HUNTER, KA, AND PS LISS. Organic matter and the surface ... - ASLO

Limnol. Oceanogr., 27(2), 1982, 322-335 

@ 1982, by the American Society of Limnology and Oceanography, Inc. 

Organic matter and the surface charge of suspended 

particles in estuarine waters1 

K. A. Hunter2 and P. S. Liss 

School of Environmental Sciences, University of East Anglia, 

Norwich NR4 7TJ, United Kingdom 

Abstract 

The surface electrical charge on suspended particles in four estuaries of the U.K. has been 

measured as a function of salinity by the technique of particle microelectrophoresis. Two 

characteristic types of behavior were found. In rivers low in dissolved cations, especially 

Caz+ (Conwy, Beaulieu), the electrophoretic mobility uE was negative in sign at all salinities, 

increasing slightly in magnitude from the seawater end member to lower salinities of 5-10%0, 

with a more pronounced increase toward the river water end member. In rivers draining 

calcareous terrain and having relatively high concentrations of Ca2’ (Alde, Orwell) uE showed 

a similar dependence on salinity above 5-l% but no marked increase in magnitude at lower 

salinities. Ionic composition of the water appears to be the major factor controlling changes 

in ug with salinity. Positively charged particles were entirely absent. The charge distribution 

of all samples was highly unifoml, in spite of the mixed nature of the suspended matter, 

indicating a dominant control of surface properties by adsorbed organic matter, metallic ox- 

ides, or both. This implies that differential flocculation of different suspended minerals is 

largely suppressed in the estuarine zone. Measurements of dissolved organic carbon (DOC) 

and surface-active substances (by suppression of polarographic maximum) in the same estu- 

aries indicate a sufficient supply of organic matter for the adsorption process. No evidence 

for nonconservative removal of DOC or surface-active substances was found. Sewage inputs 

into some of the estuaries are clearly seen by the measurements of surface-active substances. 

Significant quantities of surface-active materials are injected into the Alde estuary through 

tidal flushing of a salt marsh area. 

The transport of riverborne suspended 

matter of colloidal dimensions through 

the estuarine mixing zone into shelf 

waters is a classical problem of colloid 

stability superimposed on a regime of 

complex water advection, turbulence, 

and changing ionic composition. The ag- 

gregation of colloids in such systems is 

controlled by hydrodynamic factors 

(Brownian diffusion, velocity shear, sedi- 

mentation scavenging) that influence the 

rate of collision of primary particles (Ler- 

man 1979) and by interfacial effects that 

stabilize or destabilize aggregates (Hahn 

and Stumm 1968, 1970; Stumm et al. 

1970; Edzwald et al. 1974). Of the inter- 

facial mechanisms for colloid stability, 

the best understood is the electrostatic 

stabilization resulting from the mutual 

’ Financial assistance was provided by the U.K. 

Natural Environment Research Council. 

2 Present address: Department of Chemistry, 

University of Otago, P.O. Box 56, Dunedin, New 

Zealand. 

322 

repulsion of the ionic atmospheres sur- 

rounding particles of like surface charge. 

The Derjaguin-Landau-Verwey-Over- 

beek (DLVO) theory shows that electro- 

static stabilization is sensitive to the 

changes in surface charge density caused 

by the adsorption of ions, especially of 

higher valency. Colloids become unsta- 

ble in sufficiently concentrated electro- 

lytes where the extent of the ionic diffuse 

layer becomes small or where ion adsorp- 

tion neutralizes the surface charge (Da- 

vies and Rideal 1963). Since the electro- 

lyte composition controlling this ion 

adsorption undergoes large changes early 

in the estuarine mixing zone, the view 

that riverborne colloidal materials are 

flocculated by sea salt early in the salinity 

range of an estuary has gained wide- 

spread acceptance. It is likely, however, 

that many examples of upstream deposi- 

tion ascribed to this mechanism may 

arise largely because of hydrodynamic 

factors (Postma 1967; Dyer 1973). 

Some attempts have been made to ac-

count for the distribution of clay minerals - 

in shelf sediments adjacent to a major riv- 

er source by using differential floccula- 

tion rates derived from the application of 

the DLVO theory to measured stabilities 

of the different clay minerals in saline 

waters (Whitehouse and Jeffrey 1953; 

Whitehouse et al. 1960; Hahn and Stumm 

1970; Edzwald et al. 1974). However, the 

validity of such predictions is in doubt 

when organic material is adsorbed on the 

mineral surfaces, giving rise to hydro- 

philic or steric stabilization (Narkis and 

Rebhin 1975; Horzempa and Helz 1979) 

and changes in the mechanism of coag- 

ulation (Kranck 1973). This objection is 

supported by the work of Gibbs (1977) 

and others (Meade 1972; Manheim et al. 

