Sorption Properties of a Magnetic Composite Based on Modified Carbon Nanotubes: Influence of the Synthesis Conditions

The performance of a magnetic composite sorbent consisting of magnetic nanoparticles and carbon nanotubes (CNTs) in sorption of toxic metal ions [Be(II), Bi(III), Cd(II), Cr(III)] was studied in relation to the conditions of preparing the composite and its constituents. The pH dependence of the sorption of the metal ions was studied, and the sorption capacity of the composites containing CNTs of different morphology was determined. Be(II) and Cr(III) are taken up from aqueous solutions at pH 6, and Cd(II) and Bi(III), at pH 4.0–5.0. The composite containing CNTs prepared by catalytic pyrolysis of ethanol vapor on the Ni catalyst, CNT(Ni), exhibits the highest, and the sorbent containing CNTs synthesized on the Fe catalyst, CNT(Fe), the lowest sorption capacity. The sorption capacity of the composite containing CNTs prepared on the Co catalyst, CNT(Co), is intermediate. The dependence of the sorption ability of the composite on the ratio of its constituents was studied. The minimal content of magnetic nanoparticles ensuring the magnetism required for the efficient phase separation is 10% relative to the total sorbent weight. At this composition, the sorption capacity of the composite for Be(II), Bi(III), Cd(II), and Cr(III) is the highest: 8.0, 7.0, 6.2, and 6.7 mg g−1, respectively. Despite lower sorption capacity of magnetic sorbents based on CNT(Fe) and CNT(Co), they can also be successfully used for removing toxic elements from aqueous media by static magnetic solid-phase extraction both in sample preparation for analysis and in treatment of various water reservoirs.