Chemistry 2nd Paper · Classes 11-12 · Chapter 1 · 1.22

Heavy Metals and the Food Chain.

The chicken on your plate may carry chromium, though the bird never went near a tannery. The metal came along the food route. In this note we follow arsenic, chromium, lead and cadmium into the food chain, and see which one ruins which part of the body.

1. The food chain

Producers, the autotrophs, make their own food. Energy then flows from them to one consumer after another, and that flow is the food chain. Consumers come in three levels: herbivores are the first, carnivores the second, omnivores the third. Along with energy, poison can travel down the chain too, and the poisons of this note are four heavy metals.

2. Arsenic

Soil holds on average 5.6 ppm of arsenic. Plants absorb it, it passes to herbivores, and then through fish and meat it reaches humans as As³⁺. From surface water it enters plankton, and from plankton it enters fish. Pesticides and wood preservatives are sources of arsenic as well.

Air carries arsenic too, but it varies a great deal with place: 1 to 4 ng per cubic metre in villages, 200 ng in cities, and above 1000 ng in industrial areas.

The WHO tolerance is 0.05 ppm. Above it the damage begins: black-foot disease in the feet, fat collecting in the liver as fatty liver, and lung and skin cancer. A pregnant mother's foetus grows poorly, and birth defects and gene mutation are seen as well.

3. Chromium

Chromium has four main sources. Tanneries cure hides with chromic sulphate, which is called chrome tanning. Copper dichromate serves as a wood preservative. In chrome pigments the yellow is CrO₄²⁻ and the orange is Cr₂O₇²⁻. And stainless steel welding gives off chromium as well.

Cr enters the body by breathing, water and food, and through the liquid waste of tanneries. At Hazaribagh 200 tanneries send out 77 lakh litres of liquid waste with chromic salt every day.

Waste leather has a path into the food chain. It goes into poultry feed, Cr³⁺ collects in ducks and hens, and humans eat them. Put the path in order yourself.

Chromium harms the digestive and respiratory systems and reproduction. Cr(VI) is a carcinogen, causing lung cancer, and it is more deadly than Cr(III). About 4000 Hazaribagh workers are affected, and workers in the Kanpur tannery get respiratory disease.

The cause of the anaemia is chemical. Cr³⁺ blocks the uptake of Fe²⁺ in red blood cells, and it also takes the place of Fe²⁺ in the haem of haemoglobin. With chromium sitting where iron should be, haemoglobin cannot do its work.

There are four remedies: reduce Cr(VI) to Cr(III), adsorb it on bone charcoal, use Arthrobacter bacteria, and fit an ETP in the tannery.

4. Lead

Lead comes from lead-acid storage batteries (which hold PbO₂), burning coal, and lead arsenate sprayed as a herbicide. Tetraalkyl lead (PbR₄) is added to petrol to raise the octane number, and it settles as PbO after burning.

The WHO says lead poisoning begins above 50 ppb. Signs: the gums turn bluish, haemoglobin production is blocked and anaemia follows, a pregnant woman may bear a stillborn child, and a child's IQ falls.

5. Cadmium

Cd lies mixed in with Zn, Pb and Cu ores. It also comes from solder, phosphate fertiliser, and PVC stabiliser and pigments. One NiCd battery holds about 5 g of Cd.

Cd enters the body through the air we breathe, potatoes and cereals, fish and meat (it collects in the kidney), smoking and tobacco. A smoker's blood carries 4 to 5 times more Cd. The WHO lethal dose is just 1 gram.

Brittle bones come from a match in size. Ca²⁺ and Cd²⁺ are the same size, so Cd takes the place of Ca in bone. The form of this osteoporosis is Itai-Itai (Ouch-Ouch) disease, seen near the Jinzu river in Japan in 1968, where about 600 µg of Cd a day went into the body.

Cd damages the kidney; urinary Cd of 2 to 3 µg/g creatinine is the sign of harm. By the IARC it is a human carcinogen, group I, causing prostate and lung cancer. In zinc enzymes Cd²⁺ replaces Zn²⁺, because the two metals are alike.

Now match the sources. Batteries are a source of two metals, so the two kinds of battery are separate chips.

6. Four metals, four diseases

The book's summary at a glance: As³⁺ causes black foot and keratinisation. Cr³⁺ blocks haemoglobin. Pb²⁺ blocks haemoglobin too, with stillbirth and lower IQ. Cd²⁺ causes osteoporosis, lung cancer and kidney stones, and Ca²⁺ and Zn²⁺ are lost from the body. Haemoglobin trouble belongs to two metals, Cr and Pb, so that effect appears twice below.

7. Check yourself

Three questions in the exam's style. Start with the first.

In the second, keep straight which metal does what.

The last is one more pairing of metal and effect.

The whole note on seven cards.

Source: Chemistry 2nd Paper, Classes 11-12 (NCTB), Chapter 1 “Environmental Chemistry”, section 1.22.