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Sorption and desorption of iridium by coastal sediment: effects of iridium speciation and sediment components
Chemical Geology 166 Ž2000. 15–22 www.elsevier.comrlocaterchemgeo
Sorption and desorption of iridium by coastal sediment: effects of iridium speciation and sediment components Xiongxin Dai 1, Zhifang Chai ) , Xueying Mao, Hong Ouyang Institute of High Energy Physics and Laboratory of Nuclear Analysis Techniques, Academia Sinica, P.O. Box 2732, Beijing 100080, China Received 28 October 1998; accepted 18 August 1999
Abstract The sorption and desorption behaviors of trace iridium in a coastal seawater–sediment system were studied via batch experiments. The linear sorption and desorption isotherms were observed in the range of 0.1–1.0 ngrml Ir concentration, and the sorption–desorption process is irreversible. By using selective extraction method, the effects of Ir speciation in seawater and of sediment components on Ir sorption and desorption were investigated. The results indicate that the tetravalent Ir is easier to be sorbed by sediment than the trivalent iridium. Regarding the coastal sediment used in our study, the residue is the most important component for Ir sorption, whereas carbonate is not an effective absorbent component for iridium in sediment. The irreversible sorption–desorption process and very low desorption ability of iridium might lead to Ir enrichment on the sediment. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Iridium; Sorption; Desorption; Coastal sediment; Seawater; Speciation
1. Introduction It is well known that iridium is an important geochemical marker in sedimentary environments ŽChai, 1988; Sawlowicz, 1993.. The discovery of an Ir anomaly in CretaceousrTertiary ŽKrT. boundary clay and sediments engendered the hypothesis that a meteorite collided with Earth 65 million years ago, being in coincidence with the global mass extinction occurred at that time ŽAlvarez et al., 1980.. But it is not imperative that anomalous Ir enrichment is only caused by an extraterrestrial impact. Large-scale vol-
) Corresponding author. Fax: q86-10-62573660; e-mail: [email protected] 1 E-mail: [email protected].
canic eruption ŽOfficer and Drake, 1985; Officer et al., 1987. and geochemical deposition could also create or modify these unusual Ir spikes ŽSchmitz, 1985; Hallam, 1987; Strong et al., 1987; Colodner et al., 1992; Evans and Chai, 1997.. However, a critical evaluation of the above-mentioned interpretations needs the proper understanding of enrichment mechanisms and geochemical behaviors of iridium, particularly, in marine environment. On the other hand, iridium is also one of the least abundant elements in the marine environment and has always escaped systematic study by environmental and marine scientists. So far there are few data available for Ir abundance in marine system, especially in seawater ŽHodge et al., 1986; Anbar et al., 1996.. Knowledge about the chemical speciation, distribution pattern and sorption–desorption behavior
0009-2541r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 5 4 1 Ž 9 9 . 0 0 1 8 3 - 7
16
X. Dai et al.r Chemical Geology 166 (2000) 15–22
of iridium in marine environment is scarce. In the meantime, the concentration of platinum-group elements in the environment is significantly increasing with more and more use of them in industrial catalysts, autocatalytic converters and anticancer drugs, etc. Therefore, it is desirable to study their transportation and transformation mechanisms in the environment. In this paper, the sorption and desorption behaviors of iridium in seawater–sediment system were studied through the batch experiments. The effects of sediment components Žincluding carbonate, Fe–Mn oxide, organic matter, sulfide and residue. on sorption and desorption of iridium were evaluated by a selective extraction method.
2. Materials and methods
2.1. Seawater and sediments The seawater used in this study is a coastal water in the Yellow Sea and collected from the cape of Taiping of the Qingdao city, China. After filtration through a 0.45-mm membrane filter, the seawater was stored in a polyethylene bottle. The salinity, pH, and total organic carbon of the seawater are 3.10%, 7.78, and 2.11 mgrl, respectively. The sediment sample we chose is the coastal sediment reference material GBW07314, which was collected from the Hangzhou Bay in the East China Sea. A selective extraction method introduced by Tessier et al. Ž1979. was applied to successively remove carbonate, Fe–Mn oxide, organic matter and sulfide from the sediment. The experimental steps of this method are shown in Fig. 1. At each step, 5 g leftover sediment was taken out and washed with deionized water to neutral pH, ground to 200 mesh as the treated sediment for the sorption and desorption experiments. Major element compositions of the untreated and treated sediments were analyzed with a Phillips PW 1400 X-ray fluorescence system, and listed in Table 1. The analytical precision is better than 5%.
