Automatic monitoring systems for trace metals in natural and waste water: Nafion coated solid metal electrodes for sensitivity enhancement and stability improvement

Abstract

Voltammetry is one of a few analytical methods, suitable for on-line monitoring of trace metals and is sufficiently sensitive in the low μg/L range for this purpose. The advantages of in-situ use of voltammetry are low cost and no or minimal sample change, since the sample is taken automatically and analysis is performed immediately without a need to collect a sample manually risking contamination and metal loss. Automatic monitoring is essential to record the natural variations of metal concentration in natural or industrial water and registration of accident due to natural or anthropogenic reasons.

Remote monitoring of trace metals is of special importance due to their biological role. For that reason extensive research has been carried out internationally to develop suitable monitoring methods. The introduction of the innovative solid silver amalgam and other solid metal electrodes made the use of voltammetry suitable for remote monitoring without the use of toxic electrode materials.

An automatic trace metal system (ATMS) in combination with solid silver amalgam electrode has been applied for long time monitoring of zinc and iron in waste water. The system worked automatically with manual maintenance every five days. Sample taking, addition of electrolyte solution, stirring, chemical analysis, electrode cleaning and cell draining were carried out automatically, while electrode polishing, filter changing and calibration were carried out manually every five days. For comparison, verification and to provide a better insight into the complex waste water matrix manual samples have been taken regularly and analyzed with ICP-MS. From data obtained from those two analytical methods a speciation study was performed.

The automatic voltammetric system for continuous monitoring of trace metals has been applied in three different locations for monitoring of trace metals in different water samples. River water and drainage water has been used as complex natural water matrixes. A solid silver amalgam electrode has been used as a working electrode. The systems have been fully automated to measure the labile concentration of trace metals for a long time monitoring and store the collected data. Manual maintenance has been performed to calibrate the systems and to clean the working electrodes. Additionally, random samples have been collected manually and analyzed by ICP-MS for the total dissolved metal concentration for data verification and getting better insight into sample matrix.

Development of the ATMS has provided an opportunity to monitor earthquake precursor in ground water. A monitoring station has been installed in Husavik, Northern Iceland, for monitoring of trace metals in a ground water. The monitored concentration of metals clearly indicated changes in the ground water prior to the earthquake. The results revealed increased electrolabile concentrations of zinc, iron and copper 7-8 days before the earthquake. The increased concentrations lasted until the occurrence of the earthquake and were possible to monitor by the ATMS.

The characterization and application of a Nafion coated solid silver amalgam electrode has been tested for the determination of trace metals by differential pulse anodic stripping voltammetry (DPASV). Effect of model surfactants, commonly presented in natural water sources has also been investigated. It has been found that Nafion coated electrode is useful in surfactant containing samples. The performance of the Nafion coated silver amalgam electrode has been investigated and compared to uncoated silver amalgam electrodes. The modification of the electrode yields higher sensitivity. Moreover, the long time use of Nafion film and practical application has been tested by the use of DPASV in polluted river water sample and has shown to work optimal for 2 weeks.

New electrodes made of silver germanium and silver antimony have been tested and found to be suitable for determination of metals by DPASV with very good linearity. This type of electrodes extends potential window to both positive and negative direction. Additionally, very good sensitivity for cadmium has been observed. The practical application has been tested for applying the electrodes in monitoring station to monitor zinc concentration in polluted river water. Moreover, Nafion coating has been applied and has been found to be advantageous considering electrode stability.

The aim of this work has been to test silver amalgam and other solid metal electrodes for application in natural and environmental water samples for use in long time online monitoring in field. The papers included in this thesis prove successful applications of these electrodes in different environmental matrixes. The results provide positive evidence about the ability of voltammetry in combination with solid metal electrodes (bare and modified with Nafion) as a working electrodes to be employed in automatic monitoring of trace metals in polluted water with minimal sample pretreatment and minimal maintenance of the system. The use of solid metal electrodes avoids the use of liquid mercury in voltammetric analyses, and offers an environmental friendly alternative suitable for use in field. Due to ease to use, nontoxic fabrication materials, wide cathodic potential window, long time stability, and good sensitivity these electrodes can be applied for continuous online monitoring in various environmental and industrial solutions. They are also easy to modify and this extends the application even further. Several experimental projects at different locations and in different water samples (river-, sea-, ground-, drainage- and waste water) have shown sufficiently long time stability without the need for physical presence for maintenance. These results strongly indicate that further developed and improved method may in future be used as a commercial warning system for environmental pollution accidents.