A PRELIMINARY INVESTIGATION OF ORGANIC-INORGANIC

Transcription

1 952 NOTES NYGAARD, G On the productivity of the bottom vegetation in the lake Grane Langse. Int. Ver. Theor. Angew. Limnol. Verh. 13: Hydrographic studies, especially on the carbon dioxide system, in Grane Langse. Biol. Skr. Kgl. Dan. Vidensk. Selsk. Skr. lp(2): 110 p. RAALTE, M. H. VAN On the oxidation of the environments by the roots of rice (Oryza satica L. ). Ann. Bot. Gardens Buitenzong (Hors Ser. ) 1: TEAL, J. M., AND J. W. KANWISHER Gas transport in the marsh grass, Spartina alterniflora. J. Exp. Bot. 17: WIUM--kDERSEN, s Photosynthetic uptake of free CO, by the roots of Lobelia dortmanna. Physiol. Plant. 25 : AXD J. M. ASDERSEN Carbon dioxide content in the interstitial water in the sediment of Grane Langso, a Danish Lobelia lake. Limnol. Oceanogr. 17: A PRELIMINARY INVESTIGATION OF ORGANIC-INORGANIC ASSOCIATIONS IN A STAGNATING SYSTEMS ABSTRACT \dolecuiar weight fractionation procedures were used to examine the possibility of organic-inorganic complex formation during stagnation of a lake sediment system. Fe, Mg, and 1ln were found to associate with fractions >500 molecular weight. No evidence was found for the occurrence of the ionic states of these metals in the anoxic environment used in this investigation. The results are discussed in relation to the behavior of soil humic compounds. Shapiro (1958, 1964, 1966) noted that natural organic materials found in lakes may be important as chelating agents capable of keeping iron in a nonprecipitable state at high ph and redox potential values. He also speculated that these colored ( and colorless) organics could originate in the soils of the watershed and from lake sediments. The most intensive research on these naturally occurring organics has been focused on their role in anoxic waters (Hutchinson 1957). However, their capacity to hold ions in solution suggests a probable role in the movement of trace elements from lake sediments during stagnation. The work of Szalay (1964) indicated that the limitation in the geochemical enrichment process lies in the slow leaching and migration of cations from source rocks and not in their sorption by the humic acids of soils. If naturally 1 This work was supported by the Office of Water Resources Research, as authorized under the Water Research Act of occurring organics found in lakes behave like soil humic compounds, it would be expected that trace elements mobilized during stagnation (Mortimer 1941, 1942; Hutchinson 1957) may be sorbed onto organics present in the water overlying the sediments. The significance of this type of behavior would be that metals mobilized from the sediments would be less susceptible to precipitation by any sulfide evolved from the sediments during the later stages of stagnation and that trace elements would be less likely to precipitate if transported to areas of higher ph and redox potential such as in the productive euphotic zone of lakes. This association in turn would increase their availability to the producer components of the lake, This report describes a preliminary investigation of the behavior of the metals Fe, Mn, Mg, and Zn in the presence of naturally occurring organics during the process of stagnation. METHODS Undisturbed sediment-water samples were collected from Lago Pond, a small Georgia fishpond. The intact mud-water samples and the overlying waters were obtained by means of a free-fall corer with a transparent plastic insert. Uniformly spaced holes were drilled into the insert and filled with silicon rubber to form septa. A one-way flow valve mounted above the insert in the corer developed a negative hydrostatic pressure as the corer

