**4. Conclusions**

Glucose hydrochar was studied as a model renewable sorbent for heavy metals, using Cu(II) as a test case for custom-designing a hydrochar sorbent. Glucose hydrochar required alkali activation to exhibit Cu(II) sorption capacity; a strong base (hydroxide) was more effective than a weak base (carbonate) and K<sup>+</sup> counter ions were more effective than Na<sup>+</sup> for activation. In comparison, activated carbon sorption was less than observed for activated glucose hydrochar, despite significant differences in their measured BET surface areas (<10 m<sup>2</sup> g−<sup>1</sup> compared with >800 m2 g−1). Similarly, activated carbon did not require alkali treatment to promote cation sorption, consistent with entirely different sorption mechanisms for these two common sorbents. Zeta potential measurements indicated that Cu(II) sorption to hydrochar was due to electrostatic interactions and FT-IR analysis implicated a key role for carboxylate groups. DFT simulations provided further information on Cu-hydrochar binding to the carboxylate site, suggesting beneficial synergy with nearby furan groups.

These results were used as the basis for molecular level design of two hydrochars bearing either carboxylate or sulfonate groups. The designer hydrochars exhibited approximately 50 m<sup>2</sup> g−<sup>1</sup> Cu(II) sorption capacity, consistent with the role of carboxylate and sulfonate groups in cation binding. Nonetheless, the capacity of the hydrochar sorbents failed to yield the expected increase in sorption capacity. In accordance with the electrostatic binding mechanism, sorption capacity of two ion exchange resins, Amberlyst®-15 and AG® 50W-X4, was studied for comparison with hydrochar. The ion exchange resins outperformed hydrochar on a per mass basis. Interestingly, the hydrochars bound approximately 0.75 Cu ions per acid site, whereas Amberlyst®-15 bound only 0.5 Cu ions per acid site. The superior performance of Amberlyst®-15 compared with hydrochar was therefore attributable entirely to differences in acid site density. AG® 50W-X4, on the other hand, bound nearly 2 Cu ions per acid site, which accounted for its superior performance compared with hydrochar. DFT simulations confirmed that bidentate binding is preferred whenever possible, meaning, uniform spacing of the binding groups will maximize their binding efficiency on a per site basis. Similarly, cooperative effects from nearby aromatic groups (either furan or arene) can promote cation-π interactions that improve binding and acid site utilization. The combination of experimental investigation and computer simulation provide a clear starting point for molecular-level design of hydrochar for use as sorbents, that can be used in future work.

**Author Contributions:** Conceptualization, M.T.T. and N.A.D.; Methodology, A.B.B., and G.A.T.; Validation, L.D. and B.S.; Formal Analysis L.D. and M.P.R.; Investigation J.T.H., L.D., A.A., and M.P.R.; Data Curation, L.D.; Writing (original draft), Writing (review and editing), N.A.D., M.T.T., A.B.B., A.A, J.T.H. and G.A.T.; Visualization, B.S., L.D., M.T.T.; Supervision, M.T.T., N.A.D.; Project Administration, M.T.T., A.B.B., N.A.D.; Funding Acquisition, M.T.T. All authors have read and agreed to the published version of the manuscript.

**Funding:** The work was funded by the U.S. National Science Foundation (ENG/CBET 1605916).

**Acknowledgments:** Rediet Tegegne and Maksim Tyufekchiev supported experimental efforts by synthesizing and titrating several of the hydrochars.

**Conflicts of Interest:** The authors declare no conflict of interest.
