Hydrophobic organic pollutants in industrial wastewater and oil spillage have attracted wide attention. It is thus important to develop a variety of adsorbents like activated carbon, biochar, zeolite, graphene, etc.. However, carbon-based porous materials exhibit poor thermal stability and flammability, having secure risks during high-temperature applications and thermal regeneration processes. Silicon-based porous materials like zeolite are complex to prepare and difficult to effectively remove multiple organic pollutants simultaneously. Montmorillonite-based adsorbents become popular due to their low cost and environmental friendliness. However, montmorillonite as a powder has challenges in practical applications such as clogging, leakage, and recovery difficulties. It is thus effective in converting montmorillonite into fixed-form aerogel materials.

Polyurethane sponge (PU) as an ideal substrate for developing porous aerogels can enhance the mechanical properties of the adsorbents due to its biodegradability, affordability, low density, and customizable dimensions. In this paper, low-density, recyclable, and cost-effective organic montmorillonite/polyurethane sponge composite aerogels were prepared via ultrasonic swelling and freeze-drying. The composite aerogel was evaluated for its adsorption/absorption capabilities towards benzene, bisphenol A, nitrobenzene, oils, and organic solvents. In addition, the reusability and flammability characteristics of the aerogel were also analyzed.

An organic montmorillonite (i.e., C16Mnt) was prepared with cetyltrimethylammonium bromide to modify montmorillonite. 1 g C16Mnt was added to 20 mL organic solvent (the mass ratio of cyclohexane to tertiary butanol is 9∶1) to prepare a suspension C16Mnt. Chitosan sol was then introduced into the suspension C16Mnt and agitated in a water bath at 50 ℃ for 3 h, yielding a homogeneous sol. Subsequently, PU was immersed in the sol for 5 min to ensure a complete absorption. The sol-loaded PU was subsequently aged at 25 ℃ for 12 h, facilitating the transformation of the sol into a gel. The organic montmorillonite/polyurethane sponge composite aerogel (i.e., PU/CS-C16Mnt) was fabricated via freezing the gel in liquid nitrogen for 10 min and subsequently freeze-drying for 48 h. The composite aerogels, differing in densities (PU25 and PU35), were denoted as PU25/CS-C16Mnt and PU35/CS-C16Mnt, respectively.

PU has a three-dimensional porous network structure and a smooth surface on its skeleton. The pore structure of PU undergoes a significant change after loading with CS-C16Mnt. The pores are filled with numerous organic montmorillonite clusters, resulting in a rougher surface, which is favorable for enhancing the adsorption performance. The incorporation of CS-C16Mnt also improves the hydrophobicity of PU. Furthermore, the infrared spectroscopic analysis reveals that CS-C16Mnt is loaded onto the PU skeleton through covalent bonds and hydrogen bonds formed between PU and CS-C16Mnt.

The PU/CS-C16Mnt exhibits exceptional adsorption capabilities for organic pollutants, based primarily on organic distribution and pore-filling mechanisms. The adsorption results indicate that PU25/CS-C16Mnt has the maximum static adsorption performance for benzene (i.e., 682.3 mg/g). This superior performance can be attributed to the intense interactions with the benzene molecules and C16Mnt, as well as the presence of an abundance of macropores. Besides, PU25/CS-C16Mnt has excellent adsorption properties for bisphenol A and nitrobenzene, achieving adsorption capacities of 339.8 mg/g and 137.0 mg/g, respectively. The infrared spectroscopic results show that O―H and N―H in PU25/CS-C16Mnt participate in the adsorption process. The cyclic adsorption results show that the adsorption capacity of PU25/CS-C16Mnt for bisphenol A decreases from 208.4 mg/g to 154.7 mg/g, and the adsorption capacity of nitrobenzene decreases from 57.9 mg/g to 41.6 mg/g, which is still higher than that of original PU25.

Furthermore, PU25/CS-C16Mnt has good hydrophobicity and lipophilicity, which can selectively absorb oil and organic solvents with the maximum absorption capacity (i.e., 20.8~37.0 g/g) of xylene, gasoline, liquid paraffin, dimethyl silicone oil and carbon tetrachloride. After 10 repeated experiments, the absorption capacity can still maintain 82.8% of the first absorption capacity. Also, PU25/CS-C16Mnt has a good absorption performance (i.e., 7.12 g/g) for grease in actual sewage, and it is easily recyclable. The combustion behavior of PU25/CS-C16Mnt reveals that the incorporation of organic montmorillonite significantly enhances the flammable retardancy of pure PU.

Organic montmorillonite/polyurethane sponge composite aerogel with low-density (i.e., 34.61 mg/cm³), highly hydrophobic and low flammability was synthesized via ultrasonic swelling and freeze-drying. The montmorillonite/polyurethane sponge composite aerogel exhibited exceptional adsorption capabilities for organic pollutants, relying primarily on organic distribution and pore-filling mechanisms. The aerogel demonstrated a static adsorption capacity of 682.3 mg/g for benzene vapor and respective adsorption capacities of 339.8 mg/g for bisphenol A and 137.0 mg/g for nitrobenzene. The absorption capacity for various oils and organic solvents ranged from 20.8-37.0 g/g, with the excellent reusability. Furthermore, montmorillonite/polyurethane sponge composite aerogel exhibited a good adsorption/absorption performance (i.e., 7.12 g/g) for organic contaminants in actual wastewater. This aerogel could be used as a promising montmorillonite-based adsorption material for the treatment of organic pollution.