Incorporation of trivalent chromium into riverine and estuarine colloidal material

Gmhimrco PI Cosmochmrca Acln Vol. 48. PP. I1 I7Q Pewmon Preu Ltd. 1984. Printedin U.S.A.

Incorporation

0016-7037/84/C

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of trivalent chromium into riverine and estuarine colloidal material LAWRENCEM. MAYER’,LINDA L. SCHICK'and C. ALLEN CHANG'

‘Program in Oceanography and Department of Geological Sciences, Ira C. Darling Center, University of Maine, Walpole, ME 04573 %epartment of Chemistry, University of Texas, El Paso, TX 79968

(ReceivedAugust 16, 1983; accepted in revisedfirm May 24. 1984) Abstract-The binding of dissolved trivalent chromium by dissolved and colloidal substrates at the riverestuary interface was studied using a combination of product and reactant mode experiments, at concentrations of materials typical of estuarine conditions. Using spikes of I-20 &I C?+, about one third of the Cr’+ was scavenged by that fraction of riverine colloidal material which flocculated upon mixing of river water and seawater. Reactant mode experiments, using chemiluminescence as a speciation technique, showed that virtually all of the spiked Cr” was bound by dissolved or colloidal substrates, but that the higher molecular weight fractions were able to kinetically outcompete the lower molecular weight fractions. There was no effect of salinity or the flocculation process on the binding of Cr by riverine substrates at natural concentrations. However, salinity did produce a strong kinetic inhibition of binding if the river water was first diluted. This salinitv_ reswnse is likely a result of a wide variety of Cr binding .

site energies on the substrates. INTRODUCFION

REACTIVE TRACE metals are known to interact extensively with dissolved and colloidal (e.g. organic and iron oxyhydroxide) substrates in fresh water systems (recently reviewed by NEUBECKER and ALLEN, 1983). It is also well established that a significant portion of the colloidal material present in river waters can flocculate upon mixing with seawater (SHOLKOVITZ, 1976; BOYLEet al., 1977; SHOLKOVITZet al., 1978). This flocculation process can result in the conversion of dissolved, riverine trace metals to a particulate form in the estuarine zone. The incorporation of metals into floes has been demonstrated in product mode (SHOLKOVITZ, 1976) experiments in which a number of trace metals have been found accompanying iron and organic matter floes (SHOLKOVITZ, 1976, 1978; CRANSTON and MURRAY, 1980; SHEN efal., 1983; FEELY et al., 1983). In these experiments, however, it seems likely that the metals found in the floes were associated with the iron or organic colloids prior to the mixing experiment. Scavenging of ionic metals added to estuarine mixtures, in contrast, has not been well studied. Dissolved trivalent chromium levels of as much as several &l, an order of magnitude above background concentrations, have been found in waters at the head of the Saco estuary, Maine, as a result of tannery activities (unpub. data). The purpose of this study is to examine the complexation and adsorption reactions (hereafter represented by the term “binding”) that may incorporate initially “free” C? into the dissolved or colloidal materials present in riverine and estuarine waters. In particular, we have studied the effect of salinity on these binding reactions, both in terms of (1) the influence of salinity on binding kinetics of Cr and (2) the incorporation of Cr into those fractions of riverine material susceptible to flocculation upon mixing with seawater. An important feature of this work is the assessment of these reactions using con-

centrations of the various reacting species that are typically present in the estuary. METHODS

AND MATERIALS

“Product” mode experiments (SHOLKOVITZ, 1976) were carried out to assess the amount of initially free 0” that could be incorporated into flocculant material. These experiments were performed by spiking aliquots of Saco river water with Cti’ (as Cr(NO)j or CrCl,), allowing the water to react for 5 min. mixing the river water with seawater, and agitating the sample to promote flocculation (MAYER, 1982b). The river water and seawater endmembers were

