Groundwater arsenic in the Red River delta, Vietnam ... - Fiva

Jessen et al., manuscript to be submitted to Geochim. Cosmochim. Acta
state transported to Denmark and stored at 10 C. A subsample of the core has been
dated by optical luminescence to 460±30 years before present (Larsen et al., 2008).
2.2. Isotherm batch experiment
2.2.1. Reagents and materials
Reagent grade or higher purity chemicals were used for all experiments. A 5% CO2 in
N2 gas mixture was deoxygenated either by bubbling through an acetate buffered FeSO4
solution or, when preparing deoxygenated water, by passage through a Cu oxygen trap.
All solutions were prepared from deoxygenated H2O, boiled and bubbled with the
appropriate gas mixture during cooling. Stock solutions of As(III) were prepared in
septum bottles from NaAsO2 and 10 mM NaHCO3. Solution filtration was conducted
using Minisart cellulose acetate (Sartorius) or Acrodisc Supor hydrophilic
polyethersulfone membrane (PALL) 0.20 μm syringe filters.
2.2.2. Experimental procedure
A sample of the core was distributed over five 100 mL septum bottles. A sub sample
was heated to 110 C for 24 hour to determine the water content. Each of the five bottles
received between 25 and 100 mL of 10 mM NaHCO3 equilibrated with the 5%
CO2/95% N2 gas mixture, corresponding to the 10 meq/L alkalinity and pH 7 observed
in the field at the coring site (Postma et al., 2007). A sixth bottle received only the
solution, providing a blank. The bottles were capped by 10 mm thick rubber stoppers,
removed from the glove box, and stripped for glove box-H2 by bubbling with the
CO2/N2 gas. The bottles were shaken side-to-side for two hours, and then left in an
upright position for settling of the suspended particles for one to five days. Samples of
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