Treatment methods of acid mine
drainage and a case study on
selective recovery of metals
Overview of AMD, its generation and
• Acid mine drainage refers to the outflow of acidic water
from (usually abandoned) metal mines or coal mines.
• a serious long-term environmental problem associated with
• Its causes, prediction and treatment have become the focus
of a number of research initiatives, the mining
industry, universities and research
establishments, environmental groups.
• highly acidic discharges where the ore is a sulfide mineral
or is associated with pyrite EX: Zn, Cu, Ni.
• ore of copper is chalcopyrite. Thus copper mines are often
major culprits of acid mine drainage.
Chemistry of AMD
General equations for this process are:
• 2FeS2 + 7O2+ 2H2O → 2Fe2+ + 4SO42- + 4H+
• 4Fe2+ + O2 + 4H+ 4Fe3+ + 2H2O
• 4Fe3+ + 12H2O 4Fe(OH)3 + 12H+
• FeS2 + 14Fe3+ + 8H2O → 15Fe2++ 2SO42- + 16H+
The net effect of these reactions is to release H+, which lowers the
pH, produces sulphate ions.
Effect scenarios in some places
• About half of the coal mine discharges in Pennsylvania have pH under 5.
• In US >12,000 miles of river and 180,000 acres of lakes/reservoirs are
• US companies now spend >$1million/day to treat CMD prior to
• In the coal belt around the south Wales valleys in the UK highly acidic
nickel-rich discharges from coal stocking sites have proved to be
Effects of Acid mine drainage
• causes disrupted growth and reproduction of aquatic plants and animals.
• source of freshwater metal contamination, land contamination and sparse
• may leach into surrounding watercourses, enters waterways and disrupts
ecosystems by lowering the pH and coating stream bottoms with iron
hydroxide, aluminium, black-manganese.
• Precipitation of Fe(OH)3 resulting in Increased turbidity and decreased
• direct toxicity (benthic algae, invertebrates, and fish); Harm to fisheries.
• In low pH conditions, the metals are dissolved and leave no physical
indication of their presence. However, acid mine drainage eventually
leaves its mark.
• primary prevention of the acid-generating process;
• secondary control, deployment of acid drainage migration, prevention
• tertiary control, or the collection and treatment of effluent.
Treatment of AMD
• Varied objectives
• Recovery and reuse of mine water within the mining operations
• protection of human and ecological health
• aims to remove the pollutants contained in mine drainage to prevent or
mitigate environmental impacts
Selection of Treatment Method
• treatment approach will be influenced by a number of considerations.
• A clear statement and understanding of the objectives
• treatment must be evaluated and implemented within the context of the
integrated mine water system.
• based on mine water flow, water quality, cost, and water use(s) and
• chemical properties of mine drainage relate to acidity/alkalinity, sulfate
content, salinity, metal content, etc.
• Handling and dis-posal of treatment plant waste and residues such as
sludges and brines and their chemical characteristics
This presentation gives a review on wetlands
and mainly focuses on simple treatment
methods which are economic and integrated
methods helping in individual metal recovery
and treatment of acid mine drainage.
PASSIVE TREATMENT SYSTEMS
• compliance using passive treatment systems at low
cost, producing pollution free treated water, and fostering a
community responsibility for acid mine water treatment.
• AMD sources may remain active for decades or even
centuries after mine closure. So, we need infinite amount
of reactants in active systems which problem is solved by
• absorb and bind heavy metals and make them slowly
concentrated in the sedimentary deposits to become part
of the geological cycle
• The wetland’s ability to treat AMD depends on: water flow
distribution, residence time, seasonal, climate.
• Viable treatment system for many mining regions
• Passive, low-cost remediation
• removes iron and other metals from the water
column, sulfate reduction
• alleviation of extreme acidic conditions
• Acidity, iron, manganese and aluminum concentrations are
in AMD are declined
• Removed on average 33% of acidity and 20-40% of metal
• Enables vegetation establishment
• Precipitates the metals.
