Environ. Sci. Technol. 2000, 34, 4386-4391
Copper Phytotoxicity in a Contaminated Soil: Remediation Tests with Adsorptive Materials MURRAY B. MCBRIDE AND C A R M E N E N I D M A R T IÄ N E Z * Department of Crop and Soil Sciences, Cornell University, Ithaca, New York 14853
A long-contaminated mineral soil containing in excess of 3000 mg kg-1 total Cu was amended with several materials having metal-adsorptive properties, including humus and iron, aluminum, and manganese oxide/hydroxide, in an attempt to reduce its phytotoxicity. The total soluble (measured by ICP) and labile (measured by ASV) concentrations of Cu were reduced by amendment with 5% (by weight) noncrystalline alumina and 10% (by weight) ferrihydrite, although humus and crystalline (low surface area) iron oxide were ineffective in reducing soluble or labile Cu. None of the soil amendments was able to correct the phytotoxicity indicated by bioassays with maize seedlings. Although manganese oxide (birnessite) lowered the free Cu2+ activity in the soil, phytotoxicity persisted due to an increase in dissolved organic carbon (DOC) and both soluble and labile Cu. Toxicity of Cu to maize shoots appeared to be dependent on both labile Cu as well as free ionic Cu2+. The high surface area (noncrystalline) iron and aluminum hydroxides reduced the solubility of Mo, As, Cd, Pb, and Cr as well as Cu attributable in part to their effectiveness in removing DOC from solution. Other metal solubilities, notably those of Ni and Zn, were not decreased by these hydroxide amendments. In contrast, the manganese oxide, by increasing DOC in the soil, raised the solubility of Pb, Cd, As, and several other metals. Thus, while reactive (high specific surface area) oxides did in the short-term reduce soluble concentrations of specific metals in this contaminated soil, the effects were metal-specific and sensitive to any pH change caused by the amendment.
Introduction Numerous materials have been tested as soil amendments to reduce solubility and bioavailability of toxic metals in contaminated soils. These materials potentially reduce toxicity to plants or animals by raising soil pH, chemisorbing or precipitating the metals of concern, adding large excesses of competing metals, or some combination of these. For example, carbonate-rich materials (e.g., ground limestone) have long been used agronomically to reduce toxicity of Al in acid soils and have shown some success, albeit inconsistent, in reducing the solubility and crop uptake of Cd and Zn (1-7). Phosphate minerals such as hydroxyapatite (8, 9), oxides and hydroxides of iron, aluminum, and manganese (10-12), steel shot, and alkaline mineral products (e.g., * Corresponding author e-mail:
[email protected]; phone: (607)255-1730; fax: (607)255-8615. 4386
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beringite) (11, 13, 14) have shown ability to decrease solubility of metals such as Cd, Zn, Pb, and Ni in contaminated soils. Organic matter addition to soils has also effectively reduced soil phytotoxicity and heavy metal uptake by plants (15-17). Reduced plant uptake of metals of concern is not consistently observed with soil amendments, however, with some amendments being ineffective for particular metals or very high contamination levels (7, 11, 13). A problem with remediation technology of stabilizing heavy metals in soils is that the actual soil reactions leading to reduced metal activity or bioavailability are not well-known and therefore unpredictable. Because numerous changes in soil chemistry result from these amendments, undesirable “side effects” can arise. These include increased solubility of accessory toxic elements in the soil (e.g., As in the case of hydroxyapatite addition to an industrially contaminated soil) and decreased solubility of essential trace elements, leading to poorer plant growth (18). Increased soil pH resulting from alkaline amendments, while reducing free metal ion activity of most heavy metals, increases total dissolved organic matter (DOM) and heavy metals and therefore enhances leachability of certain metals (19, 20). Even where soil treatments initially reduce metal solubilities, it is uncertain whether this condition can be sustained. The added material may “draw down” the solubility of a number of metal cations and anions immediately, but this adsorptive “sink” could become saturated. Severely contaminated soils have large reserves of the metal of concern in bound but potentially desorbable form, allowing for a “rebound” effect in which the soil solids eventually release sufficient metals or DOM to saturate the adsorptive capacity of the added material. Recent studies indicate that, contrary to the expected behavior of chemisorbed metals, when a large sink for metal adsorption is provided and the metal activity is maintained at a low level by removal from solution, there is a relatively rapid and near-complete release of “strongly bound” metals from minerals or soils and transfer to the sink (21-24). Therefore, release of strongly bound metals such as Pb from surfaces