# Silk Bionanocomposites for Organic Dye Absorption and Degradation
## 4 Conclusions  
To our knowledge, the use of silk materials as adsorbents has rarely been studied to date. Herein, the maximum adsorption capacity of SF hydrogels has been evaluated. The better fit of the experimental data with the Langmuir adsorption model agrees with most of the studies on organic dye adsorption found in the literature. Moreover, the results found in this section strongly suggest an electrostatic-mediated interaction between MB and SF hydrogels. This interaction is in fact impaired in the presence of salt, or within an acid media due to the positive charge of SF materials at this stage. In addition, the use of an anionic dye resulted in an opposite behavior confirming the electrostatic interaction hypothesis. This work also shows the possibility of using pH 3 solutions to restore and reuse SF materials. Finally, the addition of Au NPs allows the coupling of the dye removal capacities of SF with its in-situ degradation by catalysis.  
Herein, this work provides a biodegradable material capable of removing and degrading MB from an aqueous solution within a simple procedure. It is important to note that the results presented here are just a proof of concept and that the material can be easily tuned and functionalized to optimize its performance. Future investigations could focus on the use of HRP enzyme (used for hydrogel crosslinking) for pollutant degradation as recently shown in the literature [57] as well as their possible use in potential synergy with NPs catalyst.  
The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app12189152/s1](https://www.mdpi.com/article/10.3390/app12189152/s1)]([https://www.mdpi.com/article/10.3390/app12189152/s1](https://www.mdpi.com/article/10.3390/app12189152/s1)), Video S1: methylene blue degradation in presence of NaBH4 and catalyzed by SF-AuNP (speed x8).  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Material** & **Mode of Action** & **Maximum Capacity** & **Reference** \\ \hline SF hydrogel & & 25.77 mg g\({}^{-1}\) & This work \\ Silk-graphene oxide aerogel & & 1322.71 mg g\({}^{-1}\) & [11] \\ Graphene oxide hydrogel & & 714.29 mg g\({}^{-1}\) & [10] \\ Alginate beads-magnetic & & & \\ NPs-activated carbon & Adsorption & 18.23 mg g\({}^{-1}\) & [56] \\ Polyacrylamide dextran sulfate & & 19.145 mg g\({}^{-1}\) & [41] \\ hydrogels & & & \\ Exfoliated montmorillonite & & 530 mg g\({}^{-1}\) & [47] \\ \cline{2-4}  & & & \\ \cline{2-4}  & & & \\ \cline{2-4}  & & & \\ \cline{2-4}  & & & \\ \end{tabular}
\end{table}
Table 2: Summary of MB removal materials and their maximum removal capacities.  
**Author Contributions:** Conceptualization, C.B.M. and E.G.; Formal analysis, C.B.M.; Funding acquisition, C.E., J.L. and E.G.; Methodology, C.B.M., C.E., J.L. and E.G.; Project administration, J.L. and E.G.; Supervision, J.L. and E.G.; Validation, C.E., J.L. and E.G.; Writing--original draft, C.B.M.; Writing--review & editing, J.L. and E.G. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research received no external funding.  
**Conflicts of Interest:** The authors declare no conflict of interest.