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Comparative chemistry of submicron sized organic fractions in four rivers of the southeastern United States
Comparative chemistry of submicron sized organic fractions in four rivers of the southeastern United States J. J. ALBERTSand D. W. EVANS Savannah River Ecology Laboratory, Drawer E, Aiken, S.C. 29801, U.S.A. Abstract--Investigations of 4 southeastern United States rivers show that greater than 80% of the dissolved organic carbon (DOC) is in a size fraction less than 13A in diameter. Total acidity values determined for organics less than 4500 but greater than 13 A vary by a factor of only 3 among rivers and size fractions (2.29-6.22 meq/g organic matter). Although these values appear small in comparison to inorganic anions in the systems, they become increasingly important in low pH streams, representing up to 23% of the total milliequivalents of anionic charge available for binding. INTRODUCTION Several a u t h o r s (ScHINDLER a n d ALBERTS, 1974; BECK et al., 1974; REUTER a n d PERDUE, 1977; GIESY a n d BRIESE, 1977, 1978) h a v e discussed the i m p o r t a n c e of o r g a n i c m a t t e r in the g e o c h e m i c a l cycles of i n o r g a n i c e l e m e n t s in s o u t h e a s t e r n U n i t e d States waters. T h e w o r k r e p o r t e d here is p a r t of a c o n t i n u i n g p r o g r a m to define the i m p o r t a n t p h y s i c o c h e m i c a l c h a r a c t e r i s t i c s d e t e r m i n i n g b i o g e o c h e m i c a l cycling o f metallic elements. In p a r ticular, we are a t t e m p t i n g to q u a n t i f y the relative i m p o r t a n c e of f u n c t i o n a l g r o u p s on the o r g a n i c m o l e c u l e s a n d of i n o r g a n i c a n i o n i c c o m p o n e n t s in m e t a l b i n d i n g for several t y p e s of a q u a t i c e n v i r o n m e n t s in the s o u t h e a s t e r n U n i t e d States.
METHODS Surface water samples (~ 501) were collected at 5 locations from 4 rivers in Georgia and Florida on 14 and 15 September 1978 (Fig. 1). Locations were chosen where major highways cross the rivers: Caloosahatchee River at Florida State Highway 31 near Olga, Fla.; Peace River at Florida State Highway 70 near Arcadia, Fla.; Lower Suwannee River at U.S. Highway 19 near Chiefland, Fla.; Upper Suwannee River at U.S. Highway 441 near Fargo, Ga.; and the Satilla River at Georgia State Highway 82 near Waycross, Ga. Samples were filtered on the day of collection (0.45 jum membrane filters, Millipore Corp.). Temperature, dissolved oxygen (Yellow Springs Instrument Model 54 Clark-type Polarographic 02 probe), pH (Radiometer Model PHM53 Specific Ion Meter, using Radiometer Model GK2401C combination pH glass electrode), conductivity (YSI Model 31 Conductivity Bridge) and alkalinity (AMERICANPUBLICHEALTHASSOC.,1976) were determined in the field. Aliquots of the filtered water were collected at the same time for analysis of dissolved organic carbon (Beckman Model 915 Total Carbon Analyzer), reactive phosphate (AMERICANPUBLIC HEALTH ASSOC.,1976), dissolved sulfate (BAUMAN~,1976), and chloride (AMERICANPUBLICHEALTHASSOC.,1976). Upon return to the laboratory, colloidal materials in the filtered waters were concentrated by ultrafiltration (Diaflo Hollow Fiber Bundles, Amicon Corp.) which sequentially retained particles > 52 and > 13 A diameter. Organic carbon concentrations were determined on the fractions (Beckman Model 915 Total Carbon Analyser), as were total equivalents of exchangeable protons by reaction with Ba(OH)2 and titration with standard acid (SCHNITZERand KHAN, 1972). Elemental analyses were performed on both filtered water samples and isolated size fractions. Concentrations of Si and the major cations, Na, K, Ca and Mg, were determined with a Jarrell-Ash Plasma Atomcomp 750 ICP Emission Spectrometer. Analyses for the minor elements, As, Cd, Cu, Fe, Mn and Pb, were performed with a Perkin-Elmer Model 306 Atomic Absorption Spectrophotometer equipped with an HGA-2100 Graphite Furnace. The ratios of E4/E6 were determined at 465 and 665 #m in l cm path length cuvettes (Beckman Acta CIII UV-Visible spectrophotometer).
