Heavy Metals Analysis in Seawater and Brines

Abstract

There has been growing concern during and since the 1990s about the effect that the toxicity and persistence of heavy metals will have on the biosphere. Consequently, these new environmental pressures have challenged analysts to find better experimental procedures for handling, recognition and evaluation of traces of heavy metals. Moreover, the determination of low levels of heavy metals in seawater has received particular attention as a way of assessing the early impact of human activities on the marine environment. It is necessary to protect the marine and continental waters against contamination so as to allow suitable conditions for the maintenance of aquatic life and the various uses derived from aquatic life. It is not the metal concentration alone that enables an environmental diagnosis to be made but the way metal combines with other elements to form species. These species are responsible for the biological behavior of the heavy metal under consideration, as its bioavailability will determine the amount of harm it may cause. Thus, “speciation” studies should be conducted to assess the potential environmental impact of human activities. From the analytical point of view, seawater and brines may be considered to be closely related systems in terms of matrix effects and analytical difficulties. Hence these two systems will be included in this article although the aims of heavy metal detection and determination might be diverse in each case. Analytical inaccuracies that arise from the high salt content of the seawater or brine solutions are exacerbated by the low levels of trace metals, which require careful control of contamination during sample collection and analysis and very sensitive analytical techniques. Different techniques for the determination of trace metals in seawater, such as spectroscopic and electroanalytical methods, will be discussed here in terms of their performance in dealing with the problem of trace metal detection and determination. Even the term “trace” has had different meanings over the years, normally being associated with the detection limit of the technique employed (“traces”, “footprints” or “trails” have a lot to do with the way they are observed). Amounts that would have been considered traces in the 1960s are probably no longer considered as such in the 1990s, as detection limits for most heavy metals have been lowered by three to six orders of magnitude (using “state of the art” techniques) since the late 1960s. Special attention has been paid to the preconcentration of the analyte and its isolation from the matrix constituents which might obscure its accurate determination. A variety of separation–preconcentration methods will be presented, with a strong bias towards those procedures leading to some kind of automation, because pretreatment of the samples is often time‐consuming and prone to contamination and may lead to an incomplete recovery of the analytes. Flow injection analysis (FIA) has appeared as a solution to time‐consuming and complicated separations and it will also be discussed in relation to alternative forms of analysis, especially in terms of its ability to perform speciation studies.