Study of the influence of a gel former (thickener) type on the sorbtion kinetic of long-living radionuclides and heavy metals by natural zelolite paste from an aqueous solution in vitro

Abstract

Introduction. The deterioration of the population health, due to the chemicalization of many aspects of life, requires the development and introduction of new healing technologies. One of the modern areas of efferent therapy is the oral administration of sorbents. An alternative to solid dosage forms, in some acute cases, is using enterosorbents in the form of paste, but in the Ukrainian pharmaceutical market, the range of such medicines has only several positions. The promising sources for the creation of such medicines is the natural zeolite. The aim of our study was to investigate the influence of a gel former (thickener) type on the sorption kinetics of long-living ⁹⁰Sr and ¹³⁷Cs radionuclides and Pb²⁺ and Hg²⁺ heavy metals by natural zeolite paste from an aqueous solution in vitro. Materials and methods. As objects for the study we used experimental pastes containing 25% natural zeolite each and different gel formers: 7% silicon dioxide, 3% apple pectin and 0.5% sodium alginate. As sorption medium, we used solutions 7.25 mg/l Hg(NO₃)₂·H₂O and 6.74 mg/l Pb(NO₃)₂ and radionuclide solutions ⁹⁰Sr (1.8 × 107 Bq/dm³) and ¹³⁷Cs (3.2 × 107 Bq/dm³). The sorption capacity for heavy metals was investigated by a limited volume method at pH = 2 and pH = 6. The concentration of lead was determined by atomic absorption spectroscopy (atomic absorption spectrophotometer C-115 PCS) in a flame of acetylene-air (λ = 283.3 nm). The concentration of mercury was determined by the flame atomization method using mercury device PR-115 (λ = 253.7 nm). The sorption capacity of the samples against radionuclides was studied by the static sorption method at pH values 2.0, 5.5 and 8.0. The radioactivity were measured by a α-β-automaton NRR-610 "Tesla" radiometer. Concentration of ions and radioactivity of solutions were controlled after 0.5, 1, 2, 3, 4 and 5 hours of experiment. The equilibrium capacity of sorbents coefficient (A_eq., mg/g), the coefficient of sorption (K_s,%) and the coefficient of distribution of (K_d, l/g) were calculated for evaluation the process. Results and discussion. The process of sorption includes two main stages – fast and slow. The first stage is responsible for the process of saturation of microcrystals by ions on the surface of the sorbent. The second stage, which indicates the complete saturation of sorbent surface by the ions with the subsequent slow diffusion ions inside the sorbent. For most of the investigated objects, the achievement of the adsorption equilibrium in the heterophasic system took place within first 30-60 minutes of the experiment. The sorption activity of samples against heavy metals and radionuclides varied widely and depended on a number of factors, such as the type of gel formers, pH of the solution, nature and physico-chemical properties of the ions (hydration energies, charge and radius of the ions). Significant influence on the sorption gave the pH value of the solutions. The complex nature of this dependence we explain by the influence of the acidity of the solution on the physico-chemical properties and the structure of the sorbents, as well as on their sorption-active centres. In this connection, it can be assumed that under other equal conditions, the exchange capacity of the studied sorbents in the stomach environment will be minimal, and will grow in the moderately alkaline environment of different parts of the intestine.The K_d value of studied samples against heavy metals allowed us to compare the sorption and selective properties of samples and arrange the metal ions in order of decreasing their relative affinity to sorbents as Hg²⁺>Pb²⁺. This difference in selectivity can be explained by the difference in an electronic configuration, as well as by a radius and a value of hydration energy of ions. The obtained K_d and K_sof radionuclides showed that samples better absorb¹³⁷Cs radionuclide at all pH values. The sorption characteristics of⁹⁰Sr are much lower, but they increase sharply with increasing pH values. The highest affinity with respect to¹³⁷Cs and ⁹⁰Sr radionuclides was demonstrated by samples with silicon dioxide as a gel former. The greatest activity against all metals also showed the composition of the paste containing silicon dioxide as a gel former. Its sorption coefficient for lead and mercury ions were 93.2-99% and 95.8% respectively, and for ⁹⁰Sr and ¹³⁷Cs radionuclides were 19-65% and 64-91% respectively. Conclusions. The influence of the type of gel formers (thickeners) on the sorption kinetics of long-living radionuclides and heavy metals by natural zeolite paste (clinoptilolite) from an aqueous solution in vitro was studied. The best results samples with silicon dioxide have demonstrated. The obtained results will be taken into account in the further development of medicines with natural zeolite (clinoptilolite) in the paste form.