Grathwohl Peter
University of Tübingen, Center of Applied Geosciences, Hölderlinstr. 12, 72074 Tübingen, Germany.
Waste Manag. 2014 May;34(5):908-18. doi: 10.1016/j.wasman.2014.02.012. Epub 2014 Mar 11.
Column leaching tests are closer to natural conditions than batch shaking tests and in the last years have become more popular for assessing the release potential of pollutants from a variety of solids such as contaminated soils, waste, recycling and construction materials. Uncertainties still exist regarding equilibration of the percolating water with the solids, that might potentially lead to underestimation of contaminant concentrations in the effluent. The intention of this paper is to show that equilibration of pore water in a finite bath is fundamentally different from release of a certain fraction of the pollutant from a sample and that equilibrium is reached much faster at low liquid-to-solid ratios typical for column experiments (<0.25) than in batch tests with much higher liquid-to-solid ratios (e.g., 2-10). Two mass transfer mechanisms are elucidated: First-order type release (film diffusion) and intraparticle diffusion. For the latter, mass transfer slows down with time and sooner or later non-equilibrium conditions are observed at the column outlet after percolation has been started. Time scales of equilibrium leaching can be estimated based on a comparison of column length with the length of the mass transfer zone, which is equivalent to a Damköhler number approach. Mass transfer and diffusion coefficients used in this study apply to mass transfer mechanisms limited by diffusion in water, which is typical for release of organic compounds but also for dissolution of soluble minerals such as calcite, gypsum or similar. As a conclusion based on these theoretical considerations column tests (a) equilibrate much faster than batch leaching tests and (b) the equilibrium concentrations are maintained in the column effluent even for slow intraparticle diffusion limited desorption for extended periods of time (>days). Since for equilibration the specific surface area is crucial, the harmonic mean of the grain size is relevant (small grain sizes result in high concentrations even after short pre-equilibration of a column). The absolute time scales calculated with linear sorption and aqueous diffusion aim at organic compounds and are not valid for sparingly soluble mineral phases (e.g. metal oxides and silicates). However, the general findings on how different liquid-to-solid ratios and specific surface area influence equilibration time scales also apply to other mass transfer mechanisms.
柱淋滤试验比间歇振荡试验更接近自然条件，并且在过去几年中，对于评估来自各种固体（如污染土壤、废物、回收材料和建筑材料）中污染物的释放潜力而言，其变得越来越流行。关于渗滤水与固体之间的平衡仍然存在不确定性，这可能会导致低估流出物中的污染物浓度。本文的目的是表明，有限浴中孔隙水的平衡与样品中一定比例污染物的释放从根本上是不同的，并且在柱实验典型的低液固比（<0.25）下比在液固比高得多（例如2-10）的间歇试验中达到平衡的速度要快得多。阐明了两种传质机制：一级类型释放（膜扩散）和颗粒内扩散。对于后者，传质随时间减慢，并且在开始渗滤后，迟早会在柱出口观察到非平衡条件。平衡淋滤的时间尺度可以基于柱长度与传质区长度的比较来估计，这等同于达姆科勒数方法。本研究中使用的传质和扩散系数适用于受水中扩散限制的传质机制，这对于有机化合物的释放是典型的，但对于方解石、石膏或类似的可溶性矿物的溶解也是如此。基于这些理论考虑得出的结论是，柱试验（a）比间歇淋滤试验达到平衡的速度快得多，并且（b）即使对于颗粒内扩散受限的缓慢解吸，在很长一段时间（>天）内，柱流出物中的平衡浓度也能保持。由于对于平衡而言，比表面积至关重要，因此粒度的调和平均值是相关的（即使在柱进行短时间预平衡后，小粒度也会导致高浓度）。用线性吸附和水相扩散计算的绝对时间尺度针对有机化合物，对于微溶性矿物相（例如金属氧化物和硅酸盐）无效。然而，关于不同液固比和比表面积如何影响平衡时间尺度的一般发现也适用于其他传质机制。