2.2. Preparation of
19 2
Ir radiotracer
The radiotracer 192 Ir was formed via 191 IrŽ n, g . Ir reaction by using chemical reagent ŽNH 4 . 2IrCl 6 which was irradiated in a nuclear reactor for 5 days Žneutron flux 2 = 10 13 n cmy2 sy1 .. After cooling for 1 month, the irradiated ŽNH 4 . 2 IrCl 6 was dissolved in seawater and settled for 2 months, then diluted for sorption and desorption experiments. 192
2.3. Sorption and desorption experiments The batch sorption experiments were performed in polyethylene centrifuge tube in duplicate. Samples Ž0.1 g. of the untreated or treated sediments were shaken at 25 " 18C with 5.0 ml seawater containing 192 Ir radiotracer in the Ir concentration range of 0.118–1.18 ngrml. The preliminary experiment indicated that a fast stable state of Ir sorption at marine sedimentrseawater interface was reached in 2 or 3 h. Thus, the shaking time was chosen to be 5 h. Afterwards, the suspensions were centrifuged at 4000 rpm for 10 min. Then, 1 ml supernatant was analyzed for Ir concentration with an HPGe detector at 192 Ir g-ray energy 316.50 keV, and the pH values were determined with a glass electrode pH meter. The amounts of Ir sorbed by sediments were deduced from the difference between the amounts of the added Ir and the remaining Ir in supernatants. The blank experiment was performed at the same time, and no sorption of Ir by polyethylene centrifuge tube was observed. Desorption experiments were then carried out. Supernatants Ž3 ml each. were pipetted from the centrifugation tube after the sorption experiments, and the same volume of seawater without iridium was added. A 5-h washing time was used, and other experimental steps and methods were the same as in the sorption experiments. Finally, the concentrations of Ir in solution were determined and the Ir amounts sorbed by sediments were calculated. 2.4. Near ultraÕiolet and Õisible absorption spectrum analysis The chemical reagents K 2 IrCl 6 , K 3 IrCl 6 and K 2 wIrŽH 2 O.Cl 5 x were prepared and purified through
X. Dai et al.r Chemical Geology 166 (2000) 15–22
17
Fig. 1. Flow chart of selective extraction procedure.
the methods reported by Poulsen and Clifford Ž1965. and Adpinoba Ž1964.. The iridium and chloride contents in these reagents were determined by precise coulometric titration method, the atomic ratios be-
tween Ir and Cl indicated that the purity of these compounds were more than 98%. For the near ultraviolet and visible absorption spectrum analysis, the K 2 IrCl 6 , K 3 IrCl 6 and K 2 wIr-
X. Dai et al.r Chemical Geology 166 (2000) 15–22
18
Table 1 Major element compositions of the untreated and treated coastal sediment GBW07314 Ž%.
SiO 2 Al 2 O 3 TiO 2 FeO Fe 2 O 3 MnO CaO MgO K 2O Na 2 O P2 O5 LOI Total
Untreated sediment
Fraction 2 a
Fraction 3 b
Residue c
60.61 13.10 0.85 1.86 3.45 0.13 4.19 2.40 2.50 1.61 0.15 9.06 99.91
67.86 14.16 0.88 1.27 3.20 0.10 1.08 2.08 2.29 1.12 0.14 5.51 99.69
69.34 14.81 0.96 0.63 1.69 0.06 0.83 1.81 2.65 1.40 0.13 5.24 99.55
69.48 15.10 0.93 0.71 1.65 0.06 0.75 1.77 2.71 1.42 0.15 4.85 99.58
a
Sediment treated to remove carbonate. Fraction 2 treated to remove Fe–Mn oxide. c Fraction 3 treated to remove organic matter and sulfide. b
ŽH 2 O.Cl 5 x reagents were first dissolved in seawater. Ž0.1 mg Irrml., IrCl 3y Then, 10 ml of the IrCl 2y 6 6 Ž1.34 mg Irrml. and IrŽOH 2 .Cl 2y Ž1.14 mg Irrml. 5 solution were respectively mixed with 0.2 g of the
untreated sediment for the sorption experiment. Fi3y nally, the IrCl 2y and IrŽOH 2 .Cl 2y solution 6 , IrCl 6 5 before sorption and the supernatants after centrifugation were analyzed with a BECKMAN DU 640 Spectrophotometer.
3. Results and discussion 3.1. Chemical species of Ir in seawater The speciation of iridium in the marine environment seems to have been little discussed in literature because of its very low concentration. In seawater, the species of Ir are most likely to be oxy–hydroxy and chloro complex ion forms ŽFergusson, 1992.. Comparable to Pt and Pd, Ir shows complicated chemical behaviors, and its hydrolysis reactions are of greater importance ŽGoldberg, 1987.. Therefore, in order to check the sorption ability of different Ir species by the sediment, the near ultraviolet and 3y and visible spectrum analyses of IrCl 2y 6 , IrCl 6 2y IrŽOH 2 .Cl 5 before and after sorption by the untreated sediment were carried out in relative high Ir concentration. Their near ultraviolet and visible ab-
3y Fig. 2. Near ultraviolet and visible absorption spectra of IrCl 2y and IrŽOH.Cl 2y in seawater before and after sorption by the 6 , IrCl 6 5 sediment GBW07314 at pH 7.7.
