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Effect of dissolved oxygen on chemical transformations of heavy metals, phosphorus, and nitrogen in an estuarine sediment
Estuarine
and CoastaL
Science (1978)
Marine
6, 21-35
Effect of Dissolved Oxygen on Chemical Transformations of Heavy Metals, Phosphorus, and Nitrogen in an Estuarine Sediment”
R. A. Khalid, W. H. Patrick, Jr. and R. P. Gambrels Laboratory Louisiana Baton
of Flooded Agricultural
Rouge,
Received
4_%ne
Keywords: Louisiana
La
70803
Soils and Sediments, Experiment Station,
Department, State University,
U.S.A.
1976 and
heavy coast
Agronomy Louisiana
in revised
metals,
form
estuarine
XI
October
sedimentation,
I976
redox,
pH,
dredging,
The effect of dissolved 0, on the chemical transformations of Fe, Mn, Zn, Cu, Pb, Cd, NH: -N and P in an estuarine sediment suspension was investiReduced sediment suspensions were gated under laboratory conditions. constantly purged with a gas mixture containing O-I I %, 2.1 O/$,and 21 o/o 0, at a flow rate of 20 m.l min -I. Suspension samples were taken at various time intervals and extracted for different chemical fractions. The data indicate that increasing 0% concentrations in the gas mixture resulted in lower sediment pH and higher oxidation-reduction potential (redox potential) with time. Changes in redox potential and pH strongly modified the distribution of trace metals in various chemical fractions. The hydroxylamine hydrochloride extraction failed to selectively remove oxides and hydroxides of Mn without dissolving a substantial proportion of reactive Fe, as the recovery of Fe by this extractant was 8 to IO orders of magnitude greater than Mn. The large recovery of Fe with Mn in this fraction makes it difficult to distinguish trace metals associated with Mn oxides from those associated with Fe oxides. A substantial fraction of total sediment Zn was associated with potentially bioavailable chemical forms, which may be mobilized to more readily available forms due to changes in the physicochemical properties of the sediment-water system. Significant correlation coefficients obtained between soluble Pb, Cd and Cu and different Fe fractions suggest a strong relationship between these trace metals and Fe compounds. Phosphorus and NH: -N concentrations decreased sharply with time as a result of increased redox potential levels in the z-I~/~ and 21% Oa levels. The effect of the O-I 1% 0, treatment was negligible. This indicates that oxidized sediment conditions may be an important factor in regulating eutrophication by reducing the levels of P and N available for biota. This necessitates a careful study of changes in the dissolved 0, concentration (reflected by redox potential levels) in the sediment-water system. (1This study was supported by the Dredged Materials Research Program, U.S. Army Corps of Engineers Waterways Experiment Station, Vicksburg, Ms. on Contract No. DACW3g-74-C-oo76. Mention of trade names does not constitute official endorsement by Louisiana Agricultural Experiment Station. 21
22
R. A. Khalid,
W. H. Patrick,Jr.
&3 R. P. Gambrel1
Introduction The exchange of potentially toxic metals and plant nutrients between dredged sediments and surface water is a function of several physical, chemical and biological processes. Some of the important chemical and biological reactions include adsorption and desorption processes, precipitation with oxides and hydroxides of Fe and Mn, complexation with organic matter, sulfides and carbonate reactions, and mineralization and immobilization processes. Several of these reactions may occur simultaneously in the same system. Oxidation-reduction potential (redox potential) and pH of the sediment-water system are major factors known to influence these processes. However, a lack of understanding of the opposing interacting effects of certain of these processes stems from two opposite views of the effects of redox potential. One view is that the presence of sulfide under reduced conditions will precipitate toxic metals resulting in a very low solubility and that the oxidation of sulfide to sulfate under oxidizing conditions will release these metals into the overlying water (Gardiner, q74a; Holmes et al., 1974; Engler & Patrick, 1975; Chen et al., 1976). The opposite thought is that toxic metals will be more soluble under reduced conditions due to the reduction of Fe and Mn hydrous oxides which tend to sorb or coprecipitate toxic metals under oxidized conditions (Brooks et al., 1968; Jenne, 1968; Windom, 1973). The effect of sulfide and oxides and hydroxides of Fe and Mn in controlling the solubility of trace metals in the sediment-water system is greatly modified by the presence of organics (Morel et al., 1973 ; Lindberg & Harris, 1974). It is very difficult to single out the specific effects of certain sediment components under such a complex system. This study was designed to measure the integrated effects of interaction between various regulatory processes operating in an estuarine sediment-water system on the chemical transformation of trace metals and plant nutrients. The objectives were: (I) to investigate the influence of various levels of dissolved 0, on the transformation of Fe, Mn, Zn, Cu, Pb, Cd, NH:-N and P in Barataria Bay sediments, and (2) to differentiate metal ions associated with Mn oxides from those associated with Fe oxides in the sediment-water system. We attempted to simulate dredging and dredged material disposal conditions where reduced bottom sediments are dispersed in an oxygenated surface water or transported to a land-fill area where atmospheric oxygen may diffuse into the dredge material. This study also approximates changes in the natural estuarine environment where seasonal variation in the oxidation-reduction conditions, pH and other physicochemical properties normally occur (Brannon, 1973). Such changes may influence levels of toxic metals and biostimulants affecting aquatic life.
Materials and methods Sediment sampling and incubation
Sediment and surface water samples were collected from Barataria Bay, a large shallow estuary located in the Louisiana coastal marsh area. Barataria Bay is surrounded on the east by the Mississippi River and on the west by Bayou Lafourche. The sampling site was located 3 km west of the southern tip of Grand Isle on the Louisiana Gulf Coast. Care was taken to minimize exposure of the sediment material to atmospheric 0, during transportation and storage prior to laboratory studies. Some of the properties of the sediment material and surface water are presented in Table I. Wet sediment material, equivalent to 3oo g oven dry solids, was transferred to 3-1, 3necked, flat-bottomed Pyrex flasks. Sufficient stored surface water was added to attain a solids to water weight ratio of I : 8, which approximates dredged slurry. After IO days of
Effect of dissolved oxygen on heavy metals
TABLE I. Characteristics
of sediment material
%
Cation exchange capacity (medmo d
Electrical conductivity a (millimhos cm-r)
Chloride” (ug ml-r)
5’15
IQ’2
IQ.1
9100
PH
Redox potential bv)
7’3
-60 Feb
23
y0 Clay 28.0
Mnb
Organic matter
Znb
Pbb
Cdb
Cub
SC
H2S
4.1
14.6
242
traces
(M&4 28,800
3oo
78
37
Predominant minerals um): montmorillonite (IS”), illite (IS), kaolinite Coarse clay (2-0.2 Fine clay (<0’2 mrr): montmorillonite (40), illite (s), kaolinite (5)
(IO)
“Determined in the I :8 sediment solids to surface water extract. ‘Total metal content. “Total combined sulfides. “% of total crystalline minerals identified by X-ray diffraction.
preincubation at 30 “C under an Ar atmosphere, sediment suspensions were placed under constant stirring with Oa-free Ar gas bubbling through the flasks to create a highly reduced system. Suspension pH and redox potential were recorded accordingly using the system developed by Patrick et al. (1973). After 7 days, when the suspensions reached a minimum redox potential, zero-time (pretreatment) samples of sediment suspensions were pipetted out and partitioned into various chemical fractions described later in this section. After the zero-time (pretreatment) sampling, sediment suspensions were constantly purged with 0.1 I o/o0,, 2.1 o/o0, and 21 o/o0, at a flow rate of 20 ml min-l. Changes in pH and redox potential were recorded with time. Suspension samples were taken for fractionation as a function of redox potential and time. The sampled sediment suspensions were extracted for different chemical fractions outlined below. Chemical fractionation
Sampled sediment suspensions were partitioned into : (I) water-soluble, (2) exchangeable, (3) easily reducible, (4) reducible, and (5) total fractions. Water-soluble and total fractions were determined on separate samples. Fractions 2, 3 and 4 were sequentially extracted in the indicated order on duplicate samples. Water-soluble
A rso-ml aliquot was withdrawn with a glass pipette-plastic syringe assembly which had been purged with N, immediately prior to sample removal. This suspension was transferred through a serum cap into a sealed soo-ml polycarbonate bottle purged with N,. The suspensions were centrifuged at 6000 r.p.m. for 20 min and the supernatant was transferred to polyethylene storage bottles containing sufficient 6 N-hydrochloric acid to lower the pH to ~2. This transfer was carried out in a glove bag under an N, atmosphere. The acidified extract was stored at 4 “C! prior to elemental analysis. Exchangeable
Fifty ml of sediment suspension was pipetted into each of two centrifuge bottles in which the atmosphere had been replaced with O,-free N,. The centrifuge bottles contained a
24
R. A. Kkalid,
W. H. Patrick, Jr. & R. P. Gambrel1
sufficient amount of deoxygenated 5 N-sodium acetate adjusted to pH 6.5 to obtain a final concentration of sodium acetate of 0.5 N in a I :IO solids to solution ratio (w/v). The suspension-extractant mixture was shaken for 2 h and centrifuged. The supernatant was acidified to pH z 2 and stored at 4 “C until analysis. The quantity of elements in the exchangeable fraction was calculated as follows: exchangeable element = element in the sodium acetate extract-element in the water-soluble fraction. Easily reducible
A solution of O-I M-hydroxylamine hydrochloride in 0.01 N-nitric acid, pH 2.0 was added to the residual material from the exchangeable extraction in a solids to extractant ratio of I :50 and the contents deoxygenated. The mixture was shaken for 30 min and the extraction was carried out as described for the exchangeable fraction, This extractant is reported to selectively remove Mn associated with hydrous Mn oxides from agricultural soils (Chao, 1972). The metals removed from sediments by this procedure have been termed easily reducible as the extractant is comparatively a weak reductant (Engler et al., 1974). Reducible
Two hundred and fifty ml of oxalate reagent (O-I M-oxalic acid+o*r75 M-ammonium oxalate, pH 3.25) was added to the residual material from the easily-reducible extraction and the contents deoxygenated. The suspension was shaken for 2 h and the extraction was carried out as described for the exchangeable fraction. Total elements
The sediment suspensions was digested with a hydrofluoric-nitric-perchloric acid mixture and dissolved in hydrochloric acid for total Fe, Mn, Zn, Pb, Cd and Cu analyses. This procedure is described in more detail elsewhere (Gambrel1 et al., 1977). Analytical
procedures
Iron, Mn and Zn in the various chemical fractions were determined by flame atomic absorption (Perkin-Elmer Model 107). Water-soluble Pb, Cd and Cu were measured by flameless atomic absorption (Perkin-Elmer Model 360 Atomic Absorption Spectrophotometer, Perkin-Elmer Model 2100 HGA Graphite Furnace) as described by Gambrel1 et al. (1977). Ammonium-N in the sodium acetate extracts was determined by the steam distillation method (Bremner, 1965). Distilled ammonia was collected in 0.1 N-sulfuric acid and determined calorimetrically after nesslerization (Jackson, 1958). Phosphorus was determined calorimetrically by the ascorbic acid method of Watanabe & Olsen (1965). Results and discussion The effects of various concentrations of 0, on the measured physicochemical properties and elemental transformations in the reduced Barataria Bay sediment with time are discussed in the order of redox potential, pH, Fe, Mn, Zn, Pb, Cd, Cu, P and N. It should be pointed out that Pb, Cd, Cu and dissolved PO,-P were measured in the water-soluble fraction only. The objective was to approximate levels of these toxic metals and PO,-P in the interstitial water phase which are available for direct biological uptake.
