The capacity of marine plankton, macrophytes and particulate matter to adsorb Cu2+ in the presence of Mg2+

Marine Chemistry, 12 (1983) 255--280

255

Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

THE CAPACITY OF MARINE PLANKTON, MACROPHYTES A N D PARTICULATE MATTER TO ADSORB Cu 2+ IN THE PRESENCE OF Mg 2 + CARLTON D. HUNT* and WILLIAM F. FITZGERALD

Marine Sciences Institute, University of Connecticut, Avery Point, CT 02640 (U.S.A.) (Received March 24, 1981; revision accepted October 4, 1982)

ABSTRACT Hunt, C.D. and Fitzgerald, W.F., 1983. The capacity of marine plankton, macrophytes and particulate matter to adsorb Cu 2+ in the presence of M g 2+. Mar Chem., 12: 255-280. A complexometric titration technique was employed to measure the total capacity of a varietyI of marine organisms to adsorb Cu 2+. Measured adsorption capacities were 0 . 2 2 m e q g - for phytoplankton, 0 . 3 - - 1 . 0 m e q g -1 for macrophytes, 1 . 0 - - 2 . 5 m e q g -1 for zooplankton and ~ 0 . 3 m e q g -1 for suspended particulate matter. The capacity of these materials to adsorb Cu 2+ was reduced significantly in the presence of Mg2+ at seawater concentrations. C o m p e h t m n between Mg2+ and Cu 2+ for adsorptmn sites at pH 6 is described by an average conditional equilibrium constant of ~ 103"7. This constant is such that very little Cu 2+ may be adsorbed onto particulates and marine phytoplankton in the presence of Mg2+. Further, primary productivity data and estimates of the detrital carbon sedimentation in Long Island Sound suggest that the flux of particulate carbon is insufficient to remove significant amounts of Cu from the water column to sediments by adsorption mechanisms. •

•

•

•

•

INTRODUCTION

Phytoplankton and particulate organic carbon can be significant phases involved in the transport and deposition of certain trace metals in the coastal zone (Hunt, 1979). Transfer of metals to sediments by organisms may involve direct deposition of plankton or be in association with fecal pellets. The initial step in this biotransport is incorporation of the metal into the organisms and other particulate phases. The mechanisms by which metals become associated with the biological phases are not entirely clear. Studies of marine macrophytes (Gutknecht, 1961a,b, 1963, 1965; Bryan, 1969) and phytoplankton (Krauskopf, 1956; Burton and Hostetter, 1977; Conway, 1978), suggest that incorporation of many trace elements in organisms is by an adsorptive process. This may involve either reactions at the surface of the cell or intracellular polysaccharides (Haug and Smidsrod, 1967; Bryan, 1969). Lowman et al., (1971) also invoke adsorption as an explanation for * Present address: MERL Graduate School of Oceanography, University of Rhode Island, Kingston, RI 02881 (U.S.A.)

0304-4203/83/$03.00

© 1983 Elsevier Science Publishers B.V.

256 high metal concentration observed in marine plants. Particulate matter and plankton in seawater are known to have negatively charged surfaces (Bayne and Lawrence, 1972; Neihof and Loeb, 1972, 1974; Myers et al., 1975: Hunter and Liss, 1979, 1982), consistent with the observed adsorption of metal cations. Rapid loss of metals from macroalgae is often observed when they are killed (Bryan, 1969). This would suggest that the organisms exert some control over the retention of metals while alive. In contrast to these experimental results, model calculations involving dissolved trace element speciation and adsorption onto particles across the fresh to salt-water interface (Sibley and Morgan, 1975), suggest that surface adsorption is not important in marine waters. Decreased adsorption in seawater results primarily from competition by Mg, Na and Ca for adsorption sites and alteration of dissolved metal speciation from ionic strength and changes in ligand concentration. Quantification of adsorption on all types of particulates is essential if we are to understand metal removal processes in the marine environment. Essentially, no data are available for the adsorption capacities or adsorption equilibrium constants for metals onto biogenic particulate matter, which are major constituents of the particulate load in the coastal zone. This lack of data led us to quantify the adsorption capacity of a variety of marine macrophytes, zooplankton, p h y t o p l a n k t o n and particulate matter. In addition, the competition for exchange sites between Cu 2+ and Mg 2+ and Mg 2+ plus Na + on the surfaces of these plants was determined in order to estimate the importance of the adsorption process in the marine geochemical cycle of Cu. EXPERIMENTAL

Sample collection Macrophytes, plankton and particulate samples were collected from Long Island Sound during the spring and summer of 1976.

Plankton samples Plankton samples were collected with a 50 cm diameter, 20 mesh plankton net. Samples were concentrated in bags fabricated from 20 mesh net. During collections the bags were retained in a plexiglass housing r a ~ e r than standard glass bottles. The net was pushed through the water with a Boston Whaler rather than t o w e d in the classical manner, in order to avoid trace metal contamination from the boat m o t o r (Fig. 1). The push unit was constructed from three 1 0 ' x 21~" Schedule 4 0 P V C plumbing pipes telescoped inside a 3" Schedule 40 pipe. The pipe was secured to the plankton net towing ring with PVC clamps and rope, Two push pipes were allowed to pivot on stand-offs, clamped to the forward gunnel of the Whaler. The third member was hand held for depth control.

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258

TABLE I Species composition of selected phytoplankton and August (P-11, P-12) Phytoplankton

speciesa

s a m p l e s c o l l e c t e d in M a r c h (P-6, P -8)

Sample

P-6 (%)

P-8 (%)

P-1 ] (%)

P-12 (%)

Skeletonema costatum C h a e t o c e r o s sp. N i t z s c h i a seriata B a c t e r i a s t r u m sp. Melosira sp. Thalassionema nitzsulopes Asterionella japonica Thalassiosira sp. Guinardia flaccida P e r i d i n i u m sp. Ceratium

6.5 6.1 1.3 --10.2 1.2 3 7.7 30.6 ---

1.8 12.8 2.2 ---2.0 -3 5.3 37.0 -b

62.2 19.0 7.3 6.4 1.6 1. ] 0.6 --0.6 --

48.0 40.2 4.2 4.3 0.5 1.6 0.2 ~-0.4 --

Animals

--

0.2

0.6

--

aother species were found in lesser abundance. b Occasionally found.