RESULTS AND DISCUSSION T h e rivers of the s o u t h e a s t e r n U n i t e d States h a v e diverse p h y s i c o c h e m i c a l c h a r a c t e r istics, r a n g i n g f r o m high d i s s o l v e d o r g a n i c c a r b o n ( D O C ) , low p H s t r e a m s referrred to as " b l a c k w a t e r ' , to the low D O C , high p H , high n u t r i e n t s t r e a m s often i n d i c a t i v e of a n t h r o p o g e n i c inputs. T h e 4 rivers s a m p l e d in this s t u d y are r e p r e s e n t a t i v e of t h a t range. T h e S a t i l l a River a n d U p p e r S u w a n n e e River ( s a m p l e d n e a r the river's source, the O k e f e n o k e e S w a m p ) a r e a c i d s t r e a m s of low c o n d u c t i v i t y (Table 1). T h e P e a c e River, C a l o o 463
464
J.J. ALBERTSand D. W. EVANS
GEORGIA
Wildlife-~ Refuoe
GULF OF MEXICO
2O0
0 I
I
I
kilometers
185"W Fig. 1. Locations of southeastern U.S. rivers sampled 14 and 15 September 1978.
sahatchee River and Lower Suwannee River (sampled near its mouth in central Florida) are neutral to slightly basic streams with elevated nutrient levels indicative of agroindustrial influences. Both the Peace and Lower Suwannee rivers drain the phosphate mining areas of central Florida as reflected by the high dissolved phosphate levels in those waters. The Caloosahatchee River also receives some phosphate run-off from the citrus groves of south-central Florida. All 3 of these rivers appear to be more metabolically active than the other streams as indicated by the lower dissolved oxygen concentrations of their waters. The rivers also differ in their concentrations of dissolved major and minor elements (Table 2). Differences between the characteristics of the upper and lower Suwannee River are of some interest. Large changes in pH, conductivity and DOC concentration, as well as changes in the concentration of some major and minor cations and anions along its course, make the Suwannee River an excellent area for the study of changes in the geochemistry of metals and organics within a river system. However, detailed evaluation of these differences will not be attempted in this report. Because of the complexity of the dissolved organic matter in natural waters (SCHNITZER and KHAN, 1972), quantitative prediction of the interactions between organic Table 1. Comparison of water chemistry characteristics for 5 southeastern United States river waters
Alkalinity (raM) pH Cond. (mmho) D.O. (ppm.) D O C (ppm.) CI- (mM) PO2 3 (mM) SO~ 2 (mM) Si (mM)
Caloosahatchee River
Peace River
Lower Suwannee River
Upper Suwannee River
Satilla River
2.20 7.33 400 3.7 16.0 1.1 0.81 0.14 0.15
0.92 7.21 220 5.9 15.6 0.91 21 0.25 0.18
2.54 7.74 300 5.9 -0.12 2.4 0.25 0.15
<0.01 4.20 55 6.3 25.6 0.09 < 0.001 0.004 0.02
0.15 6.49 53 8.3 9.8 0.14 0.67 0.03 0.15
Chemistry of submicron sized organic fractions in four rivers of southeastern U.S.A.
465
Table 2. "Dissolved" concentrations of some major (ppm) and minor (ppb) elements in 5 river waters
Caloosahatchee River
Lower Suwannee River
Peace River
Upper Suwannee River
Satilla River
Major elements Na K Mg Ca
32.7 11.6 7.95 53.7
As Cd Cu Fe Mn Pb
26.0 <0.01 5.6 77.0 0.29 0.58
32.7 2.65 9.25 30.5
5.05 0.52 7.06 53.7
3.87 <0.1 0.52 0.84
6,32 1,21 1.22 2A
18.0 <0.01 6.0 20.0 0.53 <0.1
8.5 0.07 0.7 240.0 21.0 <0.1
10,0 0.33 1.3 59.0 0,10 <0.1
Minor elements 14.0 0.02 3.9 70,0 0.53 <0.1
and inorganic compounds is difficult. Several parameters appear to be related to the extent of metal binding by natural organic substances. WILSON and KINNEY (1977) discuss the need to consider the bonding of both protons and metals at individual binding sites and with respect to the polymeric charge density of humic acids. The determination of the total acidity in milliequivalents of reactive proton sites per unit concentration of organic matter, allows one to consider metal binding to naturally occurring organic compounds as a series of proton replacement reactions. The total equivalents of protons exchangeable on the organic sites may be contrasted to those of other potential ligands in the system. Together with metal binding capacities and stability constants, total acidity measurements should allow one to quantify the importance of organic ligands (relative to inorganic ligands and anionic particulates) in metal interactions. Our work indicates that the range of milliequivalents of total acidity per gram of dissolved organic matter is relatively narrow (Table 3), with total acidity values varying by only a factor of 3 among rivers and among size fractions. With the exception of material isolated from the Satilla River, it appears that the smaller sized material (particles between 52 and 13 A diameter) has the higher total acidity. High total acidities are indicative of both a greater number of active sites per unit organic matter and less-condensed polymeric nuclei. Some support is given to this latter conclusion by the E4/E 6 ratios, which are larger for the smaller size material, indicating less condensation of the organic molecules (ScHNITZER and KHAN, 1972). Unfortunately, the trend of greater total acidity in smaller, less complex size ranges is not found in the case of the Satilla River, although the E4/E6 ratio continues to indicate less complexity in the smaller size fractions. Insufficient material for analysis prevented determination of the total acidity values for both size classes of the Lower Suwannee River and for the larger size class of the Caloosahatchee River. It appears that no strong relationship exists between the E4/E6 ratios and total acidity values across river systems, limiting the possibility of predicting milliequivalents of total acidity by determining the degree of complexity of the organic material. Table 3. Total acidities (meq/g. org.) and E4/E6 ratios of molecular size isolates* from river water > H10X100
Caioosahatchee River Peace River Upper Suwannee River Satilla River
> H10P5
Total acidity
EJE 6
-2.29 _ 0.04 4.68 _ 0.04 4.39 + 0.04
-2.16 4.72 2.44
Total acidity 7.16 6.22 5.44 2.45
+ + + +
0.04 0,04 0.04 0.04
E4/E6 4.56 3.79 6.19 5.00
* Size fractions refer to Amicon Corp. Diaflo Hollow Fibers with retention parameters of: HIOX100; pore diameter ~ 52 A, retains > 100,000 MW; HIOP5; pore diameter -~ 13 A, retains > 5,000 MW.
