Speciation of cobalt, zinc and mercury in marine environment

Transcription

1 Indian Journal of Chemistry Vol. 40A, December2001, pp Speciation of cobalt, zinc and mercury in marine environment R S D Toteja, M Sudersanan* & R K Iyer Analytical Chemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai , India Received 15 May 2000; revised 3/ August 2001 Speciation of cobalt, zinc and mercury has been investigated using solid and liquid ion exchangers. Batch equilibration method and column chromatography have been used to speciate the metal ions. Cobalt species is found to be present mainly as a monopositive ion with an appreciable concentration of cobalt sulphate complexes. The uptake from anion exchanger is found to be less. In case of zinc, the metal ion is found to exist partly in the cationic, anionic and hydrolysed forms which exist in labile equilibrium. Mercury has been found to exist mainly as anionic chloro complexes alongwith hydrolysed or organically bound complexes. In all the cases, there is a good agreement with the data obtained using simulated and actual sea water, indicating that the general speciation of the metal ion is governed by the anions like chloride, sulphate and carbonate/hydroxide present in sea water. Information relating to speciation ts essential for understanding the toxtctty, bioavailability, bioaccumulation and transport characteristics of a given element. One of the major challenges of environmental chemistry has been to describe the behaviour of trace metal ions in natural aquatic environment1.2. Sea water has large concentrations of some major anions and cations like chloride, sulphate, carbonate, hydroxide, sodium, magnesium etc. The ph and the composition of sea water is fairly constant in a particular region 3.4. This makes sea water an ideal system for speciation studies. Hence, the speciation of cobalt, zinc and mercury has been studied from marine environment and the results are described in this paper. The study of the nature of the species by direct analytical determination is very difficult in view of their extremely low concentrations. On the other hand, calculation of various species on the basis of equilibrium constants or stability constants is also difficult in view of non-availability of various equilibrium constants 5 6 as well as a lack of knowledge on the nature and concentration of various ligands, especially in a complex environmental matrix. In such instances, the use of simulated solutions often provides valuable information on the nature of the species. Using this approach, the speciation of cobalt, zinc and mercury has been investigated and the results are reported in thi s paper. Experimental The radioisotopes, 60 Co, 65 Zn, 201 Hg were obtained from the Isotope Group, BARC and were assayed using a single channel analyser coupled to a Nai (Tl) detector (Electronics Corporation of India, Hyderabad). Dowex 50Wx8 and Dowex l x8 were used as the cation and anion exchanger respectively and were converted to sodium or chloride form by repeated equilibration with simulated sea water. Dinonylnaphthalene sulphonic acid (RT Vanderbilt and Co., USA), a liquid cation exchanger was converted to the sodium form before use by equilibration with simulated sea water. Simulated sea water was prepared by dissolving 32 g of NaCI, 14 g of MgS047H 2 0, and 0.29 g of NaHC0 3 in I litre of water. Sea water was collected from Trombay Jetty. It was mainly surface water collected at a distance of about 1 km from coast at a depth of a few inches from the surface. The samples were collected by Environmental Assessment Division in the usual manner. Other solutions containing 16 g of NaCI, 7 g of MgS04 7H 2 0 and 0.19 g of NaHC0 3 respectively in 100 ml were prepared and were used for studies. All the chemicals used were of analytical reagent grade. Ion exchange experiments using batch equilibration method were carried out by equilibrating 0.25g of the resin with 10 ml of the solution containing either simulated or actual sea water containing the appropriate radioactive tracers. After equilibration, the ph of the aqueous phase was measured using Elico ph meter and the radioactivity was assayed using Nal (TI) detector in a single channel analyser (M/s ECIL, Hyderabad). The distribution ratio was calculated from D={ [metal in resin] /[metal in solution]} x{[volumeofsolution]/[weightofresin]}... 1 In the case of dinonylnaphthalene sulphonic acid, the extractant was converted to the sodium form by repeated equilibration with sodium chloride.

