Environmental chemistry. E 12. water and soil. Water and soil. Solve problems relating to the removal of heavy-metal ions, phosphates and nitrates from water by chemical precipitation. State what is meant by the term cation -exchange capacity (CEC) and outline its importance.
E 12. water and soil
Pb2+ (aq) + 2Cl- (aq) → PbCl2 (s)
Cr3+ (aq) + PO43- (aq) → CrPO43 (s)
MX(s) aM+ (aq) + bX- (aq)
Kc = [M+(aq)]a [X− (aq)]b
Solubility product constant
Concentration or solubility (s) of an aqueous metal ion in a saturated solution if Kspis given.
The solubility of a compound if Ksp is given
1.0 x 10-27 = [Cd2+(aq)] x 0.5
[Cd2+(aq)] = 1.0 x 10-27 / 0.5 = 2.0 x 10-27
clay- - K+ (s) + H+ (aq) clay– - H+ (s) + K+ (aq)
RCOOH (humus) + K+ (aq) RCOOK (humus) + H+ (aq)
RCOOK (humus) + H+ (aq) RCOOH (humus) + K+ (aq)
At a low pH, H+ ions displace the exchangeable cations on the surface of the clay:
clay - Mg + 2H+ (aq) clay - H+Mg2+ (aq)
As a result these essential cations are not being adsorbed by the clay, lower CEC value, and are easily leached leaving the soil with fewer nutrients.
The hydroxide ions remove H+ ions from the hydroxyl group on the clay giving the clay a negative charge increasing CEC
clay - OH + OH- (aq) clay – O-+H2O