Synthesis of Quaternary Hydrotalcite-Carbon Nanotube Composite and Its Sulfate Adsorption Performance in Cement Paste

In this paper, quaternary hydrotalcite [layered double hydroxide (LDH)] (CoFeMgAl-LDH) was first fabricated based on the coprecipitation method, and then, CoFeMgAl-LDH/carbon nanotubes (CNTs) composite was synthesized by CNTs and CoFeMgAl-LDH through the solid phase mixing method. Subsequently, the physical-chemical properties of CoFeMgAl-LDH/CNT composite were investigated by scanning electron microscope (SEM), X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, energy dispersive spectroscopy (EDS), and Brunauer-Emmett-Teller (BET) analysis. Meanwhile, the performance of CoFeMgAl-LDH/CNT composite for SO42- adsorption was evaluated under different conditions, including different initial concentration, contact time, adsorbent dosage, solution pH, temperature, and coexisting ions. Afterward, the SO42- adsorption capacity of CoFeMgAl-LDH/CNT in cement paste was further studied. The results showed that the CoFeMgAl-LDH/CNT composite exhibited a three-dimensional structure with high specific surface area. The maximum SO42- adsorption amount of the CoFeMgAl-LDH/CNT composite was 116.27 mg/g, which was significantly higher compared with other absorbents of the same type. Pseudosecond-order kinetic model could reasonably describe the adsorption kinetics, and Freundlich isotherm could fit the adsorption data accurately. The results also suggest that the synthesized CoFeMgAl-LDH/CNT composite can serve as a potential material for the sulfate binding in cementitious materials.