Effect of dissolved oxygen on heavy metals
-.-
_ 0
6
10 Tlma
IS (days1
io
40
Figure I. The effect of dissolved 02 and incubation and pH of Barataria Bay sediment suspensions.
50
time on the redox potential
Redox potential
Figure I indicates that aeration with gas mixtures containing 21”/$, and 2.176 0, completely oxidized the reduced sediment suspensions in 2.5 and 9 days, respectively. In these suspensions, the redox potential increased to around 600 mV. No further increase in redox potential was observed upon subsequent purging at these 0, levels. The redox potential of the suspension treated with O-I I o/o0, increased very slowly with time. After 5 days, the redox potential increased by only 40 mV. After 56 days, the redox potential increased by 145mV to -65 mV a moderately reduced condition. PH
The influence of varying 0, levels on suspension pH is also indicated in Figure I. In the 21% 0, treatment, the pH of the sediment suspension decreased from an initial value of 7.7 to 5.7 after 5.5 days of constant purging. Where sediment suspensions were purged with 2.1% 0,, the pH dropped from 7.7 to 6.5 after 12 days. The o*II~/~ 0, treatment reduced suspensions pH from an initial pH of 7.7 to 7.5 after 56 days of constant purging. In the O-I 1% 0, treatment, small increases in redox potential were accompanied by a small reduction in suspension pH, suggesting a relationship between these parameters. In the two higher 0, treatments, the trend of change in pH and redox potential was the same though a greater differential in pH occurred than in redox potential. Iron
The concentration of water-soluble Fe in various 0, treatments was below detection limits of atomic absorption analysis (0.4 ug Fe ml-i in solution). Purging with o*II~/( 0, did not appreciably reduce exchangeable Fe concentration with time. When sediment suspensions were purged with 2.1 y0 0, for 288 h, exchangeable Fe decreased to non-detectable levels (Table 2). This decrease may be due to oxidation during this period resulting in ferric
Exchangeable Easily reducible“ Reducibled
Fraction
5'39
'573 2320
8.05
1491 1729
1518
10’00 '5'3 235"
944 3376
8.28
1396 2188
1518 1834
(0.5)
-160
1024 2330
(0)
-210
1406 2187
(W
+640
1518 1834
+I00 (48)
(mV)
fig Fe g-’ oven dry solids 6.91 n.d.’ IO.19
-50 (48)
2
2224
5.06 1045 2459
3'78
+*so (7)
1440
-40 (3.5)
r o/o Oxygen
Bay sediment suspensions
5.86
(8)
-160
Redox potential
of Fe in Barataria
8.28
(0)
-210
form and distribution
2’ 1 o/o Oxygen
oxygen on the chemical
“Zero-time (pretreatment) sampling. *Sampling time in hours after zero-time. CNot detectable. “Mean of duplicate samples.
1834
(OY
8.28
-170
(=o)
-210
Oxygen
w
-210
0.11%
TABLE 2. The effect of dissolved
4420
15'57 781
+610 (132)
Effect of dissolved oxygen on heavy metals
27
compounds not extracted with 0.5 N-sodium acetate. In the 21 o/00, treatment, however, the exchangeable Fe at the end of 132 h purging was greater than initial levels. Apparently a decrease in pH from the initial value of 7.7 to 5.7 at the end of the incubation period may have resulted in the transformation of potentially reactive ferric hydroxides to a more available exchangeable fraction due to acidic pH. The residual solid material from the sodium acetate extraction was shaken with 0.1 Mhydroxylamine hydrochloride solution prepared in o-01 M-nitric acid to selectively dissolve Mn oxides with minimal attack on Fe oxides (Chao, 1972). However, Fe in this easily reducible fraction ranged from 780 to 1570 pg g-l (Table 2). The effect of the 0.1 I y. 0, treatment during 120 h of constant purging on easily reducible Fe was negligible. In the 2*10/ and 21% 0, treatments, easily reducible Fe decreased with increasing redox potential. The oxalate-extractable (reducible) Fe increased with incubation time, and this effect was very pronounced at the 2.1% and 21% 0, levels (Table 2). At the end of the experiment, the level of reducible Fe increased with an increase in the 0, level in the order of 21% 02>2*1 y. O,>o-I I y. 0,. These increases in reducible Fe at greater 0, levels may be due to lower suspension pH values recorded. The pH values at the end of the incubation period were 5.7, 6.5 and 7.5 for 21 y. 0,, 2.1 y. 0, and 0.1 I “/ 02, respectively. Apparently, a a lower sediment pH facilitated the dissolution of Fe compounds present in the sediment suspension, resulting in higher Fe levels extracted with oxalate solution. Our data show the easily reducible Fe fraction is indicative of the Fe compounds which are transformed into more chemically reactive forms under reduced conditions and thus are more susceptible to exchange reactions. Oxidation of the initially reduced sediment suspensions, as a result of dissolved 0, additions (indicated by redox potential levels of >600 mV in the 2.1% and 21 oh 0, treatments, Table 2) may result in the formation of ferric oxyhydroxides and ferric oxides not susceptible to hydroxylamine hydrochloride extraction. These precipitated forms of ferric Fe present in the well oxygenated system were extracted with oxalate solution as more Fe was recovered in the well oxidized treatments (>600 mV redox potential) in the reducible fraction. These freshly precipitated oxides and hydroxides of Fe represent potentially reactive forms of sediment Fe and may control the release of Fe and associated metal ions in sediment-water systems in the range of redox potential and pH conditions encountered in the environment (Jenne, 1968; Lee, 1973). The discussion on the effects of aging and the crystallinity of hydrous oxides of Fe on trace metal adsorption is beyond the scope of this paper and the reader is referred to Jenne (1968) and Lee (1973) for a detailed account. Manganese
The effect of dissolved 0, on water-soluble and exchangeable Mn was variable (Table 3). For example, in the 0.11 “/ 0, treatment, little trend in Mn levels was observed with time. Since there was only a slight increase in redox potential after 5 days, the system was still very reduced, and Mn was present mostly in the divalent cationic form (Mn2+). Purging with 2.1 yb and 21 o/o0, levels resulted in a gradual decrease in water-soluble Mn as a consequence of increased redox potential. The decrease in soluble Mn upon oxidation may be due to the formation of less soluble Mn oxides as the redox potential increases. Similar effects of redox potential on Mn concentrations have been reported by Gotoh & Patrick (1972) and Gambrel1 et al. (1977). Th ere was no consistent effect of incubation time and 0, levels in the easily-reducible (hydroxylamine hydrochloride ext.) and reducible (oxalate ext.) Mn fractions.
Water soluble Exchangeable Easily reducible” Reducibled
Fraction
Water soluble Exchangeable Easily reducible’ Reducible’
Fraction
0.11
Oxygen
(0)
8.99 2.69 103 38
14.91 2-47 73 59
-210
(120)
-170
The effect of dissolved
n.d. n.d. 4’51 17’50
(0)
-210
n.d. n.d. 2.67 15.64
(168)
2.1%
solids .63 4’27 9r 50 I
(2W
+ 640
Oxygen
+ 640
(288)
ug Zn g-r oven dry solids n.d. n.d. n.d. b.d. n.d. 1.61 3’14 9.60 16.17 15’48 8.46 5.84
(16%
(mV) + 100
form and distribution
Redox potential -so (8) (48)
-160
(48)
+100
ug Mn g-r oven-dry 8.18 I .63 3.61 4.81 77 87 51 47
oxygen on the chemical
“Zero-time (pretreatment) sampling. * Sampling time in hours after zero-time. c Not detectable. d Mean of duplicate samples.
n.d. 0.28 5.46 22.56
(120)
W
0.30 n.d. 4.28 20.24
-170
-210
0’ I I o/OOxygen
4.
n.d.c n.d. 4’5 1 15.64
(0)’
-210
TABLE
8.79 3’29 73 54
(8)
-50
Redox potential -160
(mV)
form and distribution
2. I o/o Oxygen
oxygen on the chemical
“Zero-time (pretreatment) sampling. %ampling time in hours after zero-time. ‘Mean of duplicate samples.
8.73 2.80 82 5’
w
-210
o/o
3. The effect of dissolved
8.99 3.29 ro3 38
(OY
-210
TABLE
-
n.d. n.d. 4’5’ 15-64
(0)
-210
of Zn in Barataria
8.99 3’29 ro3 38
(0)
-210
43
102
6.01 4’40
-40 (3.5)
2 1 o/OOxygen
Bay sediment
6.44
4.ro 9r 56
+*50 (7)
0.30 n.d. 5’51 15.22
(0.5)
-160
2
n.d. 0’47 5.16 13’37
-40 (3.5)
1 o/o Oxygen
2.92 0.50 4’54 13.83
+*50 (7)
15.61 10.66
4.38 1.96
(132)
i-610
63:3: 100 67
+6ro (132)
suspensions.
