provide knowledge to environmental, geology, mining and mineral processing students on the occurrence of Acid Mine drainage and remediation.

1. ACID MINE DRAINAGE
Refers to the flow of water out of a mine
that has a very high acidic(low pH) after
being in contact with air and metal.

2. Acid mine drainage occurrence
• Acid Mine Drainage results from the oxidation of sulfide
minerals inherent in some ore bodies and the
surrounding rocks.
• Iron sulfide minerals, especially pyrite (FeS2),
chalcopyrite (FeS.CuS) and also pyrrhotine(FeS)
contribute the most to formation of Acid Mine Drainage.
• Oxygen (from air or dissolved oxygen) and water (as
vapor or liquid) which contact the sulfide minerals
directly cause chemical oxidation reactions which result
in the production of sulfuric acid.

3. Acid mine drainage occurrence
• Overall chemical reactions associated with pyrites can
be described as follows;
- Pyrite is initially oxidized by atmospheric oxygen
producing sulfuric acid and ferrous iron (Fe2+) according
to the following reaction:
- FeS2 + 7/2 O2 + H2O > Fe2+ + 2SO4
2- + 2H+…….. (1)
Conversion of ferrous iron to ferric iron.
- Fe2+ + 1/4 O2 + H+ > Fe3+ + ½ H2O .........(2)

4. Acid mine drainage occurrence
• Ferrous iron may be further oxidized by oxygen releasing
more acid into the environment and precipitating ferric
hydroxide.
- Fe2+ + 1/4 O2 + 5/2 H2O > Fe (OH) 3 + 2H+........(3)
• As acid production increases and the pH drops (to less
than 4), oxidation of pyrite by ferric iron(Fe3+) becomes
the main mechanism for acid production
- FeS2 + 14Fe3+ + 8H2O > 15Fe2+ + 2SO4
2-+ 16H+……. (4)
• This reaction is catalyzed by the presence of Thiobacillus
ferrooxidans which accelerates the oxidation of ferrous
iron into ferric iron (reaction 2) by a factor of 106:1.

5. Acid mine drainage occurrence
• The sulfuric acid produced in these reactions increases
the solubility of other sulfide minerals in the solid
surfaces. Ferric iron in acidic solution can oxidize metal
sulfides per the following reaction:
• MS + 2Fe3+ > M2+ + S + 2Fe2+ …………………………..(5)
Where MS = metal sulfide (galena, sphalerite, etc.)

6. Acid mine drainage occurrence
• Metals commonly solubilized from sulfides in AMD include
aluminum, copper, lead, manganese, nickel, and zinc.
Metals in the form of carbonates, oxides, and silicates may
also be mobilized, often aided by biological catalysts.
• AMD may also leach uranium, thorium, and radium from
mine wastes and tailings associated with uranium mining
operations

7. AMD effects on environment
• AMD harms the environment by lowering soil and water
pH hence increase heavy metals drainage from rocks and
soil colloid materials into water bodies and the
environment in general
• At low pH levels heavy metals release from soil colloids
increase
• Accumulation of heavy metals in soil and water leads to
increase bio-concentration and bio-accumulation in
plants, fish, livestock and humans through the process of
eating and being eaten.

8. AMD Prevention and Mitigation
• Minimizing oxygen supply because of diffusion or
advection
• Minimizing water infiltration and leaching (water acts
as both a reactant and a transport mechanism)
• Minimizing, removing, or isolating sulphide minerals
• Controlling pore water solution pH
• Maximizing availability of acid neutralizing minerals
and pore water alkalinity
• Controlling bacteria and biogeochemical processes

9. AMD Control Strategies
• Containment and isolation
- Soil covers- by imported materials e.g. clay, soil
- low sulphide waste-rock if compactable
- Geo-textile fabrics
- Water cover- creation of a permanent lake or swamp
- use of an existing lake
- flooding of underground tunnels
- submarine disposal
-Blending- mixing of acid and non acid forming waste rock
• Treatment- Using AMD remediation technologies

10. AMD Control Strategies
Soil Covers
• Soil covers generally involve the use of granular earthen
materials placed over mine wastes. The objectives of a
soil cover varies from site to site, but generally include (i)
dust and erosion control; (ii) chemical stabilization of
acid-generating mine waste (through control of oxygen or
water ingress); (iii) contaminant release control (through
improved quality of runoff water and control of
infiltration); and/or (iv) provision of a growth medium for
establishment of sustainable vegetation

11. AMD Control Strategies
Key factors to consider in the design of a soil cover
include
• The climate regime at the site
• The reactivity and texture of the mine waste material
• The geotechnical, hydrologic, and durability properties
of economically available cover materials
• The hydrogeologic setting of the waste storage facility
• Long-term erosion, weathering, and evolution of the
cover system

12. AMD Control Strategies
Processes Affecting Long-Term Performance of Soil Covers

13. AMD Control Strategies
Limitations of soil covers
• Soil covers do not stop infiltration and may not stop acid
drainage.
• Permeability of water infiltration barriers may increase with
time when subjected to climate and vegetation.
• Oxygen barrier covers are especially vulnerable to relatively
small imperfections in the cover – such as differential
settlement, holes caused by animal burrows, desiccation
cracking etc.
• Soil covers may be prone to erosion and long-term
maintenance requirements
• Soil covers may be vulnerable to vegetation, animal, and
human activity including vehicle traffic

14. AMD Remediation Technologies
• Abiotic remediation
- Active technologies
- Passive technologies
• Biological remediation
- Passive technologies

15. AMD Remediation Technologies
Active remediation technologies
• One of the most common ways used is chemical
precipitation
• These systems usually include:
1. equipment for introducing the neutralizing agent used to
treat the Acid Mine Drainage
2. means for mixing the two streams
3. procedures for ensuring iron oxidation
4. settling ponds for removing iron, manganese, and other
co-precipitates