prefiltered through Gelman A/E glass fiber filters (MAYER, 1982a). After agitation, the resulting river water-seawater mixtures were re-filtered through Gelman A/E glass fiber filters, which were extracted with hot, concentrated nitric acid and analyzed for Cr by flame AA spectrophotometry. Control experiments consisted of (1) a Cr spike in distilled water instead of river water, which was then carried through the rest of the procedure, and (2) a river water-seawater mixture without Cr spike. “Reactant” mode experiments (SHOLKOVITZ, 1976) were carried out by determining the loss of free C?’ spiked into a sample of river or estuarine water. Analysis for free Cr’+ was performed using a modified version of the chemiluminescence (CL) method of analysis for trivalent Cr (SEITZ er al., 1972; CHANG et al., 1980). This technique provides speciation information analogous to that provided by specific ion electrodes, in that only free, aquated Cr and hydroxy complexes of Cr are detected. Most experiments were performed with river water collected from the Sheepscot River, which drains a watershed near and similar to the Saco River. Both rivers drain noncarbonate catchments and have similar bulk water chemistry (conductivity-35-80 PSiemens; pH-6.4-7.0). Samples were collected in acid-cleaned plastic bottles and filtered through 0.2 pm Nuclepore filters. To examine for the effect of molecular weight on complexation. some experiments were performed with samples that were additionally pressurefiltered through prewashed and soaked Amicon XM-50 ultrafilters.The pH was adjusted to desired levels with HCI or KOH and samples buffered with a KC&orate solution. Cr(NO& spikes were added to the river water and the reaction was then tracked with CL. Binding experiments were also carried out with other substrates. including solutions

of calcium oxalate 0; a commercially available humic acid (Aldrich).Adsorptionexperiments were conducted with iron

1717

171x

L M. Mayer. L.. L Schtck and C. A. Chang

oxyhydroxides made by adding Hz02 and KOH to an FeCI, solution. These iron oxyhydroxides were stored at pH 8 and room temperature for 10 months. and redispersed before use by means of a sonicator. The effect of salinity on Cr binding was studied by mixing river water, or solutions containing the other substrates, with filtered seawater collected from the Damariscotta estuary. Control experiments to examine wall adsorption, precipitation, and binding by seawater ligands were carried out in borate-buffered distilled water and distilled water-seawater mixtures. The optimized analytical parameten for the CL measurements were similar to those reported in CHANG et al. ( 1980). with the following differences. Sodium bromide and EDTA were added to give inanitions of 0.0267 M and 0.4 M, respectively, in the linal solution entering the spectrolluorometer. The sodium bromide acts to enhance sensitivity and linearize response curves (BAUSE and PATTERSON, 1979; CHANG and PATTERSON,1980). The EDTA serves to complex any other metal ions present in the solution that will catalyze CL (SEW et al.. 1972); this feature is of special importance in experiments involving natural waters because of (I) metal ions already present in solution. and (21 metals displaced from an adsorbed or complexed state by Cr binding. The EDTA solution is in contact with the solution being analyzed for less than a second before undergoing the CL reaction, which prevents any appreciable complexation of the free Cr” being measured owing to the slow kinetics of Cti’ complexation. Samples were infused with a Sage 351 Syringe Pump, along with the luminol reagent (pH = 11.6,using a borate rather than a bicarbonate buffer) and

the EDTA-bromide solution, into a Perkin-Elmer 6%10s spectrofluorometer, set with the excitation lamp off and the emission photodetector at 430 nm. Infusion flows were usually 30-40 mi/min. The analytical precision was i/-9%, as a reiative standard deviation, for fresh water samples at 0.2 rg-Cr/l. RESULTS

AND DISCUSSION

Producr mode experimenrs

Spikes of ionic Cr added to prefiltered river water were consistently found to be incorporated into the floes resulting from the mixing of river water and seawater. The extent of this incorporation can be gauged from an experiment in which varying levels of Cr, from 6 to 20 &I, were added to river water which was then mixed with an equal volume of

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FIG. 2. Percent incorporation ot an 8 pg-Or/l sprhc rntir llocs resulting from mixing Saco River water and seawater to a salinity of 16.1 ppt, as a function of incubation time for the Cr spike in river water before mixing with seawater. Mixture agitated for 0.5 hr.

