Colloids in the Aquatic Environment

296 Pages·1993·7.06 MB·English

by  T. F. Tadros and J. Gregory (Eds.)

Colloids in the Aquatic Environment
Edited by
Th.F.Tadros
ZEN EC A Agrochemicals, Jealott'sHill Research Station,
Bracknell, Berkshire RG12 6EY, U.K.
J. Gregory
Department of Civil and Environmental Engineering, University College London,
Gower Street, London WC1E6BT, UK.
Published for SCI
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Colloids and Surfaces A: Physicochemical and Engineering Aspects, 73 (1993) vii vu
Elsevier Science Publishers B.V., Amsterdam
Preface
Natural waters contain a wide variety of substances in the form of suspended fine particles or dissolved
macromolecules. The interactions between these colloidal materials and with trace pollutants can be of
great environmental significance, especially in determining the transport properties and ultimate fate of
aquatic contaminants.
An International Symposium on Colloids in the Aquatic Environment, organized by the SCI Colloid and
Surface Chemistry Group, was held at University College London, September 7-9, 1992, and brought
together many of the leading workers in the field from around the world. It was a special pleasure to
welcome Professor Werner Stumm as the 1992 Rideal Lecturer, who gave an outstanding presentation on
Aquatic colloids as chemical reactants: surface structure and reactivity.
Other contributions covered a very wide range of topics, including transport and deposition of colloidal
particles in porous media, properties of organic substances in water, particle size distributions, precipitation
and heterocoagulation, adsorption processes, biological aspects and several others. The meeting served to
highlight the wealth of activity in this important area and also to show where further effort is needed. As
in many other areas, application of fundamental concepts to real-world systems is by no means
straightforward and empirical data cannot always be reconciled with model predictions. Nevertheless, a
deeper understanding of mechanisms of important environmental processes, often acquired from model
studies, can be of great value in interpreting field measurements.
Many of the papers presented at the Symposium are collected here, together with some of the discussion
remarks. I hope that this record will be of lasting value to those who attended the meeting and to many
who were unable to participate.
The organizing committee would like to express their gratitude to the following organizations for their
generous sponsorship of the Symposium: European Environmental Research Organization, International
Association of Colloid and Interface Scientists, British Nuclear Fuels pic, ECC International Ltd, ICI
Corporate Colloid Science Group and Unilever Research.
The SCI Conference Secretariat at Belgrave Square carried a great deal of the administrative load in an
efficient and friendly manner, with hardly any complaint. The smooth running of the Symposium was due
very largely to their efforts.
Finally, I would like to thank Dr Tharwat Tadros for his help in processing the manuscripts and the
staff of Elsevier for their efforts in producing this Special Issue and for their forbearance over missed
deadlines.
John Gregory
Symposium Chairman
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 73 (1993) 1-18 1
Elsevier Science Publishers B.V., Amsterdam
Aquatic colloids as chemical reactants:
surface structure and reactivity*
Werner Stumm
Swiss Federal Institute of Technology (ΕΤΗ Zurich), Institute for Water Resources and Water Pollution
Control ( Ε AW AG), C Η-8600 Dubendorf Switzerland
(Received 21 September 1992; accepted 23 October 1992)
Abstract
Colloids are ubiquitous; they occur in natural waters, even in seawater, groundwater and interstitial soilwater, in
relatively large concentrations (more than 109 colloids per liter). We consider first an idealized a-Fe 0 (hematite) colloid
2 3
and consider how its surface chemistry, surface speciation and surface charge is affected by its interaction with H +, OH",
metal ions and ligands. The interaction is modeled with the help of the surface complex formation theory; effects of
electrostatic interaction are taken care of with the Gouy-Chapman diffuse double layer theory.
The surface charge of a particle can be estimated from the extent of isomorphic substitution, and from H + , OH~, metal
ions and ligands bound to the surface. Competitive surface complex formation equilibria can be used to estimate surface
charge and, in turn, surface potential. Steric stabilization by polymer segments needs to be considered when the thickness
of the polymer layer is larger than the thickness of the electric double layer, e.g. in seawater, <5 (polymer) > (5(Debye
h D
length).
