Adsorption of Aromatic Carboxylate Ions to Black Carbon (Biochar) Is Accompanied by Proton Exchange with Water

作者：Ni, Jinzhi; Pignatello, Joseph J.^*; Xing, Baoshan

来源：Environmental Science & Technology, 2011, 45(21): 9240-9248.

DOI：10.1021/es201859j

• WEAK ORGANIC ELECTROLYTES
• VARIABLE-CHARGE SOILS
• ACTIVATED CARBON
• HYDROGEN-BOND
• AQUEOUS-SOLUTION
• SORPTION
• MATTER
• ACID
• CHARCOAL
• ATTENUATION

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 AM are > 99% dissociated, are highly nonlinear, give distribution ratios as high as 10(4.8) L/kg, and are insensitive to Ca2+ or Mg2+. 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 [RCO2 center dot center dot center dot H center dot center dot center dot 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.

• 出版日期2011-11-1
• 单位福建师范大学