1972) who have concluded that field 

studies provide no real evidence for the 

importance of differential flocculation. In 

the Amazon River system, surface films 

of organic matter, metal oxides, or both, 

dominate the interfacial properties of 

transported suspended matter, removing 

the intrinsic differences in clay mineral 

stabilities found in laboratory studies 

(Gibbs 1977). 

It is evident that the surface electrical 

properties of suspended material and the 

presence of surface oxide or organic films 

are matters of central importance in the 

estuarine transport of particles. We report 

here a detailed investigation of the sur- 

face charge on suspended particles 

throughout the mixing zone of four U.K. 

estuaries carried out by the technique of 

particle microelectrophoresis (Shaw 

1969). Organic material available for ad- 

sorption onto the surfaces of suspended 

particles was examined by measuring sa- 

linity profiles of dissolved organic carbon 

(DOC) and surface-active organic mate- 

rial, the latter measured by the polaro- 

graphic method described originally by 

Zvonaric et al. (1973) and Cosovic et al. 

(1977) and extended to estuarine waters 

by Hunter and Liss (1980). Previous stud- 

ies with the microelectrophoresis tech- 

nique in coastal and offshore waters 

(Neihof and Loeb 1972, 1974; Loeb and 

Neihof 1975, 1977; Hunter 1980) have 

established that sufficient surface-active 

Surfuce electrical charge 323 

material exists in these waters to domi- 

nate the surface properties of suspended 

solids. Since estuarine and river waters 

normally contain higher levels of dis- 

solved organic matter (Head 1976), sur- 

face organic films should also be impor- 

tant in these systems. 

We thank J. D. Burton for making avail- 

able the facilities of the Department of 

Oceanography, University of Southamp- 

ton, and to P. J. le B. Williams and P. 

Statham for their assistance with the 

DOC measurements. P. Balls, D. Wright, 

I. N. McCave, and J. G. Harvey helped 

collect samples. 

Methods 

Electrophoretic measurements were 

carried out by established methods 

(Shaw 1969) with a Rank Mk II apparatus 

using a rectangular quartz cell thermo- 

statted to 25°C. The electrophoretic mo- 

bility uE’ was calculated from the mean 

velocities of at least 10 particles at each 

of the two stationary levels (40+ mea- 

surements in all). The field strength Er: 

was calculated as E: = IIKA, where A = 

cell cross-section, after measurement of 

the current 1 with a digital ammeter; the 

electrolyte conductivity K was separately 

determined by connecting a Wayne-Kerr 

transformer bridge to the cell. The con- 

ductivity data for estuarine samples were 

also used to calculate the sample salinity. 

Because higher currents were required 

for mobility measurements in seawater of 

35%0 salinity, AglAgCl reversible elec- 

trodes were used and found to be resis- 

tant to gassing up to the maximum cur- 

rent available (ca. 35 mA) with the 

standard lo- x l-mm cell. Occasional 

problems were encountered at currents 

above 20 mA with convective motion in 

the cell as a result of Joule heating. To 

help overcome this, we obtained a spe- 

cial cell of much reduced cross-section, 

4 x 0.5 mm (Rank Bros., Bottisham, Cam- 

bridge, U.K.), which enabled us to re- 

duce the current to ca. 6-9 mA. 

The polarographic method used to de- 

termine surface-active material in estua- 

rine samples has been described in detail 

elsewhere (Hunter and Liss 1980). Brief-

324 Hunter and Liss 

ly, surface-active substances are detected 

by virtue of their suppression of the 

streaming maximum on the reduction 

wave of Hg(I1) at -0.28 V (S.C.E.) in sa- 

line solution. The method was calibrated 

with the nonionic surfactant Triton-X- 

100, so that the surfactant activities of 

water samples are reported in terms of 

the concentration of this material, which 

has the same suppression effect. The 

technique therefore provides only an ar- 

bitrary measure of the concentration of 

surface-active material. Mixing experi- 

ments with river and seawaters showed 

an absence of salt effects, indicating that 

this variable should be a valid estuarine 

tracer (Hunter and Liss 1980). If samples 

are stored in the dark at 6°C there are no 

significant changes in surfactant activity 

during 1 week, within which time all the 

analyses of the present work were com- 

pleted. Samples were not filtered before 

analysis. 