Most surface-controlled processes depend on the identity of the surface species and the geometry of the coordinating
shell. The overlapping orbital of the inner-sphere surface complex interconnects the solid phase (ionic or covalent solid,
polymer) with the aqueous solution phase. Surface complex formation concepts have been extended to carbonates,
sulfides, phosphates and organic particles (cells). The surface structure can be modified by hydrophobic adsorption and
the sorption of polymers. Colloid surfaces can mediate electron transfer (including light-induced) processes. Electron
cycling mediated by surfaces often complements or substitutes for an enzymatic mechanism.
Keywords: Aquatic colloids; model; surface reactivity; surface structure.
Introduction regulating the concentrations of most reactive ele-
ments and of many pollutants in soil and natural
Colloids are ubiquitous in natural waters; they water systems, and in the coupling of various
are present in relatively large concentrations (more hydrochemical cycles. Processes with colloids are
than 106 cm-3) in fresh surface waters, in ground- also of importance in technical systems, above all
waters, in oceans, and in interstitial soil and sedi- in water technology.
ment waters. The solid-water interface established Aquatic suspended particles are usually charac-
by these particles plays a commanding role in terized by a continuous particle-size distribution.
The distinction between particulate and dissolved
Correspondence to: W. Stumm, Swiss Federal Institute of compounds, conventionally made in the past by
Technology (ΕΤΗ Zurich), Institute for Water Resources membrane filtration, does not consider organic
and Pollution Control (EAWAG), CH-8600 Dubendorf,
and inorganic colloids appropriately. Colloids of
Switzerland.
*The Rideal lecture for 1992. iron(III) and manganese(III,IV) oxides, sulfur and
0927-7757/93/S06.00 © 1993 — Elsevier Science Publishers B.V. All rights reserved.
2 W. St Colloids Surfaces A: Physicochem. Eng. Aspects 73 (1993) 1-18
sulfides are often present as submicron particles TABLE 1
that may not be retained by membrane filters (e.g.
The colloidal particle as a reactant (various combinations of
Buffle et al. [1]). Recent measurements in the ocean reactions are possible)
led to the conclusion that a significant portion of
Type of property/ Property or reaction
the operationally defined "dissolved" organic
reaction
carbon is, in fact, present in the form of colloidal
particles. Physical Collector of other particles
(aggregation of colloids, coagulation)
Conveyor of chemicals
Definition of colloids
Chemical Collector of hydrophobic solutes which
accumulate at the surface because of
Colloids are usually defined on the basis of size,
expulsion from the water
e.g. entities having at least in one direction a Organic or inorganic surface ligands
dimension between 1 nm and 1 pm. An operational (Lewis bases) that interact with protons
or metal ions
distinction on the basis of size (membrane filtration,
Lewis acids which bind ligands (anions
centrifugation, diffusion), although useful for many and weak acids) (ligand exchange)
operational questions, is not fully satisfactory. In Charged surface (mostly resulting
incipiently from the adsorption of
order to be in agreement with the thermodynamic metal ions, H+ and ligands) interacting
concept of speciation, the connotation "dissolved" with charged and polar surfaces
should be used for those species for which a Redox catalyst sorbing oxidants and
reductants and mediating their
chemical potential can be defined. Colloids are
interaction
dynamic particles; they are continuously generated,
Electron acceptors or donors oxidizing
undergo compositional changes, and are continu- or reducing solutes (Fe(III) oxides,
ously removed from the water (by coagulation, Mn(III,IV) oxides, FeS2 and sulfides,
biogenic organic particles)
attachment and settling, and by dissolution). Some
Chromophore absorbing light to
of the reactions of colloids are reviewed in Table 1. induce heterogeneous redox processes
The colloids adsorb waterborne pollutants; (including reductive dissolution of
higher-valent oxides) (semiconductors)
hence the fate of reactive elements and of many
pollutants in the environment depend to a large Chemical-biological Biological particle biochemically
processing carbon and other nutrients,
extent on the movement of colloids in the aqueous
by generating or destroying alkalinity
systems. The colloids in natural waters are charac- Extra-cellular enzymes hydrolyzing,
terized by an extreme complexity and extreme oxidizing or reducing solutes
diversity, being organisms, biological debris,
organic macromolecules, various minerals, clays
and oxides, partially coated with organic matter. matter. We characterize these colloids in terms of
In this discussion we try to abstract from the the surface charge and infer semiquantitatively the
complexity of real systems and to understand how colloidal stability from considerations of surface
idealized model-type particles behave in a solution charge and from possible effects of polymers. We
whose variables are known and can be controlled. also show how the surfaces of other minerals, such
We start with the simple and then proceed to the as carbonates and sulfides, react chemically with
more difficult. We first describe the surface of oxide H + , OH-, metal ions and ligands, and how organi-
particles in terms of functional groups and then cally coated colloids or organic colloids and even
characterize quantitatively the interaction of H + , surfaces of bacteria and algae show similar patterns
OH-, metal ions and ligands on an idealized oxide of chemical coordinative interactions.
