Seawater heavy metals

Anodic-stripping voltaimnetry (ASV) is used for the analysis of cations in solution, particularly to detemiine trace heavy metals. It involves pre-concentrating the metals at the electrode surface by reducmg the dissolved metal species in the sample to the zero oxidation state, where they tend to fomi amalgams with Hg. Subsequently, the potential is swept anodically resulting in the dissolution of tire metal species back into solution at their respective fomial potential values. The detemiination step often utilizes a square-wave scan (SWASV), since it increases the rapidity of tlie analysis, avoiding interference from oxygen in solution, and improves the sensitivity. This teclmique has been shown to enable the simultaneous detemiination of four to six trace metals at concentrations down to fractional parts per billion and has found widespread use in seawater analysis. 

Chlorine is desirable as a bulk pretreatment biocide for inlet water, but its subsequent removal upstream of the membrane is absolutely necessary ana difficult. NaHSO,3 is a common additive to dechlorinate before membranes. It is customarily added at 3-5 mg/1, an excess over the stoichiometric requirement. NH3 is sometimes added to convert the chlorine to chloramine, a much less damaging biocide. Heavy metals present in seawater seem to amplify the damaging effects of chlorine and other oxidants. 

Legin AV, Vlasov YG, Rudnitskaya AM, Bychkov EA (1996) Cross-sensitivity of chalcogenide glass sensors in solutions of heavy metal ions. Sens Actuators B 34 456 61 De Marco R, Shackleton J (1999) Cahbration of the Hg chalcogenide glass membrane ion-selective electrode in seawater media. Talanta 49 385-391... 

Bischoff, J.L. and Dickson, F.W. (1975) Seawater-basalt interaction at 200°C and 500 bars Implications for origin of seafloor heavy-metal deposits and regulation of seawater chemistry. Earth Planet. Sci. Lett., 25, 385-397. 

Fang et al. [661] have described a flow injection system with online ion exchange preconcentration on dual columns for the determination of trace amounts of heavy metal at pg/1 and sub-pg/1 levels by flame atomic absorption spectrometry (Fig. 5.17). The degree of preconcentration ranges from a factor of 50 to 105 for different elements, at a sampling frequency of 60 samples per hour. The detection limits for copper, zinc, lead, and cadmium are 0.07, 0.03, 0.5, and 0.05 pg/1, respectively. Relative standard deviations are 1.2-3.2% at pg/1 levels. The behaviour of the various chelating exchangers used was studied with respect to their preconcentration characteristics, with special emphasis on interferences encountered in the analysis of seawater. 

Down to 1 pg/1 of the aforementioned heavy metals can be determined by this procedure. However, it is emphasised that real seawater samples were not included in this study. 

Nygaard [752] has evaluated the application of the Spectraspan DC plasma emission spectrometer as an analysis tool for the determination of trace heavy metals in seawater. Sodium, calcium, and magnesium in seawater are shown to increase both the background and elemental line emission intensities. Optimum analytical emission lines and detection limits for seven elements are reported in Table 5.8. 

Although anodic stripping voltammetry is one of the few techniques suitable for the direct determination of heavy metals in natural waters [310,756-764], it is not readily adaptable to in situ measurements. Lieberman and Zirino [623] examined a continuous flow system for the anodic stripping voltammetry determination of zinc in seawater, using a tubular graphite electrode predeposited with mercury. A limitation of the approach was the need to pump seawater to the measurement cell, while the method required the removal of oxygen with nitrogen before measurements. 

Batley and Matousek [390,778] examined the electrodeposition of the irreversibly reduced metals cobalt, nickel, and chromium on graphite tubes for measurement by electrothermal atomisation. This method offered considerable potential for contamination-free preconcentration of heavy metals from seawater. Although only labile metal species will electrodeposit, it is likely that this fraction of the total metal could yet prove to be the most biologically important at the natural pH [779]. 

