The deposition of molybdenum in anoxic waters

Marine Chemistry Elsevier Publishing Company, Amsterdam-Printed in The Netherlands

THE DEPOSITION OF MOLYBDENUM IN ANOXIC WATERS

K. K. BERTINE* Yale University, New Haven, Conn. (U.S.A.) (Accepted for publication April 27, 1972)

ABSTRACT Bertine, K. K., 1972. The deposition of molybdenum in anoxic waters, Mar. Chem., 1: 43-53. Molybdenum deposition in anaerobic areas is a result of several processes. The primary one, which removes about 70 % of the molybdenum in laboratory experiments, is the coprecipitation with iron sulfide on a relatively short time scale (order of a week after the hydrogen sulfide has built up). The formation of Mo(V) by any of the following mechanisms: reduction by organic acids, reduction of molybdenum in the nitrogen cycle or by sulfate-reducing bacteria, all result in a slow sorption or coprecipitation of about 20 % of the total molybdenum after a three-week period onto almost any solid phase present. If the pH in the anoxic sediment decreases, this process becomes increasingly important. The total removal of molybdenum at any site is the sum of these processes.

INTRODUCTION It is a well-substantiated fact that m o l y b d e n u m is c o m m o n l y found concentrated in sediments associated with marine anoxic conditions (see for example, Krauskopf, 1955; Bertine, 1970). Although various mechanisms have been proposed for the removal of molybdenum from solution under anoxic conditions, the problem has not been systematically investigated. Goldschmidt ( t 9 5 4 ) and Wedepohl (1968) suggested that molybdenum is precipitated by hydrogen sulfide as the finely-dispersed m o l y b d e n u m sulfide. Sugawara et al. (1961) and Pilipchuk and Volkov (1968) did not observe the direct precipitation o f m o l y b d e n u m sulfide and suggested a coprecipitation with iron sulfides; a mechanism also suggested by Manheim (1961) from his study of Baltic Sea sediments. In contrast to these sulfide-controlled mechanisms, Szilagyi (1967) and A. Nissenbaum (personal communication, 1970) have proposed that sorption on particulate organic matter (particularly humic acids) is the principal mechanism of m o l y b d e n u m extraction from sea water and deposition in sediments. F. Richards (personal communication, 1969) suggested that a reduction of m o l y b d e n u m could take place in anoxic systems via the nitrogen cycle, and the reduced species could be removed by adsorption on whatever surfaces were available in the system. * Present address: Scripps Institution of Oceanography, La Jolla, California.

44

K . K . BERTINE

The removal of molybdenum from sea water under anoxic conditions depends upon its dissolved forms. The stability fields for M o - H 2 0 - S system are shown in Fig. 1. MoO42and possibly MoO2 + can exist under natural anoxic conditions, i.e., pH around 7 and Eh

lo ~

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,=, -0.5 ----

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pS =

Fig.lA. Stability field of m o l y b d e n u m species in water at 25°C. (Constructed from Gibb's free energy data of Garrels a n d Christ, 1965, a n d Titley, 1963). B.Eh, pS = diagram for m o l y b d e n u m in water at 25°C, pH = 8. Natural m e a s u r e m e n t s are from Berner (1964).

around - 0 . 2 V. Yatsimerskii and Zakharova (1963) suggest the occurrences of thiooxymolybdates such as Mo206S z-, Mo03S 2- and Mo02S2 z- in the presence of hydrogen sulfide. With increased sulfide concentration, MoOzS22- should become dominant. The aim of this investigation was the experimental evaluation of the various suggested processes for the transfer of dissolved molybdenum from marine waters to the sediments under anoxic conditions.