• Adaptability to acid drainage and elevated metals
• Low capital costs of natural wetland systems
• Low operational costs for constructed wetland
• Provide wildlife habitat and flood control
Heavy metal removal in Wetlands
• natural and artificially constructed wetlands offer an efficient treatment
• biological filters
• These systems rely on several basic processes to purify contaminated
wastewater: uptake of nutrients by plants; bacterial degradation and
oxidation of contaminants; sedimentation and adsorption of particles and
dissolved substances in the waste on to the substrate.
Mechanism of Wetlands
• the three main compartments of a wetland, i.e. (1) Soil and substrate, (2)
Hydrology, (3) Vegetation.
• Hydric soils long enough during the growing season to develop anaerobic
conditions in the upper portion of the soil for hydrophytic vegetation.
• plants create reduced carbon compounds that serve as food for a variety
• Plants remove trace metals from the water through biological uptake and
• Decomposition of organic matter is by reduction and accumulation on the
sediment surface. The resulting organic sediment surface is responsible for
scavenging heavy metals from influent AMD.
Processes in wetlands
The primary removal processes in these systems include
sedimentation, adsorption, ion exchange, complexation, which are finite;
removal will cease unless new removal sites are generated.
Physical removal processes
• settling and sedimentation
Chemical removal process
• Sorption, adsorption, Oxidation and hydrolysis of metals, Precipitation and
co-precipitation, Metal carbonates, Metal sulphides
Biological removal processes
• Beining and Otte (1996, 1997) reported about a natural wetland of a leadzinc mine of life 120 yrs. At surface mine sites with continual contaminant
production or at systems constructed to treat drainage from underground
mines or coal refuse disposal areas. High metal removal rates of close to
100% have been reported in both natural and artificially constructed
TREATMENT USING SULPHATE REDUCING BACTERIA
Sulphate reducing bacteria oxidize organic compounds under anaerobic conditions
and reduce SO42- to sulphides. As the organic compounds are oxidized, alkalinity in
the form of HCO3- is produced.
2CH2O + SO42- + H+ → H2S + 2 HCO3+
where CH2O represents the organic compound
The sulphides produced in the reaction (H2S, HS-, S2-) react with metals to form
insoluble metal sulphide precipitates, where M2+ represents metals such as
Cd2+, Cu2+, Fe2+, Ni2+, or Zn2+ (Kaksonen & Puhakka, 2007). Precipitation occurs
in order of increasing solubility product (Ksp), which is the product of ion
These reactions result in an increase in pH and alkalinity, and a decrease in SO 42and metals concentrations, consistent with the objectives for treatment of the site
AMD. Longevity of a system involving SRB can be limited by the depletion of the
organic sources and clogging due to precipitation of the metal sulphides (Kaksonen
& Puhakka, 2007).
organic substrates: including plant materials, and agricultural, industrial and
municipal wastes. Examples of the organic substrates include alfalfa, rye
grass, hay, straw, rice stalks, peat, spent mushroom compost, municipal and leaf
compost, molasses, sewage sludge, manure, paper products, plant
hydrolyzate, cellulose, sawdust, and wood (Kaksonen & Puhakka, 2007).
Excellent removal of metals including Cd, Cu, Ni, and Zn has been demonstrated in
a number of laboratory and field scale studies completed using low-cost carbon
sources to support SRB (Clyde et al., 2010; Costa, Martins, Jesus, & Duarte, 2008;
Waybrant et al., 1998; Zagury et al., 2006;)
Selective recovery of Cu, Zn, Fe from
acid mine drainage by intergrating
sulfidization with neutralization
The most commonly used process for acid mine drainage (AMD)
treatment today is lime neutralization.
accompanied by the treatment of produced metal hydroxide
precipitate which have costly disposal requirements.
There is a decrease in the capacity of landfill disposal site.
There is also an increase in the price of base metals such as copper
and zinc in recent years.
