Chapter 14.
Environmental Chemistry of Colloids
And Surfaces
Size classification of materials in the hydrosphere
Boundary for ‘soluble’ and ‘insoluble’
14.0 Generals
Specific surface area
14.0 Generals
Colloid surface:
Adsorption phenomena due to electrostatic attraction to a charged surface
1) clay minerals have a negatively charged surface (relatively const. in magnitude), but in usual cases,
2) the nature of the charge is affected by properties of the surrounding solution, i.e.
protonation/deprotonation with pH of solution (see fig. 14.3)
3) zero point of charge (zpc), point of zero charge (pzc), or pH zero (pH0): pH value at which surface positive and negative charges associated with protonation /deprotonation equilibria are just balanced (see fig. 14.4, tab 14.1)
-when pH of the ambient solution < the colloid pH0, then the surface becomes protonated and the colloid attains a net positive surface charge, which is able to attract electrostatically and adsorb negative species.
14.1 Surface properties of colloidal materials: surface charge
14.1 Surface properties of colloidal materials: surface charge
Many environmental colloids have a negatively charged surface, hence can ion- exchange cations
colloid-Na+ + K+(aq) colloid-K+ + Na+(aq) (14.1)
Equilibrium position of the reaction depends on the nature of the colloid, and also depends on the nature (principally, charge density) and concentration of the
aqueous species adjacent to the colloidal particle.
Cation Exchange capacity (CEC), the units of which are centimoles of positive charge on the surface per kilogram of solid (cmol(+)/kg)
This is a Physical process where charge density on both the colloid and solution species determines the extent of adsorption
14.1 Surface properties of colloidal materials: Electrostatic adsorption
This type of adsorption involves the formation of specific covalent chemical bonds b/w the solution species and the surface atoms of the colloid, e.g. adsorption of fatty acids by a hydrated iron oxide surface (rxn 14.2)
Specific adsorption: adsorption depends on a degree of chemical, as opposed to electrostatic, affinity b/w the colloid and the adsorbed species. The adsorption is favored under peculiar pH conditions, it is not simply dependent on whether there is a positive or negative charge on the surface of the solids.
When specific adsorption occurs, the nature of the surface is altered and so the extent of protonation or deprotonation as measured by pH0 also changes: covalent bonding to the surface shifts the colloid pH0 to a , while biding of an anionic species produces an .
14.1 Surface properties of colloidal materials: Specific adsorption
(14.2)
EDL: describes charge properties of a colloid surface.
EDL: not a clearly defined set of ions that are exclusively of the opposite charge to that of the surface.
Thickness of EDL: the distance at which the potential has decreased to of its value at the surface.
Colloidal system, consisting of uniformly charged particles, is stable because small charged particles repel each other. As they move about due to Brownian motion, they are unable to come sufficiently close in order to overcome the repulsive forces of the surface charge and aggregate into larger, settleable units.
14.1 Surface properties of colloidal materials: Electric double layer
Destabilization of colloids: there are several mechanisms:
Mechanism 1. presence of a high conc. of electrolyte in the water This provides a source of counterions which then effectively reduce the thickness of the double layer.
Mechanism 2. Specific binding of an ion of opposite charge to the colloid surface. E.g. Al3+
bound to a negatively charged clay surface reduce the surface charge to near zero and repulsive interactions are partially eliminated.
14.1 Surface properties of colloidal materials: Electric double layer
Langmuir relation: assumes a surface with a specific number of sites each of which is capable of reacting with and binding to a sol’n molecule. All of the sites are considered to be equivalent and when all are occupied, no further adsorption can occur, i.e. monolayer coverage.