Laboratory, Milford, Connecticut on March 9, 1976. Eight additional samples were collected in late August, 1976 (Fig. 2). Samples from five March stations and the eight August samples were preserved for species identification (Table I). Particulate matter samples were also collected at these stations (Fig. 2). Macroalgae and marsh plants Macroalgae were obtained by collecting plants washed onto Connecticut beaches following major storms. Samples were collected within 12 hours of the storms and stored frozen in clean plastic bags. A collection, consisting of eight plant species common to New England rock shores, was made on September 12, 1975 at Avery Point, Groton. Five additional samples were collected at Avery Point on September 13. These were mainly plants which had begun to degrade in the previous twenty-four hours. A set of winter samples was collected from Avery Point on March 17, 1976. Four other beaches (Branford, Hammonasett State Beach, Old Lyme Town Beach and Rocky Neck State Park) of the Connecticut shore were sampled on April 6, 1976 (Fig. 2). Marsh plants were sampled at Pawson's Marsh and Mudflat, Branford, Conn., in November 1975 and March 1976. In May 1976, several macroalgae samples carried to the water surface by a sediment grab were added to the macrophyte sample collection. These were primarily Ulva lactuca and Laminaria. Samples of Fucus vesiculosus and Ulva

259 were collected in Greenwich Harbour in early May, 1976. Six additional samples of Fucus and Ulva were collected from the surface of Long Island Sound in August, 1976. Macrophytes were stored frozen. Prior to analysis, the samples were freeze
Zooplankton Samples dominated by zooplankton were collected at stations P-16, 17 and 18 in August, 1976. Other pure zooplankton samples collected from the Northwest Atlantic in 1971 (Fitzgerald et al., 1972) were also analyzed for adsorption capacity. Adsorption capacity: me thodology General Plant surfaces are composed of several polysaccharides, proteins, fatty acids, ring compounds such as porphyrins and any of several types of cellulose found in cell walls (Mackie and Preston, 1974; Crist et al., 1981). As a result these surfaces may have a variety of ligands (e.g., amino groups, carboxyl and hydroxyl groups) which may contribute to metal adsorption. The adsorption may be described in a manner similar to that of Pb ~+ adsorption onto hydrous ~/-A1203 (Hohl and Stumm, 1976). This model allows the metal adsorption to occur in a manner similar to the formation of soluble metal complexes. That is, given a ligand, L n-, in a solution with a cation, M~ +, some portion or all of M~ ÷ will be associated with L nL n- + M~ + ~ M 1L m - n

(1)

If another cation, M~ ÷ is added, some portion of M~ + may be displaced from the ligand MI L m

-n

+ M ~ + _~ M2 Lra

-n

+ M~+

(2)

This exchange reaction is generally rapid and under appropriate conditions the surface ligands can become saturated with either cation. A n equilibrium condition can also be experimentally established and a conditional equilibrium constant derived using titration procedures. A prerequisite for any complexometric titration is the absence of other complexing agents in the system; if competing ligands are present, their effect on the test ion must be known and measured. In the present study, we chose to eliminate the effects of competing ligands by using Na or M g perchlorate media, rather than seawater. Copper was chosen as the tit~ant, over Ca 2+, NI-~4, K + and Na +, because it forms complexes with various organic ligands (Stumm and Brauner, 1975) and can be readily determined

260 with a specific ion electrode. In addition, Cu is a required nutrient for plankton which can become toxic at high concentrations (O'Kelley~ 1974)o Thus, an understanding of the adsorption of Cu on plankton has an important ecological basis. Unfortunately, there are few metals which do not form inorganic complexes at the pH of seawater, including Cu. The formation of soluble Cu hydroxide and carbonate species (Zuehlke and Kester, 1983) were undesirable for these experiments, thus the media was maintained at ~i pH 6. Below this pH, Cu exists as the free metal and competition from other complexing agents in the media are eliminated. We assume that the total capacity of the surfaces to adsorb cations is independent of pH. That is, the shifts in pH do n o t alter the total number of sites (or surface charge) for ion exchange or complexation reactions. Such assumptions are not unwarranted since the zero point of charge for algae is at pH ~ 3 and the surface charge does not appreciably change between pH 6 and 8 (Stumm and Morgan, 1970, p. 455).

Adsorption capacity determinations Phytoplankton and macroalgae All titration procedures were monitored with an Orion 14-20 Cupric specific ion electrode, referenced against a Beckman single-junction calomel electrode in a laboratory maintained at 22 + I°C. The potential was monitored with an Orion 801 digital pH/volt meter and a Beckman SS-2 Expandomatic pH meter. The adsorption capacity was determined by adding a known weight of plant material to a plastic beaker. Deionized water (98.0 ml) was added by buret and 2 ml of 1 N NaC104 pipetted into the beaker to provide a constant ionic strength (0.02) solution. 5.0 ml of Cu titrant (1.0 × 10 -2 M} were then added by pipette. The pH of the titrant was adjusted so that the final solution pH was ~ 5.9. The volume of titrant was experimentally found to be sufficient to completely saturate the adsorption sites of the sample material. The solution was stirred overnight with a teflon-coated magnetic stirring bar. The sample solution was insulated from the heat generated by the stirring unit. The following morning, a standard titration curve was established to check the electrode response. A Cu 2+ saturated sample was placed under the electrodes and, if necessary, pH adjusted to 6.0 with microliter amounts of dilute NaOH. The Cu 2+ response and pH were recorded for this addition. Three or more additional increments of titrant were then added by buret. The mV and pH resulting from each addition were recorded after a stable reading was obtained. Stability was determined when two readings taken at one minute intervals agreed within -+ 0.1 mV. The potential determined for each addition was then plotted on Gran paper (Anonymous, 1970} and the volume intercept determined. The adsorption capacity, in m e q g -~ dry weight was calculated from the Gran plot intercept and sample weight.

261

TABLE II Reproducibility of adsorption capacity method Sample

Adsorption capacity (meq g - l )

Coefficient of variation (%)

n

Chondrus crispus Ascophyllum nodosum Zostera marina Ulva lactuca

0.33 0.95 1.56 1.53

4.5 3.2 7.7 5.2 0.6--14%

4 3 3 7 12

Phytoplankton a

+ 0.014 + 0.03 -+ 0.12 + 0.08

a Coefficient of variation on twelve samples analysed in duplicate.

