Automatic monitoring of heavy metals in waters

Ernest Beinrohr

Department of Analytical Chemistry, Slovak Technical University

Radlinskeho 9, 812 37 Bratislava, Slovakia, e-mail: beinrohr@chelin.chtf.stuba.sk

ABSTRACT

An automatic system for continuous monitoring of trace concentrations of heavy metals in water is presented and characterized. The computer controlled analyzer consists of a simple flow system with a robust coulometric cell enabling the measurement of various trace elements such as Ag, As, Bi, Cd, Cr, Cu, Fe, Hg, Mn, Pb, Sb, Se, Zn, as well as chlorides, bromides, iodides, phosphates, acids, bases and ammonia. The device was tested with the monitoring of Hg in industrial waste waters and Fe and Mn in mineral water. Metal concentrations down to the mg/L (ppb) level can be measured at measurement frequencies up to 10-40 in an hour.

INTRODUCTION

The measurement of trace concentrations of heavy metals in environmental samples is regarded as a routine task for analytical laboratories. However, the continuous monitoring of heavy metals in waters, especially in waste waters and process waters of communal or industrial origin, may not be as simple as discrete laboratory analyses.

Electrochemical methods especially flow-through coulometry may offer a reasonable alternative to spectrophotometric measurements due to simpler sample pre-treatment procedures and intrinsically higher sensitivity. The usefulness of such an approach will be illustrated in this paper.

EXPERIMENTAL

The measurements were performed with an automatic monitoring station EcaMon (ISTRAN Ltd., Bratislava, Slovakia). The monitor is built in a robust 19´´ case holding an industrial PC, one or two analytical units and the reservoirs for the electrolyte and standard solutions. The block diagram of the monitor is depicted in Fig. 1.The analytical unit is in fact a flow-through system consisting of two valves, peristaltic pump, manifold, electrochemical cell and the supporting electronics. The measurement procedure is controlled by the program in the PC. The obtained data are stored on the hard disk and/or can be sent to a central computer via a serial line. The electrochemical cell was a wall-jet type cell EcaJet and a 353 cell with porous working electrode. A solid gold electrode was used in the former cell for the measurement of Hg. Other elements were measured in the latter cell by making use of a porous working electrode E56-LMF (all from ISTRAN Ltd., Bratislava, Slovakia).

Reagents and solutions: Analytical grade reagents and deionized water were used in all experiments. Commercially available electrolyte solutions were used (ISTRAN Ltd., Bratislava, Slovakia). The electrolyte solution for the determination of Hg (Reagent No. R-003) was 0.1 mol/L and 0.001 mol/L in sulfuric acid and hydrochloric acid, respectively [1].

Fig.1 The block diagram of the monitoring device

RESULTS AND DISCUSSION

The monitor virtually facilitates all types of electroanalytical measurements: voltammetry, potenciometry, coulometry, conductimetry whereas enables automatic measurements. In trace analysis, low concentrations of heavy metals such as As, Hg, Cd, Cu, etc. are mostly measured by two-step stripping procedures: The analyte species are collected at the surface of a suitable working electrode (deposition or preconcentration step) and are stripped from the electrode chemically or electrochemically in the next step - stripping step. The same principle has been used for the monitoring by means of the EcaMon device. However, the sample matrix may seriously interfere in numerous instances. Organic substances present in the sample may prevent metal species to enter the electrode surface. Dissolved oxygen interferes in the deposition of electronegative metals such as Zn, Cd, Pb, etc. [2]. Hence, the electrochemical system need to be resistant against such influences.

Most interferences can significantly be suppressed by suitable electrode materials and coatings on the electrode surface. The adverse effect of dissolved oxygen is virtually negligible if porous working electrodes are used and the stripping step in the procedure is carried out galvanostatically instead of the commonly used potentiostatic stripping.

The measurement procedure consists of the following steps: i) A given volume of the sample solution is pumped through the cell set to a suitable deposition potential. Element species of interest are deposited at the electrode surface. ii) The flow system is purged with the electrolyte solution while keeping the electrode potential at the same value. iii) The flow is stopped and the deposit is stripped by a suitable constant current. The change of the potential of the working electrode gives information about the amount of the determined element (-s). iv) The flow system is washed again with the electrolyte solution.

The calibration can be performed automatically by injecting a know volume of the standard solution to the flowing sample solution by making use of the valve 1.

The duration of the whole procedure, including the calibration step is typically 5 min, and can be significantly shorter if the calibration procedure is performed only occasionally.

Fig. 2 bring a typical recording of a sample containing Zn, Cd and Pb. The stripping peaks are evaluated by integration.

Fig. 2 Typical chart recording of a signal for water sample containing Zn, Cd and Pb.

The figures of merit of the monitoring station were tested by continuous monitoring of waste waters for Hg originating from an electrolysis plant. The composition of tested waters changes during the monitoring significantly. The pH values varied between 2 and 12, there was a high calcium chloride concentration (over 1 g/L), aromatic organic compounds and surfactants were also present. The water sample was continuously pumped through a rough filter to a siphon where the aliquots were taken from by the monitoring device.

The measurements were performed 4 to 10 times per hour, whereas the calibration was carried out once every hour.

The results were checked by the determination of Hg in the same sample solution by the cold vapor AAS technique.

The untreated sample solution gave lower contents of Hg as the AAS method. The reason was the ready reduction of Hg(II) species to elementary Hg in the reducing water environment. Unlike the AAS method, the electrochemical method response to the ionic form of Hg only, which results in virtually lower Hg contents. To avoid this phenomena, potassium permanganate was added to the electrolyte solution to oxidize all Hg species to ionic Hg(II) species which can be measured by the monitor. Since the electrolyte solution is mixed automatically to the sample solution, there was no delay in the measurement procedure.

Fig. 3 Monitoring of Hg in the waste water sample with constant and linearly rising Hg concentration, respectively [3].

Fig. 3 brings the obtained Hg concentrations measured within a week of continuous measurement. The stability and reliability of the system was checked by long-term measurements of waste water samples with constant and linearly rising Hg concentrations, respectively. The volume of the water sample was 5 ml.

REFERENCES

1. Beinrohr E, Cakrt M, Dzurov J, Kottas P, Kozakova E (1996) Fresenius J. Anal. Chem. 356, 253.

2. Beinrohr E, Cakrt M, Dzurov J, Jurica L, Broekaert JAC (1999) Electroanalysis 11, 1137.

3. Beinrohr E, Dzurov J, Annus J, Broekaert JAC (1998) Fresenius J. Anal. Chem. 362, 201.