First observation of labile arsenic stratification in aluminum sulfate-amended sediments using high resolution Zr-oxide DGT

Abstract

Arsenic contamination in sediments has received increasing attention because it may be released to the water and threaten aquatic organisms. In this study, aluminum sulfate (ALS) was used to immobilize As in sediments through dosage-series and time-series experiments. Diffusive gradients in thin films (DGT) was used to obtain labile As at a vertically 2.0mm resolution. Our results indicated that a “static” layer with extremely low labile As concentration (minimally 0.13mgL−1) with weak variation (<30% RSD) formed within the top 12mm sediment layer at the dosage of 6–12ALS/Asmobile (kmolmol−1, Asmobile means the total mobile As in top 40mm sediment) and on days 30–80 after amendment at the dosage of 9 ALS/Asmobile. The maximum labile As decreased from 1.83 to 0.99μgL−1 and from 1.96 to 1.20μgL−1 in the dosage-series (3–12 ALS/Asmobile) and time-series (10–80days) experiments, respectively, while the depths showing the maximal concentrations moved deeper from 22 to 34mm and 20 to 32mm in the sediments. It implied a reduced upward diffusion potential of labile As to the static layer in deeper sediments. Both distribution coefficient for As between sediment solid pool and pore water (Kd) and the adsorption rate constant (k1) consistently increased, reflecting that As release from sediment solid became increasingly difficult with the progress of ALS immobilization. The results of this millimeter-scale investigation showed that ALS could efficiently immobilize As in sediments under simulated conditions.