Environ. Sci. Technol. 1996, 30, 3294-3303
Mimicked In-Situ Stabilization of Metals in a Cropped Soil: Bioavailability and Chemical Form of Zinc ANNA CHLOPECKA* AND DOMY C. ADRIANO Division of Biogeochemical Ecology, The University of Georgia, Savannah River Ecology Laboratory, Aiken, South Carolina 29802
Agricultural lime, natural zeolite (clinoptilolite), hydroxyapatite, and an iron oxide waste byproduct (Ferich, a trademark name of E. I. du Pont de Nemours) were added to an artificially contaminated Appling silt loam soil to stabilize and limit the uptake of Zn by crops. A greenhouse pot study involved spiking the soil with flue dust (FD) at 0, 150, 300, 600, 1200, and 2400 mg of Zn kg-1. As much as 40% of the total Zn occurred in an exchangeable form, the form considered most bioavailable to plants, when the pH of the FDspiked soil was below 6.0. The ameliorants (lime, zeolite, apatite, and Fe-rich) decreased the concentration of the exchangeable form of Zn at each level of FD in soil; however, the largest decrease occurred with the lowest dose. Maize (Zea mays), barley (Hordeum vulgare), and radish (Raphanus sativus) were grown to determine the effects of Zn on the plant growth and its uptake. The addition of ameliorants to soil enhanced the growth and yield of maize and barley, but only Fe-rich enhanced the growth of radish at all FD rates. Lime, zeolite, and apatite significantly reduced the Zn concentration in tissues of the 3-week-old maize, in mature maize tissues (roots, young leaves, old leaves, stems, grain), and in barley. The largest reduction (over 80%) in Zn uptake by all crops was effected by Fe-rich, which is consistent with the greatest reduction in soil-exchangeable Zn by this ameliorant.
Introduction Zinc is widely recognized as an essential trace element in plant and animal nutrition. By the late 1960s, agricultural practices had led to widespread occurrence of Zn deficiency in plants. Consequently, 39 states in the United States recommended that Zn be applied to major field, forage, vegetable, fruit, and nut crops (1). Although much emphasis has been placed on deficiency, Zn phytotoxicity problems are becoming an increasing concern due to nontraditional * Corresponding author telephone: (803)725-2752; fax: (803)7253309; e-mail address:
[email protected].
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sources of this metal. Important anthropogenic sources of Zn that may lead to soil contamination include aerial deposition from the nonferric metal industry and certain agricultural practices. Accumulative emission has already caused extremely high levels of Zn in topsoils in certain areas. In the old mining area of southwest England, Zn concentrations in soil range from 220 to 66 400 mg kg-1 (2, 3), and in Upper Silesia, Poland, soil levels range from 5 to 13 250 mg kg-1 caused by the metal processing industry (4). In some urban gardens and orchards in the United States and Great Britain, Zn concentrations range from 20 to 1200 and from 250 to 1800 mg kg-1, respectively (5, 6). In several countries, Zn concentrations in sludged soils have exceeded the natural levels for Zn (7-10). In the 1980s, flue dust from a scrap metal recycling facility was mixed with lime (1:3 by weight) and sold to farmers in Tift County, Georgia, as a micronutrient supplement in soil. The flue dust which contained the following: 14% Fe, 9.7% Zn, 4.2% Ca, 3.6% Pb, and 0.03% Cd by weight) was applied to nearly 4000 ha of farmland. Of this, about 1000 ha was adversely affected, reducing the yields of crops, mainly peanuts (Arachis hypogaea), which are very sensitive to Zn. Concentrations of total Zn in soil in the affected area were quite variable ranging from 46 to 137 mg kg-1 as compared to the usual background level of 6 mg kg-1 for the area (11). In addresing metal-contaminated soils, it is essential to estimate the bioavailability of the metals. An approach commonly used for studying the mobility of metals in soils is to use selective sequential extraction procedures such as that developed by Tessier et al. (12) or Shuman (13). The procedures, through the use of extractants of increasing strength, estimate the distribution of metals among the exchangeable, carbonate, oxide, organic, and residual fractions. The procedure is essentially operational and may give rise to difficulties if this is not kept in mind (14, 15). It is dangerous to assume that the nominal forms extracted from soil samples always represent the real situation. Thus, metals supposedly associated with the carbonate fraction may be extracted with buffered acetate solution, although soil pH condition may render it highly unlikely that carbonate actually exists in the soil being extracted. In these circumstances, it must be assumed that the extracted metals are derived from non-carbonate sources that are impossible to specify. Nevertheless, despite the dangers of uncritical acceptance of results from sequential extraction analysis, the procedure is still widely used because it is a useful first approch in assessing the likehood of mobilization and biological uptake of metals from the soil. Worldwide, contaminated sites have been found that require remedial action. Such action may involve excavation and removal of the contaminated soil or in-situ stabilization of metals, rendering them immobile and minimizing the possibility of groundwater contamination or plant uptake. One way of stabilizing metals in soil is through the addition of ameliorants such as lime, phosphates, zeolites, apatites, glauconite, iron oxide material, compost, etc. Liming, presumably the most widely known ameliorant in agriculture, decreases the mobility of metals and their uptake by plants caused by the metal hydrolysis reactions and/or coprecipitation with carbonate (16).
