DETECTING ARSENIC IN GROUNDWATER USING PORTABLE...
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DETECTING ARSENIC IN GROUNDWATER USING PORTABLE DIGITAL VOLTAMETER Magda Wajrak Centre for Ecosystem Management, Edith Cowan University, Perth [email protected] Arsenic Chemistry Arsenic in its elemental state is a steel-grey, brittle, crystalline semi-metallic solid (Figure 1). It belongs to Group V of the periodic table and exists in four valency states, the most stable are; As 3+ (arsenite) and As 5+ (arsenate). Figure 1 Arsenic in its elemental state Arsenic is widely distributed in earth’s crust (average arsenic levels in soil are 1-5ppm). It is found in many minerals, most commonly in arsenopyrites (FeAsS). Arsenic can be released into the environment from several sources. Natural processes include erupting volcanoes or erosion of mineral deposits that contain arsenic, such as As 2 S 2 , As 2 S 3 , As 4 S 3 , As 4 S 4 , FeAsS. Human activities include smelters of ores such as copper, gold and lead and production of pesticides. Arsenic Toxicity Elemental arsenic is not toxic. The toxicity of arsenic in a combined state depends on the actual compound. Inorganic compounds such as As 2 O 3 are very toxic, however, organic compounds are less or non-toxic, for example arsphenamine (Figure 2) was actually used to treat syphilis. Figure 2 Arsphenamine structure The most common way that arsenic enters body is by inhalation and ingestion. Once inside the body arsenic binds to haemoglobin, plasma proteins and leukocytes. It gets distributed around the body, initially stored in the liver then kidneys, spleen and lungs. If exposure occurs over extend periods of time, deposits of arsenic can be found in the skin, hair, nails and bone muscle. Although, inorganic trivalent forms of arsenic are more readily absorbed by the body than the organic pentavalent compounds. Both forms of arsenic cause cellular damage. Arsenite interferes with enzymes within the body by binding to the HS-HO- groups forming stable rings that deactivate the enzyme. Arsenate disrupts the Krebs Cycle. This results in the inhibition of the bodies normal metabolism. Arsenate can also inhibit DNA repair. Arsenic is excreted from body by the process of methylation forming mono-, di- and tri-methylated compounds which are readily excreted in urine. Arsenic Poisoning Chronic exposure to arsenic can cause myocarditis, encephalopathy, painful paresthesia' s (abnormal nerve sensations), pericarditis, peripheral vascular disease (i.e., Blackfoot disease, see Figure 3), skin cancers (Figure 3) lung cancer, renal failure, anaemia, brittle nails exhibiting Mees lines, or hyperpigmentation (especially of the eyelids, neck, axillae, and groin). Figure 3 Squamous cell carcinoma on the heel (left) and Blackfoot disease (right) Bangladesh Story Decision made by the UN over 20 years ago to switch from bacteria-laden surface water to ground water has now created the largest case of mass poisoning in the world. The source of contamination is the pyritic sedimentary rock laid down over the centuries by rivers that run down from the Himalayas, which has released high concentrations of arsenic into groundwater, that people are now drinking (Figure 4) [1]. Figure 4 Source of arsenic contamination in the groundwater in Bangladesh It is now estimated that over 50 million people are at risk of arsenic poisoning, with about 1 million being exposed to water containing 600ppb of arsenic (Figure 5)[2], which is 60 times the safe drinking water limit set by WHO. Black < 10ppb Blue 10 – 25ppb Red > 250ppb Drinking Water Limits WHO – 10ppb Australia – 7ppb Figure 5 Concentration of arsenic in groundwater in Bangladesh. Arsenic in Australia The issue of arsenic in groundwater is now becoming a significant problem in Australia, with the discovery of acid sulphate soils, which cover more than 40,000km 2 of Australia’s coastline (Figure 6)[3]. These soils have the potential to result in acidification and cause the mobilization of heavy metals into ground and surface waters. In urban areas of Western Australia, this process has occurred resulting in high levels of arsenic detected in groundwater. Figure 6 Acid sulphate distribution in Australia Arsenic Detection Methods Detection and measurement of arsenic in groundwater is crucial for reducing human exposure to toxic levels of arsenic in Bangladesh. It is also now very important in Australia to monitor arsenic levels in groundwater. The most accurate and reliable detection methods are [4]: 1. ICP-MS (MDL: <2ppb) 2. ICP-AES (MDL: 8ppb) 3. AAS-HG (MDL: 0.5ppb) 4. AAS-GF (MDL: 0.5ppb) However, these methods are: expensive, laboratory based (ie. require chemist to run and infrastructure). Not good for frequent monitoring of over 6 million drinking wells in Bangladesh!!!!! References [1] EAWAG News 49, p.18, [2] Meharg, A.A., ‘Venomous Earth’, 2005, McMillan, New York., [3] Department of