**4. Conclusions**

In this work, a carboxyl group-functionalized starch derivative was synthesized and used as an effective flocculant for Cu(II) removal from wastewaters. Employing these Cu-contaminated sewage sludge as precursors, Cu-doped carbon materials were prepared as e fficient electrode materials for supercapacitors through one-step carbonization without any additional metal salt. The results revealed that the specific capacitance of the resulting annealed products was linearly positively correlated to the Cu(II) flocculation capacity. With respect to the environmental capacity and energy capacity, the Cu(II) removal e fficiency has been analyzed and compared to the possible energy benefits. The flocculant dosage up to 200 mg·L−<sup>1</sup> was an equilibrium point existing between environmental impact and energy, as high as 99.50% Cu(II) removal e fficiency could be achieved. Moreover, the resulting annealed product (SFC-0.6) exhibited a high specific capacity (389.9 <sup>F</sup>·g<sup>−</sup><sup>1</sup> at 1 <sup>A</sup>·g<sup>−</sup>1) and long cycling stability, with only 4% loss after 2500 cycles. This work presents a new approach to recycling heavy metal-contaminated sewage sludge to synthesize advanced energy storage materials, which is highly promising for commercial applications ranging from the energy to environment fields.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2079-4991/9/6/892/s1. Figure S1. Starch-based flocculant containing ionizable carboxyl group was synthesized using 2-chloro-4,6-diglycino-[1,3,5]-triazine (CDT) as etherification agent, Figure S2. Standard curve of Copper, Figure S3. The elemental analysis of SFC-x, Table S1. Di fferent types of carbon bonds, Table S2. Physical parameters for SFC-x, Figure S4. (a) SEM images and (b) HRTEM of SFC-0.9. (c) SEM images of and (d) HRTEM of SFC-0.25, Figure S5. GCD curves of (a) SFC-0.9 and (b) SFC-0.25 electrode at di fferent current densities in three-electrode, Figure S6. (a) CV of SFC-0.6 at various scan rates (5–50 mV s<sup>−</sup>1) in two-electrode (b) GCD curves of SFC-0.6 electrode at various current densities (0.5–10 A g<sup>−</sup>1) in two-electrode. (c) The specific capacitance of various electrodes as a function of current density based on the GCD curves. (d) Energy density with respect to the power density of the SFC-0.6.

**Author Contributions:** Z.T., F.Y., and Y.S. designed the experiments. Y.S., F.Y., and X.G. administered the experiments. Z.T. and L.L. performed experiments. Z.T. and Y.S. collected data. L.C., Y.L., and Y.X. gave conceptual advice. All authors analyzed, discussed the data, and wrote the manuscript.

**Funding:** This research was funded by National Natural Science Foundation of China (51764049, 21467024 and 21661027), International Scientific and Technological Cooperation Project of Xinjiang Bingtuan (2018BC001), Transformation of Technological Achievements in Shihezi University (CGZH201715) and the project was funded by Scientific Research Start-up Fund for High-Level Talents, Shihezi University (KX0138).

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