arsenic in drinking water - e. sormo

8/8/2019 Arsenic in Drinking Water - E. Sormo

1/24

Arsenic in drinking water

Chemistry 371

Fall 2010

Erlend Sormo


2/24

Arsenic

As, 33, 72.9216 amu

Oxidation states -3, 0,+3, +5

Metalloid


3/24

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


4/24

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%22.2


5/24

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%22.2


6/24

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%22.2


7/24

1) Henke, K. R. Arsenic: Environmental chemistry, health threats and waste treatment; Whiley: UK, 2009


Figure 1: Solubility constants for various As-salts1


8/24


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.


9/24

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/


10/24

1) http://www.physics.harvard.edu/~wilson/arsenic/countries/arsenic_project_countries.html

As-related problems

Figure 2: Arsenic related problems worldwide1


11/24

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


12/24

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


13/24

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


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-overview


14/24

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


2) Le, X. C.; Lu, X.; Li, X. F. Arsenic Speciation, Analytical Chem. 2004, 76 (1), 26A-33A.


15/24

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


16/24

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


17/24

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.


18/24

1) www.appliedspeciation.com

As speciation in water

Figure 9: Phase diagram of As species in water1


19/24

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


20/24

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


21/24

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


22/24

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.


23/24

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


24/24

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

arsenic in drinking water - e. sormo

Documents