## 1 Introduction  
Several pollutants, such as organic dyes, heavy metal ions and bacteria, can cause serious problems to human health and the environment [1; 2; 3]. For this reason, it is important that they are removed efficiently from water basins [4; 5; 6]. Among the organic dyes, crystal violet (CV) has been reported as a recalcitart dye molecule that can persist in the environment for a long period of time and can induce toxic effects [7; 8]. CV is a well-known dye since it is used for various purposes, including medicine, textile dyeing, and paper coloration [9]. It is formed by three benzene aromatic rings with six N-methyl groups that make it potentially mutagenic and carcinogenic, especially for mammalian cells [10], hence raising environmental and health concerns. Different purification techniques, such as oxidative degradation, photodegradation, and electrocoagulation, have been proposed to remove organic dyes from wastewaters [11; 12; 13]; however, they suffer from various drawbacks due to the difficulties in applying them to large-scale plants along with high energy and chemicals consumption [14]. For these reasons, adsorption is often used for dyes and organic substances removal, thanks to its easy operation, ability to treat concentrated forms of the dyes and the possibility of reusing the adsorbents after regeneration [14]. Moreover, adsorption has advantages over the other methods in terms of simple design and initial investment cost. Several authors tried to remove CV from wastewater using different solid adsorbents [14; 15; 16]. The adsorption process involves mass transfer mechanisms to transfer a substance from the liquid phase to the surface and the bulk of a solid, due to the formation of physical and/or chemical bonding. The initial high cost of the solid adsorbents and the need for an inexpensive regeneration system has led to the use of some natural materials [17; 18], characterized by a good affinity with the dye [19] and a high specific surface area [19; 20; 21]. Gels can be used as adsorbent materials since they can have the above discussed properties. They can be termed 'xerogels' when drying is carried out at room pressure, and 'cryogels' when the ice-crystals inside the hydrogels are sublimated by freeze-drying. As a rule, these two types of material have a macroporous structure, characterized by large and irregular pores. By contrast, 'aerogels' can be obtained using supercritical CO2 (SC-CO2) assisted drying technique, in which the solvent is removed thanks to the formation of a supercritical mixture CO2 plus organic solvent that works at almost zero surface tension and avoids the collapse of the native gel nanofibrous structure [22; 23; 24]. Some of the papers present in the literature dealing with CV removal are reported in Table 1.  
According to the literature investigation reported in Table 1, different types of bio-based nanocomposites were studied for CV removal from water. The most used bio-based adsorbents were in the form of hydrogels, xerogels and particles. Gels were generally functionalized using magnetic nanoparticles, to improve their removal efficiency, but time-consuming processes were employed. Particles required very simple processes for their production, but the separation from the aqueous solution was more complex and difficult to scale up to industrial applications. Some attempts have been also performed using solid adsorbents in form of cryogels to remove CV from wastewaters. These gels were prepared by freeze-drying, starting from different raw materials, such as: low-cost chitin and psyllium biopolymers [30], sugarcane bagasse-bentonite and sodium alginate [31]. However, this procedure lasted about 48 h,  
\begin{table}
\begin{tabular}{p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt}} \hline \hline
**Adsorbent Material** & **Adsorption Process** & **CV Removal Efficiency** & **Advantages** & **Disadvantages** & **Ref. ** \\ \hline \multirow{4}{*}{ \begin{tabular}{} \end{tabular}