A voltammetric method for determining free metal activities in aqueous solutions. Part 2: anodic stripping voltammetry of Cd, Cu, Pb and Zn in synthetic and soil solutions

Abstract

Theory and methodology for determining the free metal activity, (M 2+ ), of Cd, Cu, Pb and Zn by anodic stripping voltammetry is presented. Metal-buffer solutions were prepared to control (M 2+ ), which was varied by using different ligands and by changing pH. Total metal aqueous concentration of 10 -6 -10 -7 M and metal-ligand ratios of 1:20M, 0.010M acetate medium were used. Equations for reversible and irreversible systems were interpreted to explain the results. For Cd and Pb (pPb < 10), agreement was found among theory, experimental ASV and (M 2+ ), calculated by using a speciation chemical equilibria programme and stability constants reported in the literature. Kinetic effects were observed for values of pPb > 10. For Cu and Zn, good agreement was found between the theory and calculated (M 2+ ), and between experimental ASV results and calculated (M 2+ ). Experimental ASV results showed Nernstian parallel shifts with regard to theory. Free metal activities in the order of pCd = 12, pCu = 18, pPb = 10 and pZn = 9 were measurable under the experimental conditions tested. Results too showed good agreement between Cu-ISE and Cu-ASV when measuring the free Cu 2+ activity in aqueous extracts of four soils. Values of soil-(Cu 2+ ) measured were in the order of 10 -5 -10 -9 .

Heavy Metal Speciation, and the Evaluation and Remediation of Polluted Mine Wastes and Soils

“…), or organic ligands, or associated with mineral colloids [52,53]. Thus, the analytical evaluation of the free metal activity, (M 2+ )-value, is an important step in the process of chemical speciation of pollutant metals in aqueous solutions [38,[55][56][57][58]. However, in the past, the experimental determination of (M 2+ ) was restricted due to several limitations regarding sensitivity and selectivity of the applied methodologies.…”

“…Traditionally, the strict evaluation of (M 2+ ) was only possible using solid-state metal ion-selective electrodes (M-ISE), but some other analytical techniques have also been used to estimate 'free ion' concentrations of metals in solution (Donnan dialysis, resin exchange methods, and chromatographic techniques [54,59], voltammetry (e.g., Anodic Stripping Voltammetry, ASV [56], or Cathodic-Stripping Voltammetry)). However, HM ion fraction determinations, for instance, might become unreliable if concentrations in solution are below the detection limits (e.g., Donnan dialysis, resin exchange methods) necessary for the use of very sensitive techniques such as ASV [55,56]. Measurements with ion-selective electrodes (ISEs) in natural samples may be affected by the fouling of the electrode by organic matter [60].…”

“…Voltametric techniques, specifically ASV, have been successfully used to determine not only the concentrations of labile metal species from a current measured in solution as a metal is dissolved or released from a Hg-electrode, but the (M 2+ )-value too. Although analytical problems have been claimed regarding overlapping of stripping peaks, adsorption of surface-active organic compounds on the Hg-surface inhibiting the metal deposition, or formation of insoluble intermetallic compounds that affect peak size and position, other studies showed that these inconveniences can be overcome, so that free Cd 2+ ,Cu 2+ ,Pb 2+, and Zn 2+ activities [55,56] can be measured in natural polluted soil solution samples.…”

“…It is well known that the M-ISE for Cd 2+ and Pb 2+ , for instance, respond similarly to both metals; hence, Cd and Pb interfere with each other [64,65]. Aguirre et al [55,56] developed a robust method to determine (M 2+ )-values of Cd, Cu, Pb, and Zn by ASV. The method was tested using metal-buffer solutions to control (M 2+ ), by complexing metals with weak, medium, and strong ligands, and varying pH, total metal aqueous concentration (10 À6 -10 À7 M), metal-ligand ratios of 1:20 M, and 0.010 M acetate medium.…”

The Use of Biochar of High Growth Rate Plants to Agriculturally Remediate Heavy Metal Polluted Acidic Mine Wastes

“…For acidic polluted mine wastes, studies should focus mainly on lowering the mobility of Cd, Cu, Pb, and Zn below official environmentally safe values to warrant a biologically clean and sustainable ecosystem. To reach this goal, at least two schemes must be visualized; First the addition of widely used agronomic materials consisting of lime (Ca(OH) 2 ); gypsum (CaSO 4 •2H 2 O); P-fertilizer (KH 2 PO 4 ) and/or compost, and more recently with much success biochar, to "treat" a gradient of soil-fresh mine tailings mixtures in order to assess the treatment effectiveness to lower both the most toxic metal species in solution, the free-metal ion chemical activity, (M 2+ ), of metals like Cd 2+ , Cu 2+ , Pb 2+ and Zn 2+ [43] and the so called bio-available (acid-extractable) HM fraction imposed by international standards and norms. Second, a bioassay should be carried out to find the conditions to pass sensitive indicator plant response tests in these "fertile" ameliorated media.…”