E.12.1 Solve problems relating to the removal of heavy-metal ions, phosphates and nitrates from water by chemical precipitation.
For any salt (even 'insoluble salts') formed from a metal M with a non-metal X,
MX(s) ↔ M+ (aq) + X- (aq)
Ksp (solubility product) = [M+][X-]
Many metal sulphides have low solubility products so to effectively precipitate heavy metal ions, hydrogen sulphide is bubbled through polluted water. The solubility product can then be used to calculate the amount of a metal ion that will remain in solution after it has been precipitated.
MS(s) ↔ M2+ (aq) + S2- (aq) [metal M has valency 2]
Ksp = [M2+][S2-] but [M2+]=[S2-]
Therefore Ksp = [M2+]^2
[M2+] = Ksp^0.5
By doing this, the precipitation of ions reduces the amount of ions left remaining in the polluted water. However, this can be further reduced through the common ion effect by adding more sulphide ions to the solution so that the concentration of lead ions is not the same as the concentration of the sulphide ions. For example, if the concentration of sulphide ions is one, then
Ksp = [M2+][1]
[M2+]= Ksp
And since metal sulphides have low solubility products, the concentration of metal ions will be equally low. However, when reducing using the common ion effect, care must be taken to ensure that soluble complex ions are not formed (e.g. zinc hydroxide is soluble in excess).
Phosphate can be precipitated as aluminium phosphate. For many heavy metal ions found in waste water, the process is slightly more complex as the salts formed are not binary but the principal is fundamentally the same.
M2+(aq) + 2OH-(aq) ↔M(OH)2(s)
Ksp= [M2+][OH-]^2
[OH-]=2[M2+]
Ksp= [M2+][2M2+]^2
=4[M2+]^3
[M2+]= (Ksp/4)^1/3
E.12.2 State what is meant by the term cation-exchange capacity (CEC) and outline its importance.
Cation Exchange Capacity is the amount of positively-charged cations that a soil can hold. Both the SOM and clay particles in the soil are negatively charged and hence attract and bond to cations. These ions are classified either as basic (e.g. Ca, Mg, Na, K) or acidic ions such as H or Al. These ions are exchanged with cations such as hydrogen ions on the root hairs of plants and thus provide nutrients to the plant.
E.12.3 Discuss the effects of soil pH on cation-exchange capacity and availability of nutrients.
Soil pH is determined by whether more acidic or more basic cations are present, and hence affects CEC. Natural rain water is itself acidic (because of how carbon dioxide dissolves to form carbonic acid) and has a pH of 5.6. This slight acidity makes it possible for more metal ions to dissolve. However, as pH decreases, nutrient cations in the soil may be replaced.
e.g. ZnS(s) + 2H+(aq) --> Zn2+(aq) + H2S(aq)
e.g. Zn(OH)2(s) + 2H+(aq) --> Zn2+(aq) +2H2O(l)
H+ from acid deposition can replace nutrient cations in the soil such as Zn2+ and Mg2+.
Alumina (aluminium oxide) is insoluble in water and reacts as a base with H+ ions to form soluble Al3+(aq) ions. In basic conditions, Fe3+ and Al3+ react with OH- to fom insoluble hydroxides. Hydrated ions with high charge densities attract the negative part of the water dipole to produce weakly acidic solutions.
[Al(H2O)6]3+ ↔ [Al(H2O)5OH]2+ + H+
Al3+ and Mn2+ are toxic to plants. Al3+ in pHs below 5 results in the displacement of important plant nutrient cations such as Ca2+ and Mg2+ from the soil.
3Mg2+(soil) + 2Al3+(aq) --> 2Al3+(soil) + 3Mg2+(aq)
(Mg2+(aq) etc. can be washed from the soil i.e. by irrigation, rainwater etc.).
In acidic soil, carbonates are converted into soluble ions and these can be lost to water drainage.
Adding lime/other carbonates to the soil increases pH and also replenishes the concentration of basic cations held by the clay and SOM.
Acidic soil below pH 5.5. affects microbes ability to fix NH4+ to N2, and hence these weakly acidic ions accumulate in the soil. Soil below pH 4 is too acidic and results in poor plant growth.
Soil pH that is too high causes the precipitation of ions such as Fe3+ to give insoluble hydroxides, leading to a lack of important nutrients.
Soil pH also affects the amount of phosphate ion present in solution. Phosphoric acid (triprotic acid) dissociates first at low pH (<4),>10), and hence plants absorb inorganic phosphorus from the soil as H2PO4- (first dissociate) and HPO4 2- (second dissociate).
Micronutrients such as iron, manganese and zinc are available in acidic soil but decrease as pH increases as they precipitate out as hydroxides. Ammonium sulphate can be added to soil that is too basic as it is weakly acidic:
NH4+(aq) ↔ NH3(aq) + H+(aq)
SOM contains humic acids that exhibit buffering capacity over the pH 6.5 to 7.5. e.g. weakly carboxylic acids
RCOOH(aq) ↔ H+(aq) + RCOO-(aq)
E.12.4 Describe the chemical functions of soil organic matter (SOM).
SOM has the ability to increase CEC, behaves as a buffer (controls soil pH), and removes heavy metals and pesticides through its chelating abilities. The presence of anions in SOM from carboxylic acids and phenolic functional groups (RCOO- and ArO-) prevent nutrient cations from precipitating out as insoluble compounds. SOM hence acts as a nutrient reservior for plants but is unable to bind to nitrates, sulphates and phosphates and therefore these must be made available to plants through microbial activity in the SOM (enhanced by large surface area of SOM).
Phosphorus: required for early root development and growth, if not yellow leaves & stunted growth. Main source is from calcium phosphate. Organic phosphate in the soil is relatively immobile and is slowly converted into inorganic phosphate (mineralization), and SOM enhances phosphorus availability through its chelating ability.
SOM also chelates to toxic cations such as Al3+ and heavy metal ions. Pesticides are deactivated as they are broken down by soil organisms. By binding to contaminants, less pollution reaches and affects the water supply. SOM also binds to minerals, sometimes forming stable complexes with cations, to make them available to plants rather than preipitating out as insoluble salts/being leached.