Bay sediment suspensions
7’09 4’4’ 103 43
-160 (0.5)
of Mn in Barataria
Effect of dissolved oxygen on heavy metals
29
The hydroxylamine hydrochloride extraction (easily reducible) is reported to selectively extract oxides and hydroxides of Mn (Chao, 1972). These data show that ~~-75% of total reducible Mn was present in this easily reducible form. Though this fraction dissolved a greater proportion of the potentially reactive Mn, the dissolution of Fe from Fe oxides was 8 to IO orders of magnitude greater. Manganese concentration ranged from 73 to 103 pg Mn g-1 in the easily reducible fraction and from 38 to 67 pg Mn g-l in the reducible fraction (Table 3). Along with Mn, 780 to 1570 pg Fe g-l, which comprised 15-67q6 of the total reducible Fe was also extracted in the easily reducible fraction (Table 2). Oxalate extractable Fe ranged from 1730 to 4420 pg Fe g- l. The large recovery of Fe with Mn in both reducible fractions makes it difficult to distinguish between toxic metals associated with Fe oxides and Mn oxides. Zinc Water-soluble Zn ranged from nondetectable levels to over 4 pg g-l, depending on 0, treatments and time of incubation (Table 4). The highest concentrations were observed at redox potential levels of ISO and 610 mV in the 21% 0, treatment. This may be due to a decrease in suspension pH which solubilized more Zn. Exchangeable Zn exceeded I pg g-l only in the 2.11~/~and 21% 0, treatments which became well oxidized. Again, the corresponding decrease in pH may have contributed to this observation. Zinc associated with the easily reducible fraction increased slightly at the end of the incubation period in the 0.1 I o/o0, treatment. In the 2.1 o/o0, treatment, easily reducible Zn decreased initially up to - 160 mV, but a further increase in redox potential to -50, IOO and 640 mV was accompanied by an increase in easily-reducible Zn. In the 21 o/o0, treatment, easily-reducible Zn was considerably higher at 610 mV than at lower redox potential levels. Reducible Zn decreased with increasing redox potential in the 2-17~ 0, treatment. In the 0.1 I y. 0, treatment, reducible Zn increased slightly with incubation time. A slight reduction in reducible Zn was noted with time in the 21 o/00, treatment. Our data show that 23-38% of the total sediment-bound Zn was associated with chemical forms susceptible to moderate chemical extractions (hydroxylamine and oxalate extraction). This zinc is potentially bioavailable and may be mobilized to more readily available forms as a consequence of changes in physicochemical properties of the sediment-water system. This fraction of total Zn represents the proportion adsorbed by the colloidal oxides of Fe and Mn. Jenne (1968) has proposed that precipitation by hydrous metal oxides of Fe and Mn is the principle control mechanism for Zn in soils and sediments. Gambrel1 et al. (1977) reported that in other estuarine and river sediments, most of the Zn recovered in similar chemical fractionation procedures was associated with the reducible fraction. Although the contribution of insoluble organic material to Zn adsorption was not investigated in this study, the literature indicates that a relatively small proportion of Zn is associated with the insoluble organic-bound fraction (Serne & Mercer, 1975; Gambrel1 et al., 1977). Lead, cadmium and copper The data plotted in Figures z(a), 2(b) and 2(c) for Pb, Cd and Cu, respectively, indicate that concentrations of these toxic metals in the water-soluble fraction at the end of the incubation period were 25-30% higher than that at the pretreatment sampling. This increase was evident in all 0, treatments. The concentration of soluble Pb ranged from 41.2 to 196.6 pg l-l, Cd from ~9 to 6.4 pg l-1, and Cu from 4.9 to 12.2 pg 1-l. Recall from the
30
R. A. Khalid, W. H. Patrick,Jr. &’ R. P. Gambrel1
(bl Cadmium = a 3 3
!ko-
.*
2,5-
Ob
0
I 25 lncubatkm
.I 50
t
.’
P
I
150 time (h)
Figure 2. The effect of dissolved Bay sediment suspensions.
.’
250
Op on water-soluble
Pb, Cd and Cu in Barataria
previous discussion that in the O*II% 0, treatment, the sediment suspension was still very reduced (- 170 mV redox potential) at the end of 120 h. This level of reduction is well within the stability boundary for sulfide. In the 2.1 y0 and 21 y0 0, treatments, sediment suspensions were strongly oxidized (600 mV redox potential) after 288 and 132 h of aeration, respectively. Such high redox potential levels support the oxidation of insoluble sulfide to soluble sulfate, and at the same time, the transformation of soluble manganous Mn (Mna+) and ferrous Fe (Fez+) to insoluble manganic (Mn4+) and ferric (Fea+) oxides and hydroxides. Under the conditions of our experiment, an increase in soluble Pb, Cd and Cu with incubation time under reduced conditions (o*II~/~ 0, treatment) may be due to constant agitation of sediment suspensions solubilizing more organic material with time. Complexation of trace metals with soluble organics may have resulted in more metal ions in solution in spite of sulfides present in the system. Under oxidized conditions, an increase in soluble Pb, Cd and Cu at the end of the incubation period (288 and 132 h for 2.1% and 21% 0, treatments, respectively) was most likely due to the effect of lower suspension pH (Figure I) on the speciation of these trace metals. In the pH range encountered under oxidized conditions, these toxic metals are reported to be present as hydrated divalent cations, as soluble complexes of chlorides and sulfates, and as hydroxides with limited solubility (Hem, 1972; Hahne & Kroontje, 1973). Chloride complexation is known to increase the solubility of trace metals if the concentration of chloride ions is high enough to compete with hydroxide formation (Hahne & Kroontje,
E#ect of dissolved ovgen m heavy metals
31
1973). Barataria Bay is a brackish estuary containing considerable quantities of chloride (Table I) which may compete with hydroxides to keep metal ions in solution. Hem (1972) suggested the presence of a mixed complex, Cd(OH)CI, where chloride concentration exceeded 0.1 M, with a solubility greater than Cd(OH),. The probable formation of Cd(OH)Cl in the Barataria Bay sediment suspensions containing 0.3 M-sodium chloridesodium sulfate solution has also been proposed (C. A. Price, personal communication, Geology Department, Louisiana State University). The formation of insoluble metal carbonate complexes was not thought to be a significant factor regulating metal solubility in this study because: (I) constant purging of a gas mixture through the suspensions likely prevented an accumulation of CO, and thus limited carbonate availability for Pb, Cd and Cu complexes; (2) the chelation of trace metals with organics may keep them from precipitating with carbonates and hydroxides (Lisk, 1973 ; Morel et al., 1973); and (3) the relatively high solubility of carbonate below pH 7.0 (Hem, 1972) may also result in greater concentration of metal ions in solution in the 2.1 y0 and 21% 0, treatments. The role of Fe and Mn compounds in regulating Pb, Cd, and Cu solubility was also investigated by computing linear correlation coefficients for Fe and Mn extracted in different chemical fractions with water-soluble Pb, Cd and Cu (Table 5). Lead, Cd, and Cu were negatively correlated with easily-reducible Fe, whereas, a significant positive correlation between reducible Fe and the trace metals was determined. Manganese extracted in various chemical fractions was not significantly correlated with the trace metals except for the reducible Mn fraction, which was significantly correlated with Cd. It is apparent from our results that hydrous oxides of Fe and possibly Mn (in the case of Cd) may act not only as scavengers for trace metals as advocated in the literature (Jenne, 1968; Murray et al., 1969; Stumm & Morgan, x970), but also as a source of metal ion release, the mechanism of which is not very clear. It is likely that the trace metals adsorbed by the amorphous hydrated oxides of Fe and Mn or coprecipitated with ferric oxyhydroxides may be released under slightly acidic, oxidized conditions due to hydrogen ion displacement. The released metal ions may form soluble complexes with organic material or chlorides present in the system. Also evident is that the simple metal solubility and Eh-pH relationships based on pure inorganic thermodynamics of sulfides, carbonates, oxides, and hydroxides (Krauskopf, 1967; Stumm & Morgan, 1970; Hem, 1972) do not adequately explain metal-ion behavior in the natural environment. Since trace metals such as Pb, Cd, Cu, and others are toxic to terrestrial and aquatic life at very low concentrations (Khalid et al., 1977) the relative
TABLE 5. Linear relationship (I) between various fractions of Fe and Mn
Fraction Exchange Fe Easily Reducible Fe Reducible Fe Water-Soluble Mn Exchangeable Mn Easily Reducible Mn Reducible Mn
Pb 0.127~ --0*595*
o.773+*
water-soluble
Water-soluble Cd CU 0.138
-0*566* 0.670’
-~291
-0'122
o-358 -o205 -237
-0.041
#‘I’ values without asterisk, non-significant. *Significant at 5% level, n = 13. **Significant at 1 o/Olevel, n = 13.
Pb, Cd, Cu and P and
0.417 0.566.
o-405 -0.580* 0*761** -0.275
0’499 0,267 0’4.20
P -0.188
0.848** -06o5f 0.766.. -o569* -0.039 0.349
32
R. A. Khalid,
R
i
W. H. Patrick,Jr.
& R. P. Gambrel1
1
fe) Phosphorus \
\
\
\
\
\
\
\
-o~llxo* - - 02.11% 02 \
\
--A21~0%0* \\
‘-__ --.
Incubation
time Ch)
Figure 3. Effect of dissolved 0, on water-soluble PO&-P and sodium acetate extractable NH: -N in Barataria Bay sediment suspensions. contribution of possible sources and chemical and biochemical transformations affecting the availability of these metals should be more fully investigated in an integrated physical, chemical, and biological system approximating the natural environment. Phosphorus
The concentration of water-soluble P in the 0.1 I y. Oa treatment did not change appreciably with time [Figure 3(a)], due to similar reduced conditions present during the incubation period. In the 2.1 y0 0, and 21 y. 0, treatments, soluble P decreased sharply with time as a result of increasing redox potential. Figure 3(a) indicates that the rate of soluble P disappearance was much faster in the 21 y. 0, treatment as compared to 2.1 oh 0, level, although both treatments had similar P concentrations at the end of the incubation periods, at which time both suspensions were well oxidized. These results indicate that the level of dissolved 0, in the sediment-water system may have a profound effect on the exchange of P between the sediment material and the overlying water. Mortimer (1971) reported that the disappearance of dissolved 0, and subsequent reduction of sediment material resulted in a several-fold increase of dissolved P. Oxygenation of the sediment reversed the process and decreased the P concentration in solution. Since P is a key element in determining the eutrophication rate of natural waters, it is very important to study the behavior of this element in the aqueous environment. The role of Fe components, such as amorphous oxides and hydroxides of Fe and ferrous compounds, in controlling the levels of dissolved inorganic P in soils and sediments is well
Effect of dissolved oxygen on heavy metals
33
documented (Syers et al., 1973 ; Patrick & Khalid, 1974). In our study, water-soluble P was positively correlated with easily reducible Fe (Y = o-848**) (Table 5). This indicates that changes in redox potential had a similar effect on water-soluble P as well as Fe compounds extracted with hydroxylamine hydrochloride. As discussed earlier in the Fe section, easily reducible Fe probably represents Fe compounds more soluble under reduced conditions and may be in equilibrium with water-soluble fractions thus affecting P levels. Oxidation of ferrous phosphate to less soluble ferric phosphate is an important factor in regulating P solubility. A significant negative correlation (--0X105*) (Table 5) between water-soluble P and oxalate extractable Fe (reducible Fe) indicates that freshly precipitated ferric compounds control the solubility of P. Our results are in agreement with literature reports (Syers et al., 1973; Patrick & Khalid, 1974; Khalid et al., 1977) that oxides and hydroxides of Fe strongly influence P transformations, and that these reactions are greatly modified by the oxidation-reduction conditions of the sediment-water system. No significant relationship was observed for the easily reducible and reducible Mn fractions, indicating that Mn compounds had no effect on P concentration, Nitrogen
The levels of sodium acetate extractable NH:-N were influenced by the concentration of 0, and the equilibration time [Figure 3(b)]. This effect was largely a function of redox potential. In both the 2.1 y0 and 21 y0 0, treatments, the concentration of NH:-N decreased as redox potential increased. In the 2.1% 0, treatment, disappearance of NH:-N (presumably nitrification) commenced sometime between 48 and 168 h after the treatments began. It is possible that where the purging gas contained only 2.1 y0 0,, most of the initially added 0, was consumed in the oxidation of organic material, Fe and Mn, and little 0, was available for several oxidation processes to proceed simultaneously. No change in the level of NH:-N was observed in the O-I I o/o0, treatment. In this treatment, the sediment suspensions remained very reduced during the 56-day incubation (Figure I). Much of the NH:-N lost as a result of oxidation is very likely transformed to the more mobile NOT-N form and is equally bioavailable in the sediment-water system. However, under natural conditions, this NOT-N may be removed by denitrification as nitrate diffuses from surface waters and aerobic surface sediment horizons into underlying reduced sediment material. Reddy & Patrick (1975) reported losses of applied 15NH4f-N as high as 63 o/o during alternate aerobic and anerobic cycles. This phenomenon may be of great significance in dredging and disposal operations where reduced bottom sediments are mixed with aerobic surface waters and subsequently become reduced upon redeposition. Conclusions The results of this study indicate that the intensity of oxidation increased with increasing dissolved 0, concentrations used to purge the sediment suspensions. An increase in redox potential values due to dissolved 0, was accompanied by a decrease in suspension pH, and the maximum effect was observed in the 21% 0, treatment. This decrease in suspension pH from an initial value of 7.7 to a final value of 5.7 at the end of 132 h in the 21 y. 0, treatment resulted in increased levels of soluble Zn, Pb, Cd and Cu, and also in exchangeable Fe, Mn and Zn under very oxidized conditions ($610 mV redox potential). This study suggests that the expected adsorption or coprecipitation of soluble trace metal ions by oxides and hydroxides of Fe and Mn as an estuarine sediment is oxidized may be offset by concurrent redox potential mediated decreasesin pH. The results of this study strongly suggest
