Investigating the use of microdialysis and SEC-UV-ICP-MS to assess iodine interactions in soil solution

Online Microdialysis-High-Performance Liquid Chromatography-Inductively Coupled Plasma Mass Spectrometry (MD-HPLC-ICP-MS) as a Novel Tool for Sampling Hexavalent Chromium in Soil Solution

Conventional soil solution sampling of species-sensitive inorganic contaminants, such as hexavalent chromium (Cr^VI), may induce interconversions due to disruption of system equilibrium. The temporal resolution that these sampling methods afford may also be insufficient to capture dynamic interactions or require time-consuming and expensive analysis. Microdialysis (MD) is emerging as a minimally invasive passive sampling method in environmental science, permitting the determination of solute fluxes and concentrations at previously unobtainable spatial scales and time frames. This article presents the first use of MD coupled to high-performance liquid chromatography-inductively coupled plasma mass spectrometry (HPLC-ICP-MS) for the continuous sampling and simultaneous detection of Cr^VI in soil solution. The performance criteria of the system were assessed using stirred solutions; good repeatability of measurement (RSD < 2.5%) was obtained for Cr^VI, with a detection limit of 0.2 μg L^-1. The online MD-HPLC-ICP-MS setup was applied to the sampling of native Cr^VI in three soils with differing geochemical properties. The system sampled and analyzed fresh soil solution at 15 min intervals, offering improved temporal resolution and a significant reduction in analysis time over offline MD. Simple modifications to the chromatographic conditions could resolve additional analytes, offering a powerful tool for the study of solute fluxes in soil systems to inform research into nutrient availability or soil-to-plant transfer of potentially harmful elements.

Short-Term Iodine Dynamics in Soil Solution

Assessing the reactions of iodine (I) in soil is critical to evaluate radioiodine exposure and understand soil-to-crop transfer rates. Our mechanistic understanding has been constrained by method limitations in assessing the dynamic interactions of iodine between soil solution and soil solid phase over short periods (hours). We use microdialysis to passively extract soil solution spiked with radioiodine (¹²⁹I^- and ¹²⁹IO₃^-) to monitor short-term (≤40 h) in situ fixation and speciation changes. We observed greater instantaneous adsorption of ¹²⁹IO₃^- compared to ¹²⁹I^- in all soils and the complete reduction of ¹²⁹IO₃^- to ¹²⁹I^- within 5 h of addition. Loss of ¹²⁹I from solution was extremely rapid; the average half-lives of ¹²⁹I^- and ¹²⁹IO₃^- in soil solution were 4.06 and 10.03 h, respectively. We detected the presence of soluble organically bound iodine (org-¹²⁹I) with a low molecular weight (MW) range (0.5-5 kDa) in all soils and a slower (20-40 h) time-dependent formation of larger MW org-I compounds (12-18 kDa) in some samples. This study highlights the very short window of immediate availability in which I from rainfall or irrigation remains in soil solution and available to crops, thus presenting significant challenges to phytofortification strategies in soil-based production systems.