Tubewell water is the water we trust. Yet when tubewells in 64 districts were tested in 1998, arsenic turned up in 61 of them. Where does this poison come from, how much is too much, and what does it do inside the body? This note works through exactly that.
1. How arsenic gets into the water underground
This is natural pollution: the arsenic is already in the ground as minerals. The book lists seven: NiAs (niccolite), NiAsS (nickel glance), As₂S₃ (orpiment), As₂S₂ (realgar), CoAsS (cobaltite), Fe₂S₂As₂ (arsenopyrite) and As₂O₃ (arsenolite). Learn the formulae as the book prints them.
From the minerals, arsenic reaches the groundwater. Al₂O₃ and iron oxide in the soil hold arsenic back, so it is not released easily. But when the soil has less clay, there is more arsenic.
The pH of the water decides the form the arsenic takes. At pH 6.7 it is present as H₂AsO₄⁻, and at pH 8.9 as HAsO₄²⁻. The book gives these two pH values and no others.
Now the bacteria. Anaerobic bacteria reduce As(V) to arsenite AsO₂⁻ (As(III)), and that arsenite is held by Fe²⁺ and Al³⁺ in the soil. In the other direction, aerobic bacteria oxidise As(III) to As(V).
If the reduction by anaerobic bacteria goes further, organic compounds form: dimethylarsine (CH₃)₂AsH and trimethylarsine (CH₃)₃As.
Drop each of the six conditions into the box that holds its result. For the bacteria, compare the oxidation state of arsenic before and after.
match themaerobic bacteria oxidise
What happens where
Pick up a condition, then drop it in the box with its result.
Right. Next one.
Not yet. For bacteria, compare the oxidation state before and after: As(V) to As(III) is reduction, the reverse is oxidation. For pH, recall which pH goes with which form.
All in place. Anaerobic bacteria reduce, aerobic bacteria oxidise. pH 6.7 gives H₂AsO₄⁻, pH 8.9 gives HAsO₄²⁻. Al₂O₃ and iron oxide hold arsenic, and less clay means more arsenic.
2. The forms of arsenic, and how poisonous each is
The book gives five forms of arsenic: elemental As (a metalloid), arsine AsH₃, arsenic oxide As₂O₃, arsenite AsO₂⁻ and arsenate AsO₄³⁻, in which arsenic is in the +5 state.
The five forms are not equally poisonous. The order of toxicity is arsine gas > arsenite (inorganic) > arsenic oxide (organic, trivalent) > arsenate (inorganic) > arsenic (metalloid). Notice that the book labels arsenic oxide “organic, trivalent”, so the chip carries that label too. Put them in order.
which firstthe gas comes first
The most poisonous first
Start with the most poisonous form. It is a gas.
Yes. Who comes next?
Not yet. Another form comes before this one. Arsine gas is first, and the metalloid As is last.
Right order: arsine, arsenite, arsenic oxide, arsenate, arsenic.
3. How much is too much: the limits
Arsenic is never quite absent. Its concentration in soil is 5.6 mg/kg, and the average of inorganic arsenic in drinking water is 2.5 ppb.
By WHO the maximum tolerable arsenic in water is 0.05 mg/L (ppm), and the safe value is 0.01 mg/L. Do not mix up the two: 0.05 is the tolerable limit, 0.01 is the safe value. When a question says “maximum tolerable”, the answer is 0.05, following the book.
In Bangladesh the maximum acceptable arsenic in drinking water is 0.05 ppm. The international TLV is 0.04 to 0.05 ppm. Other countries set stricter limits: Germany 0.01 ppm (1996) and America 0.002 ppm (1992).
The damage has been seen in other countries too: arsenic pollution in Taiwan and Chile has caused black-foot disease.
4. The picture in Bangladesh, and the remedy
In 1998 tubewells in the 64 districts of Bangladesh were tested, 2022 in all. Arsenic was found in 61 districts. In 51% of tubewells As was above 0.01 ppm, in 35% above 0.05 ppm, and in 14% between 0.1 and 1.0 ppm. Learn the percentages as the book prints them.
The worst case is Sharsha thana in Jashore: deep tubewell water there has the highest arsenic pollution.
The simple remedy is marking. A polluted tubewell gets a red mark (dangerous), a safe one a green mark. The problem does not stop at drinking: arsenic collects in plants and from there passes into the food chain.
5. What arsenic does to the body
Arsenic is a human carcinogen: it causes cancer of the lung, liver, kidney and skin. Acute poisoning, the kind that comes quickly, harms the digestive system and causes diarrhoea.
The slow, accumulating chronic poisoning is called arsenicosis. Its signs are dark spots on the palms and soles, gangrene, black-foot disease and keratinisation. The book places black-foot disease under chronic poisoning and also under the Taiwan and Chile cases. Both are right.
Drop each effect into its box: acute, chronic or cancer.
sort themdiarrhoea comes quickly
Fast or slow
Pick up an effect, then drop it in a box. Ask: does it come quickly, build up slowly, or is it a cancer?
Right. Next one.
Not yet. Think how fast it appears: diarrhoea and digestive harm come quickly, signs like dark spots and gangrene build up slowly, and cancer has its own box.
All in place. Diarrhoea and digestive harm are acute. Dark spots, gangrene, black-foot disease and keratinisation are chronic, that is arsenicosis. Black-foot disease was also seen in Taiwan and Chile.
6. Check yourself
Three questions in the exam's style. Start with the first.
check yourselfpick one answer
Question 1
According to WHO statistics, what is the maximum level of arsenic in ppm?
0.01 mg/L is the book's WHO safe value, not the maximum tolerable value.
0.04 is the lower end of the international TLV (0.04 to 0.05), not the WHO maximum.
Right. By the book the WHO maximum tolerable level is 0.05 mg/L (ppm).
0.06 is not a limit in the book; the maximum is 0.05.
In the second, keep the numbers apart.
check yourselfpick one answer
Question 2
What change do anaerobic bacteria cause in the arsenic of the soil?
Aerobic bacteria oxidise As(III) to As(V), not anaerobic ones.
Forming As₂O₅ is not stated in the book; reduction occurs, not oxidation.
Right. Anaerobic bacteria reduce As(V) to arsenite AsO₂⁻ (As(III)).
Arsenic does not give CO₂; there is no such link.
Think the last one over a little longer.
check yourselfpick one answer
Question 3
What is the acceptable level of arsenic in the drinking water of Bangladesh?
0.01 ppm is Germany's limit and the WHO safe value, not the Bangladesh limit.
Right. The maximum acceptable arsenic in drinking water in Bangladesh is 0.05 ppm.
0.25 ppm is no limit in the book; it is far too high.
0.50 ppm is no limit in the book; it is ten times too high.
The whole note on eight cards.
cut from the notetap to turn over
Eight cards
QuestionIn what form is arsenic at pH 6.7 and pH 8.9?
AnswerH₂AsO₄⁻ at pH 6.7; HAsO₄²⁻ at pH 8.9.
QuestionWhat change do anaerobic and aerobic bacteria cause in arsenic?