Oxidation of Cinnabar by Fe(lll) in Acid Mine Waters John E. Burkstaller" and Perry L. McCarty Department of Civil Engineering, Stanford University, Stanford, Calif. 94305
George A. Parks Department of Applied Earth Sciences, Stanford University, Stanford. Calif. 94305
Fe(II1) concentrations occurring in acid mine drainage water oxidized cinnabar (HgS), the most common natural mercury mineral. With C1- present at environmental levels, significant mercury was released to solution. T h e rate of oxidation of cinnabar was much higher than t h e rate of mercury release to solution. Most of the mercury was bound to the remaining cinnabar by a n unknown mechanism. An isotopic dilution technique was used t o calculate t h e total mercury, both in solution and bound t o t h e remaining cinnabar, released by oxidation.
T h e average concentration of mercury in the earth's crust is about 80 ppb, but levels more than 10,000 times higher are found in mineable natural deposits occurring in regions of intense tectonic activity ( I ) . Mercury release from these natural sources may have been occurring throughout geologic time and may be aggravated by mining. T h e most common mercury-bearing minerals occurring in these deposits are cinnabar and metacinnabar, both isomorphs of HgS. Compared t o most sulfides, they are very insoluble ( 2 ) and appear more resistant t o oxidation (3). Saukov and Aidinyan ( 4 ) reported t h a t cinnabar could be oxidized by solutions containing 20-162 m M Fe(II1). Chloride (0.2M) increased t h e oxidation rate 14 times as inferred from observed reduction of Fe(II1) t o Fe(I1). Measured mercury in solution averaged only about l i j ~ o the concentration expected on t h e basis of the concentration of Fe(I1) formed, and they concluded t h a t the mercury released was removed from solution as HgSOdePHgO. Fe(II1) and C1- occur in acid drainage waters from mining areas, b u t in lower concentrations than investigated by Saukov a n d Aidinyan. We wished t o determine whether cinnabar oxidation might occur under these environmentally significant conditions Procedures
Cinnabar was prepared from a sample of recently mined, high-grade ore from the New Almaden mine, Santa Clara County, Calif. T h e ore was crushed and sieved to obtain the -325 $400 mesh (Tyler series) fraction. Cinnabar was hydraulically separated from other ore components, washed with 50% nitric acid to remove traces of pyrite, then reground in a porcelain ball mill t o minimize t h e amount of surface area possibly altered by the nitric acid treatment. T h e surface area of the resulting material was 1.08 m2/g as determined by B.E.T. nitrogen adsorption. A small acid ( p H 2.0) stream from a tailings pile near a n abandoned mercury mine a t Mt. Diablo, Calif. was analyzed to obtain some idea of reasonable initial Fe(II1) and C1- concentrations. I t contained 4 g/l. (72 m M ) total iron and 250 mg/l. (7 m M ) C1-. Titration with permanganate indicated t h a t a t least Y4 of the total iron was Fe(II1). Total iron was measured by atomic absorption and chloride by 676
Environmental Science & Technology
argentometric and mercuric nitrate methods. T h e soluble mercury concentration in t h e stream, measured by flameless atomic absorption (FAA), was 20 p p b (0.1 pM), T h e effect of Fe(II1) and C1- concentrations on cinnabar oxidation by Fe(II1) and mercury release to solution were evaluated using Pyrex centrifuge tubes with Teflon-lined plastic caps. Tubes containing cinnabar, t h e oxidizing solution, and a gas phase were mounted in racks on a shaking table at 25OC. Fe(II1) was added as the perchlorate salt, and C1- as NaC1. T h e p H was adjusted with perchloric acid to prevent Fe(II1) hydrolysis and to duplicate the low p H conditions in acid drainage waters. Mercury released to solution was determined by FAA analysis of the filtrate from 0.45-p Millipore filters. T h e gas phase was sampled prior to opening the tubes and analyzed by FAA; no elemental mercury was found. Losses to the container walls, as monitored by recovery of 203Hg(II) added t o samples without cinnabar and by mercury analysis of nitric acid washes from emptied tubes, proved insignificant. A NaI crystal scintillation counter determined 20"Hg(II) concentrations. Losses to Millipore filters were significant only a t very low mercury concentrations. Early experiments indicated t h a t much of t h e mercury released by cinnabar oxidation became bound to the remaining cinnabar. T o distinguish this mercury from that present as cinnabar (cinnabar-Hg) and that dissolved in t h e aqueous phase, we will refer to it as "bound-Hg." I t was possible t o differentiate between bound-Hg and cinnabarHg by adding a pool of 203Hg(II)tracer to the cinnabar-solution mixture of a n oxidation experiment. Cinnabar-Hg does not exchange with the 203Hg(II) pool, while bound-Hg does exchange. This exchange dilutes t h e specific activity of t h e tracer pool, permitting calculation of the quantity of bound-Hg present. Proof t h a t added tracer exchanges with only the boundHg was obtained as follows: 203Hg(II) was placed in a nonoxidizing solution containing HgS. T h e total Hg(I1) concentration added with the tracer was only 20 ppb, and over 98% of the tracer was removed from solution by the HgS. Next, a much larger concentration (33 mg/l.) of unlabeled Hg(I1) was added. Within 3 hr the distribution of tracer between solids and solution was the same as the distribution of the total Hg(I1) added (65% in solution). T h e amount of Hg(I1) and tracer in solution remained constant (for 45 days) indicating t h a t 203Hgwas not exchanging with mercury in the HgS crystal. In oxidation experiments results were essentially t h e same when tracer was added either prior to or after oxidation, giving further evidence of free exchange between bound-Hg and mercury in solution.
