Removal of humic acid from aqueous solution by magnetic multi-walled carbon nanotubes decorated with calcium

Abstract

Abstract Multi-walled carbon nanotubes (MWCNTs) were magnetized with hybrids of iron oxides and modified with calcium by one step chemical co-precipitation method to prepare magnetic nanocomposite for removal of humic acid (HA) from aqueous solution. The nanocomposite was characterized by field emission scanning electron microscope, X-ray diffraction, Fourier transform infrared spectroscopy, specific surface area and vibrating-sample magnetometer, respectively. The results showed that MWCNTs were coated with magnetic iron oxide and CaCO 3 . The magnetic MWCNTs coated calcium (MMC) was used to remove humic acid and the influence of exposure time and pH of solution on the removal rate was examined. The MMC had excellent properties in removal of HA, when MMC dosage was 0.5 g L − 1 the HA removal rate achieved 90.27% at HA initial concentration of 20 mg L − 1 and pH = 7.0 ± 0.2 for contacting time of 30 min. The pH slightly affected the removal efficiency of HA. The studies of adsorption kinetics indicated that pseudo-second-order kinetics model was suitable for describing the adsorption. The adsorption isothermal equations were analyzed and results revealed that the adsorption of HA on MMC followed Temkin equation. Thermodynamics analysis of the adsorption showed that Δ G 0 and Δ H 0 were negative, suggesting that the adsorption was a spontaneous and exothermic process, associated with mainly chemical adsorption. The negative enthalpy Δ S 0 illustrated that the randomness decreased after adsorption. The HA removal efficiency increased after calcium was coated on MWCNTs, which was attributed to coordination of calcium with HA by the interaction of metal cation bridging and ligand. MMC were regenerated by using pre-ultrasonic microwave technology and could remain high HA removal efficiency of 89.20% after 5 cycles of adsorption- regeneration.