Accumulation of Mg to Diffusive Gradients in Thin Films (DGT) Devices: Kinetic and Thermodynamic Effects of the Ionic Strength

Abstract

Availability of magnesium is a matter of concern due to its role in many environmental and biological processes. Diffusive Gradients in Thin Films (DGT) devices can measure Mg availability in situ. This work shows that Mg accumulation in water largely increases when ionic strength (I) decreases. This phenomenon can be explained from (i) the increase of both the association equilibrium (K) and rate (k) constants for the reaction between Mg cations and resin sites, and (ii) the growing contribution of the partitioning of Mg cations at the resin-gel interface, as I decreases. Two theoretical models that take into account electrical interactions among Mg cations, background electrolyte, and resin sites can successfully be used to determine k and K at each I. Both models yield similar k values, which fulfill an expression for the kinetic salt effect. For freshwater (with a typical salinity of 10 mM and circumneutral pH), the binding of Mg is so fast and strong that the simplest perfect-sink DGT expression can be helpful to predict (overestimation lower than 5%) the accumulation in solutions with Mg concentrations up to 1 mM whenever the deployment time is below 9 h. Perfect sink conditions can still be applied for longer times, in systems with either a lower I or a lower Mg concentration.

Extending the Use of Diffusive Gradients in Thin Films (DGT) to Solutions Where Competition, Saturation, and Kinetic Effects Are Not Negligible

“…3. Actually, pH has an important influence on the K MgR as can be seen by comparing the value reported in Table (2) of [39] (ca. 1.7 m 3 mol -1 for I=100 mol m -3 , pH =7.5) with the value here reported in When other less selective binding agents are used, competition effects are expected to be more noticeable.…”

“…Fig. 3 also shows, in continuous line, the accumulation calculated with numerical simulation after fitting the equilibrium and kinetic association constants to the experimentally obtained total accumulation and to the percentage in the back binding layer [39]. With these parameters, the simulation tool has also been used to plot the concentration profiles corresponding to different times (Fig.…”

Dynamics of trace metal sorption by an ion-exchange chelating resin described by a mixed intraparticle/film diffusion transport model. The Cd/Chelex case

“…The study of the physicochemical mechanisms that regulate the metal accumulation is essential for the interpretation of the DGT performance in varying environmental conditions (pH, ionic strength, etc.) [17][18][19][20]. However, the mechanisms that govern the kinetics of metal uptake by the free resin beads need further study.…”

Time weighted average concentrations measured with Diffusive Gradients in Thin films (DGT)

“…The fulfilment of perfect-sink conditions in simple metal solutions can be diagnosed from a negligible accumulation in the back resin disc (when a stack of two resin discs is used in the DGT device) or from the linearity in the plots accumulation vs. time, when the bulk metal concentration is constant, as prescribed by eqn. (1) [31,32].…”