seawater. Figure 1 shows that 30-42s of the sptkeo Cr was incorporated into the hots retained on the filter. No Cr was found on the filters in the control experiments, indicating that the Cr found on the filters resulted from the Cr spike and that the maieriai present in the river water was responsible for the Cr incorporation into floes retained on the lilten. There was negligible difference in the amount or Cr found on the filter if the Cr was added to the river water 5 min before or 5 min after mixing with seawater (Fig. I). BOYLE et al. (1977) and MAYER (1982b) have shown that a large proportion ot rron and organic matter flocculation occurs withtn two minutes after mixing river water with seawater. sc this result suggests that the flocculation process does not affect the Cr binding extent of the participating colloidal material. Althou~ based on only one data point, this result is corroborated by additional data presented below. An experiment was conducted to determrnr the effect of incubation time of the Cr spike wrth the river water before mixing with the seawater. The agitation time for the spiked river water with the seawater before filtration and recovery of the flocculated colloids was 0.5 hr for all spike incubation times. The results of this experiment (Fig. 2) show that there was negligible effect of varying spike tncubation time. Reactant mode experiments

Cr

SPIKE (jq,‘ll

FIG. 1. Percent of Cr scavenged by flocculating material resulting from a 50:50 mixture of Saco River water and seawater (salinity of mixture = 15.8 ppt). Concentrations of Cr spikes are given per volume of estuarine mixture. Solid circles refer to Cr spikes injected 5 min before mixing river water and seawater; hollow circle refers to spike injected 5 min after mixing. Mixture agitated for 3.5 hr.

The control experiments in the product mode work demonstrated that riverine substrates, rather than those from the seawater endmember. art’ re. sponsible for Cr binding. Similarly. we found no drop in CL reactivity with time for 0” spikes III distilled water-seawater mixtures, indicating no bmding by seawater substrates. We therefore focused our reactant mode work on river water. Loss o! i-1. activity in buffered distilled water controls. drl :ndication of either wall adsorption or hydroxide prclcq?

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Cti’ incorporation in coljoidal material

was negligible at pH values less than 7.0 for the Cr concentrations that we used. Kinetics experiments with river water samples showed that the major fraction of a C? spike was always bound within an hour or two (Fig. 3). Binding experiments were carried out in various seasons, and there was a tendency for springtime samples to show slower kinetics than those of winter or summer. With Cr spikes of 1 to 20 Icg/l, river water was invariably found to contain sufficient substrates to remove all of the spiked Cr from a state reactive to CL, demonstrating that the complexation/adsorption capacity of these filtered river waters for Cr was greater than the levels of the spikes employed in these experiments. Attempts to determine a consistent rate law for the binding kinetics were inconclusive, with zero, first, and second order reactions (with respect to C?‘) found on occasion, and undefinable rates at other times. This erratic behavior is in contrast with the consistent first order kinetics found by HAMM ef al.(1958) for Cr complexation by a variety of carboxylic acids and in our preliminary experiments with oxalic acid in distilled water. The molecular weight (MW) dependence of the binding reaction is seen, as a function of pH, in Fig. 4. At pH ranging from 5 to 7, the reaction was considerably inhibited by filtration of the water sample by an ultrafilter of nominal MW 50,000. Other binding experiments demonstrated that the <50,000 MW fraction was capable of taking up all of a 2 rg/l Cr spike, indicating that the partial binding seen for this fraction in Fig. 4 was due to slow kinetics of binding rather than lack of a sufficient number of binding sites. These results indicate that the high MW fraction has faster binding kinetics than the lower MW fraction during binding of Cr by a 0.2 pm filtered water sample. This preference of Cr for higher MW substrates may not be unique to the waters that we studied; STEINBERG (1980) found natural Cr concentrated in a higher MW fraction. The reason for the relatively rapid binding by the high MW fraction was not determined. The direction of the pH dependence of the binding kinetics is

itation,

TIME

(min)

FIG. 3. Concentration of spiked, free (CL-detectable) Cr)’ remaining with time after an 8 fig-Cr/l spike into 0.2 pm filtered Sheepscot River water, pH 6.7.