Adsorption of Cu on the beaker walls was tested by carrying five beakers, without plant tissue, through the saturation procedure. The Gran plot intercepts all test beakers were the same as a standard curve prior to the test samples.

Suspended matter samples The adsorption capacity of suspended matter was determined by procedures analogous to the plant studies. However, because sample weights were low (5--20mg), the titrant concentration was adjusted to I x 10 -a M Cu 2+. A preweighed sample was placed in 100ml of 0.02 M NaC104 and 5 ml of 10 -3 M Cu 2+ added. Samples were allowed to equilibriate for two hours. Sample titration was then completed as above and the adsorption capacity determined. The glass-fiber filters used to collect the suspended matter were tested for possible complexing of copper by carrying blank filters through the full titration procedures. The Gran plot intercepts were the same as those for standard curves.

High ionic strength media The influence of a high ionic strength medium (seawater) on the adsorption capacity was determined by replacing the NaC104 solution with a 1.1 x 10-1 N Mg (C104)2 solution. Titration procedures were identical to those above.

Reproducibility of the adsorption capacity determination The reproducibility of the plant adsorption capacity method was tested by analyzing subsamples of several macroalgae samples from three to seven times (Table II). The Ulva lactuca sample was initially used to develop the titration procedure. Seven subsamples were analyzed during the course of method development. Control of pH, concentration of titrant and saturation techniques varied with each analysis. In spite of these different treatments, the total adsorption capacity of this sample had a coefficient of variation of 5.2% (1.53 + 0.08 meq g-1, n = 7). In addition, two subsamples were analyzed

262 two months after the procedure was developed. The adsorption capacity of these two samples fell within the standard deviation previously determined. All other samples were analyzed in duplicate. The average coefficien~ of variation for the individual p h y t o p l a n k t o n samples ranged from 0.6 1o 14~;. Laminaria samples had high coefficients of variation ranging from 0.7 to 327~ {average 9.3 -* 10.5%, n :: 7). The precision of the suspended matter titration was checked by analyzing replicate samples taken from the shore at Avery Point. These samples had a 15% coefficient of variation in adsorption capacity. It was found that frequent polishing of the Cu 2÷ ion specific electrode was necessary in order to reduce electrode instability and increase reproducibility of the response. These effects were particularly n o t e w o r t h y for Laminaria, fresh Ascophyllum and Codium.

Equilibrium studies The exchangeability of the adsorbed Cu was also determined and the results used to derive an equilibrium constant. Two procedures were employed in this determination. Samples which were saturated with copper in the 0.02 N NaClO4 solution were removed from the saturating media by filtration (0.2 gm). The samples were rinsed with 2--3 ml of deionized water to remove excess solution copper. The filter and plant material were then transferred to a 0.11 N Mg(ClO4)2 solution and stirred for 3 h. Copper desorbed from the sample was determined with the Cu-specific electrode. Copper concentrations were estimated from a standard curve prepared in the Mg(C104 )2 solution. Cu standards in the 10-6--10 -s M range were corrected for a background Cu concentration of 2.0 -+ 0.2 × 10 -6 M. Standards were prepared daily. The coefficient of variation of the standard curve, expressed as a percentage, was 3.3 at 20 × 10 .6 M Cu and 1.1 at 1 x 10 .4 M Cu. Duplicate analysis of 10 samples gave a relative standard deviation of 8.3 -+ 5.1% for Cu released from the plant material. These results were used to estimate equilibrium constants. The equilibrium constants obtained by this m e t h o d were checked with batch experiments. In these experiments 1 0 0 m l of a solution containing 5.45 x 10 -2 M Mg2+ and 1.0 x 10 -4 MCu 2÷ at pH 5.5 were pipetted onto known weights of dry plant material. The samples were stirred and allowed to equilibrate overnight (15h). The following morning the free Cu 2÷ concentration and pH of the solution were measured. Free Cu was determined against calibration curves made in 5.45 x 10 -2 M Mg(C104 )2 at pH 5. The pH electrode was calibrated at 7.00 and 4.01 periodically during the analysis. Effects of pH on the equilibrium were examined by adjusting the pH of the solution to ~ 4.5, allowing 3 h for reequilibration and redetermining the Cu 2+ concentration. The pH was then lowered to ~" 3.5 and the procedure repeated. The equilibrium between dissolved and adsorbed Cu was treated as follows. The surface of the plant samples are primarily composed of cellulose or in

263 the case o f diatoms as silica casings or silica coated with a membrane (Vargo, personal communication, 1979). These materials, similar to the 7-A12Oa studied by Hohl and Stumm (1976}, have a large n u m b e r of hydroxide groups available for complexing cations (Mackie and Preston, 1974). In addition, there are m a n y other cellular components which contain N, S, O and R--COOH groups, all of which can attract cations (Haug and Smidsrod, 1967; Mackie and Preston, 1974). The complex nature of the plant material makes the choice of equations to describe the equilibrium between Mg2+ and Cu 2+ difficult. It does not seem unreasonable, however, to model the adsorption of Mg 2÷ and Cu 2÷ on the plant material as a results of interactions with the --OH group found in the cellulose and other cellular components, in the manner described by Hohl and Stumm (1976). The use of the OH group does not imply that this is the only ligand that can cause adsorption. The equilibrium constants derived from the equations become conditional equilibrium constants, which are useful, even though not thermodynamically exact. The following reactions, analogous to those described by Hohl and S t u m m {1976), can be applied to the plant samples of this study =ROH + Cu 2+ ~ ~-ROCu+ + H +

(3)

K cu = {=ROCu +} (H+)/{-ROH} (Cu 2+)

(4)

- R O H + Mg2+ ~ -=ROMg+ + H +

(5)

K Mg = {=ROMg+} (H+)/{-ROH} (Mg2+)

(6)

where {} indicate moles of cation per kg of solid and () indicate the molarity of the element. Subtracting eq. 5 from eq. 3 with a rearrangement of terms, gives the following equilibrium condition for the Cu release study - R O M g + + Cu 2+ ¢- R O C u + + M g 2+

(7)

Ka = { - R O C u +} (Mg2+)/{-ROMg +} (Cu 2+)

