Spectroscopic investigations of sequential nitric acid treatments on granulated activated carbon: Effects of surface oxygen groups on π density

Surface oxygen groups (SOGs) on granulated activated carbon were introduced using ∼10% v/v of nitric acid in a controlled, stepwise fashion. SOG peaks from DRIFTS were deconvoluted and correlated with chemical titration of acidic and basic surface sites. Total surface area was maximized at the 3rd oxidation stage with maximum absorption intensities of conjugated aromatic, aromatic, ether, phenol, and lactone groups. SEM and EDS data reveal optimal cleaning of the substrate and edge chemisorption of non-pore restricting SOGs at the 3rd oxidation stage. Increased order of the substrate is revealed with a minimum I(D):I(G) ratio, a reduced D-band FWHM and a large D-band red shift. Concurrent strengthening of conjugated olefin bonds are indicated through G-band blue shifts as well as conjugated aromatic and carboxylic acid features. Inductive effects resulting from the attachment of electron withdrawing SOGs are responsible for an aromatic-to-olefin π bond restructuring, contributing to the overall graphite-like order of the system, which is simultaneously stabilized by donating and withdrawing SOGs. Heteromaterial loss, high relative %C increases, and the presence of SOGs result in enhanced order and pore access from acid treatments. Such heteromaterial alterations and microstructure analysis shed light on chemical alterations commonly used in activated carbon pre-adsorption treatments.