2.3.1. pH at the Point-of-Zero Charge (pHpzc)

The relative surface charge of the sample was determined by estimation of the pHpzc using an adapted method reported previously [13]. First, 0.01 M NaCl aqueous solution (20 mL) was transferred into each of nine 7-dram glass vials. The solution pH of each vial was adjusted by the addition of NaOH/HCl to obtain pH values that ranged from pH 2.0 to 13.0. The sample materials (ca. 0.5 g) were added to each solution and allowed to equilibrate for 48 h before the final pH was measured. A graph of final pH versus initial pH was plotted where the intersection point was recorded as the pH for the point-of-zero charge (pHpzc) for each material.

#### 2.3.2. FTIR Spectroscopy

Fourier transform infrared (FTIR) spectra of powdered samples were obtained as 1 wt % solid samples mixed with KBr and analyzed in diffuse reflectance mode using a BIO-RAD FTS-40 spectrophotometer. Multiple (*n* = 64) scans were obtained with a 4 cm−<sup>1</sup> resolution that was corrected against a background spectrum of spectroscopic grade KBr over a defined spectral range (400−4000 cm<sup>−</sup>1).
