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seawater interfaces
Marine Chemistry, 23 (1988) 51-67
51
Elsevier Science Publishers B.V., Amsterdam-- Printed in The Netherlands
ON THE F O R M A T I O N OF ORGANIC COATINGS ON M A R I N E P A R T I C L E S : I N T E R A C T I O N S OF ORGANIC M A T T E R AT HYDROUS ALUMINA/SEAWATER INTERFACES
V E R A 2UTIC and J A D R A N K A
TOMAIC
Center for Marine Research, Zagreb 'Rudjer Bo~kovid' Institute, POB 1016, 41001 Zagreb (Yugoslavia)
(Received October 22, 1986; revision accepted June 15, 1987)
ABSTRACT ~utid, V. and Tomaid, J., 1988. O n the formation of organic coatings on marine particles:interactions of organic matter at hydrous alumina/seawater interfaces. Mar. Chem., 23: 51-67. The interaction mechanism between aquatic organic matter (AOM) and mineral particles in seawater has been studied using hydrous alumina as a model mineral phase. The electrochemical methodology allowed direct measurements in oxide suspensions and distinction between 'dissolved' and 'particulate'organic matter. Adsorption of small organic molecules, such as salicylic acid, was found to be negligible at natural concentration levels.Unsaturated lipidsare the most reactive surface-active constituents of A O M leading to multilayer formation. For humic materials an inverse relationship has been established between absorbed densities and particle concentration and it is interpreted as the fractionation effectof a polydisperse solute. A new, highly surface-active fraction of A O M has been identified as fluid surface-active aggregates, ubiquitous in productive surface waters, sea surface microlayers and in mixing zones in estuaries. Hydrophobic attraction is the most important force governing AOM-particle interaction in seawater. Reversible adsorption of organic molecules at charged marine interfaces is only the initialstep in the process of organic coating formation that involves complex structural organization phenomena well known in membrane sciences and biophysics.
INTRODUCTION The behaviour of the elements in seawater is closely linked to the extent to which they are involved in particle-water interactions, either via biological utilization or by adsorption/desorption reactions at active surfaces. A number of models have been developed to simulate the adsorption process. The most effective appears to be via the assumption that adsorption results from complexation at specific surface sites on the particle surface (Stumm and Morgan, 1981). The identification and characterization of these sites remains empirical (as reviewed by Morel and Hudson (1985) and Whitfield and Turner (1986)). The microelectrophoretic studies (Neihof and Loeb, 1972, 1977; Hunter, 1980;
0304-4203/88/$03.50
© 1988 Elsevier Science Publishers B.V.
52 Hunter and Liss, 1982) have established that surface-active molecules in estuarine and coastal seawater determine the surface properties of suspended particles. However, the extent of oxide surfaces covered by organic films and the mechanism of their formation in the marine environment is still a matter of speculation. Much more work has been done in defined model systems relevant to fresh water (Kummert and Stumm, 1980; Sigg and Stumm, 1981; Stumm et al., 1980; Tipping, 1981; Davis and Gloor, 1981; Davis, 1982, 1984). In most of the experiments described, relatively high concentrations of particles (~- 1 gl 1 ) were used (mostly due to the methodological problems inherent to adsorption studies by standard techniques, with the exception of a methodology developed by Baccini et al. (1982)). Formation of thick organic films developing over many hours, has been observed by optical methods on metal surfaces immersed in natural seawater (Loeb and Neihof, 1977; Kristoffersen et al., 1982) while the extensive evidence on the earliest phases of adsorption of marine organic matter at charged interfaces has been obtained by direct electrochemical measurements of surfactant activity of seawater samples using a mercury drop electrode (Zvonarid et al., 1973; ~uti5 et al., 1977, 1981; ~osovi~ and Vojvodi~, 1982; ~osovi~ et al., 1985). The range of surfactant activity values observed in coastal as well as deep Mediterranean waters (0.2-1.6 mg 1 1) indicates that under the conditions of maximum adsorption (positively charged and hydrophobic surfaces) significant surface coverages (5-50%) are already attained on a few second time scale by convective transport. We consider here the interactions between defined hydrous oxide particles and model organic materials (natural and artificial) in seawater. The electrochemical methodology presented in this work allows: (1) direct measurements in oxide suspensions without any previous separation that would affect the distribution between aqueous and solid phase; and (2) distinction between dissolved surface active materials and surface active aggregates. 5-alumina was chosen as the model mineral phase because of its electroinactivity, the range of positive surface charges and a significant amount of research on the interactions of this material with organic solutes in terms of adsorption (Kummert, 1979; Kummert and Stumm, 1980) and weathering (Furrer, 1985; Furrer and Stumm, 1986). ELECTROCHEMICALMETHOD In a previous paper (~utid and Stumm, 1984) we developed a direct electrochemical approach to study the interactions between dissolved organic molecules and hydrous alumina under weathering conditions. The interactions between alumina particles and organic molecules in seawater can be studied by the electrochemical method ofpolarographic maximum of mercury(II), (~utid et al., 1977; Hunter and Liss, 1980; ~uti6 et al., 1981).
53
The method is based on the interfacial instability of a liquid/liquid interface-dropping mercury electrode/aqueous electrolyte solution which is generated by the gradient of surface tension (Sorensen, 1979; Aogaki et al., 1978) manifested as convective streaming and a measurable increase in the reduction current of mercury(II) (polarographic maximum). Dissolved organic molecules (as well as micelles) adsorbed at the mercury/seawater interface decrease the gradient of surface tension and slow down the convective streaming, which is manifested as a suppression of the maximum current. This phenomenon provides a basis for a quantitative determination of surface active materials. Suspensions of alumina particle alone (~< 1 g1-1) do not affect the electrochemical response. Heterodispersions of fluid surfactants, such as unsaturated fatty acids and their esters in seawater, produce the irregular current oscillations of the polarographic maximum (Fig. 1). The perturbations in the current-time curves of variable amplitude and frequency indicate a stochastic process, which corresponds to random collisions of surface active aggregates of variable size with the mercury electrode/aqueous solution interface (Plebe and ~utid, 1984). Measurement of the amplitude and duration of perturbations allows a direct characterization of single events of coalescence and transformation of the aggregates into the adsorbed layer at the interface (relaxation times 10-500 ms, aggregation numbers > 109) (~,utid et al., 1984). The amount of the organic matter adsorbed by alumina particles during adsorption batch experiments is determined from the difference in a solution (dispersion) concentration before and after addition of alumina according to the calibration curves (Fig. 2).
i/IJ.A
1) METHYL OLEATE 14.1m g / I 2) (1) filtered 0.45 IJ.m 3) (1) • ,3-Al203 100 mg/[
15C
10(
5C
/'
t
/j**
/// /
,, . . - "
.....
II.'" l#
,/,' /
V 4
6
8
t/s
Fig. 1. Current-time curves at the dropping mercury electrode (2 s drop time) in artificial seawater (curve 0) in the presence of 14.1 mg1-1 methyl oleate before (curve 1) and after filtration (curve 2). Curve 3: current-time curves in dispersion of 14.1 mg l-1 methyl oleate + 100 mg 1-i ~.AI203 after 2 h shaking at 20°C.
54
"o, \ \ \
) ~ N N
i
i
i
i i ,Jr1
0.I
.