X. Dai et al.r Chemical Geology 166 (2000) 15–22
19
stronger than those of the trivalent Ir species. These results are similar to the sorption behaviors of Ir on the anion exchanger by which only the higher oxidation state of Ir complexes were strongly sorbed ŽYi and Masuda, 1996.. Therefore, the sorption ability of Ir should be highly dependent on the chemical species Žespecially the valent state. of Ir in sediment– seawater system. 3.2. Sorption and desorption of Ir by sediment Fig. 3 gives the sorption and desorption isotherms of Ir in the untreated and treated sediments–seawater system at pH 7.7 " 0.2. Within the range of Ir concentration used in these experiments, all the isotherms are linear and can be described as: K S s SSrCS K D s SD rC D
Fig. 3. Sorption and desorption isotherms of the untreated and treated sediments at pH 7.7 and 258C. F1: Untreated sediment; I, Sorption; B, Desorption. F2: Fraction 2; ^, Sorption; ', Desorption. F3: Fraction 3; `, Sorption; v, Desorption. F4: Residue; e, Sorption; l, Desorption.
sorption spectra were shown in Fig. 2. After sorption, the absorption peaks of the IrCl 2y species at 6 434 and 488 nm wavelength disappeared, but the intensities of the absorption peaks of IrCl 3y and 6 IrŽOH 2 .Cl 2y had no decrease. It was indicated that 5 the sorption ability of the tetravalent Ir was much
Ž 1.
where K is the distribution coefficient Žmlrg., C and S are the Ir concentration Žngrml. in solution phase and absorbed content Žngrg. by the sediment at equilibrium, the subscripts S and D represent sorption and desorption equilibrium, respectively. Through the linear fitting, we can obtain the sorption Ž K S . and desorption Ž K D . distribution coefficients, i.e., the slopes of the isotherms. For all these four sediments, the values of K D are twice the corresponding K S or more Žas seen in Table 2.. It indicates that the sorption–desorption process of Ir is irreversible. This result is likely due to the strong . binding ability of Ir Žespecially the species of IrCl 2y 6 on the sediment, thus the Ir sorbed by the sediment is
Table 2 K S , K D and DP% values of Ir in the untreated and treated sediments Sample Untreated Sediment Fraction 2 b Fraction 3 c Residued a
Sorption K S Žmlrg.
ra
Desorption K D Žmlrg.
ra
Desorption percentage ŽDP%.
19.9 " 0.6 25.6 " 0.7 22.5 " 1.0 23.0 " 0.6
0.9828 0.9865 0.9637 0.9842
43.0 " 2.8 57.7 " 2.4 51.6 " 2.2 60.6 " 2.2
0.9769 0.9903 0.9949 0.9961
9.7% 4.6% 4.1% y2.4%
Linear relation coefficient. Sediment treated to remove carbonate. c Fraction 2 treated to remove Fe–Mn oxide. d Fraction 3 treated to remove organic matter and sulfide. b
X. Dai et al.r Chemical Geology 166 (2000) 15–22
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difficult to be desorbed. The irreversible sorption– desorption mechanism and such low desorption percentage can give rise to Ir enrichment in the sediment, because the sedimentation process is a continuous sorption–desorption one. Many times of sorption but little Ir desorption amount must cause Ir accumulation in the sediment. With the slow sedimentation rate and thick water depth, the Ir enrichment will be even more obvious. According to the mass balance relation, the Ir content at the beginning of desorption experiment equals to that in desorption equilibrium, i.e.: C S V l q SSW s C D V q S D W
Ž 2.
where V1 is the leftover sorption supernatant volume Ž2 ml. at the beginning of desorption experiment; V is the total solution volume Ž5 ml. in the desorption experiment; W is the mass of sediment Žg.. The desorption percentage DP% can be described as: DP% s
SS y S D SS
100
Ž 3.
Therefore, the following equation can be derived from Eqs. Ž1. – Ž3.: DP% s
1 y u K D rK S 1 q K SWrV
100
Ž 4.
where u is the ratio of Vl to V and equals to 0.4 in our experiments. All the values of K S , K D and DP% for the untreated and treated sediments are also listed in Table 2 in the following sequence: K S : Untreated Sediment- Fraction 3 ; Residue - Fraction 2, K D : Untreated Sediment- Fraction 3 - Fraction 2 ; Residue, DP%: Residue - Fraction 3 ; Fraction 2 Untreated Sediment. The effects of various components of sediment will be individually discussed as follows. 3.2.1. Carbonate After removing carbonate from the sediment, the increments of K S and K D may be explained by no or very little sorption of Ir on the carbonate at such a neutral pH value, and the removal of carbonate means the increase of the content of effective sediment components with stronger sorption ability for
iridium. In other words, the carbonate in sediment plays a ‘‘dilution role’’ for Ir sorption. The drop of DP% value for Fraction 2 is probably resulted from the reduction of absorption capacity of exchangeable sites of the sediment after the treatment with NaOAc–HOAc buffer at pH 5.0. 3.2.2. Fe–Mn oxide, organic matter and sulfide It is well known that Fe–Mn oxide has strong scavenging efficiency for trace metals. The very high Ir contents observed in manganese nodules ŽHodge et al., 1986.. Anbar et al. Ž1996. suggested that iron–manganese oxyhydroxides could scavenge Ir under oxidizing conditions. Thus, the K S and K D values have slight decrease after the removal of Fe–Mn oxide from Fraction 2, though the content of Fe–Mn oxide in the sediment GBW07314 is very low Žsee Table 1.. Trace metals can also be bound to organic matter and sulfide in various forms. Besides, it has already been found that microbial activity may have played an important role in the formation of iridium anomalies ŽDyer et al., 1980.. The incorporation of Ir with sulfide was also supposed by Graup and Spettel Ž1989. in study of the KrT boundary section of the Lattengebirge, Bavarian Alps. In our experiments, no difference for the K S values between the Fraction 3 and residue was found within experimental errors. It could be explained that the content of organic matter and sulfide in the sediment was too low to affect the sorption of iridium. 