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The carbonation of recycled aggregate was accelerated by sparging with supercritical carbon dioxide (scCO2) to reduce the amount of time needed for carbonation, which is necessary for the pH neutralization of recycled aggregate. To accelerate the carbonation process, pressurized scCO2 was sparged into two different types of recycled aggregates immersed in water for 1 h, followed by standstill for 2 h (in total, a 3 h treatment process). The reduction of the pH of the treated aggregates due to carbonation was investigated using batch extraction experiments. A continuous column extraction experiment for the scCO2-sparged recycled aggregate was also performed to identify the effect of pH reduction under the condition of non-equilibrium reaction. From XRD, SEM/EDS, and TG/DTA analyses, much of the portlandite in the recycled aggregates was consumed. In its place, calcite was created as a secondary mineral during only 3 h of treatment (1 h scCO2 sparging and 2 h stationing), indicating satisfactory carbonation of the aggregate. The results of the batch extraction experiments for both of the two recycled aggregate types also showed that the average pH of scCO2-sparged aggregate decreased from 12.0 to <9.8 (the tolerance limit for recycling). The pH of the eluent from the column packed with the scCO2-sparged aggregate also remained as <9.8, suggesting that a 1 h scCO2 sparging process is sufficient to carbonate waste concrete aggregate and to create an alternative construction material resource. [ABSTRACT FROM AUTHOR]

Laboratory experiments were performed to measure the supercritical CO2 (scCO2) storage ratio (%) of conglomerate and sandstone in the Janggi Basin, which are classified as rock in Korea that are available for CO2 storage. The scCO2 storage capacity was evaluated by direct measurement of the amount of scCO2 replacing the pore water in each reservoir rock core. The scCO2 sealing capacity of the cap rock (i.e., tuff and mudstone) was also compared by measuring the scCO2 capillary entry pressure (Δp) into the rock core. The measured average scCO2 storage ratio of the conglomerate and the sandstone were 30.7% and 13.1%, respectively, suggesting that the scCO2 storage capacity was greater than 360,000 metric tons. The scCO2 capillary entry pressure for the tuff ranged from 15 to 20 bar and for the mudstone it was higher than 150 bar, suggesting that the mudstone layers had enough sealing capacity from the aspect of hydromechanics. From XRF analyses, before and after 90 d of the scCO2-water-cap rock reaction, the mudstone and the tuff were investigated to assure their geochemical stability as the cap rock. From the study, the Janggi Basin was considered an optimal CO2 storage site based on both its high scCO2 storage ratio and high capillary entry pressure. [ABSTRACT FROM AUTHOR]

Rhizofiltration experiments were conducted using uranium-contaminated groundwater and lettuce (Lactuca sativa), Chinese cabbage (Brassica campestris L.), radish (Raphanus sativus L.), and buttercup (Oenanthe javanica), which are commonly grown and consumed in South Korea. The results of the rhizofiltration experiments with artificial solutions with different initial uranium concentrations (18, 32, 84, 116, 173, and 263 μg/L) show that the uranium accumulation and bioconcentration factor (BCF) of plant roots increase with increasing uranium concentration in the groundwater. Among the four plants, the uranium concentration in the roots of Raphanus sativus L. is 1215.8 μg/g dry weight, with a maximum BCF value of 2692.7. The BCF value of the artificial solutions with various pH values (pH 3, 5, 7, and 9) is the highest under acidic conditions (pH 3) for all four plants. The uranium BCF values based on different hydroponic conditions range from 170.5 to 11580.3 and the results are comparable with those of other studies using similar methods; the highest BCF value was determined for Brassica campestris L. at pH 3. The BCF values of Raphanus sativus L. after the rhizofiltration experiments with genuine groundwater contaminated with uranium are the highest among the four species; that is, 1684.7 and 1700.1 in Oesam-dong and Bugokdong groundwater samples with uranium concentrations of 83 and 173 μg/L, respectively. The results of the scanning electron microscope/electron dispersive X-ray spectroscope analyses show that uranium in contaminated groundwater is adsorbed as a solid phase on the root surface. These results demonstrate that Raphanus sativus L. has a high tolerance to high concentrations of uranium and low pH conditions and a remarkable potential for uranium accumulation. [ABSTRACT FROM AUTHOR]