50
J
J
I
1 TRITON-X-IOO
c~:r,oo~
2. u.oL~mlc AClO ~. OLE,C AC,O
r
i ,J,iI
E
i
:
i
10 Concenfrai'ion ling L-I
\\, \
\
\ X
,
100
\
%
\
\ \ \
i,iiI
'\T
\]
\,
~ 0,.-__,.~1 O__ 0 ._~mm ~,,~__z~__ I 0,|
I 1 Concentration/mg
110
id, f
~0
I. -i
Fig. 2. Calibration curves: dependence of current (at the end of drop life) on the concentration of surfactants in artificial seawater. EXPERIMENTAL A d s o r p t i o n b a t c h e x p e r i m e n t s were performed in 100 cm 3 flasks at 20°C. An aliquot of freshly p r e p a r e d stock suspension of a l u m i n a particles (in distilled water) is added to the solution (or dispersion) o f o r g a n i c m a t e r i a l in the artificial or n a t u r a l seawater. T h e equilibration time of 4 h (with or w i t h o u t c o n t i n u o u s shaking) was c h o s e n for all experiments. Only freshly p r e p a r e d solutions and dispersions of s u r f a c t a n t s were used and a n a l y s e d to avoid a n y significant t r a n s f o r m a t i o n by chemical oxidation (e.g. u n s a t u r a t e d f a t t y acids) or by b a c t e r i a l activity. M e a s u r e m e n t s of unfiltered aliquots (50cm3), to which Hg(II) solution (0.5 cm 3 o f 0.I M solution) is added j u s t before the m e a s u r e m e n t , are made in a lOOcm3 all-glass cell open to air and t h e r m o s t a t e d a t 20~C w i t h a fawt d ~ p p i n g
55 mercury electrode and Ag/AgC1 reference in a three-electrode configuration. A PAR model 174 Polarographic analyzer in connection with a 7054 HewlettPackard recorder has been used for registration of polarograms, while the current-time transients (at a constant potential, E = - 300 mV) were recorded using a Gould digital storage oscilloscope with 4001 output unit. A CoulterCounter, Model TA, was used for characterization of particle distribution. Materials The commercial product 'Aluminium oxide C' (Degussa, Frankfurt) used as a model mineral phase, consists of J-Als 03 (R. Giovanolli, unpublished results, 1981) nonporous particles with a diameter of 20 n m (BET specific surface area 108 m sg-i) (Kummert, 1979) and p H zero point of charge 8.7 (Neuwinger, 1970). Commercial products (salts and surfactants) were reagent grade. Artificial seawater was of the following composition: 0.6MNaCI, 1 0 - 3 M K B r * and 5 × 10-3M N a H C O a . Tetradistilled water was used throughout. Special care was taken with respect to the purity of water, glassware and the atmosphere in the laboratory. Organic traces from commercial sodium chloride were eliminated by prolonged heating at 450°C and addition of active carbon to the stock solution. Acetic buffer (10-2M) was used for adjusting to p H 5, since in these conditions it does not adsorb on hydrous alumina (~utid and Stumm, 1984). Humic acid I, isolated from the deposits of Ruppia maritima (Lagune of Cannet, Western Mediterranean) by Faguet (1982) and Cauwet and Faguet (1982) had the molecular mass distribution shown in Fig. 3 as determined by Gloor et al. (1981) and the elemental composition: 44.84% C, 4.73% H, 4.35% N. Humic acid II was isolated from Penwhirn Reservoir, Scotland by Tipping (1981), it had a weight-average molecular weight c. 27000, 3.1 × 10-3molg -I carboxyl groups (pK < 5) and 10-3molg -1 phenolic O H groups (pK -~ 11) (Tipping, 1981). Stock dispersions of fatty acids and their esters were prepared by adding 0.1cm 3 of a commercial product into 500cm 3 5 × 10-2NaHCO3 solution and shaking for 4 h. Axenic phytoplankton cell suspensions were prepared in the Biological Institute, Dubrovnik, with the nutrient enriched (f/2 medium) Adriatic seawater (S = 38%0, DOC ~< 0.5mg1-1) as the growth medium. RESULTS The interaction between alumina particles and organic matter has been studied in artificial seawater at p H 8.7 (zero point of charge) and p H 5.0 (pos-
* While additionof Mg 2+,Ca 2+ and SO~- had no measurable effecton the reductionof Hg(II)and dissolved oxygen in air-saturated0.6M NaCl solution,the presence of Br- was important in simulating the response in natural seawater because of its strong interaction with mercury electrodesat potentials > -0.1V.
56
Z 0 m n-
HUMIC A C I D from Ruppio deposit
(,9 U
z 0 nO ILl .J
LU n~
]
I
[
I
>80
20
10
I
I|
5 1<0.2
M O L E C U L A R W E I G H T ' 10 3 Fig. 3. Molecular weight distribution of humic acid I.
itivety charged surface). The adsorption behaviour of the following natural and synthetic model materials (i) dissolved simple molecules (salicylic acid, phenylalanine, tyrosine) (ii) polymers (polyetoxynonytphenol and humic materials) (iii) heterodispersions of fluid surfactants (unsaturated fatty acids and esters) and (iv) phytoplankton cell suspensions was studied in the concentration range 10#gl-l-100mgl ~, while the concentration of alumina particle was varied in the range 10-800 mg 1-~.
Dissolved simple molecules Simple molecules, with functional groups characteristic of aquatic organic matter, and humic materials in particular, do not show measurable adsorption on positively charged or uncharged alumina/seawater interfaces. This applies to the concentration range which could be analysed by the polarographic method (Table I; calibration curves in Fig. 2a) and would also be relevant for natural conditions. This is in agreement with the published results at lower electrolyte concentrations (< 0.1 M) of Kummert and Stumm (1980) for salicylic acid, Elliot and Huang (1980) for phenylalanin and Davies and Gloor (1981). They concluded that in fresh water, molecules with a molecular weight less than 1000
57 TABLE I
Measurable concentration range for some simple molecules, and the estimated upper limits of the values for adsorption constants (K L app.)assuming Langmuir isotherms
molecule [ ~
I pK
OOH OH
salicylic
concentr, range / mg t"1
KL app. / M-1
2.8
0.01 - 5
< 10 ¢
2.2
0.01 - 8
<10 ¢
1.8
2 - 100
< 10 3
0.01 - 10
< 10 4
acid
OH
0
CH2CH COOH NH2
tyrosine
© CH2 CH-COOH NHz
phenytalanine
;AI~>-O-[CHz- CH20-] M ' ~
9-1c - -
Triton - X - 100
are not significantly adsorbed at mineral surfaces. At the mercury electrode/ seawater interface strong adsorption of salicylic acid, tyrosine and polyetoxyalkylphenols (Triton-X-100) is due to ~-electron interactions between the flat oriented phenolic ring and the positively charged mercury surface (Damaskin et al., 1971). Humic materials
Adsorption behaviour of the two substances is summarized in Figs. 4 and 5. Adsorption densities expressed as mg adsorbed humic per gram alumina are plotted against concentration in the aqueous phase measured after 4 h adsorption.
58
2o01
b
r I i
"Tm150~ -o~
HUMIC ACID I, pH 8.7
E t *~-010
I00~-
O
20
/".,
11 / / /
100
?/ , xf j /,,/~..~
.200 ~ u ===.~_ 501- ,1,/ ~ o - - - - - ~ o I :~,'" o/ 8o0._~_~ l;g," ..o/ ~ ' ~ It/,,,, o"/ / lY?~"
o
/ 10
20 30 CONCENTRATION
MIC ACID I. pH 5.0
. 10 Imgt-I
, 30
20
Fig. 4. Apparent adsorption isotherms for humic acid I at pH 8.7 and pHS.0. Concentration of 10, 20, 100, 200 and 800 mg ]-z. Dashed lines indicate adsorption experiments with shaking. 6-AI 20 3 suspensions:
,~ 20
200
a
Tc~ 150 E
100
x ~
x
/2OOO.1°
z o ~. I0(~ no u3 D
~
X
50 HUMIC ACID II, pH 5.0
10
I
20 10 3O CONCENTRATION / m g r 1
Fig. 5. Apparent adsorption isotherms for humic acid II a t p H 8.7 and pH 5.0.
I
20
3O
59
Although the two materials are of differentorigin and isolationprocedures, the following general characteristicsare apparent: (i) strong adsorption in a similar concentration range as at the positively charged mercury/seawater interface; (ii)adsorption is stronger at the lower pH; this effectis much more important for humic acid I,but is less than that observed for natural organic matter in fresh water conditions (low ionic strength) (Davis, 1982); (iii)adsorption densities approach a limiting value; (iv) shaking of suspensions during the batch adsorption experiment does not affectthe m a x i m u m adsorption density. However, the risingpart of the adsorption curve shiftstowards lower solution concentrations indicating that (at this time scale) mass transport in the aqueous phase is the limiting step; and (v) adsorption densities depend on the particle concentrations: they decrease dramatically with increasing particleconcentration, as is clearlyshown by the apparent adsorption isotherm in Fig. 5a. The values of apparent adsorption constants (subscriptapp.) evaluated from the 'Langmuir plots' (Fig. 6) decrease with increasing particle concentration, while the m a x i m u m adsorption density, P .... is independent of particle concentration in the aqueous phase, and reaches 14% of the material in the particulate phase. Such behaviour can be understood as adsorption of a polydispersed solute (Adamson, 1982), which humic material definitelyis (Fig. 3), with a preferentialadsorption of higher molecular weight fractions(adsorption constants /?iincrease with increasing molecular weight). The fractionation HUMIC ACID !
~app(Cm3/g) :
/ 6-AIzO 3 {g/cm31 : /
le=~
L
o
x
/
r"mox = 0.143 [
0
I
[
106 [CRAIg/cm31-1
Fig. 6. [ ~ n ~ u i r
plot (inverse adsorption density vs. inverse concentration) for adsorption of
humic acid I, pH 8.7 at different concentrations of aluimina suspensions (g ~-AI208 cm -s) indicated on the curves.