2 1368 INDIAN J CHEM, SEC A, DECEMBER Extraction experiments were carried out by equilibrating 10 ml of aqueous phase containing the radioactive tracers with an equal volume of the organic phase. After equilibration, the two phases were separated, ph of the aqueous phase was measured using a ph meter and the radioactivity was assayed in both the phases. Background corrections were made as needed. All experiments were carried out at 27±2 C. Column chromatographic studies were made in the usual manner usi ng a small ion exhange column contai ning 1 g of the exchanger. The exchanger was conditioned with sea water before the experiments. Results and discussion The major anions present in sea water are ch loride, sulphate, hydroxide and carbonate in addition to various organic and inorganic anions. Among these, the first four are present in greater concentrations. In addition, the organic ligands, though present in smaller concentrations exert a larger influence on complexation behaviour in view of their greater chelation efficiency and hi gher stability constants. The heavy metal ions, especially, the transition metal ions show a pronounced tendency towards the formation of complexes. Hence, it was considered desirable to in vestigate the role of organic and/or inorganic complexing ligands on speciation. Hence, si mulated sea water was used for the studies, so that the influence of inorganic li gands can be investigated. A comparison with actual sea water, which contains in addition to the inorganic ligands, various organic anions also enab les to distinguish between organic and inorganic complexation effects. Cation exchange studies Cobalt Ion exchange uptake of cobalt was studied as a function of concentrations of chloride, sulphate and carbonate. The studies were made by varying the concentration of chloride at a constant concentration of sulphate and nearly constant ph. The plot of log D versus. log [Na] obtained at different concentrations of chloride was fou nd to be linear with slope 1. This indicates the formation of a chlorocomplex [Cocn. However, one has to take into account the si multaneous variation in sodium ion concentration. Moreover, the resin takes up the magnesium ions which are present in sea water. Hence, the variation of distribution ratio in presence of chloride ions can be interpreted as due to both chlorocomplex formation and the vanat10n in sodium ion concentration. However, it is interesting to observe that the data obtained on the variation of di stribution ratio of cobalt from simulated and actual sea water were nearly the same indicating that the presence of major ions determines the speciation behaviour of cobalt and the presence of other complexing agents in sea water has no major influence on speciation. The variation of distribution ratio of cobalt was studied as a function of sulphate ion concentration at a fixed concentration of chloride ion. The data in the case of both solid and liquid cation exchanger were similar and could be attributed to the formation of sulphate complexes. The data on the equ ili bri um constants K 0 and K obtai ned from the data in the absence and presence of sulphate ion concentration were analysed to obtain the values of stability constants of Co-sulphate complexes which are related to the equilibrium constants by the relation... (2) The plot of Kof K versus [S0 4 ] was linear for both solid and liquid cation exchangers. Further, the data obtained at ph 6 and 8 were nearl y the same. These results can be interpreted on the basis of formation of sulphato complexes with the stability constant of about 30. However, this value should be considered as an apparent value since the interaction, if any, between magnesium and sulphate may reduce the free li gand concentration of sulphate which may affect the values of stability constants. However, the results indicate the formation of cobalt sulphato complexes which reduces the uptake of cobalt by a liquid or solid exchanger. The variation of carbonate ion concentration or the simultaneous variation of ph did not substantially affect the uptake of cobalt. The contribution due to the soluble complexes of carbonate can, therefore, be neglected. However, at higher concentrations of the metal, CoC0 3 may precipitate if its solubility product is exceeded. The uptake of cobalt was studied both in the absence (ph-6) and presence (ph-8 ) of NaHC0 3. The two sets of data were almost identical indicating the absence of any soluble carbonate complexes. Ion exchange studies were carried out using sea water collected from the CIRUS Jetty at BARC and simultaneously another set of experiments were carried out using simulated sea water. It was found that the distribution ratio, D, obtained was virtually the same in both the cases. The studies were also