surface. We then adsorb surfactants, humic acids In our discussion we treat, above all, the adsorp-
and peptides and "coat" the oxides with organic tion of solutes in terms of the surface complex
W. Stumm/Colloids Surfaces A: Physicochem. Eng. Aspects 73 (1993) 1-18 3
formation (SCF) model. The theory has been Inner-sphere and outer-sphere surface complexes
reviewed extensively by Schindler and Stumm [2],
The interaction at the solid-water interface can
Dzombak and Morel [3] and Stumm [4].
be characterized in terms of the chemical and
Functional groups on the interface of natural solids
physical properties of water, the solute and the
(colloids, minerals, particles) with water provide a
sorbent. Two basic processes in the reaction of
diversity of interactions through the formation of
solutes with natural surfaces are
coordinate bonds with H + , OH-, metal ions and
(1) the formation of coordinate bonds (surface
ligands. The concept of active surface sites is
complexation);
essential in understanding the mechanisms of sur-
(2) hydrophobic adsorption.
face-controlled processes (nucleation and crystal
The latter type of adsorption is primarily driven
growth, biomineralization, dissolution and weath-
by the non-compatibility of the non-polar and the
ering, soil formation, catalysis of redox processes
hydrophobic substances with water. The formation
and photochemical reactions).
of coordinate bonds is based on the generalization
that the solids can be considered as inorganic and
Objectives
organic polymers, whose surfaces can be viewed as
extending structures bearing surface functional
The objectives of this discussion are as follows.
groups. These functional groups contain the same
(1) To review the concepts of surface coordina-
donor atoms that are found in functional groups
tion and to apply these concepts to interpret
of solute ligands such as
adsorption of H + , OH~, metal ions and ligands
to idealized colloid surfaces (e.g. oxides), and to
estimate, in turn, from the adsorption data the net
-OH, -SH, -SS, -C etc.
surface charge as a function of pH and solution
OH
variables.
(2) To illustrate that the adsorption of humic
acids to oxide surfaces and the effect of humic Such functional groups provide a diversity of inter-
substances on the colloidal stability can, in a first actions through the formation of coordinate bonds.
approximation, be modeled in terms of surface In a similar way, ligands can replace surface OH
complex formation. Steric stabilization by extend- groups (ligand exchange) to form ligand-surface
ing segments is negligible in fresh waters because complexes.
the thickness of the adsorbed organic layer is small As illustrated in Fig. 1, a cation can associate
in comparison with the thickness of the diffuse with a surface as an inner-sphere or outer-sphere
double layer (Debye length). However, in seawater complex, depending on whether a chemical, i.e. a
the adsorbed layer thickness exceeds that of the largely covalent, bond between the metal and the
Debye length. electron-donating oxygen ions is formed (as in an
(3) To show that the surface reactivity of colloids inner-sphere type solute complex) or whether a
(nucleation, dissolution, surface-catalyzed redox cation of opposite charge approaches the surface
reactions (including heterogeneous photochemical groups within a critical distance; as with solute-ion
processes)) depends on surface structure, specifi- pairs, the cation and the base are separated by one
cally on the structural identity of the surface (or more) water molecules [4,5]. Furthermore, ions
species. may be in the diffuse swarm of the double layer.
