Mine water treatment

Question 1

Which are some treatment categories?

Answer 1

o Neutralization
o Metal removal
o Desalination
o Specific target pollutant removal

Question 2

What are some retention methods?

Answer 2

o Neutralization – precipitation
o Sulphur reduction
o Adsorption
o Co-precipitation
o Uptake by plants
o Sedimentation
o (complexation)

Question 3

Which elements are seen as untreatable?

Answer 3

o Na, K, Cl

Question 4

Which treatment methods are active?

Answer 4

Aeration, neutralization/hydrolysis, precipitation

Question 5

What is meant by aeration?

Answer 5

 Increases the level of dissolved oxygen
 Promotes oxidation of Fe2+ to Fe3+, Fe(II) precipitates are rather unstable compared to Fe(III) precipitates
 Increases chemical treatment efficiency
 Drives off CO2

Question 6

What is meant by neutralization/hydrolysis?

Answer 6

Neutralizing agents:
* Hydrated lime - Ca(OH)2
* Quick lime - CaO
* Limestone - CaCO3
* Coal fly ash - CaCO 3+ CaO
* Lime kiln dust - CaO + Ca(OH)2
Neutralization by carbonates – calcite (pH 7-9)
Neutralization by hydrated lime – pH 10-11
Hydrolysis – Fe, Al, Mn hydroxides formed:
* Al hydroxides has the lowest solubility at around neutral pH.
* Mn oxides not as important as sinks in groundwater (due to low redox potential) and acidic oxygenated surface waters (due to low pH)
* As, Cu, Pb adsorbed at low pH, might desorb at higher pH
* Zn, Cd adsorbed at higher pH
* Attraction between solids and ions in solution
* Adsorption of anions occur, generally at low pH, and of cations at near neutral pH
* Solids have different charges at different pH (positive low pH)
* Factors that influence adsorption; pH of solution, element concentrations, complexing agents, adsorbents

Question 7

What is meant by precipitation?

Answer 7

 Addition of carbonate
 Addition of hydrated lime
 Max pH of 8-8.5 with limestone

Question 8

Describe sulphur and its speciations

Answer 8

o Elemental S, sulphite SO32-, thiosulphate S2O32- which are then oxidized to sulphate SO42-
o Relatively conservative
o Found in e.g. gypsum and therefore affected by its solubility
o Difficult to remove from mine water

Question 9

Which metals can be treated aerobically?

Answer 9

o Al, Fe, Mn, As, V, P

Question 10

Which metals can be treated anaerobically?

Answer 10

o Cu, Co, Cd, Ni, Zn

Question 11

Which treatment methods are passive?

Answer 11

Sulphide reducing systems, aerobic wetlands, anaerobic wetlands, alkalinity producing systems, anoxic limestone drains, limestone ponds, open limestone channel, groundwater treatment

Question 12

What is meant by sulphide reducing systems?

Answer 12

 Using organic matter as reductant
 Metals precipitated as insoluble sulphides
 Bacteria needed

Question 13

What is meant by aerobic wetlands

Answer 13

 Oxygenated shallow water (0.3m) on organic substrate or soil, with or without plants to reduce flow rate, add organic matter and stabilise sludge
 Controlled inflow of mine water (net alkaline) and outflow at the surface.
 Hydrolysis produces acid, which could affect the water and plants
 Need contact time for reactions to occur
 Element retention by processes such as, oxidation and hydrolysis, plant uptake and sedimentation
 Adsorption of Me to oxides-hydroxides
 Accumulation of sludge and sediment with time

Question 14

What is meant by anaerobic wetlands?

Answer 14

 Subsurface acid mine water (depth c. 1m)
 Permeable anoxic organic matter (reducing condition)
 Promotes sulphate reducing bacteria (SRB) and the formation of H 2S=> Sulphide formation (Fe, S)
 Other reactions; oxidation/precipitation on sediment surfaces, adsorption and co-precipitation to hydroxides and particles, methanogenesis and ammonification
 Generates alkalinity through limestone dissolution.
 Plant uptake, sedimentation, organic complexation

Question 15

What is meant by anoxic limestone drain?

Answer 15

 Cheap and simple
 Common method for acidic water that adds alkalinity to the water
 Impermeable cover, decreases oxygen to limit formation of Fe-hydroxides
 Could not be used for Al3+ and Fe3+ rich water due to precipitation resulting in clogging of the pores and coatings of the limestone
 Must have outflow with sufficient area for precipitation to occur. Also rather long contact time.

Question 16

What is meant by open limestone channels?

Answer 16

 ARD flowing through limestone -> neutralized pH
 Fe-hydroxides might coat the limestone
 A decrease of porosity can occur by secondary minerals
 Not suitable for Zn and Mn due to pH

Question 17

What is meant by groundwater treatment in terms of permeable reactive barriers?

Answer 17

• Trench filled with permeable gravels and reactive materials such as
• organic matter
• calcite
• zeolites
• phosphates
• Fe- hydroxides
• The contact time is important
• The majority use iron metal, Fe(0)