(8)

The equilibrium condition in this system should be independent of p H as long as the capacity for adsorbing cations is independent of pH. The terms in this equation were evaluated as follows. The M g 2+ concentration in solution was decreased < 0.4% by adsorption onto the plant material. Thus, the concentration of M g 2+ was unchanged from bulk concentration. The Cu 2+ was determined by the cupric specific electrode as above and the concentration of Cu 2+ and M g 2+ on the solid samples were determined as follows. The total moles of copper on a sample were calculated using the adsorption capacity experimental data. The moles of copper released from the sample were then estimated from the concentration of free copper in the solution at equilibrium and the volume of solution. This is equal to the moles of M g 2+ which were adsorbed. The difference between

264 m

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Fig. 3. Range in the adsorption capacity of marine macrophytes, phytoplankton, zooplankton and suspended matter. Bleached plants were identifiable species which had lost all color. the adsorption capacity and the copper released from the samples gives the moles of copper remaining on the solid. These values were converted to a mol g- 1 basis and applied to the calculation o f the distribution constant. The equilibrium constants in the batch studies were determined in a similar manner. The solution Mg2+ concentration was considered unchanged and the Cu 2 ÷ concentration was determined from standard curves. The total a m o u n t of Cu 2 ÷ adsorbed was determined from the difference of the total moles of copper initially in solution and t h a t remaining in solution a t equilibrium. The moles of Mg2+ adsorbed were determined from the difference of the total copper adsorption capacity and the moles of Cu 2÷ adsorbed by the sample. The moles of adsorbed Cu 2+ and Mg 2÷ were divided by the sample weight to give the moles o f metal g- 1 sample.

RESULTS

Adsorption capacity The adsorption capacity determined for p h y t o p l a n k t o n , zooplankton, suspended matter and a variety o f macroatgae range from "~ 0.1 to 2.1 meq g - l d r y weight (Fig. 3). Generally, macroalgae and zooplankton have the highest adsorption capacities while p h y t o p i a n k t o n and suspended particulate matter have relatively l o w adsorption capacities. The adsorption capacity of phytoplankt4~n is relatively c o n s ~ t and

265 TABLE III Range in adsorption capacity for macrophytes and suspended particulate matter from

Long Island Sound. Bleached samples were identifiable macrophytes which had lost all pigmentation Plant

Ulva lactuca Chaetomorphia linum Codium fragile Chondrus crispus Rodymenia palmata Champia parvula Laminaeria sp. AscophyUum nodosum Fucus vesiculosus Spartina altinaflora Spartina patens Zostera marina

Suspended matter

Live

Bleached

(meqg -1 dry wt)

(meq g- 1 dry wt)

0.49--1.53 0.39 0.06 0.30--0.64 0.43 0.32--0.34 0.59--1.37 0.95--2.0 0.92--1.30 0.20--0.25 0.41 0.67--1.56 0.11--0.56

0.35 a 0.01 0.30 a 0.32 0.50 a 0.38 0.25--0.28 0.18--0.26 a

a No samples obtained.

independent of season, location and species composition. The average adsorption capacity for p h y t o p l a n k t o n obtained during the winter throughout Long Island Sound (P-1 to P-9) is 0 . 2 8 + 0 . 0 9 m e q g -1 dry weight (n = 9). Samples collected in August, taken over a 40 km stretch of western Long Island Sound (P-11 to P-15), had an average capacity of 0.23 + 0.08 meq g- l dry weight. Samples o f zooplankton from Long Island Sound (P-16 to P-18) and the open ocean had adsorption capacities averaging 1.65 + 0 . 4 7 m e q g -1 dry weight. No trends between open ocean, continental shelf and coastal populations were evident. No attempts were made to relate variability in the adsorption capacity of the zooplankton to species or size. Phytoplankton samples with high adsorption capacities frequently had small numbers of zooplankton associated with them. The average adsorption capacity for all p h y t o p l a n k t o n samples from Long Island Sound, b u t excluding those containing zooplankton, is 0.22 -+ 0.05 meq g - : dry weight. A wide range in adsorption capacities for macrophytes and vascular plants was found (Table III). Variability in the Cu adsorption capacity exists between the major divisions, individual plant species within a division and individual plants of the same species (Hunt, 1979). The variability observed within a particular algae species is n o t entirely unexpected, as other investigators have seen variations in the rate and the total a m o u n t of cations taken up during various stages of growth. Strontium uptake in

266

T A B L E IV Comparison of plant adsorption capacity in 0 . 0 2 N N a C I O 4 and 0 . 1 1 N M g ( C 1 0 4 ) 2 ; samples labeled with a P are p l a n k t o n samples, those labeled with an M or MB are macrophytes Sample

A d s o r p t i o n capacity in NaC104 (meq g- )

A d s o r p t i o n capacity in Mg(C104 )2 (meq g- l )

P-4 P-6 P-7 P-11 P-18 M-3 M-2 M-11 M-6 MB-7 M-14 M-12

0.34 + 0.04 0.27 + 0.07 0.09 0.35 -+ 0.01 1.40 1.54 -+ 0.08 0.06 1.12 -+ 0.22 0.95 -+ 0.03 0.28 0.18 1.56 + 0.11

0.36 -+ 0.09 0.20 -+ 0.03 0.33 0.35 -+ 0.01 1.42 1.10 0.0 0.56 0.43 -+ 0.17 0.19 0.14 0.69

Carteria is influenced by the rate of cell division, culture age and initial pH of the culture medium (Rice, 1965). Bryan (1969) found Zn uptake by Laminaria was influenced by the rate of growth. Ascophyllum, Fucus, Ulva, Chondrus and Laminaria have also shown high variability in adsorption capacities between individual plants, which may relate to the state of plant growth at the time of the collection. The data presented here, while equivocal, may best be explained as a result of variability in individual plants within a species. A limited number of marsh plants plus Zostera marina were also examined for adsorption capacity (Table III). Spartina altinaflora had low, but constant adsorption capacities of 0.25 + 0.03 meq g- 1 (dry weight). All but one of these plants had completed its seasonal growth. Spartina patens also had low adsorption capacities, 0.2--0.4 meqg -1 . Zostera marina appears to have a high, but variable adsorption capacity. The range in the cation adsorption capacity of suspended detritus is also shown in Table III. The adsorption capacity for these samples ranges from 0.11 to 0.56 meq g- l, average 0.27 + 0.10 meq g- 1. The adsorption capacity appears to be constant and nearly equal to that of the phytoplankton. Ionic strength effects The copper adsorption capacity for several plant samples determined in dilute Na and concentrated Mg solutions shows little difference in total adsorption capacity (Table IV). The titrations done in Mg resulted in more stable electrode potentials with the results of this test showing the total

267 adsorption capacity of the phytoplankton to be unchanged in the presence of high cation concentrations competing for adsorption sites. The macrophyte samples appear to have lower adsorption capacities in the presence of Mg.