466
J.J. ALBERTS and D. W. EVANS Table 4. Amount of dissolved organic carbon and microequivalents of total acidity in 2 size fractions of river waters > H10X100 DOC* (%) /.teq/I Caloosahatchee River Peace River Lower Suwannee River Upper Suwannee River Satilla River
3.8 3.3 . 1.8 1.1
> H10P5 DOC (%) /aeq/I
-6 .
.
17.7 5.6
16 10
17.9 12.0
49 6
.
6 2
* D O C is reported as the percentage of the total D O C which is retained by the specified filter.
REUTER and PERDUE (1977) found that, on average, 60-80% of the DOC and POC (particulate organic carbon) in rivers consist of humic substances. While the molecular weight of humic substances is in doubt (reported values range over 4 orders of magnitude (SCHNITZER and KHAN, 1972)), they are usually considered to be large molecules. SCHINDLER and ALBERTS (1974) have shown that more than 80% of the DOC in 4 southeastern U.S. lakes is removed by ultrafiltration of particles greater than 24 A diameter. GIESY and BRIESE (1978) show that greater than 60% of the DOC in 2 South Carolina streams is less than 15 A in diameter. Our work shows that more than 80% of the DOC in these rivers is less than 13 A in diameter (Table 4). In addition, most carbon retained by the ultrafilters is in the smaller sized material. We can only speculate on the reasons for the inconsistencies in the relative amount of organic matter in larger size fractions. SCHINDLERand ALBERTS(1974) separated their organic fractions by ultrafiltration from aerobic and anaerobic lake and reservoir waters. It would be reasonable to expect that these systems might have higher concentrations of larger size material based on residence times in the systems and the more refractory nature of larger molecules. The work of GIESY and BRIESE (1978), using ultrafiltration techniques in flowing systems, agrees with the observations reported here in which similar techniques were used. The apparent disagreement between both the stream data and the current work with that discussed by REUTER and PERDUE (1977) may simply be based on operational definitions and different analytical techniques. Using the total acidity values determined here for the size fractions and the percentage of organic carbon in the isolates (Table 4), the total acidities of the fractions may be calculated on a per liter basis. Calculated in this manner, these values appear small. However, when compared to the milliequivalents per liter of other anionic components in the waters (Table 5), the organic matter in the isolates makes a significant contribution to the dissolved anionic charge concentration of some rivers. This is particularly so in the low pH systems, such as the Upper Suwannee River, where the organic isolates represent greater than 33% of the anionic charge, and in the Satilla River, where the organic fractions contribute approximately 12% of the sulfate. In the latter case, the organic isolates comprise only about 2% of the anionic charge component. However, chloride and bicarbonate ions, which comprise most of the negative charge in that system, form relatively weak complexes with metal cations (HAHNE and KROONTJE, 1973). Humic type organic compounds form strong complexes (Ka ~ l0 a) with many transition metals Table 5. Anionic components (/~eq/l) of southeastern United States rivers
C1HCO~ PO~7 s SO,~ 2 Organic fraction
Caloosahatchee River
Peace River
Upper Suwannee River
Satilla River
1100 2200 2.4 270
910 920 63 500
90 < 10 <1 8.1
140 150 2.0 67
16
16
55
8.0
Chemistryof submicron sized organicfractionsin four rivers of southeastern U.S.A.