16. AMD Remediation Technologies
Passive remediation technologies
• Require little to no maintenance
• Typically in the form of a wetland
• Cuts down on the price immensely
• However must remove the buildup of metal
accumulations
• Very common method used because of its ability to
cut down on price and the utilization of nature and
not chemicals to remediate the water

17. Abiotic remediation
Active technologies
• The most widespread method used to mitigate acidic
effluents is an active treatment process involving addition
of a chemical-neutralizing agent.
• Addition of an alkaline material to AMD will raise its pH,
accelerate the rate of chemical oxidation of ferrous iron (for
which active aeration, or addition of a chemical oxidizing
agent such as hydrogen peroxide, is also necessary), and
cause many of the metals present in solution to precipitate
as hydroxides and carbonates

18. Abiotic remediation
Active technologies cont…
• Various neutralizing reagents have been used,
including lime (calcium oxide), slaked lime, calcium
carbonate, sodium carbonate, sodium hydroxide, and
magnesium oxide and hydroxide.
• Neutralizing agents vary in cost and effectiveness; for
example, sodium hydroxide is some 1.5 times as
effective but is about nine times the cost of lime.

19. Abiotic remediation
Passive technologies- Anoxic limestone drains
• An alternative approach for addition of alkalinity to AMD is
the use of anoxic limestone drains (ALD).
• The objective of the systems is to add alkali to AMD while
maintaining the iron in its reduced form to avoid the
oxidation of ferrous iron and precipitation of ferric
hydroxide on the limestone), which otherwise severely
reduces the effectiveness of the neutralizing agent.
• In an ALD, mine water is constrained to flow through a bed
of limestone gravel held within a drain that is impervious to
both air and water (generally constructed of a plastic
bottom liner and a clay cover).

20. Abiotic remediation
Anoxic Limestone drains

21. Abiotic remediation
Cross-section of an ALD

22. Abiotic remediation
Open Limestone Channels
• Open limestone channels are long channels or ditches
lined with limestone to increase the alkalinity of the
water.
• The acid water flows down the limestone bed and
AMD is treated bylimestone dissolution.
• The construction of an OLC is determined by the flow
rate, channel slope, and influent acidity concentration
and this information will dictate the weight of
limestone, the cross-sectional area and the length of
the channel, and the in-channel retention

23. Abiotic remediation
Open limestone channel cont…
• One of the primary design factors is to have a steep enough
slope and large enough limestone particle size to prevent
iron and aluminum hydroxides from plugging up limestone
pores.
• The ideal limestone size is 15-30cm in diameter.
• Another important factor when designing an OLC is the
residence time. The quantity of time that the limestone is
in contact with the AMD water is central and is a function
of influent acidity and flow. Residence time of the AMD
water with limestone is calculated as;
R.T (hours) = Length (ft). * Width (ft.) * Depth (ft)* Void Ratio (%) / Flow (cfs) * 3600

24. Abiotic remediation
An example of an open limestone channel

25. Biological remediation
• Passive biological systems- Constructed wetlands
• Constructed wetlands utilize soil- and water-borne microbes
associated with wetland plants to remove dissolved metals
from mine drainage.
• Aerobic wetlands are generally constructed to treat mine
waters that are net alkaline.
• This is because the main remediative reaction that occurs
within them is the oxidation of ferrous iron and subsequent
hydrolysis of the ferric iron produced, which is a net acid
generating reaction (Eqs. (1) and (2)).
- 4Fe2+ + O2 + 4H+> 4Fe3+ + 2H2O........(1)
- 4Fe3++ 12H2O> 4Fe (OH) 3 + 12H+ ....(2)

26. Biological remediation
Constructed wetland cont…
• If there is insufficient alkalinity in the mine water to
prevent a significant fall in pH as a result of these
reactions, this may be amended by the incorporation of,
for example, an anoxic limestone drain.
• In order to maintain oxidizing conditions, aerobic
wetlands are relatively shallow systems that operate by
surface flow.
• An aerobic wetland comprises channels or basins with an
impermeable bottom (to limit seepage loss), a medium
to sustain vegetation, and a shallow water depth (10-
50cm) in order to allow water to contact the soil medium

27. Biological remediation
Constructed wetland cont…
• Anaerobic wetlands are designed to treat net acidic and
metal-laden waters.
• They can be used only if a large enough area of land is
available.
• Anaerobic wetlands usually consist of a limestone layer
overlain by an organic compost layer, over which the
AMD flows.
• Water quality in wetlands is improved by filtration of
suspended and colloidal materials, and by adsorption of
metals to the soil substrate or other organic-based
substrates.

28. Biological remediation
Constructed Wetland

29. Biological remediation
Constructed wetland cont…
• Constructed wetlands are efficient when the effluent
is low in its acidity loading, but often show reduced
efficiency and even fail under high acid loading. For
this reason, alkalinity additions to wetland influent
may precede constructed wetland.

30. Biological remediation
Sulfate Reducing Bioreactors
• Sulfate reduction has been shown to effectively treat
AMD/ARD containing dissolved heavy metals, including
aluminum, in a variety of situations. The chemical reactions
are facilitated by the bacteria desulfovibrio in sulfate-
reducing bioreactors
• The sulfate-reducing bacterial reactions involve the
generation of:
- Sulfide ion (S-2), which combines with dissolved metals to
precipitate sulfides
- Bicarbonate (HCO3), which has been shown to raise the pH
of the effluent

31. Biological remediation

32. Scheme for passive treatment of AMD
Flowchart for selecting a passive AMD treatment system

33. AMD from waste rock piles at
Nyabigena North Mara- TZ