(4) To emphasize that new techniques for in situ It is important to distinguish between outer-
measurements, which avoid artifacts in sampling, sphere and inner-sphere complexes. In inner-sphere
storage and handling of colloids, need to be complexes the surface hydroxyl groups act as
developed. σ-donor ligands which increase the electron density
4 W. Stumm/Colloids Surfaces A: Physicochem. Eng. Aspects 73 (1993) 1-18
Oxygen _ . OH
u
Centrnalio 2
, g) OH S-OH s(°^ S^-OH
#
D i f t nu s e i o Ô v l ^ " p > ^ ^as a'reac^y been pointed out that these func-
— — tional groups have donor properties similar to
complex
ç$tp œmpiehxeer lnr-J^~V those of their corresponding counterparts in dis-
c^^^^Ç^ ^Og^^O^ ^C^Œ^ ^ Qx. P^S^_Q(l I solved solutes such as hydroxides or carboxylates,
u+
(^^_QH+ 0°©°°C) ^'e* depwtonated surface groups (S—O") behave as
( \ Ύ\ Ύ^ 0^~^\ 2 °χΫ Lewis bases and the sorption of metal ions (and
öT/^°^ule
Q X O X O X O X O^ OC~*^ H protons) can be understood in terms of competitive
_)·Η3" 0©(3 complex formation.
^ I ^1 The adsorption of ligands (anions and weak
acids) on metal oxide (and silicate) surfaces can
^ ^ also be compared with complex formation reac-
Fig. 1. (a) Surface complex formation of an ion (e.g. cation) on tions in solution, e.g.
a hydrous oxide surface (from Sposito [5]). (b) A schematic
portrayal of the hydrous oxide surface, showing planes associ- Fe(OH) 2 + + F~ = FeF2 + + OH- (la)
ated with surface hydroxyl groups (s), inner-sphere complexes
(a), outer-sphere complexes {β) and the diffuse ion swarm (d). S—OH + F~ = S—F + OH - (lb)
In the case of an inner-sphere complex with a ligand (e.g. F~,
ΗΡΟΓ) the surface hydroxyl groups are replaced by the ligand The central ion of a mineral surface (in this case
(ligand exchange) (modified from Sposito [5]). ^ f example, the surface of an Fe(III)
e { w or
oxide and S—OH corresponds to =Fe—OH) acts
as a Lewis acid and exchanges its structural OH
. _ ^ with other ligands (i.e. there is ligand exchange). A
of the coordinated central metal Λion. The Cu (I/IT)XT • · ·. • · r · 1 1 r
, . „ . ι -, LewiTs acid site is a surface site capable of receiving
bound inner-spherically is a different chemical · r 1 c 1 11 /AT
t . 11 a pair of electrons from the adsorbate. (A Lewis
entity from thΛ at b Λound ot uter-sphencally or present
Λ r t r
/ .„ 1. base is a site having a tree pair 01 electrons — like
Λ i r
in the diffuse part of the double layer; the inner-
sphere Cu(II) has chemically different properties,
e.g. a different redox potential (with respect to TABLE 2
Cu(I)) and its equatorial water is expected to Adsorption of H + , OH", cations and ligands on a hydrous
exchange faster than if the Cu(II) were bound in oxide surface (surface complex formation equilibria) 3
an outer-sphere complex. As we shall see, the
. . rr . 1 Λ il ι Type of equilibrium Reaction
reactivit y of aΓ surfacer is affected above all by inner-
sphere complexes. Acid base equilibria
Table 2 summarizes schematically the type of S-OH + H + ^S-OHj
surface complex formation equilibria that charac- ^ 1 1 2 '
terize the adsorption of Η + , OH", cations and Metal (M) binding
S—OH + MZ + -^S— OM( z _ 1+ ,+ H +
ligands at a hydrous oxide surface. The various 2S-OH + M z+-^(S-0) M ( Z _ 2+) +2H +
surface hydroxyl groups formed at a hydrous oxide S-OH + M z+ +H O^S-OMOH( z _ 2+ )+ 2H+
2
surface may not be fully equivalent structurally Ligand (L) exchange
and chemically, but to facilitate the schematic S-OH + L~^±S-L + OH~
representation of reactions and of equilibria, one 2S-OH + L ^±S -L++20H
2
usually considers the chemical reaction of a Surface Ternary surface complex formation
hydroxy, group, S-OH. "J + L." . M - J S - L ^ O H -^
The following surface groups can be envisaged
(Schindler and Stumm [2]): From Schindler anda Stumm [2] (modified).
