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Environmental Toxicology, Master Sc. in Industrial Biotechnology Silvia Gross

Environmental toxicology: chemical aspects

Aquatic chemistry (5)

Fundamentals of aquatic chemistry




Major aquatic chemical processes




Phase interactions in aquatic chemistryMost important

environmental

chemical processes

in water involve

interactions between

water itself and another

phase.




Colloids: the neglected dimension of matter

A number of substances (including organic pollutants, proteinaceous materials, some algae

and some bacteria) are suspended in water in the form of small particles.

These particles, which range in diameter from 1 nm-1 mm and scatter white light as a blue

hue, are defined as COLLOIDS . Also discontinuities in this regime (old.iupac.org)

Their properties depend on the physico-chemical characteristics, including high specific

area (m2/g), high interfacial energy and high surface/charge density ratio.





Wolfgang Ostwald (1883-1943)




Different classes of colloids (in green environmental relevant)

Source: IUPAC

In a suspension solid particles are dispersed in a liquid; a colloidal suspension is one in which the

size of the particles lies in the colloidal range.

In an emulsion liquid droplets and/or liquid crystals are dispersed in a liquid. In emulsions the

droplets often exceed the usual limits for colloids in size.

An emulsion is denoted by the symbol O/W if the continuous phase: is an aqueous solution and by

W/O if the continuous phase is an organic liquid (an ’oil'). More complicated emulsions such as

O/W/O (i.e. oil droplets contained within aqueous droplets dispersed in a continuous oil phase) are

also possible.

A latex (plural = latices or latexes) is an emulsion or sol in which each colloidal particle contains a

number of macromolecules.

A foam is a dispersion in which a large proportion of gas by volume in the form of gas bubbles, is

dispersed in a liquid, solid or gel. The diameter of the bubbles is usually larger than 1 m, but the

thickness of the lamellae between the bubbles is often in the usual colloidal size range.




Source: IUPAC

Aerosols are dispersions in gases. In aerosols the particles often exceed the usual size limits for

colloids. If the dispersed particles are solid, one speaks of aerosols of solid particles, if they are liquid

of aerosols of liquid particles. The use of the terms solid aerosol and liquid aerosol is discouraged. An

aerosol is neither `solid' nor `liquid' but, if anything, gaseous.

A great variety of terms such as dust, haze, fog, mist, drizzle, smoke, smog are in use to describe

aerosols according to their properties, origin, etc. Of these only the terms fog and smoke are included

in this nomenclature.

A fog is an aerosol of liquid particles, in particular a low cloud.

A smoke is an aerosol originating from combustion, thermal decomposition

or thermal evaporation. Its particles may be solid (magnesium oxide

smoke) or liquid (tobacco smoke).

Different classes of colloids (in green environmental relevant)




Colloids: implications in environmental chemistry

Many organic and inorganic contaminants may be transported by/as colloids (the so-called colloid-

facilitated transport). Although most pesticides and herbicides are not water soluble, they do aggregate

with humic and fulvic acids which form hydrophilic colloids in water, transporting pesticides and

herbicides in the process.

This mechanism can circumvent natural and artificial barriers in the subsurface disposal of some kinds

of wastes.

Colloids composed of a variety of organic substances (including natural humic substances), inorganic

materials (especially clays), and pollutants occur in both natural and waste waters.

Colloids may be classified as hydrophilic, hydrophobic and association colloids, differring from each

other in their chemical composition.




Primary and secondary wastewater treatmentPrimary treatment

The objective of primary treatment is the removal of settleable organic and inorganic solids by

sedimentation, and the removal of materials that will float (scum) by skimming. Approximately 25 to 50% of the

incoming biochemical oxygen demand (BOD5), 50 to 70% of the total suspended solids (SS), and 65% of the oil

and grease are removed during primary treatment. Some organic nitrogen, organic phosphorus, and heavy

metals associated with solids are also removed during primary sedimentation but colloidal and dissolved

constituents are not affected. The effluent from primary sedimentation units is referred to as primary effluent.

Secondary treatment

The objective of secondary treatment is the further treatment of the effluent from primary treatment to remove

the residual organics and suspended solids. In most cases, secondary treatment follows primary treatment

and involves the removal of biodegradable dissolved and colloidal organic matter using aerobic biological

treatment processes. Aerobic biological treatment is performed in the presence of oxygen by aerobic

microorganisms (principally bacteria) that metabolize the organic matter in the wastewater, thereby producing

more microorganisms and inorganic end-products (principally CO2, NH3, and H2O).

Several aerobic biological processes are used for secondary treatment differing primarily in the manner in

which oxygen is supplied to the microorganisms and in the rate at which organisms metabolize the organic

matter.




Primary and secondary wastewater treatment

Source of the picture: britannica.com





Hydrophilic colloids are large molecules containing, as an integral part of their

structure, functional groups that can form hydrogen bonds with water molecules

(e.g. humic acids, proteins, some synthetic polymers).





Hydrophobic colloids are substances that have charged surfaces in water, and form

an electrical "double layer" that holds them in suspension.

