Read LODER, T. C., AND P. S. LISS. Control by organic coatings of the surface charge of estuarine suspended particles. text version

418

Notes

FREDERICK,

chemical changes available for Lake Ontario (Dobson 198 1) and indicates that it may be useful in retrospective studies of lakes when detailed histories of water chemistry are not available.

E. F. Stoermer J. A. Wolin C. L. Schelske D. J. Conley

Great Lakes Research Division University of Michigan Ann Arbor 48 109

References

H. J. 1982. Algal dynamics and trophic interactions in the recent history of Frains Lake, Michigan. Ecology 63: 18 14-1826. CHAPRA, S. C. 1977. Total phosphorus model for the Great Lakes. J. Environ. Eng. Div. ASCE 103: 147-161. CLEVE-EULER, A. 19 12a,b. Das Bacillariaceen-Plankton in Gewbsern bei Stockholm. 2. Zur Morphologie und Biologie einer pleomorphen Melosiru. Archiv. Hydrobiol. 7: 119-139, 230-260. . 19 12~. Das Bacillariaceen-Plankton in Gewbsem bei Stockholm. 3. Uber Gemeinden des schwach salzigen Wassers und eine neue Charakterart derselben. Archiv. Hydrobiol. 7: 500514. DOBSON, H. F. 198 1. Trophic conditions and trends in the Laurentian Great Lakes. WHO Water Qual. Bull. 6: 146-151, 158-160.

CARNEY,

V. R. 198 1. Preliminary investigation of the algal flora in the sediments of Lake Erie. J. Great Lakes Res. 7: 404-408. HUSTEDT, F. 1930. Die Kieselalgen. Teil 1. Kryptogamen-Flora Bd. 7. Akad. Verlag. Leipzig. SCHELSKJZ, C. L., E. F. STOERMER, D. J. CONLEY, J. A. ROBBINS, AND R. M. GLOVER. 1983. Early eutrophication in the lower Great Lakes: New evidence from biogenic silica in sediments. Science 222: 320-322. STOERMER, E.F.,R.G. KREIS,JR., AND L. SICKO-GOAD. 198 1. A systematic, quantitative, and ecological comparison of Melosira islandica 0. Mtill. with M. granuluta (Ehr.) Ralfs from the Laurentian Great Lakes. J. Great Lakes Res. 7: 345-356. 1976. Apparent optimal -, AND T. B. LADEWSKI. temperatures for the occurrence of some common phytoplankton species in southern Lake Michigan. Univ. Mich. Great Lakes Res. Div. Spec. Rep. 18. 48 p. -, AND -. 1978. Phytoplankton associations in Lake Ontario during IFYGL. Univ. Mich. Great Lakes Res. Div. Spec. Rep. 62. 106 p. THERIOT, E. C. 198 3. Morphological variation in Stephanodiscus niagarae (Bacillariophyceae). Ph.D. thesis, Univ. Michigan. 145 p. -, AND E. F. STOERMER. 1984. Principal components analysis of character variation in Stephanodiscus niagurae Ehrenb.: Morphological variation related to lake trophic status, p. 97-l 11. In Proc. 7th Int. Diatom Symp. Koeltz.

Submitted: 31 May 1984 Accepted: 27 August 1984

Limnol.

Oceanogr., 30(2), 1985, 418421 0 1985, by the American Society of Limnology and Oceanography, Inc

Control by organic coatings of the surface charge of estuarine suspended particles'

Abstract- Particles in suspension in fresh, sea, and estuarine waters appear uniquitously to exhibit a small range of negative surface charge, as measured by electrophoresis. This uniformity is often attributed to the presence of organic or oxide surface coatings on the particles. Here we present experimental results which lend direct support to the idea of control of the particle surface charge by organic coatings. Iron oxide particles formed in situ in organic-free water exhibit the expected positive charge but, on immersion in water containing its natural organic material, rapidly acquire the negative charge normally found for field samples.

I This work was supported financially Natural Environment Research Council 3164).

by the U.K. (grant GR31

There is a growing body of evidence that the electrophoretic mobility ( U,), and hence the surface charge, of suspended particles from a wide variety of estuaries is always negative and falls in the range -0.7 to -2.0 x lop8 m2 s-l V-l (Hunter and Liss 1979, 1982; Pauc 1980; Loder and Liss 1982; Hunter 1983). Similarly UE measurements on suspended particles from fresh (Tipping et al. 198 1) and seawater (Neihof and Loeb 1972, 1974; Hunter and Liss 1979; Loder and Liss 1982) also indicate negatively charged particles with U, values in a somewhat narrower range than that given above for estuarine particles. Plankton in natural waters also appears to have a neg-

