The determination of metal speciation in natural waters by electrochemical techniques

1. The determination of metal speciation in natural waters by electrochemical techniques Øyvind Mikkelsen Mikkelsen 2003

2. Overview • Theoretical aspects - Natural water - Speciation, and importance of speciation studies - Available techniques for speciation studies - Electrochemical techniques • Some practical examples - Cu, Cd, Pb and Zn speciation in natural water - Fe(II) and Fe(III) speciation in seawater - Al(III) speciation in natural water • Conclusions Mikkelsen 2003

3. Theoretical considerations Mikkelsen 2003

4. Natural water • Natural water includes e.g. rivers, lakes, ground water, wells, seawater,…. Mikkelsen 2003

5. What is speciation? In water trace metals are present in a wide range of chemical forms, in both the particulate and dissolved phases. The dissolved phase comprises the hydrated ions, inorganic and organic complexes, together with species associated with heterogeneous colloidal dispersion and organometallic compounds. In some instances these metals are present in more than one valency state.

6. Possible forms of trace elemements Simple ionic species: Zn(H2O)62+ Valency states: As(III), As(V), Cr(III), Cr(IV) Weak complexes: Cu-fulvic acid Adsorbed on colloidal particles: Cu-Fe(OH)3-humic acid Lipid-soluble complexes : CH3HgCl Organometallic species: CH3AsO(OH2), Bu3SnCl Particulate : Metals adsorbed onto or contained within clay particles

7. Interactions affecting trace metal speciation G.E Batley, Trace element speciation; analytical methods and problems, CRC Press, Inc., 1989

8. An example, lead. Free metal  Pb2+ Ion pair  PbHCO3 Complexes with organic pollutants  Pb2+/EDTA SOLUTION Complexes with natural acids  Pb2+/fulvic acid SUSPENSION Ion adsorbed onto colloids  Pb2+/Fe(OH)3 COLLODIAL Metal within decomposing  Pb in organic soils organic material Ionic solids  Pb2+ held with the clay structure, PbCO3 SOLID

9. Why speciation studies? Generally basic reasons for speciation measurements: Study transport and biogeochemical cycling processes Predict biological impact (identify those metal species which are likely to have adverse effects on biota and includes measurements both of bioavailability and toxicity)

10. Toxicity In general, the toxicity of metals stems from the fact that they are biological non-degradable and have a tendency to accumulate in vital organs, e.g. brain, liver, etc. and their accumulation become progressively more toxic

11. Toxicity, some examples. Ionic copper are fare more toxic towards aquatic organisms than organically-bounded copper, and that more stable the copper complex, the lower is its toxicity. Alkyl compounds of mercury and lead are especially toxic because they are lipid-soluble As(III) is fare more toxic than As(V) Ni, Cr, Cu and Se are known to display carcinogenic effects due to their interactions with nucleic acids – e.g. whereas Cr(VI) is anionic and highly toxic Cr(III) is nontoxic, this because negative charge on CrO4- makes it able to pass the cell membrane

12. Detection of trace metal speciation? Lipid soluble forms Information of speciation can be obtained even near the total limit of detection because separation methods can be used prior to the measurements of the actual species Particle bond forms Ionic forms and labile complexes These species are in principle more difficult to measure because any separation methods or attempts of pre- concentration will shift the distribution of the species. Molecular spectroscopy ?  Fails due to detection limit Potentiometry ?  Fails due to detection limit ? Voltammetry ?   ICP-MS ?  

13. Detection of trace metal speciation?

14. Flame AAS El. term. AAS ICP/MS Voltammetri Sensitivity Interference Speciation Efficiency Capacity Cost Online Maintaince Linear range Detection of trace metal speciation?

15. Metals of common enviromental concern

16. Electrochemical methods • Principle; information about the analyte is achieved from measurements of e.g. • potential, current, resistance or conductance. There at several methods available: • Coulometry (measurements of current and time) • Conductometry (measurements of conductance) • Potentiometry (measurements of potential at zero current) • Polarography / Voltammetry (measurements of current as function of an applied potential) In particular voltammetry is suitable for analyses of trace metal and speciation studies. Detection limit for the most common heavy metals are in the range of 10-6 to 10-12 M.