For the subsequent smelting process, the major issue is how to
separate the copper, zinc and iron from AMD as selectively as
it is expected that not only to treat but also to recover these base
metals from AMD.
by modifying the present lime neutralization treatment process
with sodium hydrosulfide (NaHS) sulfidization.
Process of study:
• At first, lime neutralization was applied to the AMD to
find the precipitation behaviours of Cu, Zn, and Fe.
• Next, NaHS sulfidization as well as the integration with
lime neutralization were conducted to separately
precipitate Cu, Zn and Fe from the AMD.
• Finally, Modified treatment approaches for selectively
recovering the metals from the AMD were proposed.
Sulfidizing agent used:
• more adequate than H2S
• preferred over H2S for safety consideration
• easier to handle in practical applications than the
• An AMD sample generated from an abandoned copper mine of
Ashio Mines located in east Japan was utilized in this study.
• The Cu mine in our case study, discharges about 12 m3/min of
AMD, which is gathered and treated in the downstream purification
• AMD contains copper, zinc , and iron. Other heavy metals such as
lead, cadmium, and arsenic in the AMD are always below the
Japanese effluent standards.
Materials used in the process:
• AMD sample taken: pH 2.8, Cu 4.05mg/L, Zn 9.86mg/L, Fe 6.58
• filtration before experiment
• Lime (calcium hydroxide)
• NaHS (70%sodium hydrosulfide)
• 30 wt.% hydrogen peroxide (H2O2) solution.
• All reagents were analytical grade.
Experiment results and discussion is
Precipitation of Cu, Zn and Fe with
• Fe: pH 4 to pH 5.
• Cu: pH 5–6 to pH 8.
• Zn: pH 7, to pH 9.
• both Cu and Zn in the AMD
precipitated between pH 6–8, which
suggested that these two metals
cannot be separated from each
other in the present lime
neutralization treatment process.
• Some factors can affect the metal
precipitation process so that the
observed pH when precipitation
occurs spans a range.
2. Precipitation of Cu, Zn, Fe by NaHS sulfidization
• According to the results in 1, the sulfidization is to be conducted below pH 5
• At pH 2.8, Cu: 5 mg/L NaHS to 20 mg/L
• Fe and Zn remained unchanged.
• >20 mg/L NaHS at pH 2.8 can selectively precipitate Cu over Fe
Theoritical dosage of sulfidizing agent:
• The solubility equilibrium condition and solubility product (Ksp) of a metal
MS = M2+ + S2Ksp = [ M2+ ][ S2- ]
• The theoretical solubility products of CuS, ZnS and FeS are 4.53*1037, 1.67*10-25 and 1.18*10-19 respectively.
• The solubility equilibrium conditions of each metal sulphide are
drawn using log scales.
• Cu begins to precipitate when S2- concentration is >2.99*1029mol/L, whereas Zn when >1.1*10-21mol/L S2- is introduced, and Fe
at 1*10-15mol/L S-15 is introduced. Therefore, it is theoretically
possible to selectively precipitate Cu over Zn and Fe from the AMD
• The introduced NaHS (sulfidizing agent) exists as H2S, HS-, and S2-.
The theoretical dosages of NaHS needed are 5.26×10-19 mol/L at pH
2.8, and 3.32×10-21 mol/L at pH 5.
• The experimental dosages of NaHS for precipitating Cu are 2.5×10-4
mol/L (20mg/L) at pH 2.8 and 1.25×10-4 mol/L (10 mg/L)at pH
5,which are much higher than the theoretical values.
• Because of generated H2S and escapes into the atmosphere from
the solution and is also unfavourable.
• Although NaHS sulfidization may be conducted at a higher pH so as
to avoid H2S formation and reduce the NaHS dosage
needed, however, Cu and Zn might co-precipitate during pH
adjustment and the selective sulfidization may become difficult.
Precipitation of Zn, Fe by lime
neutralization after Cu
precipitation by sulfidization
• Zn: pH 7 to pH9.
• Fe:pH 6–7, pH 9.