Notes

ative surface charge (Bayne and Lawrence 1972; Neihof and Loeb 1972; Myers et al. 1975). Particulate matter from natural waters contains a wide variety of minerals as well as organisms. Some of these minerals (e.g. iron oxides/hydroxides) would be expected to exhibit positive U, in pure systems of similar pH (Parks and de Bruyn 1962; Tipping 198 1). Consequently, it is perhaps surprising that electrophoretic measurements of natural samples have never detected positively charged particles. Furthermore, when only particles from waters of salinities > 5% are considered, the spread of negative mobilities is rather small (-0.7 to - 1.2 x 10e8 m2 s-l V-l). Particles having more negative U, values are only found in freshwaters containing low concentrations of divalent cations (Hunter and Liss 1982). The total absence of positive values and the limited range of negative WE observed, despite the wide mineralogy of the samples examined, is usually attributed to the particles being partially or completely covered by a coating of organic or oxide material (Neihof and Loeb 1972, 1974; Hunter and Liss 1979, 1982; Hunter 1980; Tipping 198 1). We report here direct evidence strongly supporting the idea that organic coatings on the particles are instrumental in controlling their surface charge. Water samples were collected by T. Leatherland on two occasions from the lower reaches of Keithing Burn, which is located in Inverkeithing, Scotland, and drains into the Firth of Forth. These samples covered the whole salinity range of the burn as it mixes with seawater from the firth. Electrophoretic mobility of the particles was determined, without any pretreatment, by techniques developed in this laboratory (Hunter and Liss 1982; Loder and Liss 1982). Reproducibility of the U, determinations was generally +0.03 X lo+ m2 s-l V-l, but could be 3-4 times greater when particle mobilities were low. Aliquots of some samples were oxidized by UV irradiation (Loder and Liss 1982). We selected Keithing Burn because it contains suspended particles almost entirely composed of iron oxide/hydroxide resulting from natural oxidation of iron-rich runoff

419

+o.a5

0

10

15

20

25

30

35

s (%o)

Fig. 1. Electrophoretic mobilities (U,) of natural (untreated)-curve A-and treated particles as a function of salinity (so/oo) for two sets of samples from Keithing Burn (KB 1: open symbols- 3 1 March 1982; KB2: closed symbols-30 June 1982). Shaded area B indicates the spread of results from other estuaries (redrawn from fig. 3 of Hunter and Liss 1979). Curve D-suspended particles centrifuged and resuspended in UVoxidized sample supernatant and then UV-oxidized. Curve C-natural samples (particles plus supernatant) UV-oxidized. Curve E-sample supernatant UV-oxidized to form new particles (UV-PPT). Several UVPPT samples from KB2 were centrifuged and the partitles resuspended in their original untreated sample supernatant; the resulting changes in U, are indicated by the dashed lines (asterisks-final values).

from an abandoned coal mine at its head. SEM pictures show the particles to be generally oblong with dimensions of 0.2 x 0.4 pm. The particles were preliminarily iden-

420

Notes

the natural samples, the resulting supernatant was UV-oxidized, which caused "new" iron-rich particles to precipitate from the water. This new solid phase (UV-PPT) had the same XRD pattern as the natural particles. However, the electrophoretic behavior of the newly formed material (Fig. 1, curve E) was very different from that of the natural particles, with mobility always positive (except at very low salinities where U, was near zero). The fact that UV-PPT has a positive charge probably explains why oxidation of the whole sample produced particles whose charge (Fig. 1, curve C) was more positive than when just the natural suspended particles were oxidized (Fig. 1, curve D). Curves C, D, and E all show U, becoming progressively more positive as the salinity increases. This effect is reversible, so that if the salinity is decreased U, becomes less positive and is due to adsorption of cations (especially Ca2+ and Mg2+) from the seawater. The above results provide strong indications that the breakdown of organic coatings leads to more positive values for UE in this iron-rich system. In the most extreme case (Fig. 1, curve E), new iron oxide/bydroxide particles are being formed in water whose natural organic content is being simultaneously destroyed by UV irradiation. Further evidence comes from the observation that resuspension of UV-PPT from several of the KB2 samples in their own natural (untreated except for removal of particles) sample water leads to a change in U, from positive to negative values close to those of untreated particles from the bum (dashed lines in Fig. 1). This dramatic change must be due to uptake of organic material from the natural water onto the particle surface and illustrates the ability of adsorbed organic material to substantially alter the charge of even strongly positively charged particles. The kinetics of the change in U,, and hence of the adsorption process, are shown in Fig. 2, indicating that although uptake and reversal of charge is complete within minutes, it takes several hours of exposure for the UV-PPT to attain the charge of the natural particles. This would perhaps be expected since organic films increase in thick-

0

I

20

I

40 TIME (min)

60

' `W

320

Fig. 2. Change in electrophoretic mobility (U,) with time for UV-PPT formed from a Keithing Bum sample (KBl-5: salinity, 22.9%) and resuspended in untreated KB l-5 supernatant. The open circles represent more and the closed circles less dilute suspensions and indicate the relative proportions of UV-PPT to supernatant volume. A and n are the U, values for the UVPPT suspended in organic-free water and for natural (untreated) Keithing Bum particles; for both samples the salinity was about 22.9760.

