Humic Acid/Multi-Walled Carbon Nanotube Composites: Influence of Ultrasonic Treatment Duration on Structure, Physicochemical Properties, and Phenol Adsorption

Composite materials based on humic acids and multi-walled carbon nanotubes were synthesized using ultrasonic-enhanced co-precipitation. The effect of ultrasonic treatment duration on the structure and adsorption properties of the composite materials with respect to phenol was studied. The structural and functional characteristics of the materials were investigated using elemental analysis, FTIR spectroscopy, SEM, TGA/DTA, and determination of the content of oxygen-containing functional groups. It was found that the Σ(COOH+OH) values for the studied composites are in the range of 3.00–4.45 mmol/g. The highest value of this indicator was observed for the HA:MWCNTs-20 (US = 30 min) composite. The results of physicochemical studies show that the ultrasonic treatment duration has a significant effect on the morphological, functional, and thermal characteristics of the composites. Adsorption properties were studied in the phenol concentration range of 0.5–15 mg/dm3. It was shown that the HA:MWCNTs-20 (US = 30 min) composite exhibited the highest adsorption capacity for phenol among the studied samples. Analysis of adsorption isotherms revealed that the experimental data were most satisfactorily described by the Langmuir model (r = 0.996–0.999), while the kinetic data were best described by the pseudo-second-order model. These results demonstrate the potential of HA:MWCNTs composites as sorption materials for extracting phenol from aqueous solutions.