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Arsenic in drinking water
Chemistry 371
Fall 2010
Erlend Sormo
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Arsenic
As, 33, 72.9216 amu
Oxidation states -3, 0,+3, +5
Metalloid
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1) Emsley, J. "Arsenic". The Elements of Murder: A History of Poison. Oxford University Press, 2006.
As2O3:
Popular poison
Colourless, odourless and easily incorporated into food
and drink Symptoms similar to those of Cholera
Inheritance powder
Not possible to detect until the Marsh testin 1836
Historical uses of As
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Historical uses of As
Agriculture:
Pesticides, herbicides, insecticides
i.e. Cacodylic acid, (CH3)2AsOOH
Industry:
Timber treatment with Chromated Copper Arsenate (CCA)
Pharmaceuticals:
Fowler's solution (1% KH2AsO3) to treat malaria, cholera andsyphilis (predating antibiotics)
As2O3 has also been used to treat leukaemia
1) Chemical Safety Information from Intergovernmental Organizations. International Program on Chemical Safety: Arsenic and Arsenic Compounds.http://www.inchem.org/documents/ehc/ehc/ehc224.htm#3.2
http://www.inchem.org/documents/ehc/ehc/ehc224.htm%22%20/l%20%22.2http://www.inchem.org/documents/ehc/ehc/ehc224.htm%22%20/l%20%22.2http://www.inchem.org/documents/ehc/ehc/ehc224.htm%22%20/l%20%22.28/8/2019 Arsenic in Drinking Water - E. Sormo
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Natural occurrence of As
Present in the earth's crust in an average of 2 ppm, as a component in over
200 different minerals Volcanic rocks
Basalt and Volcanic Ash
Ores
Arsenopyrite (FeAsS), Realgar (AsS), Orpiment (As2S3)
Fossil fuels
As is found in trace amounts in all living organisms
Marine sediments
Marine organisms have higher As concentrations than terrestrial organisms
Present in water in trace concentrations
Seawater ~ 3 g/L
Fresh water dependent surrounding geology
1)Chemical Safety Information from Intergovernmental Organizations. International Program on Chemical Safety: Arsenic and Arsenic Compounds.http://www.inchem.org/documents/ehc/ehc/ehc224.htm#3.2
2) Wilson, J.; Schreier, H.; Brown, S. Arsenic in Groundwater in the Surrey-Langley Area; A technical report for Fraser Health Authority and EnvironmentalHealth Services B.C. and the Ministry of Environment Lower Mainland Region Surrey, B.C. 2008.
http://www.inchem.org/documents/ehc/ehc/ehc224.htm%22%20/l%20%22.2http://www.inchem.org/documents/ehc/ehc/ehc224.htm%22%20/l%20%22.28/8/2019 Arsenic in Drinking Water - E. Sormo
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Sources of As in water
Natural sources:
Weathering and erosion of As containing minerals and soils
Geothermal dissolution
High temperature geothermal waters increase solubility of As minerals
Geochemical reduction-oxidation processes
Oxyhydroxide reduction
Organic matter as electron donors, mediated by certain bacteria
Pyrite oxidation
Pyrite ores exposed to oxygen
Volcanic eruptions As containing aerosols can mix with water
By dry deposition
By getting washed out of the air by rainfall
1)Chemical Safety Information from Intergovernmental Organizations. International Program on Chemical Safety: Arsenic and Arsenic Compounds.http://www.inchem.org/documents/ehc/ehc/ehc224.htm#3.2
http://www.inchem.org/documents/ehc/ehc/ehc224.htm%22%20/l%20%22.2http://www.inchem.org/documents/ehc/ehc/ehc224.htm%22%20/l%20%22.28/8/2019 Arsenic in Drinking Water - E. Sormo
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1) Henke, K. R. Arsenic: Environmental chemistry, health threats and waste treatment; Whiley: UK, 2009
Sources of As in water
Figure 1: Solubility constants for various As-salts1
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Sources of As in water
Anthropogenic sources:
Industrial emissions and waste:
From producing i.e. herbicides or CCA
Agriculture:
Runoff from decaying plants and soils sprayed with pesticides, herbicides or
insecticides
Combustion of As containing species:
Fossil fuels or wood treated with CCA
Waste from mining operations or metal smelting plants:
Smelting of metal ores (i.e. FeAsS) produces left over sludge Sludge is deposited, but leakage is frequent
1) Wang, S.;Mulligan, C. N. Occurrence of Arsenic Contamination in Canada: Sources, Behaviour and Distribution. Sci. of the Total Environ. 2006, 366,701 721.
