Determination of Heavy-Metal Distribution in Marine Sediments Edwin S. Pilkington and Leonard J. Warren* CSlRO Division of Mineral Chemistry, P.O. Box 124, Port Melbourne, Victoria 3207, Australia
w A new method for determining the distribution of heavy metals in near-shore marine sediments has been investigated. The various sediment components were separated into distinct bands in a density gradient formed from the heavy liquid tetrabromoethane. Direct flame and graphite furnace atomic absorption spectrometry was used to analyze for the low levels of lead, cadmium, and zinc in the density subfractions. Details of the mineralogy and the concentration and distribution of heavy metals in two selected sediment types are given. Conventional methods of studying the distribution of trace elements in soils and sediments are based on the selective dissolution of the sample by a series of reagents of increasing reactivity (1-4). One of the most comprehensive schemes of this type ( 3 )involved treating the sediment successively with distilled water to dissolve soluble and weakly sorbed metal, 0.05 M calcium chloride to remove exchangeable metal, 2.5% acetic acid to desorb metal specifically sorbed on organic sites, and so on for a total of nine separate chemical steps. However, apart from problems of contamination, such chemical treatments are seldom selective enough to give an unambiguous indication of the metal distribution. Malo ( 4 ) , for example, found that acetic acid dissolved not only specifically adsorbed metal but also significant amounts of silica and aluminum, presumably from lattice sites. Subramanian ( 5 ) observed structural changes in some clay minerals after treatment with citrate, bicarbonate, dithionite, and hydrogen peroxide. Agemian and Chau (2) found that about 10% of the total aluminum in aquatic sediments was extracted by cold 0.5 N hydrochloric acid, a treatment normally presumed to dissolve only the complexed, adsorbed, and precipitated heavy metals. In the present paper another approach is reported in which the various sediment components, including the organic matter, are separated before analysis for heavy metals without chemical change. The basis of the method is that the components of a sediment, differing in specific gravity, will separate into distinct bands in a density gradient in a suitable heavy liquid. Similar methods have been developed for use in the mineral processing industry (6) and for characterizing soil clays ( 7 ) , but their potential for environmental analysis has only recently been realized (8). Using density-gradient mineral segregation in a zonal rotor, Francis and Brinkley (8)detected preferential adsorption of radioactive cesium on the micaceous component of a freshwater sediment. We have found that the density-gradient technique can be used to separate the main mineral species in a near-shore 0013-936X/79/0913-0295$01 .OO/O
sediment and that the total lead, cadmium, and zinc levels in the various sediment components can then be determined by appropriate chemical analysis. Since the technique physically separates the sediment components it is possible to study not only the total metal but also the biologically available heavy metal associated with each component. Work along these lines is in progress. In the present paper we describe the methodology of the density-gradient technique and give results for two selected sediment types. Experimental
Collection of Samples. Sediment samples were collected from Spencer Gulf, South Australia, several kilometers offshore from a large lead smelter. Divers working at depths of approximately 7 m forced the top 10 cm of sediment into plastic tubes, which were capped a t the top, withdrawn, and then capped underneath. The samples were transferred to acid-washed polythene bags, cooled to 4 "C, and filtered several days later. The damp salty sediments were dried in a clean stainless steel oven a t 100 "C. Oven drying may lead to irreversible changes in some sediment components and effects of this nature are being investigated by comparing metal distributions obtained after oven drying with those measured after freeze drying the fresh sediment. Two of the sediment samples were selected for detailed study. Sample ES 26 was a grey, gritty near-shore sediment collected at 6.7 m depth in an area covered to about 90% by broad-leafed Posidonia. Sample ES 35 was a brownish, oozy sediment collected at 7.6 m depth in a shallow depression devoid of seaweed growth. Separation into Size and Density Fractions. One-gram samples of the dried sediment were suspended in 500 mL of AR acetone and dispersed ultrasonically (1min, 20 kHz, 150 W). Acetone, rather than water, was chosen as the dispersing medium to avoid the possibility of the water removing weakly sorbed metals. The suspension was sized by repeated Stokes' law sedimentation in acetone into three size fractions: -1000 10 pm, -10 1pm, and -1 pm. No flocculation occurred during sizing of the freshly dispersed particles. Tests showed that the concentrations of lead, zinc, and cadmium in samples before and after washing with acetone were the same, indicating negligible removal of heavy metals by the acetone. The preliminary separation of the sediment samples into sized fractions greatly facilitated the subsequent separations into density subfractions. By treating each size fraction separately, problems caused by the differing behavior of the large and the very fine particles in the density gradient were avoided. Tetrabromoethane (TBE) was chosen as a base heavy liquid. Its sp gr of 2.96 is high enough to suspend most of the
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@ 1979 American Chemical Society
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Volume 13, Number 3, March 1979
295
DENSITY BANDS RETRIEVED
___________ < 2 4 __________ 2 . 4 - 2 6 5
_________ 2.55 - 2.66
_________ 2.66 - 2.75 2.75
- 2.95
_________ 5 2 . 9 5 Figure 1. Mineral bands observed after centrifuging sediment samples in the heavy liquid density gradient
minerals commonly found in near-shore sediments. Laboratory grade TBE was further purified by redistillation at reduced pressure and the colorless distillate of sp gr 2.96 mixed with acetone to give a series of liquids of sp gr 2.75, 2.66,2.55, 2.40, and 2.20. The liquids, in order of decreasing density, were carefully layered into the double-walled centrifuge tube shown in Figure 1.About 0.5 g of sample was placed on the surface of the topmost density layer and the tube centrifuged at 1500 10 pm fraction) or 60 min (-10 1 rpm for 5 min (-1000 pm fraction). Distinct bands of particles of similar density formed at the interfaces between the liquid layers (Figure 1). The system was maintained at a constant temperature during prolonged centrifuging to prevent intermixing of the bands. Sample masses greater than 0.5 g were avoided, as some of the denser particles were then entrained in the upper low-density layers. The various density bands were removed in order by displacement with pure TBE. (An appropriate apparatus is available from T. W. Wingent Ltd., Cambridge, England.) The -1000 10-pm particles were washed free of TBE on an acid-rinsed and dried Buchner funnel, weighed, and then analyzed for heavy metal content. Particles finer than 10 pm tended to flocculate in the TBE/acetone mixtures, but polyvinylpyrrolidone (PVP) was found to be an effective dispersant. A 1%solution of PVP in acetone was used as the diluent in preparing the topmost density layer. It was also found that bands formed from the 1 pm size fraction were removed more easily with a -10 Pasteur pipet. The particles were washed free of TBE by repeated centrifugation in acetone. The finest particles (-1 pm) could not be separated into density fractions with the conventional centrifuge used in this work. Tests were made to establish whether TBE extracted heavy metals from sediment particles during the density fractionation procedure. The residue of TBE and acetone after a fractionation experiment was analyzed by atomic absorption spectrometry (AAS) using a carbon furnace and standards prepared from metal salts of cyclohexylbutyric acids. The concentration of lead in the residue was
