Experimental Column Setup for Studying Anaerobic Biogeochemical Interactions Between Iron (Oxy)Hydroxides, Trace Elements, and Bacteria

Summary

Fate and speciation of arsenic and mercury in aquifers are closely related to physio-chemical conditions and microbial activity. Here, we present an original experimental column setup that mimics an aquifer and enables a better understanding of trace element biogeochemistry in anoxic conditions. Two examples are presented, combining geochemical and microbiological approaches.

Abstract

Fate and speciation of trace elements (TEs), such as arsenic (As) and mercury (Hg), in aquifers are closely related to physio-chemical conditions, such as redox potential (Eh) and pH, but also to microbial activities that can play a direct or indirect role on speciation and/or mobility. Indeed, some bacteria can directly oxidize As(III) to As(V) or reduce As(V) to As(III). Likewise, bacteria are strongly involved in Hg cycling, either through its methylation, forming the neurotoxin monomethyl mercury, or through its reduction to elemental Hg°. The fates of both As and Hg are also strongly linked to soil or aquifer composition; indeed, As and Hg can bind to organic compounds or (oxy)hydroxides, which will influence their mobility. In turn, bacterial activities such as iron (oxy)hydroxide reduction or organic matter mineralization can indirectly influence As and Hg sequestration. The presence of sulfate/sulfide can also strongly impact these particular elements through the formation of complexes such as thio-arsenates with As or metacinnabar with Hg.

Consequently, many important questions have been raised on the fate and speciation of As and Hg in the environment and how to limit their toxicity. However, due to their reactivity towards aquifer components, it is difficult to clearly dissociate the biogeochemical processes that occur and their different impacts on the fate of these TE.

To do so, we developed an original, experimental, column setup that mimics an aquifer with As- or Hg-iron-oxide rich areas versus iron depleted areas, enabling a better understanding of TE biogeochemistry in anoxic conditions. The following protocol gives step by step instructions for the column set-up either for As or Hg, as well as an example with As under iron and sulfate reducing conditions.

Introduction

Understanding and predicting trace element (TE) mobility and biogeochemistry in the environment is essential in order to monitor, develop, and apply appropriate management decisions for polluted sites. This especially applies in the case of toxic TEs such as arsenic (As) and mercury (Hg). The fate and speciation of these TEs in soil or aquifers are closely related to physico-chemical conditions, such as Eh and pH, but also to microbial activities that can play either a direct role on speciation or an indirect role on mobility.

Indeed, some bacteria can directly oxidize As(III) to As(V) or reduce As(V) to As(III). This affects As toxicity, s....

Protocol

1. Experimental Preparation

1. Acid-wash all materials (glass, polytetrafluoroethylene (PFTE)) in contact with samples (5 days in 20% nitric acid (HNO₃) v/v) followed by 5 days in hydrochloric acid (HCl) 10% v/v). Rinse several times with ultra-pure water and dry under a laminar flow hood prior to use.
2. Use polyethylene gloves (or similar) and a fume hood for all steps involving chemicals.

2. Prepare Hg and As Spiked Amorphous Iron Oxides

1. Prepare approximately 20 g of ferryhydrate (Fe(OH)₃): dissolve 50 g of FeCl₃-6H₂O in 500 mL of ultra-pure water (resistivity....

Results

Example 1. Impact of iron reduction of As mobility and speciation

The As column was directly inoculated with groundwater from a site presenting an As concentration higher than the drinking standards (Bracieux, Loire et Cher, France). Groundwater was sampled in sterile bottles, and stored at 5 °C until use. The column was fed from the bottom with this water containing the natural endogenous microbial community a.......

Discussion

The experimental column setup proved to be a convenient laboratory device to study anaerobic biogeochemical processes in continuous conditions. Continuous column systems allow working in conditions closer to those of real aquifers than slurry batch systems or microcosms. Continuous systems can simulate the movement of groundwater through aquifer sediments.

The most critical step within the protocol is preparing the TE-iron (oxy)hydroxides and the mixture with silica gel and sand, which needs t.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Glass columns	Beaucaverre, France	Specific request	columns were composed of 3 separate pieces, the main column core with the cooling jacket and the 5 sampling ports (size GL14 with olive) and a top and bottom piece that fits to the main column body and is held in place with a silicone joint and screw (RIN F 40x38 & SVL 42). note: this design was discussed directly with the company. We recommend to find a local glazier.
Septa PTFE/silicone diameter 20 mm	Sigma-Aldrich	508608
PTFE tubing ID 3 mm	VWR	228-0745
Peristaltic pump	Dominique Dutsher SAS	66493
Peristaltic pump tubing LMT 55	VWR	224-2250	Tygon® LMT 55
Fontainbleau sand D50=209 µm	SIBELCO, France
N2 for bubbling	Air Liquide, France
Gamma irradiation	Ionisos, Dagneux, France
Automatic Mercury Analyzer (AMA 254)	Courtage Analyses, France
Varian SpectrAA 300 Zeeman	Agilent
Name	Company	Catalog Number	Comments
Chemicals
HNO3 Supra pur	VWR	1.00441.1000	Manufacturer: Merck
HCL 30% Supra pur	VWR	1.00318.1000	Manufacturer: Merck
Hg(NO3)2	Merck	516953
As2O3	Merck	202673
FeCl3-6H2O	Merck	207926
silica gel	Sigma-Aldrich	336815-500G