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Arsenic in the Environment: Biochemical Effects on Plants
and Human Health
Department of Environmental Science
University of KalyaniKalyani, Nadia
S.C.Santra
ARSENICVery common in most geological environments, igneous,
metamorphic and sedimentary, causing a high background in many parts of north America
Chalcophile, oxyanionic or metalloid element often associated with sulphide ores
Crustal abundance: 1.8 ppm, ranging from 0.1 to several hundred ppm.
Major source of anthropogenic arsenic mobilization is weathering of mine waste rock and tailings as gold is often associated with arsenopyrite especially in Canada
Also common in reduced environment of coal deposits
Orpiment
Realgar
Arsenopyrite
Arsenic contamination
WHO recommended maximum in drinking water 10μg/lEU and US EPA recommended level is 50 μg/l, which is the
level detectable by ICP OES.Up to 5000 μg/l in contaminated water
Groundwater contaminationArgentina, Bangladesh, Chile, China, Hungary, Nepal, India,
Mexico, Romania, Taiwan, Vietnam, SW USA, MyanmarContamination from Geothermal Water
Argentina, Dominica, Chile, France, Japan, Iceland, New Zealand, Alaska USA
In Mining EffluentsCanada, Ghana, Greece, Italy, Russia, Thailand, USA
Arsenic in India and BangladeshWater from tube wells is contaminated with
arsenic.Surface water is contaminated with
pathogenic bacteria causing cholera etc.
The tube wells were put in to provide “safer” water with no pathogens and irrigation water for more intensive agriculture during the “Green Revolution”
People become sick with skin lesions, black skin, and eventually cancer. They are shunned by others who think that the disease is contagious.
Men and children are more affected than women.
Bangladesh about 20% of wells are contaminated and an estimated 80 million people are dependent on those wells for domestic purposes and affected by arsenic poisoning.
Periodic Table of the Elements
As is a Group V element (like N and P) Replaces S in minerals and metabolic systems replaces P in minerals and ATP energy cycle
Arsenic ChemistrySeveral oxidation states:
As-1 in sulphide minerals, As0, metal, only stable in very reduced conditions but can be reduced
to As-3 in the most toxic form of arsine gas (AsH3) As3+ As5+ are common in oxidizing conditions and soluble at all values
of Eh and pHOxidation of As3+ to less toxic As5+ is slow so usually both are
present in oxidized environments like mine tailings.
Arsenic can be removed from mine water by the addition of a solution containing FeSO4.
Fe2+ is oxidized to Fe3+ and precipitates as FEOOHArsenate is strongly absorbed by FeOOH and precipitated
Toxicity of Arsenic Historically arsenic trioxide was known as “inheritance dust”In 55 AD Nero poisoned Britannicus with arsenic to secure the Roman throne 15th/16th centuries, the Italian Borgias used arsenic for political assassinations. Napoleon may have been poisoned by arsenic-tainted wine or by the wallpaper
AsO4-3 replaces PO4
-3 and cells die
AsO4-3 inhibits oxidative phosphorylation in the ATP energy cycle
AsO3-3 replaces S in thiol groups and inhibits protein functions
Absorbed by inhalation or digestion and transferred via the bloodstream to all organs producing systemic damage.
Long term low level exposure causes hyper pigmentation (black spots on skin), followed by skin malignancy, peripheral arteriosclerosis (black foot disease)
Lung, liver and kidney cancer develop over time.
Acute arsenic exposure results in vomiting, abdominal pain and bloody diarrhea and death.
Arsenic Characteristics• Most natural waters contain
inorganic species– As (III) or arsenite predominant in
ground waters H3AsO3
– As (V) or arsenate in surface waters H2AsO4 & HAsO4
-2
Natural Arsenic LevelsCrystalline Rock
Soil
Ground Water
Surface Water
Avg. 2 ppm
1-40 ppm
0.01 – 800 ppbAs high as 40,000 in hot
springs
2.38 – 65 ppbAs high as 22,000 in river water
Some Arsenic Uses/Anthropogenic Sources
• Smelting of metals• Pharmaceutical industry (medicines)• Pesticide manufacture (very limited)• Wood preservative – CCA [in phase out]• Cattle and sheep dips• Feed additives• Dye stuffs• Petroleum, coal, and wood burning• Semiconductor manufacture• Waste incineration
More on Methylation• Reduce arsenite (via purine
nucleoside phosphorylase) to arsenate then methylation (via enzymatic transfer of the methyl group from S-adenosylmethionine (SAM) to arsenite to form monomethylarsonic acid (MMAV) )
• Gene that codes for the enzyme responsible for this reaction is just like Cyt 19
• arsenite+SAM→MMAV• MMAV+thiol→MMAIII• MMAIII+SAM→DMAV• DMAV+thiol→DMAIII
• DMA III = Dimethylarsinous Acid
• Most humans exposed to arsenic excrete 10–30% inorganic arsenic, 10–20% MMA(V+III) and 60–80% DMA(V+III),
It’s Effects
Accumulation of Arsenic in Biological system
Arsenic In Algal System• Freshwater algae has enormous capacity to
bio-accumulate and bio-transform inorganic arsenic.
