Aluminum sampling by Chelex, titanium dioxide and zirconium oxide DGT: Influence of pH on accumulation behaviors

Diffusive gradient in thin film (DGT) is used for aluminum monitoring in waters, where content can reach more than 1 mg L-1, but no information is available concerning its limit of linear accumulation, leading to a risk of an underestimation of the water concentration. Furthermore, this limit can be strongly affected by the pH due to changes of the aluminum speciation and global charge of the binding phase. The linear accumulation capacities of three types of DGT, based on Chelex-100 (R) (DGT-Ch), titanium dioxide (DGT-Ti) and zirconium oxide (DGT-Zr), were determined as a function of pH by time-series accumulation over 30 days. For DGT-Ch, it decreased with increasing pH: around 16 -g at pH 4.1, 5 -g at pH 5.3 and null at pH 8.6. The DGT-Ti and DGT-Zr had similar performances and could accumulate aluminum over the whole tested pH range, but with a significantly higher limit of linear accumulation at pH 8.6 (approximate to 15-g) compared to pH 4.1 (approximate to 1-g). The evolution of the accumulated mass over time agreed with the predicted values except for DGT-Zr at pH 4.1 and 5.3, where a ca 30% higher accumulation was observed. Application in natural freshwaters containing various aluminum species and potential competitors confirmed the linear limits for the three types of DGT and also the DGT-Zr tendency to over-estimate aluminum (up to 200% in some freshwater samples compared to DGT-Ch and DGT-Ti). This phenomenon observed for DGT-Zr cannot be explained yet. Currently, DGT-Zr is not recommended for aluminum monitoring. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

This study investigated the linear accumulation limits of aluminum in waters using three types of DGT under different pH conditions. It was found that DGT-Zr tended to over-estimate aluminum concentration in certain freshwater samples. The reason for this phenomenon remains unclear at present.