A monitorable and renewable pollution filter based on graphene nanoplatelets

Abstract

This paper deals with the fabrication, modeling and experimental characterization of a monitorable and renewable graphene-based pollution filter. The main goal is to demonstrate a method to monitor the status of such a filter in real time during its operating phases: pollutant adsorption, saturation, and regeneration. The filter is realized by a disk of pressed graphene nanoplatelets. This is a low-cost type of graphene which has recently drawn great interest due to its potential use in large scale industrial production. Here the nanomaterial is obtained through the exfoliation method assisted by microwave irradiation, by exploiting the thermal expansion of commercial intercalated graphite, according to a low-cost and ecologically friendly procedure. The filter is used here to adsorb acetonitrile, a toxic water-soluble organic compound that is present in some industrial solvents and paints. The monitoring method is based on the interpretation of the time variation of the electrical impedance measured during filter operation. There are two main results of the paper: Firstly, the graphene filter is shown to be effective in adsorbing the above pollutant, with the additional feature of being fully renewable: all the pollutant can be removed from the filter without the need of costly physical or chemical processes. Secondly, monitoring of the time-evolution of the electrical impedance allows efficient detection of the different phases of the filter life cycle: clean, polluted, saturated and regenerated. This feature is of potential interest since it enables the predictive maintenance of such filters.

Electrical Impedance Spectroscopy for Real-Time Monitoring of the Life Cycle of Graphene Nanoplatelets Filters for Some Organic Industrial Pollutants

“…Low thickness values are essential in retaining outstanding physical properties like those of 2D materials that are at the basis of the excellent adsorption performance of the graphene filters. In addition, this fabrication technique provides high-quality samples, with negligible oxidation and a good level of purity of the material, as demonstrated by the Raman and X-ray characterization carried out in [18]. The fabricated GNP flakes have been then pressed to form thin disks.…”

“…As highlighted in the Introduction, the proposed method is based on the monitoring of the time variation of the electrical impedance frequency spectrum of the filter when exposed to pollutants. These changes happen thanks to the sensitivity of the graphene electrical properties (e.g., conductivity and permittivity) with the binding of external substances [18], [28]. It is worthy to stress that the sensitivity and selectivity to target substances can be strongly enhanced by a suitable functionalization process [29], [30], so that these materials are currently proposed, for instance, as sensing elements in environmental sensor platforms [31], [32].…”

“…In this tube, the pollutant is injected and dosed in known and controlled quantities. In addition, a polytetrafluoroethylene (PTFE, also known as Teflon) layer provides the insulation between the metal contacts and the pollutant, ensuring that only the filter is in direct contact with the pollutant [18], [33].…”

“…The latter technique has been recently assessed by the Authors and has been used to produce the pollutant filters adopted in this paper [15], [16]. In addition, the Authors have successfully presented a methodology based on the Electrical Impedance Spectroscopy (EIS), for the realtime monitoring of the State of Health (SoH) of both ACF filters [5], [17], and GNP ones [18].…”

“…In this paper, starting from their experience in real-time impedance measurements [19]- [21], the authors propose a significant advancement on the above-mentioned EIS methodology, presenting a measurement approach based on the definition of suitable figures of merit that analyze the time variation of the electrical impedance frequency spectrum. Indeed, although the single-frequency method proposed in [18] highlights some benefits and theoretically allows revealing some states of the monitored filters, in many cases the uncertainties related to the filter reproducibility, the measurement repeatability, the set-up non-idealities, the interactions between the filter and the pollutant, and the aging of the filter, might reduce the state detection capability. Therefore, starting from the preliminary encouraging results presented in [22], this paper proposes the analysis of the electrical impedances at different frequencies to increase the capability to detect the operating state of the filter.…”