Colloids in water

¨ Sediments: interact with water in several ways
Substances dissolve into water
Substances precipitate from water to sediment
Ions are adsorbed by sediments and are released, as conditions change
Organisms living in sediments absorb both nutrients and pollutants and bring these into the food chain

Suspended solids: Colloidal material

Characterized by high surface/mass ratio

Formation of sediments

¨ Erosion: Clay particles, organic matter, sand, carried into water by flowing streams, and wind erosion

¨ Precipitation: When water flows into another body of water with different composition precipitation may take place

¨ 5 Ca ⁺² + H₂O + 3HPO₄^-2 ® Ca₅OH(PO₄)_3¯ + 4H ⁺
hydroxyapatite

¨ Ca ⁺² + 2HCO₃^- ® CaCO_3¯ + CO₂ + H₂O

¨ Ca ⁺² + 2HCO₃^- + hn ® {CH₂O} + CaCO_3¯ + O₂
photosynthesis biomass

¨ Humic acid (in solution) + H⁺ ® Humic acid ¯

¨ SO₄^-2 +10 H⁺ + 8 e^- ® H₂S + 4H₂O (by bacteria in anaerobic conditions)

H₂S + M⁺² ® MS ¯ (Insoluble metal sulfide)

Bacteria also reduce Fe(III) to Fe(II)

Fe(OH)₃ (s) + 3H⁺ + e^- ® Fe ⁺² (aq) + 3 H₂O

Then Fe ⁺² + S^-2 ® FeS ¯

Since the CaCO₃ deposits in summer when photosynthesis is active and the FeS is formed when water is most anaerobic in winter, sediment sometimes shows stripes of the two substances.

Sediments build up in thickness--evolve into sedimentary rocks

Solubility

Governed by K_sp when all the dissolved substance is ionized.

Solubility of Pb CO₃ ?? Ksp = 1.48 x 10^-13

[Pb⁺²][CO₃^-2] = 1.48 x 10^-13

or: PbCl₂ ® Pb⁺² + 2 Cl^-

K_sp = [Pb⁺²] [Cl^-]² K_sp = 1.6 x 10^-5

Calculate the solubility:

However, not all compounds dissociate completely, some show a solubility of the molecular form.

Where we have both ionic and molecular forms

solve Ksp equation for S, then add to the molecular species

Intrinsic solubility = [Molecular Species] + [S]

What else effects solubility?

Chelation, complexation, common ions, reaction of one of the ions with acid or base.

Colloids

Hydrophilic colloids

particles which have polar groups on their surfaces, bond easily to water--proteins, synthetic polymers with polar groups-- not easily ‘broken’ by adding salts to the suspension. Stabilized by shell of water surrounding particle -- hydration.

Hydrophobic Colloids

Particles which are stabilized by adsorbing ionic species on their surfaces forming a double layer. The charged surface keeps particles from coalescing. Salts disrupt the double layer and ‘break’ the colloid.

Association colloids or Micelles

Formed of long chain molecules with a polar head and non polar tail. These aggregate in micelles with heads in the surrounding water and tails in center. Can solubilize non polar materials (oils, etc.) into water by dissolving them in the hydrophobic interior of micelles. Soaps and detergents are common micelle-forming substances.

Stability depends on surface charge and hydration.

Why is colloid stability important?

Hydration: occurs when surface polar groups are present:

Surface charge:

· Produced when reaction at surface produces ions: as in Fe(OH)₃

Fe(OH)₃ + 3 H⁺ + 3 H₂O Û Fe( H₂O )₆⁺³

in acid solution

In basic solution extra OH^- are acquired by the Fe(OH)₃ yielding a negatively charged particle.

At some pH these processes balance, the particles become neutral (zero point of charge, ZPC) when they are likely to coagulate.

· Also can produce charge by adsorption of ions. Most commonly, crystalline particles will adsorb ions which are similar to those in their lattice, and others by dispersion forces rather than by covalent bonding

· Finally, can have charged particles when ions are replaced in the lattice by others of similar size but different charge: i.e. when an Al⁺³ is replaced by a Mg⁺² a net negative charge is produced.

Clay particles in colloids

Clays are formed from silicon - oxygen chains and tetrahedra, with Al - O structures and other metal atoms included.

Layered structure, as in montmorillonite, allows sorption of water and solutes dissolved in the water into the interlayer space.