17. Electrochemical detection of trace metals Voltammetry Anodic stripping voltammetry Adsorptive cathodic stripping voltammetry Square wave stripping voltammetry Potentiometry Ion selective electrodes

18. Some advantages for el.chem. techniques Electroanalysis is a powerful technique for the study of trace element speciation, and has been applied to over 30 elements Four to six metals of prime environmental concern; Cu, Pb, Cd, Ni, Zn an Co can be detected simultaneously and with a sensitivity in the range of ng/L Study of the kinetics of metal complex dissociation at en electrode is supported by well-established theory Electrochemical techniques requires minor sample pretreatment, resulting in fewer potential sources for contaminations Speciation study can be performed in the field within minutes, with low-cost equipment

19. Range of applicability, el.chem. speciation methods Direct applications, determination of labile and inert metal fraction redox state half wave or peak potential shifts Indirect applications, determination of fraction bound in inert organic complexes or to organic colloids, by measurements before and after UV irradiation after UV irradiation and lipid soluble complexes, after extraction of water samples with e.g. n-octanol or 20% n-butanol in hexane size distribution after ultra filtration Preconcentration prior to e.g. Carbon Furnaces AAS

20. Range of applicability; labile/inert metal fraction Discrimination between labile and inert metal fraction in the sample • Labile metal compromise free metal ion and metal that can dissociate in the double • layer (near electrode surface) from complexes or colloidal particles - For natural waters the most used techniques are ASV, AdCSV and SWV - Applied to e.g. Cu, Pb, Cd, Zn, Mn, Cr, Tl, Sb and Bi • Often the labile metals have been found to correlate well with the toxic fraction of • the metal

21. Range of applicability; redox state Determination of the redox state of an element in solution is very important because it can drastically affect the toxicity, adsorptive behavior, and metal transport Applied to distinguish between e.g. Fe(III)/Fe(II), Cr(VI)/Cr(III), Tl(III)/Tl(I), Sn(IV)/Sn(II), Mn(IV)/Mn(II), Sb(V)/Sb(III), As(V)/As(III), Se(VI)/Se(IV), V(V)/(IV), Eu(III)/Eu(II), U(VI)/U(IV)

22. Redox state, toxicity vs. el.chem. lability

23. Range of applicability; half wave potential shifts Shift in the polarographic half wave potential or ASV peak potential of metal ions in presence of complexing agents can provide information about the thermodynamic stability of complexes in solution. Quantitative deductions may be difficult due to the high number of possible present ligands and metals in natural or polluted water Sometime however it is possible to do such quantitative deductions (e.g. the ASV peak for copper(I)-chloro complex in seawater)

24. Limitations of el. chem. speciation techniques Unable to measure the concentration of individual ionic species E.g. one peak only will appear for a mixture of Cd2+, CdSO4, CdCl+, and CdCO3 (which all may coexist in a river water sample) Also, electrochemical techniques like polarography and ASV are dynamic systems which draw current through the solution and disturb ionic equilibrium. However with microelectrodes the current flowing is reduced to nA or pA Ion-selective electrode potentiometry is the only method that can measure the activity of a individual ion – but the sensitivity has up to now been poor

25. Limitations, however…. Other speciation methods, including ion exchange chromatography, Solvent extraction, dialysis, and ultrafiltration also disturb the natural ionic equilibrium in water samples during the speciation process In addition often the question is only the discrimination between to species,where one is electrochemically active (labile) and the other species inert

26. Some practical examples

27. Measurements of Cu, Pb, Cd and Zn in waters

28. Measurements of Cu, Pb, Cd and Zn in waters Heavy metals have a influence on the biological life, and may cause serious damage due to toxicity effects • free and weakly complexed metals are transported across the cell membrane and provide • bioconcentration factors between 102 and 105 -may replace Mg at sulfhydryl binding sites -possible intracellular reaction between Cu and reduced glutathione which defend the cell against peroxide damage -loss of lysosomal membrane stability, which may lead to a leakage of hydrolytic enzymes into the cytosol and catabolic breakdown of the cell -when the capacity of a cell to detoxify accumulated metal is exceeded, damage to cyroplasmic constituents will occur, e.g. ultrastructural deformities, as well as reduction of cell division rate, respiration, photosynthesis, motility, electron transport activity, and ATP production