W. Stumm/Colloids Surfaces A: Physicochem. Eng. Aspects 73 (1993) 1-18 5
the oxygen donor atom in a surface OH group — 0 10 1 1 —
that can be transferred to the adsorbate.) The ^ 80 ~ y y y / / /
extent of surface complex formation (adsorption) J o- // A / / /
6 2+ 2 +
is, for metal ions and anions (depending on the ^ cr 3*//Pb2+ / I II '
release of protons and OH" ions respectively) E40 // / // Zn+ 2 2+
strongly dependent on pH (Fig. 2). In addition to 0 "2 // y y yy
monodentate surface complexes, bidentate (mono- 0 5^ MeT J 7*" β—
nuclear or binuclear) surface complexes can be (a) pH
formed, e.g.
+2 +
2 S-OH + Cu = (S-0) Cu + 2 H (2a)
2
1001
r ^ ——
- S - OH - S - O ^ , ^ X^V
+ 2 v
2+ N +
I + Cu I Cu + 2 H (2b) 80 - \ \ \ \
- S - OH - S- 0 / V \ \ \
- c ' °~ 1 60 " SeOf \\S0 2-\crO|- VoA \ASOJ* "
0 4
=FeOH + HC 0; | • + H 0 (3) , 40- V* \ \ \
2 N 2
(Oxalate) 0— C ^ ^ \\ \\
Q
^e 0H sFe-(\ /O" J , , , , N^i , NS»ISL
u
I +Η ΡΟ^ I Pv (4) 23 4 5 6 7 8 9 10 11 12 13
2 Y
^FeOH =Fe - O * Ο (b) pH
The following criteria are characteristic for
all surface complexation models (Dzombak and
Morel, [3]): | • L1^00W- ' r -
(1) sorption takes place at specific surface coor- _ / p~ \ ^ C "N
80
dination sites; -ο \ \ \ ^2-
lir
c 60 - S \ \ \· .HPQr -
(2) sorption reactions can be described by mass J ^SiO^ \ \ /\ **•
law equations; ^ 40 " V* \ \
(3) the surface charge results from the sorption 20 - ··* \ \ \.
(surface complex formation) reaction itself; _! , χ* , \^, \
q
(4) the effect of the surface charge on sorption ^ 2 4 pH 8 °1
(extent of complex formation) can be taken into
account by applying a correction factor derived Fsi-1 <a) E x t te nof s u r fe accomplex formation (measured as
- 1 - j 1 1 1 .1 .1 mole per cent of the metal ions in the system adsorbed or
frrom the ele ctrAic double layer theory to the mass
suref abco u n) das a functjn oof p H; totla c o n c e n t r ant ioof Fe
law constants for Surface reactions. [Fe] = 1(Γ3 Μ (2· 10_* mol reactive sites per liter); metal
Tot
The extent of adsorption, or surface coordination concentration in solution, 5 · 10~ 7 M; ionic strength 1 = 0.1 M
and its pH dependence can be accounted for by "NN?/- V*™* l r %based on f t ac o mPi l de by Dzombak
f
r and Morel [3]. (b) Surface complex formation with ligands
mass law equilibria; their equilibrium constants function of H, for the binding of anions from
( a) n ias o na s P
reflect the affinity of the Surface sites for H + , metal dilute solutions (5 · 10"7 M) to hydrous ferric oxide: [Fe] =
Tot
ions and ligands. The tendency to form surface 10 " 3 M; / = 0.1 (based on data from Dzombak and Morel [3]).
