DGT Measurement of Dissolved Aluminum Species in Waters: Comparing Chelex-100 and Titanium Dioxide-Based Adsorbents

Abstract

Aluminum is acutely toxic, and elevated concentrations of dissolved Al can have detrimental effects on both terrestrial and aquatic ecosystems. Robust analytical methods that can determine environmentally relevant Al fractions accurately and efficiently are required by the environmental monitoring community. A simple, robust passive sampling method, the diffusive gradients in thin films (DGT) technique, was evaluated for the measurement of dissolved Al species in freshwater and marine water using either Chelex-100 or Metsorb (a titanium dioxide-based binding agent) as the adsorbent. Mass vs time DGT deployments at pH 5.05 (Al(3+) and Al(OH)(2+) dominate) and 8.35 (Al(OH)(4)(-) dominates) demonstrated linear uptake of Al (R(2) = 0.989 and 0.988, respectively) for Metsorb. Similar deployments of Chelex-DGT showed linear uptake at pH 5.05 (R(2) = 0.994); however, at pH 8.35 the mass of Al accumulated was 40-70% lower than predicted, suggesting that Chelex-100 is not suitable for Al measurements at high pH. The Metsorb-DGT measurement was independent of pH (5.0-8.5) and ionic strength (0.001-0.7 mol L(-1) NaNO(3)), whereas the Chelex-DGT measurement was only independent of ionic strength at pH 5.0. At pH 8.4, increasing ionic strength led to considerable underestimation (up to 67%) of Al concentration. Deployments of Metsorb-DGT (up to 4 days) in synthetic freshwater (pH range 5.4-8.1) and synthetic seawater (pH 8.15) resulted in linear mass uptakes, and the concentration measured by DGT agreed well with solution concentrations. Conversely, deployment of Chelex-DGT in synthetic seawater and freshwater (pH ≥7.7 Al(OH)(4)(-) dominant species) resulted in a decrease in accumulated mass with increasing deployment time. In situ field evaluations in fresh, estuarine, and marine waters confirmed that Metsorb-DGT was more accurate than Chelex-DGT for the measurement of dissolved Al in typical environmental waters.

A systematic determination of diffusion coefficients of trace elements in open and restricted diffusive layers used by the diffusive gradients in a thin film technique

“…The elution factors used in this study (except for Al) were those reported for the MBL by Panther et al [23]. In terms of Al, an elution factor of 0.845 (using an average of Chelex and Metsorb gels) was utilized [9].…”

“…Al(OH) 4 predominant at pH 8.30 (99%), the biggest change in speciation for all the elements studied (Table S1). Assuming the negatively charged Al(OH) 4 species are less hydrated than Al 3+ species, the size of the diffusing Al species at pH 8.30 is expected to be smaller than pH 4.01 [9]. Consequently, the D values measured for Al change substantially between pH 4.01 and 8.30 (a ratio of 0.73 with ODL and 0.77 with RDL), while the effect is less than 13% for all elements.…”

“…The binding gel consists of a mixture of Chelex and Metsorb resins. Metsorb was cleaned with HNO 3 and NaOH prior to mixing with Chelex [9]. 2 g of dry Chelex-100 (sodium form, 200-400 mesh, Bio-Rad) and 1 g of cleaned Metsorb (Graver Technologies, USA) were used per 10 ml of acrylamide gel stock.…”

“…A (cm 2 ) is the area of the gel exposed to the solution; and, M (ng) is the mass of accumulated analytes on the binding gel, determined by eluting the analytes from the binding layer [6][7][8][9] corrected for the elution efficiency. C = MΔg / DtA (Eq.…”

“…The recent mixed binding layer (MBL, containing both Chelex and Metsorb) described by Panther et al [23] was used for the DGT time-series measurement of diffusion coefficients at both acidic and neutral pH to determine the effect of speciation changes on the diffusion coefficients. The performance of this DGT-MBL was also evaluated for Al uptake, based on the findings of Panther et al [9] that DGT-Chelex measures Al in weakly acid waters and that DGT-Metsorb measures Al in seawater and other slightly basic waters. These data will support many subsequent studies in which these metals are measured and fill a critical gap in the DGT research literature.…”