We measured dissolved organic carbon 

(DOC) by th e automated photo-oxidation 

procedure of Collins and Williams (1977) 

except that no persulfate was used, allow- 

ing a significant reduction in the proce- 

dural blank (P. J. Williams pers. comm.). 

Samples were filtered as soon as possible 

after collection through precombusted 

Whatman GFlC glass-fiber filters and 

acidified to pH 2.5 with HCl. River Beau- 

lieu samples were analyzed immediately 

at Southampton University; samples from 

the Alde and Conwy were frozen after fil- 

tration 

Organic-free seawater and distilled 

water were produced by UV photo-oxi- 

dation (Armstrong et al. 1966) for periods 

of 18-24 h, found necessary to complete- 

ly oxidize surfactant in seawater (Hunter 

and Liss 1980). All glassware was pre- 

cleaned either by prolonged soaking in 

chromic acid or by baking at 550°C over- 

night. Some freshwater samples were 

analyzed for dissolved Na, K, Mg, and Ca 

by flame atomic absorption spectropho- 

tometry with a Varian 1100 instrument. 

Fluorescent organic matter (Loeb and 

Neihof 1975) was measured with a Per- 

kin-Elmer 204 fluorimeter with a lo-mm 

Table 1. Major cation compositions (in 

mmol-kg-l) of Alde, Conwy, and Beaulieu rivers 

and seawater (adapted from Riley and Skirrow 

1975). 

Alde Conwy Beaulieu Seawater 

Samples 4 2 7 - 

Na+ 2.00 0.375 0.670 482 

K+ 0.176 0.014 0.063 10.2 

Mg+ 0.434 0.077 0.169 54.9 

Ca? 4.63 0.234 0.664 10.6 

quartz cell at an excitation wavelength of 

330 nm. 

Sampling 

Samples were collected from the estu- 

aries of four rivers intended to represent 

contrasting freshwater types. The River 

Alde, in Suffolk, drains largely calcareous 

rocks and contains relatively high con- 

centrations of dissolved Ca; the River 

Orwell is next to it; the River Conwy, in 

North Wales, drains mostly igneous rocks 

and is relatively low in dissolved cations; 

the River Beaulieu, in Hampshire, drains 

sections of the New Forest, contains 

quite high levels of dissolved Fe and hu- 

mic materials (Holliday and Liss 1976; 

Moore et al. 1979), and also has quite low 

concentrations of dissolved cations, 

slightly higher than the Conwy (Table 1). 

In general, samples (18-35 per survey) 

were collected from the surface at mid- 

stream from a small boat, but on the Con- 

wy River we also used road bridges. 

Results 

Because of the variable composition of 

suspended matter in the samples (clay 

minerals, aluminosilicates, quartz grains, 

calcite, living and dead organisms, detri- 

tus) it is important to establish what pro- 

portion of the measured variation of mo- 

bility is due to experimental errors and 

what proportion arises from intrinsic vari- 

ations in the surface charge. 

If we follow the reasoning outlined by 

Shaw (1969), we would expect a standard 

error of 2-2.5% for dispersions of electro- 

phoretically identical particles, using the 

4- X 0.4~pm cell, arising largely from fo-

Table 2. Electrophoretic mobility of suspended 

particles in freshwater (Alde River) and seawater 

(North Sea, salinity 33%0) samples at different times 

after collections and storage at 6°C in the dark. “1 

day” includes some samples analyzed a few hours 

after collection. 

Days after 

collection 

14 

16 

90 

1 

1 

8 

8 

Freshwater 

Seawater 

Surface electrical charge 325 

1.01 kO.03 

1.09+0.04 

0.97+0.04 

1.01+0.03 

0.91-to.03 

0.97kO.04 

0.98+0.03 

0.89-eO.01 

0.80*0.03 

0.85+0.03 

0.88+0.04 

0.84+0.04 

0.85+0.02 

0.87+0.01 

cusing errors. This was confirmed by re- 

petitively determining uE on eight ali- 

quots of a seawater suspension, which 

yielded 2.5% C.V. in mean mobilities. In 

general, the standard error of individual 

measurements on estuarine samples 

ranged between 1 and 5%, indicating that 

the suspended matter examined showed 

little variation in surface charge in spite 

of its mixed nature. 

There is no significant additional error 

in mobilities introduced as a result of col- 

lection methods, as indicated by mea- 

surements on replicates. Samples col- 

lected at the surface and at 10-m depth in 

coastal North Sea water also showed 

good agreement, implying that there is 

no large variation in uB over short-scale 

horizontal and vertical distances in a rel- 

atively homogeneous water mass. 

