Gwo Jin-Ping, Mayes Melanie A, Jardine Philip M
University of Maryland, Baltimore County, Department of Civil and Environmental Engineering, 1000 Hilltop Circle, Maryland, MD 21250, United States.
J Contam Hydrol. 2007 Mar 20;90(3-4):184-202. doi: 10.1016/j.jconhyd.2006.09.013. Epub 2006 Nov 28.
The objective of the research is to quantify the relative contributions of physical and chemical mass transfer to the movement of Co(II/III)EDTA (chelates of Cobalt and Ethylene Diamine Tetraacetic Acid or EDTA) through a limestone-shale saprolite soil. Saprolite is a collective term referring to partially-weathered bedrock. It exists extensively in the subsurface. Because the parent bedding structures are maintained during the weathering process, saprolite soils are characterized by intensive fractures and secondary deposits of minerals such as Al-, Fe- and Mn-oxides on the fracture surfaces. Movement of reactive species through the soils may be influenced by diffusion into the rock matrix, a physical mass transfer (PMT) process, and interfacial chemical reactions, a chemical mass transfer (CMT) process. The PMT and CMT processes are phenomenologically similar but mechanistically different. In this research, previous laboratory observations from a Br and Co(II)EDTA tracer injection into an undisturbed saprolite soil column were used. Mechanistic reactive transport models were formulated to quantify the PMT and CMT processes. The PMT process was independently characterized by using the non-reactive tracer Br. Model parameters thus obtained were subsequently used as constraints to quantify the CMT processes involving Co(II)EDTA and its oxidation product Co(III)EDTA. Our calculations indicated that the PMT rates of the less reactive Co(III)EDTA were comparable with their theoretical CMT rates. In contrast, for the more reactive species Co(II)EDTA, CMT rates are higher than PMT rates. Evaluations of alternative CMT process models further confirmed one of our hypotheses on the basis of previous experimental understandings. The hypothesis suggested that competition from Fe-oxide for Co(II)EDTA may account for the majority of the decrease of Co(III)EDTA effluent concentrations that resulted in the separation of total Co and Co(III)EDTA breakthrough curves. Because Co(III)EDTA is more mobile than Co(II)EDTA in the subsurface, the results of this research suggest independent quantifications of CoEDTA PMT and CMT processes if laboratory results are to be interpreted correctly and scaled up for field and predictive uses.
该研究的目的是量化物理和化学传质对Co（II/III）EDTA（钴与乙二胺四乙酸或EDTA的螯合物）在石灰岩-页岩腐泥土中迁移的相对贡献。腐泥土是一个统称，指的是部分风化的基岩。它广泛存在于地下。由于在风化过程中母层理结构得以保留，腐泥土的特征是具有密集的裂缝以及在裂缝表面有诸如铝、铁和锰的氧化物等矿物的次生沉积。活性物质在土壤中的迁移可能会受到扩散进入岩石基质这一物理传质（PMT）过程以及界面化学反应这一化学传质（CMT）过程的影响。PMT和CMT过程在现象学上相似，但机理不同。在本研究中，使用了之前将溴和Co（II）EDTA示踪剂注入未扰动的腐泥土柱的实验室观察结果。构建了机理反应输运模型来量化PMT和CMT过程。通过使用非反应性示踪剂溴独立表征PMT过程。随后将由此获得的模型参数用作约束条件来量化涉及Co（II）EDTA及其氧化产物Co（III）EDTA的CMT过程。我们的计算表明，反应性较低的Co（III）EDTA的PMT速率与其理论CMT速率相当。相比之下，对于反应性较高的物种Co（II）EDTA，CMT速率高于PMT速率。对替代CMT过程模型的评估进一步基于先前的实验认识证实了我们的一个假设。该假设表明，氧化铁对Co（II）EDTA的竞争可能是导致Co（III）EDTA流出物浓度下降从而使总钴和Co（III）EDTA突破曲线分离的主要原因。由于Co（III）EDTA在地下比Co（II）EDTA更具迁移性，本研究结果表明，如果要正确解释实验室结果并将其扩大到野外和预测用途，需要对CoEDTA的PMT和CMT过程进行独立量化。
