ENVIRONMENTAL CHEMISTRYE 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.

Discuss the effects of soil pH on cation-exchange capacity and availability of nutrients.

Describe the chemical functions of soil organic matter (SOM).

CHEMICAL PRECIPITATION Heavy metal ions can be removed by reacting them

with anions, such as OH-, Cl- , PO43- and S2-, that

will make a salt with a low solubility; the heavy metal ions can then be filtered or precipitated out.

Examples of such precipitation reactions are:

Pb2+ (aq) + 2Cl- (aq) → PbCl2 (s)

Cr3+ (aq) + PO43- (aq) → CrPO4

3 (s)

CHEMICAL PRECIPITATION: HOW MUCH? The solubility product constant allows us to calculate how

much of the anion needs to be added to precipitate out a metal ion. This is possible because dissolution of a salt is an equilibrium process as a dynamic equilibrium is set up between the salt and its aqueous ions as shown below:

MX(s) aM+ (aq) + bX- (aq) (metal M = metal ion and X = non metal ion)

The equilibrium expression for this heterogenous system would be

Kc = [M+(aq)]a [X− (aq)]b

[MX(s)] is a constant as it is a solid!

SOLUBILITY PRODUCT Ksp only changes with temperature.

Its unit is mol2 dm-6.

The Ksp value of an ionic compound is a measure of how soluble it is in water. In general: low Ksp = low solubility high Ksp = high solubility solubiity product constants

A saturated solution is when the product of the concentrations of the aqueous ions according to the solubility product expression = Ksp at that temperature.

Amount of ion above the solubility product is precipitated.

SOLUBILITY PRODUCT CALCULATIONS

Calculating ion concentration of metal ion in a saturated solution when Ksp is givenIf MX(s) M+ (aq) + X- (aq) then…

Ksp = [M+(aq)] x [X−(aq)] or as [M+(aq)] = [X−(aq)]  then Ksp = [M+(aq)]2

and [M+(aq)] = Ksp

SOLUBILITY PRODUCT CALCULATIONS

Calculating ion concentration of metal ion in a saturated solution when Ksp is given

If MX2(s) M2+ (aq) + 2X- (aq) then …

Ksp = [M2+(aq)] x [X− (aq)]2 as [X− (aq)] = 2 x [M2+(aq)] then Ksp = [M2+(aq)] x 2 [M2+(aq)]2

 

and Ksp = 4[M2+(aq)]3 and [M2+(aq)] = (Ksp/4)1/3

SOLUBILITY PRODUCT CALCULATIONS

Calculating ion concentration of metal ion in a saturated solution when Ksp is given

If MX3 (s) M3+ (aq) + 3X- (aq) then…

Ksp = [M3+(aq)] x [X− (aq)]3 as [X− (aq)] = 3 x [M3+

(aq)] then Ksp = [M3+(aq)] x 3 [M3+(aq)]3

 

and Ksp = 27 [M3+(aq)]4 and [M3+(aq)] = (Ksp/27)1/4

COMMON ION EFFECT To precipitate out a salt, the concentrations of the

aqueous ions needs to be greater than the solubility product.!!! The equilibrium shifts to the left producing more insoluble salt (s) and decreasing the concentration of the ions.

In practical terms, if a metal ion, e.g. Cr3+, needs to be removed from a solution, then another solution with the same non-metal ion, e.g. OH-, as a chromium salt with a low Ksp, e.g. Cr(OH)3, needs to be added.

The OH- is considered the ion common to both the chromium salt with low solubility, Cr(OH)3 and the solution added, e.g. NaOH.

CATION EXCHANGE CAPACITY - CEC (1) cations such as K+, Ca2+ and Mg2+

CEC is an indicator of the fertility of a soil.

it is the clay (mainly) and humus in a soil that give the soil its CEC.

CEC is the amount of exchangeable cations a soil can hold, usually in clay

measured in millequivalent (mg) of H+ usually per 100 g of soil

the higher the CEC the more fertile the soil.

CEC

CEC (2) plants need to absorb cations they do this through nutrient exchange with the soil at

their root hairs; exchange between H+ and K+ or Mg2+ or Ca2+.

the amount of cations the soil can exchange depends on amount of cations it adsorbs in the first place.

most important factor that affects the amount of cations a soil can adsorb is the amount of clay or humus/SOM.

if cations are not adsorbed they are easily washed away (=leached) e.g. in sandy soils.

measurements of CEC

CEC (3) clay has a layer structure which has an overall

negative charge.

this negative charge attracts cations to surface of the clay sheets to balance out the negative charge.

cations can be exchanged for hydrogen ions, H+ (aq), at the roots of plants.

this exchange is facilitated by the large surface area of clay.

CEC (4) – NEGATIVE CHARGE ON CLAY The net negative charge on clay occurs as a result

of silicon atoms (oxidation number +4) being replaced by aluminium atoms (+3) or even iron atoms (+2) which do not balance out the negative charges of all the oxygen atoms.

A clay with more Fe atoms has a greater CEC than a clay with many aluminium atoms.

CEC (5) - HUMUS

Humus/SOM contains weak organic acids, RCOOH, which also can exchange cations as shown below and therefore contribute to the CEC of a soil.

RCOOH (humus) + K+ (aq) RCOOK (humus) + H+ (aq)

At the roots of plants during cation exchange the reverse reaction occurs.

EFFECT OF PH ON CEC Low pH lowers CEC 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.

  High pH increases CEC 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

EFFECT OF PH ON AVAILABILITY (1) Ionic nutrients are available if they are aqueous and

adsorbed onto clay or SOM. If the ions are insoluble they are unavailable as nutrients; they are also unavailable if they are aqueous but not adsorbed onto clay or humus as they are then likely to be leached from the soil.

The best availability of nutrients is between pH 6 and 6.5; around neutral.

PH AND AVAILABILITYion low pH neutral high pH

Ca2+/Mg2+

low availability as H+ ions displace Ca2+/Mg2+ from

clay surfaces and Ca2+/Mg2 are washed

away.maximum availability

not available as they form insoluble

carbonates or phosphates

Fe3+/Al3+ maximum availability form insoluble hydroxides

PO43-

most available apart from Fe3+/Al3+ which form

insoluble phosphates maximum availability

forms insoluble phosphates with

Ca2+

PH AND AVAILABILITYion low pH neutral high pH

NO3-

NO3- is reduced to

NH4+ which is not

available to plantsHalf-equation of reduction:NO3

- +10H++ 8e- → NH4

+ +3H2OLess nitrogen available to plants

maximum availability

maximum availability of nitrate – some NH4

+ lost as NH3 (g) at higher pH:NH4

+ (aq) + OH- (aq) → H2O (l) + NH3 (g)

K+

washed away at low pH as displaced by

H+

maximum availability

Cu2+/Zn2+

maximum availability

unavailable as forms insoluble hydroxides

CHEMICAL FUNCTIONS OF SOM

Contributes to cation-exchange capacity (CEC) as they form stable complexes with cations.

Enhances the ability of soil to buffer changes in pH.

Reduces the negative environmental effects of pesticides, heavy metals and other pollutants by binding contaminants.

Binds to organic and inorganic compounds in the soil preventing nutrients from easily being washed away