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with the results of HAMM et al. (1958), although the magnitude of this dependence that we found is somewhat less. It is presumably this preferential binding by the high MW fraction that is responsible in large part for the incorporation of Cr into the salinity-induced floes, as seen in the product mode experiments. A variety of workers (SHOLKOVITZ et al., 1978; Fox, 1983; CARLSONand MAYER, 1983) have shown that salt-induced flocculation preferentially involves the higher MW organic material. However, FOX (1983) has shown that there is an incomplete overlap between the high MW material and that material flocculated by sea salt, so there is little reason to attempt to justify the proportions of spiked Cr found in the product mode floes by their relative rates of incorporation into different MW fractions observed in the reactant mode experiments. We were unable to determine the identity of the substrates binding the Cr in these experiments. The most reasonable candidates are iron colloids and dissolved organic matter, both of which are capable of binding trivalent Cr (NAKAYAMA et al., 198 1; YAMAZAKIetaf., 1980; CHUECAS~~~RILEY,1966). JACKSONet al. (1980) found a strong association between spiked Cr and natural Fe in Sephadex sep arations of a lake water sample, but the applicability of their findings to our system is unknown. The effect of salinity on the rate of binding of an 8 &I Cr spike in river water is shown in Fig. 5. In this experiment, a sample of Sheepscot River water was split into two aliquots, one of which was diluted tenfold with distilled water. Each of these aliquots was then split again into two aliquots, one of which was mixed with enough seawater to reach a salinity of I ppt and the other with an equivalent volume of distilled water. There was no evident effect of salinity during the short time required for complete binding of the Cr spike in the undiluted river water. However, the ten-fold dilution of the river water with distilled water induced a salinity effect, in which Cr binding was inhibited at the higher salinity. Because the data consistent

L.. M. Mayer, L. L. Schick and C. A. Chang

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FIG. 5. Concentration of spiked, free (CL-detectable) Cr’remaining with time after 8 fig-Cr/l spikes into aliquots of Sheepscot River water, either undiluted (circles) or diluted ten-fold with distilled water (triangles). Each aliquot was mixed with seawater to a salinity of 1 ppt (open symbols) or an equivalent volume of distilled water (closed symbols). pH = 6.85 for all runs.

in this experiment followed second order kinetics, we can assess the impact of salinity by examining the ratios of the slopes of the data linearized by plotting as reciprocal concentrations. For the undiluted river water runs, the slopes were identical. For the diluted river water runs, the 1 ppt slope was one third of the lower salinity equivalent. Salinity inhibitions were found for the other substrates examined: commercial humic acid, at a concentration (5 mg/l) approximating that found in some rivers; oxalic acid, at concentrations below saturation for calcium oxalate; and iron oxyhydroxide coiloids, at concentrations ranging from 0.2 to 2.0 mg-Fe/l. It should be emphasized that a salinity inhibition was found only for the kinetics of the binding: we made no attempt to determine “equilibrium” levels of Cr in the presence of river water ligands. Experiments were usually carried out for several hours only. and in some experiments the extent of binding in samples containing seawater would eventually merge with the extent found for the river water endmember. in spite of slower kinetics. A binding experiment was also performed m which the Cr spike was added to two aliquots of a river water-seawater mixture-one of which had been mixed and allowed to sit overnight before the spike and the other of which was mixed at the same time as the spike. No difference was found in the binding kinetics of these two aliquots, corroborating the results discussed above that demonstrate a lack of effect of flocculation on Cr scavenging. The effect of salinity on diluted river water samples was examined at salinities ranging up to 6 ppt, above which various interferences prevented accurate CL measurements. The salinity dependence of this inhi-

