Sustainable development and the environmentMobility and Bioavailability of metals

Exposure to environment

• Overall metal bioavailability studies must take in to consideration following

• Metals can be dispersed in soil and sediment, dissolved in ground and surface water, suspended as particles in surface water, and in pore fluid in sediment .

– In addition, metals can be dispersed into the atmosphere, by natural geochemical cycling and by other anthropogenic processes (such as smelting and burning leaded gasoline and coal) and by microbial activities;

• Most adverse effects can result form bioaccumulation of metals by biota in surface water and by plants and animals in terrestrial environments.

• Bioavailability is a complex function of – total concentration and speciation (physical-chemical forms) of metals, – Mineralogy of soil, rock air particles, – pH, redox potential, temperature, total organic content (both particulate and dissolved fractions), – suspended particulate content, as well as volume of water, water velocity, and duration of water availability,

particularly in arid and semi-arid environments.

• In addition, wind transport and removal from the atmosphere by rainfall – (frequency is more important than amount) – Many of these factors vary seasonally and temporally, and most factors are interrelated.

• Consequently, changing one factor may affect several others.

• Poorly understood biological factors seem to strongly influence bioaccumulation of metals and severely inhibit prediction of metal bioavailability

Geochemical exploration data

• Geochemical exploration searches for anomalies in the concentrations of chemical substances in a region.

• In site-specific investigations, geochemistry is a routine procedure (in cold-temperate regions)

• the sampling density varies from a couple of meters to several hundred.

• In glaciated areas the most common sample material is till, – Contains ground bedrock material– indirectly reflects the composition of bedrock.

• In addition to the sampling of soil (till,) a percussion drill may be used to collect samples of drilling mud and crushed rock from the bedrock surface.

Selective/sequential leaching

• Geochemical exploration uses as a routine so called selective leaching techniques– Aqua regia: pseudo total analysis (not most resistant minerals)– Nitric acid (or other strong acid): most silicates and all ore minerals– Ammonium oxalate: iron hydroxides– Weak (organic) acids: clay minerals, absorbed metals, exchangeable

cations, calcite – Diluted salt; Water: soluble minerals salts, sulfates

• Extremely useful data for environmental assessments– Speciation of metals– Background and base line– Exposure and risk analysis

• Mandatory for design of post-operational activities at sulfide-ore mines– Treatment of tailings/remediation of soil contamination

Example: metal mining wastes with arsenic• Speciation of Fe and S in tailings

• Solubility (sulphates)

• Oxyhydroxides – solubility varies with redox-

conditions– Desorption of metals (e.g. As)

• Distinction of requires sequential/selective leaching tests besides other analyses

• Negligence of mineralogical studies has lead to “remediation and closure measures” that have just worsened the situation (mobilized As)

Environmental information from sequential leaching

– Aqua regia: pseudo-total analysis (dissolves almost all, but not all minerals): considered to provide conservative estimates of metal pollution

• Does not differentiate mobile or stable speciation– Ammonium oxalate: provides the contents of metals that can be

released if oxidized conditions change reduced – Weak (organic) acids: estimates of bioavailable concentrations

in soils– Water: soluble minerals salts, sulfates

Karabash – soil analysis

• Soil sample mineralogy was analysed using XRD, and their chemistry by selective leaching. (blue=bound by adsorption or soluble sulfates, orange=bound by iron(III)hydroxides, yellow=bound by sulphides)

Cu

0

500

1000

1500

2000

2500

K1 K2 K3 K4

Cu_+512P

Cu_224P

Cu_201P

Using modern methods to mineralogical reprocessing of the mining tailings and wastes could be used to reduce the pollution

Metal contents in some tailings and processing wastes are close to economic ore deposits!

Exposure analysis and dose assessment

Input to risk based action planning:– Distinguishes the

chemicals of concerns

– Sets priorities to remediation work

– Reflects the present knowledge on health risks

Geochemical modelling

AMD in Bolnisi Mining Area, Georgia

Physical data Hydrological data Mineralogical composition vol-% #

Drainage water chemistry in mg/L *

Areal extent 3.49e+6 m2 Precipitation 700 mm /y quartz 78 pH 2.65 Volume of waste rock

8.99e+7 m3 Evaporationx 50 % sericite 7 SO42- 14794

Mean annual temperature

+10 oC Water flow rates

pyrite 3.1 Fetot 1630

Porosity 39 % -to Kazretula river

300 m3 /d chalcopyrite 0.8 Cu2+ 1100

Bulk density 1695 kg /m3

-to Poladauri riverx

50 m3 /d sphalerite 0.1 Zn2+ 682

- discharge to groundwaterx

3000 m3 /d chlorite 11 Mg2+ 1100

 

