An Experimental Protocol for Studying Mineral Effects on Organic Hydrothermal Transformations

Summary

Earth-abundant minerals play important roles in the natural hydrothermal systems. Here, we describe a reliable and cost-effective method for the experimental investigation of organic-mineral interactions under hydrothermal conditions.

Abstract

Organic-mineral interactions are widely occurring in hydrothermal environments, such as hot springs, geysers on land, and the hydrothermal vents in the deep ocean. Roles of minerals are critical in many hydrothermal organic geochemical processes. Traditional hydrothermal methodology, which includes using reactors made of gold, titanium, platinum, or stainless-steel, is usually associated with the high cost or undesired metal catalytic effects. Recently, there is a growing tendency for using the cost-effective and inert quartz or fused silica glass tubes in hydrothermal experiments. Here, we provide a protocol for carrying out organic-mineral hydrothermal experiments in silica tubes, and we describe the essential steps in the sample preparation, experimental setup, products separation, and quantitative analysis. We also demonstrate an experiment using a model organic compound, nitrobenzene, to show the effect of an iron-containing mineral, magnetite, on its degradation under a specific hydrothermal condition. This technique can be applied to study complex organic-mineral hydrothermal interactions in a relatively simple laboratory system.

Introduction

Hydrothermal environments (i.e., aqueous media at elevated temperatures and pressures) are ubiquitous on Earth. The hydrothermal chemistry of organic compounds plays an essential role in a wide range of geochemical settings, such as organic sedimentary basins, petroleum reservoirs, and the deep biosphere^1,2,3. Organic carbon transformations in hydrothermal systems occur not only in pure aqueous medium but also with dissolved or solid inorganic materials, such as Earth-abundant minerals. Minerals have been found to dramatically and selectively influence the hydrotherm....

Protocol

1. Prepare the Sample for Hydrothermal Experiment

1. Choose the size of the quartz or silica glass tubes, e.g., 2 mm inner diameter (ID) x 6 mm outer diameter (OD) or 6 mm ID x 12 mm OD, and determine the amounts of organic compounds and minerals to use. In this work, the amounts of nitrobenzene and magnetite (Fe₃O₄) to load into the silica tube (e.g., 2 mm ID x 6 mm OD) are 3.0 µL and 13.9 mg, respectively.
   NOTE: The large diameter tubes allow easier loading of the materials but require more efforts of tube sealing.
2. Cut the clean silica glass tubing into small pieces with ~30 cm in length using a ....

Results

To demonstrate how to use this approach to study hydrothermal organic-mineral interactions, a simple experiment using a model compound, nitrobenzene, was conducted with mineral magnetite (Fe₃O₄) at a hydrothermal condition of 150 °C and 5 bars for 2 h. To show the mineral effect, an experiment of nitrobenzene without mineral was also performed under the same hydrothermal condition. As shown in Figure 1a, two silica tubes were made f.......

Discussion

In this study, we used nitrobenzene with mineral magnetite as an example to demonstrate how to evaluate mineral effects on hydrothermal organic reactions. Although the experiments are carried out in small silica glass tubes, highly reproducible results are observed in the magnetite experiments, i.e., 30.3 ± 1.4% in nitrobenzene conversion, which suggests the effectiveness and the reliability of this hydrothermal protocol. In the no-mineral experiments, the conversion of nitrobenzene is 5.2 ± 2.1%, whic.......

Materials

Name	Company	Catalog Number	Comments
Chemicals:
Dichloromethane	VWR	BDH23373.400
Dodecane	Sigma-Aldrich	297879
Nitrobenzene	Sigma-Aldrich	252379
Fe2O3	Sigma-Aldrich	310050
Fe3O4	Sigma-Aldrich	637106
Supplies:
Silica tube
Vacuum pump	WELCH	2546B-01
Vacuum line
Oven	Hewlett Packard	5890
Thermocouple	BENETECH	GM1312
Gas chromatography	Agilent	7820A