DS0104 - Innovations scientifiques et technologiques pour anticiper ou remédier les risques environnementaux

Early warning sensors for monitoring mercury and chromium (VI) in water – Captôt

Early Warning Sensors for Monitoring Mercury in Water

Electrochemical mercury sensors based on screen printing electrodes allow in situ detection but mainly suffer from a lack of sensitivity. With their 3D nanostructure, CAPTÔT sensors allow a pre-concentration of mercury ions and their rapid detection on site at concentrations of µg/l range.

Issues

The need of water quality monitoring, in different environmental and health context submitted to european and national regulations, is still increasing. It gives rise to large sampling campaigns with spot sampling, handling, preservation, shipping, storing of samples and then time consuming laboratory analysis. The trend for some application is toward real-time, on-site or in situ analyses to improve the reliability of monitoring regarding for example the representativity of sampling, get faster information and increase the capacity of real time warning system. Quality standards are now in the very low µg/L (ppb) or ng/L range. The objective of getting on site or in situ information requires fast, portable, low-cost, environmentally friendly and sensitive instruments; however, few options are readily available.<br />Among all monitored pollutants, mercury is one of the dangerous priority substances identified by the Water Framework Directive (Directive 2000/60/EC) for which discharges have to be stopped before 2020. However in some historical industrial context or illegal activities (gold panning) mercury contamination subsists

Based on ASV detection, our team has already proposed functionalized nanoporous polymer electrodes sensitive and selective for heavy metal analysis. With their 3D nanostructure, which allows in situ sampling, these membrane-electrodes are ideal for self-monitoring of metals directly on-site. This technology relies on ion-track etched nano-functionalized membranes made of a biocompatible, the polyvinylidene difluoride (PVDF). Poly-4-vinyl pyridine (P4VP), well-known to complex Hg(II) is grafted in PVDF track-etched membrane. After Au-sputtering, the resulting sensors measures mercury well below the limitations imposed by existing norms for polluted soil leachate (EU directive of the 15 march 2006). Nevertheless, the establishment on a robust protocol to obtain reliable results is necessary and was the objective of the project.
A performance evaluation on a set of natural water samples was performed to look at possible interferences. Finally, a first generation prototype exhibiting its own integrated potentiostat, its soft-ware and set of membrane-electrode pads was developed.

CAPTÔT membranes-electrodes exhibit a high sensitivity toward Hg(II) ions in water. Following the established protocol (109 nanopores per cm2, 2/3 filled glass containers, 200 rpm stirring, duration of 4h, calibration prior measurement), a limit of detection of around 0.1 ppb and a mean error in the order of 20% were determined. No interferences in mineral waters in presence of high salt contents were observed. However, in certain natural waters, weak recovery levels were evidenced probably linked to the total carbon content or to the strong affinity of mercury for suspended matters. A prototype allowing in situ analysis (portable kit) was developed.

Captôt technology benefits now from a portable prototype which detect traces of Hg(II) mercury form in water. This technology is scalable to other ions of interest (Pb(II), Cu(II), Se(IV), UO2, etc…), the priority is thus to extend the software developed by ValoTec toward the detection of these other ions. The targeted application domains are either to classify polluted soils after lixiviation in pure water or on the metal ions detection in matrices exhibiting high salt content such as seawater.

U. Pinaeva, D. Lairez, O. Oral, A. Faber, T. L. Wade, M-C. Clochard, P. Moreau, J-P. Ghestem, M. Vivier, S. Ammor, R. Nocua, A. Soulé “Early warning sensors for monitoring mercury in water” Haz. Mat. 2018 (submitted)

Climate changes increase the severity and frequency of flooding thus spreading pollution and exposing water to contamination. Oil-shale is a source of fuel but it is also a source of mercury and other heavy-metal pollution. Water control agencies are at the front-line of monitoring groundwater quality and tracking pollution. Budget and manpower cuts make these tasks more difficult. Regulations push towards real-time, on-site or in situ analyses to improve the reliability of monitoring, the representatively of sampling and faster information requires fast, portable, low-cost, environmentally friendly, sensitive sensors that are able to quantify heavy metals in different types of water. The problem with conventional sampling is the collection of water, preservation, shipping and storing. Sampling changes the water: pH and oxygen content. This changes the concentration and oxidation state of metal ions and only gives a snap-shot of quality. Thus the trend in water monitoring is passive or in situ sampling. Passive sampling is done by placing cartridges into a body of water. The cartridges contain metal ion chelating polymers that selectively adsorb and pre-concentrate metal ions. After two weeks, the samplers are recovered and sent to a lab for extraction and analysis. The advantages of passive sampling are that the limits of detection (LODs) are very low, due to metal ion pre-concentration. Analysis results better represent the water, since passive samplers are at equilibrium with it. The problem with passive samplers is the long sampling times, the extraction and expensive analysis by atomic adsorption spectroscopy (AAS) or inductively coupled plasma mass spectroscopy (ICP-MS). Two particularly difficult toxic metal ions are mercury and chromium (VI). Allowed levels for mercury are sub µg/L (0.015 µg/L potable water and 0.5 µg/L residual waters - French water norms of 27 October 2011). Due to volatility, mercury is analysed separately from other metals. Mercury analysis is usually performed by AAS or ICP-MS with cold vapour trapping. AAS and ICP-MS are time consuming and require a centralised lab with trained personnel. Chromium (VI) can be analysed by colourimetry but lacks sensitivity. It can be analysed by LC-ICP-MS but this is rare in private labs. Chromium (VI) analysis requires fast analysis due to potential changes between sampling and analysis. Most of the current technology is not adequate to meet the demands of sensor manufacturers or end users. Electrochemical sensors are cheap with fast response times. The most sensitive are based on toxic mercury and the cheapest are disposable screen printed electrodes (SPE), that are portable and non-toxic but lack sub µg/L sensitivity. Moreover, mercury based electrochemical systems cannot detect mercury. For these reasons a consortium of two public research labs, LSI and BRGM, a private lab, SGS MULTILAB, and a small enterprise which devlopes prototypes and series, VALOTEC, have joined to validate the potential of a patented 3-D membrane sensor developed by the LSI, able to quantify heavy metals, notably mercury but also Chromium (VI), at trace levels, in situ, on-site and/or on-line, using passive sampling. The new sensitive 3-D membrane sensors passively pre-concentrate metal ions by a chelating polymer and obtain equilibrium with the environment. The adsorption time is 30 minutes to twenty four hours and the analysis is rapid and on-site thus bridging the gap between passive samplers and electrochemical sensors. This environmentally friendly sensor relies on ion-track etched nano-functionalized membranes made of bio-compatible polyvinylidene fluoride (PVDF). The sensors measures heavy metals well below the limitations imposed by existing norms making them competitive with electrodes containing toxic mercury. If proven reliable and cost effective, the sensor could be integrated into existing systems, or used standalone as portable devices.

Project coordination

Travis WADE (Laboratoire des Solides Irradies)

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

CEA/IRAMIS/LSI Laboratoire des Solides Irradies
BRGM BRGM
SGS MULTILAB Société Générale de Surveillance
VALOTEC VALOTEC

Help of the ANR 320,227 euros
Beginning and duration of the scientific project: - 24 Months

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