Application of Mechanosynthesized Azine-Decorated Zinc(II) Metal-Organic Frameworks for Highly Efficient Removal and Extraction of Some Heavy-Metal Ions from Aqueous Samples: A Comparative Study

The three zinc(II) metal-organic frameworks [Zn-2(oba)(2)(4-bpdb)].(DMF)(x) (TMU-4), [Zn(oba)(4-bpdh)(0.5)]n.(DMF)y (TMU-5), and [Zn(oba)(4-bpmb)(0.5)]n.(DMF)z (TMU-6) [DMF = dimethylformamide, H(2)oba = 4,4'-oxybisbenzoic acid, 4-bpdb = 1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene, 4-bpdh = 2,5-bis(4-pyridyl)-3,4-diaza-2,4-hexadiene, and 4-bpmb = N-1,N-4-bis((pyridin-4-yl)methylene)benzene-1,4-diamine], which contain azine-functionalized pores, have been successfully synthesized by mechanosynthesis as a convenient, rapid, low-cost, solventless, and green process. These MOFs were studied for the removal and extraction of some heavy-metal ions from aqueous samples, and the effects of the basicity and void space of these MOFs on adsorption efficiency were evaluated. The results showed that, for trace amounts of metal ions, the basicity of the N-donor ligands in the MOFs determines the adsorption efficiency of the MOFs for the metal ions. In contrast, at high concentrations of metal ions, the void space of the MOFs plays a main role in the adsorption process. The studies conducted revealed that, among the three MOFs, TMU-6 had a lower adsorption efficiency for metal ions than the other two MOFs. This result can be attributed to the greater basicity of the azine groups on the TMU-4 and TMU-5 pore walls as compared to the imine groups on the N-donor ligands on the TMU-6 pore walls. Subsequently, TMU-5 was chosen as an efficient sorbent for the extraction and preconcentration of trace amounts of some heavy-metal ions including Cd(II), Co(II), Cr(III), Cu(II), and Pb(II), followed by their determination by flow injection inductively coupled plasma optical emission spectrometry. Several variables affecting the extraction efficiency of the analytes were investigated and optimized. The optimized methodology exhibits a good linearity between 0.05 and 100 mu g L-1 (R-2 > 0.9935) and detection limits in the range of 0.01-1.0 mu g L(-1). The method has enhancement factors between 42 and 225 and relative standard deviations (RSDs) of 2.9-6.2%. Subsequently, the potential applicability of the proposed method was evaluated for the extraction and determination of target metal ions in some environmental water samples.