The effects of storage on mobility re- 

sults were small for both freshwater and 

seawater suspended matter (Table 2) 

over periods considerably longer than 

those involved in sample collection and 

analysis (typically

I I I 1 1 

a) ALDE estuary 

I I I I I I L 

IO 20 20 

0 0 

0 Ooo 

. 

. . 

SALINITY %o 

c) CONWY 

0 

0 

l . . l . 

. 

estuary 

10 20 30 

SALINITY %o 

Hunter and Liss 

-20 

r----- ’ 

-1’ 

, , I 1 

b) ORWELL estuary 

I I I 

10 20 30 

SALINITY %o 

d)BEAULIEU estuary 

I 

10 20 30 

SALINITY %o 

Fig. 1. Electrophoretic mobility of estuarine particles as a function of salinity. a. O-8 February; O- 

19 March. b. 16 April. c. O-24 April; O-17 June. d. l - 14-17 May; 04 June. All dates, 1979. 

Fig. 8). The results of this mixing exper- the particulate matter within the estua- 

iment show that either the surface prop- rine zone is of other origin, e.g. resus- 

erties of riverborne suspended matter of pended marine sediment. The latter view 

the Alde are changed as soon as it enters is supported by the noticeable increase 

the saline zone, e.g. by adsorption of or- in turbidity (visual observation and mea- 

ganic or metal-oxide films, or that most of surements with fluorimeter in a nephe-

Surfuce electrical charge 327 

ALOE 1 

mixing experiments 

river water x 

particles 

I I 1 I I 1 

10 20 30 

SALINITY %o 

Fig. 2. Electrophoretic mobilities of suspended 

particles in river water and seawater end members 

of Alde estuary as a function of salinity in mixing 

experiments. Upper curves-seawater particle sus- 

pension diluted to lower salinity with distilled 

water (0) and with filtered river water (0). Lower 

curve-river water particle suspensions diluted to 

higher salinity with filtered seawater (a) and with 

filtered UV photo-oxidized seawater (x). Two other 

series of mixing experiments using samples of river 

and seawater end members collected from Alde on 

different occasions gave virtually identical results. 

lometric mode) in this system above a 

few % salinity. Identical results were 

found for river water suspensions and UV 

photo-oxidized seawater (Fig. 2, crosses), 

indicating that the major factor control- 

ling particle mobility in this case is the 

electrolyte ionic composition. 

On the other hand, addition of filtered 

river water from the Alde to suspensions 

of particulate matter collected at the sea- 

ward end of the estuary completely re- 

produced the field results (cf. Fig. la 

with Fig. 2). This provides further sup- 

port for the view that most of the partic- 

ulate matter in the saline region of the 

estuary is resuspended marine material 

transported upstream by bottom currents. 

Dilution of the same seawater-suspended 

particles with distilled water, however, 

produces an increase in the magnitude of 

uE with decreasing salinity of the same 

type as that found with the type 2 estu- 

aries (Conwy, Beaulieu). Thus, when the 

CONWY 

mlxlng experiments 

I I I I I I 

IO 20 30 

SALINITY %o 

Fig. 3. Electrophoretic mobilities of suspended 

particles in seawater end member of Conwy estuary 

as a function of salinity after dilution with distilled 

water (0) and filtered Conwy River water (0). 

Cross-hatched area encompasses in situ results of 

Fig. lc. 

river water cations are absent, ion ad- 

sorption into the electrical diffuse layer 

is less at low salinity and the negative 

charge on the electrophoretic unit is 

greater. 

Figure 3 shows the results of mixing 

experiments carried out with suspended 

matter from the seawater end member of 

the Conwy estuary. Here, because the 

concentrations of dissolved ions, espe- 

cially divalent ones (Table l), are rela- 

tively low, the difference in behavior be- 

tween the addition of filtered river water 

to the seawater suspension and addition 

of distilled water is smaller. It is clear 

nevertheless that in neither case does the 

mixing experiment completely repro- 

duce the Conwy field results. We must 

therefore conclude that the surface prop- 

erties of the suspended matter within the 

Conwy estuary differ from those of the 

particles in suspension at its mouth. Oth- 

er evidence presented below suggests 

distinct differences between the water 

masses having greater and less than ca. 

29% salinity, reinforcing this view.


L I- 

10 20 30 

SALINITY %o 

C) CONWY estuary 

10 - 20 

SALINITY %o 

I 

I 

10 20 30 

SALINITY %o 

d) BEAULIEU estuary 

LL- ’ 

IO 20 30 

SALINITY ‘ho 

Fig. 4. Surfactant activity of estuarine waters as a function of salinity. Samples are identical to those 

in Fig. 1. 