3.2.3. Residue Generally, the residue mainly consists of detrital silicate minerals, residual sulfide and small quantity of organic material ŽTessier et al., 1979., which may hold trace metals within their crystal structure. Once the metals are bound to the residue, they are not expected to be released into solution under normal conditions. High Ir concentration can be associated with silicate minerals in a number of sediments. Elliot et al. Ž1989. reported that Ir may be incorporated into smectite as it formed from vitric ash. The close association of Ir with clay minerals has been reported in the KrT boundary at Gubbio ŽRocchia et al., 1990.. Schmitz et al. Ž1988. also suggested that
X. Dai et al.r Chemical Geology 166 (2000) 15–22
kerogen recovered by leaching the clay from Stevns Klint, Denmark, in HCl and HF enriched about 50% of Ir present in the bulk sample. With regard to the sediment GBW07314, the slight changes of K S values from Fraction 2 to residue indicated that the relative low content of Fe–Mn oxide, organic matter and sulfide are not yet the main contributors of Ir sorption, whereas the residue phase is the most important component for the sorption of iridium. In addition, iridium was associated very closely with the residue. The fact that DP% of the residue is less than zero clearly indicates that this fraction of Ir is hard to be desorbed.
4. Conclusions From the Ir sorption and desorption data and the result of visible absorption spectrum analysis, the following can be concluded. Ž1. Within the range of 0.1–1.0 ngrml Ir concentration, the sorption and desorption isotherms of iridium in coastal sediment–seawater system are linear, and the sorption and desorption distribution coefficient are 20 and 43 mlrg, respectively. The sorption–desorption process is irreversible, and the desorption percentage is less than 10%. This irreversible sorption and desorption mechanism might cause the Ir enrichment in the sediment. Ž2. In seawater, iridium is mainly absorbed by sediment as the tetravalent species and the absorbed Ir amounts of the trivalent species are minor. Ž3. For the sediment GBW07314, most of Ir are closely incorporated into the residue phase, and this fraction of Ir can not be released in normal conditions in natural environment. Ž4. The sorption of carbonate is very weak, and carbonate only plays a ‘‘dilution role’’ for Ir sorption.
Acknowledgements This work is financially supported by National Natural Science Foundation of China ŽContract No. 29771031., Chinese Academy of Sciences Major Project ŽContract No. 21039751. and Foundation of Laboratory of Nuclear Analysis Techniques. [JD]
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References Adpinoba, O.N., 1964. Synthesis for Complexes of Platinum Metals Žin Russian.. Science Press, Moscow, pp. 236. Alvarez, L.W., Alvarez, W., Asaro, F., Michel, H.V., 1980. Extraterrestrial cause for the Cretaceous–Tertiary extinction. Science 208, 1095–1108. Anbar, A.D., Wasserburg, G.J., Papanastassion, D.A., Andersson, P.S., 1996. Iridium in natural waters. Science 273, 1524–1528. Chai, C.F., 1988. Neutron activation analysis of platinum group elements as indicators of extraterrestrial materials. Isotopenpraxis 24, 257–272. Colodner, D.C., Boyle, E.A., Edmond, J.M., Thomson, J., 1992. Post-depositional mobility of platinum, iridium and rhenium in marine sediments. Nature 358, 402–404. Dyer, B.D., Lyalikova, N.N., Murray, D., Doyle, M., Kolesov, G.M., Krumbein, W.E., 1980. Role of microorganisms in the formation of iridium anomalies. Geology 17, 1036–1039. Elliot, W.C., Aronson, J.L., Millard, H.T. Jr., GierlowskiKordesch, E., 1989. The origin of the clay minerals at the CretaceousrTertiary boundary in Denmark. Geol. Soc. Am. Bull. 101, 702–710. Evans, N.J., Chai, C.F., 1997. The distribution and geochemistry of platinum-group elements as event markers in the Phanerozoic. Palaeogeogr., Palaeoslimatol., Palaeoecol. 132, 373–390. Fergusson, J.E., 1992. Noble metals in the environment. In: Brooks, R.R. ŽEd.., Noble Metals and Biological Systems. CRC Press, FL, USA, pp. 246–247. Goldberg, E.D., 1987. Comparative chemistry of the platinum and other heavy metals in the marine environment. Pure Appl. Chem. 59, 565–571. Graup, G., Spettel, B., 1989. Mineralogy and phrase-chemistry of an Ir-enriched pre-KrT layer from the Lattengebirge, Bavarian Alps, and significance for the KTB problem. Earth Planet. Sci. Lett. 95, 271–290. Hallam, A., 1987. End-Cretaceous mass extinction event, argument for terrestrial causation. Science 238, 1237–1242. Hodge, V., Stallard, M., Koide, M., Goldberg, E.D., 1986. Determination of platinum and iridium in marine waters, sediments, and organisms. Anal. Chem. 58, 616–620. Officer, C.B., Drake, C.L., 1985. Terminal Cretaceous environmental events. Science 277, 1161–1167. Officer, C.B., Hallam, A., Draker, C.L., Devine, J.D., 1987. Late Cretaceous and Paroxysmal CretaceousrTertiary extinctions. Nature 326, 143–149. Poulsen, I.A., Clifford, S.G., 1965. A thermodynamic and kinetic study of hexachloro and aquopentachloro complexes of iridiumŽIII. in aqueous solutions. J. Am. Chem. Soc. 84, 2032– 2037. Rocchia, R., Boclet, D., Bonte, Ph., Jehanno, C., Chen, Y., Courtillot, V., Mary, C., Wezel, F., 1990. The Cretaceous– Tertiary boundary at Gubbio revisited, vertical extent of the Ir anomaly. Earth Planet. Sci. Lett. 99, 206–219. Sawlowicz, Z., 1993. Iridium and other platinum-group elements as geochemical marks in sedimentary environments. Palaeogeogr., Palaeoclimatol., Palaeoecol. 104, 253–270. Schmitz, B., 1985. Metal precipitation in the Cretaceous–Tertiary
