive strength of solidified packages can exceed 31 MPa at the highest amount of 42 wt% UPS. The addition of NaF powder into the solidified packages generates more hybrid type gels, which are more conducive to partial dissolution and bonding UPS particles, thereby producing stable and stronger GP. Leaching results of solidified GP in presence of up to 13 wt% SORs showed that only 0.15 % of total ¹³⁴Cs was leached, even under aggressive solutions. Solidification mechanism revealed that activation of UPS-MK blend forms N,K-A-S-H, (N,K,C)-A-S-H/C-S-H gels coexisting with unreacted particles, thereby solidify/stabilize metal contaminants and Cs⁺ by a synergetic immobilization action of hydration products via substitution and encapsulation. This study provides a promising paradigm for effective solidification of nuclear-grade resins and synergetic harmless treatment of industrial/phosphate mine solid wastes.

Simultaneous stabilization/solidification of arsenic in acidic wastewater and tin mine tailings with synthetic multiple solid waste base geopolymer

Stabilization/Solidification (S/S) is considered as a feasible technology for the treatment of arsenic (As) in acidic wastewater (AW) and tin mine tailings (TMTs); however, high cost, high carbon footprint, and strict reaction conditions are the main limitations. Herein, a novel alkali-activated geopolymer material (AAGM) for S/S As was synthesized by combining AW, TMT, gypsum (GP), and metakaolin (MK). At room temperature, an initial As concentration of 3914 mg/L, a NaOH content of 4.98%, and an MK content of 20% decreased the As leaching concentration to 1.55 mg/L (<5 mg/L). The main S/S mechanisms of As included physical encapsulation of C-(A)-S-H and geopolymer structures, ion exchange of ettringite, and formation of Fe-As and Ca-As precipitates. Further studies showed that increasing initial As concentration and MK content facilitated the formation of Ca-As precipitates and C-(A)-S-H gels. The semi-dynamic leaching tests revealed that the leaching mechanism of As was surface wash-off. The effective diffusion coefficients of the samples were less than 10^-13 cm²/s, and the respective leachability indexes were greater than 9, indicating that AAGM was effective in preventing the leaching of As. Therefore, this study provides a green and low cost solution for the synergistic utilization of AW, TMT, GP, and MK.