Effects of Instream pH Modification on Transport of Sulfide-Oxidation

Chapter 16

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on March 7, 2018 | https://pubs.acs.org Publication Date: December 20, 1993 | doi: 10.1021/bk-1994-0550.ch016

Effects of Instream pH Modification on Transport of Sulfide-Oxidation Products 1

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Briant A.Kimball ,Robert E.Broshears ,Diane M.McKnight ,and Kenneth E. Bencala 3

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U.S. Geological Survey, 1745 West 1700 South, Room 1016, Salt Lake City, UT, 84104 U.S. Geological Survey, Box 25046, MS 415, Denver, CO 80225 U.S. Geological Survey, 345 Middlefield Road, MS 496, Menlo Park, CA 94025 2

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We studied the dynamic response in aqueous concentrations of metals and sulfate to an experimental increase of pH in a mountain stream affected by acidic mine drainage. Downstream from mine spoils in St. Kevin Gulch, Colorado, U.S.A., ambient pH was 3.5; filtered concentrations of Al, Cu, Fe, andSO 2-were 3.1, 0.18, 1.1, and 128 mg/L, respectively. Injection of Na CO caused pH to increase to 4.2 and eventually to 5.9 at a site 24 meters downstream from the injection. Filtered Al decreased to 0.07 mg/L; Cu, to 0.12 mg/L; Fe, to 0.41 mg/L; and SO 2- to 122 mg/L. Particulate metal concentrations increased as filtered concentrations decreased, indicating processes of partitioning. Measured concentrations compared favorably with concentrations calculated by a geochemical equilibrium model that simulated precipitation of amorphous Fe and Al hydroxysulfates and sorption of Cu to the precipitating Fe phase. Differences between measured values and concentrations simulated with a conservative solute transport model indicated the substantial buffering capacity of streambed sediments. Particulate concentrations of Al, Cu, and Fe decreased downstream because of sedimentation to the bed. After the injection, concentrations of filtered Al and Cu exceeded background levels. At a site 24 meters downstream, 100 percent of the particulate Al that had settled from the water column was returned after the injection; 53 percent of the Cu was returned, but only 19 percent of the Fe was returned. These observations are attributed to dissociation of the settled polynuclear complexes of Al and to desorption of Cu from the settled Fe phases. 4

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Diverse interactive physical, chemical, and biological processes affect the mobility and ultimate fate of sulfide-oxidation products in surface water. Advection, solute dispersion, and inflow mixing contribute to patterns of instream concentrations downstream from contaminant sources such as acidic mine drainage. In addition, This chapter not subject to U.S. copyright Published 1994 American Chemical Society Alpers and Blowes; Environmental Geochemistry of Sulfide Oxidation ACS Symposium Series; American Chemical Society: Washington, DC, 1993.


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patterns in solute concentrations are affected by biological cycles and by chemical processes including complexation and speciation of metals leading to precipitationdissolution, sorption-desorption, and oxidation-reduction reactions. While field studies are necessary to characterize physical transport in streams, the determination of chemical equilibria and rates of chemical and biological reactions often is relegated to the controlled conditions of the laboratory. A complete understanding of natural conditions, however, requires that interactive processes be studied in the field, where the complexities of stream history, variable boundary conditions, and distributions of residencetimesin biogeochemical microenvironments combine in ways not duplicated in the laboratory. Field studies of the transport and fate of sulfide-oxidation products in surface water have been conducted by several investigators. Dilution by inflows, precipitation of amorphous Fe and A l hydroxides, and sorption of Cu to Fe precipitates were indicated in two mountain streams affected by acidic mine drainage (L2). Linked watershed, geochemical, and solute transport codes were used to simulate the measured behavior of Cu mobilized from mine wastes Q). The importance of uptake by periphyton and sorption to sediments on the mobility of Cu in a stream was demonstrated in field experiments (4). Radio-labeled Fe in limnocorrals helped to determine the partitioning of Fe among dissolved, non-settling colloidal, and settling particulate phases (S). Instream measurement of Fe has shown the importance of diel fluctuations due to photoreduction (©. Photosynthetically-induced changes in pH affected instream As concentration (2). Chapman (£) used a geochemical speciation code coupled with a physical transport model to simulate the responses of Zn, Al, Cu, and Fe to an experimental increase of pH in a stream. Elevation of instream pH from 3 to 10 was accompanied by formation of chemical precipitates and base-neutralizing surface reactions. Experimental decrease of instream pH was used to detail Fe transformations and transport in an acidic, mountain stream (2). This paper provides a preliminary analysis of an experiment conducted in St. Kevin Gulch, an acidic, mountain stream in Colorado, U.S.A. (Figure 1). St. Kevin Gulch is a pool-and-riffle stream receiving acidic, metal-rich water of pH 2.6 from mine drainage. To study responses to pH changes, we injected Na2C0 to raise pH from a background value of 3.5 to a maximum value of 5.9. Our experiment differed from the experiment of Chapman (£) in that: (1) the pH range induced by the experiment was of more environmental relevance because it resembles the pH changes the stream encounters during downstream and seasonal changes; (2) both filtered and unfiltered samples were obtained to evaluate variations offilteredand particulate concentrations; and (3) the injection period was sustained for 6 hours at two pH levels rather than a slug exceeding pH 10. Datafromour experiment will be used to evaluate a computer code that is being developed to couple kinetic effects of hydrologie transport and chemical equilibrium. The purposes of this paper are to describe the chemical variations during the instream experiment, to propose likely processes causing those variations, and to test the hypothesis that chemical equilibrium calculations and physical transport can account for these changes. Remediation of streams and lakes affected by metals requires an understanding of the geochemical reactions that determine responses to varying environmental conditions. Among the metals sensitive to changes in pH, Fe tends to 2+

