Environ. Sci. Technol. 2002, 36, 1699-1704
Transport of Humic and Fulvic Acids in Relation to Metal Mobility in a Copper-Contaminated Acid Sandy Soil LIPING WENG, ELLEN P. M. J. FEST, JEROEN FILLIUS, ERWIN J. M. TEMMINGHOFF,* AND WILLEM H. VAN RIEMSDIJK Wageningen University, Department of Environmental Sciences, Subdepartment of Soil Quality, P.O. Box 8005, 6700 EC Wageningen, The Netherlands
The transport of inorganic and organic pollutants in water and soil can be strongly influenced by the mobility of natural dissolved organic matter (DOM). In this paper, the transport of a humic acid (HA) and a fulvic acid (FA) in a copper-contaminated acid sandy soil was studied. The data showed that the transport behavior of HA differed from that of FA. The breakthrough curves (BTCs) of HA were characterized by a rapid relatively sharp front followed by a plateau at a lower HA concentration than in the influent solution. The increase of the Ca concentration decreased the HA concentration further. Compared to HA, the BTCs of FA were retarded and showed an extended tailing, approaching complete breakthrough. The increase of the Ca concentration decreased the FA concentration only temporarily. On the basis of our model calculation, the characterization of HA transport could be explained by the coagulation of HA largely upon the binding of Al. The increase of the Ca concentration resulted in further coagulation of HA because of the increased Ca adsorption, which occurred mainly in the Donnan phase. For FA, the adsorption to the soil matrix was more likely the process that controls its solubility and mobility. The mobility of Al and Cu in the soil column was closely related to the solubility and transport of the DOM in soil solution. The concentration of Ca in the effluent was lower than in the influent because Ca was retained in the soil due to the retardation of HA and FA and due to the compensation of the other cations released from the soil to the solution.
Introduction The partitioning and mobility of organic matter play an important role in the transport of contaminants (inorganic and organic) in the natural environment. Organic matter can enhance (when dissolved) or retard (when in the solid matrix) the transport of the contaminants through the soil (1-6). Risks of enhanced transport of contaminants occur when the concentration of dissolved organic matter (DOM) in soil increases. The mobility of DOM in soil is governed by both the chemistry of the bulk solution and the composition of the soil solid phase, where the former determines DOM dissolution properties and the latter controls the extent of DOM adsorption (7). * Corresponding author phone: +31 317 48 2357; fax: +31 317 48 3766; e-mail:
[email protected]. 10.1021/es010283a CCC: $22.00 Published on Web 03/08/2002
2002 American Chemical Society
DOM is a complex mixture of many molecules and is usually operationally defined as the organic matter that passes a 0.45-µm filter. A large portion of the DOM in the soil solution is present as humic acid (HA) and fulvic acid (FA) (8). The other organic compounds include macromolecular hydrophilic acids and identifiable organic compounds such as carbohydrates, carboxylic acids, and amino acids (9). Despite the complex nature of DOM, it is often treated as a single component in the studies of its adsorption and transport on various soil, clay, and oxides surfaces; relatively few studies have explicitly considered DOM as a complex mixture, with adsorption behavior varying from one subcomponent to another (10). In the breakthrough curves (BTCs) of the natural organic matter, shoulder formation is a common phenomenon (7), which may be due to a fractionation of DOM into subcomponents according to differences in reactivity during the transport. In the study of the transport of the DOM in columns containing aquifer sediments, Dunnivant et al. (11) found that the hydrophobic constituents of DOM were preferentially adsorbed, while hydrophilic components were rapidly transported. Reduction of DOM concentration, in the presence of many different surfaces including iron and aluminum hydroxides (12-16), clay minerals (17, 18), and soils (19, 20) have been reported. The immobilization of DOM by soils, especially mineral soils, has been mostly ascribed to its adsorption to soil surfaces. Dunnivant et al. (11) speculated that, because the hydrophobic DOM was preferentially adsorbed in the column experiment, the multiprocess adsorption of DOM via direct association with the solid phase and the hydrophobic binding processes between DOM components on the solid and DOM components in solution were important in controlling DOM transport. Despite intensive research in the past decade, our knowledge of the formation and fate of DOM in soils and its response to changing environmental conditions is still fragmented and often inconsistent (21). In this paper, we studied the transport behavior of HA and FA in an acid sandy soil and their response to the changes in the Ca concentration in the bulk solution. It is not our intention to simulate the transport process with transport modeling. Instead, our focus is to understand the major factors that are involved in the HA and FA transport with the help of the BTCs and speciation model calculations. With this study, we aim to get more insight into the various mechanisms that control the solubility and mobility of different DOM subcomponents.
Materials and Methods Humic and Fulvic Acids. The materials of HA and FA used here were purified from the B horizon of a forest soil (Tongbersven in Oisterwijk, The Netherlands) and the Bs horizon of a peat Podzol (Strichen Association), respectively, following the IHSS (International Humic Substance Society) procedures. The carbon content in the HA is 54%, while in the FA it is 40%. More extensive description of the materials can be found elsewhere (22, 16). Soil Sample. The soil sample was collected in June 1998 from a field near Wageningen in The Netherlands (Wildekamp site). In 1982, the field site was established as a randomized block design of four pH adjustments (nominal pH: 4.0, 4.7, 5.4, and 6.1) and four copper concentrations (0, 250, 500, and 750 kg of CuSO4 ha-1). The pH levels were established using calcium carbonate or sulfur. In 1988, the pH levels were readjusted to their nominal values (23, 24). The surface layer (0-20 cm) sample from the treatment 4A (nominal pH 4.0, 750 kg of CuSO4 ha-1) was used in the VOL. 36, NO. 8, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Soil Characteristics
soil
pH
iron total Cu clay oxalate (2 M CEC (