S0013-936X(96)00072-7 CCC: $12.00
1996 American Chemical Society
Zeolites are crystalline, hydrated aluminosilicates of alkali and alkaline earth cations that possess infinite, threedimensional crystal structures. Nearly 50 natural species of zeolites have been recognized, and more than 100 species have been synthesized in the laboratory (17). Zeolites are widely used in industry (drying of gases and liquids, gas purification systems, oxygen enrichment of air, deactivation of radioactive effluents, filtration of drinking water, and purification of effluents, extraction of metals from complex solutions and industrial wastes, paper and rubber industries, etc.) and agriculture (animal husbandry, aviculture, plant growth, fish breeding, and environmental protection). The use of zeolites for pollution control depends primarily on their ion-exchange capabilities. Clinoptilolite, chabazite, and phillipsite are natural zeolites that have been evaluated as ameliorants for environmental cleanup (17). Studies have been conducted to determine the effect of zeolite on metal uptake by plants (18-21). Mineyev et al. (18) found zeolite application into soil reduced the Zn contents of barley tissues and grain as well as the Pb and Cd contents of strawberries and cherries. Gworek (19, 20) amending polluted soils with synthetic zeolites found significant reductions in the Cd and Pb contents of several pot-grown crops. Similarly, Rebedea and Lepp (21) reported that synthetic zeolites added to a lead/zinc mine spoil and a soil polluted by Cu and Cd reduced plant uptake of metals in pot studies. Hydroxyapatite [Ca5(PO4)3OH] minerals are found in soils, sediments, and suspended particulate matter in aquatic systems. Phosphate minerals are capable of immobilizing Pb in soils and wastes due to the low solubility of lead orthophosphate complexes (22-24). Zn2+ and Cd2+ could coprecipitate with Ca2+ on hydroxyapatite surfaces (25-27). The capacity of hydroxyapatite to stabilize metals, especially Pb, in soil indicates its potential as a remedial agent, but its role in soil cleanup needs further exploration. Other soil constituents, such as the hydrous oxides of Al, Fe, and Mn are also known to sorb metals in soils (28, 29). Zinc can be adsorbed onto a matrix containing clay and aluminum hydroxide polymers on clay surfaces (30). The enhanced sorption of metals by hydrous oxides are known to have facilitated metal binding through specific adsorption (29). Mench et al. (31) used hydrous iron oxide as an ameliorant to stabilize Cd and Pb in polluted soil and observed substantial decrement in Cd and Pb contents in ryegrass. Their study indicates hydrous iron oxide (i.e., amorphous iron oxide [Fe2O3 H2O(am)] or ferrihydrite) as a potential ameliorant as it effectively stabilized metals in soil. This material can be synthesized and produced commercially and is a common constituent in natural soils (29, 31). With soil quality playing an important role in food production and quality, especially in certain areas, such as Eastern Europe and the Newly Independent States, it is essential to employ efficient and cost-effective “stabilization” techniques for metals in soil. To this end, a greenhouse study was conducted to evaluate the stabilization potential for metals of several widely available soil ameliorants. The objectives were as follows: (i) to compare the efficacy of selected ameliorants as a metal stabilizer in contaminated soil; (ii) to determine the influence of ameliorants on the chemical form and bioavailability of Zn in soil; and (iii) to determine the effect of crop species on Zn phytotoxicity.
Materials and Methods Greenhouse Pot Experiment. To mimic treating a cropped contaminated soil, pots containing 7 kg of surface soil from an Appling silt loam (Typic Paleudult, fine, acid, thermic, Athens, GA) were used. The soil possessed the following properties: pH (H2O) 5.4; particles
