Detecting heavy metals in water with a new device

Materials World magazine
31 Mar 2020

A new device could detect heavy metals in water and dry preserve them for improved quality monitoring. Idha Valeur reports. 

A testing device, resembling a whisk in shape, has been developed for easy detection and dry preservation of contaminants in water in rural areas. Researchers from the Massachusetts Institute of Technology (MIT), USA, drew inspiration for the instrument from working in the field in India where it became evident that there was a lack of field-testing kits for quality monitoring of contaminants.

The mesh panels of the ‘whisk’, which the contaminants stick to, and the centre stick are both made from polypropylene. The adsorbent is a commercially available strong acid cation exchange resin. The mesh encapsulates the resin making it more user-friendly, with the sewn compartments in the mesh spacing out the resin for better direct contact with the liquid. 

MIT Fellow Emily Hanhauser told Materials World, ‘Polypropylene was chosen to be compatible with the entire processing protocol. The resin was chosen for its high heavy metal adsorption capacity in varying water compositions, its regenerable nature and its low cost. […] Other sorbents could adsorb heavy metals, but their adsorption capacity was negatively affected by naturally occurring cations in water samples, especially the main hardness cations, calcium and magnesium.’

Dry state 

‘Trace contaminants are substances that are harmful to human and environmental health at parts per billion (ppb) concentrations and include heavy metals and pesticides. There are currently no commercially available field-testing kits that can detect heavy metal contaminants at the low ppb levels necessary and with adequate sensitivity,’ Hanhauser said. ‘Because of this, large volume – around 250 ml to 1L – water samples must be collected, preserved using chemical additives and transported over long distances to centralised laboratories, where the spectroscopy instruments for quantitative analysis are located.’

To create the device, the team looked to the established technique of dried blood spotting where a finger-prick volume of blood is preserved in a dry state on a cellulose card. This is then stored and shipped for testing without hassle. ‘Our hypothesis is that, by replacing liquid sampling and transport with an easy-to-use, compact, light-weight, cheap and dry device, more water samples for trace analysis will make it to the lab, producing the large-scale water quality monitoring data for effective pollution control and remediation,’ Hanhauser said.  

Swirling around 

To test a water sample for contaminants, the whisk-esque device is inserted into the collected water sample, stirred around by hand for a specified time for proper adsorption of heavy metals. On removal it is either air-dried or blotted off and returned to the original packaging for transportation or shipping to a lab. 

To remove the heavy metals from the adsorbent, 10% hydrochloric acid will be used to create a sample that is ready for analysis. These tests will be conducted using standard high-throughput spectroscopy techniques, such as inductively coupled plasma optical emission spectroscopy and inductively coupled plasma mass spectrometry. 

Water samples were collected from around Boston and in trials, spiked with a specific concentration of heavy metal cations. The team then followed the steps of measuring adsorption, preserving cations in a dry state and releasing them in the acid. ‘For each water sample, we tested three devices for statistics on repeatability and reproducibility of results ‘Our results show that our device can predict the concentration of some heavy metals in the original liquid sample within 6% error after dry preservation periods of two weeks,’ Hanhauser stated. 

Since proving that the method works, the team has been collaborating with a group in India to field test the device with a wide range of samples. Hanhauser added, ‘Additionally, we have identified another sorbent material that could be suitable for the dry sampling of arsenic and are exploring devices and protocols for using this sorbent in a dry sampling device.’