Use of Partial Extractions to Study Mercury Partitioning on Natural Sediment Particles—A Cautionary Note

Abstract

Inorganic and organic mercury forms (²⁰³HgCl₂ and CH₃ ²⁰³HgCl) were added to a natural sediment maintained in dilute suspension under oxic conditions, and partitioning was followed over time with the aid of phase separation techniques (centrifugation–filtration) and chemical extractions (1 N HCl; 0.1 N NaOH; 30% H₂0₂). Sorption of inorganic Hg onto the suspended sediment was essentially complete (>99% of the added HgCl₂ was associated with the sediment particles) and very rapid (t_1/2>> 1 d). The nature of the Hg-particle association, as probed by partial extractions with HCl or NaOH, showed no discernible time trend (t = 1, 3, 8, 16, 32 d). Partitioning of CH₃ ²⁰³HgCl also favoured the particulate phase, but the relative amounts present in the dissolved phase (3–5%) and in the HCl extract were higher than with HgCl₂. Results obtained with hydrogen peroxide, a reagent often used in the past to oxidize sedimentary organic matter and solubilize the Hg associated with this sink, proved unreliable. Under the extraction conditions the H₂O₂ acts not only as an oxidizing agent for the organic matter, but also as a reducing agent for the mercury. leading to volatilization and loss of the metal, presumably as Hg°.

Key Words:

• Mercury
• sediment
• partitioning
• extraction
• hydrogen peroxide