34
R. A. Kkalid,
W. H. Patyick,Jr.
W R. P. Gambrel1
that chemical transformations of metal ions should be further investigated under controlled pH and redox potential conditions. An attempt was made in this study to selectively extract Mn oxides with minimal attack on Fe oxides as suggested by Chao (1972). The data show that hydroxylamine hydrochloride extraction failed to selectively remove oxides and hydroxides of Mn without dissolving a substantial proportion of reactive Fe, as the recovery of Fe by this extractant was 8-10 orders of magnitude greater than Mn. This large recovery of Fe with Mn in the easily reducible fraction makes it difficult to distinguish trace metals associated with Mn oxides from those associated with Fe oxides. A substantial fraction of total sediment Zn was associated with potentially bioavailable chemical forms which may be mobilized to more readily available forms due to changes in the physicochemical properties of the sediment-water system. An increase in soluble concentrations of Pb, Cd and Cu with time in the 0.11 y. 0, treatment indicates that the precipitation of toxic metal ions as insoluble sulfides under reduced conditions may not fully explain observed concentrations of these metal ions. It is probable that complexation of trace metals with soluble organics may have resulted in more metal ions in solution in spite of the presence of sulfides in the system. Under oxidized conditions, an increase in soluble Pb, Cd and Cu with time may be due to the release of metal ions adsorbed by hydrated oxides as the system becomes more acidic. The released metal ions may form soluble complexes with organics or chlorides present in the system. The relatively high concentration of soluble Pb determined in Barataria Bay sediment suspensions in this experiment suggests a possible Pb toxicity problem and should be more fully investigated. It is evident from our results that simple metal ion solubility calculations and Eh-pH relationships based on pure inorganic thermodynamic considerations of the chemistry of sulfides, carbonates, oxides and hydroxides do not adequately explain metal-ion behavior in the natural environment. The presence of organics and high concentration of chlorides in the estuarine sediments and dredged materials may modify the existing relationships and enhance metal ion release. Since trace metals such as Pb, Cd, Cu and others may accumulate to toxic levels in estuarine food webs, the relative availability of these metals should be more fully investigated in an integrated physical, chemical, and biological system approximating the natural environment. Dissolved inorganic P and NH,+-N concentrations decreased sharply with time as a result of increased redox potential levels in the 2.1% and 21% 0, levels. The effect of the o.IIO/~ 0, treatment was negligible. This indicates that oxidized sediment conditions may be an important factor in regulating eutrophication by reducing the levels of P and N available for biota. This necessitates a careful study of changes in the oxidation-reduction intensity of sediment-water systems affecting availability of biostimulants. References Brannon, J. 1974 Seasonalvariation of nutrients and physicochemical properties in the salt marsh soils of Barataria Bay, Louisiana. M.S. Thesis, Agronomy Department, Louisiana State University, Baton Rouge, Louisiana. 130 pp. Bremner, J. M. 1965 Inorganic forms of nitrogen. In Methods of Soil Analysis, Part z (Black, C. A., ed). American SOCicty of Agronomy, Inc., Madison, Wisconsin. pp. 1179-1237. Brooks, B. B., Presley, J. J. & Kaplan, I. R. 1968 Trace elements in the interstitial waters of marine sediments. Geochimica et Co-himica Acta p, 397-414. Chao, L. L. 1972 Selected dissolution of manganese oxides from soils and sediments with acid&d hydroxalamine hydrochloride. Soil Science Society of America Roceedings 36,764768. Chen, K. Y., Gupta, S. K., Sycip, A. Z., Lu, J. C. S. & K nezevic, M. 1976 The efIects of dispersion, settling, and resedimentation on migration of chemical constituents during open water disposal of
Efect of dissolved oxygen on heavy metals
3.5
dredged materials. Contract Report, U.S. Army Engineer Waterways Experiment Station, Vicksburg, 221 pp. Mississippi Contract No. DACW3974-C-077. Engler, R. P.&Patrick, W. H., Jr. 1975 Stability of sulfides of manganese, iron, zinc, copper, and mercury in flooded and nonflooded soil. Soil Science 119, 217-221. Engler, R. P., Brannon, J. M., Rose, J. & Bigham, G. 1974 A practical selective extraction procedure for sediment characterization. Proceedings r68th American Chemical Society National Meeting, Atlantic City, New Jersey, September 1974. (In press). Gambrel& R. P., Khalid, R. A., Verloo, M. G. & Patrick, W. H. Jr. 1977 Transformations of heavy metals and plant nutrients in dredged sediments as affected by oxidation-reduction potential and pH. Part II, materials and methods, results and discussion. U.S. Army Engineer Waterways Experiment Station, Vicksburg, Mississippi Contract Report D-77-4, 336 pp. Gardiner, J. 1974.0 The chemistry of cadmium in natural water. I. A study of cadmium complex formation using the cadmium specific-ion electrode. Water Research 8, 23-30. Gardiner, J. 1974b The chemistry of cadmium in natural water. II. The adsorption of cadmium on river muds and naturally occurring solids. Water Research 8, 157-164. Gotoh, S. & Patrick, W. H., Jr. 1972 Transformation of iron in a waterlogged soil as influenced by redox potential and pH. Soil Science Society of America Proceedings 38, 6671. Hahne, H. C. H. & Kroontje, W. 1972 Significance of pH and chloride concentration on behavior of heavy metal pollutants: Mercury (II), cadmium (II), zinc (II), and lead (II). Journal of Enwironmental Quality 2,444-450. Hem, J. D. 1972 Chemistry and occurrence of cadmium and zinc in surface water and ground water. Water Resources Research 8, 661-679. Jackson, M. L. (ed.) 1958 Soil Chemical Analysis. Prentice Hall, Inc., Englewood Cliffs, New Jersey. pp. X83-204. Jenne, E. A. 1968 Controls of Mn, Fe, Co, Ni, Cu, and Zn concentrations in soils and water: The significant role of hydrous Mn and Fe oxides. In Trace Inorganics in Water, Advances in Chemistry Series 73 (Gould, R. F., ed.) pp. 337-387. Khalid, R. A., Gambrell, R. P., Verloo, M. G. & Patrick, W. H., Jr. 1977 Transformations of heavy metals and plant nutrients as affected by oxidation-reduction potential and pH. Part I. Literature review. U.S. Army Engineer Waterways Experiment Station, Vicksburg, Mississippi Contract Report D-77-4. 238 pp. Krauskopf, K. B. (ed.) Introduction to Geochemistry. McGraw-Hill, New York, New York. pp. 236-280. Lindberg, S. E. & Harriss, R. C. 1974 Mercury-organic matter associations in estuarine sediments and interstitial water. Environmental Science and Technology 8, 459-462. Lisk, D. J. 1972 Trace metals in soils, plants, and animals. Adwances in Agronomy 24, 267-325. Morel, F., McDuff, R. E. & Morgan, J. J. 1973 Interactions and chemostatis in aquatic chemical systems : Role of Eh, pE, solubility, and complexation. In Trace Metals and Metal Organic Interactions in Natural Water. (Singer, P. C., ed.). Ann Arbor Science Publ., Ann Arbor, Michigan. pp. 157-200. Mortimer, C. H. 1971 Chemical exchanges between sediments and water in the Great Lakes-speculations on probable regulatory mechanisms. Limnology and Oceanography 16, 387-404. Murray, D. J., Healy, T. W. & Fuerstenau, D. W. 1968 The adsorption of aqueous metal on colloidal hydrous manganese oxide. In Adsorption from Aqueous Solution, Advances in Chemistry Series 79 (Gould, R. F., ed.) pp. 74-81. Patrick, W. H., Jr. & Khalid, R. A. 1974 Phosphate release and sorption by soils and sediments: effect of aerobic and anaerobic conditions. Science 186, 53-55. Patrick, W. H., Jr., Williams, B. G. & Moraghan, J. T. 1973 A simple system for controlling redox potential and pH in soild suspensions. Soil Science Society of America Proceedings 37, 331-332. Reddy, K. R. & Patrick, W. H., Jr. 1975 Effect of aerobic and anaerobic conditions on redox potential, organic matter decomposition, and nitrogen loss in a flooded soil Soil Biology and Biochemistry 7, 87-94. Seme, R. J. & Mercer, B. W. 1975 Characterization of San Francisco Bay dredge sediments-crystalline matrix study. Contract Report, U.S. Army Engineer District San Francisco, California. Contract No. DACW 07-73-c-080. 102 pp. Syers, J. K., Haris, R. F. & Armstrong, D. E. 1973 Phosphate chemistry in lake sediments. ~ournul of Environmental Quality 2,1-14. Stumm, W. & Morgan, J. J. (eds) 1970 Aquatic Chemistry Wiley Interscience, New York, New York, pp. 300-382. Watanabe, F. W. & Olson, S. R. 1965 Test of ascorbic acid for determining phosphorus in water and NaHCO I extracts from soil. Soil Science Society of America Proceedings 29,677-678. Windom, H. L. 1973 Investigations of changes in heavy metals concentrations resulting from maintenance dredging of Mobile Bay Ship Channel, Mobile Bay, Alabama. Report submitted to U.S. Army Engineer District Mobile, Alabama. Contract No. DACor-73-C-136. 46 pp.
and CoastaL
Science (1978)
Marine
6, 21-35
Effect of Dissolved Oxygen on Chemical Transformations of Heavy Metals, Phosphorus, and Nitrogen in an Estuarine Sediment”
R. A. Khalid, W. H. Patrick, Jr. and R. P. Gambrels Laboratory Louisiana Baton
of Flooded Agricultural
Rouge,
Received
4_%ne
Keywords: Louisiana
La
70803
Soils and Sediments, Experiment Station,
Department, State University,
U.S.A.