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boundary clay at Stevns Klint, Denmark. Geochim. Cosmochim. Acta 49, 2361–2370. Schmitz, B., Andersson, P., Dahl, J., 1988. Iridium, sulfur isotopes and rare earth elements in the Cretaceous–Tertiary boundary clay at Stevns Klint, Denmark. Geochim. Cosmochim. Acta 52, 229–236. Strong, C.P., Brooks, R.R., Wilson, S.M., Reeves, R.D., Orth, C.J., Mao, X.Y., Quintana, L.R., Anders, E., 1987. A new Cretaceous–Tertiary boundary site at Flaxbourne River, New
Zealand, biostratigraphy and geochemistry. Geochim. Cosmochim. Acta 51, 2769–2777. Tessier, A., Campbell, P.G.C., Bisson, M., 1979. Sequential extraction procedure for the speciation of particulate trace metals. Anal. Chem. 51, 844–850. Yi, Y.V., Masuda, A., 1996. Simultaneous determination of ruthenium, palladium, iridium, and platinum at ultratrace levels by isotope dilution inductively coupled plasma mass spectrometry in geological samples. Anal. Chem. 68, 1444–1450.
Sorption and desorption of iridium by coastal sediment: effects of iridium speciation and sediment components Xiongxin Dai 1, Zhifang Chai ) , Xueying Mao, Hong Ouyang Institute of High Energy Physics and Laboratory of Nuclear Analysis Techniques, Academia Sinica, P.O. Box 2732, Beijing 100080, China Received 28 October 1998; accepted 18 August 1999
Abstract The sorption and desorption behaviors of trace iridium in a coastal seawater–sediment system were studied via batch experiments. The linear sorption and desorption isotherms were observed in the range of 0.1–1.0 ngrml Ir concentration, and the sorption–desorption process is irreversible. By using selective extraction method, the effects of Ir speciation in seawater and of sediment components on Ir sorption and desorption were investigated. The results indicate that the tetravalent Ir is easier to be sorbed by sediment than the trivalent iridium. Regarding the coastal sediment used in our study, the residue is the most important component for Ir sorption, whereas carbonate is not an effective absorbent component for iridium in sediment. The irreversible sorption–desorption process and very low desorption ability of iridium might lead to Ir enrichment on the sediment. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Iridium; Sorption; Desorption; Coastal sediment; Seawater; Speciation
1. Introduction It is well known that iridium is an important geochemical marker in sedimentary environments ŽChai, 1988; Sawlowicz, 1993.. The discovery of an Ir anomaly in CretaceousrTertiary ŽKrT. boundary clay and sediments engendered the hypothesis that a meteorite collided with Earth 65 million years ago, being in coincidence with the global mass extinction occurred at that time ŽAlvarez et al., 1980.. But it is not imperative that anomalous Ir enrichment is only caused by an extraterrestrial impact. Large-scale vol-
) Corresponding author. Fax: q86-10-62573660; e-mail: [email protected] 1 E-mail: [email protected].
canic eruption ŽOfficer and Drake, 1985; Officer et al., 1987. and geochemical deposition could also create or modify these unusual Ir spikes ŽSchmitz, 1985; Hallam, 1987; Strong et al., 1987; Colodner et al., 1992; Evans and Chai, 1997.. However, a critical evaluation of the above-mentioned interpretations needs the proper understanding of enrichment mechanisms and geochemical behaviors of iridium, particularly, in marine environment. On the other hand, iridium is also one of the least abundant elements in the marine environment and has always escaped systematic study by environmental and marine scientists. So far there are few data available for Ir abundance in marine system, especially in seawater ŽHodge et al., 1986; Anbar et al., 1996.. Knowledge about the chemical speciation, distribution pattern and sorption–desorption behavior
0009-2541r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 5 4 1 Ž 9 9 . 0 0 1 8 3 - 7
16
X. Dai et al.r Chemical Geology 166 (2000) 15–22
of iridium in marine environment is scarce. In the meantime, the concentration of platinum-group elements in the environment is significantly increasing with more and more use of them in industrial catalysts, autocatalytic converters and anticancer drugs, etc. Therefore, it is desirable to study their transportation and transformation mechanisms in the environment. In this paper, the sorption and desorption behaviors of iridium in seawater–sediment system were studied through the batch experiments. The effects of sediment components Žincluding carbonate, Fe–Mn oxide, organic matter, sulfide and residue. on sorption and desorption of iridium were evaluated by a selective extraction method.