bition was not simple (Fig. 6): the effect wab otten somewhat reduced above a salinity of about 1 ppt. producing an oscillatory response to salinit). This salinity effect is qualitatively in agreement with prrvlous studies of the effect of seawater on metal binding to humic substances (MIJSANI et ui i980. HULJEVand STROHAL, 1983). in that higher salinities inhibited binding. ‘The reason for this salinity Inhibition wits nl)r determined, but is likely at least in part duz to competition with calcium ions. as evidenced h! Ed. periments in which oxalate binding was inhibited b) seawater but not by univalent electrolyte (KCl. tiNOrb solutions (unpub. data; HAMM ef al.. 1958) WC? hypothesize that the requirement for dilution ui the river water in order to detect the salinity effect results from the heterogeneous set of binding sites available for Cr binding by river water substrates. Such hrtetogeneities have been established for humic substances (LANGFORD et al., 1983) and iron oxyhydroxldes (BENJAMINand LECKIE, 1981). Sites with relatIveI) high specificities for Cr are not likely to exhtbit d salinity effect resulting from competition between t’i and the alkali earth cations of seawater, while sites with lower specificities for Cr will permit such I.ompetition. Dilution of the river water lowers the suhstrate:Cr ratio and therefore reduces the relative number of high specificity sites. The binding m the experiments with diluted river water therefore involves a greater proportion of lower specificity sites and allows the salinity effect to become evident. Heterogeneous site distributions, with the nature :li the distributions probably changing from sample to sample, are likely also responsible for the lack of conststent rate law observed among river water samples. These dilution experiments imply that caution must be used m applying conclusions from metal-organic complexation studies done at one substrate-metal ratlrr tcl

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FIG. 6. Concentration of Cr bound from an Il-rg-Cr:i spike into Sheepscot River water, which was first diluted ZO-fold and then mixed with seawater or distilled water tc achieve various salinities but constant dilution of original river water. The amounts of Cr bound are shown for 15 min (triangles) and 2 hr (circles) samplings. pH i:

Cr’+ incorporation in colltiidal material

field situations ratio. Implications

involving

a different substrate-metal

for Cr transport

The incorporation of Cr into iron/organic colloids which flocculate to form particulate materials raises the possibility of retention of Cr in the estuary by settling of the resultant floes. Flocculated iron has been shown to pass through some estuaries with little removal from the water column by either gravitational settling or sediment scavenging (MAYER, 1982a; WILKE and DAYAL, 1982), suggesting that this incorporation will be. ineffective in causing retention of Cr in the estuarine zone. However, the effluent of dissolved Cr in the Saco is likely accompanied by considerable amounts of other tannery wastes, which are capable of scavenging iron colloids from the water column (MAYER, 1982a). The efficiency of the estuarine trace metal trap (TUREKIAN, 1977) may be reduced by complete binding of Cr by dissolved and colloidal material, if this prior binding renders the Cr unavailable to sediment adsorption reactions. We have found that suspended sediments at concentrations found in the Saco estuary are capable of complete adsorption of Cr spikes at the levels used in this study (unpub. data), but it is not known whether Cr already attached to dissolved and colloidal materials will be protected from sediment adsorption. This type of protection has been demonstrated for other metal-sedimentorganic ligand systems (SHOLKOVITZand COPLAND, 198 1: YAMAZAKIet al., 1980; MCLAREN et al., 198 1; JAMES and BARTLETT, 1983). CONCLUSIONS The results of these reactant and product mode experiments indicate that Cr added to waters near a river-estuary interface may be bound within a few hours by dissolved or colloidal material present in the river water. On the order of a third of this scavenged material will be incorporated into the floes that form as a result of destabilization of riverine colloidal material by seasalt, owing to the high reactivity of the riverine colloidal material to Cr binding. The increase in salinity encountered during passage of the river water through the estuary will probably not kinetically inhibit this binding, nor will it have much effect on the partitioning of the Cr between flocculated and unflocculated substrates. Acknowledgements-We thank P. Roui for technical ass& tance. This work was partially supported by the U.S. De-

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