6O
effect should increase with increasing particle concentration. A simple model with a two-component solute (ill > f12, C2.0 > C~.0, F, .... > F2.... ) reversibly adsorbed on a homogenous surface
['1+2
=
F, .... /~1C, + F~.... /~2C2 1 ~ /?,C, + /72C2
(Ci being the equilibrium and Ci.o the initial concentration of the ith component) gives the same effect if particle concentration is introduced as a variable and only total solute concentration (C1 + C2) experimentally measured. The fractionation effect has been observed for adsorption of organic material isolated from a Swiss lake (Davis and Gloor, 1981; Davis, 1982)*. The increase in particulate organic carbon with decreasing particle concentration was detected for various particles (with homogeneous as well as heterogeneous surfaces) in adsorption experiments with natural lake water (Baccini et al., 1982), and in sedimentological studies of Western Mediterranean estuaries (Monaco, 1983, unpublished). Thus, it is inappropriate to predict the adsorption behaviour of humic material, or any other polydispersed organic material in natural aquatic systems by mere extrapolation of laboratory experiments performed with high particle concentrations (commonly ~>I g-1). Such extrapolations have resulted in tremendous underestimations of the importance of organic adsorption in the sea, where particle concentration is only a few milligrams per liter or less. Adsorption of humic material at alumina particles is much stronger than that of simple molecules with identical surface complexing groups (such as salycilate or phtalate) and is comparable to adsorption at a hydrophobic metal surface such as mercury. Surface complexation does not seem to be the only mechanism of adsorption of humic material to mineral particles in the marine environment, and the hydrophobic interaction (Tanford, 1980; Israelachvili and Pashley, 1983) should be considered.
Heterodispersions of fluid surfactants Typical molecules were chosen which differ in their structure and charge: at pH 8.7 oleic and linoleic acids are negatively charged (pK ~< 5), while methyl oleate remains uncharged. In seawater, methyl oleate and oleic acid are predominantly present as heterodispersions, while linoleic acid is significantly more soluble due to increased polarity of the alkyl chain with three conjugated double bonds (Tanford, 1980). However, the exact data on their solubilities in concentrated electrolyte solutions, such as seawater, are lacking in the literature. By measuring size distributions and settling velocities it was proved that heterodispersions of these materials in artificial seawater did not change * It is important to note that the authors could not detect any fractionation effect according to the acidity constants of the organic material, but only according to molecular weight.
61
significantly during the period of 4 h, nor did the addition of Hg(II) solution have a measurable effect during the time period of polarographic analysis (< 15 rain). Figure I illustrates typical features of dispersions of fluid surfactants: (i) at the mercury electrode coalescence of a fluid aggregate (manifested as a sharp current spike) is followed by a fast reorganization into the adsorbed layer (decay of current after the spike) at the mercury electrode/seawater interface. The process takes place on the millisecond time scale; and (ii)fluid aggregates are quantitatively removed from the aqueous phase by strong interaction with the alumina particles (curve 3), comparable to the effect of filtration (>/0.45 #m pore size). The interaction between alumina particles and heterodispersions of methyl oleate (Fig. 7) can be summarized as follows: (i) fluid aggregates are preferentially adsorbed by alumina particles; (ii) adsorption densities increase dramatically if the dispersion is agitated or particle concentration decreased; (iii) adsorption curves do not show limiting values even after the adsorbed densities correspond to formation of many monolayers of methyl oleate; (iv) adsorption is stronger at a less polar (more hydrophobic) surface at pH 8.7 than at pH 5; and (v) after a monolayer coverage is achieved, at pH 5, a steep increase in adsorption is observed, which indicates the facilitated adsorption at a covered surface. IMPORTANCE OF HYDROPH( IBIC INTERACTION
METHYLOLEATE.pH 8.7
by
METHYLOLEATE.
- 3000
a- At2031
zO - 2000
I---
Q_ 0C O (/I a
- ooo
~
=~_~_ _ mo~noiclyer I
10
CONCENTRATION/mg[ -1
I
10
Fig. 7. Interaction of heterodispersions of methyloleate and 6-A12Os at (a) pH8.7 and (b) pH 5.0.
62
Oleic acid shows a qualitatively similar behaviour. However, linoleic acid that absorbs strongly at the fluid mercury surface (Fig. 2b), does not show measurable adsorption at the alumina surface in the concentration range accessible by polarography, at pH8.7 (0.03-10mgl ~) or p H 5 (3 80mgl 1), where fluid aggregates are formed at higher concentrations (> 10mgl 1). These experimental facts indicate that steric effects are of utmost importance in adsorption of fatty acids at the alumina surface: an alkyl chain with three conjugated double bonds cannot adapt to the alumina surface due to its rigid structure. Coordination between surface aluminium atoms and the carboxylic groups seems to be of secondary importance, while hydrophobic interactions at the alumina/seawater interface predominate. Natural heterodispersions Natural heterodispersions, such as phytoplankton cell suspensions (Cryptomonas sp., Dunaliella tertiolecta) that contain phytoplankton cells and fragments, such as fluid vesicles (Aaranson, 1971; ~utid et al. 1981; Patton and Burris, 1983) show a similar irregular pattern in the current-time curves at the mercury electrode (Fig. 8, curve 1) as oleic aggregates (Fig. 1, curve 1). However, the individual perturbations are less pronounced, due to a lower fluidity (Zutid et al., 1984). The production of surface active vesicles seems to be inherent to flagellates because of their characteristic cell membrane and the secretion of membrane structures with a high lipid content (~< 10%). It seems also that the living cells themselves coalesce and partly reorganize into an adsorbed layer at the interface electrode/seawater in the time scale of the drop life ( < 2 s). The effect of hydrous alumina on Dunaliella cell suspension (Fig. 8, curve 2) is similar to the
150
1) DUNALIELLA TERTIOLECTA (1.4.108 cet~ 14 ) 2) H) * 8-At203 5 0 m g l -~
i / IJ,A !
/
,oo~
0
50
2
6
8
t/s
Fig. 8. Effect of addition/~-AI~O 3 (50mg I 1) to the cell suspension ofDunaliella tertiolecta (1.4 × 108 cell 1-1), 20°C, pH 8.7. C u r r e n t time curves in (1) a r t i f i c i a l s e a w a t e r ( 2 ) D u n a l i e a l l a cell suspension (3) Dunaltella cell suspension + 50 mg l- 1, 6-A12 O z a f t e r 2 h shaking.
63
experiment with dispersion of methyl oleate (Fig. 1), demonstrating clearly that natural, as well as artificial fluid aggregates are preferentially adsorbed at mineral interfaces. Fluid surface active aggregates, similar to those of methyl oleate, according to the type of perturbation in the current-time curves, were detected in the mixing zone in estuarine (~uti6 et al., 1984; ~uti6 and Legovi6, 1987) (Fig. 9) as well as in sea surface microlayer and subsurface waters of the North Adriatic (Novakovi6 and ~uti6, 1983, Marty et al., 1987). In estuarine mixing zones the aggregates seem to be generated by physicochemical processes from dissolved organic molecules and smaller aggregates, and not by a direct in situ primary production. High surface activity of these aggregates and a high rate of their reorganization into adsorbed layers at interfaces has important implications in the transport and scavenging of hydrophobic organic pollutants (Imboden and Schwarzenbach, 1985) and reactive trace elements (Cauwet and Faguet, 1982; Hunt, 1983). COALESCENCE FREQUENCY / 0.1
I
IxI07
0.2
'
s -I
o.3
I
5,x 107
NUHBER OF SURFACE ACTIVE AGGREGATES I N t-~
E "r bfl. i,i
S '
lb
io
S 1%o
~'o
Fig. 9. Relative distribution of surface active aggregates along the depth profile of a stratified estuary (River Aude, Western Mediterranean, May 1983). The number of coalescences is defined as the number of well defined perturbations on current-time curves.