2. Materials and methods
2.1. Seawater and sediments The seawater used in this study is a coastal water in the Yellow Sea and collected from the cape of Taiping of the Qingdao city, China. After filtration through a 0.45-mm membrane filter, the seawater was stored in a polyethylene bottle. The salinity, pH, and total organic carbon of the seawater are 3.10%, 7.78, and 2.11 mgrl, respectively. The sediment sample we chose is the coastal sediment reference material GBW07314, which was collected from the Hangzhou Bay in the East China Sea. A selective extraction method introduced by Tessier et al. Ž1979. was applied to successively remove carbonate, Fe–Mn oxide, organic matter and sulfide from the sediment. The experimental steps of this method are shown in Fig. 1. At each step, 5 g leftover sediment was taken out and washed with deionized water to neutral pH, ground to 200 mesh as the treated sediment for the sorption and desorption experiments. Major element compositions of the untreated and treated sediments were analyzed with a Phillips PW 1400 X-ray fluorescence system, and listed in Table 1. The analytical precision is better than 5%.
2.2. Preparation of
19 2
Ir radiotracer
The radiotracer 192 Ir was formed via 191 IrŽ n, g . Ir reaction by using chemical reagent ŽNH 4 . 2IrCl 6 which was irradiated in a nuclear reactor for 5 days Žneutron flux 2 = 10 13 n cmy2 sy1 .. After cooling for 1 month, the irradiated ŽNH 4 . 2 IrCl 6 was dissolved in seawater and settled for 2 months, then diluted for sorption and desorption experiments. 192
2.3. Sorption and desorption experiments The batch sorption experiments were performed in polyethylene centrifuge tube in duplicate. Samples Ž0.1 g. of the untreated or treated sediments were shaken at 25 " 18C with 5.0 ml seawater containing 192 Ir radiotracer in the Ir concentration range of 0.118–1.18 ngrml. The preliminary experiment indicated that a fast stable state of Ir sorption at marine sedimentrseawater interface was reached in 2 or 3 h. Thus, the shaking time was chosen to be 5 h. Afterwards, the suspensions were centrifuged at 4000 rpm for 10 min. Then, 1 ml supernatant was analyzed for Ir concentration with an HPGe detector at 192 Ir g-ray energy 316.50 keV, and the pH values were determined with a glass electrode pH meter. The amounts of Ir sorbed by sediments were deduced from the difference between the amounts of the added Ir and the remaining Ir in supernatants. The blank experiment was performed at the same time, and no sorption of Ir by polyethylene centrifuge tube was observed. Desorption experiments were then carried out. Supernatants Ž3 ml each. were pipetted from the centrifugation tube after the sorption experiments, and the same volume of seawater without iridium was added. A 5-h washing time was used, and other experimental steps and methods were the same as in the sorption experiments. Finally, the concentrations of Ir in solution were determined and the Ir amounts sorbed by sediments were calculated. 2.4. Near ultraÕiolet and Õisible absorption spectrum analysis The chemical reagents K 2 IrCl 6 , K 3 IrCl 6 and K 2 wIrŽH 2 O.Cl 5 x were prepared and purified through
X. Dai et al.r Chemical Geology 166 (2000) 15–22
17
Fig. 1. Flow chart of selective extraction procedure.
the methods reported by Poulsen and Clifford Ž1965. and Adpinoba Ž1964.. The iridium and chloride contents in these reagents were determined by precise coulometric titration method, the atomic ratios be-
tween Ir and Cl indicated that the purity of these compounds were more than 98%. For the near ultraviolet and visible absorption spectrum analysis, the K 2 IrCl 6 , K 3 IrCl 6 and K 2 wIr-
X. Dai et al.r Chemical Geology 166 (2000) 15–22
18
Table 1 Major element compositions of the untreated and treated coastal sediment GBW07314 Ž%.