467
Table 6. Amount* of "dissolved" metal retained by ultrafiltration
As Cd Cu Fe Mn Pb
Caloosahatchee River
Peace River
Upper Suwannee River
2.3 -9.3 61.0 38.0 52.0
3.9 32.0 16.0 57.0 ---
1.9 9.7 -16.0 6.7 --
* Values expressed as percentage of metal concentrationdetermined for 0.45/~mfilteredriver water. (GuY and CHAKRABARTI, 1976; GIESY and ALBERTS, in preparation) and would be expected to influence metal binding disproportionately to their actual concentrations. Further evidence of the importance of isolated fractions on the geochemistry of "dissolved" metals may be seen in Table 6. In some systems, considerable quantities of metals may be removed from the water column by ultrafiltration. These include metals such as Fe which would be expected to form colloidal particles, especially in elevated pH systems, as shown in the Caloosahatchee and the Peace Rivers, or may represent occluded material in organic aggregates (ALBERTS et al., 1976). Cadmium and lead may also be found in significant quantities in the particles separated by ultrafiltration. The exact nature of these particles remains unknown, but they may consist of large organic-metal complexes, organic-metal colloids, inorganic-metal colloids, or organic-clay-metal colloids. However, data we have gathered indicate that the possible clay contribution to the total acidity of these particles is less than 10% (ALBERTS,unpublished data). Whatever the composition, the fact that these metals are removed from natural water systems by ultrafiltration clearly indicates that they do not exist as simple dissolved ions or ionic complexes, but rather, that they are found in colloidal particle size ranges. A final point of interest is the difference in water chemistry of the Satilla River, when our data are compared with values reported for samples from that river system collected 9 years earlier (BECK et al., 1974). Comparison shows that the Satilla River, at our sampling site downstream of Waycross, Ga., has increased in pH from 4.45 to 6.49. Other changes include apparent 3-fold increases in SO4 and K, and 2-fold increases in dissolved Na, Mg, Ca and SiO 2. Iron and manganese concentrations have decreased by factors of 27 and 500, respectively, while organic carbon has decreased 2-fold to the world river water average of 10ppm (GARRELS and MACKENZIE, 1971). These changes probably result from the growth of anthropogenic influences on the river, most likely due to increases in population and industry in the region. It is interesting to note the similarity of the values reported by BECKet al. (1974) for the Satilla River, to those we found in the Upper Suwannee River, which has almost no anthropogenic inputs upstream of our sampling point. Indeed, the values are almost identical, including the ratios of major cations to chloride ion as used by BECK et al. (1974) to indicate the similarity in composition of the Satilla River water to rainfall in southeastern Georgia. These comparisons emphasize the need for care in attempting to correlate new data with older values from the literature, especially in areas where regional agro-industrial growth may have a major impact on relatively clean systems.
SUMMARY
We have observed that only a small percentage of total dissolved organic carbon may exist in size fractions greater than 13 A diameter (nominal molecular weight ~ 5000), yet the importance of this material in binding metals and contributing to the overall net negative charge of these natural waters can be considerable. Since ~ 80% of the total DOC was not measured, the contribution of these smaller molecules can only increase the total binding of metals by organic matter in these systems. Total acidity values of
468
J.J. ALBERTS and D. W. EVANS
organic fractions in these rivers vary by a factor of only three, but the proportion of/~eq/l of negative charge contributed to the system is dependent on the river type and becomes increasingly important in the low pH, "blackwater" streams common in the southeastern United States. Acknowledoement--This research was supported by Contract DE-AC09-76SR00819 between the University of Georgia and the U.S. Department of Energy. REFERENCES
ALBERTS J. J., SCHINDLER J. E., NUTTER, JR. D. E. and DAVIS E. (1976) Elemental, infra-red spectrophotometric and electron spin resonance investigations of non-chemically isolated humic material. Geochim. Cosmochim. Acta, 40, 369-372. AMERICAN PUBLIC HEALTH ASSOCIATION 0976) Standard Methods for the Examination of Water and Waste Water. 14th Ed. 1193 pp. BAUMANN E. W. (1976) Nephelometric determination of microgram quantities of sulfate with 2-aminopermidine. Savannah River Laboratory, ERDA DP-1437. 17 pp. BECr K. C., REUTER J. H. and PERDUE E. M. (1974) Organic and inorganic geochemistry of some coastal plain rivers of the southeastern United States. Geochim. Cosmochim. Acta, 38, 341-364. GARRELS R. M. and MACKENZIE F. T. (1971) Evolution of Sedimentary Rocks, 450 pp. Norton. GIESV, JR. J. P. and BRIESE L. A. 0977) Metals associated with organic carbon extracted from Okefenokee Swamp water. Chem. Geol. 20, I09-120. GIESV, JR. J. P. and BRIESEL, A. (1978) Trace metal transport by particulates and organic carbon in two South Carolina streams. Verh. laternat. Verein. Limnol. 20, 1401-1417. Guy R. D. and CHAKRABARTIC. L. 0976) Studies of metal-organic interactions in model systems pertaining to natural waters. Can. J. Chem. 54, 2600-2611. HAHNE H. C. H. and KROONTJE W. (1973) Significance of pH and chloride concentration on behaviour of heavy metal pollutants: mercury (II), cadmium (II), zinc (II) and lead (II). J. Environ. Qual. 2, 444-450. REUTER J. H. and PERDUE E. M. (1977) Importance of heavy metal-organic matter interactions in natural waters. Geochim. Cosmochim. Acta, 41,325-334. SCHINDLER J. E. and ALBERTSJ. J. 0974) Analysis of organic-inorganic associations of four Georgia reservoirs. Arch, Hydrobiol. 74, 429-440. SCHNITZER M. and KHAN S. U. (1972) Humic Substances in the Environment. 327 pp. Marcel Dekker. WILSON D. E. and KINNEY P. 0977) Effects of polymeric charge variations on the proton-metal ion equilibrium of humic materials. Limnol. Oceanoor. 22, 281-289.
