Probing Surface Functionality on Amorphous Carbons Using X‑ray Photoelectron Spectroscopy of Bound Metal Ions

The surface functionality of amorphous carbons is difficult to directly measure because of a lack of crystallinity and overwhelming signals derived from the bulk material. Biochar, a form of amorphous carbon containing considerable oxygen surface functionality, was probed using metal ions and X-ray photoelectron spectroscopy to simultaneously measure the presence and proximity of functional groups and determine the preferred binding modes of a variety of metal ions. These binding motifs were correlated to the efficiency of metal adsorption as determined using the Langmuir isotherm and stability with respect to leaching. Three binding motifs were apparent: physisorption (Cd<sup>2+</sup>, Mn<sup>2+</sup>, and Zn<sup>2+</sup>), chelation (Cu<sup>2+</sup>, Ni<sup>2+</sup>, and Zn<sup>2+</sup>), and hydrolysis/precipitation (Cu<sup>2+</sup>, Ni<sup>2+</sup>, and Pb<sup>2+</sup>).