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Scope of the As problem
As is found in both surface and groundwater Usually higher concentrations in aquifers release from sediments
Runoff and leaking usually responsible for contaminating surface waters
Runoff is often a incidental, localized problem while aquifers affected
by sediments is found worldwide and often over large areas Biggest problem as of today is shallow tube wells in areas with high
concentrations of As in the soil
WHO estimates that there are 10 million such tube wells in Thailand, Cambodia,Bangladesh, Myanmar, India and Nepal
Exposes between 40-50 million humans to unsafe levels of As on a daily basis
Wells in the Surrey-Langley area have also shown unsafe levels of As.
1) Hopenhayn, C. Arsenic In Drinking Water: Impact On Human Health. Elements2006, 2 (2), 103.
2) http://www.who.int/en/
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1) http://www.physics.harvard.edu/~wilson/arsenic/countries/arsenic_project_countries.html
As-related problems
Figure 2: Arsenic related problems worldwide1
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1) Polya et al. Arsenic in shallow Cambodian groundwater Mineralogical mag. (2005), 69(5), 807.
Wells in Cambodia
Figure 3: Arsenic containing wells in Cambodia, concentrated around the capital Phnom Pehn1
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1) Wilson et. al. Arsenic in Groundwater in the Surrey-Langley Area. Fraser Health and the Ministry of Environment B.C. 2008.
Wells in Surrey-Langley
Figure 4: Arsenic containing wells in the Surrey-Langley area, BC, Canada1
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Scope of the problem
Arsenic is one of the most serious environmentalcontaminants in the world:
High toxicity
Lethal dose for adults is 120 to 200 mg (As2O
3)
Chronic low level poisoning results in a wide spectre of
diseases
Vast amounts of people affected
And new areas are being discovered
In many countries drinking water is not routinely tested for As
1) Hopenhayn, C. Arsenic In Drinking Water: Impact On Human Health. Elements2006, 2 (2), 103.
2) Emedicine. Medscape's Continually Updated Clinical Reference. http://emedicine.medscape.com/article/1174215-overview
http://emedicine.medscape.com/article/1174215-overviewhttp://emedicine.medscape.com/article/1174215-overview8/8/2019 Arsenic in Drinking Water - E. Sormo
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Health effects
Inorganic forms of As are the most toxic and the prevalent species in drinking water
WHO has set the safe limit for As in drinking water to 10 mg/L
Canada lowered the limit from 50 mg/L to 10 mg/L in 2006
The mechanisms of the toxicity of As species have not yet been fully understood
Health effects have not been completely mapped out
Evidence mainly comes from epidemiological work
Animal studies have not provided good models
Inorganic As is quickly absorbed in the gastrointestinal tract and goes through aseries of metabolic steps before excretion
About 80-70% of ingested inorganic As species are excreted as dimethylarsinic acid (DMA)which is used as a tracer
~70% is excreted through urine
1) Hopenhayn, C. Arsenic In Drinking Water: Impact On Human Health. Elements2006, 2 (2), 103.
2) Le, X. C.; Lu, X.; Li, X. F. Arsenic Speciation, Analytical Chem. 2004, 76 (1), 26A-33A.
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Health effects
Ingestion of As species through drinking water is known to cause:
Cancer of skin, bladder, lungs and kidneys
As ingestion has also been associated with elevated risks of:
Vascular diseases
Vascular disease (PVD), Hypertension, Cerebrovascular disease (CVD),Coronary heart disease (CHD)
Diabetes
Neurological disorders
Reproductive problems
Stillbirths, infant mortality, low birth weight and prematurity
1) Hopenhayn, C. Arsenic In Drinking Water: Impact On Human Health. Elements2006, 2 (2), 103
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1) Hopenhayn, C. Arsenic In Drinking Water: Impact On Human Health. Elements 2006, 2 (2), 103
Health effects
Figure 6: External signs of As poisoning - skin keratoses andskin cancer1
Figure 5: Blackfoot disease1
Figure 7: External signs of As poisoning - skin cancer1
Figure 8: Patient from India with tumor due to As exposure1
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As speciation in water
Elemental As is not soluble in water
A wide range of As salts are soluble in water depending on pH,pE and ionic environment
Main forms of As in water are As(III) and As(V)
These ions react with water to produce:
Arsenous acid H3AsO3 [ As(III) ]
Anoxic conditions
Arsenic acid H3AsO4 [ As(V) ]
Oxic conditions
1) Uddin, M. T.; Mozumder, M. S. I.; Islam, M. A.; Deowan, S. A.; Hoinkis, J. Nanofiltration Membrane Process. Chem. Eng. Technol. , 30(), 12481254.