Where Cs = quantity adsorbed by the suspended solid, soil, or sediment/mol g-1; Caq
= equilibrium aqueous solution conc./mol L-1; b = binding const./L mol-1 (depends on the physical and chemical nature of the solid material; Csm = max. quantity adsorbable/mol g-1 (depends on the nature of the solid as well as the conc. of surface sites). =fraction of active sites occupied by adsorbate under specific conditions.
b C bC C
C
aq sm aq
s
1 C b
bC C
C
aq aq sm
s
1
C b
C C bC
aq sm aq
s
1 Cs Csm bCaqCsm
1 1
1
14.2 Quantitative descriptions of adsorption I: Langmuir relation
14.2 Quantitative descriptions of adsorption I: Langmuir relation
Global phosphorus cycle
14.3 Phosphorus environmental chemistry
Global phosphorus cycle
In water, phosphorus solubility is controlled by the availability of iron and Al under acid
conditions and Cd under alkaline conditions; each of these metals forms insoluble phosphate
Phosphorus: major nutrient for both plants and microorganisms.
-eurotrophic water body:
excessive biological productivity due to nutrient-rich conditions -oligotrophic water body: the
growth of aquatic organisms, esp. algae, occurs only to a limited extent
14.3 Phosphorus environmental chemistry
Two categories of sources of phosphorus: point source and diffuse source
point source - discharges from factories or outlets of municipal sewer systems.
Within the municipal discharges, the first source is from commercial soaps and detergents, which contain condensed polyphosphases to react with Ca2+, thus preventing it from causing surface-active ingredients to be removed from solution by precipitation.
E.g. condensed polycondensates
14.3 Phosphorus environmental chemistry
Freundlich relation: empirical relations
Where Cs=quantity adsorbed per unit mass/mol g-1; Caq=equil. solution conc./mol L-1; and KF/L g-1 and nF (a dimensionless number) are empirical Freundlich const.(nF is usually < 1)
nF
aq F
s
K C
C log C
s log K
F n
Flog C
aq Freundlich relation differs from Langmuir in that not all sites on the surface are considered equal but rather that adsorption becomes progressively more difficult as more and more adsorbate accumulates. It is assumed that once the surface is covered, additional adsorbed species can still be accommodated. i.e. no maximum (monolayer) adsorption is predicted by this relation. The eqn. does not imply any particular mechanism of adsorption, and is most satisfactory in the
14.4 Quantitative descriptions of adsorption II: Freundlich relation
In Freundlich relation, nF=1 for very low solute conc.
Where Cs=quantity adsorbed per unit mass/mol g-1; Caq=equil. solution conc./mol L-1; and KF/L g-1 and nF (a dimensionless number) are empirical Freundlich const.(nF is usually < 1)
Kd depends on the organic solute itself, on the chemical and physical nature of the solid phase, and on other environmental properties such as T and solution ionic strength
14.5 Partitioning of small organic solutes b/w water and soil or sediment:
The distribution coefficient, K
d In Freundlich relation, nF=1 for very low solute conc.
Where Cs= equil. conc. of a substrate in the soil or sediment, COM and CMM are conc.
of the solute in the organic matter and mineral matter respectively; fOM and fMM are the fractions of these two phases in the whole soil or sediment.
In many cases, CMM is so small,
14.5 Partitioning of small organic solutes b/w water and soil or sediment:
Sorption of organic species by soils
n-octanol is an amphiphilic.
Where CO= equil. molar solubility of the solute in octanol, Caq = the corresponding solubility in water.
14.5 Partitioning of small organic solutes b/w water and soil or sediment:
The octanol-water partition coefficient, K
ow14.5 Partitioning of small organic solutes b/w water and soil or sediment:
The octanol-water partition coefficient, K
ow Where COM = conc. of the organic comp’d of interest in the solid NOM component of a soil or sediment [mol of compound per kg of soil or sediment organic matter], and COC [mol per kg of organic carbon]. Caq = conc. of the comp’d in the equilibrated aq.
solution [mol L-1].
KOC=1.7KOM
aq OM
OM
C
K C
aq OC
OC
C
K C
OM OM
OM aq
OM aq
s d
K f
C f C C
K C
aq OM
OM
s