Aged macrophyte plants Eventual transport of particulate carbon derived from macroalgae to the sediments will not necessarily be in the form of the original plant. Certainly degradation of this material will occur. Thus, it seems unreasonable to assume that the adsorption capacity of the original plants and detritus formed from them will be the same. Some assessment of the change in adsorption capacity between a plant and its detrital stage is necessary if the effects of metal adsorption onto plants, in relation to metal deposition, is to be properly evaluated. Changes in adsorption capacity as plant material degrades was examined by determining the adsorption capacity of aged, or partially degraded plant samples. The aged macrophyte samples collected from beaches in a bleached, but identifiable condition generally had lower exchange capacities than fresh samples of the same species (Table III), tending towards an adsorption capacity of 0.3 meq g-~ dry weight. Laminaria appears to be an exception, although this sample was not as bleached as the others were and may not have had time to reach the same state of decay. The vascular plants and red algae do not appear to lose adsorption capacity as they degrade. The retention of at least some adsorption capacity as the plants degrade is significant. Retention of adsorption ability means that a plant, on death, may not exchange all of the metal ions it has adsorbed during its life. It also means that additional metal adsorption may occur on a plant even after death, which may continue to effect particulate metal chemistry. Adsorption of Zn 6s (Gutknecht, 1961a, 1965) and Sc 4s (Gutknecht, 1961b) on killed macrophytes suggests adsorption on thekilled plants may be greater than that adsorbed on Iiving organisms. Williams (1960) has :reported adsorption of Cs 137 by dead Euglena and Chlorella cells. Thus, it is not unexpected that organisms retain the ability to adsorb cations in the detrital phase.

Comparison to sedimentary organic carbon A comparison of the adsorption capacity (cation exchange capacity) of sedimentary organic carbon to source carbon was also undertaken. In order to facilitate the comparison, the results had to be normalized to organic carbon (Table V). The adsorption capacity of suspended matter ranges from 0.7 to 3.5 meq g- 1 Corg, average 1.8 -+ 0.4 meq g- 1 Corg, while that of phytoplankton ranges from 1.1 to 3 . 7 m e q g -1 Corg and averages 2.1 + 0.7 meq g-l Cong. The adsorption capacity of the macrophyte samples, on a Corg basis, is much larger (0.3--11.1 meq g-1 Co,g) than that of the plankton. Sedimentary organic matter from Long Island Sound has an adsorption capacity of ~ 6.4 meqg -1 Co~g (Hunt, 1981). Thus sedimentary organic matter

268 TABLE V C o m p a r i s o n of t h e average a d s o r p t i o n c a p a c i t y in t h e m a j o r p l a n t groups a n d s u s p e n d e d m a t t e r o n a w e i g h t basis a n d per u n i t organic c a r b o n Plant type

T o t a l w e i g h t basis ( m e q g- 1 )

Organic c a r b o n basis ( m e q g- 1 Corg }

Phytoplankton, n = 9 Zooplankton, n = 4 Macrophytes, normal , bleached S u s p e n d e d m a t t e r , n = 15

0.22 1.5 0.79 0.34 0.26

2.1 4.9 2.8 1.2 1.8

+ 0.05 -+ 0.5 -+ 0.59 + 0.04 + 0.11

+ + + + +

0.7 1.2 2.5 0.13 0.4

has a higher capacity to adsorb cations than source organic material, probably reflecting diagenic alterations within the sediments.

Equilibrium constants The experiments designed to determine the exchangeability of Cu showed that 20--50% of the Cu adsorbed to the phytoplankton was displaced in the presence of 5.4 × 10-2MMg 2+ (Table VI). Release of copper from the macrophyte samples ranged from 20 to 100% of that initially adsorbed (Table VII). Ulva samples released from 40 to 98% of the adsorbed copper; Fucus, from 34 to 91% and Ascophyllum from 43 to 100%. The amount of copper released by the macrophytes varied between samples of the same species, but was generally within 10% for replicate analysis on an individual sample. Estimates of t h e equilibrium constant, reported as log K, ranged from 3.2 to 4.0 (average 3.6 + 0.3) for the phytoplankton (Table VI). The macrophytes display substantial differences (range 0.9--4.2; average 2.8 + 0.8 as log K) in the conditional equilibrium constants (Table VII). Thus, the macrophyte samples, which have generally higher adsorption capacities than the phytoplankton, apparently have less ability to hold the Cu in the presence of competition from other cations. The differences in the conditional equilibrium constants for various macrophyte species may be associated with the types of cellulose and cellular compounds found between plant species. The higher conditional equilibrium constants for the plankton are surprising, but may result from adsorption onto the silica casings of the diatoms. Myers et al., (1975), do not see large variations in the electrophoretic mobility (surface change} of three estuarine plankton specms having different cell wall compositions. The results axe consistent with the total Cu 2÷ adsorption seen in this study, in that no change in total exchange capacity is apparent even with large changes in species composition (Table I).

