engleski

Evaluation of the partial extraction capacity of hydrochloric acid for the solid speaciation of mercury

Early diagenetic reactions occurring in the upper layers of sediment are important in the remobilisation of heavy metals. The mobilisation of sediment-bound metals during diagenesis could have two consequences: the release of dissolved metal from the sediment to the overlaying water, or change in the association of metal from one sediment phase to another. Many sediment-bound metals are affected by redox cycling of iron and manganese during early diagenesis. Recently has been suggested that mercury in sediment is also affected by early diagenesis and particularly associated with iron and manganese oxyhydroxides. Information about diagenetic behaviour of trace metals in sediments could be obtained by simultaneous analysis of elements of interest (Hg, Fe, Mn) in both solid and dissolved sediment phases, or alternatively, by partial extraction of the solid phase. Two kinds of partial extractions have been used to evaluate the association of mercury with oxide phases: extraction with hydroxilamine-acetic acid solution which extract Mn and Fe oxides and carbonates and extraction with diluted hydrochloric acid which extract Mn and Fe oxides and AVS-Fe (acid volatile sulphides). Both extractions could suffer from artefacts introduced during the extraction procedure. The extraction with a reducing hydroxilamine solution, which should be, in addition, performed at elevated temperature, can result in loss of mercury trough volatilisation. The extraction with dilute HCl can suffer from serious post extraction readsorption, when sulphides are present in sediments. Upon addition of acid, dissolved mercury can precipitate as HgS by reaction with H2S and may be readsorbed onto sulphide minerals, both reactions occurring even at very low pH values. Therefore, if extraction efficiency of mercury depends on the sulphide level in sediment (which normally increase with sediment depth), it would be difficult to make conclusions on the mercury association with oxides from such set of data. In this work the use of partial extraction with dilute HCl as a tool for the assessment of the mercury association with oxide sediment phases will be evaluated. The work will be performed by analysis of acid-extractable mercury in natural sediments containing different amounts of AVS. Also preextraction spikes will be used to assess the accuracy of mercury extraction and dependence of spikes recoveries on the sediment composition.

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