Suitability of using diffusive gradients in thin films (DGT) to study metal bioavailability in mine tailings: possibilities and constraints

Background, aim, and scope Diffusive gradients in thin films (DGT) have been recognized as a suitable tool to assess in situ metal bioavailability in soils. Mine tailings have some singular characteristics such as high heavy-metal concentrations, low pH, or absence of water retention capacity that may compromise the correct application of this technique whose applicability is known to be pH dependent. The goal of this study was to determine the response of DGT devices in heavy-metal-polluted mine tailings with different pH. In addition some experiments were performed in order to determine the effect of acidic pH and dissolved ions on the binding properties of the chelating resin. Materials and methods We tested DGT devices on three different mine tailings: acid pH 3, acid tailing limed to pH 5.5, and neutral pH 7.2. The tailings showed high metal concentrations, e.g., 7,000 mg kg⁻¹ Pb, 9,000 mg kg⁻¹ Zn, and 380 mg kg⁻¹ Cu. Diffusive and Chelex resin gels were prepared according to previously published methods. Two chelating resins and diffusive gels thicknesses (0.4 and 0.7 mm) were tested. Four DGT devices of each type were placed during 24 h in pots (one device per pot) containing 1 kg mine tailings in a climate chamber with humidity (50-90%) and controlled temperature conditions (night 16°C and day 23°C). Pots were irrigated with deionized water to field capacity, and then two different experiments were performed: (a) allowing free drainage and (b) maintaining the water saturation. In addition, we tested DGT devices in solutions at pH 3 with similar properties to the soil solution measured in the acid tailing. Eluted Zn, Cd, Pb, and Cu from the chelating resins were measured using inductively coupled plasma-optical emission spectrometer (ICP-OES; Vista-MPX Varian). Results and discussion The metal concentrations taken up by the DGT devices were affected by the different pH values of the tailings. The highest metal concentrations measured with DGT (C DGT) were obtained in the pH 3 treatments (both saturated and free drainage). Significant differences for C DGT were observed between water-saturated and free drainage treatments in the acid pH 3 tailing. When limed pH 5.5 tailing and neutral pH 7 tailing were considered, these differences were lower and not significant. In pH 3 tailings low values for C DGT/C soil solution were obtained (<0.06), indicating that these soils have a low capacity to resupply depleted metals to the solution. The limed acid tailing and the neutral tailing showed values between 0.05 and 0.94 indicating a much more rapid resupply from the solid phase. Deployment under water-saturated conditions yielded much higher C DGT values than under free drainage, indicating the importance to adequately control the moisture content in these soils with poor water retention capacity. In solutions with pH 3 mimicking the soil solution composition of the tailings, a loss of the binding capacity of the resin of 50-60% and 60-80% for Zn in 0.7-mm DGT and 0.4-mm DGT devices, respectively, was observed. As a consequence, 0.7-mm DGT devices had better reliability to carry out in situ determinations in solutions with high metal concentrations and low pH. Conclusions The use of DGT in mining soils can be a promising tool to study bioavailable metals concentrations in mine tailings but it has to be used carefully under acidic pH. Competition with other cations that are present at very high concentrations may hinder the accumulation of metals by the chelating resins, which should be tested under the conditions of the particular mine tailing.