Probing Zinc Speciation in Contaminated Sediments by Square Wave Voltammetry at a Hg/Ir Microelectrode

Square wave voltammetry at a mercury plated indium (Hg-Ir) microelectrode was used to probe directly the chemical speciation of zinc in contaminated sediments. The electrochemically labile fraction of zinc was quantified in sediments from Lake DePue (IL), a lake heavily impacted by former zinc smelting activities. Eight samples were analyzed across a metal contamination gradient where total zinc concentrations ranged from 6,000 to 190,000 ppm and the corresponding peak currents from 0.1 to 62 nA. Under the analytical conditions chosen, the Zn²⁺/Zn system behaved reversibly, as demonstrated by the forward and reverse current signals in square wave. The presence of the sediment had little effect on the electrochemical response, and Hg recovery experiments demonstrated that the mercury film remained on the electrode and that it was not fouled during analysis. An external calibration was performed using a background electrolyte that mimicked in situ conditions and in presence of a 10 mM TES buffer. The concentrations of labile zinc varied according to the locations and ranged from 0.8 to 9 ppm that represented between 50 to 90 % of the total dissolved zinc present in the porewater.