Clays form a negative charge on their surface

when placed in water, and remain in

suspension by the electrostatic interaction

between the negative surface charge and polar

nature of water.





Association colloids are formed by aggregation of molecules constituted by a

hydrophobic part and a hydrophilic part (e.g. soaps and detergents, surfactants).




SurfactantsANIONIC

AOT = sodium di(ethyl-2-hexyl) succinate

CATIONIC

Cetyltrimethylammonium bromide

NON IONICS

Triton X100

Brij 76




Association colloids: implications in environmental chemistry

As a result, stearate anions in water tend to form clusters consisting of as many as 100 anions

clustered together with their hydrocarbon “tails” on the inside of a spherical colloidal particle and

their ionic “heads” on the surface in contact with water and with Na+ counterions.

The stearate ion has both a hydrophilic -CO2- head and a long organophilic tail,

CH3(CH2)16–.




Association colloids: implications in environmental chemistry

When placed in water, these aggregates of molecules form a structure in which the

hydrocarbon "tails" collect into tiny oil droplets and the charged carboxylate groups

interact with the water through an electrostatic interaction to keep the droplet

suspended (micelles).




Colloids in water: implications in environmental chemistryThis results in the formation of micelles. Micelles can be

visualized as droplets of oil about 3-4 nanometers (nm) in

diameter and covered with ions or polar groups.

According to this model, micelles form when a certain

concentration of surfactant species, typically around 1 x 10-3, is

reached. The concentration at which this occurs is called the

critical micelle concentration (cmc).

Micelle

An aggregate of surfactant molecules or ions in solution. Such aggregates form spontaneously at or

above a surfactant concentration called the critical micelle concentration.




Colloidal dispersion stabilityThe stability of colloids is a prime consideration in determining their behaviour. It is involved

in important aquatic chemical phenomena including

• the formation of sediments,

• dispersion and agglomeration of bacterial cells,

• dispersion and removal of pollutants (such as crude oil from an oil spill).

Two main phenomena contributing to the stabilization of colloids in water :

HYDRATION

SURFACE CHARGE

The layer of water on the surface of hydrated colloidal particles prevents contact, which would result in the formation of larger units. A surface charge on colloidal particles may prevent aggregation, since like-charged particles repel each other.




Colloidal dispersion: degradation phenomena

coagulation (flocculation) in colloid chemistry

when a sol is colloidally unstable (i.e. the rate of aggregation is not

negligible) the formation of aggregates is called coagulation or flocculation

coalescence in colloid chemistry

The disappearance of the boundary between two particles (usually droplets

or bubbles) in contact, or between one of these and a bulk phase followed

by changes of shape leading to a reduction of the total surface area. The

flocculation of an emulsion, viz. the formation of aggregates, may be

followed by coalescence.

IUPAC GOLD BOOK (http://goldbook.iupac.org/)

http://goldbook.iupac.org/S05727.html

http://goldbook.iupac.org/U06569.html

http://goldbook.iupac.org/A00184.html




http://goldbook.iupac.org/E02065.html





Colloidal dispersion: degradation phenomena




Colloidal dispersion: aggregation phenomenaThe processes by which particles aggregate and precipitate from colloidalsuspension are quite important in the aquatic environment.

For example, the settling of biomass during biological waste treatment depends uponthe aggregation of bacterial cells.

Other processes involving the aggregation of colloidal particles are the formation ofbottom sediments and the clarification of turbid water for domestic or industrial use.




Colloidal dispersion stability

The surface charge is frequently pH dependent;

around pH 7 most colloidal particles in natural waters are negatively charged.

Negatively charged aquatic colloids include algal cells, bacterial cells, proteins, and

colloidal petroleum droplets.

One of the three major ways in which a particle may acquire a surface charge is by

chemical reaction at the particle surface.

SURFACE CHARGE





Flocculation of Colloids by Polyelectrolytes

Polyelectrolytes of both natural and synthetic origin may cause colloids to flocculate. Polyelectrolytes are polymers with a high formula weight that normally contain ionisable functional groups. - Anionic- Cationic

Anionic polyelectrolytes have negatively charged functional groups, such as –SO3

- and –CO2-.

Cationic polyelectrolytes have positively charged functional groups, normally H+ bonded to N.

Nonionic polymers that serve as flocculants normally do not have charged functional groups.





Flocculation of Colloids by Cationic Polyelectrolytes




Flocculation of Colloids by Anionic Polyelectrolytes

Anionic polyelectrolytes may flocculate negatively charged colloidal particles.

The mechanism by which this occurs involves bridging between the colloidal particles by way of the polyelectrolyte anions.

Strong chemical bonding has to be involved, since both the particles and the polyelectrolytes are negatively charged.

The flocculation process induced by anionic polyelectrolytes is greatly facilitated bythe presence of a low concentration of a metal ion capable of binding with thefunctional groups on the polyelectrolyte. The positively charged metal ion serves toform a bridge between the negatively charged anionic polyelectrolytes and negativelycharged functional groups on the colloidal particle surface

---- Ca2+ ----




Colloidal dispersion stability SURFACE CHARGE

This phenomenon, which frequently involves

hydrogen ion and is pH-dependent

It is typical of hydroxides and oxides and is

illustrated for manganese dioxide, MnO2.