THE DEPOSITIONOF MOLYBDENUMIN ANOXICWATERS

45

EXPERIMENTALPROCEDURES Molybdenum-99 (half-life, 68 hours) with attendant stable molybdenum was used as a tracer in the following experiments. The molybdenum activity was determined over its 0.74 Mev peak on a Ge-Li detector, except when the activity level was too low, in which case a Nal detector was used. Molybdate was the starting ion in all of the experiments. Artificial sulfate-free sea water was prepared following the recipe of Lyman and Fleming (1940). Six beakers containing one liter of the sulfate-free artificial sea water were prepared. To each of two of the beakers, the equivalent of 2,000/2g SO42-/1 were added; to each of two other beakers, 2.8 g SO42-/1 (the amount found in normal sea water) and two were left sulfate-free. To each of the six beakers, 2,000 #g molybdenum containing 99Mo was added followed by 0.5 g K2HPO4, 0.4 g NH4C1 and 5 g Ca-lactate (a sludge was formed upon the lactate addition which did not dissolve) as nutrients for microbiological activity. The beakers were heated to boiling for one hour to eliminate any bacteria which might have been present. A culture of sulfate-reducing bacteria was prepared in the following way: 1 ml of Long Island Sound sea water, after shaking with anoxic sediments containing sulfatereducing bacteria and after settling of the solids, was filtered through a 0.45 ~t Millipore membrane. The membranes, containing the bacteria, were washed with 200 ml of sulfatefree artificial sea water and then added to the beakers. The beakers were swirled and 5 ml aliquots removed from each as controls. The solutions were then subdivided as described below for the different experiments. One 250-ml aliquot of each was removed, exposed to air and stirred daily. The remaining solutions were covered and scrubbed with nitrogen for 30 min to remove the oxygen. In order to determine the molybdenum speciation during the course of the experiment, 25 ml aliquots were taken from the sulfate-free and "sea-water" sulfate stock solutions at two-day intervals, and shaken with 2 g of Dowex I-X8 (200 mesh) anion exchange resin to measure the percentage of anionic molybdenum. The amounts of molybdenum able to be sorbed on different solids were measured in the following experiment. After six and twelve days, 25 ml aliquots from the sulfate-free and 2,000/~g 8042-/1 and "sea water" sulfate stock solutions were added to 50 mg of one of the following solids: freshly-precipitated iron sulfide; illite (API 35, Fithian, Illinois); peat (Oyster Pond, Massachusetts); montmorillonite (API 22A, Mississippi); or kaolinite (API 7, South Carolina). The mixtures were stirred for one hour under a nitrogen atmosphere, then passed through a Whatman No. 42 filter paper. Five ml aliquots of the filtrate were assayed for 99Mo activity and the values compared to that of the standard. The interactions between molybdenum and various solids that might occur during the course of the experiment were examined by adding 50 mg of one of the above solids (except iron sulfide was replaced by freshly-precipitated iron hydroxide) to 250 ml aliquots of the sulfate-free and "sea water" sulfate stock solutions. At two-day intervals, 10 ml aliquots from the ten mixtures were filtered through Whatman No. 1 paper. Five ml

46

K.K. BERTINE

of the filtrate were pipetted into counting vials and the activity compared to the control solutions to which no solids had been added. RESULTS AND DISCUSSION

Molybdenum speciation in bacterial reduction experiments Fig.2 shows the results of the ion exchange experiments. In both the sulfate-free and "sea water" sulfate solutions, the behavior of molybdenum was similar for the first 10 days, The pH decreased to about 6.5 during this time and a non-anionic molybdenum species appeared and increased in concentration as the experiment progressed. On the tenth day about 35 % - 4 0 % of the molybdenum in both experiments was non-anionic. In the "sea ]oo

