Geosciences Institute, Johannes Gutenberg-University, Mainz 55099, Germany.
UFZ - Helmholtz Centre for Environmental Research, Department of Analytical Chemistry, Permoserstrasse 15, 04318 Leipzig, Germany.
Sci Total Environ. 2015 Mar 1;508:199-205. doi: 10.1016/j.scitotenv.2014.11.093. Epub 2014 Dec 5.
A surface complexation modeling approach was used to extend the knowledge about processes that affect the availability of dimethylarsinic acid (DMA) in the soil rhizosphere in presence of a strong sorbent, e.g., Fe plaques on rice roots. Published spectroscopic and molecular modeling information suggest for the organoarsenical agent to form bidentate-binuclear inner-sphere surface complexes with Fe hydroxides similar to the inorganic As oxyanions. However, since also the ubiquitous silicic acid oxyanion form the same bidentate binuclear surface complexes, our hypothesis was that it may have an effect on the adsorption of DMA by Fe hydroxides in soil. Our experimental batch equilibrium data show that DMA is strongly adsorbed in the acidic pH range, with a steep adsorption edge in the circumneutral pH region between the DMA acidity constant (pKa=6.3) and the point of zero charge value of the goethite adsorbent (pHpzc=8.6). A 1-pK CD-MUSIC surface complexation model was chosen to fit the experimental adsorption vs. pH data. The same was done for silicic acid batch equilibrium data with our goethite adsorbent. Both model parameters for individual DMA and silicic acid adsorption were then merged into one CD-MUSIC model to predict the binary DMA+Si adsorption behavior. Silicic acid (500 μM) was thus predicted by the model to strongly compete for DMA with up to 60% mobilization of the latter at a pH6. This model result could be verified subsequently by experimental batch equilibrium data with zero adjustable parameters. The thus quantified antagonistic relation between DMA and silicic acid is discussed as one of factors to explain the increase of the DMA proportion in rice grains as observed upon silica fertilization of rice fields.
采用表面络合模型方法，扩展了在存在强吸附剂（例如水稻根表面的铁斑块）的情况下影响土壤根际中二甲基胂酸（DMA）有效性的过程的知识。已发表的光谱和分子建模信息表明，有机胂试剂与铁氢氧化物形成双齿双核内圈表面络合物，类似于无机砷氧阴离子。然而，由于普遍存在的硅酸氧阴离子也形成相同的双齿双核表面络合物，我们的假设是它可能会影响土壤中铁氢氧化物对 DMA 的吸附。我们的实验批量平衡数据表明，DMA 在酸性 pH 范围内强烈吸附，在中性 pH 范围内（DMA 酸度常数（pKa=6.3）和针铁矿吸附剂的零电荷值（pHpzc=8.6）之间的陡峭吸附边缘。选择 1-pK CD-MUSIC 表面络合模型来拟合实验吸附与 pH 值数据。对于我们的针铁矿吸附剂，硅酸的批量平衡数据也是如此。然后，将 DMA 和硅酸单独吸附的模型参数合并到一个 CD-MUSIC 模型中，以预测二元 DMA+Si 吸附行为。模型预测，硅酸（500 μM）在 pH6 时会强烈竞争 DMA，可使后者的迁移率高达 60%。随后，可以通过零可调参数的实验批量平衡数据来验证该模型结果。讨论了 DMA 和硅酸之间的这种定量拮抗关系，作为解释在稻田硅肥施肥后观察到水稻籽粒中 DMA 比例增加的因素之一。