2 NOTES IO e to MUD FEB II I1 I I I I1 I,, l,, l l,, 1, I IO II I2 I3 14 I DAYS FROM COLLECTION FIG. 1. Core A, oxidation-reduction (electrode potential) isopleths. was raised from the lake bottom, preventing loss of the sample. Rubber stoppers were used to seal the ends of the plastic insert for transport to the laboratory; these sealed tubes were then used as experimental microcosms. Oxidation-reduction potentials (redox) were determined with an Ag/AgCl electrode and a bright platinum electrode (Schindler and Honick 1971) and were used with ph measurements to determine sampling frequency. Samples (25 ml) of the aqueous phases were withdrawn with a hypodermic syringe and passed through a membrane ultrafiltration system ( Amicon model 202, UF cell). The five Diaflo membrane filters used here were designed to retain material with molecular weights of about 100,000, 50,000, 10,000, 2,000, and 500. (Samples were transferred and filtered under nitrogen to prevent oxygenation. ) Replicate aliquots of these water fractions were analyzed for organic carbon (Menzel and Vaccaro 1964). The remainder of each water fraction was analyzed for Fe, Mg, Mn, and Zn with an atomic absorption spectrophotometer (Perkin Elmer model 214). Unfiltered samples were also analyzed for organic carbon and the above metallics and the concentration of each element retained in the molecular weight fractions was calculated by subtracting from the concentration in the next larger size sample. The total inorganic carbon of the aqueous phase was determined by infrared gas analysis.

3 954 NOTES SAW&E 4 ph 6.3 SAMP E5 L ph58 I4 FE6 DAYS FROM COLLECTION FIG. 2. Core B, oxidation-reduction (electrode potential) isopleths /h---- I I I I I I6 I7 18 I9 20 &? RESULTS Two cores were collected on 14 February 1971 from a depth of 4 m. Oxidationreduction potential isopleths ( SO mv) for two experimental sediment-water systems held at 22C are shown in Figs. 1 and 2. The position (with respect to the interface) and time of the sample withdrawal are indicated on the figures, along with the ph of the sample. The results of the molecular weight distribution of the organic carbon (210%) and the distribution of the metals ( 210%) corresponding to the respective carbon fractions from the two cores are shown in Figs. 3, 4, and 5. Although the results of this study are not definitive, the following observations can be made: 1. The concentration of inorganic carbon increased in both core systems during reduction. 2. The organic carbon fractions became more evenly distributed with respect to molecular weight during the process of reduction. 3. The concentrations of Fe, Mg, and W-r increased and appeared to associate with the lo,ooo-50,000 molecular weight fraction during reduction. 4. Except for Zn (sample 5), no metals were found in the O-500 molecular fraction. DISCUSSION weight The above results indicate that metals mobilized from Iake sediments during stagnation may be sorbed onto larger molecular weight substances present in the waters

4 NOTES ll ZINC 95s 20 I 1 -I FIG. 3. Fractionation results for core A, 16 February (Eh > 200 mv, ph = 6.71, inorganic carbon = 219 pg-atoms liter-l) and 18 February (Eh = 150 mv, ph = 6.46, inorganic carbon = 383 pg-atoms liter-l). Organic carbon molecular weight fractions are as follows: A-unfiltered- 100,000; B-100,000-50,000; C-50,000-10,000; D-10,000-2,000; E-2, ; F overlying the sediments. Whether these substances are strictly organic is debatable since it is possible that colloid-forming substances such as clays (size < 100 A: M. Whitfield, personal communication) could also be retained by the ultrafilters and no attempt was made in this study to determine the quantities of clays present in the molecular weight array. Table 1 lists the characteristics of the Amicon ultrafilters we used; it is evident that clays could have been retained by the filters. Obviously clays cannot be ignored in future studies of this problem. Otsuki and Hanya (1972) noted that when the method of Menzel and Vaccaro (1964) is applied to the determination of dissolved organic carbon in anoxic waters, there may be a loss of volatile organic acids during the bubbling with Nz under acidic conditions to remove inorganic carbon. This problem as well as the efficiency of the persulfate CARBON 1 I I I 1 A B C D E F FIG. 4. Fractionation results for core A, 22 February (Eh = 100 mv, ph = 6.84, inorganic carbon = 467 pg-atoms liter-l) and core B, 25 February (Eh = 100 mv, ph = 6.31, inorganic carbon = 600 pg-atoms liter-l). Organic carbon molecular weight fractions as in Fig. 3. oxidation for refractory organics may produce discrepancies in the results not readily explained. It has been suggested that organic substances found in lakes are similar to the humic components of soils ( Hutchinson 1957). Although this generalization may not hold in all situations, the number of excellent studies that have been conducted on soil humic compounds can be used for speculation. For example, Szalay ( 1964) considered Fe and Zn huminophilic elements within soils. If this behavior were maintained in aquatic systems, Fe and Zn would be expected to be sorbed onto humic substances in the water as they mobilized from the sediments during stagnation; the rate of supply of the elements to the humic compounds would depend on their rate of movement from the sediments. Szalay felt