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,

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THE DEPOSITIONOF MOLYBDENUMIN ANOXICWATERS

47

water" sulfate experiment, on about the eleventh day, the smell of H2S became apparent, coinciding with the observation of a decrease in the percentage of non-anionic molybdenum with time. This decrease continued to the 17th day whereupon the percentage of non-anionic molybdenum began to increase again. Since the above behavior occurred only when hydrogen sulfide was present, it may be ascribable to the formation of thiooxymolybdates in the "sea water" sulfate solution after the tenth day and decreasing the percentage of non-anionic molybdenum. On the thirteenth day in the sulfate-free sea water, the pH was adjusted from 6.5 to 8.0, resulting in a significant decrease in the percentage of non-anionic molybdenum which then proceeded to increase again with time as the pH again decreased. The above action with the same consequences was repeated on the 20th day. From Fig.l, the non-anionic molybdenum species that increases with decreasing pH is MoO2+. To verify the presence of this ion a similar experiment was set up. After twelve days, the amount of MoO~+ in sulfate-free and 2,000 #g SO42-/1 solutions was measured directly. Using the MoO2÷ test with thiocyanate in acid solution to produce an orange color (Sandell, 1950), 10 ml were pipetted under a nitrogen atmosphere from each solution, acidified, thiocyanate added, and the Mo(V)-thiocyanate complex extracted into a 1 : 1 mixture of isoamyl alcohol and carbon tetrachloride. The absorbance was measured at 465 m# on a Beckman DU spectrophotometer. Comparisons were carried out with 5-ml and 10-ml solutions made by reducing all of the molybdate by the addition of asparagine. In this manner, it was determined that the percentage of MoO2 + was 53 % after twelve days in a sulfate-flee solution and 40 % in a 2,000 gg so4Z-/1 solution. The difference in the two values may reflect the formation of some thiooxymolybdates in the 2,000 #g SO4Z-/1 solution. The value of 53 % is in good agreement with that of 60 % non-anionic molybdenum found after twelve days in the ion exchange experiment (Fig.2) as additional evidence that the non-anionic molybdenum species is MoO2+. Sorpt&n o f molybdenum upon solid materials on the time scale o f one hour The results of the molybdenum sorption experiments on peat, illite, montmorillonite, kaolinite and freshly-precipitated iron sulfide are given in Table I. From the lack of sorption of molybdenum in the sulfate-free, aerobic solution, it may be concluded that a negligible amount of molybdate is taken up on any of the solids in agreement with the results of Kharkar et al. (1968). From the anion exchange experiment (Fig.2), it was shown that about 60 % of the molybdenum existed as MoO2+ and about 40 % as M o O 4 2at the end of twelve days in the sulfate-free solutions. Since molybdate is not the ion being sorbed, and there is no evidence for any other molybdenum species except MoOz+, MoO2+ should be the ion being taken up. These results indicate about 10 % of the total molybdenum or 17 % of MoO2 + (using the value of 60 % MoO2+ from the anion exchange experiments) is precipitated or sorbed on the Ca-lactate and about another 10 % of the total molybdenum or 17 % of the MoO2+ is attached to the peat and illite at the end of 12 days. There was no measurable sorption on kaolinite, montmorillonite or freshlyprecipitated iron sulfide. Similar results were obtained for the sorption of molybdenum

48

K . K . BERTINE

TABLE I Activity 99Mo sample/activity 99Mo standard in sorption experiments (see text for description) Sofid added

Days 6

12

"Sea water" sulfate solution (anoxic): None Illite* Peat* Iron sulfide* Kaolinite* Montmorillonite*

~+0.04 -+ 0.04 -+ 0.04 ± 0.04 ± 0.04 ± 0.04

-0.90 0.88 1.02 0.98 1.04

+ 0.04 ± 0.04 + 0.04 ± 0.04 ± 0.04

2,000 vg sulfate/liter solution (anoxic): None 0.99 ± 0.04 Illite* 1.00 ± 0.04 Peat* 1.00 ± 0.04 Iron sulfide* 0.98 +- 0.04 Kaolinite* 0.98 ± 0.04 Montmorillonite* 0.97 ± 0.04

0.79 (0.41 0.85 0.98 0.97 1.00

+- 0.04 ± 0.03) ± 0.04 ± 0.04 ± 0.04 ± 0.04

± 0.04 -+ 0.04 -+ 0.04 ± 0.04 -t 0.04 +~0.04

0.87 0.92 0.91 1.04 0.99 1.01

± 0.05 +- 0.05 ± 0.05 ± 0.05 ± 0.05 + 0.05

0.98 ~+0.04 ------

1.05 1.00 1.02 1.00 0.98 0.96

± 0.05 -+ 0.04 -+ 0.04 + 0.04 -+ 0.04 -+ 0.04

Sulfate-free solution (anoxic): None Illite* Peat* Iron sulfide* Kaolinite* Montmorillonite*

