Electromembrane extraction and electrochemical measurement system for heavy metal ions detection in aquatic environmental samples

Abstract

Water contamination is a worldwide problem which deserves attention due to its negative impact on ecosystem, human health as well as economic growth. Heavy metals are a group of the pollutants that have received particular attention due to their high toxicity even at concentration as low as parts per billion (ppb). Technology advancement in the field of separation and detection of heavy metals has introduced sensitive and selective analytical instruments for real aquatic environmental samples. However, real sample matrices can reduce the quality of results. In modern analytical chemistry, there is a high demand for accurate quantification of trace and ultra-trace of heavy metals from real aqueous samples. In the present study, electromembrane extraction (EME) and electrochemical techniques were combined to develop effective electrodes which can separate, pre-concentrate and determine heavy metals such as Pb(II), Cr(VI) and Cd(II) in real aqueous samples. Electrochemically reduced graphene oxide-graphite reinforced carbon (ErGO-GRC) was utilised in conjunction with square wave anodic stripping voltammetry (SWASV) for the determination of Pb(II). Meanwhile, selective and sensitive determinations of Cr(VI) was carried out using ex-situ prepared nafion-coated antimony film on graphite reinforced carbon (NSbFE-GRC) by square wave adsorptive stripping voltammetry (SWAdSV) in the presence of diethyltriamine pentacetic acid (DTPA). Ex-situ prepared NSbFE-GRC was also used for simultaneous determination of Pb(II) and Cd(II) by SWASV. Simple polyvinylidene fluoride (PVDF) flat sheet membranes were synthesised and characterised in order to combine these developed electrochemical techniques with EME. Heavy metals were extracted from an aqueous sample solution into an acidic acceptor phase in the lumen of a PVDF membrane bag by the application of voltage across the supported liquid membrane (SLM), consisting of organic solvent and complexing carriers. Parameters affecting the EME were optimised for heavy metals. The PVDF–ErGO–GRC electrode system attained enrichment factors of 40 times and 80% extraction with relative standard deviation (n = 5) of 8.3% for Pb(II). Good linearity in the range of 0.25-2 nM was obtained with correlation coefficient of 0.999. The Pb(II) ions detection limit of PVDF–ErGO–GRC electrode was 0.09 nM. Meanwhile, the PVDF–NSbFE–GRC system attained enrichment factors of 86.6 times, 95.6% extraction, and good linearity in the range of 10-60 pM with correlation coefficient of 0.9933. Furthermore, the limit of Cr(VI) detection was found to be around 0.83 pM for the developed PVDF–NSbFE–GRC electrode. On the other hand, the PVDF–NSbFE–GRC was able to attain enrichment factors of 49.3 and 68.4 times, 82.6% and 114.0% extractions, and good linearity ranging from 2 to 10 pM with correlation coefficients of 0.9953 and 0.9883 for Pb(II) and Cd(II), respectively. Furthermore, the limits of detection for Pb(II) and Cd(II) were found to be around 0.65 pM and 0.60 pM, respectively. A chargeable battery operated portable EME system was developed for quantitative determination of heavy metals. The newly developed single setup electrochemical system was applied to the analysis of real aqueous samples such as tap water, industrial waste water, river water and sea water, and it was able to extract with percentage of extraction in the range of 78.7 -103.0% compared to commercially available direct current power supply.