does not require extraction with aggressive chemicals, even though desorption is much slower than adsorption. In a contaminated soil, 96 h of extraction with cation-exchange resin removed Cd and Pb to the greatest degree (>50%); Cu, Zn, and Ni to a smaller degree (15-30%); and Cr the least (3%) (22). This observation illustrates the metal-specific nature of sorption and “aging” processes in soils, where Cd in particular remains highly accessible to reaction after long periods of time. In fact, isotope exchange experiments reveal that only a small fraction if any of the total Cd in soils is “fixed” into nonexchangeable or inaccessible forms by aging over the long term (25, 26). However, a small aging effect in reducing labile Cd from 72% to 60% of the total Cd in soils moderately contaminated by long-term P fertilization was measured by Hamon et al. (27). These observations support a dynamic view of metal release in soils, with metal removal from solution using soil additives or plant roots expected to initiate release from the strongly adsorbed forms. It was decided to test this concept of dynamic response using a soil severely contaminated with Cu to determine whether amendment with adsorptive materials, such as organic matter, manganese and iron oxide, and noncrystalline (high surface area) iron and aluminum hydroxides, could effectively reduce Cu solubility, lability, Cu2+ ion activity, or phytotoxicity. 10.1021/es0009931 CCC: $19.00
2000 American Chemical Society Published on Web 09/09/2000
TABLE 1. Selected Total Metal and Nutrient Concentrations in the Cu-Contaminated Soil element
concn (mg kg-1 )
Cu Pb Zn Cd As
3430 522 203 3.0 136
element
concn (mg kg-1 )
Ni Cr S P
30.6 21.7 508 2200
Experimental Section Soil Amendments. The near-neutral mineral soil (Hapludalf loam) used for this remediation study was collected from a cultivated field near Hamilton, NY. This soil is severely contaminated with Cu (probably as a result of a spillage during use of CuSO4 as a fungicide on potato crops) and to a smaller degree with Pb, As, and Cd; the contamination having occurred in the field at least 60 years earlier (28). The total metal analysis, by nitric/perchloric acid digestion and ICP (inductively coupled plasma emission spectrophotometry) analysis of the digest, are presented in Table 1. A homogenized sample of this soil was weighed out in 6 sets of duplicate 100 g (air-dry) quantities, 0.25 g of a commercial fertilizer (N-P2O5-K of 12-16-16) added to the soil, and either 6 g (dry wt) of organic (peat) soil (OM), 10 g of Fe2O3 (FE, surface area ) 9.2 m2/g), 10 g of manganese oxide (MN, K-saturated birnessite, surface area ) 28 m2/g), 5 g of noncrystalline alumina (ALUM, surface area ) 111 m2/g), and 10 g of noncrystalline iron hydroxide (FERRI, surface area ) 170 m2/g) to test the effectiveness of these materials in limiting Cu toxicity. The high surface area hydroxides of Al (ALUM) and Fe (FERRI) were prepared as described by McBride (29) and Martı´nez et al. (30), respectively. These soil mixtures were blended thoroughly with a large mortar and pestle, placed in small Styrofoam pots, wetted up to field capacity with distilled water, and allowed to equilibrate for 1 week. A noncontaminated soil (Caldwell) containing 13 mg of Cu (kg of soil)-1 was collected in Ithaca, NY, and used as a reference soil. Maize Bioassay and Solution Speciation. Four maize seeds (Zea mays, Dekalb 560GR) were planted in the prepared pots and later thinned to 3 plants per pot. The corn shoots from each soil treatment [no amendment (NO), peat soil (OM), iron oxide (FE), manganese oxide (MN), noncrystalline alumina (ALUM), and ferrihydrite (FERRI)] were harvested after 4 weeks growth under artificial lighting and weighed. The plant samples were then oven-dried, ground, digested in nitric/perchloric acid, and analyzed for trace elements by ICP spectrophotometry. At the time of harvest, duplicate 15 g (field-moist) soil samples were taken from each treatment, mixed with 30 mL of 0.01 M CaCl2, and shaken for 30 min. The pH and Cu2+ activity of these suspensions were measured with a glass electrode and Cu ion-selective electrode (ISE), respectively, as described by Sauve´ et al. (31). The suspensions were then filtered through 0.2-µm cellulosic filters. A portion of each of these solutions was analyzed for labile Cu by differential pulse anodic stripping voltammetry (ASV) as described by Martı´nez and McBride (32). Labile metals represents the fraction of total soluble metals that includes the free hydrated metal ions and metal complexes that dissociate very rapidly to yield the free metal. It is often assumed that the labile metal represents the metal fraction that is bioavailable and therefore similar to the toxic fraction of the soluble metal. The remainder of each solution was stabilized with a drop of ultrapure concentrated HNO3 and analyzed for total soluble Cu and other trace metals by ICP. Dissolved organic carbon (DOC) was measured by persulfate
oxidation/CO2 analysis (OI Analytical model 1010 total carbon analyzer).