METHODS Surface water samples (~ 501) were collected at 5 locations from 4 rivers in Georgia and Florida on 14 and 15 September 1978 (Fig. 1). Locations were chosen where major highways cross the rivers: Caloosahatchee River at Florida State Highway 31 near Olga, Fla.; Peace River at Florida State Highway 70 near Arcadia, Fla.; Lower Suwannee River at U.S. Highway 19 near Chiefland, Fla.; Upper Suwannee River at U.S. Highway 441 near Fargo, Ga.; and the Satilla River at Georgia State Highway 82 near Waycross, Ga. Samples were filtered on the day of collection (0.45 jum membrane filters, Millipore Corp.). Temperature, dissolved oxygen (Yellow Springs Instrument Model 54 Clark-type Polarographic 02 probe), pH (Radiometer Model PHM53 Specific Ion Meter, using Radiometer Model GK2401C combination pH glass electrode), conductivity (YSI Model 31 Conductivity Bridge) and alkalinity (AMERICANPUBLICHEALTHASSOC.,1976) were determined in the field. Aliquots of the filtered water were collected at the same time for analysis of dissolved organic carbon (Beckman Model 915 Total Carbon Analyzer), reactive phosphate (AMERICANPUBLIC HEALTH ASSOC.,1976), dissolved sulfate (BAUMAN~,1976), and chloride (AMERICANPUBLICHEALTHASSOC.,1976). Upon return to the laboratory, colloidal materials in the filtered waters were concentrated by ultrafiltration (Diaflo Hollow Fiber Bundles, Amicon Corp.) which sequentially retained particles > 52 and > 13 A diameter. Organic carbon concentrations were determined on the fractions (Beckman Model 915 Total Carbon Analyser), as were total equivalents of exchangeable protons by reaction with Ba(OH)2 and titration with standard acid (SCHNITZERand KHAN, 1972). Elemental analyses were performed on both filtered water samples and isolated size fractions. Concentrations of Si and the major cations, Na, K, Ca and Mg, were determined with a Jarrell-Ash Plasma Atomcomp 750 ICP Emission Spectrometer. Analyses for the minor elements, As, Cd, Cu, Fe, Mn and Pb, were performed with a Perkin-Elmer Model 306 Atomic Absorption Spectrophotometer equipped with an HGA-2100 Graphite Furnace. The ratios of E4/E6 were determined at 465 and 665 #m in l cm path length cuvettes (Beckman Acta CIII UV-Visible spectrophotometer).
RESULTS AND DISCUSSION T h e rivers of the s o u t h e a s t e r n U n i t e d States h a v e diverse p h y s i c o c h e m i c a l c h a r a c t e r istics, r a n g i n g f r o m high d i s s o l v e d o r g a n i c c a r b o n ( D O C ) , low p H s t r e a m s referrred to as " b l a c k w a t e r ' , to the low D O C , high p H , high n u t r i e n t s t r e a m s often i n d i c a t i v e of a n t h r o p o g e n i c inputs. T h e 4 rivers s a m p l e d in this s t u d y are r e p r e s e n t a t i v e of t h a t range. T h e S a t i l l a River a n d U p p e r S u w a n n e e River ( s a m p l e d n e a r the river's source, the O k e f e n o k e e S w a m p ) a r e a c i d s t r e a m s of low c o n d u c t i v i t y (Table 1). T h e P e a c e River, C a l o o 463
464
J.J. ALBERTSand D. W. EVANS
GEORGIA
Wildlife-~ Refuoe
GULF OF MEXICO
2O0
0 I
I
I
kilometers
185"W Fig. 1. Locations of southeastern U.S. rivers sampled 14 and 15 September 1978.
