Chemists often have to resort to destructive methods to analyse samples. For instance, they need to extract samples and apply substances like nitric acid to measure the concentration of metals in sediments. Ainara Gredilla of the University of the Basque Country (UPV/EHU) has laid the first foundations so that "greener" techniques can be used in the future.
She has analysed the concentration of metals in the estuary of Bilbao using traditional methods, and has created statistical models with the help of these data; she is hoping that the data obtained through traditional techniques in today's sediments can be used for tomorrow's sediments, and that way, further extractions of samples and the use of destructive substances can be avoided.
Her thesis is entitled Metalak eta metaloideak itsasadarretan: kutsadura jarraitzeko erreminta analitikoen garapena (Metals and metalloids in estuaries: development of analytical tools for monitoring contamination).
The work was carried out by the Analytical Research and Innovation (IBeA) group. In actual fact, her thesis supervisor and IBeA colleague Silvia Fernandez studied the concentrations of metal in the waters and sediments of the Nerviσn-Ibaizabal estuary (Bilbao) between 2005 and 2007, and Gredilla has followed up that piece of research. She gathered samples every three months, between April 2008 and October 2010. "I conducted a more specific spatial observation. We usually take samples of sediments and water in eight spots, but in this case we did so in 49," she explains.
The researcher has confirmed that the estuary is improving, but it still contains a higher concentration of metal than it should. She has come across peaks and troughs similar to those of between 2005 and 2007, but there are subtle differences. "As far as metal contaminants are concerned, in the waters an upward trend can be found, whereas in the sediments there is a downward trend," she says. The most robust hypothesis to account for this fact is the movement of contaminants between the sediments and the water: "The contamination comes from a spot located upriver from the estuary, and when salt water enters, the metal particles accumulate in the sediments. But if physico-chemical changes take place in the water, the particles could shift and return to it."
Spectra and chemometrics
However, the advances in 'green' chemistry constitute the main contribution of this work. She has obtained data from the Nerviσn-Ibaizabal estuary by means of the usual methods, but at the same time these data have enabled her to open up alternative techniques. These consist of infrared spectroscopy and X-rays, and this way it is possible to obtain data on a sediment sample without using chemical substances that destroy the sample.
In actual fact, this researcher has taken steps to interweave the results obtained in the traditional method with those from the alternative method. What happens is that a certain concentration of metal in a sedimentary sample (traditional method) corresponds to a specific projection when infrared spectroscopy or X-rays are applied (alternative method). So if information on the relation between the two values were to be compiled, there would be a possibility of predicting the data on the concentration of metal in each case using 'green' methods alone.
And how does one weave the relational network between these two types of values? By means of chemometrics. In other words, by applying mathematical and statistical methods to chemical data. "We combine the spectra deriving from the X-rays and the infrared spectroscopy with the concentrations of metal obtained through the previous method. So by adding the use of chemometric techniques we have developed some mathematical models. For example, they enable me to analyse 14 metals and come up with a model for each one and thus predict the concentration existing in each case," says Gredilla. This researcher spent three months at the University of Copenhagen during which she learnt the details of chemometric techniques for identifying groups.
So far she has only been able to develop the mathematical models for predicting the metal content on a theoretical level, but the aim is that this should be applicable in the future, "that one day it will be possible to determine the metal content of a sediment sample just by focussing X-rays on it. It would not be a question of taking fewer samples, but of making the analysis more straightforward and cutting the cost of it, as well as making it greener." Gredilla is now in fact getting ready to check the reliability of the models developed by applying them to sediments in other rivers worldwide. And she has just been offered the chance to participate in an international project alongside various European and South American researchers.
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