Environmental Chemistry: Equilibrium, Partitioning and Transformation
Environmental Chemistry in GATE ES tests how contaminants speciate, partition between phases, and transform in air, water and soil. Expect numerical questions on equilibrium, redox, acid-base chemistry and kinetics rather than descriptive facts.
Core rules and concepts
- Equilibria: use Ksp for sparingly soluble salts (CaCO3, Al(OH)3, Fe(OH)3), Ka for acid-base speciation, and the carbonate system defined by KH, K1 and K2.
- Alkalinity vs hardness: alkalinity is acid-neutralising capacity (HCO3-, CO3^2-, OH-, minus H+); hardness is Ca2+ plus Mg2+ reported as CaCO3. They are independent measures.
- Redox: pe = -log{a(e-)}; at 25 °C pe = Eh/0.0592. Pourbaix (pe-pH) diagrams delimit stable species.
- Partitioning: KOW for bioconcentration, KOC for soil sorption (Kd = KOC x fOC), Henry's law for air-water exchange, and fugacity for multi-phase equilibrium.
- Kinetics: first-order decay C = C0 e^(-kt), half-life t1/2 = 0.693/k; BOD exertion is the same exponential form.
Worked example
Trichloroethylene has H = 0.0098 atm m3/mol at 25 °C. Convert to a dimensionless air-water ratio: Hcc = H/(RT) = 0.0098/(0.08206 x 298) = 4.0 x 10^-4. If the water concentration is 100 ug/L, equilibrium air concentration is 4.0 x 10^-4 x 100 = 0.04 ug/L. The reasoning step that matters is converting a pressure-based Henry constant into a concentration ratio; skipping it produces an error of roughly a factor RT (about 24.5).
Traps that cost marks
Treating alkalinity and hardness as synonyms; ignoring activity corrections at high ionic strength; equating log KOW with KOC without the organic-carbon fraction; forgetting that H and Ksp are temperature dependent; and confusing total concentration with dissolved concentration when sorption is significant.