sahatchee River and Lower Suwannee River (sampled near its mouth in central Florida) are neutral to slightly basic streams with elevated nutrient levels indicative of agroindustrial influences. Both the Peace and Lower Suwannee rivers drain the phosphate mining areas of central Florida as reflected by the high dissolved phosphate levels in those waters. The Caloosahatchee River also receives some phosphate run-off from the citrus groves of south-central Florida. All 3 of these rivers appear to be more metabolically active than the other streams as indicated by the lower dissolved oxygen concentrations of their waters. The rivers also differ in their concentrations of dissolved major and minor elements (Table 2). Differences between the characteristics of the upper and lower Suwannee River are of some interest. Large changes in pH, conductivity and DOC concentration, as well as changes in the concentration of some major and minor cations and anions along its course, make the Suwannee River an excellent area for the study of changes in the geochemistry of metals and organics within a river system. However, detailed evaluation of these differences will not be attempted in this report. Because of the complexity of the dissolved organic matter in natural waters (SCHNITZER and KHAN, 1972), quantitative prediction of the interactions between organic Table 1. Comparison of water chemistry characteristics for 5 southeastern United States river waters
Alkalinity (raM) pH Cond. (mmho) D.O. (ppm.) D O C (ppm.) CI- (mM) PO2 3 (mM) SO~ 2 (mM) Si (mM)
Caloosahatchee River
Peace River
Lower Suwannee River
Upper Suwannee River
Satilla River
2.20 7.33 400 3.7 16.0 1.1 0.81 0.14 0.15
0.92 7.21 220 5.9 15.6 0.91 21 0.25 0.18
2.54 7.74 300 5.9 -0.12 2.4 0.25 0.15
<0.01 4.20 55 6.3 25.6 0.09 < 0.001 0.004 0.02
0.15 6.49 53 8.3 9.8 0.14 0.67 0.03 0.15
Chemistry of submicron sized organic fractions in four rivers of southeastern U.S.A.
465
Table 2. "Dissolved" concentrations of some major (ppm) and minor (ppb) elements in 5 river waters
Caloosahatchee River
Lower Suwannee River
Peace River
Upper Suwannee River
Satilla River
Major elements Na K Mg Ca
32.7 11.6 7.95 53.7
As Cd Cu Fe Mn Pb
26.0 <0.01 5.6 77.0 0.29 0.58
32.7 2.65 9.25 30.5
5.05 0.52 7.06 53.7
3.87 <0.1 0.52 0.84
6,32 1,21 1.22 2A
18.0 <0.01 6.0 20.0 0.53 <0.1
8.5 0.07 0.7 240.0 21.0 <0.1
10,0 0.33 1.3 59.0 0,10 <0.1
Minor elements 14.0 0.02 3.9 70,0 0.53 <0.1
and inorganic compounds is difficult. Several parameters appear to be related to the extent of metal binding by natural organic substances. WILSON and KINNEY (1977) discuss the need to consider the bonding of both protons and metals at individual binding sites and with respect to the polymeric charge density of humic acids. The determination of the total acidity in milliequivalents of reactive proton sites per unit concentration of organic matter, allows one to consider metal binding to naturally occurring organic compounds as a series of proton replacement reactions. The total equivalents of protons exchangeable on the organic sites may be contrasted to those of other potential ligands in the system. Together with metal binding capacities and stability constants, total acidity measurements should allow one to quantify the importance of organic ligands (relative to inorganic ligands and anionic particulates) in metal interactions. Our work indicates that the range of milliequivalents of total acidity per gram of dissolved organic matter is relatively narrow (Table 3), with total acidity values varying by only a factor of 3 among rivers and among size fractions. With the exception of material isolated from the Satilla River, it appears that the smaller sized material (particles between 52 and 13 A diameter) has the higher total acidity. High total acidities are indicative of both a greater number of active sites per unit organic matter and less-condensed polymeric nuclei. Some support is given to this latter conclusion by the E4/E 6 ratios, which are larger for the smaller size material, indicating less condensation of the organic molecules (ScHNITZER and KHAN, 1972). Unfortunately, the trend of greater total acidity in smaller, less complex size ranges is not found in the case of the Satilla River, although the E4/E6 ratio continues to indicate less complexity in the smaller size fractions. Insufficient material for analysis prevented determination of the total acidity values for both size classes of the Lower Suwannee River and for the larger size class of the Caloosahatchee River. It appears that no strong relationship exists between the E4/E6 ratios and total acidity values across river systems, limiting the possibility of predicting milliequivalents of total acidity by determining the degree of complexity of the organic material. Table 3. Total acidities (meq/g. org.) and E4/E6 ratios of molecular size isolates* from river water > H10X100
Caioosahatchee River Peace River Upper Suwannee River Satilla River
> H10P5
Total acidity
EJE 6
-2.29 _ 0.04 4.68 _ 0.04 4.39 + 0.04
-2.16 4.72 2.44
Total acidity 7.16 6.22 5.44 2.45
+ + + +
0.04 0,04 0.04 0.04
E4/E6 4.56 3.79 6.19 5.00
* Size fractions refer to Amicon Corp. Diaflo Hollow Fibers with retention parameters of: HIOX100; pore diameter ~ 52 A, retains > 100,000 MW; HIOP5; pore diameter -~ 13 A, retains > 5,000 MW.