1976 and
heavy coast
Agronomy Louisiana
in revised
metals,
form
estuarine
XI
October
sedimentation,
I976
redox,
pH,
dredging,
The effect of dissolved 0, on the chemical transformations of Fe, Mn, Zn, Cu, Pb, Cd, NH: -N and P in an estuarine sediment suspension was investiReduced sediment suspensions were gated under laboratory conditions. constantly purged with a gas mixture containing O-I I %, 2.1 O/$,and 21 o/o 0, at a flow rate of 20 m.l min -I. Suspension samples were taken at various time intervals and extracted for different chemical fractions. The data indicate that increasing 0% concentrations in the gas mixture resulted in lower sediment pH and higher oxidation-reduction potential (redox potential) with time. Changes in redox potential and pH strongly modified the distribution of trace metals in various chemical fractions. The hydroxylamine hydrochloride extraction failed to selectively remove oxides and hydroxides of Mn without dissolving a substantial proportion of reactive Fe, as the recovery of Fe by this extractant was 8 to IO orders of magnitude greater than Mn. The large recovery of Fe with Mn in this fraction makes it difficult to distinguish trace metals associated with Mn oxides from those associated with Fe oxides. A substantial fraction of total sediment Zn was associated with potentially bioavailable chemical forms, which may be mobilized to more readily available forms due to changes in the physicochemical properties of the sediment-water system. Significant correlation coefficients obtained between soluble Pb, Cd and Cu and different Fe fractions suggest a strong relationship between these trace metals and Fe compounds. Phosphorus and NH: -N concentrations decreased sharply with time as a result of increased redox potential levels in the z-I~/~ and 21% Oa levels. The effect of the O-I 1% 0, treatment was negligible. This indicates that oxidized sediment conditions may be an important factor in regulating eutrophication by reducing the levels of P and N available for biota. This necessitates a careful study of changes in the dissolved 0, concentration (reflected by redox potential levels) in the sediment-water system. (1This study was supported by the Dredged Materials Research Program, U.S. Army Corps of Engineers Waterways Experiment Station, Vicksburg, Ms. on Contract No. DACW3g-74-C-oo76. Mention of trade names does not constitute official endorsement by Louisiana Agricultural Experiment Station. 21
22
R. A. Khalid,
W. H. Patrick,Jr.
&3 R. P. Gambrel1
Introduction The exchange of potentially toxic metals and plant nutrients between dredged sediments and surface water is a function of several physical, chemical and biological processes. Some of the important chemical and biological reactions include adsorption and desorption processes, precipitation with oxides and hydroxides of Fe and Mn, complexation with organic matter, sulfides and carbonate reactions, and mineralization and immobilization processes. Several of these reactions may occur simultaneously in the same system. Oxidation-reduction potential (redox potential) and pH of the sediment-water system are major factors known to influence these processes. However, a lack of understanding of the opposing interacting effects of certain of these processes stems from two opposite views of the effects of redox potential. One view is that the presence of sulfide under reduced conditions will precipitate toxic metals resulting in a very low solubility and that the oxidation of sulfide to sulfate under oxidizing conditions will release these metals into the overlying water (Gardiner, q74a; Holmes et al., 1974; Engler & Patrick, 1975; Chen et al., 1976). The opposite thought is that toxic metals will be more soluble under reduced conditions due to the reduction of Fe and Mn hydrous oxides which tend to sorb or coprecipitate toxic metals under oxidized conditions (Brooks et al., 1968; Jenne, 1968; Windom, 1973). The effect of sulfide and oxides and hydroxides of Fe and Mn in controlling the solubility of trace metals in the sediment-water system is greatly modified by the presence of organics (Morel et al., 1973 ; Lindberg & Harris, 1974). It is very difficult to single out the specific effects of certain sediment components under such a complex system. This study was designed to measure the integrated effects of interaction between various regulatory processes operating in an estuarine sediment-water system on the chemical transformation of trace metals and plant nutrients. The objectives were: (I) to investigate the influence of various levels of dissolved 0, on the transformation of Fe, Mn, Zn, Cu, Pb, Cd, NH:-N and P in Barataria Bay sediments, and (2) to differentiate metal ions associated with Mn oxides from those associated with Fe oxides in the sediment-water system. We attempted to simulate dredging and dredged material disposal conditions where reduced bottom sediments are dispersed in an oxygenated surface water or transported to a land-fill area where atmospheric oxygen may diffuse into the dredge material. This study also approximates changes in the natural estuarine environment where seasonal variation in the oxidation-reduction conditions, pH and other physicochemical properties normally occur (Brannon, 1973). Such changes may influence levels of toxic metals and biostimulants affecting aquatic life.
Materials and methods Sediment sampling and incubation
Sediment and surface water samples were collected from Barataria Bay, a large shallow estuary located in the Louisiana coastal marsh area. Barataria Bay is surrounded on the east by the Mississippi River and on the west by Bayou Lafourche. The sampling site was located 3 km west of the southern tip of Grand Isle on the Louisiana Gulf Coast. Care was taken to minimize exposure of the sediment material to atmospheric 0, during transportation and storage prior to laboratory studies. Some of the properties of the sediment material and surface water are presented in Table I. Wet sediment material, equivalent to 3oo g oven dry solids, was transferred to 3-1, 3necked, flat-bottomed Pyrex flasks. Sufficient stored surface water was added to attain a solids to water weight ratio of I : 8, which approximates dredged slurry. After IO days of
Effect of dissolved oxygen on heavy metals
TABLE I. Characteristics
of sediment material
%
Cation exchange capacity (medmo d
Electrical conductivity a (millimhos cm-r)
Chloride” (ug ml-r)
5’15
IQ’2
IQ.1
9100
PH
Redox potential bv)
7’3
-60 Feb
23
y0 Clay 28.0
Mnb
Organic matter
Znb
Pbb
Cdb
Cub
SC
H2S
4.1
14.6
242
traces
(M&4 28,800
3oo
78
37
Predominant minerals um): montmorillonite (IS”), illite (IS), kaolinite Coarse clay (2-0.2 Fine clay (<0’2 mrr): montmorillonite (40), illite (s), kaolinite (5)
(IO)
“Determined in the I :8 sediment solids to surface water extract. ‘Total metal content. “Total combined sulfides. “% of total crystalline minerals identified by X-ray diffraction.
preincubation at 30 “C under an Ar atmosphere, sediment suspensions were placed under constant stirring with Oa-free Ar gas bubbling through the flasks to create a highly reduced system. Suspension pH and redox potential were recorded accordingly using the system developed by Patrick et al. (1973). After 7 days, when the suspensions reached a minimum redox potential, zero-time (pretreatment) samples of sediment suspensions were pipetted out and partitioned into various chemical fractions described later in this section. After the zero-time (pretreatment) sampling, sediment suspensions were constantly purged with 0.1 I o/o0,, 2.1 o/o0, and 21 o/o0, at a flow rate of 20 ml min-l. Changes in pH and redox potential were recorded with time. Suspension samples were taken for fractionation as a function of redox potential and time. The sampled sediment suspensions were extracted for different chemical fractions outlined below. Chemical fractionation
Sampled sediment suspensions were partitioned into : (I) water-soluble, (2) exchangeable, (3) easily reducible, (4) reducible, and (5) total fractions. Water-soluble and total fractions were determined on separate samples. Fractions 2, 3 and 4 were sequentially extracted in the indicated order on duplicate samples. Water-soluble
A rso-ml aliquot was withdrawn with a glass pipette-plastic syringe assembly which had been purged with N, immediately prior to sample removal. This suspension was transferred through a serum cap into a sealed soo-ml polycarbonate bottle purged with N,. The suspensions were centrifuged at 6000 r.p.m. for 20 min and the supernatant was transferred to polyethylene storage bottles containing sufficient 6 N-hydrochloric acid to lower the pH to ~2. This transfer was carried out in a glove bag under an N, atmosphere. The acidified extract was stored at 4 “C! prior to elemental analysis. Exchangeable
Fifty ml of sediment suspension was pipetted into each of two centrifuge bottles in which the atmosphere had been replaced with O,-free N,. The centrifuge bottles contained a
24
R. A. Kkalid,
W. H. Patrick, Jr. & R. P. Gambrel1
sufficient amount of deoxygenated 5 N-sodium acetate adjusted to pH 6.5 to obtain a final concentration of sodium acetate of 0.5 N in a I :IO solids to solution ratio (w/v). The suspension-extractant mixture was shaken for 2 h and centrifuged. The supernatant was acidified to pH z 2 and stored at 4 “C until analysis. The quantity of elements in the exchangeable fraction was calculated as follows: exchangeable element = element in the sodium acetate extract-element in the water-soluble fraction. Easily reducible
A solution of O-I M-hydroxylamine hydrochloride in 0.01 N-nitric acid, pH 2.0 was added to the residual material from the exchangeable extraction in a solids to extractant ratio of I :50 and the contents deoxygenated. The mixture was shaken for 30 min and the extraction was carried out as described for the exchangeable fraction, This extractant is reported to selectively remove Mn associated with hydrous Mn oxides from agricultural soils (Chao, 1972). The metals removed from sediments by this procedure have been termed easily reducible as the extractant is comparatively a weak reductant (Engler et al., 1974). Reducible
Two hundred and fifty ml of oxalate reagent (O-I M-oxalic acid+o*r75 M-ammonium oxalate, pH 3.25) was added to the residual material from the easily-reducible extraction and the contents deoxygenated. The suspension was shaken for 2 h and the extraction was carried out as described for the exchangeable fraction. Total elements
The sediment suspensions was digested with a hydrofluoric-nitric-perchloric acid mixture and dissolved in hydrochloric acid for total Fe, Mn, Zn, Pb, Cd and Cu analyses. This procedure is described in more detail elsewhere (Gambrel1 et al., 1977). Analytical
procedures
Iron, Mn and Zn in the various chemical fractions were determined by flame atomic absorption (Perkin-Elmer Model 107). Water-soluble Pb, Cd and Cu were measured by flameless atomic absorption (Perkin-Elmer Model 360 Atomic Absorption Spectrophotometer, Perkin-Elmer Model 2100 HGA Graphite Furnace) as described by Gambrel1 et al. (1977). Ammonium-N in the sodium acetate extracts was determined by the steam distillation method (Bremner, 1965). Distilled ammonia was collected in 0.1 N-sulfuric acid and determined calorimetrically after nesslerization (Jackson, 1958). Phosphorus was determined calorimetrically by the ascorbic acid method of Watanabe & Olsen (1965). Results and discussion The effects of various concentrations of 0, on the measured physicochemical properties and elemental transformations in the reduced Barataria Bay sediment with time are discussed in the order of redox potential, pH, Fe, Mn, Zn, Pb, Cd, Cu, P and N. It should be pointed out that Pb, Cd, Cu and dissolved PO,-P were measured in the water-soluble fraction only. The objective was to approximate levels of these toxic metals and PO,-P in the interstitial water phase which are available for direct biological uptake.