, , , · ι , , (c) Binding of phosphate, silicate and-fluoride on goethite (a-
complexes may be compared With the tendency to FeOOH). The species shown are surface species: FeOOH
form the corresponding (inner-sphere) solute com- concentration, 6gl" 1; concentrations of total phosphate and
plexes [2,3,6]. Figure 3 shows the relationship s ie l iace ra tP = 10"3M, Si = 8-l0"4M (Sigg and Stumm
b. , etween th, e, solu,,t e comple x . fo .rmatio. ,n of FΓeΤO^HΤ^+. [6]). (The curTves in (a), (b) da nTedq u(ci)li barreiu cma lccounlasttaend tws.i)t h the help
2 f e xo p e r i m ey n dt ae lt le r m i n e
or A10H2+ with various ligands and the surface
complexation of =FeOH and ΞΑΙΟΗ surface
6 W. Stumm/Colloids Surfaces A: Physicochem. Eng. Aspects 73 (1993) 1-18
groups with the same ligands. The reasonably good isomorphic substitutions or from the adsorption
correlation obtained in this and in similar linear of charged solutes, above all the interactions of
free energy relationships (LFER) plots indicates surface functional groups with ions in aqueous
that the same chemical mode of interaction occurs solutions. The surface charge, in the latter case, is
in solution as at the surface and that the available accessible from the quantities of H + , OH-, cations
sorption data are consistent with one another; and anions that are bound to the surface, e.g. in
therefore such LFERs may be used to predict the case of the adsorption of M 2 +
(intrinsic) sorption constants from solute complex
Q = {S-OH2 +} + {S-OM +} - {S-O -} (7)
formation constants and vice versa. p
where Q is the surface charge (mol kg'1) accumu-
p
Surface complex formation on carbonates
lated at the interface and braces indicate concen-
trations in mol kg 1; Q can be converted into
p
Various possible functional groups may occur the net total particle charge σ (Cm-2) (σ =
ρ ρ
on the surface of carbonates, sulfides, phosphates
QFjs, where F is the Faraday constant and s is
p
etc. Using a very simple approach similar to that the specific surface area (m2 kg-1)). If a sufficient
for hydrous oxides (chemisorption of H 20) one amount of M 2+ sorbs, the surface will have a more
could postulate surface groups for carbonates (e.g.
positive charge than if proton exchange reactions
FeC0 ) as follows:
3 alone were governing the surface charge.
Correspondingly, surface-bound ligands tend to
H OH H OH H OH water Î decrease the net surface charge. The quantities
C0 Fe C0 Fe C0 Fe solid I bound to the surface can be determined experimen-
3 3 3
Fe C0 Fe C0 Fe C0 * tally or can be calculated from mass law considera-
3 3 3
C0 Fe C0 Fe C0 Fe (5) tions, e.g. for the interaction of M 2+ with the
3 3 3
surface
As indicated in the scheme of Eqn (5) it is
reasonable to assume that H + , OH-, HCO3, {S-OM + }[H + ]/{S-OH} [M2 + ]
C0 (aq) and Fe 2+ can interact with MeC0 (s)
2 3
and effect its surface charge. Surface complex for- = KS = X^(intr) Qxp(-AZFW/RT)
M
mation of the surface groups with ligands and
In other words, XS , the apparent constant, is the
metal ions can occur (Van Cappellen et al. [9]). M
product of an intrinsic constant /^(intr) ( a c o _n
stant valid for a hypothetical uncharged surface)
and a Boltzmann factor. The variable Ψ is the
surface potential, F the Faraday and A Ζ is the
change in the charge of the surface species of the
reaction for which the equilibrium constant is
defined. The intrinsic constant is experimentally
^ 0- fi82003 accessible by extrapolating experimental data to
the surface charge where σ = 0 and where Ψ = 0.
3 ρ
The correction, as given above (Stumm et al. [10])
=MeC0 HMeHC0°
3 3
assumes the classical diffuse double layer model (a
planar surface and a diffuse layer of counter-ions).
The surface charge of the colloids
The value of Ψ can be estimated from σ on the
ρ
basis of the Gouy-Chapman model:
Solid particle surfaces can develop an electric
surface charge in two principal ways: either from At25°C: σ = 0.1174(/)°-5(sinh Z¥> · 19.46) (8)
ρ
W. Stumm/Colloids Surfaces A: Physicochem. Eng. Aspects 73 ( 1993) 1-18 7
. . . ..