0.98 0.99 1.00 0.96 0.99 1.01

1.01 0.98 0.93 1.00 1.02 0.98

Sulfate-free (aerobic): None lllite* Peat* Iron sulfide* Kaolinite* Montmorillonite*

* Activity 99Mo standard for these experiments is the activity in the respective none solution. ( ) = anomalous value. Errors are counting errors.

f r o m t h e " s e a w a t e r " s u l f a t e a n d t h e 2 , 0 0 0 / ~ g SO42-/1 s o l u t i o n s . T h e s i m i l a r i t y o f t h e r e s u l t s in t h e t h r e e s o l u t i o n s s i g n i f y t h a t a n y t h i o o x y m o l y b d a t e s

p r e s e n t are n o t b e i n g

significantly sorbed.

Interactions between molybdenum and various solids T h e r e s u l t s o f t h e s e e x p e r i m e n t s (given in T a b l e II a n d ( F i g . 3 a n d 4 ) are in general a g r e e m e n t w i t h t h e r e s u l t s o f t h e p r e v i o u s e x p e r i m e n t . M o O 2 ÷ is s l o w l y s o r b e d o n m o s t o f t h e solids w i t h t h e a m o u n t s i n c r e a s i n g w i t h t i m e a n d d e c r e a s i n g p H .

49

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50

K.K. BERT1NE

Because of the similar behavior of molybdenum in the "sea water" sulfate solutions and sulfate-free solutions containing Ca-lactate, peat and kaolinite, it may be concluded that sorption of thiooxymolybdates on the various solids is not significant. This observed similarity also excludes the direct precipitation of molybdenum sulfide in natural systems on the time scale of weeks.

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Fig.3. Percentage of molybdenum in sulfate-free sea water in contact with various solids after inoculation with sulfate-reducing bacteria. In the course of the "sea water" sulfate solution experiments, it was observed that both the montmorillonite and illite contained varying amounts of iron which dissolved and formed iron sulfide during the experiment. It is perhaps significant that the montmorillonite, which was visually observed to have had less iron sulfide formed, also had less molybdenum coprecipitated. Up to approximately 70 % of the total molybdenum was coprecipitated with the iron sulfide in the illite and iron (originally added as ferric hydroxide which reacted with hydrogen sulfide to form iron sulfide) experiments (Fig.4). An effort was made to determine in what form the molybdenum was precipitated.

THE D E P O S I T I O N O F M O L Y B D E N U M I N A N O X I C W A T E R S

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The iron sulfide precipitate from the iron experiment and the illite plus iron sulfide precipitate from the illite experiment were filtered through 0.45 # Millipore filters. The total activities of the 99Mo on each of the membranes were determined. The filters were then treated with 100 ml of HC1 (pH = 4) and boiled for one hour to dissolve the iron sulfide. The resulting solutions were then filtered through 0.45/~ Millipore filters. The 99Mo activities on the filters were counted and compared to those before dissolution of the iron sulfide. Seventy-two percent of the molybdenum was left on the membrane from the iron experiment and 68 % on the membrane from the illite experiment. The membrane from the iron experiment was then boiled with 1N HNO3 for 15 minutes, and then the solution was filtered through 0.45/a Millipore filter. The membrane was then counted for 99Mo activity. There was no significant activity over background.

52

K . K . BERTINE

MoS3 does not dissolve in HC1 as does FeS but does dissolve in 1N HNO3 (Hillebrand and Lundell, 1953). It is therefore probable that most of the final precipitate of molybdenum was this compound. From the information cited above, and the lack of sorption on freshly-precipitated iron sulfide in the sorption experiments, the most probable method of molybdenum deposition may involve solid solution as: FeSamorphou s + MOO2S22- + H2S + 2H ÷ = FeS • MOS3amorphous + 2H20 As this amorphous compound ages: FeS

• MOS3amorphous =

FeStetr"

+ M o S 3.

Since it appears that the dissolved molybdenum slightly increases with time (Fig.4), it seems probable that some of the MoS3 is dissolving.

