Environ. Sci. Technol. 2004, 38, 156-162
Understanding the Effects of Soil Characteristics on Phytotoxicity and Bioavailability of Nickel Using Speciation Models LI PING WENG,* ANKE WOLTHOORN, THEO M. LEXMOND, ERWIN J. M. TEMMINGHOFF, AND WILLEM H. VAN RIEMSDIJK Sub-department of Soil Quality, Department of Environmental Science, Wageningen University, P.O. Box 8005, 6700 EC Wageningen, The Netherlands
Acidity (pH) has been realized to be the most important soil characteristic that modulates bioavailability of heavy metals by affecting both the chemical speciation of metals in soil and the metal binding to the active sites on biota. In this work, we show that besides soil pH, metal bioavailability also depends to a certain extent on the type of soil. A better understanding of the role of soil type in regulating metal availability can be achieved with the analysis of soil composition and with calculations using chemical speciation models. Results of pot experiments, in which three different soils were spiked with nickel, show that the EC50 of total nickel in decreasing the biomass production of oats varies widely (0.7-22.5 mmol kg-1 soil, more than 30 times). pH (4.7-7.0) is the most important factor, explaining up to a factor of 14 difference of nickel bioavailability in the soils. The remaining variation is caused by other differences in soil composition (soil type). The bioavailability and toxicity of nickel in the organic matter-rich soil studied is less than half of that in the sandy and clay soil studied at a similar pH. The chemical calculations using a multi-surface speciation model show that soil organic matter binds Ni much stronger than clay silicates and iron (hydr)oxides within the acidic pH range, which supports the experimental findings. In all three soils, the EC50 of Ni expressed in terms of Ni in 0.01 M CaCl2 soil extraction is rather stable (24-58 µM), suggesting the possibility to use this extraction as an estimation of metal availability in soil.
Introduction Soil pollution by heavy metals is a potential threat to the environment. It can be harmful to plants and other organisms, and it can lead to contamination of groundwater and surface water. When a certain limit is exceeded, remediation measures have to be taken to remove the pollutants or to reduce the risk. Common techniques of soil remediation include excavation, extraction, phytoremediation, and immobilization. The remediation of soil is mostly destructive and expensive, especially when dealing with extensive areas (1). Therefore, the decision-making needs to be based on a more accurate assessment of the risk that the specific metal * Corresponding author phone: +31-317-483327; fax: +31-317483766; e-mail: liping
[email protected]. 156
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 38, NO. 1, 2004
posts on the environment rather than using total metal concentration as a criteria. The bioavailability and toxicity of heavy metals in soil are modulated by both biotic factors (species, uptake pathway, etc.) and soil characteristics (pH, soil type, etc.). On the biotic side, metal interactions with organisms can be understood by considering an organism as an assemblage of reactive ligands (2-4). Free metal ion is the key chemical species that determines metal uptake (2, 5, 6). Apart from the free metal ion activity, other solution parameters such as pH and Ca concentration will also affect metal bioavailability (3, 5, 7, 8). Cations such as proton and calcium may compete with a metal ion for the active sites on organisms; Therefore at a fixed free metal ion activity, the bioavailability and toxicity of a metal will be reduced by a decrease in pH and an increase in Ca concentration in soil solution. On the soil side, the activity of the key chemical species, the free metal ion, depends on the distribution of the metal over its various chemical forms (chemical speciation). Metal in a soil can be in the form of precipitated, adsorbed, complexed, dissolved, or free ion in soil solution. The metal loading (total metal concentration) and soil characteristics determine to a large extent the chemical speciation or distribution of the metal. Soil pH is connected closely to the chemical processes of precipitation, sorption, and complexation. Researches showed that soil pH is the most important soil characteristics affecting bioavailability of heavy metals (e.g., ref 7). Contribution of the other soil characteristics in determining metal speciation and bioavailablity is not as well-established as that for pH due to experimental and statistical difficulties (7, 9, 10). In a separate paper (11), we illustrated the pH effect on nickel bioaccumulation in plant and on nickel binding to soil. Both plant uptake and nickel speciation are pHdependent. Using a quantitative approach (3, 12), we showed that the magnitude of pH effect on Ni speciation was bigger than that on Ni uptake. Therefore at an increased pH level, the bioavailability and toxicity of Ni are intensified when the plant is grown in nutrient solution, whereas the opposite effect is observed when the plant is grown in soil. When based on the free Ni2+ concentration in soil solution, the effect of pH in the nutrient solution experiment and in the pot experiment is in the same direction, which supports the idea that the free metal ion is the key chemical species that determines bioavailability of a metal. In the above-mentioned paper (11), one sandy soil was used. The effects of soil composition on the bioavailablity and toxicity of Ni were not considered. In the current paper, attention is paid to the role of soil type and soil composition in regulating the bioavailability and toxicity of Ni. These results will be helpful to decide if soil type (composition) should be included in the standards for risk assessments of heavy metals. We will also demonstrate that, with soil composition analysis and chemical speciation calculation using a multi-surface model (10, 13), the differences in metal bioavailability of various soil types can be better understood.
Materials and Methods Soil Pot Experiments. Three different soils were investigated: a sandy soil (SS), a sandy soil with a relatively high content of organic matter and iron (hydr)oxides (OMFe), and a clay soil (CS). Soil samples were taken from various experimental fields in The Netherlands. The field soil samples 10.1021/es030053r CCC: $27.50
2004 American Chemical Society Published on Web 11/21/2003
TABLE 1. Soil Characteristics
soil
organic mater (%) (loss on ignition)
SS OMFe CS
4 8 4
clay Fe total Ni total field moist (%) (%) iron (mmol kg-1) (aqua pH capacity (