DISCUSSION In natural waters, and in seawater in particular, hydrophobic compounds commonly exist in a micetlar or colloid accommodated state rather than as true molecular solutes; critical micellar concentration for a number of lipids has been reported to be below 10 7M (Tanford, 1980). Previous studies (Loeb and Neuhof, 1977; Kristofferson et al., 1982) have demonstrated a continuous growth of organic coatings on platinum surfaces immersed in natural seawater. Among all the model substances we have studied, only sparingly soluble hydrophobic materials, such as unsaturated fatty acids and esters, demonstrated an ability to form multilayer coatings on alumina particles, with no indication of a saturation coverage (Fig. 7). Adsorption of simple carboxylic acids, such as salicylic acid, or amino acids with functional groups characteristic of aquatic organic matter was found to be negligible for natural concentration levels (Table I). Humic materials showed saturation at a monolayer surface coverage (Fig. 6) of a similar value (corresponding to 27 C atoms per nm 2) to one reported for lake organic matter (Baccini et al., 1982). The important dependence of adsorption densities on particle concentration (Figs. 4-6) agrees well with the findings in lake waters: (Baccini et al., 1982) and estuaries. This seems to be a common phenomenon in natural aquatic systems, where AOM has all the characteristics of a polydisperse (multicomponent) solute with a span of adsorption constants, analogously to its complexation behaviour to heavy metals (Turner et al., 1986). Hydrophobic attraction (Tanford, 1980; Israelachvili and Pashley, 1983) seems to be the most important mechanism of AOM interaction at marine interfaces. It is stronger than van der Waals forces and of significantly longer range than any 'bond' and highly unspecific. Unsaturated lipid molecules, micelles and aggregates represent the most reactive surface active materials. They also seem to mediate interaction of other AOM constituents with particles once a first monolayer, or its fraction, has been formed. We have characterized a highly reactive fraction of AOM: fluid surface active aggregates that are ubiquitous in productive surface waters and sea surface microlayers (Novakov[c and ZutiS, 1983; Plebe and Zuti6, 1984) and in mixing zones in estuaries (Fig. 9) (Zuti6 et al., 1986; Zutid and Legovi6, 1987) and is not amenable to analysis by standard techniques that involve separation to a ~dissolved' and ~particulate' AOM fraction. Although unsaturated lipids can constitute only a minor fraction of an aggregate material (Marty et al., 1987), they seem to determine its structure and fluidity as in the case of biological materials, such as membrane structures (Tanford, 1980). This highly reactive and unstable fraction of AOM seems to be a precursor of SEM visible organic coatings, persistent gel-like structures (~voiles organiques', Alo~si et al. (1975)) observed on mineral particles in coastal estuarine zones. A fast and reversible adsorption of AOM at charged marine interfaces might only constitute an initial step in the process of organic coating formation that
65
in its advanced steps should involve the complex structure formation and organization phenomena well known in membrane science and biophysics (Tanford, 1980). The closest analogue of this process is bubble dissolution in seawater (Johnson and Cooke, 1980). Thus, reversible adsorption models should be critically applied to marine processes involving AOM and particles. ACKNOWLEDGEMENT
Dr. Damir ViliSid is thanked for the preparation of the phytoplankton cultures, Dr. Gustave Cauwet and Dr. Edward Tipping for their generous gifts of the humic materials, and Ralph Gloor for the molecular mass distribution analysis. The financial support of the Self-Managing Community of Interest for Scientific Research of S.R. Croatia, Yugoslavia, is gratefully acknowledged. REFERENCES Aaranson, S., 1971. The synthesis of extracellular macromolecules and membranes by a population of the phytoflagelate Ochromonas danica. Limnol. Oceanogr., 16: 1-9. Adamson, A.W., 1982. Physical Chemistry of Surfaces, 3rd Edn. Wiley-Interscience, New York, 663 pp. Aloisi, J.C., Monaco, A. and Pauc, H., 1975. Mechanism de la formation des prodeltas dans le Golf de Lion. Example de l'embouchure de l'Aude (Languedoc). Bull. Geol. Bassin Aquitaine, 18: 3-12. Aogaki, R., Kitzawa, K., Iukai, K. and Mukaibo, T., 1978. Theory of polarographic maximum current. Growth or decay of the electrochemical and hydrodynamic instability. Electrochim. Acta, 23: 875~880. Baccini, P., Grieder, E., Stierli, R. and Goldberg, S., 1982. The influence of natural organic matter on the adsorption properties of mineral particles in lake water. Swiss J. Hydrol., 44: 99-116. Cauwet, G. and Faguet, D., 1982. The role of organic matter in transport processes of metals in estuarine environments. Thalassia Jugosl., 18: 379-392. Cosovid, B. and Vojvodid, V., 1982. The application of a.c. polarography to the determination of surface-active substances in seawater. Limnol. Oceanogr., 27: 361-369. Cosovid, B., ~utid, V., Vojvodid, V. and Ple§e, T., 1985. Determination of surfactant activity and anionic detergents in seawater and sea surface microlayer in the Mediterranean. Mar. Chem., 17: 127-139. Damaskin, B.B., Petrii, O.A. and Batrakov, V.V., 1971. Adsorption of Organic Compounds on Electrodes. Plenum Press, New York, 499 pp. Davis, J.A. and Gloor, R., 1981. Adsorption of dissolved organics in lake water by aluminium oxide. Effect of molecular weight. Environ. Sci. Technol., 15: 1223-1229. Davis, J.A., 1982. Adsorption of natural dissolved organic matter at the oxide]water interface. Geochim. Cosmochim. Acta, 46: 2387-2393. Davis, J.A., 1984. Complexation of trace metals by adsorbed natural organic matter. Geochim. Cosmochim. Acta, 48: 679-691. Elliot, H.A. and Huang, C., 1980. Adsorption of some copper(II) amino acid complexes at the solid-solution interface: effect of ligand and surface hydrophobicity. Environ. Sci. Technol. 14: 87-93. Faguet, D. 1982. Influence des substances humiques sur les formes dissoutes et particulaires de quelques metaux dans la milieux marine et lagunaire. M.Sc. Thesis, Perpignan. Furrer, G., 1985. Die Oberfl~ichenkontrollierte Aufl5sung von Metalloxiden: Ein Koordinationschemischer Ansatz zur Verwitterungskinetik. Ph.D. Thesis, ETH, Zurich.
6~ Furrer, G. and Stumm, W., 1986. The coordination chemistry of weathering. I. Dissolution kinetics of ~$-A1203 and BeO. Geochim. Cosmochim. Acta, 50:1847 1860. Gloor, R., Leidner, H. and Wuhrman, K., 1981. Exclusion chromatography with carbon detection. A tool for characterization of dissolved organic carbon. Water Res., 15: 457-463. Hunt, C.D., 1983. Incorporation and deposition of Mn and other trace metals by flocculant organic matter in a controlled marine ecosystem. Limnol Oceanogr., 28:302 308. Hunter, K.A., 1980. Microelectrophoretic properties of natural surface.active organic matter in coastal seawater. Limnol. Oceanogr., 25:807 822. Hunter, K.A. and Liss, P.S., 1980. Polarographic measurement of surface-active material in natural waters. Water Res., 15: 203-205. Hunter, K.A. and Liss, P.S., 1982. Organic matter and the surface charge of suspended particles in estuarine waters. Limnol. Oceanogr., 27: 322-355. Imboden, D. and Schwarzenbach, R., 1985. Spatial and temporal distribution of chemical substances in lakes: modelling concepts. In: W. Stumm (Editor), Chemical Processes in Lakes. WileyInterscience, New York, pp. 1-28. Israelachvili, J.N. and Pashley, R.M., 1983. Measurement of the hydrophobic interaction between two hydrophobic surfaces in aqueous electrolyte solutions. J. Colloid. Interface Sci., 98: 500514. Johnson, B.D. and Cooke, R.C., 1980. Organic particles and aggregates formation resulting from the dissolution of bubbles in seawater. Limnol. Oceanogr., 25: 653-661. Kristoffersen, A., Rolla, G., Skjorland, K., Glantz, P.O. and Ivarsgon, B., 1982. Evidence for the formation of organic films on metal surfaces in seawater. J. Colloid. Interface Sci., 75: 373-385. Kummert, R., 1979. Die Oberfl~ichenkomplexbildung von organischen S~iuren mit GammaAluminiumoxid under ihre Bedeutung fur natiiurliche Gew~iser. Ph.D. thesis, ETH, Zurich. Kummert, R., and Stumm, W., 1980. The surface complexation of organic acid on hydrous y-A120a . J. Colloid Interface Sci., 75; 375-385. Loeb, G.I. and Neihof, R.A., 1972. The surface charge of particulate matter in seawater. Limnol. Oceanogr., 17: 16-27. Loeb, G.I. and Neihof, R.A., 1977. Adsorption of an organic film at the platinum-seawater interface. J. Mar. Res., 35: 283-291. Marty, J.-C., ~utid, V., Precali, R., Cosovid, B., Saliot, A., Smodlaka, N. and Cauwet, G., 1988. Mar. Chem., submitted. Morel, M.M. and Hudson, J.M., 1985. The geobiological cycle of trace elements in aquatic systems: Redtield revisited. In: W. Stumm (Editor), Chemical Processes in Lakes. WiIey-Interscience, New York, pp. 251-281. Neuwinger, H.D., 1970. Darstellung und Oberflacheneigenschaften verschiedener Aluminiumoxidformen. Ph.D. Thesis, Heidelberg. NovakoviS, T. and ~utiS, V., 1983. Electrochemical characterization of unsaturated lipid dispersions in marine aqueous samples. Rapp. Comm. Int. Mer Medit., 28: 109-113. Patton, J.S. and Burris, J.E., 1983. Lipid synthesis and extrusion by freshly isolated zooxantellae (symbiotic algae). Mar. Biol., 75:131 136. Plebe, T. and ~utid, V., 1984. On irregular pattern of polarographic maxima in surfactant dispersions. J. Electroanal. Chem., 175: 299-312. Sigg, L. and Stumm W., 1981. The interactions of anions and weak acids with the hydrous geothite surface. Colloid. Surf. 2: 101-117. Sorensen, T.S., 1979. Dynamics and Instability of Fluid Interfaces. Lecture Notes on Physics. Springer-Verlag, Berlin, 327 pp. Stumm, W. and Morgan, J.J., 1981. Aquatic Chemistry, 2nd Edn. Wiley-Interscience, New York, 780 pP. Stumm, W., Kummert, R. and Sigg, L., 1980. A ligand exchange model for the adsorption of inorganic and organic ligands at hydrous oxide interfaces. Croat. Chem. Acta 53: 291-312. Tanford, C., 1980. The Hydrophobic Effect: Formation of Micelles and Biological Membranes, 2nd Edn. Witey-Interscience, New York.