Surfuctant uctivity und 

DOC measurements 

Salinity profiles of surfactant activity 

for the four estuaries are presented in 

Fig. 4, to be compared with salinity pro- 

files of DOC for three of these estuaries 

in Fig. 5. 

The DOC results for the Beaulieu sys- 

tem, particularly the second set (4 June), 

show good evidence for conservative 

mixing of this component, in support of 

previous work in the same estuary 

(Moore et al. 1979; P. J, Williams pers. 

comm.). Although there is apparent 

downward curvature in the DOC profile 

for the first data set (14-17 May), this is 

made uncertain by the lack of data be- 

tween 0 and loo/,, salinity. The surfactant 

activity data for this estuary show good 

evidence for conservative behavior, ex- 

cept for a number of samples, mostly 

from the second set of results (Fig. 4d: 4 

June), of low and intermediate salinity 

which have anomolously high surfactant 

activity in relation to conservative behav- 

i,or and the DOC results. There was good 

visual evidence at the time of sampling 

(toilet paper fragments, etc.) that many of 

these anomolous samples may have been 

affected by untreated sewage effluent 

which had undergone only moderate di- 

lution. 

The DOC results for the Conwy estu- 

ary (Fig. 5b) show fairly good evidence 

of conservative behavior down to 29%0 

salinity, with an abrupt increase to the 

single seawater point. At low salinities, 

however, several samples show relative- 

ly high DOC concentrations in relation 

to conservative behavior. The surfactant 

I

1 

I I I I I I 

10 20 30 

SALINITY %o 

SALINITY %o 

IO 20 30 

SALINITY %o 

Fig. 5. Dissolved organic carbon concentration 

of estuarine waters as a function of salinity. a. 19 

March. b. 17 June. c. O-14-17 May; 04 June. 

All dates, 1979. 

activity profile for 17 June closely resem- 

bles that of DOC for the same period. 

The first set of surfactant activity results 

(24 April), for which there are no corre- 

sponding DOC data, also appears to show 


conservative mixing from ca. 2%0 up to 

29”/00 salinity, but this is less certain ow- 

ing to the lack of data between 1 and 12%0 

salinity. There is also the same distinct 

change in the profile at 29%0, but this 

time toward lower surfactant activities. 

These results, like those of the mixing 

experiments with suspended particles 

from the estuary mouth (Fig. 3), tend to 

suggest that the high salinity water sam- 

pled there is not continuously related 

through estuarine mixing to the water 

sampled farther upstream. 

The greatest contrasts between DOC 

and surfactant activity are for the Alde 

estuary. Although there is a fair degree of 

scatter in the DOC results (Fig. 5a), in- 

dications are that the mixing is near-con- 

servative, with perhaps a slight upward 

curvature. The surfactant activity results 

(Fig. 4a) for both surveys show a clear 

input of surface-active material at 4-7%0 

salinity. This probably comes from an ad- 

jacent salt marsh which is separated from 

the main river by banks. Two major 

breaks in the banks have developed at 

positions where the high-tide salinities 

are ca. 15 and 34%0, and these allow con- 

siderable tidal flushing of the marsh. 

During floodtide, water enters the down- 

stream breach, floods the marsh, and 

flows rapidly into the lower salinity up- 

stream part of the estuary through the 

other breach. Presumably, in this process 

considerable quantities of surface-active 

material are leached from the regularly 

exposed mud and vegetation of the 

marsh, since these systems can be signif- 

icant sources of such material (de la Cruz 

and Poe 1975; Pellenbarg and Church 

1979; Gardner 1975). 

A similar surfactant activity profile for 

the Orwell estuary (Fig. 4b) also suggests 

large-scale input of surfactants at low sa- 

linity. The town of Ipswich is located in 

this part of the estuary, so that the behav- 

ior seen for the Orwell may well be due 

to industrial or domestic effluent. The ab- 

solute surfactant activities found in this 

system, together with those for sewage- 

dominated samples from the Beaulieu 

estuary, are among the highest we ob- 

served.


I I I I I 1 I 

ALDE estuary 

- I I 1 I I 

10 20 30 

SALINITY %o 

Fig. 6. Blue fluorescence of Alde estuarine 

waters as a function of salinity: O-8 February 

1979; O-19 March 1979. Samples prefiltered 

through 0.45-pm Millipore filters. 

