Simple and Equipment-Free Paper-Based Device for Determination of Mercury in Contaminated Soil

Abstract

This work presents a simple and innovative protocol employing a microfluidic paper-based analytical device (µPAD) for equipment-free determination of mercury. In this method, mercury (II) forms an ionic-association complex of tetraiodomercurate (II) ion (HgI₄^2-_(aq)) using a known excess amount of iodide. The residual iodide flows by capillary action into a second region of the paper where it is converted to iodine by pre-deposited iodate to liberate I_2(g) under acidic condition. Iodine vapor diffuses across the spacer region of the µPAD to form a purple colored of tri-iodide starch complex in a detection zone located in a separate layer of the µPAD. The digital image of the complex is analyzed using ImageJ software. The method has a linear calibration range of 50-350 mg L^-1 Hg with the detection limit of 20 mg L^-1. The method was successfully applied to the determination of mercury in contaminated soil and water samples which the results agreed well with the ICP-MS method. Three soil samples were highly contaminated with mercury above the acceptable WHO limits (0.05 mg kg^-1). To the best of our knowledge, this is the first colorimetric µPAD method that is applicable for soil samples including mercury contaminated soils from gold mining areas.

Keywords: iodometry; mercury; paper-based; soil; tetraiodomercurate; water.

Figure 1

(a) The µPAD pattern: (i) acceptor layer A with circular-shaped barrier and (ii) donor layer D with dumbbell-shaped barrier. (b) The three layers of the membraneless gas-separation µPAD, showing alignment of the donor layer, the spacer layer with circular hole and the acceptor layer. (c) 3D-view of assembled device from both the acceptor and donor sides, with position of the transparent tapes.

Figure 2

Illustration of the operating steps for the determination of mercury using the membraneless gas-separation µPAD.

Figure 3

Effect of physical parameters on analysis of Hg(II): (a) reaction time and (b) spacer thickness. Experimental conditions: 0.2 mol L⁻¹ KIO₃ in 0.2 mol L⁻¹ H₂SO₄, 10 mmol L⁻¹ KI and 1% (w/v) of starch in 0.1 mmol L⁻¹ KI. For the spacer thickness study, the reaction time is 4 min.

Figure 4

Effect of iodide concentration on analysis of Hg(II): (a) images of purple iodine-starch complex for various concentrations of iodide analysis using a standard solution of 150 mg L⁻¹ Hg and (b) plots of effect of iodide concentrations on the green intensity values (left ordinate) using a standard solution of 150 mg L⁻¹ Hg and sensitivity of Hg(II) analysis (right ordinate). Experimental conditions: 0.2 mol L⁻¹ KIO₃ in 0.2 mol L⁻¹ H₂SO₄, 10 mmol L⁻¹ KI, 1% (w/v) of starch in 0.1 mmol L⁻¹ KI and reaction time of 4 min.

Figure 5

Calibration line using the membraneless gas-separation µPAD for the determination of mercury and the corresponding image of the purple iodine-starch complex.

Figure 6

Bar plots of the concentrations of mercury in digested soil and water samples as determined using the membraneless gas-separation µPAD and the reference ICP-MS method. The digested soil samples, S1–S3, were analyzed directly using ICP-MS with appropriate dilution. The other digested soil samples, S4–S10, were spiked at 2500 mg kg⁻¹ Hg. The water samples, W1–W4, were spiked at 100 mg L⁻¹ Hg.