Comparison of different aerogels derived from Eichhornia crassipes for efficient oil removal from water

Abstract

Oil-related water pollution originates from various sources, including industrial discharge, accidental oil spills, road runoff, etc. In recent years, it has become one of the most severe global challenges due to its detrimental effects on the environment and the economy. Typical oil clean-up techniques from water are categorized as physical/mechanical, chemical, biological, and physiochemical methods. These conventional methods have several limitations such as being labour intensive, high cost, timeconsuming and less effective for some oil types. The current study aimed to synthesize environmentally friendly and hydrophobic aerogels with low density, high porosity, high oil absorption capacity, rapid oil absorption rates, and enhanced reusability by using Eichhornia crassipes (Water hyacinth-WH), an aquatic invasive plant as the raw material. Aerogel is a special sponge-like porous sorbent material with excellent physiochemical properties enabling effective adsorption of oil. In this study, aerogels were prepared in two forms: WH cellulose-based aerogel and WH biochar-based aerogel. Both aerogels were produced using the same method, including alkaline/H2O2 pre-treatment, crosslinking with Polyvinyl Alcohol (PVA), freeze-drying, and finally silanization with Methyltrimethoxysilane (MTMS) to produce hydrophobic aerogels. Among these 2 aerogels, WH biochar-based aerogel showed the highest oil adsorption capacity (19.10 ± 0.35 g g-1) and the highest oil adsorption rate (3.82 ± 0.07 g s-1), while 16.31 ± 0.69 g g-1 of oil adsorption capacity of cellulose-based aerogel for different oils including crude oil, gasoline, diesel, etc. High adsorption capacity allows the sorbents to hold more oil, reducing the frequency of replacement or regeneration needed and high adsorption rate ensures rapid response to oil spills, minimizing the duration of environmental exposure and potential damage. Moreover, this biochar-based aerogel showed the lowest density (0.30 ± 0.01 g cm-3), highest porosity (80.78±0.82%) and excellent reusability. Low density, high porosity, and high reusability are essential for oil sorbents as they ensure lightweight handling, increased oil adsorption capacity, and cost-effective, sustainable operation. Furthermore, the physicochemical properties of the developed aerogels, including chemical structure and surface morphology were thoroughly characterized using Fourier Transform Infrared Spectroscopy (FTIR) and Scanning Electron Microscopy (SEM). The findings revealed that the aerogels exhibited a complex, interconnected three-dimensional pore structure consisting of multiple layers. These pores were connected by flexible sheets and contained a large number of macro and micro pores throughout the aerogels. Additionally, various functional groups such as alcohol (-OH), alkane (C-H), carboxylic group (R-COOH), ether (R-O-R), aromatic, etc., were observed distributed on the surface of the aerogel. Together with these properties, both aerogels are suitable for oil clean-up applications, while biochar-based aerogel exhibited superior performances in addressing oil pollution challenges. Therefore, the present study proposes a low-cost, sustainable, green method for the effective utilization of aquatic invasive plants for oil clean-up approaches.