Production Union Prydniprovsk Chemical Plant (PCP) was among the first Soviet facilities involved in the processing of uranium ores and concentrates. During the factory operation (1948 to 1991), nine radioactive waste tailing sites were formed with a total capacity of 42 million tons and a total activity of 2.7×10¹⁵ Bq. In 1992, uranium production was stopped at PCP. However, the uranium complex was not decommissioned according to the regulatory requirements in effect at that time. As a result, protective barriers were partially broken at the tailings sites, causing contaminant migration into the environment [1].
The Centralnyi Yar tailings site operated from 1950 to 1954; its parameters are specified in [2]. The high waste acidity (pH = 2.5 to 4.0) is a distinctive feature of the waste accumulated in it, probably due to insufficient pulp neutralization after ore processing, and contributes to uranium migration into groundwater beyond the tailings site.
This paper presents the research results on the equilibrium and kinetics of uranium recovery by using various granular ion-exchange resins and sorbents based on modified vegetal raw materials from the solution simulating radioactively contaminated groundwater accumulated in the Centralnyi Yar tailings site [2].
Based on literature data, the following domestic and foreign grained ion-exchange resins were selected for the study:
- Smoly JSC: AMP, AM-p, AM-p-2;
- Lanxess: Lewatit MonoPlus M 500, Lewatit CNP 80, Lewatit MDS TP 208;
- Purolite: Puromet MTA6002PF, Purolite A530E, Puromet MTS9300, Purolite C115;
- DuPont: Ambersep 920U.
Besides, the sorption materials based on phosphorylated vegetal materials – crushed kernel of apricot Prunus Armeniaca L. and Juglans Regia L. walnut shell – were prepared for the study by their mercerization and phosphorylation.
The acidic solution simulated radioactively contaminated groundwater in the Centralnyi Yar tailing site was prepared based on the data given in [2]: U – 20.0 mg/dm3, NO3- – 7.81 mg/dm3, MgCl2 – 93.3 mg/dm3, CaCl2 – 332 mg/dm3, KCl – 19.1 mg/dm3, MgSO4 – 1 535 mg/dm3, Na2SO4 – 297 mg/dm3, H2SO4 – 68.4 mg/dm3, рН = 3.0 to study uranium recovery with different sorption materials.
Uranium concentration in the aqueous phase was measured by photocolorimetry with Arsenazo III [3]. The moisture weight fraction was defined in sorbents by drying sorbent portions at 105 °С in a drying oven until constant weight and weighing on an analytical balance.
Uranium equilibrium distribution was studied using the different portion technique in a static mode by contacting sorbent portions with 20 cm3 of solution for 24 hours at 20±2 °С and agitation. Equilibrium capacity was calculated by the difference of uranium concentrations in the aqueous phase before and after recovery on a dry sorbent basis. Experimental data were processed using the most common Freundlich, Langmuir, Sips, Henry, and Dubinin-Radushkevich models. The highest determination factor value was the used as a criterion for the best fit of an equilibrium model to experimental data.
It was found [3, 4] that AM-p resin with nitrogen-containing functionalities and Lewatit MDS TP 208 with iminodiacetic ones featured the highest sorption capacity, while carboxylic resins demonstrated the lowest one. Sorbents based on phosphorylated apricot kernels and walnut shells demonstrated a lower sorption capacity compared to granular ion-exchange resins.
Comparison of experimental data with the results obtained using the Langmuir, Sips, Freundlich, and Dubinin-Radushkevich models showed that the Sips and Freundlich equations fitted the experimental data with a high determination factor within the studied equilibrium concentrations for most sorbents. Characteristic sorption energy by the Dubinin-Radushkevich model (ranging from 14.87 kJ/mole to 21.68 kJ/mol) indicated an ion-exchange mechanism involving chemisorption [4].
Kinetics of uranium recovery was studied using the limited volume solution technique by contacting a sorbent portion with 200 cm³ of simulated solution for 24 hours at 24±2 °C in a thermostated cell with agitation using a propeller-type mixer at a rotation speed exceeding 200 min-1.
Sorption kinetics featured a high speed of uranium recovery during the initial step; at the same time, the half-reaction time calculated by experimental data ranged ~36 minutes to ~175 minutes depending on the sorbent type.
To estimate the contribution of chemical interaction to uranium sorption, a quantitative description of the kinetics was carried out using the most common chemical kinetics models: pseudo-first-order, pseudo-second-order, and Yelovich exponential model [5]. It was found that the pseudo-second-order model fitted the experimental data with a higher determination factor compared to the pseudo-first-order model. The high agreement between the calculated and experimental capacity, as well as the high determination factor provide basis for concluding that chemisorption (ion-exchange interaction of uranyl ions with the functional groups of the resin) was the limiting step. The satisfactory approximation of the experimental data by the Elovich model confirmed the energy heterogeneity of the surface of the studied materials. Comparative analysis of the model parameters allowed us to quantitatively assess the differences in the kinetic characteristics and the heterogeneity degree for the studied materials.
Based on mathematical models, we have calculated equilibrium kinetic parameters of uranium recovery, which could be used when designing sorption equipment.
References:
1. Korovin V., Korovin Y., Laszkiewicz G. [et. al]. Problem of radioactive pollution as a result of uranium ores processing. Scientific and Technical Aspects of International Cooperation in Chernobyl: Collection of Scientific Articles. Kyiv, 2001. P. 461–469. https://inis.iaea.org/records/f9bs9-6hh59.
2. Speciation and mobility of uranium in tailings materials at the U-production legacy site in Ukraine / K. O. Korychenskyi et al. Nuclear Physics and Atomic Energy. 2018. Vol. 19, no. 3. P. 270–279. URL: https://doi.org/10.15407/jnpae2018.03.270.
3. Uranium sorption from the solutions simulated radioactively contaminated water using sorbents of different origin / V. Korovin et al. Geo-Technical Mechanics. 2024. No. 168. P. 61–70. URL: https://doi.org/10.15407/geotm2024.168.061.
4. Equilibrium of uranium recovery with ion-exchange resins from the solution simulated radioactively contaminated groundwater / V. Korovin et al. Geo-Technical Mechanics. 2025. No. 172. P. 86–92. URL: https://doi.org/10.15407/geotm2025.172.086.
5. Cheira M. F., Atia B. M., Kouraim M. N. Uranium (VI) recovery from acidic leach liquor by Ambersep 920U SO4 resin: Kinetic, equilibrium and thermodynamic studies. Journal of Radiation Research and Applied Sciences. 2017. Vol. 10, no. 4. P. 307–319. URL: https://doi.org/10.1016/j.jrras.2017.07.005.
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