University of Maryland Center for Environmental Science, Chesapeake Biological Laboratory, Solomons, MD 20688, USA.
J Colloid Interface Sci. 2011 Sep 1;361(1):137-47. doi: 10.1016/j.jcis.2011.05.029. Epub 2011 May 15.
In controlled metal sorption experiments, the equilibrium distribution coefficient is a key variable quantifying sorbate partitioning across the solid-solution interface. Separation of metals into 'dissolved' and 'particulate' fractions is commonly achieved with syringe filtration, where the boundary is somewhat arbitrarily dictated by the limited selection of available pore sizes. Investigations involving natural organic matter, such as bacterial cells or plant tissues, are especially prone to experimental artifacts if the substrate releases abundant colloidal compounds that contribute to sorption by binding free metal cations in a pH-dependent fashion yet pass through conventional filters, causing the truly dissolved fraction to be grossly overestimated. We observed this phenomenon during a study of lanthanide sorption on a marine macroalga, Ulva lactuca, as a function of pH. At low ionic strength, distribution coefficients calculated for a 0.22-μm size cutoff falsely imply that metal sorption reverses to gradual release above pH 4.6, instead of continuing to increase. Centrifuging the filtrates in Amicon® Ultra units (30 and 3 kDa molecular weight cutoff) revealed a mounting proportion of colloid-bound metal, constituting up to 95% of the 'dissolved' (<0.22 μm) fraction near pH 8. Measurements of DOC concentrations suggest this being due to pH-dependent binding of free metal cations to a fixed pool of organic colloids. The process is well described with a simple 2-site Langmuir isotherm in 0.05, 0.5, and 5.0M NaCl. Using this model to correct the original distribution coefficients not only removed the spurious reversal at low ionic strength, but also uncovered a prominent suppressive effect on the intermediate and high ionic strength data that had initially gone undetected. Ultra-filtration may thus be an essential analytical tool for proper characterization and interpretation of metal sorption on organic matter over a wide range of experimental conditions. Some implications are discussed for the use of biosorbents in the remediation of metal-contaminated waste waters.
在受控的金属吸附实验中，平衡分配系数是量化溶质在固液界面分配的关键变量。通过注射器过滤将金属分离为“溶解”和“颗粒”部分是常见的方法，其中边界是通过有限的可用孔径选择来任意规定的。如果基质释放出大量胶体化合物，这些化合物通过结合自由金属阳离子以 pH 依赖的方式对吸附产生贡献，并且可以通过常规过滤器，从而导致真正的溶解部分被严重高估，那么涉及天然有机物（如细菌细胞或植物组织）的研究尤其容易受到实验假象的影响。在研究镧系元素在海洋大型藻类浒苔（Ulva lactuca）上的吸附时，我们观察到了这种现象，这是 pH 的函数。在低离子强度下，对于 0.22-μm 大小截止值计算的分配系数错误地暗示，金属吸附在 pH 4.6 以上会逆转并逐渐释放，而不是继续增加。在 Amicon® Ultra 装置（30 和 3 kDa 分子量截止值）中离心过滤液，发现胶体结合的金属比例不断增加，在 pH 8 附近，高达 95%的“溶解”（<0.22 μm）部分由其构成。DOC 浓度的测量表明，这是由于自由金属阳离子与固定有机胶体的 pH 依赖性结合所致。在 0.05、0.5 和 5.0M NaCl 中，使用简单的 2 位 Langmuir 等温线可以很好地描述该过程。使用该模型校正原始分配系数不仅消除了低离子强度下的虚假逆转，而且还揭示了对中间和高离子强度数据的明显抑制作用，这些作用最初未被发现。因此，超滤可能是在广泛的实验条件下正确描述和解释有机物上金属吸附的必要分析工具。讨论了在处理金属污染废水时使用生物吸附剂的一些影响。
