Adsorption as a Control of Metal Concentrations in Sediment Extracts Paul S. Rendell' and Graeme E. Batley Analytical Chemistry Section, Australian Atomic Energy Commission Research Establishment, Lucas Heights, 2232, New South Wales, Australia
Alan J. Cameron School of Chemical and Earth Sciences, New South Wales Institute of Technology, Broadway, 2007, New South Wales, Australia
w The adsorption of Cu, Pb, and Cd from selected extractant solutions, onto uncontaminated river sediments, has been investigated under typical extraction conditions. Significant adsqrption of added metal was found to occur during overnight extraction with dilute HC1 (pH >1.5), 0.1 M hydroxylamine hydrochloride (pH 2), 0.1 M sodium citrate ( p H 4.6), 1 M ammonium acetate, 10%sodium citrate-1% sodium dithionite, and 25% acetic acid. Adsorption also occurred during a hydrogen peroxide digestion procedure. T h e inability of reagents to prevent losses of soluble metal in these experiments strongly suggests that a proportion of the metal actually released from a sediment sample during an extraction will be readsorbed. This may lead to serious misinterpretation of extraction data because the metal concentrations determined in the extract do not represent metal levels in the sediment fractions attacked. Chemical extraction techniques, used either to estimate trace element bioavailability or trace element associations in sediments and soils, have been extensively applied in environmental studies, agronomy, and exploration geochemistry. A wide range of reagents has been used, encompassing most modes of chemical attack ( I , 2). The amount of metal in the final extract is generally assumed to be equivalent to the amount of previously bound metal released from the sediment or soil, Le., the amount associated with the sediment or soil components attacked by the reagent. The possibility of metal, released from certain components during a weak extraction, being readsorbed by other sediment or soil components or freshly exposed surfaces has received little attention. Sediment and soil components differ in their susceptibility to various types of chemical attack and in their ability t o adsorb or complex metals. Guy et al. ( 3 ) ,investigating the extraction of metal from individual model sediment components and two-tomponent mixtures, found that less than the expected amount of metal was extracted by hydrogen peroxide and ammonium chloride during extractions of the two-component mixtures. This loss of metal was attributed t o the readsorption of released metal by the second component. Gupta and Chen ( 4 ) suggested that adsorption of metal leached during hydrogen peroxide digestion may have been responsible for the different extract metal concentrations observed when the oxidized sediment was subsequently treated with 0.01 M nitric acid or 1M ammonium acetate (pH 2). Readsorption has also been used to explain an apparent loss of copper a t p H 7 from sodium citrate-sodium dithionite extracts ( 5 ) .The adsorption of metal released by weak extractants could be an important control of metal concentrations in sediment and soil extracts, and hence have a significant bearing on the interpretation of extraction results. In this paper, extractants spiked with Cu, P b , or Cd were shaken overnight with river sediment, and the solution metal concentrations compared before and after extraction. To give a n indication of the range of reagents t h a t might be affected, adsorption was investigated as a function of pH, and in the presence of reducing agents, strong and weak complexing agents, a weak acid, an oxidizing agent, and a cation ex314
Environmental Science & Technology
changer. The adsorption of metal added in these experiments was assumed to be evidence for the readsorption of metal released during sediment extractions. I t should be noted that, although readsorption is used as a synonym for loss of metal from solution, metal losses could be the result of a combination of processes such as complexation, precipitation, coprecipitation, and adsorption.
Experimental Sample Collection and Preparation. Surface aerobic sediments were collected from shallow water areas in four freshwater rivers in New South Wales (Table I). A polyethylene scoop was used to transfer sediment (predominantly the top 1 cm) to acid-rinsed polyethylene bottles for transport to the laboratory. In the laboratory, samples were wet-sieved through a nylon screen (1-mm aperture) to remove pebbles, coarse sand, and organic debris. The sediment was then allowed to settle for 1 to 2 days, before overlying water was removed by suction and the sample thoroughly mixed. During settling, and between subsequent investigations, samples were stored at 4 "C. Total concentrations of Fe, Mn, Cu, P b , and Cd in the sediment samples were determined by atomic absorption analysis after hydrofluoric acid-nitric acid dissolution of ashed samples. Particle size distributions for the dried sediments were obtained by screening through 180- and 53-pm sieves. Total organic carbon analyses were carried out on dried samples using a Hewlett-Packard carbon analyzer. Adsorption Studies. Sediment extractions were carried out, in the presence and absence of added Cu, Pb, and Cd, using the following reagents: dilute hydrochloric acid (pH 1-3.5); 0.1 M hydroxylamine hydrochloride acidified to p H 2 with nitric acid; 0.05M disodium EDTA; 0.1 M trisodium citrate acidified to p H 4.6 with nitric acid; 1 M ammonium acetate; 30% hydrogen peroxide 0.01 M nitric acid; 25% (v/v) acetic acid; 1%(w/v) sodium dithionite in 10% (w/v) trisodium citrate. Only analytical grade reagents, except sodium dithionite which was laboratory grade, were used for the preparation of extractant solutions. With the exception of the H2O2 treatment, 40 mL of extractant was added to wet weight subsamples equivalent to 2 g of dry sediment, contained in screw-topped polyethylene centrifuge tubes of 50-mL capacity. Sufficient distilled water (0-1.5 mL) was added to eliminate volume variations caused by the differing amounts of water associated with each sediment. The wet weight of the subsample and the distilled water volumes were based on regular dry weight to wet weight determinations. T h e total solution volume was 42 mL. Added metal concentrations, unless otherwise stated, were such that t h e final test solutions contained initially 2 pg of Cu mL-', 2 pg of P b mL-', or 0.2 pg of Cd mL-*. Each metal was investigated individually. In the citrate-dithionite extraction, 10% sodium citrate was spiked with metal, and the sodium dithionite added as a solid immediately before commencing the extraction. The sediment-extractant mixtures were shaken for 16 h and centrifuged, and the solutions decanted carefully into polyethylene bottles for subsequent analysis and p H determina-
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@ 1980 American Chemical Society
Table 1. Sources and Size Distributions of Sediment Samples sample no.
5
sampling slte
Nepean River, near Penrith, NSW Nepean River, Camden, NSW Georges River, Ingelburn, NSW Shoalhaven River, near Marulan South, NSW Hacking River, Royal National Park, south of Sydney, NSW
slze distribution, % 1000-180 180-53