67 Tipping, E., 1981. The adsorption of aquatic humic substances by iron oxides. Geochim. Cosmochim. Acta, 45: 191-199. Turner, D.R., Varney, M.S., Whitfield, M., Mantoura, R.F. and Riley, J.P., 1986. Electrochemical studies of copper and lead complexation by fulvic acid. I. Potentiometric measurements and a comparison of metal binding models. Geochim. Cosmochim. Acta, 50: 289-297. Whitfield, M. and Turner, D.R., 1986. The role of particles in regulating the composition of natural waters. In: W. Stumm (Editor), Proceedings in Aquatic Surface Chemistry: Chemical Processes at the Particle/Water Interface. Ermatingen, pp. 106--112. Zvonari6, T., ~uti6, V. and Branica, M., 1973. Determination of surfactant activity of seawater samples by polarography. Thalassia Jugosl., 9: 65-73. ~uti6, C. and Legovi6, T., 1987. A film of organic matter at the freshwater/seawater interface of an estuary. Nature, 328: 612-614. ~uti6, V. and Stumm, W., 1984. Effect of organic acids and fluoride on the dissolution kinetics of hydrous alumina. A model study using the rotating disc electrode. Geochim. Cosmochim. Acta, 48: 1493-1503. ~uti6, V., Cosovid, B. and Kozarac, Z., 1977. Electrochemical determination of surface active substances in natural waters. On the adsorption of petroleum fractions at mercury/seawater interface. J. Electroanal. Chem., 78: 113-121. ~utid, V., Cosovid, B., Mar6enko, E., Bihari, N. and Kr~ini6, F., 1981. Surfactant production by marine phytoplankton. Mar. Chem., 10: 505-520. ~uti6, V., Ple§e, T., Tomaid, J. and Legovid, T., 1984. Electrochemical characterization of fluid vesicles in natural waters. Mol. Cryst. Liq. Cryst., 113: 131-145. ~,uti6, V., Cauwet, G. and Monaco, A., 1986. Role of organic aggregates in the transport of pollutants in Mediterranean Estuaries. Proc. 7th Workshop on Marine Pollution of the Mediterranean, Luzern, 1984, ICSM/IOC/UNEP, pp. 173-180.
51
Elsevier Science Publishers B.V., Amsterdam-- Printed in The Netherlands
ON THE F O R M A T I O N OF ORGANIC COATINGS ON M A R I N E P A R T I C L E S : I N T E R A C T I O N S OF ORGANIC M A T T E R AT HYDROUS ALUMINA/SEAWATER INTERFACES
V E R A 2UTIC and J A D R A N K A
TOMAIC
Center for Marine Research, Zagreb 'Rudjer Bo~kovid' Institute, POB 1016, 41001 Zagreb (Yugoslavia)
(Received October 22, 1986; revision accepted June 15, 1987)
ABSTRACT ~utid, V. and Tomaid, J., 1988. O n the formation of organic coatings on marine particles:interactions of organic matter at hydrous alumina/seawater interfaces. Mar. Chem., 23: 51-67. The interaction mechanism between aquatic organic matter (AOM) and mineral particles in seawater has been studied using hydrous alumina as a model mineral phase. The electrochemical methodology allowed direct measurements in oxide suspensions and distinction between 'dissolved' and 'particulate'organic matter. Adsorption of small organic molecules, such as salicylic acid, was found to be negligible at natural concentration levels.Unsaturated lipidsare the most reactive surface-active constituents of A O M leading to multilayer formation. For humic materials an inverse relationship has been established between absorbed densities and particle concentration and it is interpreted as the fractionation effectof a polydisperse solute. A new, highly surface-active fraction of A O M has been identified as fluid surface-active aggregates, ubiquitous in productive surface waters, sea surface microlayers and in mixing zones in estuaries. Hydrophobic attraction is the most important force governing AOM-particle interaction in seawater. Reversible adsorption of organic molecules at charged marine interfaces is only the initialstep in the process of organic coating formation that involves complex structural organization phenomena well known in membrane sciences and biophysics.
INTRODUCTION The behaviour of the elements in seawater is closely linked to the extent to which they are involved in particle-water interactions, either via biological utilization or by adsorption/desorption reactions at active surfaces. A number of models have been developed to simulate the adsorption process. The most effective appears to be via the assumption that adsorption results from complexation at specific surface sites on the particle surface (Stumm and Morgan, 1981). The identification and characterization of these sites remains empirical (as reviewed by Morel and Hudson (1985) and Whitfield and Turner (1986)). The microelectrophoretic studies (Neihof and Loeb, 1972, 1977; Hunter, 1980;
0304-4203/88/$03.50
© 1988 Elsevier Science Publishers B.V.
52 Hunter and Liss, 1982) have established that surface-active molecules in estuarine and coastal seawater determine the surface properties of suspended particles. However, the extent of oxide surfaces covered by organic films and the mechanism of their formation in the marine environment is still a matter of speculation. Much more work has been done in defined model systems relevant to fresh water (Kummert and Stumm, 1980; Sigg and Stumm, 1981; Stumm et al., 1980; Tipping, 1981; Davis and Gloor, 1981; Davis, 1982, 1984). In most of the experiments described, relatively high concentrations of particles (~- 1 gl 1 ) were used (mostly due to the methodological problems inherent to adsorption studies by standard techniques, with the exception of a methodology developed by Baccini et al. (1982)). Formation of thick organic films developing over many hours, has been observed by optical methods on metal surfaces immersed in natural seawater (Loeb and Neihof, 1977; Kristoffersen et al., 1982) while the extensive evidence on the earliest phases of adsorption of marine organic matter at charged interfaces has been obtained by direct electrochemical measurements of surfactant activity of seawater samples using a mercury drop electrode (Zvonarid et al., 1973; ~uti5 et al., 1977, 1981; ~osovi~ and Vojvodi~, 1982; ~osovi~ et al., 1985). The range of surfactant activity values observed in coastal as well as deep Mediterranean waters (0.2-1.6 mg 1 1) indicates that under the conditions of maximum adsorption (positively charged and hydrophobic surfaces) significant surface coverages (5-50%) are already attained on a few second time scale by convective transport. We consider here the interactions between defined hydrous oxide particles and model organic materials (natural and artificial) in seawater. The electrochemical methodology presented in this work allows: (1) direct measurements in oxide suspensions without any previous separation that would affect the distribution between aqueous and solid phase; and (2) distinction between dissolved surface active materials and surface active aggregates. 5-alumina was chosen as the model mineral phase because of its electroinactivity, the range of positive surface charges and a significant amount of research on the interactions of this material with organic solutes in terms of adsorption (Kummert, 1979; Kummert and Stumm, 1980) and weathering (Furrer, 1985; Furrer and Stumm, 1986). ELECTROCHEMICALMETHOD In a previous paper (~utid and Stumm, 1984) we developed a direct electrochemical approach to study the interactions between dissolved organic molecules and hydrous alumina under weathering conditions. The interactions between alumina particles and organic molecules in seawater can be studied by the electrochemical method ofpolarographic maximum of mercury(II), (~utid et al., 1977; Hunter and Liss, 1980; ~uti6 et al., 1981).
53
The method is based on the interfacial instability of a liquid/liquid interface-dropping mercury electrode/aqueous electrolyte solution which is generated by the gradient of surface tension (Sorensen, 1979; Aogaki et al., 1978) manifested as convective streaming and a measurable increase in the reduction current of mercury(II) (polarographic maximum). Dissolved organic molecules (as well as micelles) adsorbed at the mercury/seawater interface decrease the gradient of surface tension and slow down the convective streaming, which is manifested as a suppression of the maximum current. This phenomenon provides a basis for a quantitative determination of surface active materials. Suspensions of alumina particle alone (~< 1 g1-1) do not affect the electrochemical response. Heterodispersions of fluid surfactants, such as unsaturated fatty acids and their esters in seawater, produce the irregular current oscillations of the polarographic maximum (Fig. 1). The perturbations in the current-time curves of variable amplitude and frequency indicate a stochastic process, which corresponds to random collisions of surface active aggregates of variable size with the mercury electrode/aqueous solution interface (Plebe and ~utid, 1984). Measurement of the amplitude and duration of perturbations allows a direct characterization of single events of coalescence and transformation of the aggregates into the adsorbed layer at the interface (relaxation times 10-500 ms, aggregation numbers > 109) (~,utid et al., 1984). The amount of the organic matter adsorbed by alumina particles during adsorption batch experiments is determined from the difference in a solution (dispersion) concentration before and after addition of alumina according to the calibration curves (Fig. 2).
i/IJ.A
1) METHYL OLEATE 14.1m g / I 2) (1) filtered 0.45 IJ.m 3) (1) • ,3-Al203 100 mg/[
15C
10(
5C
/'
t
/j**
/// /
,, . . - "
.....