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1) www.appliedspeciation.com
As speciation in water
Figure 9: Phase diagram of As species in water1
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1) Hll, W. H. Mechanisms Of Arsenic Removal From Water. Environ. Geochem. Health 2010, 32, 287-290.2) Johnston, R.; Heijnen, H. Safe water technology for Arsenic removal. Environment and Sustainable Development Program (ESD) 2008.
http://unu.edu/env/Arsenic/Han.pdf
Removal techniques
Precipitation by Calcium oxides: Precipitation by metal salts used since 1934
CaO or Ca(OH)2 added = various precipitates
i.e. Ca(AsO4)2 very low solubility
Will also remove other ions like phosphates, sulphate, fluoride, iron andmanganese
Precipitates collected by filtration
Operated at pH > 10.5
Works well for high conc., As > 50 mg/L
Hard to get final yield below 1 mg/L
Precipitates are not physically stable
Leaching frequent from deposits
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1) Hll, W. H. Mechanisms Of Arsenic Removal From Water. Environ. Geochem. Health 2010, 32, 287-290.
Removal techniques
Synthetic ion exchange resins:
Resins are made of cross-linked polymer skeletons (polystyrene anddivinylbenzene) with charged functional groups at the surface
Arsenite and arsenate exchanged with Cl- onto surface of highly basic quaternaryamines
R-[N(CH3)3]+Cl- + H2AsO4
- R-[N(CH3)3]+H2AsO4
- + Cl- (R=matrix)
Should be operated at pH close to neutral to avoid competition by OH-
Shows good yields for removing As species
Strong interference from sulphates and possibly nitrates
Efficient elimination for sulphate conc. < 50 mg/L
Regeneration by addition of NaCl
Widely used technique for removal of many ionic contaminants
Functional groups can be changed to acidic to select for cations
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1) Hll, W. H. Mechanisms Of Arsenic Removal From Water. Environ. Geochem. Health 2010, 32, 287-290.
Removal techniques
Sorption with hydrous metal oxides:
Hydrolysed metal oxide surfaces contain hydroxyl groups
At pH < pzc the hydroxyl groups are protonated
Me-OH + H+ Me-OH2+
As anions can then be adsorbed onto the surface
Me-OH2+ + H2AsO4
- Me-OH2-H2AsO4
The metal oxides is then filtered out and deposited
Ferric oxides, titanium oxide and cerium oxide are commonly used
Granular Ferric Oxide (GFO) common in water treatment plants and householddevices
Metal oxides have slow sorption due to low surface area
Combination of nanomaterials and ferric oxides proved very efficient
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Removal techniques
Nanofiltration membrane process:
Water is filtrated through a charged nanomaterial membrane under highpressure
Size/charge exclusion
Nanoscale membrane pores filtrate As species by size
Hydrous As ions are to small to be excluded by micro or ultra filtration
A highly negatively charged membrane excludes As species by charge
Ions of same charge as membrane (co-ions) are inhibited most successfully
Has shown excellent yields for As(V)
Unacceptable yields for As(III)
At pH = 5-7 predominant specie is neutral H3AO3
Effect of charge exclusion lost
Can be solved by oxidizing As(III) to As(V) before filtration by i.e. KMnO4
Technique still at developmental stage, but shows great promise
1) Uddin, M. T.; Mozumder, M. S. I.; Islam, M. A.; Deowan, S. A.; Hoinkis, J. Nanofiltration Membrane Process. Chem. Eng. Technol. , 30(9),12481254.
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Removal techniques
Figure 10: Arsenic removal plant1
1) Johnston, R.; Heijnen, H. Safe water technology for Arsenic removal. Environment and Sustainable Development Program (ESD) 2008.
http://unu.edu/env/Arsenic/Han.pdf
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Future outlook
Surge of As related diseases will come longterm effects
Mechanism of toxicity has to be figured out
Problem bigger for underdeveloped countries
Less frequent testing of drinking water
Lack of technology for both testing and removal
Cheap and efficient removal techniques has tobe made available
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