Effect of dissolved oxygen on heavy metals
-.-
_ 0
6
10 Tlma
IS (days1
io
40
Figure I. The effect of dissolved 02 and incubation and pH of Barataria Bay sediment suspensions.
50
time on the redox potential
Redox potential
Figure I indicates that aeration with gas mixtures containing 21”/$, and 2.176 0, completely oxidized the reduced sediment suspensions in 2.5 and 9 days, respectively. In these suspensions, the redox potential increased to around 600 mV. No further increase in redox potential was observed upon subsequent purging at these 0, levels. The redox potential of the suspension treated with O-I I o/o0, increased very slowly with time. After 5 days, the redox potential increased by only 40 mV. After 56 days, the redox potential increased by 145mV to -65 mV a moderately reduced condition. PH
The influence of varying 0, levels on suspension pH is also indicated in Figure I. In the 21% 0, treatment, the pH of the sediment suspension decreased from an initial value of 7.7 to 5.7 after 5.5 days of constant purging. Where sediment suspensions were purged with 2.1% 0,, the pH dropped from 7.7 to 6.5 after 12 days. The o*II~/~ 0, treatment reduced suspensions pH from an initial pH of 7.7 to 7.5 after 56 days of constant purging. In the O-I 1% 0, treatment, small increases in redox potential were accompanied by a small reduction in suspension pH, suggesting a relationship between these parameters. In the two higher 0, treatments, the trend of change in pH and redox potential was the same though a greater differential in pH occurred than in redox potential. Iron
The concentration of water-soluble Fe in various 0, treatments was below detection limits of atomic absorption analysis (0.4 ug Fe ml-i in solution). Purging with o*II~/( 0, did not appreciably reduce exchangeable Fe concentration with time. When sediment suspensions were purged with 2.1 y0 0, for 288 h, exchangeable Fe decreased to non-detectable levels (Table 2). This decrease may be due to oxidation during this period resulting in ferric
Exchangeable Easily reducible“ Reducibled
Fraction
5'39
'573 2320
8.05
1491 1729
1518
10’00 '5'3 235"
944 3376
8.28
1396 2188
1518 1834
(0.5)
-160
1024 2330
(0)
-210
1406 2187
(W
+640
1518 1834
+I00 (48)
(mV)
fig Fe g-’ oven dry solids 6.91 n.d.’ IO.19
-50 (48)
2
2224
5.06 1045 2459
3'78
+*so (7)
1440
-40 (3.5)
r o/o Oxygen
Bay sediment suspensions
5.86
(8)
-160
Redox potential
of Fe in Barataria
8.28
(0)
-210
form and distribution
2’ 1 o/o Oxygen
oxygen on the chemical
“Zero-time (pretreatment) sampling. *Sampling time in hours after zero-time. CNot detectable. “Mean of duplicate samples.
1834
(OY
8.28
-170
(=o)
-210
Oxygen
w
-210
0.11%
TABLE 2. The effect of dissolved
4420
15'57 781
+610 (132)
Effect of dissolved oxygen on heavy metals
27
compounds not extracted with 0.5 N-sodium acetate. In the 21 o/00, treatment, however, the exchangeable Fe at the end of 132 h purging was greater than initial levels. Apparently a decrease in pH from the initial value of 7.7 to 5.7 at the end of the incubation period may have resulted in the transformation of potentially reactive ferric hydroxides to a more available exchangeable fraction due to acidic pH. The residual solid material from the sodium acetate extraction was shaken with 0.1 Mhydroxylamine hydrochloride solution prepared in o-01 M-nitric acid to selectively dissolve Mn oxides with minimal attack on Fe oxides (Chao, 1972). However, Fe in this easily reducible fraction ranged from 780 to 1570 pg g-l (Table 2). The effect of the 0.1 I y. 0, treatment during 120 h of constant purging on easily reducible Fe was negligible. In the 2*10/ and 21% 0, treatments, easily reducible Fe decreased with increasing redox potential. The oxalate-extractable (reducible) Fe increased with incubation time, and this effect was very pronounced at the 2.1% and 21% 0, levels (Table 2). At the end of the experiment, the level of reducible Fe increased with an increase in the 0, level in the order of 21% 02>2*1 y. O,>o-I I y. 0,. These increases in reducible Fe at greater 0, levels may be due to lower suspension pH values recorded. The pH values at the end of the incubation period were 5.7, 6.5 and 7.5 for 21 y. 0,, 2.1 y. 0, and 0.1 I “/ 02, respectively. Apparently, a a lower sediment pH facilitated the dissolution of Fe compounds present in the sediment suspension, resulting in higher Fe levels extracted with oxalate solution. Our data show the easily reducible Fe fraction is indicative of the Fe compounds which are transformed into more chemically reactive forms under reduced conditions and thus are more susceptible to exchange reactions. Oxidation of the initially reduced sediment suspensions, as a result of dissolved 0, additions (indicated by redox potential levels of >600 mV in the 2.1% and 21 oh 0, treatments, Table 2) may result in the formation of ferric oxyhydroxides and ferric oxides not susceptible to hydroxylamine hydrochloride extraction. These precipitated forms of ferric Fe present in the well oxygenated system were extracted with oxalate solution as more Fe was recovered in the well oxidized treatments (>600 mV redox potential) in the reducible fraction. These freshly precipitated oxides and hydroxides of Fe represent potentially reactive forms of sediment Fe and may control the release of Fe and associated metal ions in sediment-water systems in the range of redox potential and pH conditions encountered in the environment (Jenne, 1968; Lee, 1973). The discussion on the effects of aging and the crystallinity of hydrous oxides of Fe on trace metal adsorption is beyond the scope of this paper and the reader is referred to Jenne (1968) and Lee (1973) for a detailed account. Manganese
The effect of dissolved 0, on water-soluble and exchangeable Mn was variable (Table 3). For example, in the 0.11 “/ 0, treatment, little trend in Mn levels was observed with time. Since there was only a slight increase in redox potential after 5 days, the system was still very reduced, and Mn was present mostly in the divalent cationic form (Mn2+). Purging with 2.1 yb and 21 o/o0, levels resulted in a gradual decrease in water-soluble Mn as a consequence of increased redox potential. The decrease in soluble Mn upon oxidation may be due to the formation of less soluble Mn oxides as the redox potential increases. Similar effects of redox potential on Mn concentrations have been reported by Gotoh & Patrick (1972) and Gambrel1 et al. (1977). Th ere was no consistent effect of incubation time and 0, levels in the easily-reducible (hydroxylamine hydrochloride ext.) and reducible (oxalate ext.) Mn fractions.
Water soluble Exchangeable Easily reducible” Reducibled
Fraction
Water soluble Exchangeable Easily reducible’ Reducible’
Fraction
0.11
Oxygen
(0)
8.99 2.69 103 38
14.91 2-47 73 59
-210
(120)
-170
The effect of dissolved
n.d. n.d. 4’51 17’50
(0)
-210
n.d. n.d. 2.67 15.64
(168)
2.1%
solids .63 4’27 9r 50 I
(2W
+ 640
Oxygen
+ 640
(288)
ug Zn g-r oven dry solids n.d. n.d. n.d. b.d. n.d. 1.61 3’14 9.60 16.17 15’48 8.46 5.84
(16%
(mV) + 100
form and distribution
Redox potential -so (8) (48)
-160
(48)
+100
ug Mn g-r oven-dry 8.18 I .63 3.61 4.81 77 87 51 47
oxygen on the chemical
“Zero-time (pretreatment) sampling. * Sampling time in hours after zero-time. c Not detectable. d Mean of duplicate samples.
n.d. 0.28 5.46 22.56
(120)
W
0.30 n.d. 4.28 20.24
-170
-210
0’ I I o/OOxygen
4.
n.d.c n.d. 4’5 1 15.64
(0)’
-210
TABLE
8.79 3’29 73 54
(8)
-50
Redox potential -160
(mV)
form and distribution
2. I o/o Oxygen
oxygen on the chemical
“Zero-time (pretreatment) sampling. %ampling time in hours after zero-time. ‘Mean of duplicate samples.
8.73 2.80 82 5’
w
-210
o/o
3. The effect of dissolved
8.99 3.29 ro3 38
(OY
-210
TABLE
-
n.d. n.d. 4’5’ 15-64
(0)
-210
of Zn in Barataria
8.99 3’29 ro3 38
(0)
-210
43
102
6.01 4’40
-40 (3.5)
2 1 o/OOxygen
Bay sediment
6.44
4.ro 9r 56
+*50 (7)
0.30 n.d. 5’51 15.22
(0.5)
-160
2
n.d. 0’47 5.16 13’37
-40 (3.5)
1 o/o Oxygen
2.92 0.50 4’54 13.83
+*50 (7)
15.61 10.66
4.38 1.96
(132)
i-610
63:3: 100 67
+6ro (132)
suspensions.
