Environ. Sci. Technol. 2001, 35, 908-916
Thermally Induced Changes in Metal Solubility of Contaminated Soils Is Linked to Mineral Recrystallization and Organic Matter Transformations C A R M E N E N I D M A R T IÄ N E Z , * ASTRID JACOBSON, AND MURRAY B. MCBRIDE Department of Crop and Soil Sciences, Cornell University, Ithaca, New York 14853
Soils are biogeochemical systems under continual modification by biological and chemical processes. Trace element solid-solution partitioning is thus influenced by longterm changes to these solid phases. We study Pb, Cd, Zn, and Cu solution speciation and solid-phase dynamics in two soils of volcanic origin (Te Akatea and Egmont, high in noncrystalline aluminosilicates), an oxisol from Brazil (Oxisol, high in oxides of Al and Fe), and several sludgetreated soils (labeled NYS soils, high in organic materials). Total soluble (by ICP) and labile (by ASV) concentrations of Pb, Cd, Zn, and Cu were determined after incubation of the soils for about 1.5 yr at room (23 °C) and elevated (70 °C) temperatures. Changes occurring to the solid phases were monitored by FTIR and extraction with oxalate and pyrophosphate. It is shown that induced hydrolysis or decomposition of organic materials in soils results in increases in both labile and total soluble concentrations of Pb, Cd, Cu, and Zn in solution. Labile and total soluble concentrations of Cu and Zn increase concomitantly with dissolved organic carbon (DOC); the nonlabile soluble fraction also increases with increasing DOC. Similarly, the concentration of Cd and Pb in solution increases with increasing DOC; however, most soluble Cd and Pb is asv-labile. Only in the Egmont soil (mineralogy dominated by proto-imogolite allophane) was reduced Pb solubility observed after prolonged equilibration and heating. Lead solubility increased after partial crystallization of amorphous minerals in the Te Akatea and the Oxisol. Thus, for most of the metal-soil systems studied, prolonged thermal treatment at 70 °C increased total soluble and asv-labile metals, suggesting that aging effects on metals in contaminated soils could release metals to labile forms in some cases.
Introduction Trace elements are retained by both organic and inorganic (mineral) solid phases present in soils. Yet mineral transformations and the hydrolysis and/or decomposition of organic materials are potential long-term processes controlling the release of trace metals from soil solids to the solution phase. These processes influence the amount of metal available to biological systems. Potentially, changes to the mineral and organic components of soils could result in an * Corresponding author phone: (607)255-1728; fax: (607)255-8615; e-mail:
[email protected]. 908
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 35, NO. 5, 2001
aging effect that either increased or decreased metals in solution over time. Studies of pure mineral systems or soils usually display a decreasing solubility of trace metals with time after addition. However, recent evidence has demonstrated that aging does not necessarily decrease metal lability or solubility for Pb in model mineral systems by both Pb exchange onto resin beads and by Pb ejection from a noncrystalline solid after its transformation (1-3). Among the mineral constituents that contribute to metal retention in soils are aluminum, iron, and manganese oxides (4-6), phyllosilicates (7, 8), carbonates (9, 10), and phosphates (11). Oxide minerals are common components of the soil environment. In the highly weathered soils of the tropics, for example, they constitute a high proportion of the solid phases and provide important reactive surfaces for trace elements (12). Trace elements may be highly concentrated in the high surface area, noncrystalline fractions of iron and aluminum hydroxides (13). Noncrystalline aluminosilicates (allophane, proto-imogolite allophane, and imogolite) are important reactive components in soils of volcanic origin (14). Allophane and imogolite are weathering products of feldspars (primary mineral) and can form gibbsite (Si depletion), kaolinite, and halloysite upon subsequent weathering. Despite their common structural unit [HOSiO3Al2(OH)3], however, allophane and imogolite differ in charge and surface properties (16). Allophane is an amorphous material characterized by the predominance of Si-O-Al bonds and a variable structure that appears to consist of discrete hollow spherules or polyhedra 3.5-5.0 nm in diameter. Imogolite is a mineral related to allophane but having a higher degree of structural order. It consists of a tubular aluminum silicate polymer structure approximately 2 nm in diameter with a repeat distance of 0.84 nm along the tube axis. An important characteristic of imogolite is that its inner surface contains isolated Si4+ atoms with the fourth coordination position occupied by an OH- directed inward toward the center of the tube. These hydroxyl groups have been implicated in the ability of imogolite to trap ions inside its tubular structure (15). By selecting soils with widely varying mineralogies, including soils rich in allophanic and oxide minerals, we intended to discern the effect of these minerals on metal (Pb) solubility under experimental conditions of elevated temperature that would accelerate mineral dissolution/ reprecipitation or crystallization. Soil organic matter has been increasingly