• Majority of arsenic accumulates in the organisms as dimethyl arsenic compounds and are mainly found in algal body.
• Among the different algal strains , blue green algal species Oscillatoria –Lyngbya mixed culture showed high efficiency in removing arsenic ( Samal et al, 2004).
• A simple one-celled algae called Cyanidioschyzon sp can withstand extremely harsh conditions and is able to chemically modify arsenic that occurs naturally around hot springs. Cyanidioschyzon sp. isolate oxidized arsenite [As(III)] to arsenate [As(V)], reduced As(V) to As(III), and methylated As(III) to form trimethylarsine oxide (TMAO) and dimethylarsenate [DMAs(V)]. (Qin et al., 2009).
Oscillatoria –Lyngbya spp
Algae detoxify As
Arsenic Volatilization By Fungi
• Fungi are capable of transforming inorganic and organic arsenic compounds into highly volatile trimethyl-arsine (TMA) which could thereby lost into air.
• A wide variety of fungi including the strains of Aspergillus sp, Fusarium sp, Penicillium sp, Candida sp, Humicola sp and Gliocladium roseum were found to capable of converting mono-methyl arsenate(MMAA) and dimethyl arsenate(DMAA) into trimethyl-arsine (TMAA).
Penicilium sp
Fusarium sp
Humicola sp
Gliocladium roseum Candida sp
Fusarium sp
The Fungal Methylation Pathway For The Formation Of Trimethyl
Arsine
Mono-methyl arsenate (MMAA)
Dimethyl arsenate (DMAA)
Trimethylarsine oxide
Trimethyl arsine (TMAA)
Arsenic Accumulation In Ferns• Arsenic can be hyper accumulated by ferns
(Ma et al., 2001; Wang et al 2002; Zhao et al 2002).
• Hyper accumulating ferns identified to date are all located in the order Pteridales, and include a number of Pteris species.
• The sporophyte of the fern Pteris vittta is known to hyper accumulate As in its fronds to >1% of its dry weight.
• Hyper accumulation of As by plants has been identified as a valuable trait for the development of a practical phyto-remediation process for removal of this potentially toxic trace element from the environment.
Pteris vittata
Arsenic bioaccumulation in vegetables• Arsenic is a highly toxic element and its presence in food composites is a matter of
concern to the wellbeing of both humans and animals.
• The vegetables are important food crops and are rich in vitamins and minerals, which are very essential for maintaining good health.
• Widespread uses of As contaminated ground water for irrigation suggested that
ingestion of irrigated crops and vegetables could be another major exposure route for arsenic (WHO, 2001; Duxbury et al., 2004).
• Arsenic in the environment will be leached into the soil, absorbed by plants and further entering the food chain and affecting food safety.
• In comparison to other types of vegetable, root and tuber vegetables contained more inorganic arsenic, MMA (mono-methyl arsenate and DMA( di –methyl arsenate).
Fig. 2: Accumulation of arsenic in vegetables (Mean± SD)
050
100150200250300350400450
As
conc
in µ
g/kg
Arsenic concentration in rice and vegetables collected from the study area (µg kg-1 dry weight basis ± SD)
Sam
al e
t al.,
201
1
Normal daily average food intake of adults and children
Samal et al., 2011
Daily average intake of As by adults and children through consumption of contaminated rice, vegetables and pulses in the study area
Average daily intake of As through drinking and cooked water
Samal et al., 2011
Samal et al., 2011
Daily total arsenic intake through drinking, cooking water and foodstuffs in the study area (µg day-1 person -1)
Samal et al., 2011
Arsenic accumulation by the adults and children in the study area
Samal et al., 2011
(Bhattacharya, et al. 2010)
Concentrations of arsenic in the tissues of two varieties of rice plant (Red Minikit and Megi) in the five blocks of Nadia district
Arsenic contamination in rice and cooked rice
• The second-largest contributor of The second-largest contributor of Arsenic intake is food, notably rice after Arsenic intake is food, notably rice after As contaminated drinking water.As contaminated drinking water.
• High arsenic irrigated water and soil High arsenic irrigated water and soil appears to result in higher concentration appears to result in higher concentration of arsenic in root, stem and leaf of rice of arsenic in root, stem and leaf of rice plants (Abedin et al., 2002). plants (Abedin et al., 2002).
• Even if a rice sample does not contain Even if a rice sample does not contain any detectable amount of As, the cooked any detectable amount of As, the cooked rice however, contains a substantial rice however, contains a substantial amount of the element when it is cooked amount of the element when it is cooked with As contaminated waterwith As contaminated water
. Meharg et al., (2003) observed that . Meharg et al., (2003) observed that both As (III) and MMA are phytotoxic to both As (III) and MMA are phytotoxic to rice plants grown on nutrient solutions rice plants grown on nutrient solutions and the degree of arsenic uptake by rice and the degree of arsenic uptake by rice followed as As (III)>MMA>As(V)>DMA.followed as As (III)>MMA>As(V)>DMA.