Solute flows to Kazretula River Solute flows to the streams entering the Poladauri River

Solute flows infiltrating to the groundwater

Mol /s Tonnes /y Mol /s Tonnes /y Mol /s Tonnes /y

SO42- 0.532 1620 SO4

2- 0.089 270 SO42- 5.315 16200

Fetot 0.101 178 Fetot 0.017 30 Fetot 1.013 1784

Cu2+ 0.060 120 Cu2+ 0.010 20 Cu2+ 0.601 1204

Zn2+ 0.036 75 Zn2+ 0.006 13 Zn2+ 0.362 746

Mg2+ 0.157 120 Mg2+ 0.026 20 Mg2+ 1.571 1204

Mineral contents Mineral weathering rates Mineral lifetimes

Mol kg Mol /s Years

pyrite 3.94e +10 4.72e +9 pyrite 2.095 pyrite 596

chalcopyrite 6.64e +9 1.22e +9 chalcopyrite 0.671 chalcopyrite 314

sphalerite 1.56e +9 1.52e +8 sphalerite 0.404 sphalerite 123

chlorite 3.02e +10 1.68e +10 chlorite 0.351 chlorite 2724

georgiamodel0905_Heads

554.900867

574.178188

593.455509

612.73283

632.0101509

651.287472

670.564793

689.842113

X

Y

Z

Bolnisi-mining area

Contaminated streams from tailings

Contaminated river

Irrigation channels

Scope modelling

new species

0.0050.0750.1450.2150.2850.3550.4250.4950.5650.6350.7050.7750.8450.9150.985

X

Y

Z

new species

0.010.080.150.220.290.360.430.50.570.640.710.780.850.920.99

X

Y

Z

Toxic metals

• Subjects of particular concern include various metals, – chromium, nickel, copper, manganese, mercury, cadmium, and lead, (not

forgetting uranium, thorium, radium)

• and metalloids, including – arsenic, antimony, and selenium

• Near former mine sites, dumps, tailing piles, and impoundments,

• In urban areas and industrial centers

• Higher than average abundances of these elements in soil, sediment, water, and organic materials,

• In some cases due to past mining and (or) industrial activity, may cause the formation of the more bioavailable forms of toxic heavy metals.

• Bioavailability is a complex function of – total concentration and speciation (physical-

chemical forms) of metals, – Mineralogy of soil, rock air particles, – pH, redox potential, temperature, total organic

content (both particulate and dissolved fractions),

– suspended particulate content, as well as volume of water, water velocity, and duration of water availability, particularly in arid and semi-arid environments.

• In addition, to aqueous transport, wind transport and removal from the atmosphere by rainfall

– (frequency is more important than amount) – Many of these factors vary seasonally and

temporally, and most factors are interrelated.

• Consequently, changing one factor may affect several others.

• Poorly understood biological factors seem to strongly influence bioaccumulation of metals and severely inhibit prediction of metal bioavailability

Isotopes as tools to assess exposure

Toxicity of Zn depends substantially on the hardness of water!!

Partitioning of metals in surface waters and sediments• After discharge to an aquatic environment but before uptake by organisms, metals are partitioned

between solid and liquid phases. • Within each phase, further partitioning occurs

– determined by ligand concentrations and metal-ligand bond strengths.

• In solid phases, soil, sediment, and surface water particulates, metals may be partitioned into six fractions: – (a) dissolved, – (b) exchangeable, – (c) carbonate, – (d) iron-manganese oxide, – (e) organic,– (f) crystalline

• Various metals partition differently among these fractions • Partitioning is affected strongly by variations in pH, redox state, organic content, and other environmental

factors • The relative mobility and bioavailability of trace metals associated with different fractions are shown in the

next slide• The dissolved fraction consists of • carbonate complexes, whose abundance increases with pH, • metals in solution, including metal cation and anion complexes and hydrated ions

– whose solubilities are affected strongly by pH and tend to increase with decreasing pH

Metal mobility and bioavailabilityMetal species and association Mobility

Exchangeable (dissolved) cations High.Changes in major cationic composition may cause a release due to ion exchange (estuarines, mixing of saline groundwater with surface water)

Metals associated with Fe-Mn oxides Medium. Changes in redox conditions may cause a release but some metals precipitate if sulfide mineral present is insoluble

Metals associated with organic matter Medium/High Decomposition/oxidation of organic matter occurs in time

Metals associated with sulfide minerals Strongly dependent on environmental conditions. Under oxygen-rich conditions, oxidation of sulfide minerals leads to release of metals.