5 956 NOTES ZINC 0. I I - 20 < IRON 10 <- 0, I I 1 MANGANESE 20 * IO * 0 I- 1 I 50, _ TABLE 1. Characteristics of the Amicon ultrafilters zwxl in this study. A-Amicotz filter designation; B-retains approximate molecular weight; C--pore size (radius in A); D-charge A B C D XM 100 A 100, nonionic; negligible charge XM 50 50, nonionic; negligible charge UM 10 10, ionic sites available; net neutral charge um2 2, ionic sites available; net neutral charge UM ionic sites available; net negative charge CARBON FIG. 5. Fractionation results for core B, 19 March (Eh = mv, ph = , inorganic carbon = 717 pug-atoms liter-l). Organic carbon molecular weight fractions as in Fig. 3. that Mg and Mn were not adequately characterized at that time to comment on their behavior toward humic compounds. However, he did indicate that P was huminophobic ; in other words, that it would be repelled by humic compounds. Schnitzer (1969) also showed that P would not react with fulvic acid ( a soil humic compound) but could react via a metallic, forming a fulvic acid-metal-phosphate complex. If this kind of reaction occurred within aquatic systems, it would undoubtedly complicate the phosphorus cycle. Unfortunately, studies of naturally occurring organics in aquatic systems have not kept pace with their counterpart in soils and parallels between the behavior of soil humics and aquatic humics are questionable. Our work demonstrates an association of the elements studied with definite fractions in the array of molecular weights. This interaction between the elements and colloid-forming organics ( or inorganics ) could impart a stability to the elements which would permit their transport to the oxygen-rich, productive areas of lakes, Unfortunately, we do not have sufficient data to comment on the specific behavior of the elements in our study. We cannot explain the peculiar concentration of elements in the lo,ooo-50,000 molecular weight fraction or the apparent movement of Zn toward the lower molecular weight fraction. Our use of molecular weights in this paper is for convenience only and our designation of specific molecular weight fractions is in reference to the Amicon Corporation s claims for each of the individual ultrafilters ( Table 1). However, our results indicate that organic substances may play an important role in the movement of elements from the sediments to the water. More information on the chemical composition and behavior of the organic material, as well as the complexes, must be obtained for an understanding of the role of organic material in this transport process. Furthermore, since this study was conducted on only one lake system, additional lakes must be investigated before a more generalized hypothesis can be tendered. Department of Zoology, University of Georgia, Athens Division of Natural Sciences, New College Sarasota, Florida J. E. SCHINDLER J. J. ALBERTS K. R. HONICK

6 NOTES 957 REFERENCES HUTCHINSON, G. E A treatise on limnology, v. 1. Wiley p. MENZEL, D. W., AND R. F. VACCARO The measurement of dissolved organic and particulate carbon in seawater. Limnol. Oceanogr. 9 : MORTIh%ER, c. H The exchange of dissolved substances between mud and water in lakes. 1 and 2. J. Ecol. 29: The exchange of dissolved substances between mud and water in lakes. 3 and 4. J. Ecol. 30: OTSUKI, A., AND T. HANYA Production of dissolved organic matter from dead green algae cells. 2. Limnol. Oceanogr. 17 : SCHJNDLER, J. E., AXD K. R. HOSICK Oxidation-reduction determinations at the mud-water interface. Limnol. Oceanogr. 16: SCHSITZER, M Reactions between fulvic acid, a soil humic compound and inorganic soil constituents. Soil Sci. Sot. Amer. Proc. 33: SHAPIRO, J Yellow acid-cation complexes in lake water. Science 127: Effect of yellow organic acids on iron and other metals in water. J. Amer. Water Works Ass. 56: The relation of humic color to iron in natural waters. Int. Ver. Theor. Angew. Limnol. Verh. 16: SZALAY, A Cation exchange properties of humic acids and their importance in geochemical enrichment of UO, and other cations. Geochim. Cosmochhn. Acta 28 :