title: Adsorption of aromatic carboxylate ions to black carbon (biochar) is accompanied by proton exchange with water.
abstract: We examined the adsorption of the allelopathic aromatic acids (AA), cinnamic and coumaric, to different charcoals (biochars) as part of a study on bioavailability of natural signaling chemicals in soil. Sorption isotherms in pH 7 buffer, where the AAs are >99% dissociated, are highly nonlinear, give distribution ratios as high as 10(4.8) L/kg, and are insensitive to Ca(2+) or Mg(2+). In unbuffered media, sorption becomes progressively suppressed with loading and is accompanied by release of OH(-) with a stoichiometry approaching 1 at low concentrations, declining to about 0.4-0.5 as the pH rises. Sorption of cinnamate on graphite as a model for charcoal was roughly comparable on a surface area basis, but released negligible OH(-). A novel scheme is proposed that explains the pH dependence of adsorption and OH(-) stoichiometry and the graphite results. In a key step, AA(-) undergoes proton exchange with water. To overcome the unfavorable proton exchange free energy, we suggest AA engages in a type of hydrogen bond recognized to be of unusual strength with a surface carboxylate or phenolate group having a comparable pK(a). This bond is depicted as [RCO(2)···H···O-surf](-). The same is possible for AA(-), but results in increased surface charge. The proton exchange pathway appears open to other weak acid adsorbates, including humic substances, on carbonaceous materials.
authors: College of Geographical Sciences, Fujian Normal University, Fuzhou 350007 China.
authors: Jinzhi
authors: J
authors: Ni
authors: Joseph J
authors: JJ
authors: Pignatello
authors: Baoshan
authors: B
authors: Xing
mesh_terms: D000327:Adsorption; D002264:Carboxylic Acids / Q000737:chemistry*; D002606:Charcoal / Q000737:chemistry*; D008956:Models, Chemical; D011522:Protons; D014867:Water / Q000737:chemistry*
chemical_list: D002264:Carboxylic Acids; D011522:Protons; C540010:biochar; D014867:Water; D002606:Charcoal