3 NOTES 1369 made as a function of ph in both simulated and actual sea water. The studies were also made as a function of NaCI concentration by dilution of actual or simulated sea water with di stilled water so as to vary the sodium ion concentrations. In all the cases, the data on actual and simulated sea water were identical indicating that the cation exchange behaviour of cobalt is determined only by the nature of predominant ions in solution and that the simulated solutions represent fairly accurately the behaviour of cobalt in sea water (Fig.l) Zinc Preliminary experiments indicated an adsorption of zinc on the walls of the glass container when the ph of the solution was high, while such losses were negligible when polythene containers were employed. Hence, all experiments were carried out using polythene containers when the ph of the solution was high. Ion exchange uptake of zinc was studied using both Dowex 50Wx8 and sodium form of dinonylnaphthalene sulphonic acid. The studies were carried out at a ph of about 3 when hydrolysis effects can be neglected and also at normal sea water ph. The plot of log D versus log[na] resulted in a line of slope 2 for the exchange of zinc from solutions containing only sodium chloride. However, in the presence of a mixture of NaCl and MgS0 4, as is typical of normal sea water environment, a line of slope one was obtained for the above plot suggesting the presence of zinc as a monopositive ionic species. In solutions containing both sodium and magnesium ions, the effective ionic concentration was taken as the sum of concentrations of sodium and magnesium, both the concentrations being expressed in equivalents. The results indicated an average charge of zinc species as one. The variation of the concentration of chloride at a constant concentration of magnesium was also studied at a higher ph of 8.2. The apparent distribution ratios were found to be higher, suggesting the possibility of hydrolysis of zinc at higher ph values. Studies carried out in glass containers at higher ph values also suggested an appreciable amount of hydrolysed species which may be adsorbed on a suitable solid matrix. Variation of the distribution ratio of zinc was also studied in presence of varying amounts of magnesium sulphate at a constant concentration of NaCI. Studies were carried out at a ph of 3.2 and 8.2. The distribution ratio for zinc decreased with an increase in concentration of sulphate due to the formation of sulphato complexes. The variation of NaHC0 3 was also investigated using simulated sea water. The results showed that the distribution ratios were nearly constant upto a ph of 4.0 whereas at ph above 7.0, there was a systematic enhancement in uptake. This increase in uptake can be attributed to the formation of hydrolysed forms of zinc as the ph of the solutions simultaneously increased with increase in concentration of NaHC0 3. The cation exchange studies, therefore, suggest the presence of monopositive species of zinc with appreciable presence of both sulphate and hydroxy species. The existence of such species has been reported by other workers also 5 6. Mercury The uptake of mercury by a cation exchanger was studied as a function of concentration of chloride ions. When the ph of the solution was low (3 or less) the uptake of mercury was poor. There was some uptake of mercury when the solution did not contain any chloride ion, but the uptake decreased on the addition of even O.lM NaCI. The uptake was practically independent of the concentration of NaCI. These results suggested the uptake of only neutral complexes, if any by the cation exchanger. The presence of magnesium in the sea water marginally reduced the uptake of mercury. The uptake of mercury was also studied as a function of ph. The uptake increased with ph and was about 50% when the ph of the solution was 8.3 corresponding to the normal sea water ph. These results can be explained on the basis of hydrolysis of mercury and are in broad agreement with the chemistry of mercury...,.x a 0. :::> ;:;, 50.0 a c..,.., 0.. o Actual Simulated 0 O.OL--L--L-L--LL-L Percentage OF Sea Water Fig.!-Uptake of cobalt (top), zinc (middle) and mercury (bottom) by cation exchanger. i 0

4 1370 INDIAN J CHEM, SEC A, DECEMB ER The uptake of mercury was also investigated from the si mulated and actual sea water at various ph conditions. The studies were made as a function of sea water content, expressed as a function of percentage of actual or simulated sea water. These studies provided information on the variation of uptake as a function of sodium ion concentration. The studies were made both at a relatively low ph (where the hydrolysis effects can be neglected) and at normal sea water ph in order to investi gate the possible role of hydrolysis on speciation. In the case of simulated sea water, at a ph of 2-3, the uptake of mercury was low ( 5 %). The uptake was nearly constant with change in sea water content. This indicates the absence of cationic species. Thus mercury may be present mainly as neutral or anionic species. This is also in agreement wi th the observations obtained in presence of onl y NaCI. Simil ar results were obtained in the case of actual sea water also. Studi es were al so made with actual and simulated sea water at higher ph. The uptake of mercury was dependent on ph. In