Fluorescent organic matter 

Samples from the Alde estuary were 

analyzed for blue fluorescent organic 

compounds after filtration through Milli- 

pore 0.45pm membrane filters. Species 

such as humic acids contribute strongly 

to this fluorescence and are implicated in 

the surface adsorption of organic matter 

from seawater onto solid surfaces (Loeb 

and Neihof 1975; Hunter 1980). The re- 

sults (Fig. 6) show that the variation of 

fluorescence intensity with salinity falls 

midway between that of the DOC and 

surfactant activity; i.e. there is conserva- 

tive mixing above ca. loo/o0 salinity and 

input below this, which appears, how- 

ever, less great than in the case of surfac- 

tant activity. Separate experiments with 

filtered river and seawater samples con- 

firmed that mixing of the two surfactant 

sources had an additive effect on fluores- 

cence. 

Discussion 

Our results provide no conclusive evi- 

dence for large-scale removal of DOC 

during mixing in any of the estuaries 

studied, in agreement with earlier work 

on the Beaulieu estuary (Moore et al. 

1979) and elsewhere (Sholkovitz et al. 

1978). Most of the DOC in ocean waters 

is thought to be of in situ origin (Head 

1976) and differs in properties from ter- 

restrial organic materials such as humic 

acids (Stuermer and Harvey 1974; 

Stuermer and Payne 1976; Gagosian and 

Stuermer 1977) that make up a major part 

of riverborne DOC, This has led to the 

belief that most of the riverborne mate- 

rial is flocculated in estuaries as mixing 

with salt water brings about destabiliza- 

tion of colloidal organic materials. Clear- 

ly the removal of organic matter by this 

process affects only a small proportion of 

the total DOC present. 

It is more surprising that the surfactant 

activity results also show no evidence for 

removal of surface-active species during 

estuarine mixing. The salt marsh of the 

Alde system acts as a source of high con- 

centrations of surface-active material at 

about 5% salinity, but mixing down- 

stream of this point was conservative on 

both sampling occasions. The mixing in 

the Beaulieu estuary is similarly conser- 

vative, with the exception of low salinity 

samples probably affected by sewage ef- 

fluent, as is that in the Conwy estuary 

downstream to 29%0 salinity. Clearly, the 

property of surface activity as measured 

by the polarographic technique used 

here is quite generally distributed 

throughout the range of organic matter 

found in estuarine waters. This is best 

shown by the comparison of surfactant 

activity and DOC results in Fig. 7. Ex- 

cept for the anomalous Beaulieu and 

Alde estuary samples already discussed, 

there is a clear linear relationship be- 

tween the two quantities that can be ex- 

pressed as: 

Surfactant activity (mg. dm-“) 

= 1.3 DOC (mg. dme3). 

Since only a few points fall below this 

line, Fig, 7 provides no evidence for any 

substantial removal of surface-active ma- 

terial relative to the bulk of the DOC. 

The electrophoretic results show that, 

as far as surface electrical properties are 

concerned, suspended particles in these 

four estuaries are highly uniform. Above 

a few so salinity, the same mobility trend 

is observed for duplicate surveys on the 

same estuary and for the results of the 

different estuaries. In all, these results

20 - 

I I 1 I I I 

o Alde 

l Conwy 

x Beaulieu 1 

0 Beaulieu 2 0 

6 

DISSOLVED ORGANIC CARBON mg dme3 

Fig. 7. Composite plot comparing measured 

surfactant activities (Fig. 4) and dissolved organic 

carbon concentrations (Fig. 5). (Beaulieu 1: 14-17 

May 1979; Beaulieu 2: 4 June 1979.) 

encompass a wide selection of particle 

type and differences in geology, water 

compositi on and flow properties, but 

show remarkably little dispersion. This 

uniformity points toward the dominant 

role of surface coatings of organic matter 

or metal oxides (Gibbs 1977) in deter- 

mining the surface electrical properties 

of the suspended matter. Previous work 

in coastal North Sea waters (Hunter 1980) 

has established that naturally occurring 

organic surfacta .nts are adsorbed readily 

onto a wide variety of solid surfaces, pro- 

ducing particles of uniform electropho- 

retic behavior. The DOC levels and sur- 

factant activities of coastal North Sea 

waters were in the range l-2 mgedm-“, 

considerably less than those observed in 

these estuaries. Provided that the adsorp- 


0 

0 

tive properties of the estuarine and ma- 

rine organic matter are similar, there 

would appear to be more than ample op- 

portunity for the complete formation of 

surface organic films on estuarine sus- 

pended matter by such a mechanism. 