recognized as a critical component in heavy metal retention in soils (1719). Soil organic materials include living organisms, biomolecules, and humic substances (18). At low pH (pH 4-6), organic matter is relatively more effective than minerals at sorbing heavy metals. At high pH (pH 7-8), the effectiveness of organic matter as a sorbent is reduced. High pH favors organic matter dissolution and an increase in heavy metal solubility (20). Soil microbial biomass also provides adsorption surfaces for metals with reportedly higher retention capacities than some minerals or humified organic materials (21-23). Solid organic matter is however subject to chemical and biological modification, such as oxidation or depolymerization, resulting in the release of dissolved organic matter (DOM) and a potential increase in metal mobility (24, 25). Although low molecular weight organic acids are produced continuously in soils through microbial activity, they are sensitive to microbial degradation and are therefore shortlived in soil solution (26). Since organic soil constituents may have a transient but important effect on heavy metal retention, we determine metal solubility and solution speciation in soils rich in organic materials under conditions 10.1021/es001647m CCC: $20.00
2001 American Chemical Society Published on Web 02/02/2001
TABLE 1. Soil Characteristics: Origin, Mineralogy, Soil pH,a Organic Carbon Contentb (g of C (kg of soil)-1), and Total Elemental Analysesc (mg kg-1) Origin minerals pH org C Pb Cd Cu Zn Ni Fe Ald Sid S P Mn Mo
Te Akatea
Egmont
Oxisol
Caldwell
Master Old Site
Cornell 23
Cornell 24
N. Zealand allophane 4.20 7.22 5.76 0.06 4.88 9.80 0.74 29 000e 196 000e 154 000e 1 718 199 60.9 2.82
N. Zealand imogolite 4.86 20.4 3.52 0.13 23.5 35.5 1.42 33 000e 176 000e 87 000e 472 419 183 1.18
Brazil oxides 3.81 23.1 0.42 0.51 7.99 11.9 6.89 63 750 48 230 442 73.5 202 20.9 2.69
NYS illite/chlorite 5.17 25.3 12.9 0.37 13.1 61.5 20.9 21 550 15 290 193 241 613 286 0.91
NYS illite/chlorite 6.50 123 122 39.3 415 1 478 100 18 873 8 616
NYS illite/chlorite 6.19 47.7 132 21.8 219 942 69.4 26 066 15 979 235 837 3 156 552 1-3
NYS illite/chlorite 5.95 44.3 263 24.9 228 1 127 71.4 21 780 15 786 271 803 3 332 716 1-3
923 6 228 798 1-3
a Soil pH was measured in 2:1 KNO :soil suspensions. b The organic carbon content was measured using a combustion method or a TOC 3 analyzer (OI Analytical, solid module). c Total elemental analyses were performed by inductively coupled plasma emission spectrometry (ICP, Thermo Jarrell Ash IRIS ICP-OES) of HNO3/HClO4 digests. d Only a fraction of the total Al and Si in soils is dissolved by HNO3/HClO4 digestion. e Data from ref 16. Fe, Al, and Si analyses of HF digests for Te Akatea and Egmont soils.
favorable to decomposition to test if binding to organic matter provides long-term protection. Studies on metal solid-phase dynamics have shown that long-term reaction of heavy metal coprecipitates with ferrihydrite (1, 27, 28) or noncrystalline alumina (29) results in reduced Cd, Zn, and Cu solubility after transformation to more crystalline materials. In contrast, increases in soluble Pb have been observed in thermally treated lead-ferrihydrite coprecipitates (1) and Pb adsorbed onto ferrihydrite and pedogenic iron oxide-containing materials (3). An increase in the crystallinity of the iron oxides in the pedogenic (heterogeneous) materials did not however account for the total release of Pb into solution. We hypothesized that alterations to soil constituents other than iron oxides, such as organic materials or noncrystalline aluminosilicates, could contribute to the release of Pb into solution. The present study tests this hypothesis by measuring the total soluble and labile concentrations of Pb after long-term equilibration at room temperature (23 °C) and at 70 °C in soils containing noncrystalline aluminosilicates (volcanic soils), oxides of Al and Fe (tropical soil), or organic materials (sludge-treated soils) as their predominant reactive surfaces. We relate changes that occur to mineral and organic soil constituents to metal solubility behavior. The sludge-treated soils have elevated concentrations of Cd, Zn, and Cu as well as Pb. Thus, the long-term solubility (total soluble and labile) of Cd, Zn, and Cu is also studied in these soils. Thermal treatment is used to simulate spontaneous long-term transformations of the solid phases.
Experimental Section Soils. The soils were selected to contain specific mineralogical characteristics (noncrystalline aluminosilicates and oxides of Al and Fe) and a range of organic matter contents. Two volcanic ash soils from New Zealand were selected because of their high allophanic and varied imogolite content. One of these soils, Te Akatea, was collected from an allophanic bed in the Hamilton Ash Formation. Its mineralogy is dominated by allophane (noncrystalline aluminosilicate) but contains some imogolite (30). The second volcanic ash soil, Egmont, formed from Stratford Pumice parent material and was collected at Mount Egmont. The Egmont soil contains proto-imogolite allophane (30). A variable charge soil from the Cerrado Region, Brazil (Typic Acrustox, USA soil taxonomy; Dark Red Latosol, Brazilian classification scheme)
was selected to represent a soil rich in aluminum and iron oxides. This clay textured (59.4%) soil is labeled Oxisol. The whole soil (
