LITERATURE REVIEW: The Use of Cation-Exchange Resins in Natural Water Trace Metals Research. ARTICLE: Determination of Free Ion Trace Metal Concentrations in Freshwaters Using Cation-Exchange Resin Equilibrium. A Preliminary Investigation.

摘要

LITERATURE REVIEW: The desire to measure low metal concentrations in natural waters stems principally from the aim to understand the effects of trace metals on aquatic biota and to understand the fate of metal pollutants. Trace metals rarely exist in the free ion form, with distribution of the different species being dependent upon pH, Eh, and the types of organic/inorganic ligands and colloidal surfaces. Use of cation exchange resins, particularly Chelex-100, has been developed over the past few decades and enables preconcentration to yield, upon elution, measurable concentrations of trace metals. These resins can also yield data on the speciation (by measuring lability) of a metal in solution by adjusting the contact time of the sample with the resin. Several speciation schemes have been developed where separation techniques (such as dialysis) are coupled with Chelex-column methods (short contact time) and Chelex-batch methods (long contact time). With the exception of recent equilibrium based studies Chelex is always used in an environment where the number of binding sites is in large excess compared to the number of competing cations, meaning competition effect are not significant, although kinetic factors are important is Chelex lability studies. Nitric acid elution generally yield good recovery rates (>90%) and stepwise elution has been shown to be the most effective for both metal recovery and minimising the eluent volume. The development of DGT speciation studies may bring about a technique that is less complex but easier to undertake and yields simple yet useful complexation data. The potential for IDA-resins to yield free-ion concentrations based on equilibrium-based sorption is a beneficial development that is currently being investigated. This review investigates the use of cation exchange resins by assessing the resin properties, the techniques developed and the optimum sorption conditions. ARTICLE: The dissolved free metal ion in natural waters is often related to bioavailability. However, no method currently exists which allows quick, simple and routine determination of free ion concentrations. This study aims to investigate the potential for an ion exchange resin (Chelex-100) to determine free ion concentrations by allowing the resin to equilibrate in situ with a natural water. The environmental concentrations can then subsequently be determined by back calculation of the concentrations eluted from the resin. The sampler employs resin beads retained between two layers of PE mesh, thus allowing flow through the device when deployed vertically in the flow of a stream. Metals are back extracted into a known volume of 2M HNO3. The Windermere Humic Aqueous model (WHAM6.1) was used to predict values for the equilibrium concentration of resin bound metals. Experimental Mn & Cd values were higher than predicted and Cu & Ni values were three orders of magnitude lower than predicted. However, these data are consistent with WHAM modelled data indicating the trend of resin-metal binding during a deployment. These data indicated that the deployment time was insufficient. The suitability of free ligand stability constants to resin binding and possible method developments are also discussed.