• Fe co-precipitated with Zn at
• introducing H2O2 (30 wt.%
hydrogen peroxide solution).
By adding more than 0.02
vol.% H2O2, all Fe
precipitated at pH 5 whereas
Zn did not.
• For the purpose of selectively recovering Cu and Zn over Fe
from AMD, the feasibility of integrating NaHS sulfidization
with conventional lime neutralization treatment was
• Cu, Zn, and Fe in the AMD were removed and separated
into individual precipitates and the concentrations of
Cu, Zn, and Fe were able to meet the Japanese effluent
• The present study was a collabora tive research project
between the University of Tokyo, Japan and the Furukawa
Co., Ltd., Japan & supported in part by the 21st Global COE
program, ‘‘Mechani cal Systems Innovatio n’’, MEXT,Japan.
A novel low pH sulfidogenic bioreactor using activated sludge as
carbon source to treat AMD and selective recovery of metal
• In this work, a pilot scale sulfidogenic bioreactor was used to treat acid
mine drainage from Zijinshang copper mine using less expensive &
easily available carbon source and economic treatment process
• Zijinshan copper mine is the largest secondary copper sulfide mine in
China (Ruan et al., 2011), large amount of AMD was produced.
• The AMD produced by mining activity at Zijinshan plant was treated by
membrane technology with a capacity of 3300–3600 m3/days (Ruan et
al., 2011). Due to relative high cost of membrane technology, other
alternative AMD treatment process is needed.
• Bacterial sulfate reduction has been applied.
• The major goal of the study aims at evaluating the feasibility of the
system, with real AMD from Zijinshan Copper Mine, activated sludge as
carbon source and no pH control, and to provide data for the technical
and economical evaluation for a larger scale.
• In this process, S2- produced in the Up-flow Anaerobic Sludge Bed (UASB) reactor
were recycled in the two precipitation tanks for copper and iron
precipitation, activated sludge from local wastewater treatment plant was used as
the carbon source.
• The reactor were steady operated in acid condition (with no pH control) for 4
• AMD with a Cu concentration of 100–120 mg/L, Fe 170–200 mg/L, sulphate of
2000–2500 mg/L and pH 2.34–2.56, were feeding into the reactor under a feed
rate of 1 m3/days and HRT of 3 days
Materials used in the process:
• The feeding AMD of this study was from Zijinshan Copper Mine. The chemical
characteristic of the influent were as follows: pH 2.34–2.56; Cu2+ 100–120 mg/L; Fe
170–200 mg/L; SO4 2- 2000–2500 mg/L.
• Two stainless precipitation tanks with a working volume of 1.5 m3
• The bioreactor used in this research was a stainless UASB reactor with a working
volume of 3 m3.
• AMD was fed into copper precipitation tank by pump, and from there to iron
precipitation tank by gravity. A pump fed AMD from iron precipitation tank to the
UASB reactor, and the water was re-circulated back to the iron precipitation tank
by gravity. The H2S gas produced from the UASB reactor was directly fed into the
copper precipitation tank. Excess activated sludge from Shanghang wastewater
treatment plant with a water content of 70% was used as carbon source and
directly feeding into the UASB reactor. The feeding rate was 25 kg/day.
Sulfate was determined by ion
chromatography. Copper and total iron
concentrations were determined by
atomic absorption spectrophotometer.
Results obtained in process
1. Cultivation of sulfate reducing bacteria
• AMD was neutralized to pH 7 by NaOH
and precipitated to removal copper
• water only contains SO42- with a
concentration range from 1000 to
Sulfate removal was over 80% in the
first 10 days, then decreased to
around 40% and maintain steady.
2. Sulfate removal during the start-up
stage (69–210 days)
In the period I, the AMD was diluted by
equal volume of water before feeding
into the system. In period II, the
volume of water add into AMD was
stepwise decreased, and in period
III, the AMD was directly feeding into
the system without dilution.