Varietal variation of Arsenic Accumulation in Amon and Boro Rice
0
0.5
1
1.5
2
2.5
Rice straw Rice husk Rice grain
As c
onc
(mg/
kg)
Jaya
Nayanmani
0
0.5
1
1.5
2
2.5
Ricestraw
Rice husk Rice grain
As C
onc
(mg/
kg)
Jaya
Nayanmani
Amon Rice
Boro Rice
Arsenic Contamination Through Food Chain
• Arsenic in irrigation water poses a potential threat to soils and crops, the food chain generally, and consequently to human health
• Arsenic ingestion in human body besides drinking water is through food chain
• Arsenic transfer through aquatic food chains is the primary cause of observed impacts of arsenic on the higher trophic levels of aquatic systems.
• Crops receiving arsenic contaminated irrigation water take up this toxic element and accumulate it in different degrees depending on the species and variety
Toxicokinetics• Absorption
– Soluble forms• Humans – 40 % to complete absorption• Animals – 50% to complete absorption
– Insoluble forms• Limited absorption
Toxicokinetics cont.• Distribution
– Found in all humans – Blood conc. (1-5 ppb)• Smokers (2 – 10 ppb)• Occupational exposure (10 ppb)• Taiwan (20 – 60 ppb)• Poisonings (1,000 – 2,000 ppb)
Distribution• Highest levels (ppb)
– Nails (0.89)– Hair (0.18)– Bone (0.07 – 0.12)– Heart, kidney, liver, lung (0.03 – 0.05)
Metabolism of Inorganic Arsenic
ReductionMethylation
SAM
SAH
SAH
SAM
SAH
SAM
iAs5
iAs3
MAs5
MAs3
DMAs5
DMAs3
TMAs3
TMAs5
Excretion• Primarily via urine
– 60% - 95% in 5 days• Fecal excretion low
Acute ToxicityAnimal
RatsMice
Guinea pigsHumans
LD50 (mg/kg)
15 - 29326 - 43
91 - 4 (approx)
Acute Effects – Humans(est. LD50 1-4 mg/kg)
• Peripheral neuropathy• Anemia• Renal and liver dysfunction• Skin pigmentation• EKG abnormalities• Severe GI effects
Chronic Toxicity: HumansVascular
• Taiwan – Blackfoot disease
• Poland – Vintners – 6 cases of gangrene
• Chile– Raynaud’s disease
Chronic Toxicity: Humans• Nervous system
– Peripheral neuropathy – legs and arms• Cranial nerves
– Loss of hearing in Japanese infants
Normal Human Levels of Arsenic
• Source- (U.S. Department of Health and Human Services Public Health Service Agency for Toxic Substances and Disease Registry,2007 )
Levels of arsenic in unexposed individuals: < 1 μg/L in blood
<100 μg/L in urine
≤ 1 ppm in nails
≤ 1 ppm in hair
Arsenic Essentiality: A Role Affecting Methionine Metabolism
• Numerous studies with rats, hamsters, minipigs, goats and chicks have indicated that arsenic is an essential nutrient but Arsenic has not been tested for essentiality in humans nor has it been found to be required for any essential biochemical processes.
• Although there is no known biological function of arsenic, considerable evidence suggests that arsenic has a physiological role related to methionine metabolism in animals.
• Recent studies have suggested that arsenic has a physiological role that affects the formation of various metabolites of methionine metabolism including taurine and the polyamines, especially when methionine metabolism is stressed (e.g. pregnancy, lactation, methionine deficiency, vitamin B6 deprivation). The concentration of plasma taurine is decreased in arsenic-deprived rats and hamsters (Uthus ,2005)
Health Risk Of Arsenic Contamination
• Arsenic is one of the most important environmental global toxicants that cause acute and chronic adverse health effects, including cancer.
• In many countries exposure to arsenic is a daily occurrence because of its environmental pervasiveness and millions of people around the world have been, and are, exposed to arsenic through geologically contaminated drinking water.
• Epidemiological studies conducted since 1960s indicated that ingestion of inorganic As is linked to internal cancers in humans, including lung, bladder and kidney cancer.
• The evidence of health risk from As contamination is so compelling that in 2002 the EPA recommended lowering of the maximum contaminant level of As from 50 to 10 ug/L.
Countries Reporting Tumors After Arsenic Exposure
• Taiwan• Mexico• Argentina• Chile• China• Mongolia• Japan
Cancers Associated with Exposure to Arsenic in Drinking
Water• Skin• Bladder• Lung• Kidney• Liver• Prostate
Lifetime Risk of Cancer (per 1000)
0
10
20
30
40
50
Exce
ss L
ifetim
e R
isk
(x10
00) ED01 = Effective dose (central estimate) at which 1% of
population is affected by the contaminantLED01 = Lower limit of range with 95% certainty of being the effective dose for 1%MOE = Ratio of LED01 divided by MCL option (300/50) = 6
MCL50
LED01300
ED01
Point of Departure (PoD)
- - Margin of Exposure - -(MOE)
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