SiO 2 Al 2 O 3 TiO 2 FeO Fe 2 O 3 MnO CaO MgO K 2O Na 2 O P2 O5 LOI Total
Untreated sediment
Fraction 2 a
Fraction 3 b
Residue c
60.61 13.10 0.85 1.86 3.45 0.13 4.19 2.40 2.50 1.61 0.15 9.06 99.91
67.86 14.16 0.88 1.27 3.20 0.10 1.08 2.08 2.29 1.12 0.14 5.51 99.69
69.34 14.81 0.96 0.63 1.69 0.06 0.83 1.81 2.65 1.40 0.13 5.24 99.55
69.48 15.10 0.93 0.71 1.65 0.06 0.75 1.77 2.71 1.42 0.15 4.85 99.58
a
Sediment treated to remove carbonate. Fraction 2 treated to remove Fe–Mn oxide. c Fraction 3 treated to remove organic matter and sulfide. b
ŽH 2 O.Cl 5 x reagents were first dissolved in seawater. Ž0.1 mg Irrml., IrCl 3y Then, 10 ml of the IrCl 2y 6 6 Ž1.34 mg Irrml. and IrŽOH 2 .Cl 2y Ž1.14 mg Irrml. 5 solution were respectively mixed with 0.2 g of the
untreated sediment for the sorption experiment. Fi3y nally, the IrCl 2y and IrŽOH 2 .Cl 2y solution 6 , IrCl 6 5 before sorption and the supernatants after centrifugation were analyzed with a BECKMAN DU 640 Spectrophotometer.
3. Results and discussion 3.1. Chemical species of Ir in seawater The speciation of iridium in the marine environment seems to have been little discussed in literature because of its very low concentration. In seawater, the species of Ir are most likely to be oxy–hydroxy and chloro complex ion forms ŽFergusson, 1992.. Comparable to Pt and Pd, Ir shows complicated chemical behaviors, and its hydrolysis reactions are of greater importance ŽGoldberg, 1987.. Therefore, in order to check the sorption ability of different Ir species by the sediment, the near ultraviolet and 3y and visible spectrum analyses of IrCl 2y 6 , IrCl 6 2y IrŽOH 2 .Cl 5 before and after sorption by the untreated sediment were carried out in relative high Ir concentration. Their near ultraviolet and visible ab-
3y Fig. 2. Near ultraviolet and visible absorption spectra of IrCl 2y and IrŽOH.Cl 2y in seawater before and after sorption by the 6 , IrCl 6 5 sediment GBW07314 at pH 7.7.
X. Dai et al.r Chemical Geology 166 (2000) 15–22
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stronger than those of the trivalent Ir species. These results are similar to the sorption behaviors of Ir on the anion exchanger by which only the higher oxidation state of Ir complexes were strongly sorbed ŽYi and Masuda, 1996.. Therefore, the sorption ability of Ir should be highly dependent on the chemical species Žespecially the valent state. of Ir in sediment– seawater system. 3.2. Sorption and desorption of Ir by sediment Fig. 3 gives the sorption and desorption isotherms of Ir in the untreated and treated sediments–seawater system at pH 7.7 " 0.2. Within the range of Ir concentration used in these experiments, all the isotherms are linear and can be described as: K S s SSrCS K D s SD rC D
Fig. 3. Sorption and desorption isotherms of the untreated and treated sediments at pH 7.7 and 258C. F1: Untreated sediment; I, Sorption; B, Desorption. F2: Fraction 2; ^, Sorption; ', Desorption. F3: Fraction 3; `, Sorption; v, Desorption. F4: Residue; e, Sorption; l, Desorption.
sorption spectra were shown in Fig. 2. After sorption, the absorption peaks of the IrCl 2y species at 6 434 and 488 nm wavelength disappeared, but the intensities of the absorption peaks of IrCl 3y and 6 IrŽOH 2 .Cl 2y had no decrease. It was indicated that 5 the sorption ability of the tetravalent Ir was much
Ž 1.
where K is the distribution coefficient Žmlrg., C and S are the Ir concentration Žngrml. in solution phase and absorbed content Žngrg. by the sediment at equilibrium, the subscripts S and D represent sorption and desorption equilibrium, respectively. Through the linear fitting, we can obtain the sorption Ž K S . and desorption Ž K D . distribution coefficients, i.e., the slopes of the isotherms. For all these four sediments, the values of K D are twice the corresponding K S or more Žas seen in Table 2.. It indicates that the sorption–desorption process of Ir is irreversible. This result is likely due to the strong . binding ability of Ir Žespecially the species of IrCl 2y 6 on the sediment, thus the Ir sorbed by the sediment is
Table 2 K S , K D and DP% values of Ir in the untreated and treated sediments Sample Untreated Sediment Fraction 2 b Fraction 3 c Residued a
Sorption K S Žmlrg.
ra
Desorption K D Žmlrg.
ra
Desorption percentage ŽDP%.
19.9 " 0.6 25.6 " 0.7 22.5 " 1.0 23.0 " 0.6
0.9828 0.9865 0.9637 0.9842
43.0 " 2.8 57.7 " 2.4 51.6 " 2.2 60.6 " 2.2
0.9769 0.9903 0.9949 0.9961
9.7% 4.6% 4.1% y2.4%
Linear relation coefficient. Sediment treated to remove carbonate. c Fraction 2 treated to remove Fe–Mn oxide. d Fraction 3 treated to remove organic matter and sulfide. b
X. Dai et al.r Chemical Geology 166 (2000) 15–22
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difficult to be desorbed. The irreversible sorption– desorption mechanism and such low desorption percentage can give rise to Ir enrichment in the sediment, because the sedimentation process is a continuous sorption–desorption one. Many times of sorption but little Ir desorption amount must cause Ir accumulation in the sediment. With the slow sedimentation rate and thick water depth, the Ir enrichment will be even more obvious. According to the mass balance relation, the Ir content at the beginning of desorption experiment equals to that in desorption equilibrium, i.e.: C S V l q SSW s C D V q S D W
Ž 2.
where V1 is the leftover sorption supernatant volume Ž2 ml. at the beginning of desorption experiment; V is the total solution volume Ž5 ml. in the desorption experiment; W is the mass of sediment Žg.. The desorption percentage DP% can be described as: DP% s
SS y S D SS
100
Ž 3.