29. Cu, Pb, Cd and Zn The surface area of an organism is critical to the passive metal diffusion process into cell, therefore bacteria and algal communities frequently have the highest metal concentrations in the food web. There are a magnifying through the food web E.g. Periphyton has been found to contain up to 1g/kg Cd, while the normal Concentration are 22 mg/kgindigenous bryophyte populations in rivers draining old metal mines have been shown to contain up to16 mg/g Pb and 7 mg/g Zn

30. Cu, Pb, Cd and Zn In fresh water - inorganic fraction computed to be present mainly as CuCO3 (over 90%), colloidal particles and hydrated iron oxides In seawater - dominant inorganic species computed to be carbonato and hydroxy complexes (CuCO3 up to 80%), in addition CuOH+ and Cu(OH)20 (approx. 6,5%), Cu(OH)(CO3)- (approx. 6,5%), CuHCO3+ (approx. 1%) and Cu Coastal surface seawater usually has 40 to 60% of total copper present as inert organic complexes. In unpolluted seawater ASV-labile copper is usually less than 50% of dissolved copper, even at pH as low as 4,7. Most freshwater streams also has little ASV-labile copper (organically bound)

31. Cu, Pb, Cd and Zn In fresh water - Computed to exist as PbCO3 and Pb2(OH)2CO3 (often > 90% of the inorganic leadspecies) - in general lead has a stronger affinity for some inorganic adsorbents, especially iron oxide (pH 7), than for organic ligands, - at pH 6.0 or lower most lead is found as electro inactive Pb2(OH)2CO3 In seawater - Pb is found as carbonato complexes (83%) and chloro species (11%) - 40 to 80% of dissolved lead is found in the inorganic colloid fraction Alkyllead in natural waters may be determined by ASV after selective organic phase extraction

32. Cu, Pb, Cd and Zn In fresh water - Dominant form is computed to be Cd2+ and CdCO3 depending on pH - Cd adsorbs to colloidal particles only at relatively high pH values, so very little Cd is present as pseudocolloids In seawater - Cd is computed to exist as CdCl+ and CdCl20 complexes (92%) A high portion (over 70%) of Cd is found to be ASV labile both in seawater and freshwater. In anoxic water Cd may exist as no-labile CdHS+

33. Cu, Pb, Cd and Zn In fresh water - dominant inorganic forms are computed to be Zn2+ (50) and ZnCO3 (38%) In seawater - main species are computed to be Zn2+ (27%), chloro complexes (47%), and ZnCO3 (17%) - open ocean waters contains as little as 10 ng/L Zn at the surface The carbonato complexes of Zn, especial the basic carbonates, may have low ASV lability. About 59% of the total zinc in seawater and river water is ASV labile.

34. Cu, Pb, Cd and Zn Most suitable techniques are ASV and AdCSV AdCSV ASV 1. Step The electrode are set to a potential about 300 mV more negative than the first expected metal peak Mn+ + ne- M (deposited on the electrode) 2. Step Cd is than stripped of by reverse the potential over the electrode towards more positive value M  Mn++ ne- 1. Step Cations are complexed with surface active complexing agents (L) Mn+ + xL  MLxn+ 2. Step Metal-complex adsorbs to the electrode surface MLxn+ + Met  MLxn+,ads(Met) 3. Step The cation is released from complex by reduction MLxn+,ads(Met) + me-  M(n-m)+ + xL + Met

35. Speciation scheme for Cu, Pb, Cd and Zn in waters Sample (unacidified), filter through a 0.45-mm membrane filter, reject particulates and store filtrate unacidified at 4C a) Bring to pH 4.7 with acetate buffer, b) Not valid if Fe > 100 mg/L, c) Optional step, d) Solvent dissolved in aqueous phase must be removed first