Sorption of molybdenum on peat Szilagyi (1967) showed that humic acids from peats have the ability to reduce molybdate to a cationic form (presumably MoO2 +) and hence to sorb the species on the time; scale of hours without the presence of sulfate-reducing bacteria (Fig.5). This effect is

100-

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This work

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.~ 5 0 o 40O :502010I

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Fig.5. The adsorption of molybdenum on peat as a function of pH on the time scale of one hour.

strongly pH dependent. Two additional points at marine values of pH indicate a confirmation of Szilagyi's experiment and indicate that the effect is fairly small in most marine humic sediments.

THE DEPOSITION OF MOLYBDENUM IN ANOXIC WATERS

53

CONCLUSIONS The results of these experiments clearly point up the importance of the time scale. On a short time scale (days) the coprecipitation o f m o l y b d e n u m with iron sulfide is most important, occurring where you have reducing conditions at the sediment-water interface, in the interstitial water, or in the water column itself. However, if the slow sorption of Mo(V) cited previously continues for years, the removal o f m o l y b d e n u m from the water column by this process may be more important than the coprecipitation of molybdenum with iron sulfide. Evidence for this contention is the high m o l y b d e n u m concentrations found in marine humic acids (up to 2,700 p.p.m. Mo, A. Nissenbaum, personal communication, 1970). Experiments on the time scale o f years are needed to clarify this situation. ACKNOWLEDGEMENTS I would like to thank K. K. Turekian and R. Berner for their advice and support during this research, which was supported b y AEC Grant AT(30-1)-2912. REFERENCES Berner, R. A., 1964. Stability fields of iron minerals in anaerobic marine sediments. J. Geol., 72: 826834. Bertine, K. K., 1970. The Marine Geochemical Cycle of Chromium and Molybdenum. Thesis. Yale University, New Haven, Conn., 67 pp. Garrels, R. M. and Christ, C. L., 1965. Solutions, Minerals and Equilibria. Harper Row, New York, N.Y., 450 pp. Goldschmidt, V. D., 1954. Geochemistry. Clarendon, Oxford, 730 ppo Hillebrand, W. F. and Lundell, G. E. F., 1958. Appliedlnorganic Analysis. Wiley, New York, N.Y., 1034 pp. Kharkar, D. P., Turekian, K. K. and Bertine, K. K., 1968. Stream supply of dissolved Ag, Mo, Sb, Se, Cr, Co, Rb and Cs to the oceans. Geochim. Cosmochim. Acta, 32: 285-298. Krauskopf, K. B., 1955. Sedimentary deposits of rare metals. In: A. M. Bateman (Editor), Econ. Geol., 50th Anniversary Volume, 1905-1955~ Part 1 : 411-463. Lyman, J. and Fleming, R. H., 1940. Composition of sea water. J. Mar. Res., 3: 134-146. Manheim, F. T., 1961. A geochemical prof'fle in the Baltic Sea. Geochim. Cosmochim. Acta, 25: 52-70. Pilipchuk, M. F. and Volkov, I. I., 1968. The geochemistry of molybdenum in the Black Sea. Lith. Miner. Res., 4: 389-407. Sandell, E. B., 1950. Colorimetric Determination of Traces of Metals. Interscience, New York, N.Y., 2nd ed., 673 pp. Sugawara, K., Okabe, S. and Tanaka, N., 1961. Geochemistry of molybdenum in natural waters (II). J. Earth. Sci.,Nagaya Univ., 9: 114-128. Szilagyi, M., 1967. Sorption of molybdenum by humus preparations. Geochem. ln t., 4:1165-1167. Titley, S. R., 1963. Some behavioural aspects of molybdenum in the supergene environment. Trans. AIME, 226: 199-204. Wedepohl, K. H., 1968. Chemical fractionation in the sedimentary environment. In: L. H. Ahrens (Editor), Origin and Distribution of the Elements. Pergamon, Oxford, pp. 999-1016. Yatsimerskii, K. B. and Zakharova, L. A., 1963. Spectrophotometric study of thiosalts of molybdenum in solution. Russ. J. Inorg. Chem., 8: 48-50.