Introduction

Background

Rationale:

Due to the minimal steps that have been taken globally to mitigate anthropogenic CO2 emissions and inhibit climate change, simply reducing emissions is no longer a viable option to avoid catastrophic impacts. As such, the active removal of CO2 from the atmosphere is necessary in conjunction with a reduction in emissions. This is what motivates the development of carbon dioxide removal technologies, and more specifically, enhanced rock weathering, which I am studying. Enhanced rock weathering refers to the repurposing of agricultural infrastructure to spread finely crushed alkaline silicate minerals, such as wollastonite (CaSiO3) onto agricultural land. This draws down and captures CO2 through reactions in the soil’s aqueous phase, forming orthosilicic acid and calcite while removing CO2 from air:

CaSiO3 + 2CO2 + 3H2O → Ca2+ + 2HCO3– + H4SiO4

Ca2+ + 2HCO3– → CaCO3 + CO2 + H2O

This carbon is then stored in one of two ways: as secondary carbonate minerals in the soils, or as carbonate and bicarbonate ions in soil water. In addition to carbon drawdown, the application of these silicate rocks can provide many ancillary benefits:

1. Nutrient introduction: The breakdown of these silicate rocks releases Calcium, Magnesium, and Silicon, all of which are valuable macronutrients for plant growth.

2. Pest and fungi deterrence: Amending silicates to agricultural soil natural deters the growth/herbivory of insects and fungi.

3. Soil acidification reduction: The breakdown of silicate rocks introduces alkalinity into the soil which naturally increases the soil's pH, which can improve plant nutrient availability.

4. Ocean acidification reduction: The alkalinity introduced into the soil porewater flows out through groundwater, eventually transporting to the ocean where it can help to inhibit ocean acidification, by reacting with free hydrogen ions.

Research Objectives

Concerns with Enhanced Rock Weathering:

While enhanced rock weathering is an excellent form of carbon dioxide removal in theory, there are still some concerns with the technique that I hope to address through my research:

1. How much carbon dioxide does enhanced rock weathering of mine wastes remove, and how do we experimentally measure the carbon dioxide removal?

We will look to estimate carbon dioxide removal through the measurement of dissolved carbonate (alkalinity) in the leachates from the soils.
It is believed that the mine wastes will demonstrate carbon capture potential similar to the rocks mined freshly for ERW

2. Many mine wastes proposed for use in enhanced rock weathering contain high concentrations of Nickel (with other elements of concern occurring at much lower concentrations), an element of environmental and ecological concern. How much of this nickel will be released into groundwater through the process of enhanced rock weathering?

It is believed that the weathering of mine wastes will result in an increase in aqueous nickel, but that this increased level will still remain below regulatory standards.

Figure 1. Enhanced rock weathering reaction of wollastonite to calcite. (Created by Author)