36. Measurements of Fe in seawater

37. Fe in seawater Iron is one of the most important bioactive trace metal in the oceans. The first-row transition metal plays a key role in the biochemistry and physiology of oceanic phytoplankton. Low iron concentrations are suggested to limit phytoplankton growth and biomass in certain oceanic regions

38. Fe in seawater The oceanic chemistry is highly complicated, and still not fully understood. Dissolved iron can exist in two different oxidation states, Fe(III) and Fe(II). Thermodynamically Fe(III) is the stable form in oxygenated water, however several processes reduce Fe(III) to Fe(II). Fe(II) may exist for several minutes in surface water(pH 8) before it is oxidized back to Fe(III). Presence of Fe2+ may cause an increase in the dissolved iron fraction making more iron available for use by biota.

39. Fe in seawater Inorganic speciation of dissolved Fe(III) and Fe(II) differ considerably. Inorganic Fe(III) species are dominated by hydrolysis products, Fe(OH)2+, Fe(OH)30, and Fe(OH)4-. Free hydrated Fe3+ ion is extremely rare. Inorganic Fe(II) however exists in primarily as Fe2+ ion. Evidence is also found for complexing of Fe(III) and Fe(II) with organic ligands.

40. Fe in seawater Since total dissolved iron in oceanic surface waters can be very low (down to a few pM), there is a need for highly sensitive techniques. Iron(II) at nanomolar levels has been determine by e.g. and colorimetry preceded by preconcentration of iron(II) using octadecyl silica as stationary phase However the most suitable technique is AdCSV

41. Fe in seawater Fe(III) complexed with 1-nitroso-2-napthol is preconcentrated onto a hanging mercury drop electrode (adsorption). (Addition of H2O2 secures that all iron is oxidized to Fe(III) Concentration of Fe(II) is calculated from the difference between analyses with and without added 2,2-dipyridyl, which masks Iron(II).

42. Fe in seawater Recent results from our laboratory has shown a new ASV technique that can be used for detection of Fe(II) down to 50 ng/L on solid dental amalgam electrode. Analyses can be performed directly in the sample with only the additions of citrate or oxalate

43. Fe in seawater Detection of iron (II) with DPASV in tri-sodium citrate-5,5-hydrate (0.02M) solution. Addition of iron (II) standard to solutions of 1,67 ppb, 3,34 ppb, 5 ppb, 15 ppb, 25 ppb, 50 ppb, pre-deposition time 180 s.

44. Measurements of Hg in water

45. Hg in water Mercury has no known essential functions, though it has been used to treat syphilis, actuallywith some success. Mercury probably affects the inherent protein structure which may interfere with functions relating to protein production. Mercury has a strong affinity for sulfhydryl, amine, phosphoryl, and carboxyl groups, and inactivates a wide range of enzyme systems, as well as causing injury to cell membranes. Main problems seem to result from its attack on the nervous system. Mercury may also interfere with some functions of selenium, and can be an immunosuppressant

46. Hg in water Mercury dissolved in water is present in many forms, including organomercurials, such as methylmercuric chloride, phenylmercuric chloride and other alkyl- and arylmercury compounds. Among the co-existing forms of mercury in natural water the most toxic to man and biota are organomercurials (up to 46% of the total mercury content has been found in this form in river water samples, and up to 63% in unfiltered samples)

47. Hg in water Organomercurials as methyl-mercury has high lipid solubility, something that makes bioaccumulation a serious problem. Bioaccumulation up to 103 to 104 have been reported for mercury in fish .

48. Hg in water LD50 of different organomercuric compounds

49. Hg in water Organic and inorganic mercury can be detected with a glassy carbon electrode modified with thiolic resin. Detection limits in low g/L

50. Hg in water, detection of Hg2+, MeHg+, EtHg+, PhHg+ Sample E1= -0,5V E3= -1,35V Hg2+ total Hg2+ MeHg+ Hg2+ (Hg2+) (EtHg2+) (PhHg2+) (TMS) (MeHg2+) not treated treated E2= -1,0V Hg2+ MeHg+ EtHg+ PhHg+ R. Agraz et al.