269 TABLE VI Copper retained by plankton in 1.1 × 10-1N Mg(C104)2 and the conditional equilibrium constants derived from the release of Cu Sample

Copper retained (%)

Log K

P-1 P-2 P-3 P-4 P-5 P-6 P-8 P-9 P-11 P-12 P-13 P-14 P-15 P-16 P-17 P-18

48.5±12.0 80.5±6.4 73.5±5.0 76.0±7.0 58.5±12.0 47.2 77.5±3.5 72.0±2.8 87.5±1.4 56.5±7.8 76 63.5±12.0 78.8±1.8 85 94 96

3.2±0.1 4.0±0.1 3.4±0.4 3.7±0.1 3.4±0.3 3.2 3.8±0.2 3.7±0.1 4.3±0.1 3.3±0.1 4.0 3.6±0.3 4.0±0.2 ----

63.8 -+ 11.9 (n = 9)

3.6 -+ 0.3 (n = 9)

Average Dr pure phytoplankton samples

Average for zooplankton samples 92 -+ 6 (n = 3)

Selected p h y t o p l a n k t o n and m a c r o p h y t e samples were tested, using the batch mo d e, f o r comparison with the previous results, Table VIII. T he mean conditional equilibrium cons t a nt f or t he m a c r o p h y t e samples increased f r o m 2.8 to 3.5 between pH 4.3 and 5.1 b u t did n o t increase significantly f r o m pH 5.1 to 6.0. Conditional equilibrium constants det erm i ned for these same m a c r o p h y t e samples at pH ~ 6, show t he mean equilibrium c o n s t a n t in the batch m e t h o d to be 3.4 + 0.4. This is an increase of 1.0 log units from the mean log K d eter m i ned in t he filtration technique. T he variability bet w een species is also less in the batch analysis. Th e p h y t o p l a n k t o n do n o t show a clear increase in the conditional equilibrium constant as t h e pH is raised f r om 4.3 t o 5.1. T he m ean log K at pH 6 is slightly higher than at pH 4 and 5 (Table VIII). F r o m t h e r m o d y n a m i c considerations, it does n o t seem probable t h a t the observed increase in log K at pH 6 is due to Cu 2+ complexing by CO~-. T he reasons for the increase in th e log K at pH are n o t identifiable f r o m this work. F u r t h e r experiments

270 T A B L E VII C o p p e r r e t a i n e d b y m a c r o p h y t e samples in 1.1 x ] 0 - t N Mg(ClO4)2 and the c o n d i t i o n a l e q u i l i b r i u m c o n s t a n t derived f r o m the release o f Cu Species

Sample designation

Copper r e t a i n e d (%)

Log K

C h o n d r u s c. Ulva I.

M-I M-3 MB-5 M-37 M-11 M-4 M-35 DS-4-1 DS-4-3 M-5 DS-I-1 M-39a M-12 DS-3-1 DS-3-2 M-6 DS-2-1 DS-2-2 M-8 M-13 M-39c M-7 MB-2 MB-3 M-44

5.3 6.0 2.5 58.0 59 51 56 32 62 65.8 67 0.0 33.5 67 63 0.0 57 50 55 80 67 46 74 53 76

1.0 3.0 0.9 3.3 2.9 3.9 2.8 2.] 3. l 3.0 2,8 2.2 2.8 2,9 .... 2.8 2.0 3.0 4.2 3.3 2.6 3.4 2.9 3.8

Laminaria sp.

F u c u s v.

Zostera m.

A s c o p h y l l u m n.

C h a e t o m o r p h i a I. Spar±±nap. Champiap. S p a r t i n a a. R o t a m e n i a p.

± 1.6 ± 4.2

± 14 ± 9 ± 17

-+ 2.8

-+ 6.5

± 6 -+ 8 -+ 7

Average

± 0.2

-+ 0.3 ± 1.4 -+ 1.4

+ 0.1

-+ 0.2

+ 0.1 ± 0.6 ± 0.1

2~8 +- 0.8

T A B L E VIII Comparison of conditional equilibrium constants from batch equilibrium studies under variable pH Plant type

Phytoplankton Zooplankton Macrophytes

n

9 2 11

Log K pH 4.3 + 0.2

pH 5.1 + 0.05

pH 6.0 -+ 0.1

3.8 + 0.5 3.0 + 0.3 2.8 + 0.5

3.6 + 0.03 3.6 -+ 0.4 3.5 -+ 0.6

4.3 + 0.3 4.2 + 0.2 3.4 -+ 0.4

271 are required to determine the conditional equilibrium constant between the substrate and H + in solution for precise understanding of the effects of pH on the Cu adsorption conditional equilibrium constant. The conditional equilibrium constant for the macrophyte samples determined by batch analysis is less variable than that found in the filtration technique. These constants also compare more favorably with the results from the phytoplankton analysis than those done by the filtration techniques. From these experiments, it is possible to generalize the competition between the free Mg2+ and Cu 2÷ in seawater, for adsorption sites on organic detritus, with a conditional equilibrium constant based in surface complexation reactions. A reasonable value for log K is 3.7 + 0.4 in 5.5 x 10 -2 M Mg(C104 )2 at pH 6. The combined effect of Na and Mg on the adsorption of Cu on these substrates was examined in batch studies in which both Mg and Na perchlorates were combined at seawater concentrations. The total Cu 2÷ adsorbed per gram of sample in this study is shown in Table IX. Generally plankton adsorbed more Cu in the Na plus Mg solution than in Mg alone, while the macrophytes adsorbed less Cu. Experimental error may contribute up to 15% of the observed results. The causes for increased adsorption of Cu by the phytoplankton are not clear.

DISCUSSION

Comparison of the exchange capacity of several solid phases found in seawater with adsorption capacities determined for the plant species, reveals that the adsorption capacity for all these solids does not vary greatly, even though the substrates have different chemical compositions (Table X). The clay minerals, montmorillonite and vermicullite, have consistently higher adsorption capacities than the phytoplankton, but are comparable to the green and brown algae. The other major clay minerals have equal or lower adsorption capacities than the phytoplankton and macrophytes. Silica gel and MnO: commonly used as model solid substrates have adsorption capacities from three to five times greater than the phytoplankton used in the present study. These substrates generally have adsorption capacities comparable with macrophyte samples. Alon (A12O3 ) has an adsorption capacity similar to the phytoplankton. Iron and manganese oxides are generally considered to be primary solid phases responsible for heavy metal adsorption in natural aqueous systems (Jenne, 1968). The adsorption of Co, Zn, Ca and Na on MnO2 does not have a single value, but has a range equal to or greater than the macrophyte samples and from 4 to 6 times that of the phytoplankton. Boyle et al. (1977) reported that Fe-organic colloids formed during fresh water--seawater mixing have exchange capacities of about 0 . 5 m e q g -1 . Open ocean particulate matter (1--53/~m; 90% biogenic) is reported to have

