Water and environmental chemistry

Water from a river or reservoir is not safe to drink straight away. To make it potable (safe to drink) it is treated in stages:

• Sedimentation and filtration: grit settles out, then the water passes through beds of sand and gravel.
• Sterilisation: chlorine, ozone or ultraviolet light kills bacteria. A little chlorine is left in so the water stays clean in the pipes.

Where fresh water is scarce, sea water can be desalinated by distillation or reverse osmosis. Both need a lot of energy, so they are expensive.

Hard water contains dissolved calcium and magnesium ions, picked up as rain trickles through limestone and chalk. It makes a grey scum with soap instead of a lather and leaves limescale (mostly calcium carbonate) in kettles and boilers. On the plus side, the calcium is good for teeth and bones.

Pollutants include fertiliser run-off, which feeds algae in rivers, and metals such as lead and mercury, which are toxic in tiny amounts. The most troublesome pollutants do not break down and slowly build up in the fat of animals.

Hardness

Rain carrying dissolved CO_2 slowly dissolves limestone:

Temporary hardness comes from these hydrogencarbonate ions. Boiling drives off CO_2, the equilibrium shifts left and CaCO_3 precipitates as scale. Permanent hardness comes from calcium and magnesium sulfates or chlorides, which boiling cannot remove. Both kinds are removed by an ion-exchange resin, which swaps each Ca^{2+} for two Na^+, or by adding washing soda so calcium precipitates as the carbonate. Hardness is measured by titration with EDTA at about pH 10 using Eriochrome Black T as indicator.

Chlorination

Chlorine disproportionates in water:

Chlorine goes from oxidation state 0 to -1 in HCl and +1 in chloric(I) acid, HClO, which is the disinfectant. The catch is that HClO also attacks natural organic matter, giving small amounts of chlorinated by-products such as trichloromethane, so their levels are monitored.

Fate of pollutants

Whether a pollutant does long-term harm depends on how slowly it breaks down (persistence), how much more soluble it is in fat than in water, and how easily it moves through air and water. A fat-soluble, persistent compound collects in tissue faster than the organism can excrete it (bioaccumulation), and each step up a food chain concentrates it further (biomagnification). Levels are usually tiny, so they are quoted in ppm (mg kg^{-1}) or ppb.

Partitioning

Fate is predicted from a few equilibrium constants. The octanol-water partition coefficient

stands in for fat solubility. Compounds with \log K_{\text{ow}} of about 5 to 7, such as DDT and many PCBs, partition into lipids and sediment, while polar compounds stay dissolved and travel with the water. K_{\text{oc}} describes binding to soil organic carbon, and the Henry's law constant sets the air-water split. Semi-volatile compounds evaporate in warm regions and condense in cold ones, hopping repeatedly until they reach the Arctic.

The bioconcentration factor, \text{BCF} = c_{\text{organism}}/c_{\text{water}} at steady state, covers uptake from water alone; biomagnification adds diet, so a predator can carry concentrations orders of magnitude above its surroundings. Methylmercury, made from inorganic mercury by sediment bacteria, binds to thiol groups in proteins rather than dissolving in fat, and accumulates in tuna and swordfish.

Persistence as kinetics

Hydrolysis, photolysis and microbial breakdown are often treated as pseudo-first-order: c = c_0 e^{-kt}, t_{1/2} = \ln 2/k.

Worked example. A herbicide has a soil half-life of 60 days, so k = 0.693/60 = 0.0116 day^{-1}. After a year, c/c_0 = e^{-0.0116 \times 365} \approx 0.015: about 1.5% remains. A PCB with a half-life of several years barely changes in the same time. The 2001 Stockholm Convention restricts persistent organic pollutants on these criteria: persistence, bioaccumulation, long-range transport and toxicity.

PFAS take persistence to the extreme. The C-F bond is among the strongest single bonds carbon forms, and perfluorinated chains offer no C-H bonds for radicals or enzymes to attack. PFOS joined the Stockholm list in 2009. Being surfactants, PFAS bind to blood proteins such as serum albumin instead of dissolving in fat, so K_{\text{ow}} predicts their behaviour poorly.

Trace analysis

Parts-per-trillion work needs extraction, preconcentration and a selective detector. Organochlorine pesticides are classically measured by gas chromatography with an electron capture detector: a \beta source (usually ^{63}Ni) maintains a standing electron current, and halogenated analytes capture electrons so the current dips. It is far more sensitive to halogenated compounds than a flame ionisation detector and almost blind to hydrocarbons. GC-MS confirms identity from fragmentation; ionic, involatile PFAS need LC-MS/MS, and metals are measured by ICP-MS.