Chromium speciation in drinking waters

Chromium has two very different sides as far as human health is concerned. In its trivalent (III) form, chromium constitutes an important micronutrient, and its deficiency negatively affects the human organism. However, the hexavalent (VI) form of the element is toxic, carcinogenic, and mutagenic. Both these variants occur naturally in water, which makes it paramountly important to monitor whether the concentration of dangerous sunstances does not exceed norms. This is exactly what the scientists from the AGH UST have investigated by analysing chromium speciation in drinking water of various types. A team led by Professor Ewa Kmiecik from the Faculty of Geology, Geophysics, and Environmental Protection has also used this opportunity to check the efficiency of popular water filter cartridges.

Chemical speciation is the occurrence of individual concentrations of various chemical forms of an element in a sample, which vary in terms of physicochemical properties or physiological activity. Chromium in surface or underground waters occurs primarily in two oxidation states: III and IV. The speciation (Cr(III) and Cr(VI)) varies both in terms of the physicochemical properties and the impact on the human organism. For example, Cr(III) is poorly soluble in aquatic environments and is frequently found to be bound to organic matter, whereas CR(VI) shows good solubility in water and is more mobile. Additionally, the former constitutes an important element of the human diet, providing, among other things, support for healthy glucose metabolism. The latter has been proved to be mutagenic, carcinogenic, and toxic to the internal and external organs of living organisms.

Both forms occur in aquatic environments. The sources of chromium can be multifarious – both natural (soil) and related to anthropogenic activity. The toxic Cr(VI) occurs in compounds used to produce pigments that find application in the ceramic, textile, and tanning industries. As an industrial waste, the hexavalent form of chromium gets to rivers and contaminates the environment. Some chromium-containing substances are also used in galvanisation processes and for decorative or protective purposes because adding it to steel impedes corrosion. With the negative influence of Cr(VI) on human health in mind, it was necessary to establish norms determining the maximum permissible concentrations of this element in drinking water.

Ambitious research project

The goal of scientists from the AGH UST Department of Hydrogeology and Engineering Geology at the Faculty of Geology, Geophysics, and Environmental Protection who make up the Water Research Group is to analyse chromium speciation in drinking water of various types and to verify whether the norms have not been exceeded. As part of a university grant, the team has studied more than 70 brands of bottled waters that – as it turned out – conform to the norms. The following stage of the investigation is the analysis of chromium in treated tap water, underground waters, and surface waters, which will require field research. However, as far as laboratory tests are concerned, the method of measurement for each case and type of water will look exactly the same. After a meticulous preparation of samples, they will be analysed using ion chromatography with mass spectrometry, which will allow scientists to determine precise concentrations of substances in a given solution.

‘It can so happen that the speciation of chromium will be so unremarkable that it will be difficult to identify, separate and determine it directly using ion chromatography with mass spectrometry. In such cases, we need an extra stage of sample preparation for analysis, relying on solid-phase extraction and solution evaporation. In actuality, we percolate a 5-litre sample through extraction columns with a medium that caputres cation forms of Cr(III) or anion forms of Cr(VI). After we percolate the sample through the columns, the separated trivalent and hexavalent forms of chromium are eluted to small-volume solutions. This means that out of 5 litres of starting sample, we get 5 ml of a thousand-fold concentrate. As a result, the substance under investigation will be much more easily identifiable on a mass spectrometer, which will allow us to determine even low-level concentrations of chromium in the various types of drinking water that we study. The procedure discussed takes into account the separation of chromium speciation in extraction columns, which is why the analysis of Cr(III) and Cr(VI) concentrations will be performed only using a mass spectrometer this time.

We will also test a procedure for simultaneous sedimentation of chromium speciation in extraction columns after percolating Cr(III) to its anion form. These adequately prepared samples can now be analysed in an ion chromatograph, which will separate these two forms, trivalent and hexavalent chromium, and subsequently, their concentration will be determined in a mass spectrometer. The procedure will allow us to standardise the way of sample preparation and analysis regardless of the chromium concentration in the studied waters’, says Piotr Rusiniak, MSc, one of the chief researchers in the project.

 

However, the so-called instrumental analysis described above is not everything the AGH UST scientists have in store. Data collected in the laboratory will be processed in hydrogeochemical modelling using Geochemist’s Workbench (GWB) software. On the basis of the physicochemical parameters of all solutions, the scientists will conduct a chromium speciation modelling in the water-bearing layer and a phase portrait modelling presenting the most prevalent Cr speciation in water. Furthermore, the researchers plan to perform a risk analysis for human health, related to both the total concentration of chromium and Cr(III) and Cr(VI) forms in the investigated waters.

Water filter cartridges tests

Taking advantage of this opportunity, scientists have also decided to test popular water filter cartridges. Experiments have shown their high efficiency – according to the information provided by the producers, filters actually lower the contents of toxic metals in water. The investigation began with an analysis of the metal concentration of the water supply system, which acted as the background for further measurements. For more than a month, scientists have been percolating a specific volume of water through filters, testing not only their effectiveness but also their durability. The researchers wanted to pinpoint the exact moment of filter usage when the reduction in metal concentration deteriorates with respect to its initial capabilities. During the presentation of research results at a scientific conference held at the AGH UST in December, a photo of cut-up water filters was shown, which attracted some interest from the participants.

‘Where did the cut-up water filter cartridges come from? We have a clever student who took part in the implementation of water filter jug experiments under the supervision of Dr Katarzyna Wątor. The student came to a realisation that it may be worth checking what’s inside such filters and how they actually differ from one another. She took it upon herself to dismantle those filters and concluded that they’re mostly filled with activated carbon or other materials patented by a given company’, explains Piotr Rusiniak.

 

Water filter cartridges cut-up by the student.

The project was funded by a university grant within the framework of the “Initiative for Excellence – Research University” project (the AGH UST 2020-2022, PRA-3).