II.'" l#
,/,' /
V 4
6
8
t/s
Fig. 1. Current-time curves at the dropping mercury electrode (2 s drop time) in artificial seawater (curve 0) in the presence of 14.1 mg1-1 methyl oleate before (curve 1) and after filtration (curve 2). Curve 3: current-time curves in dispersion of 14.1 mg l-1 methyl oleate + 100 mg 1-i ~.AI203 after 2 h shaking at 20°C.
54
"o, \ \ \
) ~ N N
i
i
i
i i ,Jr1
0.I
.
50
J
J
I
1 TRITON-X-IOO
c~:r,oo~
2. u.oL~mlc AClO ~. OLE,C AC,O
r
i ,J,iI
E
i
:
i
10 Concenfrai'ion ling L-I
\\, \
\
\ X
,
100
\
%
\
\ \ \
i,iiI
'\T
\]
\,
~ 0,.-__,.~1 O__ 0 ._~mm ~,,~__z~__ I 0,|
I 1 Concentration/mg
110
id, f
~0
I. -i
Fig. 2. Calibration curves: dependence of current (at the end of drop life) on the concentration of surfactants in artificial seawater. EXPERIMENTAL A d s o r p t i o n b a t c h e x p e r i m e n t s were performed in 100 cm 3 flasks at 20°C. An aliquot of freshly p r e p a r e d stock suspension of a l u m i n a particles (in distilled water) is added to the solution (or dispersion) o f o r g a n i c m a t e r i a l in the artificial or n a t u r a l seawater. T h e equilibration time of 4 h (with or w i t h o u t c o n t i n u o u s shaking) was c h o s e n for all experiments. Only freshly p r e p a r e d solutions and dispersions of s u r f a c t a n t s were used and a n a l y s e d to avoid a n y significant t r a n s f o r m a t i o n by chemical oxidation (e.g. u n s a t u r a t e d f a t t y acids) or by b a c t e r i a l activity. M e a s u r e m e n t s of unfiltered aliquots (50cm3), to which Hg(II) solution (0.5 cm 3 o f 0.I M solution) is added j u s t before the m e a s u r e m e n t , are made in a lOOcm3 all-glass cell open to air and t h e r m o s t a t e d a t 20~C w i t h a fawt d ~ p p i n g
55 mercury electrode and Ag/AgC1 reference in a three-electrode configuration. A PAR model 174 Polarographic analyzer in connection with a 7054 HewlettPackard recorder has been used for registration of polarograms, while the current-time transients (at a constant potential, E = - 300 mV) were recorded using a Gould digital storage oscilloscope with 4001 output unit. A CoulterCounter, Model TA, was used for characterization of particle distribution. Materials The commercial product 'Aluminium oxide C' (Degussa, Frankfurt) used as a model mineral phase, consists of J-Als 03 (R. Giovanolli, unpublished results, 1981) nonporous particles with a diameter of 20 n m (BET specific surface area 108 m sg-i) (Kummert, 1979) and p H zero point of charge 8.7 (Neuwinger, 1970). Commercial products (salts and surfactants) were reagent grade. Artificial seawater was of the following composition: 0.6MNaCI, 1 0 - 3 M K B r * and 5 × 10-3M N a H C O a . Tetradistilled water was used throughout. Special care was taken with respect to the purity of water, glassware and the atmosphere in the laboratory. Organic traces from commercial sodium chloride were eliminated by prolonged heating at 450°C and addition of active carbon to the stock solution. Acetic buffer (10-2M) was used for adjusting to p H 5, since in these conditions it does not adsorb on hydrous alumina (~utid and Stumm, 1984). Humic acid I, isolated from the deposits of Ruppia maritima (Lagune of Cannet, Western Mediterranean) by Faguet (1982) and Cauwet and Faguet (1982) had the molecular mass distribution shown in Fig. 3 as determined by Gloor et al. (1981) and the elemental composition: 44.84% C, 4.73% H, 4.35% N. Humic acid II was isolated from Penwhirn Reservoir, Scotland by Tipping (1981), it had a weight-average molecular weight c. 27000, 3.1 × 10-3molg -I carboxyl groups (pK < 5) and 10-3molg -1 phenolic O H groups (pK -~ 11) (Tipping, 1981). Stock dispersions of fatty acids and their esters were prepared by adding 0.1cm 3 of a commercial product into 500cm 3 5 × 10-2NaHCO3 solution and shaking for 4 h. Axenic phytoplankton cell suspensions were prepared in the Biological Institute, Dubrovnik, with the nutrient enriched (f/2 medium) Adriatic seawater (S = 38%0, DOC ~< 0.5mg1-1) as the growth medium. RESULTS The interaction between alumina particles and organic matter has been studied in artificial seawater at p H 8.7 (zero point of charge) and p H 5.0 (pos-
* While additionof Mg 2+,Ca 2+ and SO~- had no measurable effecton the reductionof Hg(II)and dissolved oxygen in air-saturated0.6M NaCl solution,the presence of Br- was important in simulating the response in natural seawater because of its strong interaction with mercury electrodesat potentials > -0.1V.
56
Z 0 m n-
HUMIC A C I D from Ruppio deposit
(,9 U
z 0 nO ILl .J
LU n~
]
I
[
I
>80
20
10
I
I|
5 1<0.2
M O L E C U L A R W E I G H T ' 10 3 Fig. 3. Molecular weight distribution of humic acid I.
itivety charged surface). The adsorption behaviour of the following natural and synthetic model materials (i) dissolved simple molecules (salicylic acid, phenylalanine, tyrosine) (ii) polymers (polyetoxynonytphenol and humic materials) (iii) heterodispersions of fluid surfactants (unsaturated fatty acids and esters) and (iv) phytoplankton cell suspensions was studied in the concentration range 10#gl-l-100mgl ~, while the concentration of alumina particle was varied in the range 10-800 mg 1-~.
Dissolved simple molecules Simple molecules, with functional groups characteristic of aquatic organic matter, and humic materials in particular, do not show measurable adsorption on positively charged or uncharged alumina/seawater interfaces. This applies to the concentration range which could be analysed by the polarographic method (Table I; calibration curves in Fig. 2a) and would also be relevant for natural conditions. This is in agreement with the published results at lower electrolyte concentrations (< 0.1 M) of Kummert and Stumm (1980) for salicylic acid, Elliot and Huang (1980) for phenylalanin and Davies and Gloor (1981). They concluded that in fresh water, molecules with a molecular weight less than 1000
57 TABLE I
Measurable concentration range for some simple molecules, and the estimated upper limits of the values for adsorption constants (K L app.)assuming Langmuir isotherms
molecule [ ~
I pK
OOH OH
salicylic
concentr, range / mg t"1
KL app. / M-1
2.8
0.01 - 5
< 10 ¢
2.2
0.01 - 8
<10 ¢
1.8
2 - 100
< 10 3
0.01 - 10
< 10 4
acid
OH
0
CH2CH COOH NH2
tyrosine
© CH2 CH-COOH NHz
phenytalanine
;AI~>-O-[CHz- CH20-] M ' ~
9-1c - -
Triton - X - 100
are not significantly adsorbed at mineral surfaces. At the mercury electrode/ seawater interface strong adsorption of salicylic acid, tyrosine and polyetoxyalkylphenols (Triton-X-100) is due to ~-electron interactions between the flat oriented phenolic ring and the positively charged mercury surface (Damaskin et al., 1971). Humic materials
Adsorption behaviour of the two substances is summarized in Figs. 4 and 5. Adsorption densities expressed as mg adsorbed humic per gram alumina are plotted against concentration in the aqueous phase measured after 4 h adsorption.
58
2o01
b
r I i
"Tm150~ -o~
HUMIC ACID I, pH 8.7
E t *~-010
I00~-
O
20
/".,
11 / / /
100
?/ , xf j /,,/~..~
.200 ~ u ===.~_ 501- ,1,/ ~ o - - - - - ~ o I :~,'" o/ 8o0._~_~ l;g," ..o/ ~ ' ~ It/,,,, o"/ / lY?~"
o
/ 10
20 30 CONCENTRATION
MIC ACID I. pH 5.0
. 10 Imgt-I
, 30
20
Fig. 4. Apparent adsorption isotherms for humic acid I at pH 8.7 and pHS.0. Concentration of 10, 20, 100, 200 and 800 mg ]-z. Dashed lines indicate adsorption experiments with shaking. 6-AI 20 3 suspensions:
,~ 20
200
a
Tc~ 150 E
100
x ~
x
/2OOO.1°
z o ~. I0(~ no u3 D
~
X
50 HUMIC ACID II, pH 5.0
10
I
20 10 3O CONCENTRATION / m g r 1
Fig. 5. Apparent adsorption isotherms for humic acid II a t p H 8.7 and pH 5.0.