Results Figure 1 illustrates the rate of release of soluble mercury from New Almaden cinnabar exposed to Fe(II1) and C1-. The system containing both Fe(II1) and C1- gave rates of mercury release to solution more than an order of magni-
tude greater than a similar system without Fe(III), and two orders of magnitude greater t h a n a system with Fe(II1) but no C1-. Figure 2 illustrates the mercury released t o solution from New Almaden cinnabar using a range of C1- concentrations and 10 m M Fe(II1). T h e amount released was strongly affected by C1- concentration. Figure 3 illustrates the rates of mercury release to solu-
tion as a function of cinnabar concentration. Contrary to original expectations, larger concentrations of cinnabar released less mercury to solution. Small amounts of Hg(I1) (20 ppb) containing ro"Hg a t high specific activity were added t o these samples to confirm t h e presence of boundHg and estimate its quantity, thus allowing calculation of t h e true cinnabar oxidation rate. T h e results are shown in Figure 4. Larger concentrations of cinnabar result in greater calculated oxidation rates. T h e calculated amounts re-
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Figure 1. Effect of Fe(lll) and CI- on release of soluble mercury from New Almaden cinnabar oxidation in systems with 3 g/l HgS and pH between 1.8 and 2.0 /
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Figure 3. Effect of varied New Almaden cinnabar concentrations on release of soluble mercury in pH 1.5 solutions containing 10 mM Fe(lll) and 7 rnM CI-
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Figure 2. Effect of varying CI- concentration on release of soluble mercury from 2 g/l New Almaden cinnabar in systems containing 10 mM Fe(lll) at pH 1 5
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Figure 4. Effect of varied New Almaden cinnabar concentrations on calculated oxidation rate Experimental conditions are the same as for Figure 3
Volume 9, Number 7, July 1975
677
leased by oxidation were several orders of magnitude greater than the measured amounts in solution. Controls in which no tracer was added gave t h e same mercury release to solution, indicating t h a t Hg(I1) added with the tracer had little effect on oxidation or the partition between solution and bound-Hg. Evidently, the large amount of boundHg made a 20-ppb addition insignificant.
Discussion These laboratory studies indicated t h a t significant rates of oxidation of cinnabar and release of mercury to solution can occur under conditions prevalent in acid mine drainage waters. Other studies in our laboratory have shown t h a t Hg(I1) is strongly adsorbed by HgS, and that, in the process, anions such as SO*’-, NO,j-, and C1- may also be removed from solution. Since mercury released by oxidation is largely held on the remaining cinnabar, the oxidation rate cannot be directly inferred from mercury released to solution. Larger concentrations of cinnabar increase not only the surface area available for oxidation but also t h a t available
for removal of Hg(I1) released by oxidation. T h e results illustrated in Figures 3 and 4 can be explained if the effect from increasing surface area was greater on Hg(II) removal than on oxidation. Additional work is needed to identify the nature of the bound-Hg. Studies are continuing on this problem and on the kinetics of cinnabar oxidation over a range of pertinent variables broader than those in acid drainage environments. L i t e r a t u r e Cited ( 1 ) Bailey, E. H., Clark. A. I,.. Smith. R. M., l T . S Geol. SurL’e) Prof’. Paper, 820, 401 (1973). (2) Krauskopf. K.,Econ. G P O / . . 46,498 (1951). ic‘i) Tunell, G., Mercury in “Handbook of Geochemistry.” LToi. 11/2, p p 80B-M, Springer-Verlag, New E’ork. N.Y., 1970. (4) Saukov, A. A., Aidinyan, N. Kh., Acad. N a u k . S.S.R., Inst. Geol. N a u k . , 39 Min.-Geokhim., 8,37 (1940) (Russ).
