**4. Conclusions**

Exergy analysis was used for the comparative evaluation of emerging alternatives of chitosan-based bio-absorbent production. The exergy analysis results were not conclusive for the selection of a promising alternative given the similar values obtained for the exergy efficiencies of the three routes evaluated (0.04%, 2.83% and 2.50%), being the production of chitosan microbeads modified with TiO2 and magnetite nanoparticles, the route with the highest overall exergy efficiency (2.83%). However, the analysis presented in this work allowed to diagnose the emerging production processes from the exergetic point of view as sustainability criteria related to resources conservation. All the routes evaluated showed a similar poor exergetic performance. This behavior of the topologies assessed is related to the huge amount of water that are lost (and not recovered) in the bio-adsorbent purification stages along with the low energetic efficiency of the separation units. The use of a neutralization unit instead of washing stages might contribute to save large quantities of water, and improve the exergy efficiency of the processes. Route 2 also showed the highest irreversibilities of the three cases evaluated. The highest exergy destruction for production of chitosan microbeads modified with TiO2 and magnetite nanoparticles was found in the microbeads-drying unit, but also, this alternative presents the lowest exergy flow associated to utilities. On the other hand, CMTiO2 route presented the lowest exergy efficiency (0.04%), and an exergy loss of 181,665.63 MJ/h. For the case of CMThio route, it was determined that this process has high energy demands in the drying stage, resulting in a high exergy flow by utilities. For all routes evaluated is recommendable to apply process optimization techniques as mass and energy integration to decrease the requirement of utilities for separation/purification stages. For future work, it is recommended to develop studies related to economic and environmental impacts of the routes evaluated in order to obtain more sustainability parameters for selection of the most promising alternative under parameters evaluated.

**Author Contributions:** S.M.-H., A.H.-B. and A.G.-D. conceived and designed the paper, and wrote the Introduction and Materials and Methods. S.M.-H. and A.G.-D. wrote the Results and prepared figures and tables. Discussions and Conclusions were the collective work of all authors. The writing-review & editing was performed by A.G.-D. and A.H.-B., A.G.-D. supervised the development of this paper.

**Funding:** This research was funded by the Colombian Administrative Department of Science, Technology and Innovation COLCIENCIAS, code 110748593351 CT069/17. The APC was funded by COLCIENCIAS.

**Acknowledgments:** The authors thank to the Colombian Administrative Department of Science, Technology and Innovation COLCIENCIAS and the Doctoral Engineering program of the University of Cartagena, for its support with project "Removal of polycyclic aromatic hydrocarbons (PAHs), present in coastal waters Cartagena bay by using shrimp exoskeleton as a source of nanoparticle-modified bioadsorbents", code 1107748593351 CT069/17.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
