Abstract
Microwave spectroscopy has been identified as a novel and inexpensive method for the monitoring of water pollutants. Integrating microwave sensors with developed coatings is a novel strategy to make the sensing system more specific for a target contaminant. This study describes the determination of copper and zinc concentration in water in both laboratory-prepared and acquired mine water samples from two abandoned mining areas in Wales, UK. Uncoated sensors immersed in samples spiked with 1.25 mg/L concentrations of copper and zinc, using the standard addition method, were able to quantify the concentration at 0.44 GHz with a strong linear correlation (R2 = 0.99) for the reflection coefficient magnitude (|S11|). Functionalised microwave sensors with l-cysteine, chitosan and bismuth zinc cobalt oxide-based coatings have shown improvement in the sensing performance. Specifically, the linear correlation at 0.91–1.00 GHz between |S11| and a polluted water sample spiked with Cu showed a higher (R2 = 0.98), sensitivity (1.65 ΔdB/mg/L) and quality factor (135) compared with uncoated sensors (R2 = 0.88, sensitivity of 0.82 ΔdB/mg/L and Q-factor 30.7). A Lorentzian peak fitting function was applied for performing advanced multiple peak analysis and identifying the changes in the resonant frequency peaks which are related to the change in metal ion content. This novel sensor platform offers the possibility of in situ monitoring of toxic metal concentrations in mining-impacted water, and multiple peak features, such as area, full width half maximum, centre and height of the peaks, have the possibility to offer higher specificity for similar toxic metals, as between copper and zinc ions.
Highlights
Metal pollution and monitoring challengesMining is an important contributor to global wealth (Younger et al 2002)
The major mechanisms associated with the mobilisation of metal ions in mining areas are: (i) the oxidation and consequent hydrolysis of sulphide minerals in mine waters (Johnson 2003), (ii) the leaching from deposits of mine tailings (Perkins et al 2016) and (iii) fluvial transport of mobilised metals from headwater catchments to coastal areas (Mayes et al 2013)
The aims of this paper are: (1) to demonstrate the feasibility of using microwave spectroscopy to quantify changes in Cu and Zn concentration in water using the standard addition method; (2) to evaluate the sensitivity for Cu ion concentration using the functionalised electromagnetic (f-EM) sensors based on mixtures of l-cysteine, In this work, 8-pair gold interdigitated electrodes (IDE) printed on PTFE substrates (Fig. 1a) were functionalised using a semi-automatic screen printer (Super Primex) as described by Frau et al (2018b) with a paste mixture based on l-cysteine (168149 Sigma-Aldrich, 40%), chitosan (448869 Sigma-Aldrich, 40%) and bismuth cobalt zinc oxide (631930 Sigma-Aldrich, 10%)
Summary
Metal pollution and monitoring challengesMining is an important contributor to global wealth (Younger et al 2002). International Journal of Environmental Science and Technology (2020) 17:1861–1876 resources (Environment Agency 2008) They are essential elements for living organisms, but high concentrations, especially with long-term exposure, can generate significant health problems, such as respiratory, gastrointestinal and neuronal disorders (Agency for Toxic Substances and Disease Registry (ATSDR) 2004). For this reason, the EU Water Framework Directive (EU WFD) and the US Environmental Protection Agency (US EPA) have developed environmental quality standards (EQS) that specify that the concentration of dissolved Cu and Zn in freshwater should not exceed 28–34 μg/L and 125–210 μg/L, respectively (UK Technical Advisory Group on the Water Framework Directive 2008; United States Environmental Protection Agency 1986). Thousands of abandoned mining areas need to be monitored in more detail for better prioritisation of effective remedial actions (Environment Agency 2012)
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