--0.2 --0,2 0.0 0.0 --0.3 +0.1 +0.2 --0.2 3- 0.3 0.0 +0.2 + 0.2 + 0.1 +0.5

5.9. 5.8 5.7

ApH a

5.9 5,8 6.0 5.8 5.8 5.8 5.8 6.1 6.1 5.8 5.8

pH of Na + Mg solution

0.7 2.3 1.7

1.2 1.2 1.4 1.0 1.0 2.6 3.1 2.2 2.0 1.4 0.7

(Mg + Na)

1.3 2.3 2.2

1.0 1.0 1.2 0.7 0.95 1.8 2.9 2.1 2.3 1.4 2.6

(Mg)

Cu adsorbed by sample (10 -4 tool Cu g-I sample)

0 0 0

--22 --11 --14 --31 --2 --15 + 3 +2 3- 20 +4 +9

Xb (mg)

a The difference in pH between the (Mg + Na) and (Mg) solution. b Change in sample weight between experiments in ( W M g + N a - - WMg ) z X. c Difference in Cu adsorbed per unit weight in the two media expressed as percent change from the Mg solution alone.

Plankton P-4 P-9 P-11 P-13 P-15 P-17 Ulva lactuca Ascophyllum nodosum Zostera marina Spartina p a t e n s Chondrus crispus Spartina altinaflora Fueus vesiculosus ZOstera marina (aged)

Sample

- 45 0 + 23

+20 +20 +17 +48 +5 +44 3- 7 3- 5 - 13 0 76

(%)

Change in Cu adsorption c

Comparison of the Cu 2+ adsorbed by plant materials in 0.055 M Mg(C104 )2 and in 0.45 M NaClO4 plus 0.055 M Mg(ClO4 )2 at pH 5.8

T A B L E IX

Soil, lake, and marine humic acids Soil humic acid Soil humic acid Lake humic acid Soil organic matter Marine sedimentary humic acid

Fe(OH)3

0.34--2.83, average 1.9 0.62--2.8, average 1.3 2.5 1.5 --5.0 2.5 --3.75

0.80 0.25--5.4 0.25 0.9 - - 1.4 mmol g- 1 is cation dependent Comparable to clay minerals

0.53 2.3 --5.0 5.0 --10.0

Heavy minerals and rock Quartz (<: 2 / a n ) Zeolites Feldspathoids

Other inorganic materials Silica gel Diatomaceous earth 8- Al2 03 MnO2

0.70--1.0 0.03--0.15 0.1 --0.4 0.1 --0.4 0.11--0.20 0.2 --0.3 1.0 --1.5 0.05--0.10 0.4 --0.5

A d s o r p t i o n capacity (meq g- l )

Clay minerals Montmorillonite Kaolinite Illite Chlorite Glauonite Attaputget Vermicullite Halloysite ( 2 H 2 O) I-Ialloysite (4H 2 O)

Solid phase

Adsorption capacities (cation exchange capacity) o f minerals, soil organic matter and humics

TABLE X

(1969) (1969) (1969) (1969) (1969) (1969) (1969) (1969) (1969)

Olson and Bray (1938) Kamprath and Welch (1962) Reimer and Toth (1970) Carroll (1969) Rashid (1969)

S t u m m and Morgan (1970)

Carroll (1969) Carroll (1969) Hohl and S t u m m (1976) Loganathan and Burau ( 1973 )

Carroll (1969) Carroll (1969) Carroll (1969)

Carroll Carroll Carroll Carroll Carroll Carroll Carroll Carroll Carroll

Reference

O~

t,O

Particulate matter Open ocean F o r m e d during river seawater mixing (Fe--organic colloids) _. Particulate matter of Long Island Sound

Peat (England)

Terrestrialplants Spagna

Marine organisms Phytoplankton Zooplankton Green algae Red algae Brown algae

Solid phase

T A B L E X (cont.)

study study study study study

Bishop et al. (1977) Boyle et al (1977) This study

0.47--0.54 0.15--0.5 meq g - 1

Clymo (1963) Calculated from Ong and Swanson (1966)

This This This This This

Reference

0.71--0.96 (90% biogenic)

1.2 0.32

0.22±0.05 1.7 ± 0 , 5 0.5--1.5 0.4 ± 0 . 1 1.0 ± 0 . 2

Adsorption capacity (meq g- l )

275 exchange capacities of 0.71~-0.96meqg -1 (Bishop et al., 1977). This is 2--3 times greater than found for plankton and suspended matter in the present study, but agrees well with the reported capacity of silica gel and the macrophytes. The majority of the particulates analyzed by Bishop et al. (1977) were diatom casings and silica flagellates in the 1--53 pm size fraction. The agreement between the adsorption capacities of the silica gel and biogenic silica is not unexpected. From this comparison, it is evident that in aquatic systems with high phytoplankton and macrophyte production, the potential ability of these organisms and their detritus to provide a surface for the adsorption of soluble metals and ultimate removal from the water column by a variety of mechanisms must be considered equally with other major phases, notably clay minerals and the oxides of Fe and Mn. In the nearshore environment, the contribution of mineral phases to the total suspended matter can be significantly greater than in the open ocean (Wallace, 1976). Yet, even with the higher mineral content of nearshore waters, the suspended load can be dominated by organic material. This is true of Long Island Sound (Riley, 1970; Bohlen, personal communication, 1977). The generally high productivity of nearshore waters (Smayda, 1974), acts to maintain a high level of biologically derived particles in the suspended matter. In those systems, in which particulate matter is dominated by biologically produced solids, the biologically derived solids may become a primary phase responsible for trace metal deposition (Calvert and Price, 1970; Bostrom et al., 1973; Hunt, 1979). Thus, the importance of biologically derived particulates in metal deposition cannot be neglected even in the nearshore. The adsorption capacity of the various solid phases found in seawater and the relative concentration of each, indicates that the organic phases must be considered equally with the inorganic phases in order to properly define the system and evaluate geochemical processes. Bishop et al. (1977) have indicated the importance of major cation transport by biologically derived particulate matter in the upper 60 m of the ocean. The relatively shallow depth (~ 100 m) and high productivity of the nearshore may serve to accent the biological role in trace metal transport and deposition in the nearshore, in spite of inorganic inputs from terrestrial runoff and atmospheric fallout. Table X also compares the adsorption capacity of the plant species with the terrestrial plants and organic matter found in soils and sediments. Generally, the adsorption capacity of soil and sedimentary matter is 2--20 times greater than that of the plant material. The large increase in adsorption capacity in sedimentary organic matter over the source plants is not unexpected. After plants are deposited they are degraded by microbial and chemical processes to form fulvic and humic acids c o m m o n to all softs and sediments (Schnitzer and Kahn, 1972). These compounds, which have large numbers of associated ligands, have the potential to bind larger amounts of cation than the original plant matter (Rashid, 1969). An immediate result