Prange et al. [809,810] carried out multielement determinations of the stated dissolved heavy metals in Baltic seawater by total reflection X-ray fluorescence (TXRF) spectrometry. The metals were separated by chelation adsorption of the metal complexes on lipophilised silica-gel carrier and subsequent elution of the chelates by a chloroform/methanol mixture. Trace element loss or contamination could be controlled because of the relatively simple sample preparation. Aliquots of the eluate were then dispersed in highly polished quartz sample carriers and evaporated to thin films for spectrometric measurements. Recoveries (see Table 5.10), detection limits, and reproducibilities of the method for several metals were satisfactory. 

Sibley TH, Morgan JJ (1975) Equilibrium speciation of trace metals in freshwater-seawater mixtures. In Hutchinson HC (ed) Proceedings of international conference on heavy metals in the environment, University of Toronto, Toronto, Ontario pp 310-338... 

Davies, M.S. 1992. Heavy metals in seawater effects on limpet pedal mucus production. Water Res. 26 1691-1693. 

Other components likely to be found in both seawater and brackish water are heavy metals and organic matter, both of which will impact the composition of the concentrate. Heavy metals and other toxic constituents are found in all water sources, but they are more likely to found in inland water sources rather than... 

Concentrate can be harmful to the environment due to either its higher than normal salinity, or due to pollutants that otherwise would not be present in the receiving body of water. These include chlorine and other biocides, heavy metals, antisealants, coagulants and cleaning chemicals. Of particular concern is the effect of pollutants on delicate ecosystems and endangered or threatened species. However, with appropriate measures in place, the discharge of concentrate to surface water can remain a viable method for seawater desalination plants. 

Raspor, B., Valenta, P., Nurnberg, H.W. and Branica, M., 1978. The chelation of cadmium with NTA in seawater as a model for the typical behaviour of trace heavy metal chelates in natural waters. Sci. Total Environm., 9 87-109. 

Sibley, T.H. and Morgan, 3.3., 1975. Equilibrium speciation of trace metals in freshwater -seawater mixtures. In T. Hutchinson (ed.), Proc. Int. Conf. Heavy Metals in the Environment. Univ. of Toronto, Ontario, p. 319. 

The process of diffusion of heavy metals in seawater depends on their state. The dissolved fraction of heavy metals (ip) takes part in the biogeochemical processes more intensively than suspended particles (e). But unlike suspended particles, the heavy metals fall out more rapidly to the sediment. A description of the entire spectrum of these processes in the framework of this study is impossible. Therefore, block MMT describes processes that can be estimated. The transport of heavy metals in seawater includes absorption of the dissolved fraction ip by plankton (11%) and by nekton (H%), sedimentation of the solid fraction (//) ), deposition with detritus (11%), adsorption by detritophages from bottom sediments (Iff), and release from bottom sediments owing to diffusion (Hef). As a result, the dynamic equations for heavy metals become ... 

Batley, G.E. and Matousek, J.R (1977) Determination of heavy metals in seawater by atomic absorption spectroscopy after electrodeposition on pyrolytic graphite coated tubes. Anal. Chem., 49, 2031-2035. 

Interestingly, many hyperthermophiles also are highly salt tolerant. This is an adaptation to life involving aqueous systems that evolve with high-pressure liquid/vapor and supercritical fluid-phase separation of hydrother-mally heated seawater. Both psychrophiles and hyperthermophiles have large numbers of species that also require heavy-metal tolerance, due to the concentration of heavy metals by the thermodynamic phase-separation processes operative in both very cold and very hot aqueous systems (Breezee et al. 2004 Kaye and Baross 2002 Summit and Baross 1998). 

Leal, M.C.F., M.T. Vasconcelos, I. Sousa-Pinto, and J.P.S. Cabral. 1997. Biomonitoring with benthic macroalgae and direct assay of heavy metals in seawater of the Oporto coast (Northwest Portugal). Mar. Pollut. Bull. 34 1006-1015. 

This approach is used to remove traces of toxic heavy metals from industrial effluents and to preconcentrate trace substances (e.g., recover valuable metals from seawater). 

Burton J. D. and Statham P. J. (1990) Trace metals in seawater. In Heavy Metals in the Marine Environment (eds. P. S. Rainbow and R. W. Furness). CRC Press, Boca Raton, FL. 

---