466
J.J. ALBERTS and D. W. EVANS Table 4. Amount of dissolved organic carbon and microequivalents of total acidity in 2 size fractions of river waters > H10X100 DOC* (%) /.teq/I Caloosahatchee River Peace River Lower Suwannee River Upper Suwannee River Satilla River
3.8 3.3 . 1.8 1.1
> H10P5 DOC (%) /aeq/I
-6 .
.
17.7 5.6
16 10
17.9 12.0
49 6
.
6 2
* D O C is reported as the percentage of the total D O C which is retained by the specified filter.
REUTER and PERDUE (1977) found that, on average, 60-80% of the DOC and POC (particulate organic carbon) in rivers consist of humic substances. While the molecular weight of humic substances is in doubt (reported values range over 4 orders of magnitude (SCHNITZER and KHAN, 1972)), they are usually considered to be large molecules. SCHINDLER and ALBERTS (1974) have shown that more than 80% of the DOC in 4 southeastern U.S. lakes is removed by ultrafiltration of particles greater than 24 A diameter. GIESY and BRIESE (1978) show that greater than 60% of the DOC in 2 South Carolina streams is less than 15 A in diameter. Our work shows that more than 80% of the DOC in these rivers is less than 13 A in diameter (Table 4). In addition, most carbon retained by the ultrafilters is in the smaller sized material. We can only speculate on the reasons for the inconsistencies in the relative amount of organic matter in larger size fractions. SCHINDLERand ALBERTS(1974) separated their organic fractions by ultrafiltration from aerobic and anaerobic lake and reservoir waters. It would be reasonable to expect that these systems might have higher concentrations of larger size material based on residence times in the systems and the more refractory nature of larger molecules. The work of GIESY and BRIESE (1978), using ultrafiltration techniques in flowing systems, agrees with the observations reported here in which similar techniques were used. The apparent disagreement between both the stream data and the current work with that discussed by REUTER and PERDUE (1977) may simply be based on operational definitions and different analytical techniques. Using the total acidity values determined here for the size fractions and the percentage of organic carbon in the isolates (Table 4), the total acidities of the fractions may be calculated on a per liter basis. Calculated in this manner, these values appear small. However, when compared to the milliequivalents per liter of other anionic components in the waters (Table 5), the organic matter in the isolates makes a significant contribution to the dissolved anionic charge concentration of some rivers. This is particularly so in the low pH systems, such as the Upper Suwannee River, where the organic isolates represent greater than 33% of the anionic charge, and in the Satilla River, where the organic fractions contribute approximately 12% of the sulfate. In the latter case, the organic isolates comprise only about 2% of the anionic charge component. However, chloride and bicarbonate ions, which comprise most of the negative charge in that system, form relatively weak complexes with metal cations (HAHNE and KROONTJE, 1973). Humic type organic compounds form strong complexes (Ka ~ l0 a) with many transition metals Table 5. Anionic components (/~eq/l) of southeastern United States rivers
C1HCO~ PO~7 s SO,~ 2 Organic fraction
Caloosahatchee River
Peace River
Upper Suwannee River
Satilla River
1100 2200 2.4 270
910 920 63 500
90 < 10 <1 8.1
140 150 2.0 67
16
16
55
8.0
Chemistryof submicron sized organicfractionsin four rivers of southeastern U.S.A.
