Geochemistry – Part I: What is metal leaching and acid rock drainage (ML/ARD), and how does it occur?

How exactly does ML/ARD occur? If not managed or neutralized, the process of acid generation begins with a relatively slow reaction process. After the series of reactions gains momentum, the latter stages of acid generation can be especially devastating to the environment and difficult to remediate (Figure 1).

The following three generalized reactions are detailed below to help clarify the sequence of reactions, which determines acid generation and the potential for metal leaching through the initial oxidation of pyrite:

In this initial reaction, the sulphide mineral (most commonly pyrite), is broken down through fresh exposure to oxygen and water from the atmosphere, which often occurs through the crushing or blasting of waste rock material. As a result, the reduced iron (ferrous iron) is separated from sulphur to produce sulphate and acidity. Relatively minor amounts of acidity are produced during this first step.

The separated ferrous iron from dissolved pyrite begins to react with oxygen and acidity to oxidize the ferrous iron to ferric iron (Fe²⁺ to Fe³⁺). On its own, this rate-limiting reaction typically takes a long time to oxidize and generate ferric iron. However, this oxidation reaction is often significantly accelerated from the presence of naturally occurring bacteria (i.e., thiobacillus ferrooxidans), which serve to catalyze the rate of reaction by approximately 100,000 times. It is important to prevent this reaction because once Fe²⁺ is oxidized to Fe³⁺, acid generation can become rapid and very difficult to manage.

In this late-stage reaction, oxygen is no longer required, and pyrite can be simply oxidized in the presence of Fe³⁺ and water. This stage can be devastating as the acid generation process becomes cyclic and self propagating, leading to very rapid rates of acid generation, and continues until either ferric iron or pyrite is fully consumed. At this stage, affected waters are strongly acidic with pH values lower than 3, which has significant implications for metal leaching transport as described below.

Acid rock drainage is particularly dangerous because low pH (highly acidic) waters can dissolve and transport high concentrations of heavy metals, such as copper (Cu), lead (Pb), zinc (Zn) and several others (Figure 2). This dissolution and transport process is called “metal leaching”.

When metals are present in their dissolved form, they are considered toxic, as they can be readily absorbed by plant and animal life. Metal accumulation can be passed along the food chain through biomagnification, leading to further toxicity throughout these ecosystems.

While acid rock drainage is the most common mechanism for metal leaching, it is important to note that other potentially toxic compounds, such as arsenic (As), selenium (Se) and sulphate (SO₄ (2-)), can be effectively transported at near-neutral and alkaline pH conditions. Similarly, if the drainage chemistry is not properly managed, this can lead to neutral mine drainage (NMD) and saline drainage (SD).

ML/ARD starts with the generation of acidic waters, often caused by improperly managed sulphidic mine material. Acidic waters at low pH levels can dissolve heavy metals from the mine site and transport them into the surrounding environment. In high concentrations, these metals become toxic to the environment and ecosystems.

In the following article “Geochemistry Part II: Predicting metal leaching and acid rock drainage (ML/ARD)”, we discuss a scientific and multidisciplinary approach used to predict ML/ARD. With effective predictions established prior to mine development, solutions can be engineered to manage and prevent ML/ARD from occurring in the first place, thereby protecting the environment and potentially saving mining companies several millions of dollars in environmental liabilities.

1 - Price (2009) Prediction Manual for Drainage Chemistry from Sulphidic Geologic Materials. MEND Report 1.20.1.

2 - Geocon – SNC Lavalin Environment (1995) Economic Evaluation of Acid Mine Drainage Technologies. MEND Report 5.8.1.

This blog article was written in collaboration with Neal Sullivan