Redox Buffering for Contamination Prevention and Water Treatment
Buffering is a common concept in water treatment and prevention control, the most common application being pH buffering (e.g., adding lime to increase the pH). Another very important form of buffering is redox buffering. You can learn more about redox (oxidation-reduction) reactions in our previous post here.
Applications of Redox Buffering
Redox buffering can be applied to mine water treatment in many types of bioreactors and constructed wetlands to aid treatment. Redox buffering can also be applied in many ways for 'source control’, that is, to prevent the oxidation of minerals and release of contaminants into water. This approach to redox buffering can be done in pit lakes, saturated rock fills, and flooded underground mine pools.
What are Acid Volatile Sulphides (AVS)?
When iron sulphide minerals are made by microbes in semi-passive water treatment, they are referred to as “biogenically formed amorphous iron sulphides”. These have been widely studied in the field of toxicology where they are called acid volatile sulphides (AVS) and the metals stabilized by the AVS are called simultaneously extracted metals (SEM). There is a Wikipedia article about their role in toxicology, but little exists online explaining their role in semi-passive water treatment and prevention of contamination.
acid volatile sulphides (AVS): biogenically formed amorphous iron sulphide mineral
simultaneously extracted metals (SEM): Metals stabilized by the AVS. Often with a charge of 2+ such as Cadmium, Copper, Lead, Mercury, Nickel, and Zinc
Figure 1. Stability of metal-sulphide minerals when exposed to oxygen. Compounds at the top of the list are more reactive when exposed to oxygen while compounds at the bottom of the list are more stable. This stability means that the compounds at the bottom of the list will dissolve after those at the top of the list if they are all exposed to oxygen and water
How AVS help Semi-Passive Water Treatment
There are two ways that AVS are important to semi-passive water treatment.
a) Serves as a redox buffer, preventing the oxidation of SEM
b) Regulates treatment rates in cold temperature or high metals concentrations
In the first case, because the iron in AVS has less strength of holding the sulphide than other metal-sulphides, the AVS is the first to oxidize when oxygen enters the system. That is, the AVS separates back to iron and sulphate. In doing so, the oxygen is consumed, and the SEM are protected from oxidizing.
Figure 2. Biogenic FeS (AVS) providing redox buffering against the oxidation of metal sulphides (SEM). Biogenically formed FeS (AVS) protects other metal sulphide minerals from dissolving when exposed to oxygen in water. When these sulphide minerals are exposed to oxygen in water, the FeS is sacrificial, protecting the other metal sulphides and leaving them in a stable reduced state.
In the second case, the tug-of-war analogy discussed in Section 2 applies. The iron currently holds the sulphide but it isn’t as strong as the other metals at holding onto it, so it loses the ‘tug of war’ for the sulphide and gives it up to the other metals. When conditions are such that sulphide is limited in the system (i.e., not enough sulphide to match the amount of sulphide binding metals coming into the system), the AVS exchanges the iron for the other metal that has a stronger affinity for the sulphide (creating SEM). This can happen in cold temperatures where biological activity and production of sulphide is lower, or when an elevated concentration of sulphide binding metals enters the system. The ratio of SEM to AVS must be monitored to ensure there is excess AVS being produced under normal operating conditions to provide a sufficient amount to aid in treatment when needed.
Figure 3. Biogenic FeS (AVS) providing sulphide for enhanced metal/metalloid treatment in cold temperatures or when there are excess metals present. When sulphide metals/metalloids encounter biogenic FeS and there is no free sulphide to bind, the metal/metalloid can displace the iron from the AVS, resulting in free soluble iron and an insoluble metal-sulphide mineral.