Keceii’ed /or rwieu Aug 29, 3974. Accepted Februarj, 13, 1975. This u,ork u‘as supportsd bi. ths Nationai Science Foundation through the Tracs Contaminants Program of’ the R A N N Dicision, Grants GI-32943 and 40614.
Interference of Sulfate Ion on SPADNS Colorimetric Determination of Fluoride in Wastewaters Richard F. Devine* and Gerald L. Partington Calgon Laboratories, Calgon Corp., P.O. Box 1346, Pittsburgh, Pa. 15230
Serious errors have been experienced in the analysis of wastewater samples for fluoride concentration by the SPADNS colorimetric method. T h e source of positive interference in this method was due primarily to sulfate ion carry-over during the preliminary distillation step.
In accordance with the Federal Water Pollution Control Act Amendments of 1972, the October 16, 1973, F e d e r a l R e g i r i e r ( I ) specifies the approved test procedures for the analysis of water pollutants. T h e approved method for the determination of fluoride ion is the SPADNS colorimetric procedure preceded by distillation from a sulfuric acid solilt ion. T h e SPADNS method and the fluoride electrode method we had been using are both described in detail in Standard Methods (2) and Part 23 of ASTM Annual Book of Standards ( 3 ) . In the process of adopting t h e SPADNS method for routine laboratory use, comparative data were gathered on prepared standard fluoride solutions and actual wastewater samples using both methods. T h e data showed t h a t after sample distillation, the SPADNS method gave consistently higher results than the electrode method. A study was initiated to discover the source and extent of the apparent interference. Since it had previously been reported that a t t h e 1.0-mg/l. fluoride level, 200 mg/l. of sulfate ion will cause a positive error of 0.1 mg/l. in the SPADNS results (2, 31,the study was concentrated on sulfate ion. This a p peared to be a logical beginning because of the possibility of sulfate carry-over during distillation. Methodology
T h e colorimetric procedure is based upon the measurement of the loss of color of a zirconium-dye lake owing to t h e reaction of fluoride ion with zirconium to form a color678
Environmental Science & Technology
less complex ion. T h e dye used is sodium 2-(p-sulfophenyla z o ) - 198 - d i h y d r o x y n a p h t h a l e n e - 3 , 6 - d i s u l f o n a t e (SPADNS). T h e range of t h e test is 0.1-1.4 mg/l. of fluoride. The method is subject to many interferences, most of which are supposed to be eliminated by the preliminary distillation stem T h e distillation procedure was first proposed by Bellack (1)in 1958. I t involves the distillation of 300 ml of sample from a water solution of sulfuric acid which has previously been heated to 180°C and cooled. Distillation is continued until the temperature reaches 180°C and approximately :100 ml of distillate have been collected. This procedure is supposed to assure complete recovery of u p to 3 mg/l. of fluoride ion and removal of most of the interfering constituents in the water sample. Sulfate carry-over is reported to be minimal as long as the temperature in the distillation flask does not exceed 180OC. T h e electrode method requires the use of a fluoride ion selective electrode, a calomel reference electrode, and a p H meter with an expanded millivolt scale. A buffer solution is added to a portion of the raw sample or preferably to the distillate to adjust pH, to complex iron and aluminum which would interfere, and to provide a similar ionic strength background between samples and standards. T h e fluoride electrode develops a potential which is specific for fluoride ion and is proportional to the fluoride ion activity ( i f the sample. T h e range of this method is 0.1-1000 mg/l. fluoride. E s p P ri m e n t a 1
Excellent comparative data were obtained when the two methods were used t o analyze undistilled prepared standard fluoride solutions in the range 0.0-1.4 mg/l. Marked differences were found, however, when actual wastewater iarnples were analyzed following distillation. T h e results ohtained by colorimetric analysis were in all cases higher
