On-line and Remote Monitoring of Heavy Metal Pollutants

Very few analytical methods are suitable to be used outside the laboratories, and this is a great limitation for environmental monitoring. A high number of samples fre-quently have to be brought from the field. This is not only a question about the costs, but also to obtain reliable results sufficiently fast for immediate action due to envi-ronmental accidents or by other reasons. Normally the reason for the methods to be unsuitable for off-laboratory measurements is too low sensitivity with not sufficient simplicity and robustness to be applied in the field. Of particular importance is that maintenance is not needed frequently or it is possible to robotize this process. It is also important that the prices are moderate to allow a great number of remote environmental monitoring systems. With very few exceptions, the present off-laboratory methods for remote use are limited to measure-ment of temperature, conductivity, pH and some physical properties. For measuring heavy metal at trace levels several laboratory methods are available for such purposes, most frequently used are Atomic Absorption Spectroscopy (AAS) and Inductively Coupled Plasma Mass Spectrometry (ICP-MS). Electroanalytical methods are also available; voltammetry with liquid mercury drop electrodes being sufficiently sensitive for its purpose, and the drop can be renewed to ensure long-term use. However, the off-laboratory use of liquid mercury as environmental monitor is unac-ceptable for environmental reasons, but we have succeeded to find alternative materi-als, alloys of two or more metals, one being silver, gold or another noble metal and the second one being a small amount of mercury, bismuth etc. giving the alloy the re-quired overpotential to avoid the interfering formation of hydrogen gas. This works very well under laboratory conditions, with required manual maintenance each month or even more seldom. Electrochemical electrode cleaning is easily performed auto-matically. However, it is a great difference between methods that work under laboratory condi-tions and remote online environmental monitoring. The present paper deals with veri-fying our voltammetric measurements for such purposes. In addition to some indus-trial applications, two different automatic monitoring stations are evaluated. One sta-tion is analysing heavy metals in the river mouth of Nidelven, in Trondheim, Norway, consisting partially of river water and brackish water. The content of Zn, Cd, Pb and Cu has been measured continuously and satisfactory for three months using a solid dental amalgam sensor. Furthermore the scrubbing wastewater at Heimdal Waste In-cineration Plant in Norway has been monitored continuously for Zn, Cd, Pb and Hg using a double sensor system using a solid dental amalgam sensor and a solid gold-bismuth alloy sensor. Both systems have been operated for about six months without any manual maintenance except from polishing the electrodes in 30 seconds twice a week at the Incineration Plant and once a month at the river station. In the paper, fur-ther advancements of the method will be discussed, including more metals and differ-ent water systems.