:Fe-S
s a
\-/--'.'
• catechol
:Fe-HS h P04 y fJsaHc S catechol i
::FFeeHHAAss0044 7TH3Si( ^4 ' >-V """" K -
r
Cr04 if/ H P/0 4 >. \ !
cc::FFFFeeeeHH--HH22AA22BBss00003344-- SS(( >J4 g V^Jt_JA ^ phtal ^.oxalate Cr04 H3Si04
aacceettaattee ——··Hy SrI \eS^03 S04 sHH4a plyhct al (K- \ \ 1 henolF • — - —
1
• j—|benzoat( S- φa. tΟ3V . . .. V Ζ~ ) C. . .. V ω?o' Fe-tiron> ; " χ b\e nzo ate \ \
0 5 10 15 20 25
(b) pH
(a) log Κ aq
Fig. 3. (a) Linear free energy relationship between the tendency to form solute complexes of Fe(III)(aq) and Al(III)(aq)
2+ +
MeOH +H +A = MeA + H 0 Kj(aq)
2
and the tendency to form surface complexes (intrinsic equilibrium constant) on y-Al 0 and hydrous ferric oxide or goethite surfaces
2 3
+ s
=MeOH + H + A = ^MeA + H Ο K(surf )
2
where A is the actual species that forms the complex, e.g. A = H3S1O4 and =FeA = ^FeH Si0 ; for simplicity charges are omitted.
3 4
3+
Equilibrium constants in solution {1 = 0) are from Smith and Martell [7] (constants valid for Fe were converted into constants
2+ 3+ 2+ +
valid for FeOH using log Κ = —2.2 for the reaction Fe +H 0 = FeOH +H ). Data for surface complex formation on
2
hydrous ferric oxide (O) are from Dzombak and Morel [3], for goethite (g) are from Sigg and Stumm [6] and for y-Al 0 (•) from
2 3
Kümmert and Stumm [8]. These data are intrinsic equilibrium constants, i.e. extrapolated to zero surface charge. At the ordinate
and abscissa a few relevant surface complex formation constants and solute equilibrium constants, respectively, are listed for which
the constants in solution or at the surface are not known; they may be used to estimate the corresponding unknown constant.
(b) Fractional surface coverage of =Fe(III) surface complexes as a function of pH. The calculation is based on the condition:
6 2
[>FeOH] - [A] - ΙΟ" M / = 1(Γ
Tot Tot
Electrostatic correction was made with the diffuse (Gouy-Chapman) double layer model. The figure shows the effect of pH on the
relative extent of surface complexation.
where / is the ionic strength (M). It is also possible hematite surface in the absence of specific adsor-
to assume for the interface a constant capacitance bates. Obviously ligands decrease the surface
model [2] to correct for the electrostatic effects on charge and lower the pH at the point of zero
equilibrium constants, but the results are usually charge (pH ), while metal ions increase the sur-
PZC
not very different from those obtained by using face charge and raise the pH . As shown in
PZC
the diffuse model. Fig. 4(b) some of the charge vs pH curves may
Routine computer programs are available to display minima. This is merely a consequence of
make equilibrium calculations where the correc- competitive proton-metal ion or ligand-hydroxide
tions for charge effects are iteratively considered. ion equilibria that are pH dependent.
Figures 4(a) and 4(b) show mass law calculations Different types of surface charge contribute to
on the surface site density and the net surface the net total particle charge on a colloid, σ :
ρ
charge, respectively, for hematite colloids that have
σ = σ + σ + σ + σ (9)
interacted at various pH values with different ρ 0 Η ί5 08
ligands or with Cu 2+ or Cd2 + . Equilibrium con- where σ is the total net surface charge, σ is the
ρ 0
stants were corrected for electrostatic effects by the permanent structural charge (usually of a mineral)
Gouy-Chapman model (/=10~ 2). These calcu- caused by isomorphic substitutions in minerals (a
lated curves may be compared with the proton- significant charge is produced primarily in the 2:1
dependent charge (H/OH only) obtained for the phyllosilicates), σ is the net proton charge, i.e. the
Η