I
20
3O
59
Although the two materials are of differentorigin and isolationprocedures, the following general characteristicsare apparent: (i) strong adsorption in a similar concentration range as at the positively charged mercury/seawater interface; (ii)adsorption is stronger at the lower pH; this effectis much more important for humic acid I,but is less than that observed for natural organic matter in fresh water conditions (low ionic strength) (Davis, 1982); (iii)adsorption densities approach a limiting value; (iv) shaking of suspensions during the batch adsorption experiment does not affectthe m a x i m u m adsorption density. However, the risingpart of the adsorption curve shiftstowards lower solution concentrations indicating that (at this time scale) mass transport in the aqueous phase is the limiting step; and (v) adsorption densities depend on the particle concentrations: they decrease dramatically with increasing particleconcentration, as is clearlyshown by the apparent adsorption isotherm in Fig. 5a. The values of apparent adsorption constants (subscriptapp.) evaluated from the 'Langmuir plots' (Fig. 6) decrease with increasing particle concentration, while the m a x i m u m adsorption density, P .... is independent of particle concentration in the aqueous phase, and reaches 14% of the material in the particulate phase. Such behaviour can be understood as adsorption of a polydispersed solute (Adamson, 1982), which humic material definitelyis (Fig. 3), with a preferentialadsorption of higher molecular weight fractions(adsorption constants /?iincrease with increasing molecular weight). The fractionation HUMIC ACID !
~app(Cm3/g) :
/ 6-AIzO 3 {g/cm31 : /
le=~
L
o
x
/
r"mox = 0.143 [
0
I
[
106 [CRAIg/cm31-1
Fig. 6. [ ~ n ~ u i r
plot (inverse adsorption density vs. inverse concentration) for adsorption of
humic acid I, pH 8.7 at different concentrations of aluimina suspensions (g ~-AI208 cm -s) indicated on the curves.
6O
effect should increase with increasing particle concentration. A simple model with a two-component solute (ill > f12, C2.0 > C~.0, F, .... > F2.... ) reversibly adsorbed on a homogenous surface
['1+2
=
F, .... /~1C, + F~.... /~2C2 1 ~ /?,C, + /72C2
(Ci being the equilibrium and Ci.o the initial concentration of the ith component) gives the same effect if particle concentration is introduced as a variable and only total solute concentration (C1 + C2) experimentally measured. The fractionation effect has been observed for adsorption of organic material isolated from a Swiss lake (Davis and Gloor, 1981; Davis, 1982)*. The increase in particulate organic carbon with decreasing particle concentration was detected for various particles (with homogeneous as well as heterogeneous surfaces) in adsorption experiments with natural lake water (Baccini et al., 1982), and in sedimentological studies of Western Mediterranean estuaries (Monaco, 1983, unpublished). Thus, it is inappropriate to predict the adsorption behaviour of humic material, or any other polydispersed organic material in natural aquatic systems by mere extrapolation of laboratory experiments performed with high particle concentrations (commonly ~>I g-1). Such extrapolations have resulted in tremendous underestimations of the importance of organic adsorption in the sea, where particle concentration is only a few milligrams per liter or less. Adsorption of humic material at alumina particles is much stronger than that of simple molecules with identical surface complexing groups (such as salycilate or phtalate) and is comparable to adsorption at a hydrophobic metal surface such as mercury. Surface complexation does not seem to be the only mechanism of adsorption of humic material to mineral particles in the marine environment, and the hydrophobic interaction (Tanford, 1980; Israelachvili and Pashley, 1983) should be considered.
Heterodispersions of fluid surfactants Typical molecules were chosen which differ in their structure and charge: at pH 8.7 oleic and linoleic acids are negatively charged (pK ~< 5), while methyl oleate remains uncharged. In seawater, methyl oleate and oleic acid are predominantly present as heterodispersions, while linoleic acid is significantly more soluble due to increased polarity of the alkyl chain with three conjugated double bonds (Tanford, 1980). However, the exact data on their solubilities in concentrated electrolyte solutions, such as seawater, are lacking in the literature. By measuring size distributions and settling velocities it was proved that heterodispersions of these materials in artificial seawater did not change * It is important to note that the authors could not detect any fractionation effect according to the acidity constants of the organic material, but only according to molecular weight.
61
significantly during the period of 4 h, nor did the addition of Hg(II) solution have a measurable effect during the time period of polarographic analysis (< 15 rain). Figure I illustrates typical features of dispersions of fluid surfactants: (i) at the mercury electrode coalescence of a fluid aggregate (manifested as a sharp current spike) is followed by a fast reorganization into the adsorbed layer (decay of current after the spike) at the mercury electrode/seawater interface. The process takes place on the millisecond time scale; and (ii)fluid aggregates are quantitatively removed from the aqueous phase by strong interaction with the alumina particles (curve 3), comparable to the effect of filtration (>/0.45 #m pore size). The interaction between alumina particles and heterodispersions of methyl oleate (Fig. 7) can be summarized as follows: (i) fluid aggregates are preferentially adsorbed by alumina particles; (ii) adsorption densities increase dramatically if the dispersion is agitated or particle concentration decreased; (iii) adsorption curves do not show limiting values even after the adsorbed densities correspond to formation of many monolayers of methyl oleate; (iv) adsorption is stronger at a less polar (more hydrophobic) surface at pH 8.7 than at pH 5; and (v) after a monolayer coverage is achieved, at pH 5, a steep increase in adsorption is observed, which indicates the facilitated adsorption at a covered surface. IMPORTANCE OF HYDROPH( IBIC INTERACTION
METHYLOLEATE.pH 8.7
by
METHYLOLEATE.
- 3000
a- At2031
zO - 2000
I---
Q_ 0C O (/I a
- ooo
~
=~_~_ _ mo~noiclyer I
10
CONCENTRATION/mg[ -1
I
10
Fig. 7. Interaction of heterodispersions of methyloleate and 6-A12Os at (a) pH8.7 and (b) pH 5.0.
62
Oleic acid shows a qualitatively similar behaviour. However, linoleic acid that absorbs strongly at the fluid mercury surface (Fig. 2b), does not show measurable adsorption at the alumina surface in the concentration range accessible by polarography, at pH8.7 (0.03-10mgl ~) or p H 5 (3 80mgl 1), where fluid aggregates are formed at higher concentrations (> 10mgl 1). These experimental facts indicate that steric effects are of utmost importance in adsorption of fatty acids at the alumina surface: an alkyl chain with three conjugated double bonds cannot adapt to the alumina surface due to its rigid structure. Coordination between surface aluminium atoms and the carboxylic groups seems to be of secondary importance, while hydrophobic interactions at the alumina/seawater interface predominate. Natural heterodispersions Natural heterodispersions, such as phytoplankton cell suspensions (Cryptomonas sp., Dunaliella tertiolecta) that contain phytoplankton cells and fragments, such as fluid vesicles (Aaranson, 1971; ~utid et al. 1981; Patton and Burris, 1983) show a similar irregular pattern in the current-time curves at the mercury electrode (Fig. 8, curve 1) as oleic aggregates (Fig. 1, curve 1). However, the individual perturbations are less pronounced, due to a lower fluidity (Zutid et al., 1984). The production of surface active vesicles seems to be inherent to flagellates because of their characteristic cell membrane and the secretion of membrane structures with a high lipid content (~< 10%). It seems also that the living cells themselves coalesce and partly reorganize into an adsorbed layer at the interface electrode/seawater in the time scale of the drop life ( < 2 s). The effect of hydrous alumina on Dunaliella cell suspension (Fig. 8, curve 2) is similar to the
150
1) DUNALIELLA TERTIOLECTA (1.4.108 cet~ 14 ) 2) H) * 8-At203 5 0 m g l -~
i / IJ,A !
/
,oo~
0
50
2
6
8
t/s
Fig. 8. Effect of addition/~-AI~O 3 (50mg I 1) to the cell suspension ofDunaliella tertiolecta (1.4 × 108 cell 1-1), 20°C, pH 8.7. C u r r e n t time curves in (1) a r t i f i c i a l s e a w a t e r ( 2 ) D u n a l i e a l l a cell suspension (3) Dunaltella cell suspension + 50 mg l- 1, 6-A12 O z a f t e r 2 h shaking.
63
experiment with dispersion of methyl oleate (Fig. 1), demonstrating clearly that natural, as well as artificial fluid aggregates are preferentially adsorbed at mineral interfaces. Fluid surface active aggregates, similar to those of methyl oleate, according to the type of perturbation in the current-time curves, were detected in the mixing zone in estuarine (~uti6 et al., 1984; ~uti6 and Legovi6, 1987) (Fig. 9) as well as in sea surface microlayer and subsurface waters of the North Adriatic (Novakovi6 and ~uti6, 1983, Marty et al., 1987). In estuarine mixing zones the aggregates seem to be generated by physicochemical processes from dissolved organic molecules and smaller aggregates, and not by a direct in situ primary production. High surface activity of these aggregates and a high rate of their reorganization into adsorbed layers at interfaces has important implications in the transport and scavenging of hydrophobic organic pollutants (Imboden and Schwarzenbach, 1985) and reactive trace elements (Cauwet and Faguet, 1982; Hunt, 1983). COALESCENCE FREQUENCY / 0.1
I
IxI07
0.2
'
s -I
o.3
I
5,x 107
NUHBER OF SURFACE ACTIVE AGGREGATES I N t-~
E "r bfl. i,i
S '
lb
io
S 1%o
~'o
Fig. 9. Relative distribution of surface active aggregates along the depth profile of a stratified estuary (River Aude, Western Mediterranean, May 1983). The number of coalescences is defined as the number of well defined perturbations on current-time curves.
