Abstract

In recent years, aerogel, an ultra-lightweight, porous nanostructured functional material has attracted significant industrial applications, including polluted water treatment. Their high surface area can be modified to integrate more surface functional groups to become super absorbers for different water pollutants such as heavy metals, oils, organic materials, pesticides, herbicides, toxic and pharmaceutical materials. Significant advances have also been made in the regeneration and reusability of these functional materials, making them smart materials able to respond to specific stimuli. Smart nanostructured aerogels can be designed to reversibly adsorb and desorb a specific water pollutant under various external stimulations. This review comprehensively discusses the evolution, types, and properties of aerogel functional materials. It highlights the modification approaches on aerogels to suit the desired applications of the water treatment field. The review also critically discusses the latest advances of nanostructured aerogels in removing different water pollutants and presents aerogels’ future potentials in water treatment applications.

Introduction

The past two decades witnessed the massive development in both industrial and agricultural sectors, increasing the pollution level of water bodies and landfills. Water pollution with toxic substances such as heavy metal ions, organic dyes, pesticides, herbicides, crude oils, pharmaceuticals, and toxic substances has always been one of the most significant challenges that have long plagued human and animal lives [1]. However, various techniques have been developed to adsorb, filtrate, separate, or remove these pollutants from aqueous solutions such as ion exchange, redox, membrane filtration, chemical deposition, and adsorption using various materials . Adsorption materials have attracted tremendous attention due to their effectiveness, ease of fabrication, and use . The adsorption method of purifying water relies mainly on the adsorbent to adsorb the impurities in the water, using numerous materials for water purification from different impurities because of its good processing effect, less secondary pollution to the environment, and low cost . The adsorption method relies on numerous materials such as biomass activated carbon, zeolite, chitosan, graphene, clay minerals that are plant-based and industrial wastes, etc. These materials can be used as adsorbents for purifying water in different forms, including powder, films, hydrogel, and aerogels. Aerogels have several advantages, including the high surface area, easily scale up, and high adsorption efficiency. It can work as adsorbent material and filter simultaneously, depending on the precursor material/s. A great development in aerogel fabrication techniques has been recently made that minimize the production cost of aerogel from different materials .

Aerogels are obtained by removing the pore fluid from a gel without changing the material’s structural integrity, resulting in ultralight weight and high porous material. The porosity of aerogels is an essential parameter that highly affects its functionality. Numerous factors such as the precursor material, its concentration, the cross-linkers, drying method, and other additives greatly impact the porosity and pore size of the aerogel . Although many aerogels already exist, the authors are convinced that a lot more new aerogel types will be discovered in the following years, especially in their use in polluted water treatment . Many other hybrid material aerogels combinations are yet to be discovered. The interest in aerogels just kept growing, as indicated by the number of publications each year . Many aerogels composites can be produced with modifying properties by adding new functional material for water treatment and other applications.

The past few years have witnessed great advances in modifying different types of aerogels, including biomass-based aerogels, inorganic carbon-based, polymer-based, silica-based, and other aerogels. These modifications were made to customize aerogels to suit the polluted water treatment applications and enhance their performance as functional materials. The ability to modify the surface chemistry, porosity, and pore size, has resulted in the introduction of various functionalities into the nanostructured aerogels, which significantly improves their adsorption capacity. The chemistry of aerogels is very flexible and can be altered by many factors such as the precursor materials , their ratio , preparation approaches , and so on. The pore size and surface area of aerogels can also be tailored . Many reviews have been published in recent years discussing one type of aerogels, one application, or multiple directions of interest. Chen et al.  recently reviewed the preparation approaches and the modification strategies of nanocellulose aerogels for general application without highlighting other types of aerogels. Jatoi et al. discussed the synthesis and processing of aerogels and their latest applications in air purification. Maleki also reviewed the synthesis of aerogels, processing, and recent applications in air cleaning and polluted water treatment. The present review comprehensively discusses the latest advances regarding the role of nanostructured aerogels in polluted water treatment applications. The review covered the evolution of aerogel as functional material since its first discovery till the current age, properties of different aerogel materials, and novel modification of aerogel to suit its applications in polluted water treatment. The review also critically discusses the latest investigations of nanostructured aerogels in removing different water pollutants, including heavy metals, oils and organic materials, pesticides, herbicides, toxic and pharmaceutical materials. Finally, highlight the prospective of such material and its potentials in future generations in water treatment applications.