the case of si mulated sea water, the uptake was low upto a ph of 4.0 but increased at higher ph values. The uptake increased to about 60% when the ph of the solution was about 8.0. In the case of actual sea water, the uptake was low at a ph of 3.2 but increased when the ph was above 5:2 and was practically complete at ph -7. The above findings suggest that the species of mercury changes with a change in ph. With increase in ph, the hydrolysis effect of mercury increases which results in an enhanced uptake by ion exchangers. However, the cationic form of mercury is not significant or probably absent in sea water conditions. The uptake of mercury was more from actual sea water compared to that from the simulated sea water, especially at hi gh ph, although the uptake was essentially the same at low ph from both the matrices. These results suggest the possibility of organic complexing and the formation of neutral complexes. This may be attributed to the greater tendency of mercury to form complexes with organic ligands. This tendency, alongwith the possibility of interconversion and methylation has been responsible for some trgedies involving mercur/. Anion exchange studies Cobalt The anion exchange of cobalt was studied using Dowex 1 x8 in chloride form as a function of concentration of NaCl and MgS0 4. The uptake decreased with increase in concentration of chloride in accordance with the usual ion exchange equilibrium.... (3) but the overall uptake was quite low indicating the lower predominance of anionic complexes. The uptake also decreased with an increase in the concentration of sulphate. An increase in the concentration of NaHC0 3 or ph resulted in a slight increase in uptake, suggesting the formation of hydrolysed species hi gher ph. Zinc Anion exchange studies of zinc were also carried out using Dowex Ix8 in chloride form by batch equilibration technique. The studies were made as a function of concentration of NaCl and MgS0 4 at ph of 3.6, 6.8 and 7.8. At ph 3.6 in the presence of onl y NaCl uptake of zinc increased with an increase in concentration of NaCI. The uptake increased at higher ph values of 6.8 and 7.8 which suggested the formation of hydrolysed forms of zinc. The uptake of zinc was studied at a constant concentration of chloride in presence of varying concentrations of sulphate. The studies were also made as a function of ph and concentration of NaHC0 3 and at lower and high ph in presence of only NaCI and magnesium sulphate. The general trend observed from the studies indicated that the formation of anionic or hydrolysed species of zinc increased and at a ph greater than 7, the entire uptake could be attributed to these species. The uptake of zinc by anion exchanger was also studied using simulated and actual sea water. These , (]\ "' c "" 0.. "'....., -----J A _. ---'---'----' 0. 0 eo.o Percentage OF Sea Wa ter Fig. 2-Uptake of mercury (top), zinc (middle) and cobalt (bottom) by anion exchanger.

5 NOTES 1371 studies carried out at different ph showed excellent agreement between simulated and actual sea water showing that simulated solutions adequately describe the behaviour of zinc in marine environments. (Fig. 2). Mercury The uptake of mercury by anion exchangers was studied from solutions containing only sodium chloride. The uptake was found to be complete even at low concentrations of NaCI. The studies were made at a low ph in order to differentiate between chloro and hydrolysed species of mercury. It was found that complete uptake of mercury occurred even at low ph suggesting the formation and uptake of mainly chloro complexes. This corroborates the general tendency of mercury to form strong chloro complexes and the low uptake of mercury by cation exchangers. Thus the major amount of mercury is present in the form of anionic complexes and the presence of mercury in the cationic form is relatively unimportant. The uptake of mercury was also studied from simulated and actual sea water as a function of the percentage of sea water. In case of simulated sea water, at low ph, the uptake was complete. Similar was the case with actual sea water also at lower ph, obtained by the addition of acid to actual sea water. The uptake was also studied at a higher ph of about 8. Although, the uptake was fairly high, of the order of 90%, the uptake decreased somewhat at higher ph. The change in ph increases both the hydrolysis and formation of complexes with organic ligands. Hydrolysis generally leads to an increase in uptake because the ion exchanger acts as a site for precipitation while the formation of strong soluble complexes with organic ligands leads to a decrease in uptake due to a reduction in the proportion of ion exchangeable species. Since a decrease in uptake was observed in the present case, complexation with organic ligands is more likely in addition to possible hydrolysis. These observations are in general agreement with the observations made from cation exchange studies. Column chromatographic studies Column chromatography combines the advantages of specificity of the ion exchangers