4. Mercury in drinking water Data from: “Mercury in Drinking-water\" by the WHO

Question

4. Mercury in drinking water Data from: “Mercury in Drinking-water" by the WHO On the assumption of an ambient air level of 10 ng/m3, the average daily intake of inorganic mercury by inhalation would amount to about 0.2 g. If a level in drinking-water of 1 g/liter is assumed, the average daily intake of inorganic mercury from this source would amount to about 2 g. The average daily intake of mercury from food is in the range 2-20 Activated carbon, both powdered and granulated, is effective for the removal of inorganic and organic mercury. Ion exchange and reverse osmosis are also useful for mercury removal to reach a concentration below 1 g/liter by treatment of raw waters that are not grossly contaminated with mercury a) Express 1 ug/liter in ppm b) Express 1 g/liter in nM c) For regions with mercury contaminated waterways with 10 ug/liter mercury and a heavy fishing population, approximate the percent increase in daily mercury intake. d) Approximate how water filtration to 1 g/liter would impact daily mercury intake. Discuss

Explanation / Answer

(a). 1 microgram = 0.001 mg that means

1 mg = 1000 micrograms

and 1ppm = 1mg/litre=1000microgram/litre

Therefore, 1 microgram/litre=0.001ppm

(b). In molar terms,

1 microgram/litre=0.000001gram/litre = (0.000001gram/200.59gram)/litre = 4.98529e-9 moles/litre = 4.98529 nM (M is moles/litre)

Note here first we get no of moles by dividing mass by atomic mass of mercury i.e. 200.59 then convert Molarity 'M' into nano(10^-9)Molarity i.e. nM

(c). For mercury contaminated waterways, increase in mercury = (10-1) microgram/litre = 9 microgram/litre

Percentage Increase = (9 microgram/litre)*100/(1 microgram/litre) = 90%

Note here only water contamination is taken in consideration as fish consumption may or may not affect the increase.This is fine.

We can solve it as under also:

Normal mercury intake = (from air)+(from water)+(from food) =(0.2+2+2) = 4.2 microgram/litre

For contaminated waterways,Total Mercury intake= 0.2+20+2=22.2 microgram/litre

Then percent increase in total mercury intake = (22.2-4.2)*100/4.2 = 428.57%

Also if only water contamination is considered then percent increase = (20-2)*100/2= 90%

(d). If filtration is done to this contaminated waterways then mercury intake from drinking water will reduce back to 2 microgram/litre and total to 4.2 microgram/litre.

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