# Application of Three-Dimensional Porous Aerogel as Adsorbent for Removal of Textile Dyes from Water
## 1 Introduction  
Due to the expansion of the industrial sector, large amounts of wastewater are generated every year [1]. Untreated wastewater released into the natural environment can cause significant harm to humans, animals, plants and microorganisms [2]. Organic dyes can block the penetration of sunlight, reducing the photosynthesis efficiency of aquatic plants. Dyes can also accumulate in animals and cause serious health problems due to their mutagenic and/or carcinogenic properties [2]. Dyes are organic pollutants with a complex chemical structure that is characterized by high stability [3]. Dyes absorb light in the visible spectrum (400-700 nm) and contain one or more chromophores. Chromophores contain heteroatoms such as N, O and S, which include bonds such as -N=N-, =C=O, NO or N-OH, -NO\({}_{2}\) or NO-OH, and C=S [4]. Groups of chromophores are unsaturated and consist of atoms or groups of atoms in which single and double bonds are arranged one after the other and resonate, thus allowing the absorption of light rays. Synthetic dyes are characterized by great structural diversity, resulting in significant differences in chemical and physical properties [3]. Congo red is a dye belonging to the group of azo dyes with the molecular formula of C\({}_{32}\)H\({}_{22}\)N\({}_{6}\)Na\({}_{2}\)O\({}_{6}\)S\({}_{2}\). It is a water-soluble chemical that forms a red colloidal solution when the dye dissolves [5]. Due to its color change from blue to red at pH 3.0-5.2, Congo red can be used as a pH indicator. Naphthol green B is a coordination iron complex used as a dye. Its molecular formula is C30H15 FeN3 Na3O15S3. The absorption maximum of Naphthol green B in water is at a wavelength of 714 nm [6]. Rhodamine B is often used to determine the speed and direction of flow. Rhodamine dyes fluoresce [7]. The molecular formula of the dye is C28H31 ClN2O3. Methylene blue is a heterocyclic aromatic chemical compound with the molecular formula of C16H18N3 SCl. The dye is widely used in various fields, such as biology and chemistry [8]. At room temperature, Methylene blue is a solid, odorless, dark-green powder that dissolves in water to form a blue solution. Among the wastewater treatment methods most often mentioned in the scientific literature, adsorption is considered an effective option for wastewater treatment [9]. Adsorption efficiency depends on the type of adsorbent [10]. Currently, the most commonly used adsorbents are made of carbon. Activated carbon is characterized by a large specific surface area, low cost and high porosity [11]. However, this material has poor regenerative properties and reusability. Cellulose is a renewable and naturally degradable biomass material [12]. The aerogels obtained not from petroleum but rather from, for example, cellulose, chitosan, lignin, pectin or other materials have high adsorption properties for removing organic pollutants from wastewater [13]. This is due to the high porosity, high specific surface area and low density of aerogels [14]. Cellulose-based aerogel materials are made from regenerated cellulose or nanocellulose, which requires special solvents and the treatment of plant fibers with acids and bases or formaldehyde, glutaraldehyde and epichlorohydrin [15]. There are various methods of recovery or reuse of aerogel capsules. One of the simplest methods to recover the capsules is mechanical compression [16]. A chemical method using ethanol and a physical method of distillation are both possible options [17]. The disposal of aerogel is not regulated, but the most common practice is burning the used capsules of sorbent. The scientific literature increasingly describes the use of microorganisms for the decomposition of pollutants deposited on aerogel capsules and the decomposition of cellulose, since it is a biodegradable material [18]. It is also possible to recycle aerogel capsules at special sites where, for example, soil contaminated with petroleum products is recycled. Storage of this material in landfills or storage in special biodegradable containers is possible.