In a relatively acidic medium, the reaction

MnO2(H2O)(s) + H+ MnO2(H3O)+

(s)

may occur on the surface giving the particle a net

positive charge.

In a more basic medium, hydrogen ion may be

lost from the hydrated oxide surface to yield

negatively charged particles: MnO2(H2O)(s) MnO2(OH)-(s) + H+





At some intermediate pH value, called the zero point of charge (ZPC),

colloidal particles of a given hydroxide will have a net charge of zero,

which favours aggregation of particles and precipitation of a bulk solid:

Number of MnO2(H3O)+ sites = Number of MnO2(OH)- sites





Isoelectric point (IEP)

The value of pH at which the net electric charge of an

elementary entity is zero.

IEP TiO2 = 5.8 → at pH = 5.8 the charging is 0

→ at pH > 5.8 the charging is negative

→ at pH < 5.8 the charging is positive

Ti-OH + H+ → Ti-OH2+ pH < 5.8

Ti-OH + OH- → Ti-O- + TiO2 pH > 5.8







Ion absorption is a second way in which colloidal particles become charged. This phenomenon

involves attachment of ions onto the colloidal particle surface by means other than conventional

covalent bonding, including hydrogen bonding and van der Waals interactions.

Ion replacement is a third way in which a colloidal particle may gain a net charge; for example,

replacement of some of the Si(IV) with Al(III) in the basic SiO2 chemical unit in the crystalline lattice of

some clay minerals as shown:

[SiO2] + Al(III) [AlO2-] +Si(IV)

yields sites with a net negative charge.

Similarly, replacement of Al(III) by a divalent metal ion such as Mg(II) in the clay crystalline lattice

produces a net negative charge.


Charges on surface and electric

double layer

Isomorphous ion replacement

Replacement of one ion by another of similar size in a lattice

- clay (< 2 mm) minerals: isomorphic substitution of ions (Si4+ vs Al3+ or Al3+

vs Mg2+)

Source of image:

www.tankonyvtar.hu

http://www.tankonyvtar.hu/en/tartalom/tamop425/0032_talajtan/ch05s04.html

http://www.tankonyvtar.hu/en/tartalom/tamop425/0032_talajtan/ch05s04.html




Clays as hydrophobic colloids

Clays are the most common hydrophobic colloids in natural waters.

Clay is a generic term used to describe a class of secondary minerals produced from the

weathering of aluminum silicate rocks.

Clay type Chemical formula

Montmorillonite Al2(OH)2Si4O10

Illite K0-2Al4(Si8-6Al0-2)O20(OH)4

Kaolinite Al2(OH)4Si2O5

Nontronite Fe2(OH)2Si4O10

Hydrous Mica KAl2(OH)2(AlSi)3O10




Clays as hydrophobic colloidsClays are flat sheets of alternating layers of silicon oxides and aluminum oxides, held together

by ionic attraction for cations sandwiched between the sheets.

The negative surface charge of clay

particles results when an aluminum

(+3) or silicon (+4) is replaced with a

sodium (+1), potassium (+1) or

ammonium (+1) ion.





Because of their chemical composition, clays have a strong tendency to adsorb insoluble

chemical species from water and transport them.

Both organic and inorganic materials can be trapped between the sheets of aluminum and

silicon oxides, providing an effective mechanism for transporting these materials in the

aqueous environment.

These properties make clays also useful agents for cleaning contaminated water (clay liners

are placed at the bottom of landfills so as to retain pollutants, preventing them from getting

into the groundwater).

Because of their structure and high surface area per unit weight, clays have a strong tendency to absorb chemical species from water.

Thus, clays play a role in the transport and reactions of biological wastes, organic chemicals, gases, and other pollutant species in water.

Clay minerals also may effectively immobilize dissolved chemicals in water and so exert a purifying action.





Two important properties:

cation exchange capacity (CEC): milliequivalents of cations exchanged per 100 g of dry

clay;

zero point of charge (ZPC): the pH value where the negative surface charge of the clay has

been neutralized, and has a value between 0 and 7.

Clay type Particle size (mm) CEC (mequiv/100 g)

Montmorillonite 0.01-1.0 80-100

Illite 0.1-2.0 15-40

Kaolinite 0.1-5.0 3-15





The binding of positive ions to the surface of an initially negatively charged colloid can result in precipitation followed by colloid re-stabilization. This kind of behaviour is explained by an initial neutralization of the negative surface charge on the particles by sorption of positive ions, allowing coagulation to occur. As more of the source of positive ions is added, their sorption results in the formation of positive colloidal particles.

Hydrophobic colloids are often readily coagulated by the addition of small quantities of salts that contribute ions to solution.

Such colloids are stabilized by electrostatic repulsion.

The simple explanation of coagulation by ions in solution is that the ions reduce the electrostatic repulsion between particles.

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