Selective Sequential Extraction analysis ( SSE ) on Estuarine Alluvium Soils

Selective Sequential Extractions (SSE) were used to study the retention mechanisms of heavy metals on three estuarine alluvium soil columns obtained from leaching experiment for up to 5 pore volumes (PV). Leaching results indicate that almost 99 % of heavy metals (Pb, Cu, and Zn) were retained in the soils with the Ce/Co values in the order of 10 -3 to 10. This is in accord with the results obtained in respect to the resultant pH of the effluents and pore waters. The heavy metal extraction profiles from SSE show very similar trends with the retention profiles from the leaching experiment, where heavy metals were retained mainly at the top part (Entry of leachate) of the column. SSE indicates qualitatively that heavy metals precipitated with carbonates and amorphous materials (oxides/hydroxides) are higher than heavy metal retention via exchangeable mechanisms. The mass balance calculation gives range of deviation of 35-80% of the total soil extractions. The retention mechanisms of the heavy metals in soil solids are ranked in the following order: Carbonates>Amorphous>Organics>Exchangeable phases. INTRODUCTION The use of clay soils as impermeable or attenuating barriers is becoming more and more popular as the material of choice in landfill liner systems. Many researchers (Griffin et al., 1976; Yanful et al. 1988; Yong et al. 1992, 1993, etc.) have discussed the different aspects and potential use of soil material not only for liners, but also as substrate material under landfills. Heavy metals (H.M.) such as Pb, Cu and Zn that are commonly found in leachates from landfills can be effectively attenuated by such soils. The amounts of heavy metal retained depends on the pH of the soil-water system and the soil buffer capacity (Yong and Phadungchewit, 1993). Heavy metals are retained in soils by hydroxide and carbonate when the pH of the soil solution is higher than 4. The primary mechanism for H.M. retention in clay soils is through precipitation of the metal ions with carbonates and amorphous oxides or hydroxides (Griffin et al. 1977). Yong and Phandungchewit (1993) have shown that the presence of carbonates in a soil contributes significantly to the retention capability of the soil. This study was undertaken to determine the retention capability of the heavy metals Pb, Cu and Zn by soils in South Wales using leaching column experiments and selective sequential extraction analyses of the contaminated column samples. The role of the various soil fractions in sorbing the contaminants is examined in relation to the capacity of the soil for H.M. retention. MATERIALS AND METHODS The soil samples used in the study were collected from three different sites adjacent to an active landfill sites around South Wales, United Kingdom (Figure 1, Table 1). Five samples from each site were randomly taken from shallow trenches and pits. All samples were air-dried and sieved through 20mm sieve in the laboratory, to remove all coarse pebbles. Four separate sets of tasks were undertaken during the course of the study: • Basic characterisation of all soils using physical and chemical tests. • Determination of the attenuation characteristics of these soils via leaching column tests conducted on a selected sample from each group (NEA4 for NEA samples, PEA3 for PEA samples and CEA3 for CEA samples). • Acid digestion on slice soil samples obtained from leaching column tests to determine the gross amount of heavy metals retained. • Confirmation studies of retention mechanism by determine the heavy metals extracted from different soil solids via SSE. Figure 1. Study area and sampling location Table 1. Samples descriptions Group Samples Description NEA NEA1-NEA5 Estuarine Alluvium from Neath PEA PEA1-PEA5 Estuarine Alluvium from Newport CEA CEA1-CEA5 Estuarine Alluvium from Cardiff Physico-chemical tests Physical tests of all samples were conducted according to British Standard, BS 1377 (1990). These included natural moisture content, specific gravity test, particle size distribution, compaction test and permeability test. Physico-chemical tests performed according to procedures detailed in the Laboratory Manual of the Geotechnical Research Centre of McGill University, included organic content determination, specific surface area (SSA), cation exchange capacity (CEC) and pore fluid chemistry. Mineralogy of the soil samples was determined using x-ray diffractometry. Carbonate content was determined using the titration method by Hesse (1972), and amorphous oxides/hydroxides content was obtained using the procedure described by Segalen (1968). Leachate solutions and pore water were analysed using atomic absorption spectrometry (AAS) for Si, Al, and Fe. Leaching column test The leaching cell used consisted of a Plexiglas cylinder with diameter 115 mm and height 125 mm. The soil samples were compacted at maximum dry density and optimum moisture United Kingdom content into triplicate soil columns i.e., soil series as illustrated in Figure 2. The leaching experiments were conducted under air pressure at 10psi (68Kpa) to reduce the time factor. The cells were leached with a test leachate that was obtained from a Municipal Solid Waste Landfill (MSWL) and spiked with heavy metals Pb, Cu and Zn. The pH of the test leachate was reduced up to 1.45 to promote increased mobility of the heavy metals in the soil columns. Figure 2. Three series of leaching column experiments The effluents (leachate discharge) were collected and analysed at every 0.5 PV using an ICPMass Spectrometer (ICP-MS). At the end of the experiments the columns were extruded and the columns were sliced into six equal slices (~20 mm each). The soil slices were tested for exchangeable cations using ammonium acetate (pH 7.0) and pore water analysis. All solutions were analysed using the ICPMS. Acid digestion About 200 mg of dry sample was placed into a clean dry savillex vial. The vial was placed in a fume cupboard before addition of 5 ml of Romil HF to the vial. It was then left overnight to digest on the hotplate. The solution obtained after digested was reduced by partial evaporation, following which about 5 ml of aqua regia was add to the reduced solution. The aqua regia solution was prepared by mixing 60 ml of conc. HNO3 and 180 ml of conc. HCl. The sample was then digested for another 24 hours and subsequently evaporated to obtain a dry sample. Following this, 5-6 ml of 5M primar HCl was added to the sample which was left on a hotplate for several hours. The solution obtained therefrom was diluted using 100ml of plastic volumetric flask prior to analysis using the ICP-MS. Selective sequential extraction Methods of extraction have been adopted from Yong and Phandungchewit (1993), Yanful et al. (1988), Tessier et al. (1979) and Gupta and Chen (1975). The basic utility of selective sequential extraction is its use of appropriate reagents to release different heavy metal fractions from soil solids by destroying the binding agents between the metals and the soil solids and permitting the metal species to be detected (Yong and Phandungchewit, 1993). The soil samples from the leaching column tests which were used for the SSE analysis were sliced into six equal slices. For the SSE analysis, 1 g of the soil slice was weighed into the 50ml polypropylene centrifuge tubes to avoid any loss of soil sediment during the extraction NEA4 series CEA3 series PEA3 series LEACHATE