Environmental Protection, “Investigation of soil and groundwater acidity”, Stirling, 2002, Retrieved from: //portal.environment.wa.gov.au/pls/portal/docs [4] Ahmed, M. F., “Arsenic Contamination: Bangladesh Perspective’, 2003, ITN-Bangladesh, Bangladesh [5] Wajrak, M., “Infield Detection of Arsenic using Portable Digital Voltameter’, As-2006 Balkema Publication: Natural Arsenic in Groundwaters of Latin America - Occurrence, Health Impact, Remediation and Management, Eds. J. Bundschuh, M.A. Armienta, P. Bhattacharya. & J. Matschullat (in print). Anodic Stripping Voltammetry (ASV) Voltammetry offers an alternative technique for arsenic detection. ASV measures very small current (micro Amps) produced when a metal is converted to its most stable ion after being concentrated onto an electrode. Solution contains Reduction (deposition) Oxidation (stripping) different ions As 3+ + 3e - -> As (s) As (s) -> As 3+ + 3e - Portable Digital Voltameter (PDV6000) Development of a less expensive, accurate, reliable and infield method for arsenic detection was done using PDV6000 (Figure 7), built by Monitoring Technologies International Pty Ltd, Osborne Park, WA. Figure 7 PDV6000 instrument (left) consists of hand-held control unit, electrochemical cell (middle), three electrodes (right). Method Development Involved investigating different working electrodes, various electrolyte solutions, deposit and strip potentials and electrode conditioning techniques. Best results (in terms of linearity, reproducibility and detection limit) were found using solid gold electrode, 0.25M CH 3 COOH electrolyte and conditioning the electrode with 1ppm arsenic standard. Linearity Reproducibility Detection Limit Method Validation Currently the PDV6000 method is being validated by comparing it with ICP-MS. Groundwater samples were collected every two weeks from a monitoring site in Osborne Park, between March to May 2005. Preliminary results are very promising, ASV method is within 4ppb of the ICP-MS results [5]. 3(MW4) data analysis 3(MW5) data analysis 27ppb 25ppb 5 17ppb 17ppb 5 28ppb 27ppb 4 17ppb 16ppb 4 21ppb 25ppb 3 16ppb 15ppb 3 29ppb 32ppb 2 15ppb 15ppb 2 ASV ICP - MS Sample Round ASV ICP - MS Sample Round As=200ppb, Deposition=60 sec 1run = 53A 10run = 49A As=2.5ppb, Deposition=300 sec Height = 1.5A
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DETECTING ARSENIC IN GROUNDWATER USING PORTABLE DIGITAL VOLTAMETER
Magda WajrakCentre for Ecosystem Management, Edith Cowan University, Perth
Arsenic ChemistryArsenic in its elemental state is a steel-grey, brittle, crystalline semi-metallic solid (Figure 1). It belongs to Group V of the periodic table and exists in four valency states, the most stable are; As3+ (arsenite) and As5+
(arsenate).
Figure 1 Arsenic in its elemental state
Arsenic is widely distributed in earth’s crust (average arsenic levels in soil are 1-5ppm). It is found in many minerals, most commonly in arsenopyrites (FeAsS). Arsenic can be released into the environment from several sources. Natural processes include erupting volcanoes or erosion of mineral deposits that contain arsenic, such as As2S2, As2S3, As4S3, As4S4, FeAsS. Human activities include smelters of ores such as copper, gold and lead and production of pesticides.
Arsenic ToxicityElemental arsenic is not toxic. The toxicity of arsenic in a combined state depends on the actual compound. Inorganic compounds such as As2O3 are very toxic, however, organic compounds are less or non-toxic, for example arsphenamine (Figure 2) was actually used to treat syphilis.
Figure 2 Arsphenamine structure
The most common way that arsenic enters body is by inhalation and ingestion. Once inside the body arsenic binds to haemoglobin, plasma proteins and leukocytes. It gets distributed around the body, initially stored in the liver then kidneys, spleen and lungs. If exposure occurs over extend periods of time, deposits of arsenic can be found in the skin, hair, nails and bone muscle.
Although, inorganic trivalent forms of arsenic are more readily absorbed by the body than the organic pentavalent compounds. Both forms of arsenic cause cellular damage. Arsenite interferes with enzymes within the body by binding to the HS-HO- groups forming stable rings that deactivate the enzyme. Arsenate disrupts the Krebs Cycle. This results in the inhibition of the bodies normal metabolism. Arsenate can also inhibit DNA repair. Arsenic is excreted from body by the process of methylation forming mono-, di-and tri-methylated compounds which are readily excreted in urine.
Arsenic PoisoningChronic exposure to arsenic can cause myocarditis, encephalopathy, painful paresthesia's (abnormal nerve sensations), pericarditis, peripheral vascular disease (i.e., Blackfoot disease, see Figure 3), skin cancers (Figure 3) lung cancer, renal failure, anaemia, brittle nails exhibiting Mees lines, or hyperpigmentation(especially of the eyelids, neck, axillae, and groin).
