CAESIUM REMOVAL | WASTE MANAGEMENT


Above: The disaster at Fukushima Diiachi resulted in a release of caesium 137


ions, could also contribute to adsorption by being leached out under acidic conditions. However, presumably owing to the significantly smaller size of the dopant cation (Al3+ r = 0.54 Å)) than that of Cs+ the original Al3+


, (r = 1.67 Å), Cs+ cannot reside in sites, and protons, which are much smaller


and abundant, are deposited instead. This presumption suggests that if dopant cations are more comparable in ionic size to Cs+


doped with an intermediate-sized dopant cation will provide much higher capacity for Cs+


and leachable in acid, the dopant lattice


positions in the crystal structure of adsorbent can operate as additional active adsorption sites for Cs+


adsorption in acidic


conditions as long as the crystal structure is stable in acid. To address this issue a novel layered metal sulfide,


potassium calcium thiostannate (KCaSnS), was designed by introducing Ca2+


(r = 1.00 Å) provides a large enough space for Cs+


into the Sn-S matrix, assuming that Ca2+ to reside


after its release from the lattice structure. Calcium sulphide is readily dissolvable under acidic conditions, and Ca2+ much harder Lewis acid than Cs+ that the meta-stable structural Ca2+ interlayered K+


is a


. Therefore, it is conjectured in addition to the


will be exchanged for Cs+ at low pH, possibly


resulting in a notable increase in the adsorption capacity. As hypothesised, experiments demonstrate a high adsorption capacity of 620 mg/g for Cs+


at pH 2, the highest value to


date. Even though the result was obtained at very high Cs+ concentration, which is not seen in practical situations, this approach is original in its design of novel chalcogenide adsorbents for highly acidic wastewater treatment. A new KCaSnS adsorbent was synthesized by a


hydrothermal method by mixing potassium carbonate (K2


CO3 ), calcium chloride dihydrate (CaCl2 ⋅ 2H2 O), tin powder


(Sn), sulphur (S) and deionized water. The mixture was then heated in a furnace at 200°C for 24h. Considering the hazard of 137


Cs, all experiments in this study were performed using its nonradioactive isotope.


Adsorption characteristics of KCaSnS The adsorption kinetics were studied to know how fast the Cs+


. Metal sulphides


from 20 to 0.136 mg/L. This indicates a removal efficiency of 99.3% in 1 min, which is higher than that of KTS-3, SbS-1 K, AgSnSe-1, FJSM-4, and NVPC. Under acidic conditions, slow kinetics has generally been observed for the adsorption of Cs+


because of severe


competition with protons. Notably, KCaSnS exhibited fast adsorption kinetics at pH 2. The Cs+


adsorption reached


equilibrium within 1 min, with a removal efficiency of 95.4%, which is substantially faster than that of FJSM-4 and DB-ZrP. To investigate the maximum Cs+


adsorption capacities


of KCaSnS under neutral and acidic conditions, an isotherm study was conducted using low to extremely high concentrations of Cs+


solutions. Under neutral


conditions, equilibrium adsorption capacity (qe, mg/g) rapidly increased with the equilibrium concentration of Cs+


in solution (Ce, mg/ L) up to 370 mg/g or 2.78 mmol/g.


The qe value is lower than ~460.8 mg/g for hf-TiFC (hollow flower-like titanium ferrocyanide), 453 mg/g for MIL-101- SO3H, 437.5 mg/g for NVPC, and 400 mg/g for FJSM-SnS-4. But it is higher than that of most other adsorbents, such as ~252.1 mg/g for NH4V4O10 (layered ammonium vanadate), 250.3 mg/g for KAlSnS-3, ~222 mg/g for KMS-1, and 174 mg/g for AgSnSe-1, and much higher than the ~82.7 mg/g for commercial AMP-PAN (ammonium molybdophosphate– polyacrylonitrile). In acidic solutions, KCaSnS showed a significantly enhanced adsorption capacity toward Cs+


. Unlike at


neutral conditions, qe increased stepwise with Ce in acidic solutions. The sites, which protons would occupy, appeared to be adsorbed by Lewis softer Cs+ Cs+


above the concentration of 8250 mg/L. The saturated adsorption


adsorption takes place. KCaSnS displayed quick adsorption kinetics in both neutral and acidic environments with an initial concentration of ~20 mg/L. Cs+


concentration


achieved equilibrium in the first minute under neutral circumstances, showing a steep decrease in concentration


capacity was as high as 620 mg/g, or 4.66 mmol/g, which is the highest value among those measured at a pH value near 2.0. The other adsorbents with high capacities include KAlSnS-3 (170 mg/g at pH 2), KMS-1 (168 mg/g at pH 2.5), and DB-ZrP (155.42 mg/g at pH 1). KCaSnS has a substantially greater adsorption capacity at pH 2 than FJSM-SnS-4 (144.8 mg/g at pH 1.6) and NVPC (65 mg/g at pH 2.05), which have higher adsorption capacities than KCaSnS in neutral conditions. To further investigate the influence of solution acidity or basicity on the Cs+


adsorption, adsorption experiments were


performed over the pH range of 1–13. More than 97.8% of Cs+ was removed in the pH range of 4–12. Meanwhile, at the


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