Environmental Chemistry III: Water and Soil Chemistry — the Hydrological Cycle and Water as a Solvent, DO, BOD and COD, the Chemistry of Coagulation, Sedimentation and Filtration, pH and Redox Potential, Soil Components and the Nitrogen, Carbon, Phosphorus and Sulphur Cycles, and Pesticides, Heavy Metals and Metalloids

Dissolved oxygen (DO) is the oxygen dissolved in water, measured by the Winkler iodometric titration (with the azide modification to remove nitrite interference) or by a membrane electrode. Its saturation value is only about 9 mg/L at 20 °C, and most fish need more than about 4 to 5 mg/L, so DO is the single best indicator of the health of a stream. Biochemical oxygen demand (BOD) is the oxygen consumed by micro-organisms while they oxidise the biodegradable organic matter in a sample, measured as the fall in DO over an incubation in the dark, conventionally 5 days at 20 °C (BOD₅); Indian standard methods also use 3 days at 27 °C, which gives a comparable figure in a warmer climate. Because a strong waste would use up all the oxygen in the bottle, the sample is diluted with aerated, seeded dilution water, and BOD = (DOᵢ − DOf) ÷ P, where P is the decimal fraction of sample in the bottle. If 5 mL of wastewater is diluted to 300 mL, P = 5 ÷ 300 = 0.0167; if DO falls from 8.8 to 3.8 mg/L, BOD = 5.0 ÷ 0.0167 = 300 mg/L.

BOD is exerted gradually and is modelled as a first-order reaction: BODt = L₀(1 − e^(−kt)), where L₀ is the ultimate (carbonaceous) BOD and k the rate constant, typically about 0.23 per day (base e) at 20 °C for domestic sewage. Then BOD₅ ÷ L₀ = 1 − e^(−1.15) ≈ 0.68, so about two-thirds of the ultimate demand is exerted in five days; a sewage with L₀ = 300 mg/L has a BOD₅ of about 205 mg/L. After about 8 to 10 days nitrifying bacteria add a second, nitrogenous stage of demand, which is one reason the test is stopped at five days or nitrification is inhibited. Chemical oxygen demand (COD) is the oxygen equivalent of the organic matter oxidised by boiling acid potassium dichromate for two hours, with silver sulphate as catalyst and mercuric sulphate to bind chloride. It takes hours rather than days and oxidises nearly all organics, biodegradable or not, so COD is always greater than or equal to BOD. The BOD/COD ratio indicates biodegradability: around 0.5 or more for domestic sewage, readily treatable biologically; below about 0.3 for many industrial wastes, which need chemical treatment.

Turbidity in natural water is caused largely by colloidal particles, roughly 1 nm to 1 µm, that carry a negative surface charge. Each is surrounded by an electrical double layer, and the repulsion between like charges, measured by the zeta potential, keeps them apart so that they never settle. Coagulation destabilises them by adding a coagulant, usually alum (aluminium sulphate) or ferric chloride, whose trivalent cations compress the double layer and neutralise the charge; at the doses normally used, the metal hydroxide precipitate also enmeshes particles as it forms, which is sweep flocculation, and long-chain polyelectrolytes add interparticle bridging. With natural alkalinity, Al₂(SO₄)₃·14H₂O + 3Ca(HCO₃)₂ → 2Al(OH)₃ + 3CaSO₄ + 6CO₂ + 14H₂O, so alum consumes alkalinity, about 0.5 mg/L as CaCO₃ for each mg/L of alum, and lowers pH; lime or soda ash is added when the water is soft. The optimum dose and pH are found by a jar test. Flocculation follows: slow, gentle mixing lets the destabilised particles collide and grow into settleable flocs.

Sedimentation removes the flocs by gravity. A discrete particle settles at a terminal velocity given by Stokes' law, vs = g(ρp − ρw)d² ÷ 18µ, so settling depends on the square of diameter: doubling the size quadruples the speed, which is why coagulation, by making particles bigger, makes them settle. A sand grain of 20 µm and density 2,650 kg/m³ in water at 20 °C (µ ≈ 1.0 × 10⁻³ Pa s) settles at 9.81 × 1650 × (2 × 10⁻⁵)² ÷ (18 × 10⁻³) ≈ 3.6 × 10⁻⁴ m/s, about 0.36 mm/s. Filtration then removes what remains. Slow sand filters work at low rates with a biological layer, the schmutzdecke, that forms on the sand surface and removes pathogens; rapid sand filters work many times faster, rely on physical straining and adsorption within the bed after coagulation, and are cleaned by backwashing.

pH, the negative logarithm of hydrogen-ion activity, controls the solubility of metals (most are more soluble in acid), the speciation of ammonia (toxic un-ionised NH₃ rises with pH), the effectiveness of chlorination and the carbonate equilibria. The redox potential, Eh, measured in volts against the standard hydrogen electrode, expresses the tendency of an environment to accept or donate electrons. It follows the Nernst equation, Eh = E° − (0.0592 ÷ n) log Q at 25 °C, and is sometimes written as pe = Eh ÷ 0.0592. A well-aerated surface water is oxidising, with Eh around +0.4 to +0.6 V; waterlogged soils, sediments and landfills are reducing, with Eh that can fall below −0.2 V. Eh-pH (Pourbaix) diagrams show which form of an element, iron for example, is stable under each combination.

The ladder explains one of the most serious water-quality problems of the Indian subcontinent. In the young deltaic aquifers of the Bengal basin, organic matter drives the groundwater down the ladder until iron(III) oxyhydroxides, onto which naturally occurring arsenic is adsorbed, are reductively dissolved, releasing arsenic into wells. Arsenic enrichment is therefore associated with reducing, iron-rich groundwater rather than with any single industrial source.

A typical mineral soil is roughly half solids and half pore space shared between water and air; of the solids most is mineral and a few per cent organic. The inorganic fraction consists of primary minerals such as quartz and feldspars inherited from rock and secondary minerals, above all the clays and the oxides of iron and aluminium, formed by weathering; clays and humus carry the negative charge that gives soil its cation exchange capacity. The organic fraction ranges from fresh residues to humus, a dark, resistant mixture conventionally separated by solubility into fulvic acid (soluble in acid and alkali), humic acid (soluble in alkali only) and humin (insoluble). Soil organic matter is about 58% carbon, which is why organic carbon is multiplied by the van Bemmelen factor of 1.724 to estimate organic matter. Humus binds metals and pesticides, holds water and nutrients and stabilises soil structure.

Pesticides are also classified by hazard. The WHO classes run from Ia (extremely hazardous) and Ib (highly hazardous) through II (moderately) and III (slightly) to U (unlikely to present acute hazard). In India the Insecticides Rules 1971 require a coloured triangle on the label: red for extremely toxic, yellow for highly toxic, blue for moderately toxic and green for slightly toxic. Rachel Carson's Silent Spring (1962), on the effects of DDT on birds, began the modern environmental movement.