Figure 3 Squamous cell carcinoma on the heel (left) and Blackfoot disease (right)
Bangladesh StoryDecision made by the UN over 20 years ago to switch from bacteria-laden surface water to ground water has now created the largest case of mass poisoning in the world. The source of contamination is the pyriticsedimentary rock laid down over the centuries by rivers that run down from the Himalayas, which has released high concentrations of arsenic into groundwater, that people are now drinking (Figure 4) [1].
Figure 4 Source of arsenic contamination in the groundwater in Bangladesh
It is now estimated that over 50 million people are at risk of arsenic poisoning, with about 1 million being exposed to water containing 600ppb of arsenic (Figure 5)[2], which is 60 times the safe drinking water limit set by WHO.
Black < 10ppbBlue 10 – 25ppbRed > 250ppb
Drinking Water LimitsWHO – 10ppbAustralia – 7ppb
Figure 5 Concentration of arsenic in groundwater in Bangladesh.
Arsenic in AustraliaThe issue of arsenic in groundwater is now becoming a significant problem in Australia, with the discovery of acid sulphate soils, which cover more than 40,000km2 of Australia’s coastline (Figure 6)[3]. These soils have the potential to result in acidification and cause the mobilization of heavy metals into ground and surface waters. In urban areas of Western Australia, this process has occurred resulting in high levels of arsenic detected in groundwater.
Figure 6 Acid sulphate distribution in Australia
Arsenic Detection MethodsDetection and measurement of arsenic in groundwater is crucial for reducing human exposure to toxic levels of arsenic in Bangladesh. It is also now very important in Australia to monitor arsenic levels in groundwater. The most accurate and reliable detection methods are [4]:1. ICP-MS (MDL: <2ppb)2. ICP-AES (MDL: 8ppb)3. AAS-HG (MDL: 0.5ppb)4. AAS-GF (MDL: 0.5ppb)
However, these methods are: expensive, laboratory based (ie. require chemist to run and infrastructure). Not good for frequent monitoring of over 6 million drinking wells in Bangladesh!!!!!
References
[1] EAWAG News 49, p.18, [2] Meharg, A.A., ‘Venomous Earth’, 2005, McMillan, New York., [3] Department of Environmental Protection, “Investigation of soil and groundwater acidity”, Stirling, 2002, Retrieved from: //portal.environment.wa.gov.au/pls/portal/docs
[4] Ahmed, M. F., “Arsenic Contamination: Bangladesh Perspective’, 2003, ITN-Bangladesh, Bangladesh
[5] Wajrak, M., “Infield Detection of Arsenic using Portable Digital Voltameter’, As-2006 Balkema Publication: Natural Arsenic in Groundwaters of Latin America - Occurrence, Health Impact, Remediation and Management, Eds. J. Bundschuh, M.A. Armienta, P. Bhattacharya. & J. Matschullat (in print).
Anodic Stripping Voltammetry (ASV)Voltammetry offers an alternative technique for arsenic detection. ASV measures very small current (micro Amps)produced when a metal is converted to its most stable ion after being concentrated onto an electrode.
Solution contains Reduction (deposition) Oxidation (stripping)different ions As3+ + 3e--> As(s) As(s) -> As3+ + 3e-
Portable Digital Voltameter (PDV6000)Development of a less expensive, accurate, reliable and infield method for arsenic detection was done using PDV6000 (Figure 7), built by Monitoring Technologies International Pty Ltd, Osborne Park, WA.
Figure 7 PDV6000 instrument (left) consists of hand-held control unit, electrochemical cell (middle), three electrodes (right).
Method DevelopmentInvolved investigating different working electrodes, various electrolyte solutions, deposit and strip potentials and electrode conditioning techniques. Best results (in terms of linearity, reproducibility and detection limit) were found using solid gold electrode, 0.25M CH3COOH electrolyte and conditioning the electrode with 1ppm arsenic standard.
Linearity Reproducibility Detection Limit
Method ValidationCurrently the PDV6000 method is being validated by comparing it with ICP-MS. Groundwater samples were collected every two weeks from a monitoring site in Osborne Park, between March to May 2005. Preliminary results are very promising, ASV method is within 4ppb of the ICP-MS results [5].
3(MW4) data analysis 3(MW5) data analysis
27ppb25ppb517ppb17ppb5
28ppb27ppb417ppb16ppb4
21ppb25ppb316ppb15ppb3
29ppb32ppb215ppb15ppb2
ASVICP - MSSample Round
ASVICP - MSSample Round
As=200ppb, Deposition=60 sec1st run = 53�A10th run = 49�A
As=2.5ppb, Deposition=300 secHeight = 1.5�A