Metals fixed in crystalline phase Low. Only available after weathering or decomposition

Spread of metals in surface water

• In mining areas soils have commonly high concentrations of metals

• Sediments in streams and lakes in the catchments areas of mineral deposits provide information about– Transport and distribution and partitioning of metals in

sediments• Eg. Some of the springs existing in Talvivaara catchment area

where discharging acid drainages (pH 3.5!) before the mine was build providing (with black shales present in the region) “an natural analog” for the mine pollution spread (of Ni and U)

Particle size effects and “mineral liberation factor”• Particulate size and resulting total surface area available for adsorption

– important factors in adsorption processes – affect metal bioavailability

• Small particles with large surface-area-to-mass ratios allow more adsorption than an equivalent mass of large particles with small surface-area-to-mass ratios.

– Reduced adsorption can increase metal bioavailability by increasing concentrations of dissolved metals in associated water.

• The size of particles released during mining depends on mining and beneficiation methods. Finely milled ore may release much smaller particles that can

– both be more widely dispersed by water and wind, – serve as sites of enhanced adsorption.

• Consequently, mine tailings released into fine-grained sediment such as silty clays found in many playas can have much lower environmental impact than those released into sand or coarse-grained sediment with lower surface area and adsorption.

• (Natural attenuation by sorption)

Sulfide OxidationFeS2 + 7/2 O2 + H2O Fe2+ + 2 SO4

2- + 2 H+ (1)

Fe1-xS + (2-x/2) O2 + x H2O (1-x) Fe2+ + SO42- + 2x H+ (2)

Fe2+ + 1/4 O2 + 5/2 H2O Fe(OH)3 + 2 H+

Complete oxidation of pyrite:

FeS2 + 15/4 O2 + 7/2H2O Fe2+ + 2 SO42- + 4 H+

If oxygen is limited (maximum solubility to groundwater) incomplete oxidation can take place according to (1)If pH is > 3.5, Fe(OH)3 will precipitate

• Iron crust can stop neutralization by carbonates• iron-oxy-hydroxides can absorb metal (and they stay there as long as

Fe(OH)3 (or other Fe-oxy-hydroxides) are stable: changes in conditions can release them.

If however, pH is low (< 5) and oxygen is present the much more acid generating reactions with FeIII can take place !

• In stead of ferric hydroxide (metastable) more complex reactions of ferric oxy-hydroxides and hydroxysulfates can take place

Secondary MineralsFerric hydroxides & hydroxysulfates

Schwertmannite Fe8O8 (SO4)(OH)6Jarosite KFe3 (SO4)2(OH)6Ferrihydrite Fe5OH8•4H2OGoethite a-FeO(OH)

Oxidation with Ferric IronFeS2 + 14 Fe3+ + 8 H2O

15 Fe2+ + 2 SO42- + 16 H+

Fe1-xS + (8-2x) Fe3+ + 4 H2O (9-3x) Fe2+ + SO4

2- + 8 H+

Fe2+ + ¼ O2 + H+ Fe3+ + ½ H2O

These reactions are commonly cathalysed by microbes!

Acid generating and secondary minerals producing reactions

Sorption of metals on hydrated ferric oxides

Representative curves on sorption of metals on ferrous oxy-hydroxidesModelled for 1g/L of ferric oxide-sorbent

Representative curves on sorption of oxyanions on ferrous oxy-hydroxides. Modelled for 1g/L of ferric oxide-sorbent

Compositional fractioning

• Adsorption is pH-dependent taking place (only) in a certain range of pH

• The adsorption edge, the pH range over which the rapid change in sorption capacity occurs, varies among metals,

– precipitation of different metals over a large range of pH units.

• mixing metal-rich + acidic water with higher pH+ metal-poor water

• results in fractionation and separation of metals as different metals are adsorbed onto various media over a range of pH values.

• Cadmium and zinc tend to have adsorption edges at higher pH than iron and copper, and consequently they are likely to be more mobile and more widely dispersed.

• Adsorption edges also vary with concentration of the complexing agent; thus, increasing concentrations of complexing agent increases pH of the adsorption edge

• Major cations such as Mg+2 and Ca+2 also compete for adsorption sites with metals and can reduce the amount of metal adsorption

Metals in sulphides

• In reducing aquatic environments metals from mining activities are commonly associated with sulfide minerals

• Either primary (in the ore deposit) or formed by bacterial reduction of the (secondary) sulfates in oxidized tailings.– These reactions are applied in remediation!