The principal differences in behavior 

of the four estuaries (types 1 and 2) occur 

at low salinity and can be accounted for 

by differences in ionic composition of the 

river waters. The adsorption of an ion of 

charge x by a charged surface through 

purely electrical effects is proportional to 

a term exp(xFSblRT), where 4 is the sur- 

face potential at the plane of adsorption 

and F, R, and T have their usual mean- 

ings (Oldham 1975). Electrophoretic 

studies indicate that for particle surfaces 

covered by marine organic films, 4 is 

only marginally greater than 5, the elec- 

trokinetic potential (Hunter 1980). The 

data of Fig. 1 suggest, therefore, a value 

of about -20 mV for 4. The quantitative 

preference for 2 + cations over 1+ cations 

under these conditions amounts to a fac- 

tor of about 2.2. In the Alde river system 

(type 1) this would imply strong adsorp- 

tion of 2+ cations, and a consequently 

reduced 1 UE I, even in the river water por- 

tion (2+ ions = 5.1 mmolakg-l; l+ 

ions = 2.2 mmol. kg-‘). In contrast, the 

type 2 rivers have the same concentra- 

tions of 2+ cations as the Alde river water 

at a salinity of 4%0 where, in this case, 

these ions derive largely from the minor 

seawater component. Thus, purely on the 

basis of electrical effects, a difference in 

UE vs. salinity behavior is expected below 

this salinity between the calcareous 

rivers (type 1: Alde and Orwell, Fig. la, 

b) and those with low dissolved cations 

(type 2: Conwy and Beaulieu, Fig. lc, d). 

These differences may be further en- 

hanced by specific adsorption of ions 

such as Ca2+. The Alde and Orwell would 

be expected to show little increase in 

1 UV 1 below 4%0 salinity owing to the ad- 

sorption of Mg2+ and, increasingly, Ca2+ 

into the fixed part of the double layer. It 

can be seen by inspection of Fig. la, b 

that this and the corresponding increase 

in 1 uti I at low salinities for the type 2 

rivers, is what is observed. The behavior


predicted for the Alde is supported by 

the uE’ vs. salinity behavior of the sea- 

water particulates in mixing experiments 

involving distilled water and river water 

for this estuary (Fig. 2). In such mixtures, 

UE shows the expected increase in mag- 

nitude with dilution that is typical of the 

in situ data for the rivers with much low- 

er dissolved cation concentrations. 

The electrophoretic results for all four 

estuaries (Fig. la-d) also show that the 

adsorption of seawater cations by sus- 

pended particles with increase in salinity 

is not great enough to give rise to reversal 

of surface charge to positive values. Sim- 

ilar results have been reported by Pauc 

(1980) and Beckett (pers. comm.). Only 

negatively charged particles have also 

been observed in a variety of seawater 

samples with salinities >30%0 (Neihof 

and Loeb 1972; Loeb and Neihof 1975; 

Hunter 1980). 

However, all of these results differ sub- 

stantially from those obtained with the 

streaming potential method of Pravdic 

(1970) for sand-sized particles (> 100 

pm). He found a reversal of charge from 

negative to positive at salinities near 2%0, 

using a variety of marine sediments in 

mixtures of seawater and distilled water. 

Martin et al. (1971) found a similar charge 

reversal at much higher salinities with 

high-silica sands in Adriatic seawater/dis- 

tilled water mixtures. Several possibili- 

ties could account for this charge rever- 

sal: removal by acid pretreatment and 

storage conditions of natural organic or 

metal oxide films on the sediments used, 

adsorption of cationic substances such as 

long-chain amines, surface hydrolysis of 

alumina, or experimental errors arising 

from asymmetry potentials on the elcc- 

trodes used to measure the streaming 

current. 

More recent data of Pravdic et al. 

(1981) show a charge reversal for silica 

particles in mixtures of Adriatic seawater 

and distilled water. For comparison, we 

measured the electrophoretic mobilities 

of quartz particles in seawater/distilled 

water solutions that had been extensively 

photo-oxidized to remove organic matter. 