467
Table 6. Amount* of "dissolved" metal retained by ultrafiltration
As Cd Cu Fe Mn Pb
Caloosahatchee River
Peace River
Upper Suwannee River
2.3 -9.3 61.0 38.0 52.0
3.9 32.0 16.0 57.0 ---
1.9 9.7 -16.0 6.7 --
* Values expressed as percentage of metal concentrationdetermined for 0.45/~mfilteredriver water. (GuY and CHAKRABARTI, 1976; GIESY and ALBERTS, in preparation) and would be expected to influence metal binding disproportionately to their actual concentrations. Further evidence of the importance of isolated fractions on the geochemistry of "dissolved" metals may be seen in Table 6. In some systems, considerable quantities of metals may be removed from the water column by ultrafiltration. These include metals such as Fe which would be expected to form colloidal particles, especially in elevated pH systems, as shown in the Caloosahatchee and the Peace Rivers, or may represent occluded material in organic aggregates (ALBERTS et al., 1976). Cadmium and lead may also be found in significant quantities in the particles separated by ultrafiltration. The exact nature of these particles remains unknown, but they may consist of large organic-metal complexes, organic-metal colloids, inorganic-metal colloids, or organic-clay-metal colloids. However, data we have gathered indicate that the possible clay contribution to the total acidity of these particles is less than 10% (ALBERTS,unpublished data). Whatever the composition, the fact that these metals are removed from natural water systems by ultrafiltration clearly indicates that they do not exist as simple dissolved ions or ionic complexes, but rather, that they are found in colloidal particle size ranges. A final point of interest is the difference in water chemistry of the Satilla River, when our data are compared with values reported for samples from that river system collected 9 years earlier (BECK et al., 1974). Comparison shows that the Satilla River, at our sampling site downstream of Waycross, Ga., has increased in pH from 4.45 to 6.49. Other changes include apparent 3-fold increases in SO4 and K, and 2-fold increases in dissolved Na, Mg, Ca and SiO 2. Iron and manganese concentrations have decreased by factors of 27 and 500, respectively, while organic carbon has decreased 2-fold to the world river water average of 10ppm (GARRELS and MACKENZIE, 1971). These changes probably result from the growth of anthropogenic influences on the river, most likely due to increases in population and industry in the region. It is interesting to note the similarity of the values reported by BECKet al. (1974) for the Satilla River, to those we found in the Upper Suwannee River, which has almost no anthropogenic inputs upstream of our sampling point. Indeed, the values are almost identical, including the ratios of major cations to chloride ion as used by BECK et al. (1974) to indicate the similarity in composition of the Satilla River water to rainfall in southeastern Georgia. These comparisons emphasize the need for care in attempting to correlate new data with older values from the literature, especially in areas where regional agro-industrial growth may have a major impact on relatively clean systems.
SUMMARY
We have observed that only a small percentage of total dissolved organic carbon may exist in size fractions greater than 13 A diameter (nominal molecular weight ~ 5000), yet the importance of this material in binding metals and contributing to the overall net negative charge of these natural waters can be considerable. Since ~ 80% of the total DOC was not measured, the contribution of these smaller molecules can only increase the total binding of metals by organic matter in these systems. Total acidity values of
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J.J. ALBERTS and D. W. EVANS
organic fractions in these rivers vary by a factor of only three, but the proportion of/~eq/l of negative charge contributed to the system is dependent on the river type and becomes increasingly important in the low pH, "blackwater" streams common in the southeastern United States. Acknowledoement--This research was supported by Contract DE-AC09-76SR00819 between the University of Georgia and the U.S. Department of Energy. REFERENCES
ALBERTS J. J., SCHINDLER J. E., NUTTER, JR. D. E. and DAVIS E. (1976) Elemental, infra-red spectrophotometric and electron spin resonance investigations of non-chemically isolated humic material. Geochim. Cosmochim. Acta, 40, 369-372. AMERICAN PUBLIC HEALTH ASSOCIATION 0976) Standard Methods for the Examination of Water and Waste Water. 14th Ed. 1193 pp. BAUMANN E. W. (1976) Nephelometric determination of microgram quantities of sulfate with 2-aminopermidine. Savannah River Laboratory, ERDA DP-1437. 17 pp. BECr K. C., REUTER J. H. and PERDUE E. M. (1974) Organic and inorganic geochemistry of some coastal plain rivers of the southeastern United States. Geochim. Cosmochim. Acta, 38, 341-364. GARRELS R. M. and MACKENZIE F. T. (1971) Evolution of Sedimentary Rocks, 450 pp. Norton. GIESV, JR. J. P. and BRIESE L. A. 0977) Metals associated with organic carbon extracted from Okefenokee Swamp water. Chem. Geol. 20, I09-120. GIESV, JR. J. P. and BRIESEL, A. (1978) Trace metal transport by particulates and organic carbon in two South Carolina streams. Verh. laternat. Verein. Limnol. 20, 1401-1417. Guy R. D. and CHAKRABARTIC. L. 0976) Studies of metal-organic interactions in model systems pertaining to natural waters. Can. J. Chem. 54, 2600-2611. HAHNE H. C. H. and KROONTJE W. (1973) Significance of pH and chloride concentration on behaviour of heavy metal pollutants: mercury (II), cadmium (II), zinc (II) and lead (II). J. Environ. Qual. 2, 444-450. REUTER J. H. and PERDUE E. M. (1977) Importance of heavy metal-organic matter interactions in natural waters. Geochim. Cosmochim. Acta, 41,325-334. SCHINDLER J. E. and ALBERTSJ. J. 0974) Analysis of organic-inorganic associations of four Georgia reservoirs. Arch, Hydrobiol. 74, 429-440. SCHNITZER M. and KHAN S. U. (1972) Humic Substances in the Environment. 327 pp. Marcel Dekker. WILSON D. E. and KINNEY P. 0977) Effects of polymeric charge variations on the proton-metal ion equilibrium of humic materials. Limnol. Oceanoor. 22, 281-289.