*2.1. Materials*

All reagents were analytical grade, including: D-(+)-glucose (≥99.5%-Sigma Aldrich), acrylic acid monomer (≥99.0%-TCI chemicals), vinyl sulfonic acid (≥99.0%-TCI Chemicals), hydrochloric acid (0.1 M, ≥99.0%-Acros), sodium hydroxide (0.1 M, ≥95.0%), and the anhydrous salts sodium carbonate (≥99.5%-Sigma Aldrich), sodium hydroxide (≥97.0%-EM Science), potassium carbonate (≥99.0%-Alfar Aesar), and potassium hydroxide (≥85%-Sigma Aldrich). The carbon materials were donations from Norit (since acquired by Cabot) and MeadWestvaco. Specific activated carbon samples included wood-based carbons, Norit Darco® KB-G, Norit Darco® KB-WJ and Nuchar® (MeadWestvaco), and a peat-based carbon, Norit® SX-1. Amberlyst® 15 was used in its hydrogen form and AG® 50 W-X4 in its hydrogen form was purchased from Bio-Rad. Cu(NO3)2 · 2.5 H2O and elemental standards, 5% HNO3, 10 μg/mL were acquired from PerkinElmer. Deionized (DI) water was purified to a minimum resistivity of 17.9 MΩ cm prior to use.

#### *2.2. Hydrochar Synthesis*

Hydrochar was prepared from a precursor solution formed by dissolving 28.152 g of D-(+)-glucose in 100 mL of DI water. The solution was loaded into a 160 cm3 PTFE-lined, stainless-steel autoclave, which was then placed in a room-temperature oven that was heated to 180 ◦C at a heating rating of approximately 10 ◦C min<sup>−</sup>1, held at 180 ◦C for 8 h, before allowing to cool for 12 h. The reaction protocol was selected to replicate those that favor OFG generation, as reported previously by Brown et al. [61] and others [46]. After reaction, the resulting slurry was mixed first with a solution of 100 mL of ethanol and 100 mL of water, and then filtered to remove soluble organic materials from the solid hydrochar. The solid hydrochar was recovered by filtration and rinsed again with ethanol and water. The washing and filtering steps were repeated twice. The hydrochar was then placed in a crucible, dried in an oven at 65 ◦C for 24 h, and stored in airtight vials before further analysis or use.

Functionalized hydrochars were custom synthesized by preparing a precursor solution consisting of glucose and either acrylic acid or vinyl sulfonic acid and subjecting it to a modified HTC treatment. For the synthesis of acrylic acid-hydrochars (AA-hydrochar), the precursor solution consisted of 10 g of glucose, 10 g of acrylic acid, and 80 g of DI water. The HTC reaction time was extended to 16 h for AA-hydrochar synthesis (at 190 ◦C), as 8 h reaction time yielded a material that could not be recovered by filtration [56]. For synthesis of vinyl sulfonic acid-hydrochars (VSA-hydrochar), the precursor solution consisted of 36.1 g of glucose, 7.22 g of vinyl sulfonic acid, and 150 mL of water. After some preliminary trials to evaluate the effects of reaction temperature and time on hydrochar yield, the synthesis of VSA-hydrochar was performed at 190 ◦C for 24 h to yield a solid that could be recovered by filtration. As described previously for hydrochar, both AA-hydrochar and VSA-hydrochar were recovered by filtration, with ethanol and water washing, and oven drying at 65 ◦C.

Hydrochars were activated by mixing 2.0 g of the synthesized material with 500 mL of alkali solution (2 N). The effects of alkali solutions of Na2CO3, K2CO3, NaOH, KOH were evaluated. After several hours at room temperature, the material was recovered by filtration, and placed in DI water where the pH was neutralized by dropwise addition of HCl (1 N) and NaOH (1 N) until the pH stabilized. The final product was washed 3 times with DI water, dried in a 100 ◦C oven, ground, and stored in airtight glass vials.

#### *2.3. Hydrochar Characterization*

The surface areas of the samples were determined by N2 physisorption at 77 K on a Micromeritics ASAP 2000 apparatus, using N2 as adsorbate. N2 physisorption on similar instruments has been reported in more detail elsewhere [67]. Surface areas were determined using the Brunauer–Emmett–Teller (BET) model [68]. Prior to adsorption-desorption experiments, all the samples were degassed at 120 ◦C for 12 h.

Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) was performed on powder samples using a Thermo-Fisher FT-IR 6700 with DRIFTS accessory, described previously in the literature [69]. The spectral resolution was 2 cm−<sup>1</sup> and all samples were purged with N2 gas for 2 min before analysis to exclude atmospheric CO2 and H2O from the sample space. A background spectrum was obtained prior to each measurement and results were obtained by scanning 1024 times and taking their average. Spectra were analyzed using MagicPlot software and plotted by normalization with the baseline.

Zeta potential measurements were determined using a zeta meter (Malvern Zetasizer-Nano-Z) that has been previously described elsewhere [70]. For each test, 0.005 g of the solid sample was suspended in 100 cm<sup>3</sup> of de-ionized water containing 0.1 N NaCl followed by homogenization for 2 h in an ultrasonic bath. After ultrasonication, the aqueous suspension was equilibrated at different pH values for 30 min. Zeta potential results are reported as the average and standard deviation of three measurements.

The combined densities of strong and weak acid groups were determined using the Boehm titration method, described previously in the literature [29,71,72]. In brief, a carbon sample (0.5 g) was placed in NaHCO3 solution (20 mL, 0.1 N), agitated for 48 h, and the carbon was removed by filtration. The resulting filtrate was degassed for at least 30 min using N2 to remove CO2 and was then titrated to determine the acid site density of carbon-rich materials [73].