276 of this increased potential for binding cations by sedimentary organic matter is a greater capacity for retaining metals in sediments with high organic carbon content (Hunt, 1981). The importance of sediment organic matter to the present discussion lies with the source of the organic carbon in the sediments. Transfer of organic carbon to sediments can result by direct incorporation of plants, fecal pellets, particulate matter from sewage outfalls and possibly precipitation of soluble organic matter. Generally, high sedimentary organic carbon concentrations indicate that the waters overlying the sediment are highly productive, except where man has added organic materials from sources such as sewage. If metals are taken up from the water column during the growth cycle of the plants and remain with the solid phase after the organism expires, then the metals may reach the sediment in association with the plant, as long as total degradation o f the organisms does not occur prior to deposition. The capacity of plants to adsorb cations, as determined in this study, suggests that the surface adsorption process may be an important mechanism for removing metals from the water. Evaluation of the importance of plant adsorption capacity in metal deposition is essential if the process is to be put into proper perspective. Quantification of the importance o f macrophytes would require data on growth and detachment rates, in addition to metal content, and spatial distributions. Such information is not available for Long Island Sound and is b e y o n d the s c o p e o f this paper. The discussion which follows considers the importance of the p h y t o p l a n k t o n alone to the geochemical cycle of Cu. Stumm and Brauner (1975) have modeled major and minor element speciation under seawater conditions. From their work, it is evident that any extension of the adsorption equilibrium determined for divalent metal ions under natural conditions, must consider the speciation of that element in seawater. For instance, the equilibrium constants determined in the present adsorption study are based on the premise that total Cu (CUT) is equal to free copper (Cu~+). In seawater, at pH 8.0 Cu~+ is calculated to be 1.4% of the total soluble copper concentration at pCu w - - 8 . 0 (Zuehlke and Kester, 1983). Thus very little free copper is available for adsorption reactions. If Cu 2+ adsorption onto p h y t o p l a n k t o n is significant in seawater, the equilibrium constant must be large. The conditional equilibrium constant derived in the present investigation does not appear to be sufficiently high, in the face of competition from Mg 2+, to permit significant adsorption to occur. If the adsorption constant of species such as CuOH ÷, CuCO3 or Organically complexed copper is substantially greater, additional copper could be adsorbed. The effect of metal speciation considerations on Cu 2 ÷ adsorption is most dramatically shown by the following calculations. From the equilibrium conditions previously established we can write

277 K 3 = [ { - ROCu +} (Mg2+)]/[{= ROMg +} (Cu2+)] = 103.7

(9)

If typical coastal Mg and Cu concentrations (4.7 x 1 0 - 2 M and 2 x 10-SM respectively) are substituted into the equation, w i t h o u t considering the effects o f speciation and assuming this equilibrium constant is applicable to systems at pH 8, the ratio {-- ROCu+}/{ - ROMg +} is found to be 2.2 x 10 -3 . Thus, only 0.22% of the total adsorption sites will be occupied b y copper. If the same calculation is made with the more appropriate concentrations of free Mg 2+ (4.2 x 10-2M) and Cu 2+ (2.8 x 10-1°M), the ratio of Cu 2+ to Mg 2+ on the solid decreases to 3.3 x 10 -s or 67 times less. Thus, only 0.003% of the adsorption sites m a y be occupied b y Cu. The adsorption constant is here assumed to result from a single ligand type. Ligands on the surface of the p h y t o p l a n k t o n may, in fact, be heterogeneous in nature, with each ligand having a different conditional equilibrium constant. The relative number of each ligand t y p e and the associated equilibrium constant should be considered in order to properly define the o p t i m u m equilibrium constant. This becomes particularly important at very low Cu concentrations, where the ligand with the highest equilibrium constant would be expected to dominate adsorption of Cu. If so, the derived conditional equilibrium constant may be low because stronger ligands m a y be dominated b y numerous weaker ligand sites. Thus, the adsorption at low Cu concentrations may in fact be greater than estimated here. The geochemical role of Cu adsorption onto the s u r f a c e o f p h y t o p l a n k t o n based on these results can also be estimated. The net p h y t o p l a n k t o n productivity in Long Island Sound is ~ 205 g C m -2 y - 1. Combining this with the total adsorption capacity gives an annum production of new adsorption sites of 0 . 4 3 e q m -2 y - 1 . If Cu occupies 3.3 x 10 -3 percent of these sites, approximately 1.4 x 10 -s e q C u 2÷ m -2 y - I can be adsorbed b y the phytoplankton. Estimates from Riley (1956) indicate that up to 57% of this productivity may reach the sediments annually. Thus, ~ 4 x 10 -6 mol Cu m -2 y-1 may be removed from the water column b y adsorption and deposition of phytoplankton. This potential removal compares to the standing crop of Cu in a 20 m 3 water column of ~ 3.2 x 10 -4 mol, based on a typical estuarine dissolved Cu concentration of 16 nM (Hunt and Smith, 1982). Thus, a small percentage ( ~ 1%) of the standing crop of Cu may be removed b y adsorption alone. Estimates for Cu removal from a 20 m 3 water column, using the total Cu c o n t e n t of the p h y t o p l a n k t o n samples studied here, is ~ 1.5 x 10 -4 M Cu m -2 y-1 (Hunt, 1979). Obviously, Cu deposition based on the measured Cu content of p h y t o p l a n k t o n is greater than if only adsorption was occurring. Thus, Cu m a y be incorporated within the plankton by processes other than surface adsorption or extension of the equilibrium constant derived at pH 6 underestimates Cu adsorption on these surfaces at pH 8. Further studies are necessary to examine the effect of pH on the adsorption of Cu 2+ and adsorption constant of other dissolved Cu species.

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