Bay sediment suspensions
7’09 4’4’ 103 43
-160 (0.5)
of Mn in Barataria
Effect of dissolved oxygen on heavy metals
29
The hydroxylamine hydrochloride extraction (easily reducible) is reported to selectively extract oxides and hydroxides of Mn (Chao, 1972). These data show that ~~-75% of total reducible Mn was present in this easily reducible form. Though this fraction dissolved a greater proportion of the potentially reactive Mn, the dissolution of Fe from Fe oxides was 8 to IO orders of magnitude greater. Manganese concentration ranged from 73 to 103 pg Mn g-1 in the easily reducible fraction and from 38 to 67 pg Mn g-l in the reducible fraction (Table 3). Along with Mn, 780 to 1570 pg Fe g-l, which comprised 15-67q6 of the total reducible Fe was also extracted in the easily reducible fraction (Table 2). Oxalate extractable Fe ranged from 1730 to 4420 pg Fe g- l. The large recovery of Fe with Mn in both reducible fractions makes it difficult to distinguish between toxic metals associated with Fe oxides and Mn oxides. Zinc Water-soluble Zn ranged from nondetectable levels to over 4 pg g-l, depending on 0, treatments and time of incubation (Table 4). The highest concentrations were observed at redox potential levels of ISO and 610 mV in the 21% 0, treatment. This may be due to a decrease in suspension pH which solubilized more Zn. Exchangeable Zn exceeded I pg g-l only in the 2.11~/~and 21% 0, treatments which became well oxidized. Again, the corresponding decrease in pH may have contributed to this observation. Zinc associated with the easily reducible fraction increased slightly at the end of the incubation period in the 0.1 I o/o0, treatment. In the 2.1 o/o0, treatment, easily reducible Zn decreased initially up to - 160 mV, but a further increase in redox potential to -50, IOO and 640 mV was accompanied by an increase in easily-reducible Zn. In the 21 o/o0, treatment, easily-reducible Zn was considerably higher at 610 mV than at lower redox potential levels. Reducible Zn decreased with increasing redox potential in the 2-17~ 0, treatment. In the 0.1 I y. 0, treatment, reducible Zn increased slightly with incubation time. A slight reduction in reducible Zn was noted with time in the 21 o/00, treatment. Our data show that 23-38% of the total sediment-bound Zn was associated with chemical forms susceptible to moderate chemical extractions (hydroxylamine and oxalate extraction). This zinc is potentially bioavailable and may be mobilized to more readily available forms as a consequence of changes in physicochemical properties of the sediment-water system. This fraction of total Zn represents the proportion adsorbed by the colloidal oxides of Fe and Mn. Jenne (1968) has proposed that precipitation by hydrous metal oxides of Fe and Mn is the principle control mechanism for Zn in soils and sediments. Gambrel1 et al. (1977) reported that in other estuarine and river sediments, most of the Zn recovered in similar chemical fractionation procedures was associated with the reducible fraction. Although the contribution of insoluble organic material to Zn adsorption was not investigated in this study, the literature indicates that a relatively small proportion of Zn is associated with the insoluble organic-bound fraction (Serne & Mercer, 1975; Gambrel1 et al., 1977). Lead, cadmium and copper The data plotted in Figures z(a), 2(b) and 2(c) for Pb, Cd and Cu, respectively, indicate that concentrations of these toxic metals in the water-soluble fraction at the end of the incubation period were 25-30% higher than that at the pretreatment sampling. This increase was evident in all 0, treatments. The concentration of soluble Pb ranged from 41.2 to 196.6 pg l-l, Cd from ~9 to 6.4 pg l-1, and Cu from 4.9 to 12.2 pg 1-l. Recall from the
30
R. A. Khalid, W. H. Patrick,Jr. &’ R. P. Gambrel1
(bl Cadmium = a 3 3
!ko-
.*
2,5-
Ob
0
I 25 lncubatkm
.I 50
t
.’
P
I
150 time (h)
Figure 2. The effect of dissolved Bay sediment suspensions.
.’
250
Op on water-soluble
Pb, Cd and Cu in Barataria
previous discussion that in the O*II% 0, treatment, the sediment suspension was still very reduced (- 170 mV redox potential) at the end of 120 h. This level of reduction is well within the stability boundary for sulfide. In the 2.1 y0 and 21 y0 0, treatments, sediment suspensions were strongly oxidized (600 mV redox potential) after 288 and 132 h of aeration, respectively. Such high redox potential levels support the oxidation of insoluble sulfide to soluble sulfate, and at the same time, the transformation of soluble manganous Mn (Mna+) and ferrous Fe (Fez+) to insoluble manganic (Mn4+) and ferric (Fea+) oxides and hydroxides. Under the conditions of our experiment, an increase in soluble Pb, Cd and Cu with incubation time under reduced conditions (o*II~/~ 0, treatment) may be due to constant agitation of sediment suspensions solubilizing more organic material with time. Complexation of trace metals with soluble organics may have resulted in more metal ions in solution in spite of sulfides present in the system. Under oxidized conditions, an increase in soluble Pb, Cd and Cu at the end of the incubation period (288 and 132 h for 2.1% and 21% 0, treatments, respectively) was most likely due to the effect of lower suspension pH (Figure I) on the speciation of these trace metals. In the pH range encountered under oxidized conditions, these toxic metals are reported to be present as hydrated divalent cations, as soluble complexes of chlorides and sulfates, and as hydroxides with limited solubility (Hem, 1972; Hahne & Kroontje, 1973). Chloride complexation is known to increase the solubility of trace metals if the concentration of chloride ions is high enough to compete with hydroxide formation (Hahne & Kroontje,
E#ect of dissolved ovgen m heavy metals
31
1973). Barataria Bay is a brackish estuary containing considerable quantities of chloride (Table I) which may compete with hydroxides to keep metal ions in solution. Hem (1972) suggested the presence of a mixed complex, Cd(OH)CI, where chloride concentration exceeded 0.1 M, with a solubility greater than Cd(OH),. The probable formation of Cd(OH)Cl in the Barataria Bay sediment suspensions containing 0.3 M-sodium chloridesodium sulfate solution has also been proposed (C. A. Price, personal communication, Geology Department, Louisiana State University). The formation of insoluble metal carbonate complexes was not thought to be a significant factor regulating metal solubility in this study because: (I) constant purging of a gas mixture through the suspensions likely prevented an accumulation of CO, and thus limited carbonate availability for Pb, Cd and Cu complexes; (2) the chelation of trace metals with organics may keep them from precipitating with carbonates and hydroxides (Lisk, 1973 ; Morel et al., 1973); and (3) the relatively high solubility of carbonate below pH 7.0 (Hem, 1972) may also result in greater concentration of metal ions in solution in the 2.1 y0 and 21% 0, treatments. The role of Fe and Mn compounds in regulating Pb, Cd, and Cu solubility was also investigated by computing linear correlation coefficients for Fe and Mn extracted in different chemical fractions with water-soluble Pb, Cd and Cu (Table 5). Lead, Cd, and Cu were negatively correlated with easily-reducible Fe, whereas, a significant positive correlation between reducible Fe and the trace metals was determined. Manganese extracted in various chemical fractions was not significantly correlated with the trace metals except for the reducible Mn fraction, which was significantly correlated with Cd. It is apparent from our results that hydrous oxides of Fe and possibly Mn (in the case of Cd) may act not only as scavengers for trace metals as advocated in the literature (Jenne, 1968; Murray et al., 1969; Stumm & Morgan, x970), but also as a source of metal ion release, the mechanism of which is not very clear. It is likely that the trace metals adsorbed by the amorphous hydrated oxides of Fe and Mn or coprecipitated with ferric oxyhydroxides may be released under slightly acidic, oxidized conditions due to hydrogen ion displacement. The released metal ions may form soluble complexes with organic material or chlorides present in the system. Also evident is that the simple metal solubility and Eh-pH relationships based on pure inorganic thermodynamics of sulfides, carbonates, oxides, and hydroxides (Krauskopf, 1967; Stumm & Morgan, 1970; Hem, 1972) do not adequately explain metal-ion behavior in the natural environment. Since trace metals such as Pb, Cd, Cu, and others are toxic to terrestrial and aquatic life at very low concentrations (Khalid et al., 1977) the relative
TABLE 5. Linear relationship (I) between various fractions of Fe and Mn
Fraction Exchange Fe Easily Reducible Fe Reducible Fe Water-Soluble Mn Exchangeable Mn Easily Reducible Mn Reducible Mn
Pb 0.127~ --0*595*
o.773+*
water-soluble
Water-soluble Cd CU 0.138
-0*566* 0.670’
-~291
-0'122
o-358 -o205 -237
-0.041
#‘I’ values without asterisk, non-significant. *Significant at 5% level, n = 13. **Significant at 1 o/Olevel, n = 13.
Pb, Cd, Cu and P and
0.417 0.566.
o-405 -0.580* 0*761** -0.275
0’499 0,267 0’4.20
P -0.188
0.848** -06o5f 0.766.. -o569* -0.039 0.349
32
R. A. Khalid,
R
i
W. H. Patrick,Jr.
& R. P. Gambrel1
1
fe) Phosphorus \
\
\
\
\
\
\
\
-o~llxo* - - 02.11% 02 \
\
--A21~0%0* \\
‘-__ --.
Incubation
time Ch)
Figure 3. Effect of dissolved 0, on water-soluble PO&-P and sodium acetate extractable NH: -N in Barataria Bay sediment suspensions. contribution of possible sources and chemical and biochemical transformations affecting the availability of these metals should be more fully investigated in an integrated physical, chemical, and biological system approximating the natural environment. Phosphorus
The concentration of water-soluble P in the 0.1 I y. Oa treatment did not change appreciably with time [Figure 3(a)], due to similar reduced conditions present during the incubation period. In the 2.1 y0 0, and 21 y. 0, treatments, soluble P decreased sharply with time as a result of increasing redox potential. Figure 3(a) indicates that the rate of soluble P disappearance was much faster in the 21 y. 0, treatment as compared to 2.1 oh 0, level, although both treatments had similar P concentrations at the end of the incubation periods, at which time both suspensions were well oxidized. These results indicate that the level of dissolved 0, in the sediment-water system may have a profound effect on the exchange of P between the sediment material and the overlying water. Mortimer (1971) reported that the disappearance of dissolved 0, and subsequent reduction of sediment material resulted in a several-fold increase of dissolved P. Oxygenation of the sediment reversed the process and decreased the P concentration in solution. Since P is a key element in determining the eutrophication rate of natural waters, it is very important to study the behavior of this element in the aqueous environment. The role of Fe components, such as amorphous oxides and hydroxides of Fe and ferrous compounds, in controlling the levels of dissolved inorganic P in soils and sediments is well
Effect of dissolved oxygen on heavy metals
33
documented (Syers et al., 1973 ; Patrick & Khalid, 1974). In our study, water-soluble P was positively correlated with easily reducible Fe (Y = o-848**) (Table 5). This indicates that changes in redox potential had a similar effect on water-soluble P as well as Fe compounds extracted with hydroxylamine hydrochloride. As discussed earlier in the Fe section, easily reducible Fe probably represents Fe compounds more soluble under reduced conditions and may be in equilibrium with water-soluble fractions thus affecting P levels. Oxidation of ferrous phosphate to less soluble ferric phosphate is an important factor in regulating P solubility. A significant negative correlation (--0X105*) (Table 5) between water-soluble P and oxalate extractable Fe (reducible Fe) indicates that freshly precipitated ferric compounds control the solubility of P. Our results are in agreement with literature reports (Syers et al., 1973; Patrick & Khalid, 1974; Khalid et al., 1977) that oxides and hydroxides of Fe strongly influence P transformations, and that these reactions are greatly modified by the oxidation-reduction conditions of the sediment-water system. No significant relationship was observed for the easily reducible and reducible Mn fractions, indicating that Mn compounds had no effect on P concentration, Nitrogen
The levels of sodium acetate extractable NH:-N were influenced by the concentration of 0, and the equilibration time [Figure 3(b)]. This effect was largely a function of redox potential. In both the 2.1 y0 and 21 y0 0, treatments, the concentration of NH:-N decreased as redox potential increased. In the 2.1% 0, treatment, disappearance of NH:-N (presumably nitrification) commenced sometime between 48 and 168 h after the treatments began. It is possible that where the purging gas contained only 2.1 y0 0,, most of the initially added 0, was consumed in the oxidation of organic material, Fe and Mn, and little 0, was available for several oxidation processes to proceed simultaneously. No change in the level of NH:-N was observed in the O-I I o/o0, treatment. In this treatment, the sediment suspensions remained very reduced during the 56-day incubation (Figure I). Much of the NH:-N lost as a result of oxidation is very likely transformed to the more mobile NOT-N form and is equally bioavailable in the sediment-water system. However, under natural conditions, this NOT-N may be removed by denitrification as nitrate diffuses from surface waters and aerobic surface sediment horizons into underlying reduced sediment material. Reddy & Patrick (1975) reported losses of applied 15NH4f-N as high as 63 o/o during alternate aerobic and anerobic cycles. This phenomenon may be of great significance in dredging and disposal operations where reduced bottom sediments are mixed with aerobic surface waters and subsequently become reduced upon redeposition. Conclusions The results of this study indicate that the intensity of oxidation increased with increasing dissolved 0, concentrations used to purge the sediment suspensions. An increase in redox potential values due to dissolved 0, was accompanied by a decrease in suspension pH, and the maximum effect was observed in the 21% 0, treatment. This decrease in suspension pH from an initial value of 7.7 to a final value of 5.7 at the end of 132 h in the 21 y. 0, treatment resulted in increased levels of soluble Zn, Pb, Cd and Cu, and also in exchangeable Fe, Mn and Zn under very oxidized conditions ($610 mV redox potential). This study suggests that the expected adsorption or coprecipitation of soluble trace metal ions by oxides and hydroxides of Fe and Mn as an estuarine sediment is oxidized may be offset by concurrent redox potential mediated decreasesin pH. The results of this study strongly suggest
