## 3 Results and Discussion
### Adsorption Tests in Multi-Step Mode: Aerogels (A) vs. Cryogels (C)  
To improve the adsorption performance, adsorption tests in multi-step mode were realized, using again different amounts of aerogel (150, 200 and 300 mg for the Set A1, A2 and A3, respectively). The apparatus used for these experiments is reported in Figure 1. The filtered flow rate was about 0.1 mL/min and the resulting concentrations, after each step of filtration and for each experiment, are also plotted in Figure 5, in which CV concentration is graphed against the number of steps (N). A qualitative way to highlight the different adsorbent performance between aerogels and cryogels is reported in Figure S1. It shows photos of the solutions before adsorption (Figure S1a) and after some steps of adsorption (Figure S1b,d,f). Cryogel adsorption related solutions are also reported for comparison purposes (Figure S1c,e,g). It is relevant to note that the solution reported in Figure S1f is practically transparent, indicating the disappearance (adsorption) of the dye, using 300 mg of aerogels as adsorbent materials. Operating in the multi-step mode, the adsorption performance was larger than using the previous batch-mode. This result can be highlighted by comparing the experiments in which the same adsorbent quantity was used (see Figure 5). This operation mode allowed a more effective contact between the solution and the active sites of the solid material: using 300 mg of aerogel, 96% of CV removal was achieved after eight adsorption steps. Comparing this result with the others reported in the literature [40], it is possible to claim that the same value of removal efficiency of the tested dye has been reached in this work, without using a combination of processes. For example, [PERSON] et al. [40] used adsorption in combination with photocatalysis. However, photocatalysis can be considered as a promising alternative for dye removal when it is able to provide a self-cleaning property of the solid adsorbent; but, in this case, the regeneration step was required using another solvent. Therefore, the regeneration step was not immediate and post-treatments were also required [41] to separate the two phases. Figure 5: Adsorption kinetics of aerogel samples analyzed in the experiments of Set A (Aerogels) performing a multi-step adsorption (T = 25 \({}^{\circ}\)C, flow rate = 0.1 mL/min) and using three different amounts of aerogels: 150 mg (black line), 200 mg (red line) and 300 mg (blue line). Cryogels were also used to perform multi-step filtration tests and concentration values, as a function of the number of steps, N, are reported in Figure 6.  
Comparing Figure 6 with Figure 4, it is possible to identify the same trend as in batch-mode operation; but, again, aerogels showed a better overall performance with respect to cryogels. Therefore, also in this adsorption experiment, the comparison between the adsorbent performance of the materials obtained using the two different preparation processes confirms a better performance of agarose aerogels with respect to agarose cryogels, resulting in a higher removal efficiency of the dye present in the solution. In the case of cryogels, the highest removal (set C3) was 74%; whereas, in the case of aerogels (set A3) was 96% after eight steps. This result could be again attributed to the higher specific surface area of the aerogels than cryogels and, therefore, to their larger number of accessible active sites for CV adsorption. Moreover, the multi-step procedure allowed the liquid access to a larger number of active sites; some of them were not accessed in batch-mode. Therefore, operating in batch-mode, pseudo-asymptotes were observed that were not related to the complete saturation of the adsorbent material.