Extraction of Heavy Metal Ions from Aqueous Solutions by Frame Sorbents Based on Benzene-1,3,5-tricarboxylate (MBTC) and Benzene-1,4-dicarboxylates (MBDC) of Various Metals

Pollution of soils and water sources with heavy metals exerts negative impact on the environment related to the disturbances of ecosystem balance and harm to the health of living organisms and man. Organometallic frame sorbents capable to efficiently extract heavy metal ions from aqueous solutions have been synthesized to solve this problem. The authors have studied the regularities of the adsorption of cadmium, lead, copper, cobalt, and nickel ions using synthesized frame sorbents based on benzene-1,3,5-tricarboxylates (MBTC) and benzene-1,4-dicarboxylates (MBDC). The identification analysis by diffractograms of CoBTC and NiBTC powders shows the presence of structures [Co₃(BTC)₂⋅12H₂O] and [Ni₃(BTC)₂⋅12H₂O]. Contrary to NiBTC, NiBDC dicarboxylate is crystallized in triclinic syngony (spatial group P$\bar {1}$, Z = 1) and corresponds to the crystal structure [Ni₃(OH)₂(BDC)₂⋅4H₂O]. The sample of CuBTC is crystallized in cubic symmetry with the space group Fm$\bar {3}$m (Z = 16) and corresponds to the crystal structure [Cu₃(BTC)₂⋅3H₂O], while the structure of the CuBDC compound is assigned to the monoclinic symmetry. The results of the analysis of isotherms of low-temperature nitrogen adsorption, using synthesized MOFs, enable the determination of important textural characteristics of sorbents. It is revealed 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 Brunauer–Emmett–Teller (BET) method varies widely: 276.0 and 9.0 m²/g for CoBTC and NiBTC compounds, respectively. These differences are related to a large number of micropores in the CoBTC sorbent. In most cases, the kinetic patterns of the adsorption of heavy metal ions may be described by a pseudo-second-order equation. Adsorption of copper ions on the NiBDC sorbent is the only example of the process, proceeding according to the kinetic equation of the pseudo-first order. Cobalt, nickel, and copper ions are better absorbed by sorbents containing the corresponding same-name ions according to the Peneth–Fajans 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 physicochemical properties of the ions themselves. The developed organometallic frame compounds may be effectively used in technologies for purification of water resources from toxic heavy metal ions.