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Alpers and Blowes; Environmental Geochemistry of Sulfide Oxidation ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

ENVIRONMENTAL GEOCHEMISTRY OF SULFIDE OXIDATION


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form hydrous Fe oxide precipitates with extensive reactive surface area that can influence the mobility of other metals in streams. Thus, Fe can function as a controlling variable as it responds to changes in pH. Mobility of Al and Cu are sensitive to pH and the cycling of Fe (10-13^. They are also of biogeochemical importance in acidic waters because of their effects on aquatic organisms (14.151


METHODS Modification experiment. Instream pH of St. Kevin Gulch was increased from 3.5 to 5.9 by pumping a concentrated solution of Na C0 , with NaCl included as a conservative tracer, into the stream. As the pulse of increased pH was transported downstream, the response of major ions and trace metals was documented by collecting samples at four sites located 24, 70, 251, and 498 m from the injection site (Figure 1). These sites are numbered 1 through 4, respectively, in this paper. A control or background site (site 0) was sampled upstream from the injection point. The changes in pH (Figure 2) defined three steps in the experiment For site 1, 24 m downstream, step 1 began at 0900 hours with the initial injection of base and continued for 3 hours. Step 2 began at 1200, when pH was brought above 5.3 and eventually to 5.9 at site 1. Step 3 began at 1500 when the injection of base ended. Appearance of the three steps occurred successively later downstream because of the time of transport. 2

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The experimental modification added Na, which was geochemically conservative in the stream throughout the measured range of pH in the experiment Thus, aqueous,filteredNa served as a tracer in defining physical transport through the study reach. Injection of Na from NajCOg and NaCl sufficiently elevated instream concentrations of Na above background values so that a distinct concentration pulse delineated the injection period (Figure 3). The sequential arrival of the Na pulse at each sampling site permitted definition of subreach travel times, while the asymmetry of the arriving shoulders and departing tails of the pulse allowed calibration of parameters of transient storage in immobile zones adjacent to the main stream channel (16. 11). Instream flow ranged from 12.3 IVs at site 1 to 15.9 L/s at sites 3 and 4. Travel time through the reach was approximately 100 minutes. Parameters of transient storage were comparable to those calibrated in studies of similar mountain streams (18). Instream concentrations of Al, Cu, Fe, and S0 experienced the same regime of physical transport documented for Na. By accounting for these physical processes in arigorousmanner, we are better able to address the chemical and biological processes that affect the reactive metals as they move downstream. 2_

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Sampling and analytical methods. In describing the behavior of Al, Cu, and Fe, we distinguish between two operationally defined phases. First, a Ο.Ι-μπι-filtered phase was obtained by using pressure filtration through a nitro-cellulose membrane. Second, a particulate phase was obtained by subtracting the Ο.Ι-μπι-filtered concentrations from the concentrations in an unfiltered, acidified sample. The particulate concentration represents an acid-soluble concentration, principally consisting of aggregated colloids of hydrous Fe or other metal oxides greater than 0.1



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Figure 2.--Experimental response of pH to the injection of base into St. Kevin Gulch. At site 1 (solid squares), step 1 was from 0900 to 1200 hours, step 2 was from 1200 to 1500 hours, and step 3 was after 1500 hours when the injection of base ended. Arrival of these modifications at downstream sites is delayed by travel time to the sites (site 0, open squares; site 2, solid circles; site 3, upward triangles; site 4, downward triangles).

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Figure 3.~Concentration history of sodium during the pH modification experiment Instream sodium concentration was from injection of sodium carbonate and sodium chloride. Site 0, open squares; site 1, solid squares; site 2, solid circles; site 3, upward triangles; site 4, downward triangles.