• In period I, the sulfate removal is
maintained around 40%
• In the beginning of period II, the
sulfate removal rate is decreased
• decrease of influent sulfate
concentration at period II
• At the end of the whole stage, sulfate
removal was only 38%
• carbon source fed & Relative high
Cu, Fe in the influent low pH
Copper removal during the startup stage (69–210 days):
• The high copper removal rate
up to 80% in period I
• At the initial stage of period
II, the copper removal
decreased significantly from
80% to 10%
• At the end of period II, copper
removal increased to around
• At period III, copper removal
was maintained around 60%
Iron removal during the start-up stage (69–210
• Due to the pH decrease, the influent iron
concentration was significantly increased from
10 to 160 mg/L in the period I
• While effluent iron concentration was
maintain at low level (0.1– 5.8 mg/L)
• In the initial stage of period II, when the
feeding iron concentration were around 80–
120 mg/L, the effluent iron concentration
increased to 29 mg/L
• in the following stage of period II, when the
feeding iron concentration increased to 104
mg/L, the effluent iron concentration
decreased from 29 mg/L to 13 mg/L
• iron removal of 92.85% and was maintained.
In terms of toxicity and H2S effects:
• In all literature on SRB bioreactor: the Biosulfide and the Thiopaq processes
(Johnson and Hallberg, 2005).
• In the Biosulfide- like system, AMD is directly fed into a precipitation tank; H2S
produced from the biological reactor was fed into precipitation tank for the metal
recovery. High concentration of water soluble H2S will poison microbial community
in the bioreactor and thus decrease the sulfate reduction rate.
• In the Thiopaq-like processes, AMD and carbon source were directly feeding into
bioreactor, and in the reactor both sulfate reduction and metal precipitation
occurs. High concentration of metal ion will have toxic effect on the microbial
community, and the recovery of metal will be a problem.
• In our novel process, by using water recycle, the produced water soluble H2S
entered precipitation tank and precipitated with metal to form metal sulfides, thus
the H2S will maintain lower than water soluble concentration. Meanwhile, the
metal was removed from the AMD at precipitation tanks, thus when the AMD
entering bioreactor (UASB), only low concentration of metal ion within the AMD.
Such process will decrease the toxicity of both metal ion and H2S to the microbial
community and increase the sulfate reduction rate
In terms of sulfate reduction and carbon source
• It is documented that SRB were sensitive to acidity (Hard et
al., 1997), and most of the reported papers were operated at neutral
• While other researchers reported that SRB could remove 38.3% of
sulfate at pH of 3.25 and 14.4% at pH of 3.0 by using lactate as carbon
• In our novel process, though feeding pH lower to 2.5 and effluent pH
lower to 3.8, still 38% of sulfate removal was reached, which is
comparable with Elliott’s results. Consider the carbon source used in
our process were activated sludge, the sulfate reduction is promising.
• Attempts are made at isolating pure culture of acid-tolerant SRB failed.
Four 16S rRNA gene clone libraries have be constructed to analyze the
microbial community structure inside the UASB reactor, results showed
the existence of SRB including Desulfomicrobium, Desulfoviobrio
psychrotolerans, Desulfovibrio aminophilus and some other uncultured
sulfate-reducing bacteria (unpublished data). Works have been
carrying on isolating the acid-tolerant SRB from this reactor.
In terms of High iron removal
• Our work showed relative high iron removal by the system.
• Adsorption might be one reason that previous research
noted that Fe2+ could be bound to the solid phase that
contains a residual negative charge on its surface
(Cohen, 2006; Sheoran et al., 2010).
• At lower pH conditions, substitutes of SO42- for OHoccurs, resulting in the formation of ‘‘oxyhydroxysulphate’’
minerals such as schwartmanite (Sheoran et al., 2010).
• Another thing worthy to note is that Na+ and K+ and other
monovalent cation from the broken cell by the sludge
digestion in the bioreactor could form ferrite and jarosite
and other iron complex with iron ion. Hydrolysis of iron (III)
during the pH increase and produced water soluble CO2
also could attribute to the high iron removal.