College of Geographical Sciences, Fujian Normal University, Fuzhou 350007 China.
Environ Sci Technol. 2011 Nov 1;45(21):9240-8. doi: 10.1021/es201859j. Epub 2011 Oct 14.
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.
我们研究了化感芳香酸（AA）肉桂酸和香豆酸在不同活性炭（生物炭）上的吸附，这是研究土壤中天然信号化学物质生物有效性的一部分。在 pH 7 缓冲液中，AA 几乎完全离解，吸附等温线高度非线性，分配比高达 10(4.8) L/kg，对 Ca(2+)或 Mg(2+)不敏感。在未缓冲的介质中，随着负载的增加，吸附逐渐受到抑制，并伴随着 OH(-)的释放，在低浓度下接近 1 的化学计量比，随着 pH 的升高，降至约 0.4-0.5。以石墨为模型的肉桂酸在活性炭上的吸附在表面积基础上大致相当，但释放的 OH(-)可忽略不计。提出了一种新的方案，解释了吸附和 OH(-)化学计量比以及石墨结果的 pH 依赖性。在关键步骤中，AA(-)与水进行质子交换。为了克服不利的质子交换自由能，我们认为 AA 与表面羧酸盐或酚盐基团形成一种氢键，这种氢键具有不寻常的强度，其 pK(a)相当。AA(-)也可以发生这种情况，但会导致表面电荷增加。质子交换途径似乎对包括腐殖质在内的其他弱酸性吸附剂在碳质材料上是开放的。
