Colorimetric measurement of copper(II) in sand using direct electrokinetic extraction into a polymer inclusion membrane

Soil pollution by heavy metals from extensive exploration and processing has increased the demand for heavy metal analysis in soil. Traditional analysis is mostly laboratory-based due to the requirement for sample preparation, e.g., utilising HNO 3 to digest heavy metals in solid soil matrix to liquid matrix. This additional step not only lengthens the analysis process but also reduces the feasibility of in-field analysis because of the use of hazardous chemicals. Alternative approaches that are low-cost and environmentally friendly, while also allowing in-field analysis with minimal skill requirements need to be developed for better screening and monitoring purposes. A 3D electrokinetic extraction system was developed for the direct quantification of Cu (II) in silica sand and tailings samples using a colorimetric polymer inclusion membrane (PIM). Using an electric field, heavy metals are mobilised from the silica sand and move into the PIM where they complex with the colorimetric reagent, 1-(2-pyridylazo)-2-naphthol (PAN), causing a colour change proportional to the Cu(II) concentration. This was explored with Cu(II) spiked into silica sand, whereby a linear detection range of 0.30 μg to 2.35 μg of Cu(II) (equivalent to 1.20 to 9.40 μg of Cu(II) per gram of sand) was achieved. This is the first demonstration of electrokinetic extraction of ions from a solid material and shows potential for minimising sample pretreatment processes for in-field chemical analysis. The system is small in size and dry-to-touch and enables quantification using a low sample volume while remaining easily portable. Quantification through image analysis reduces the requirement for specialised skills • Electrokinetic extraction system can extract Cu(II) from soil without liquid reagent. • The colour change of polymer inclusion membrane refers to the Cu(II) concentration. • The linear detection range of the system was 0.30 μg to 2.35 μg of Cu(II).