Si⁺⁴ and Al⁺³ may be replaced by other metals, such as Fe ⁺², leaving a net negative charge, which then attracts a sorbed layer of positive ions. These are not bonded and can be exchanged with other cations:

Cation exchange capacity of clay (CEC)

Milliequivalents of cations exchangeable per 100 g of dry clay.

Aggregation of colloids

Coagulation: When electrostatic repulsion is overcome, particles can aggregate. Hydrophobic particles often will coagulate when salt is added. (note -- a mechanism in delta formation as streams enter ocean)

If the neutralization of charge is exceeded the colloid can pick up the opposite charge and redisperse.

Flocculation: bridges are formed between particles by polyelectrolytes. (Polymers with ionizable or strongly polar groups along the chain) Important in removing bacterial colloids during water treatment.

Mechanisms of surface sorption

Atoms at surfaces are more ‘active’ because of imbalance of forces.

Excess surface energy is reduced by agglomeration or by adsorption of other molecules or ions onto the surface.

E.g.: On a freshly precipitated metal oxide particle, surface energy can be reduced by:

Sorption of metal ions:

Chelation with metal ions:

Displacement of H⁺ or OH^- by a ligand-complexed metal ion:

Sorption of anions by ion exchange with OH^- from surface

Ion Exchange with Sediments

Cation exchange capacity(CEC) measures the capacity for exchanging cations.

Exchangeable cation status (ECS) measures the amounts of specific cations bonded to the surface.

ECS is a fragile status--easily changed. Samples will rapidly react with oxygen on contact with air (especially if samples were anaerobic)

Fe ⁺² (exchangeable ion) ® Fe₂O_{3 (S)} (not exchangeable)

To determine CEC: Exchange all sites with NH₄⁺. Displace the NH₄⁺ with NaCl solution. Determine the NH₄⁺ in the solution. Report as meq of NH₄⁺ per 100 g sample (on dry basis).

To determine the ECS: Strip all metal ions off by exchanging with ammonium acetate. Then analyze solution for all metals of interest, Fe⁺², Mn⁺², Zn ⁺², Cu ⁺², Ni⁺², Na ⁺, K⁺, Ca⁺², Mg⁺² . Any difference between the total ECS due to these metals and the CEC is assumed to be due to exchangeable H⁺

Sediments act as pH buffers and also metal ion buffers

Trace metals in sediments

Actual metal species present depends on pH and pE. The equilibrium between sorbed metals and solubilized metals depends on the sorption characteristics of the sediment and the presence of complexing agents in solution.

Sources of trace metals: from parent minerals of the sediments in unpolluted water. From anthropogenic sources in polluted waters.

Are these trace metals bioavailable?

May be held as sulfides in anaerobic conditions but if oxidized, may be freed.

Phosphorous in bottom sediments:

From:

· Phosphate minerals

· Phosphate ions bound to surface on particles (nonoccluded )

· Phosphate ions contained within particles of hydrated metal oxides (occluded phosphorous)

· Organic Phosphorous--usually part of biomass

· Polyphosphates from heavy field runoff or waste streams containing domestic phosphate detergent

Phosphate exchanges from sediment to water, as an important nutrient for algae, may contribute to eutrophication.

Organic Compounds in Sediments and on Colloids

Cationic organic substances are readily sorbed on clays and humic solids. In clays these are sorbed between layers--not available for bio-activity. Sorption in general may decrease biodegradability.

Anion exchange not so strong, so anionic organics are more mobile.

Volatile organics lost by evaporation.

Water insoluble (hydrophobic or lipophyllic) may sorb to humic solids.

Sorption often is described by the Freundlich eqn:

where X = amount sorbed per unit weight of solid

K , n = constants

C = Conc. of sorbed species in solution at equilibrium

(Plot of log X vs log C gives line with slope of n and intercept of log K)

Sorption of hydrocarbons:

Attracted to nonpolar parts of humic substances where they are bound by ‘dispersion or van der Waals forces. Also may become covalently bound to some group on humic material, and so become very hard to remove. (Usually mediated by bacterial enzymes)

Can distinguish between petroleum and biologically generated hydrocarbons: Petroleum has smooth distribution of chain lengths, biogenerated materials are generally more specific to certain isomers.

For nonpolar (hydrophobic) organics the most important factors in sorption are the organic content of the soil or sediment and the K_ow of the compound.

K_ow is the octanol/water partition coefficient of the compound

General effects of sorption of organics:

· May reduce biodegradation rates substantially

· May remove organics such as pesticides from surface water an its way to groundwater aquifers

· May purify leachates from landfills somewhat

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