The quartz particles were baked at 550°C 

QUARTZ particles 

Fig. 8. Electrophoretic mobility of organic-free 

quartz particles as a function of salinity in mixtures 

of UV photo-oxidized distilled and seawaters. 

for 12 h to remove adsorbed organic ma- 

terial. The results (Fig. 8) indicate no 

charge reversal up to 33%0 salinity in the 

absence of organic material. Further- 

more, data of Neihof and Loeb (1972), 

Loeb and Neihof (1975), and Hunter 

(1980) show that silica particles exhibit 

an altered but still negative mobility after 

exposure to a variety of natural seawater 

samples, owing to adsorption of organic 

acids. This suggests that the Adriatic sea- 

water used for the second streaming po- 

tential study may have been contaminat- 

ed by long-chain amines or similar 

cationic material. Alternatively, asym- 

metry potentials may have led to inad- 

vertent errors in the measurements. In 

either case, we do not believe that the 

existing streaming current data for sand- 

sized sediments can be reliably applied 

to suspended matter in estuaries, partic- 

ularly in view of the discrepancy with 

this and other recent work using particle 

electrophoresis (Pauc 1980; Beckett pers. 

comm.). 

Conclusions 

The results presented here show that 

suspended matter in estuarine waters has

a high degree of surface uniformity with 

respect to electrical properties and, by 

inference, with respect to the chemical 

and physical factors controlling the sur- 

face electrical state. We believe this to 

result from the formation of ubiquitous 

surface coatings on suspended particles 

of metal oxides, organic surface-active 

matter, or both. These two possibilities 

are in substantial accord with measured 

levels of DOC and surfactant activities of 

the water samples, known estuarine re- 

moval of oxide-forming metals such as 

iron, and a large body of ancillary evi- 

dence concerned with the formation and 

occurrence of such surface films. 

It seems reasonable to conclude that 

the basic principles revealed by the pres- 

ent electrophoretic studies are sufficient- 

ly general to apply to most other estua- 

rine systems. Surface-active organic 

matter is omnipresent in these natural 

waters, and the formation of oxide films 

by metals such as iron is a common fea- 

ture of the soil weathering environment 

feeding the water system (Carroll 1958). 

The uniformity of surface-electrical 

charge for different particle types ac- 

counts for the lack of field evidence for 

the differential flocculation of minerals in 

studies of mineral distributions (Meadc 

1972; Manheim et al. 1972; Gibbs 1977). 

Although charge neutralization does not 

occur during mixing of seawater in the 

estuaries, colloid flocculation can still 

take place because increasing salinity 

leads to compression of the electrical dif- 

fuse layer to within the range of intcr- 

particle attractive forces. Our work indi- 

cates that flocculation caused by this 

process will be independent of the par- 

ticulate matrix because of the uniform 

nature of the surfaces of different min- 

erals in situ. However, when organic 

films are present on suspended particles 

other mechanisms can affect colloid sta- 

bility as well: interparticle bridging by 

metal complex formation, surface hydro- 

philization, and steric stabilization (Nar- 

kis and Rebhin 1975; Horzempa and 

Helz 1979). In this case, the detailed 

physiochemical mechanism for floccula- 

tion is difficult to model quantitatively in 

Surfuce electrical churge 333 

the way that is possible with electrostat- 

ically stabilized colloid systems. Certain- 

ly, surface-active organic films or envel- 

opes appear to control the stability and 

size of floc aggregates in seawater and are 

essential for maintenance of the steady 

state population of aggregate sizes in the 

presence of turbulent forces (Kranck 

1973). 

Recently there has been considerable 

emphasis on the importance of interfacial 

reactions in the geochemical transforma- 

tions of trace elements involving sus- 

pended matter in estuarine systems (Bur- 

ton 1976; Goldberg 1978). The results of 

Gibbs (1973, 1977) and Moore et al. 

(1979) indicate that considerable quan- 

tities of some trace metals are transported 

through estuaries within particle surface 

coatings. Because of this, the geochemi- 

cal changes subsequently undergone by 

these species and their original adsorp- 

tion by the suspended matter may have 

little or nothing to do with the chemical 

nature of the underlying particulate ma- 

trix. Of equal importance is the suppres- 

sion of mineral-solution reactions also 

brought about by the blanketing effect of 

surface films. Organic coatings are known 

to control the stability and mineralogy of 

carbonate minerals (Suess 1970, 1973). 

Films of natural marine surfactants on 

alumina cause complete suppression of 

the ion-exchange equilibrium between 

this solid and both cations (Zn2+) and an- 

ions (SeOd2-) at trace levels in seawater 

media (Jaffresic-Renault et al. 1979). All 

these results serve to show how inappro- 

priate are laboratory studies of solid/so- 

lution reactions in estuarine systems that 

fail to reproduce the natural surface state 

of the suspended matter. 

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Submitted: 4 September 1980 

Accepted: 28 September 1981

HUNTER, KA, AND PS LISS. Organic matter and the surface ... - ASLO

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