DISCUSSION In natural waters, and in seawater in particular, hydrophobic compounds commonly exist in a micetlar or colloid accommodated state rather than as true molecular solutes; critical micellar concentration for a number of lipids has been reported to be below 10 7M (Tanford, 1980). Previous studies (Loeb and Neuhof, 1977; Kristofferson et al., 1982) have demonstrated a continuous growth of organic coatings on platinum surfaces immersed in natural seawater. Among all the model substances we have studied, only sparingly soluble hydrophobic materials, such as unsaturated fatty acids and esters, demonstrated an ability to form multilayer coatings on alumina particles, with no indication of a saturation coverage (Fig. 7). Adsorption of simple carboxylic acids, such as salicylic acid, or amino acids with functional groups characteristic of aquatic organic matter was found to be negligible for natural concentration levels (Table I). Humic materials showed saturation at a monolayer surface coverage (Fig. 6) of a similar value (corresponding to 27 C atoms per nm 2) to one reported for lake organic matter (Baccini et al., 1982). The important dependence of adsorption densities on particle concentration (Figs. 4-6) agrees well with the findings in lake waters: (Baccini et al., 1982) and estuaries. This seems to be a common phenomenon in natural aquatic systems, where AOM has all the characteristics of a polydisperse (multicomponent) solute with a span of adsorption constants, analogously to its complexation behaviour to heavy metals (Turner et al., 1986). Hydrophobic attraction (Tanford, 1980; Israelachvili and Pashley, 1983) seems to be the most important mechanism of AOM interaction at marine interfaces. It is stronger than van der Waals forces and of significantly longer range than any 'bond' and highly unspecific. Unsaturated lipid molecules, micelles and aggregates represent the most reactive surface active materials. They also seem to mediate interaction of other AOM constituents with particles once a first monolayer, or its fraction, has been formed. We have characterized a highly reactive fraction of AOM: fluid surface active aggregates that are ubiquitous in productive surface waters and sea surface microlayers (Novakov[c and ZutiS, 1983; Plebe and Zuti6, 1984) and in mixing zones in estuaries (Fig. 9) (Zuti6 et al., 1986; Zutid and Legovi6, 1987) and is not amenable to analysis by standard techniques that involve separation to a ~dissolved' and ~particulate' AOM fraction. Although unsaturated lipids can constitute only a minor fraction of an aggregate material (Marty et al., 1987), they seem to determine its structure and fluidity as in the case of biological materials, such as membrane structures (Tanford, 1980). This highly reactive and unstable fraction of AOM seems to be a precursor of SEM visible organic coatings, persistent gel-like structures (~voiles organiques', Alo~si et al. (1975)) observed on mineral particles in coastal estuarine zones. A fast and reversible adsorption of AOM at charged marine interfaces might only constitute an initial step in the process of organic coating formation that
65
in its advanced steps should involve the complex structure formation and organization phenomena well known in membrane science and biophysics (Tanford, 1980). The closest analogue of this process is bubble dissolution in seawater (Johnson and Cooke, 1980). Thus, reversible adsorption models should be critically applied to marine processes involving AOM and particles. ACKNOWLEDGEMENT
Dr. Damir ViliSid is thanked for the preparation of the phytoplankton cultures, Dr. Gustave Cauwet and Dr. Edward Tipping for their generous gifts of the humic materials, and Ralph Gloor for the molecular mass distribution analysis. The financial support of the Self-Managing Community of Interest for Scientific Research of S.R. Croatia, Yugoslavia, is gratefully acknowledged. REFERENCES Aaranson, S., 1971. The synthesis of extracellular macromolecules and membranes by a population of the phytoflagelate Ochromonas danica. Limnol. Oceanogr., 16: 1-9. Adamson, A.W., 1982. Physical Chemistry of Surfaces, 3rd Edn. Wiley-Interscience, New York, 663 pp. Aloisi, J.C., Monaco, A. and Pauc, H., 1975. Mechanism de la formation des prodeltas dans le Golf de Lion. Example de l'embouchure de l'Aude (Languedoc). Bull. Geol. Bassin Aquitaine, 18: 3-12. Aogaki, R., Kitzawa, K., Iukai, K. and Mukaibo, T., 1978. Theory of polarographic maximum current. Growth or decay of the electrochemical and hydrodynamic instability. Electrochim. Acta, 23: 875~880. Baccini, P., Grieder, E., Stierli, R. and Goldberg, S., 1982. The influence of natural organic matter on the adsorption properties of mineral particles in lake water. Swiss J. Hydrol., 44: 99-116. Cauwet, G. and Faguet, D., 1982. The role of organic matter in transport processes of metals in estuarine environments. Thalassia Jugosl., 18: 379-392. Cosovid, B. and Vojvodid, V., 1982. The application of a.c. polarography to the determination of surface-active substances in seawater. Limnol. Oceanogr., 27: 361-369. Cosovid, B., ~utid, V., Vojvodid, V. and Ple§e, T., 1985. Determination of surfactant activity and anionic detergents in seawater and sea surface microlayer in the Mediterranean. Mar. Chem., 17: 127-139. Damaskin, B.B., Petrii, O.A. and Batrakov, V.V., 1971. Adsorption of Organic Compounds on Electrodes. Plenum Press, New York, 499 pp. Davis, J.A. and Gloor, R., 1981. Adsorption of dissolved organics in lake water by aluminium oxide. Effect of molecular weight. Environ. Sci. Technol., 15: 1223-1229. Davis, J.A., 1982. Adsorption of natural dissolved organic matter at the oxide]water interface. Geochim. Cosmochim. Acta, 46: 2387-2393. Davis, J.A., 1984. Complexation of trace metals by adsorbed natural organic matter. Geochim. Cosmochim. Acta, 48: 679-691. Elliot, H.A. and Huang, C., 1980. Adsorption of some copper(II) amino acid complexes at the solid-solution interface: effect of ligand and surface hydrophobicity. Environ. Sci. Technol. 14: 87-93. Faguet, D. 1982. Influence des substances humiques sur les formes dissoutes et particulaires de quelques metaux dans la milieux marine et lagunaire. M.Sc. Thesis, Perpignan. Furrer, G., 1985. Die Oberfl~ichenkontrollierte Aufl5sung von Metalloxiden: Ein Koordinationschemischer Ansatz zur Verwitterungskinetik. Ph.D. Thesis, ETH, Zurich.
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67 Tipping, E., 1981. The adsorption of aquatic humic substances by iron oxides. Geochim. Cosmochim. Acta, 45: 191-199. Turner, D.R., Varney, M.S., Whitfield, M., Mantoura, R.F. and Riley, J.P., 1986. Electrochemical studies of copper and lead complexation by fulvic acid. I. Potentiometric measurements and a comparison of metal binding models. Geochim. Cosmochim. Acta, 50: 289-297. Whitfield, M. and Turner, D.R., 1986. The role of particles in regulating the composition of natural waters. In: W. Stumm (Editor), Proceedings in Aquatic Surface Chemistry: Chemical Processes at the Particle/Water Interface. Ermatingen, pp. 106--112. Zvonari6, T., ~uti6, V. and Branica, M., 1973. Determination of surfactant activity of seawater samples by polarography. Thalassia Jugosl., 9: 65-73. ~uti6, C. and Legovi6, T., 1987. A film of organic matter at the freshwater/seawater interface of an estuary. Nature, 328: 612-614. ~uti6, V. and Stumm, W., 1984. Effect of organic acids and fluoride on the dissolution kinetics of hydrous alumina. A model study using the rotating disc electrode. Geochim. Cosmochim. Acta, 48: 1493-1503. ~uti6, V., Cosovid, B. and Kozarac, Z., 1977. Electrochemical determination of surface active substances in natural waters. On the adsorption of petroleum fractions at mercury/seawater interface. J. Electroanal. Chem., 78: 113-121. ~utid, V., Cosovid, B., Mar6enko, E., Bihari, N. and Kr~ini6, F., 1981. Surfactant production by marine phytoplankton. Mar. Chem., 10: 505-520. ~uti6, V., Ple§e, T., Tomaid, J. and Legovid, T., 1984. Electrochemical characterization of fluid vesicles in natural waters. Mol. Cryst. Liq. Cryst., 113: 131-145. ~,uti6, V., Cauwet, G. and Monaco, A., 1986. Role of organic aggregates in the transport of pollutants in Mediterranean Estuaries. Proc. 7th Workshop on Marine Pollution of the Mediterranean, Luzern, 1984, ICSM/IOC/UNEP, pp. 173-180.