and the multistage separation capabilities of chromatography. This study also gives information on the practical separation, pre-concentration or removal of various species. The studies were carried out using a small column of lg of cation or anion exchanger as well as silica gel. The retention of cobalt by cation, anion or silica gel column was studied by passing 60 Co in simulated or actual sea water through the column. The amount of cobalt remaining in the effluent was measured. The column was then washed with 10 mj of sea water and the metal ion remaining in the column was eluted by 1M HCI in the case of cation exchanger or silica gel and by 0.0 I M HCI in the case of anion exchanger. The uptake of cobalt was found to be similar for simulated and actual sea water in all the cases. There was an uptake of cobalt by cation exchanger but the uptake by anion exchanger was Jess in conformity with the results obtained by batch experiments. In the case of silica gel, there was change in final ph, probably due to the replacement of H+ on the silica gel surface by Na+. However, the passage of sea water (simulated or actual) through the column did not indicate any substantial uptake of cobalt by the column. Hence, at least upto a ph of 6.6, the presence of hydrolysed species may not be important which is in agreement with the identical uptake of cobalt by cation exchanger at ph of 6 and 8. Uptake of zinc by Dowex 50 Wx8, Dowex Jx8 and silica gel was studied by column chromatography as in the case of cobalt. It was found that the entire amount of zinc from simulated or actual sea water could be retained on the cation or anion exchange column whereas in the case of silica gel, the uptake was negligible at low ph but was complete at high ph. The results on the behaviour of zinc suggest that the zinc species are labile in nature and interconversion to anionic or cationic form occurs as one of the species is removed from the sea water by the resin. The uptake of zinc by silica gel indicates the presence of hydrolysed forms of zinc which also contributes to an increased uptake by cation or anion exchanger at higher ph. In the case of mercury there was some uptake by a cation exchanger at normal sea water ph which may be attributed to the presence of hydrolysed species but it was somewhat surprising that the adsorbed mercury was not eluted out by passing IN HCI through the column. In the case of actual sea water also, there was an uptake of mercury at a ph of 7.5 and the uptake was higher than that from simulated sea water. About 70-80% of mercury was retained by the column but the amount of mercury released by the acid elution was quite low. These studies suggest that kinetics of elution are extremely slow. The above observations can be attributed to the tendency of mercury for forming strong complexes

6 1372 INDIAN J CHEM, SEC A, DECEMBER 2001 with sulphur contamtng ligands. Mercury is also known to be adsorbed by microcrystalline oxides and soils rapidll. About 10-20% of mercury was desorbed from the manganese oxide samples in 1 h by filtered tap water while 30-40% was desorbed by IM saline solutions. Subsequent desorption in the saline solution released only small amounts of mercury. Thus the adsorption and desorption of mercury were kinetically slow processes. At a ph of about 6, adsorption was found to be maximum. All these observations are in general agreement with the slow desorption behaviour of mercury as found in the present work. Uptake of mercury was also studied from simulated and actual sea water using anion exchanger. It was found that a major portion of mercury was retained by the anion exchanger both from simulated and actual sea water but the retained activity was not released by elution with acid. The adsorption of -mercury was also studied using silica gel as an adsorbent. It was found that there was no uptake of mercury by silica gel at low ph from simulated or actual sea water but at normal sea water ph, there was substantial uptake. As in the other cases, the mercury adsorbed was not released by elution with acid. Acknowledgement The authors feel grateful to Dr. P.K. Mathur, Head (Retd ), Analytical Chemistry Division for giving useful suggestions and also for critically gomg through the manuscript. References I Metal speciation in the environment ', edited by J A C Brockaert, S Gucer & F Adams (Springer Verlag, Berlin), Fergusson J E 'The heavy elements: Chemistry, environmental impact and health effects', (Pergamon Press, Oxford), West T S & Nurnberg H W, 'The determination of trace metals in natural water', (Blackwell Science Pub!., Oxford, UK) Grasshoff K, Ehrhardt M & Krembling K, 'Methods of sea water analysis,' (Verlag Chemie, Weinhejm), Hogfeldt E, 'Stability constants of metal ion complexes, Part A, Inorganic ligands,' (Pergamon Press, Oxford), Smith R E & Martell A E, 'Critical stability constants,' Vol.4, Inorganic Complexes, (Ple num Press, New York), Hutchinson T C & Meema K M, 'Lead, mercury, cadmium and arsenic in the environment,' (John Wiley and Sons, Chichester), 1987.