Arsenic in the Environment: Biochemical Effects on Plants

and Human Health

Department of Environmental Science

University of KalyaniKalyani, Nadia

scsantra@yahoo.com

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)

Thank You