34
R. A. Kkalid,
W. H. Patyick,Jr.
W R. P. Gambrel1
that chemical transformations of metal ions should be further investigated under controlled pH and redox potential conditions. An attempt was made in this study to selectively extract Mn oxides with minimal attack on Fe oxides as suggested by Chao (1972). The data show that hydroxylamine hydrochloride extraction failed to selectively remove oxides and hydroxides of Mn without dissolving a substantial proportion of reactive Fe, as the recovery of Fe by this extractant was 8-10 orders of magnitude greater than Mn. This large recovery of Fe with Mn in the easily reducible fraction makes it difficult to distinguish trace metals associated with Mn oxides from those associated with Fe oxides. A substantial fraction of total sediment Zn was associated with potentially bioavailable chemical forms which may be mobilized to more readily available forms due to changes in the physicochemical properties of the sediment-water system. An increase in soluble concentrations of Pb, Cd and Cu with time in the 0.11 y. 0, treatment indicates that the precipitation of toxic metal ions as insoluble sulfides under reduced conditions may not fully explain observed concentrations of these metal ions. It is probable that complexation of trace metals with soluble organics may have resulted in more metal ions in solution in spite of the presence of sulfides in the system. Under oxidized conditions, an increase in soluble Pb, Cd and Cu with time may be due to the release of metal ions adsorbed by hydrated oxides as the system becomes more acidic. The released metal ions may form soluble complexes with organics or chlorides present in the system. The relatively high concentration of soluble Pb determined in Barataria Bay sediment suspensions in this experiment suggests a possible Pb toxicity problem and should be more fully investigated. It is evident from our results that simple metal ion solubility calculations and Eh-pH relationships based on pure inorganic thermodynamic considerations of the chemistry of sulfides, carbonates, oxides and hydroxides do not adequately explain metal-ion behavior in the natural environment. The presence of organics and high concentration of chlorides in the estuarine sediments and dredged materials may modify the existing relationships and enhance metal ion release. Since trace metals such as Pb, Cd, Cu and others may accumulate to toxic levels in estuarine food webs, the relative availability of these metals should be more fully investigated in an integrated physical, chemical, and biological system approximating the natural environment. Dissolved inorganic P and NH,+-N concentrations decreased sharply with time as a result of increased redox potential levels in the 2.1% and 21% 0, levels. The effect of the o.IIO/~ 0, treatment was negligible. This indicates that oxidized sediment conditions may be an important factor in regulating eutrophication by reducing the levels of P and N available for biota. This necessitates a careful study of changes in the oxidation-reduction intensity of sediment-water systems affecting availability of biostimulants. References Brannon, J. 1974 Seasonalvariation of nutrients and physicochemical properties in the salt marsh soils of Barataria Bay, Louisiana. M.S. Thesis, Agronomy Department, Louisiana State University, Baton Rouge, Louisiana. 130 pp. Bremner, J. M. 1965 Inorganic forms of nitrogen. In Methods of Soil Analysis, Part z (Black, C. A., ed). American SOCicty of Agronomy, Inc., Madison, Wisconsin. pp. 1179-1237. Brooks, B. B., Presley, J. J. & Kaplan, I. R. 1968 Trace elements in the interstitial waters of marine sediments. Geochimica et Co-himica Acta p, 397-414. Chao, L. L. 1972 Selected dissolution of manganese oxides from soils and sediments with acid&d hydroxalamine hydrochloride. Soil Science Society of America Roceedings 36,764768. Chen, K. Y., Gupta, S. K., Sycip, A. Z., Lu, J. C. S. & K nezevic, M. 1976 The efIects of dispersion, settling, and resedimentation on migration of chemical constituents during open water disposal of
Efect of dissolved oxygen on heavy metals
3.5
dredged materials. Contract Report, U.S. Army Engineer Waterways Experiment Station, Vicksburg, 221 pp. Mississippi Contract No. DACW3974-C-077. Engler, R. P.&Patrick, W. H., Jr. 1975 Stability of sulfides of manganese, iron, zinc, copper, and mercury in flooded and nonflooded soil. Soil Science 119, 217-221. Engler, R. P., Brannon, J. M., Rose, J. & Bigham, G. 1974 A practical selective extraction procedure for sediment characterization. Proceedings r68th American Chemical Society National Meeting, Atlantic City, New Jersey, September 1974. (In press). Gambrel& R. P., Khalid, R. A., Verloo, M. G. & Patrick, W. H. Jr. 1977 Transformations of heavy metals and plant nutrients in dredged sediments as affected by oxidation-reduction potential and pH. Part II, materials and methods, results and discussion. U.S. Army Engineer Waterways Experiment Station, Vicksburg, Mississippi Contract Report D-77-4, 336 pp. Gardiner, J. 1974.0 The chemistry of cadmium in natural water. I. A study of cadmium complex formation using the cadmium specific-ion electrode. Water Research 8, 23-30. Gardiner, J. 1974b The chemistry of cadmium in natural water. II. The adsorption of cadmium on river muds and naturally occurring solids. Water Research 8, 157-164. Gotoh, S. & Patrick, W. H., Jr. 1972 Transformation of iron in a waterlogged soil as influenced by redox potential and pH. Soil Science Society of America Proceedings 38, 6671. Hahne, H. C. H. & Kroontje, W. 1972 Significance of pH and chloride concentration on behavior of heavy metal pollutants: Mercury (II), cadmium (II), zinc (II), and lead (II). Journal of Enwironmental Quality 2,444-450. Hem, J. D. 1972 Chemistry and occurrence of cadmium and zinc in surface water and ground water. Water Resources Research 8, 661-679. Jackson, M. L. (ed.) 1958 Soil Chemical Analysis. Prentice Hall, Inc., Englewood Cliffs, New Jersey. pp. X83-204. Jenne, E. A. 1968 Controls of Mn, Fe, Co, Ni, Cu, and Zn concentrations in soils and water: The significant role of hydrous Mn and Fe oxides. In Trace Inorganics in Water, Advances in Chemistry Series 73 (Gould, R. F., ed.) pp. 337-387. Khalid, R. A., Gambrell, R. P., Verloo, M. G. & Patrick, W. H., Jr. 1977 Transformations of heavy metals and plant nutrients as affected by oxidation-reduction potential and pH. Part I. Literature review. U.S. Army Engineer Waterways Experiment Station, Vicksburg, Mississippi Contract Report D-77-4. 238 pp. Krauskopf, K. B. (ed.) Introduction to Geochemistry. McGraw-Hill, New York, New York. pp. 236-280. Lindberg, S. E. & Harriss, R. C. 1974 Mercury-organic matter associations in estuarine sediments and interstitial water. Environmental Science and Technology 8, 459-462. Lisk, D. J. 1972 Trace metals in soils, plants, and animals. Adwances in Agronomy 24, 267-325. Morel, F., McDuff, R. E. & Morgan, J. J. 1973 Interactions and chemostatis in aquatic chemical systems : Role of Eh, pE, solubility, and complexation. In Trace Metals and Metal Organic Interactions in Natural Water. (Singer, P. C., ed.). Ann Arbor Science Publ., Ann Arbor, Michigan. pp. 157-200. Mortimer, C. H. 1971 Chemical exchanges between sediments and water in the Great Lakes-speculations on probable regulatory mechanisms. Limnology and Oceanography 16, 387-404. Murray, D. J., Healy, T. W. & Fuerstenau, D. W. 1968 The adsorption of aqueous metal on colloidal hydrous manganese oxide. In Adsorption from Aqueous Solution, Advances in Chemistry Series 79 (Gould, R. F., ed.) pp. 74-81. Patrick, W. H., Jr. & Khalid, R. A. 1974 Phosphate release and sorption by soils and sediments: effect of aerobic and anaerobic conditions. Science 186, 53-55. Patrick, W. H., Jr., Williams, B. G. & Moraghan, J. T. 1973 A simple system for controlling redox potential and pH in soild suspensions. Soil Science Society of America Proceedings 37, 331-332. Reddy, K. R. & Patrick, W. H., Jr. 1975 Effect of aerobic and anaerobic conditions on redox potential, organic matter decomposition, and nitrogen loss in a flooded soil Soil Biology and Biochemistry 7, 87-94. Seme, R. J. & Mercer, B. W. 1975 Characterization of San Francisco Bay dredge sediments-crystalline matrix study. Contract Report, U.S. Army Engineer District San Francisco, California. Contract No. DACW 07-73-c-080. 102 pp. Syers, J. K., Haris, R. F. & Armstrong, D. E. 1973 Phosphate chemistry in lake sediments. ~ournul of Environmental Quality 2,1-14. Stumm, W. & Morgan, J. J. (eds) 1970 Aquatic Chemistry Wiley Interscience, New York, New York, pp. 300-382. Watanabe, F. W. & Olson, S. R. 1965 Test of ascorbic acid for determining phosphorus in water and NaHCO I extracts from soil. Soil Science Society of America Proceedings 29,677-678. Windom, H. L. 1973 Investigations of changes in heavy metals concentrations resulting from maintenance dredging of Mobile Bay Ship Channel, Mobile Bay, Alabama. Report submitted to U.S. Army Engineer District Mobile, Alabama. Contract No. DACor-73-C-136. 46 pp.