• Most metal sulfide minerals are quite immobile, as long as they remain in a chemically reducing environment,– may have little impact on biota despite of anomalous metal

concentrations

Oxidation with Dissolved O2

ZnS + 2 O2 Zn2+ + SO42-

PbS + 2 O2 PbSO4

Oxidation with Ferric Iron

ZnS + 8 Fe3+ + 4 H2O Zn2+ + 8 Fe2+ + SO4

2- + 8 H+

• In recent organic carbon-rich sediments, trapped interstitial fluids can commonly form a strongly reducing (anoxic) environment. Low redox potential in this environment can promote sulfate reduction and sulfide mineral deposition. – Much of the non-silicate-bound fraction of potentially toxic metals such as

arsenic, cadmium, copper, mercury, lead, and zinc, can be co-precipitated with pyrite, form insoluble sulfides, and become unavailable to biota

– Seasonal variation in flow rates or storms, floods that induce an influx of oxygenated water can result in rapid reaction of this anoxic sediment and thereby release significant proportions of these metals.

• Also co-precipitation of As in oxic environments can be used to bound it efficiently to non-bioavailable form (Ca-Fe-arsenate with Ca/Fe about 1/6)

Metal uptake by plants

• Plant species and relative abundance and availability of necessary elements also control metal uptake rates.

• Abundant bioavailable amounts of essential nutrients, including phosphorous and calcium, can decrease plant uptake of non-essential but chemically similar elements, including arsenic and cadmium, respectively.

• More complex interactions are also observed: bioavailability may be related to multi-element amounts or ratios. For example, copper toxicity is related to low abundances of zinc, iron, molybdenum and (or) sulfate).

• Widely studied in agricultural sciences• In the scientific literature, many studies describe anthropogenic (industrial or

mining) contributions to elemental abundances, and their bioavailability controls, in the environment. E.g.

– occurrence of heavy metals in soil near and far from urban pollution; – formation of acid mine drainage; uptake of heavy metals by plants in lab experiments and – uptake of metals by vertebrates in the vicinity of zinc smelters– Arsenic in groundwater e.g. Finland, public health impacts of arsenic groundwater

Hungary

Metal up take into aquatic organism

• Metal uptake by plants and partitioning in the soil are combined in the aquatic environment.

• Two major pathways (uptake vectors) are available for metal incorporation in deposit- and (or) 13 detritus-feeding aquatic species:

• (1) ingestion of metal-enriched sediment and suspended particles during feeding, and

• (2) uptake from solution

• Consequently, knowledge of geochemical reactions of metals in both water and sediment is necessary to understand controls on metal bioavailability in natural water.

• Many biological factors controlling metal bioaccumulation in a aquatic organisms are not understood; this fact severely limits our understanding of metal bioavailability – Regulations can be expected to change

Acid NeutralizationCaCO3 + H+ Ca2+ + HCO3

2-

KAlSi3O8 + H+ + 7 H2O

K+ + 3H4SiO4 + Al(OH)3

CaAl2Si2O8 + 2 H+ + H2O

Ca2+ + Al2Si2O5(OH)4

Acid-Base AccountingFeS2 + 2CaCO3 + 3.75O2 + 1.5H2O = Fe(OH)3 +

2SO42- + Ca2+ + CO2

AP: Acid-producing potentialNP: Acid-neutralizing potentialNNP: Net neutralizing potential

NNP = NP - AP

Secondary MineralsSulfates

Highly solubleMelanterite FeIISO4•7H2ORozenite FeIISO4•4H2OCopiapite FeIIFeIII

4(SO4)6(OH)2•20H2OHalotrichite FeIIAl2(SO4)4•22H2OGoslarite ZnSO4•7H2O

Moderately solubleGypsum CaSO4•2H2O

Contrary Creek,

Virginia

Climatic conditions effect the solubility of secondary minerals, particlularly

Vermont: peaks after spring dischargeVirginia peaks during hot summers

Secondary MineralsAluminum hydroxides & hydroxysulfates

Amorphous Al(OH)3Gibbsite g-Al(OH)3Jurbanite Al(SO4)(OH)6•5H2OBasaluminite Al4 (SO4)(OH)10•5H2O

Secondary Minerals

• Metal-sulfate salts– Highly soluble– Store acidity and metals– Cycle metals via evaporation and

dissolution• Ferric hydroxides

– pH-dependent sorption of metals

Classification of Seafloor Massive Sulfide Deposits

VolcanicAssemblage

Sediments>Volcanics

Volcanics=Sediments

Volcanics>Sediments

BimodalFelsic>Mafic

Bathurst Kuroko

BimodalMafic>Felsic

Sedimentary-Exhalative

Besshi Noranda

Mafic andUltramafic

Cyprus

Uses of Models

Mitigation and planning at future minesRemediation at abandoned minesLand-use planning