Therefore, the following equation can be derived from Eqs. Ž1. – Ž3.: DP% s
1 y u K D rK S 1 q K SWrV
100
Ž 4.
where u is the ratio of Vl to V and equals to 0.4 in our experiments. All the values of K S , K D and DP% for the untreated and treated sediments are also listed in Table 2 in the following sequence: K S : Untreated Sediment- Fraction 3 ; Residue - Fraction 2, K D : Untreated Sediment- Fraction 3 - Fraction 2 ; Residue, DP%: Residue - Fraction 3 ; Fraction 2 Untreated Sediment. The effects of various components of sediment will be individually discussed as follows. 3.2.1. Carbonate After removing carbonate from the sediment, the increments of K S and K D may be explained by no or very little sorption of Ir on the carbonate at such a neutral pH value, and the removal of carbonate means the increase of the content of effective sediment components with stronger sorption ability for
iridium. In other words, the carbonate in sediment plays a ‘‘dilution role’’ for Ir sorption. The drop of DP% value for Fraction 2 is probably resulted from the reduction of absorption capacity of exchangeable sites of the sediment after the treatment with NaOAc–HOAc buffer at pH 5.0. 3.2.2. Fe–Mn oxide, organic matter and sulfide It is well known that Fe–Mn oxide has strong scavenging efficiency for trace metals. The very high Ir contents observed in manganese nodules ŽHodge et al., 1986.. Anbar et al. Ž1996. suggested that iron–manganese oxyhydroxides could scavenge Ir under oxidizing conditions. Thus, the K S and K D values have slight decrease after the removal of Fe–Mn oxide from Fraction 2, though the content of Fe–Mn oxide in the sediment GBW07314 is very low Žsee Table 1.. Trace metals can also be bound to organic matter and sulfide in various forms. Besides, it has already been found that microbial activity may have played an important role in the formation of iridium anomalies ŽDyer et al., 1980.. The incorporation of Ir with sulfide was also supposed by Graup and Spettel Ž1989. in study of the KrT boundary section of the Lattengebirge, Bavarian Alps. In our experiments, no difference for the K S values between the Fraction 3 and residue was found within experimental errors. It could be explained that the content of organic matter and sulfide in the sediment was too low to affect the sorption of iridium. 3.2.3. Residue Generally, the residue mainly consists of detrital silicate minerals, residual sulfide and small quantity of organic material ŽTessier et al., 1979., which may hold trace metals within their crystal structure. Once the metals are bound to the residue, they are not expected to be released into solution under normal conditions. High Ir concentration can be associated with silicate minerals in a number of sediments. Elliot et al. Ž1989. reported that Ir may be incorporated into smectite as it formed from vitric ash. The close association of Ir with clay minerals has been reported in the KrT boundary at Gubbio ŽRocchia et al., 1990.. Schmitz et al. Ž1988. also suggested that
X. Dai et al.r Chemical Geology 166 (2000) 15–22
kerogen recovered by leaching the clay from Stevns Klint, Denmark, in HCl and HF enriched about 50% of Ir present in the bulk sample. With regard to the sediment GBW07314, the slight changes of K S values from Fraction 2 to residue indicated that the relative low content of Fe–Mn oxide, organic matter and sulfide are not yet the main contributors of Ir sorption, whereas the residue phase is the most important component for the sorption of iridium. In addition, iridium was associated very closely with the residue. The fact that DP% of the residue is less than zero clearly indicates that this fraction of Ir is hard to be desorbed.
4. Conclusions From the Ir sorption and desorption data and the result of visible absorption spectrum analysis, the following can be concluded. Ž1. Within the range of 0.1–1.0 ngrml Ir concentration, the sorption and desorption isotherms of iridium in coastal sediment–seawater system are linear, and the sorption and desorption distribution coefficient are 20 and 43 mlrg, respectively. The sorption–desorption process is irreversible, and the desorption percentage is less than 10%. This irreversible sorption and desorption mechanism might cause the Ir enrichment in the sediment. Ž2. In seawater, iridium is mainly absorbed by sediment as the tetravalent species and the absorbed Ir amounts of the trivalent species are minor. Ž3. For the sediment GBW07314, most of Ir are closely incorporated into the residue phase, and this fraction of Ir can not be released in normal conditions in natural environment. Ž4. The sorption of carbonate is very weak, and carbonate only plays a ‘‘dilution role’’ for Ir sorption.
Acknowledgements This work is financially supported by National Natural Science Foundation of China ŽContract No. 29771031., Chinese Academy of Sciences Major Project ŽContract No. 21039751. and Foundation of Laboratory of Nuclear Analysis Techniques. [JD]
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