Extraction of heavy metal ions from aqueous solutions by frame sorbents based on benzene-1,3,5-tricarboxylate (MBTC) and benzene-1,4-dicarboxylates (MB DC) of various metals

Pollution of soils and water sources with heavy metals leads to negative consequences for the environment associated with disruption of ecosystem balance, harm to the health of living organisms and humans. To solve this problem, organometallic frame sorbents capable of efficiently extracting heavy metal ions from aqueous solutions have been synthesized. During the conducted research, the regularities of the adsorption of cadmium, lead, copper, cobalt and nickel ions were studied using synthesized frame sorbents based on benzene-1,3,5-tricarboxylates (MBTC) and benzene-1,4-dicarboxylates (MBDC). The identification analysis performed on diffractograms of CoBTC and NiBTC powders showed the presence of structures [Co₃(BTC)₂·12H₂O] and [Ni₃(BTC)₂·12H₂O]. Unlike NiBTC, NiBDC dicarboxylate crystallizes in triclinic syngony (spatial group P$\overline{1}$, Z = 1) and corresponds to the crystal structure [Ni₃(OH)₂(BDC)_2··4H₂O], the sample of the CuBTC compound crystallizes in cubic symmetry with the space group Fm$\overline{3}$m (Z = 16) and corresponds to the crystal structure [Cu₃(BTC)₂·3H₂O], and the CuBDC compound has a structure belonging to the monoclinic symmetry. The results of the analysis of isotherms of low-temperature nitrogen adsorption using synthesized MOFs made it possible to determine important textural characteristics of sorbents. It was noted that a strong adsorbate-adsorbent interaction is realized for CoBTC in the micropore region. It is shown that the specific surface area of synthesized sorbents, calculated by the Brunnauer-Emmett-Teller (BET) method, varies widely. Thus, for CoBTC and NiBTC compounds, it was 276.0 and 9.0 m²/g, respectively. The noted differences are due to the presence of a large number of micropores in the sorbent CoBTC. In most cases, the kinetic patterns of the adsorption of heavy metal ions can be described by a pseudo-second-order equation. The only example of the process proceeding according to the kinetic equation of the pseudo-first order is the adsorption of copper ions on the NiBDC sorbent. It is noted that cobalt, nickel and copper ions are better absorbed by sorbents containing the corresponding ions of the same name according to the Paneta-Faience rule. The linear relationship found between the sorption capacity and the logarithm of the ratio of the radius of ions to their electronegativity implies that the mechanism of adsorption of metal ions on MOFs is determined by the physicochemical properties of the ions themselves. The developed organometallic frame compounds can be effectively used in technologies for purification of water resources from toxic heavy metal ions.