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μπι (19). Although this membrane pore size may not remove all the colloidal Fe (2Q), it provides an operationally defined, dissolved-metal concentration that was necessary for rapid sampling during the experiment. Metal concentrations were determined by inductively coupled argon plasma atomic emission spectroscopy on samples acidified to less than pH 2.0 with ultrapure nitric acid in the field (21). Determination of S0 was by ion chromatography. Detection limits, accuracy, and precision for chemical determinations are presented elsewhere (22). Relative standard deviation for Al was 5.4 percent, for Cu was 5.3 percent, for Fe was 3.7 percent, and for S0 - was 1.1 percent. To account for the Fe /Fe redox couple in thermodynamic calculations, Fe was determined colorimetrically using a bipyridine method (22). 2_

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RESULTS AND DISCUSSION In interpreting the results of the experiment, we asked two questions. First, can the initial effects of the pH modification as measured at site 1 be explained in terms of an equilibrium model? Second, are concentration histories at sites 2-4 simply the result of physical transport of the concentrations at site 1, or are other reactions involved? Results will be presented to address these questions. Initial effects of pH modification. Prior to the injection, background pH values were 3.5, with an increase to 3.8 at site 4. Pre-injection concentrations offilteredAl were nearly equal at each of the sites, with a concentration of about 3.1 mg/L. Filtered Cu concentrations before the injection were approximately 0.18 mg/L; no trend of concentration with downstream distance was evident. Pre-injection concentrations offilteredFe declined gradually from 1.1 mg/L at site 1 to 0.75 mg/L at site 4; however, background particulate Fe concentration was essentially the same at each site, at about 0.06 mg/L. Thus, particulate Fe likely was forming in the stream prior to the injection, and there were approximately equal rates of aggregation of Fe precipitates and physical settling of Fe particulates along the reach. With the injection of base at 0900 hours, pH increased to 4.2 at site 1 (Figure 2). An increased injection rate begun at 1200 hours eventually resulted in a pH of 5.9 at this site. Temporal changes that occurred forfilteredand particulate Al, Cu, and Fe as pH increased at site 1 were substantial (Figure 4). The primary changes occurred during step 2 from 1200 to 1500 hours, when filtered concentrations decreased and particulate concentrations increased. Total concentrations (filtered plus particulate) declined only slightly at site 1, consistent with the formation of a precipitate in the water column. A slight decrease in S0 - during the same time interval indicated that the precipitate might also contain S0 -. At the end of the injection, filtered concentrations of Al and Cu briefly increased above background levels. This rebound or spike became more pronounced at each site downstream. 2

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Geochemical reactions. The substantial loss of filtered Al, Cu, and Fe upstream from site 1 can be evaluated in terms of chemical equilibrium by plotting metal activities versus pH (Figure 5). Each plot includes a line that represents equilibrium of the aqueous phase with respect to a solid phase. Phases were amorphous Al(OH) for A l activity, Cu(OH) for Cu activity, and ferrihydrite, Fe(OH) , 3+

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(a) ALUMINUM r

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Figure 4.--Temporal variations of filtered and particulate (a) aluminum, (b) copper, and (c) iron at site 1. Filtered concentrations (solid squares) are from Ο.Ι-μιη pressure filtration. Particulate concentrations (upward triangles) are from unfiltered concentrations less the 0.1 |imfilteredconcentration.



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Figure 5.~Variation of calculated activities of (a) A l , (b) Cu , and (c) Fe with pH. Activities were calculated with the WATEQ4F chemical equilibrium model. 3+


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ENVIRONMENTAL GEOCHEMISTRY OF SULFIDE OXIDATION 3+

for Fe activity; thermodynamic data from the chemical equilibrium model WATEQ4F were used (24). Higher pH caused A l activity to approach equilibrium; the slope of the points changed above pH 5.0. Values of C u activity never approached equilibrium. Activity of Fe trended along the equilibrium line over the entire range of pH. The slope of a regression line for Fe activity is -2.23, substantially different from the -3.0 slope that would result from precipitation of a pure hydrous Fe oxide phase. Values of an Fe:S ratio for Fe precipitates in acidic mine drainage have been measured from 8 to 5 (25). The Fe:S ratio of a precipitating Fe phase from the slope of the regression line is about 2.6, indicating substantial sulfur in the Fe phase. Perhaps the initial precipitation of the phase is high in S 0 because the initial Fe:S ratio in the water column is 0.015. To answer our first experimental question, we suggest plausible reactions to account for the chemical changes. Overall, the chemical changes from background conditions to the conditions during step 2 (from 1200 to 1500 hours) may be described by the following chemical reactions: 3 +

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Na C0 +H 0

Effects of Instream pH Modification on Transport of Sulfide-Oxidation

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