Results Soil Amendment Effects on Total Soluble Metals. Addition of the amendments to the Cu-contaminated soil had effects in changing the solubility of several elements (Table 2). Regarding the macronutrients, soluble concentrations of K, P, Ca, Mg, and S were affected by the manganese oxide (MN) as well as by the high-surface area hydroxide (FERRI, ALUM) treatments. The MN treatment increased soluble K and P markedly and reduced Ca, Mg, and S. The effect of MN on the cations is explained by the high exchangeable K in the form of manganese oxide used, K-saturated birnessite, which presumably adsorbed Ca and Mg from the soil solution by cation exchange. The increase in soluble P may be related to the higher pH in the soil due to the alkaline reaction of K-birnessite with the soil, which increased dissolved organic carbon and presumably soluble organic P (see pH and DOC data in Table 2). However, calcium phosphates may control P solubility in this near-neutral soil, and the reduction of soluble Ca by K+-Ca2+ exchange would be expected to release phosphate into solution. The decrease in soluble S from the MN treatment is not consistent with an explanation based on dissolved organic sulfur and suggests that the manganese oxide may have adsorbed sulfate or possibly promoted the oxidation of organosulfur compounds to sulfate. The FERRI and ALUM treatments markedly lowered DOC and soluble P and S (Table 2), suggesting that the oxide surfaces effectively removed both inorganic anions (sulfate, phosphate) and organics from solution. The soil amendments had large effects on numerous trace elements (Table 2). Generally, the MN and OM treatments increased the solution concentrations of Al, Fe, Mo, As, Cr, Cd, Ni, and Pb. For most of these elements, the effect can be attributed to higher DOC caused by the added organic matter (OM treatment) or the higher pH (MN treatment). Both Mo and As became more soluble with the MN treatment, the effect being most pronounced for As. This could be a pH effect, as arsenate and molybdate adsorb less strongly at higher pH. Conversely, the FE treatment lowered Mo and As solubility, attributable to the ability of iron oxides to adsorb molybdate and arsenate (33). Although Zn solubility was not substantially affected by the soil amendments, Mn solubility was reduced by all treatments and, interestingly, was the lowest in the MN treatment. The higher pH and presence of manganese oxide may have favored Mn(II) adsorption and oxidation, as manganese oxide adsorbs Mn(II) quite strongly. Copper solubility was increased substantially by the MN treatment (Table 2), despite the ability of manganese oxides to selectively chemisorb Cu (34). The high-surface area oxide soil treatments (FERRI, ALUM) reduced the solubility of Cu and several other elements, including As, Mo, Cd, and Pb, but not Zn or Ni (Table 2). Thus, anionic as well as cationic trace elements were removed from solution by these reactive oxides. In contrast, the low surface area iron oxide (FE) had little effect on the solubility of Cu and other trace metals. Soil Amendment Effects on Cu Solution Speciation. A detailed speciation of soluble Cu in this soil, which has been demonstrated to be phytotoxic (28), was undertaken by determining total soluble Cu (ICP emission spectrometry), labile Cu (differential pulse anodic stripping voltammetry, ASV), and free Cu2+ ion (ISE, ion selective electrode) concentrations. The result of this speciation (conducted at the time the maize was harvested) is summarized in Figure 1 where concentration units of micromolar (µM) are used to allow direct comparison of free cationic Cu2+, ASV-labile Cu, and total soluble Cu. The speciation data reveal that >99.9% of the total soluble Cu in soil solution is in complexed form and that labile Cu VOL. 34, NO. 20, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Suspension pH and Concentrations (mg L-1) of Dissolved Organic Carbon (DOC) and of Elements in 0.01 M CaCl2 Extracts of the Unamended (NO), Iron Oxide (FE), Manganese Oxide (MN), Peat Soil (OM), Ferrihydrite (FERRI) and Alumina (ALUM)-Amended Cu-Contaminated Soilsa measurement
NO
FE
MN
OM
FERRI
ALUM
pH DOC K P Ca Mg S Al Fe Mn Cu Zn Mo As Cd Pb Ni Cr
6.8 (0.07) 16 (5) 58 (21) 4.9 (1.7) 380 (28) 18 (0.3) 120 (7) 0.36 (0.13) 0.055 (0.032) 0.97 (0.14) 0.99 (0.18) 0.031 (0.001) 0.034 (0.009) 0.16 (0.01) 0.009 (0.003) 0.078 (0.001) 0.039 (0.003) 0.060 (0.001)
6.8 (0.02) 26 (0.5) 53 (1) 5.0 (0.5) 420 (10) 18 (1.0) 128 (2) 0.44 (0.05) 0.098 (0.081) 0.14 (0.3) 0.91 (0.01) 0.028 (0.006) 0.018 (0.000) 0.13 (0.00) 0.008 (0.001) 0.072 (0.006) 0.040 (0.003) 0.062 (0.003)
7.4 (0.09) 120 (6) 830 (43) 23 (0.7) 130 (2) 5.5 (0.1) 51.0 (1.2) 0.78 (0.01) 0.21 (0.01) 0.021 (0.000) 3.0 (0.12) 0.044 (0.014) 0.051 (0.002) 1.36 (0.07) 0.021 (0.001) 0.200 (0.005) 0.077 (0.003) 0.11 (0.00)
6.4 (0.02) 25 (0.1) 47 (2) 7.3 (0.1) 410 (3) 26 (0.3) 130 (0.1) 1.1 (0.02) 0.12 (0.01) 0.10 (0.00) 0.81 (0.00) 0.042 (0.002) 0.052 (0.001) 0.33 (0.00) 0.027 (0.001) 0.230 (0.003) 0.096 (0.001) 0.14 (0.00)
7.4 (0.01) 6.2 (0.8) 180 (15) 0.11 (0.01) 220 (2) 16 (0.6) 4.0 (0.6) 0.23 (0.03) 0.012 (0.001) 0.33 (0.15) 0.076 (0.02) 0.072 (0.04)