tified as akaganeite (P-FeOOH), based on the occurrence of an XRD peak at d = 7.5 A (Jt. Comm. 1974); more positive identification will have to await further work. The pH of the water samples was in the range 7.6-8.0. U, values for both sets of samples are plotted against salinity in Fig. 1 (curve A). Although the UE values for samples with salinity > 15%~ are in agreement with the results from other estuaries, particle mobilities below 15o/oo significantly less negare ative than found elsewhere (Fig. 1, shaded area B). No positive U, values were observed in any of the natural samples despite the large amounts of potentially positively charged iron-rich material. On the presumption that this was due to organic coatings on the particle surfaces, we did several experiments using UV irradiation for about 25 h to break down organic material. When whole natural samples (water plus suspended particles) were photooxidized, U, for all the particles became more positive (Fig. 1, curve C). Similarly, when natural particles were resuspended in precipitate-free UV-oxidized sample water and then photooxidized, their U, values also became positive (Fig. 1, curve D), but less so than with whole sample oxidation (curve C). Finally, after removing particles from

Notes

ness for several hours after initial surface exposure (Loeb and Neihof 1975). The degree of change in U, for newly formed or immersed particles also depends on the amount of organic material in the water, since repeated soakings in new water stabilize the charge (Hunter 1980; Loder and Liss 1982). This suggests that natural Keithing Bum samples of salinity < 15o/oo have significantly less negative U, values than expected because there is insufficient organic matter present to coat the large amount of iron-rich particles being formed there. The suspended load in the bum was 40-50 mg liter-`. Presumably, as the salinity increases and U, attains normal levels, the seawater is providing enough organic material to coat the diluted particulate load. We have clearly demonstrated the ability of dissolved organic matter to moderate the surface charge on particles. What remains to be established is the extent to which surface coverage by the organic coatings is complete and the role that such films play in ion exchange, in adsorption of trace substances, and in particle aggregationdisaggregation phenomena.

HUNTER,

421

K. A. 1980. Microelectrophoretic properties of natural surface-active organic matter in coastal seawater. Limnol. Oceanogr. 25: 807-822. 1983. On the estuarine mixing of dissolved substances in relation to colloid stability and surface properties. Geochim. Cosmochim. Acta 47: 467-473. AND P. S. LISS. 1979. The surface charge of St.&ended particles in estuarine and coastal waters. Nature 282: 823-825. AND -. 1982. Organic matter and the surface charge on suspended particles in estuarine and coastal waters. Limnol. Oceanogr. 27: 322335.

-.

-,

JOINT COMMITTEE ON POWDER DIFFRACTION STAN1974. Selected powder diffraction data DARDS.

T. C. Loder2 P. S. Liss

School of Environmental Sciences University of East Anglia Norwich NR4 7TJ, United Kingdom

References

BAYNE, D. R., AND J. M. LAWRENCE.

1972. Separating constituents of phytoplankton populations by continuous particle electrophoresis. Limnol. Oceanogr. 17: 481-490.

for minerals. 160 1 Park Lane, Swarthmore, Pennsylvania. L~DER, T. C., AND P. S. LISS. 1982. The role of organic matter in determining the surface charge of suspended particles in estuarine and oceanic waters. Thalassia Jugosl. 18: 433-447. LOEB, G. I., AND R. A. NEIHOF. 1975. Marine conditioning films. Adv. Chem. Ser. 145, p. 3 19-335. MYERS, V. B., R. L. IVERSON, AND R. C. HARRISS. 1975. The effect of salinity and dissolved organic matter on surface charge characteristics of some euryhaline phytoplankton. J. Exp. Mar. Biol. Ecol. 17: 59-68. NEIHOF, R. A., AND G. I. LOEB. 1972. The surface charge of particulate matter in seawater. Limnol. Oceanogr. 17: 7-16. -, AND -. 1974. Dissolved organic matter in seawater and the electric charge of immersed surfaces. J. Mar. Res. 32: 5-12. PARKS, G. A., AND P. L. DE BRUYN. 1962. The zero point of charge of oxides. J. Phys. Chem. 66: 967972. PAUC, H. 1980. Floculation et potentiel de surface des materiaux en suspension en environnement d'embouchure. C.R. Hebd. Seances Acad. Sci. 290: 175-178. TIPPING, E. 198 1. The adsorption of aquatic humic substances by iron oxides. Geochim. Cosmochim. Acta 45: 191-199. -, C. WOOF, AND D. COOKE. 198 1. Iron oxide from a seasonally anoxic lake. Geochim. Cosmochim. Acta 45: 1411-1419.

2 Permanent address: Department of Earth Sciences, University of New Hampshire, Durham 03824.

Submitted: 30 January 1984 Accepted: 13 August 1984

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