Flow and Reactivity Effects on Dissolved Organic Matter Transport in Soil Columns Harald Weigand and Kai U. Totsche* ABSTRACT
Van Riemsdijk, 1990), metal oxides and hydroxides are known as effective soil sorbents for DOM (Tipping, 1981; Davis and Gloor, 1981; Murphy et al., 1992). For these, DOM is a reactive component of the soil solution and affects contaminant mobility twofold: enhancement due to the formation of a mobile associate (Johnson and Amy, 1995) or reduction due to either increased sorption capacity of the solid phase or cosorption of DOM-associated contaminants (Murphy and Zachara, 1995, Totsche et al., 1997). For a given mineralogical composition, DOM sorption will be modified by the flow process. Especially under the conditions of varying pore water velocities, one may expect rate-limited sorption behavior (Brusseau and Rao, 1989) to affect the mobility of DOM. Most transport experiments involving DOM have been performed under saturated conditions. The results obtained are therefore limited to saturated porous media, e.g., aquifers and sediments. However, soils of the terrestrial environment are predominantly unsaturated. The degree of saturation affects the flow regime and thereby possible rate constraints to sorption process. This is enhanced by the restricted accessibility of sorption sites under the prevailing moisture conditions of soils. To account for these effects, our experiments were performed under unsaturated flow conditions. Our study addressed the influence that flow regime and solid-phase reactivity have on DOM mobility in the unsaturated zone. Special consideration was given to features of sorption nonideality, such as sorption irreversibility, isotherm nonlinearity, rate-limited sorption, sorption hysteresis, and the effect of the composite nature of DOM.
Dissolved organic matter (DOM) plays a prominent role in the transport of contaminants in porous media. As DOM has to be considered as a reactive component, flow regime and sorbent reactivity should affect overall DOM transport in an important way. We focused on DOM transport in unsaturated column experiments using quartz sand (QS) and goethite-coated quartz sand (GS). Rate constrictions to DOM sorption were investigated by varying the volumetric flow rate, while extent and reversibility of sorption were studied in consecutive adsorption and desorption steps. In the QS, DOM retention was low and unaffected by changes in flow rate. Desorption-step breakthrough curves (BTCs) and mass balances show full reversibility of the sorption process. However, DOM retention in GS was significant and sensitive to flow variation, indicative of nonequilibrium sorption. At lower flow rates, DOM breakthrough exhibited a change in curvature (shoulder) due to the superimposition of two BTCs representing reactive and nonreactive DOM fractions. Transport was successfully modeled assuming these two fractions governed overall DOM mobility. At higher flow rates, the ETC shoulder vanished due to reduced contact time between the DOM and the solid phase (ratelimited sorption). Sorption of DOM on GS is accompanied by a marked rise in effluent pH, indicative of a ligand-exchange mechanism. Recovery of DOM during desorption was incomplete due to either partially irreversible sorption or strongly rate-limited desorption. Increased DOM mobility in the consecutive adsorption step resulted from partial blocking of sorption sites by the initial pulse of DOM.
D
ISSOLVED ORGANIC MATTER plays a prominent role in the understanding of transport and reaction processes in soils. Dissolved organic matter is known to be involved in the processes of podsolization (Dawson et al., 1978; Buurmann, 1985), soil acidification (Brown and Sposito, 1991), and mineral weathering (Heyes and Moore, 1992). Recently, increasing interest has arisen in understanding the role of DOM as a transport-facilitating agent of nutrients (Quails et al., 1991) and contaminants (McCarthy and Zachara, 1989; Knabner et al., 1996). Dissolved organic matter can be conceived as a continuum of substances of biotic origin, which are partially or fully degraded and transformed (Quails et al., 1991; Guggenberger et al., 1994). The compounds involved show a wide range of ionization constant (p/Q and point of zero charge (PZC) values, molecular sizes, and functional groups (Gu et al., 1994). The mobility of DOM in soils is governed by both the chemistry of the bulk liquid and the composition of the soil mineral phase, where the former determines DOM's dissolution properties (Schlautmann and Morgan, 1994) and the latter controls the extent of DOM adsorption. Besides edges of layer silicates and quartz grains (Hiemstra and
MATERIALS AND METHODS Materials Quartz sand (AKW, Amberg, Germany) and goethitecoated quartz sand (provided by the Dep. of Soil Science and Plant Nutrition, University of Wageningen, Wageningen, the Netherlands) were used as model porous media. Pedogenic Fe and Al (hydr)oxides were determined as the dithionitecitrate-bicarbonate-extractable fraction of metal oxides according to the method introduced by Mehra and Jackson (1960). Porosity was calculated from bulk densities and the respective substance densities. Table 1 lists the physicochemical features of the solid phases. A DOM stock solution was prepared by extracting forest-floor organic matter on a 1:10 solid/liquid ratio. The forest-floor material was sampled at the Waldstein experimental site, northeast Bavaria, Germany. The DOM was subject to operational fractionation according Abbreviations: ETC, breakthrough curve; DOM, dissolved organic matter; GS, goethite-coated quartz sand; LEM, linear equilibrium model; PV, pore volume; QS, quartz sand; TSTR, two-site, tworegion model.
Soil Physics Div., Univ. of Bayreuth, Germany. Received 21 July 1997. * Corresponding author (
[email protected]). Published in Soil Sci. Soc. Am. J. 62:1268-1274 (1998).
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Table 1. Physicochemical properties of quartz sand and goethitecoated quartz sand. Goethite-coated quartz sand Quartz sand Grain size distribution, % 2-0.63 mm IS NDf 0.63-0.2 mm 33 85 0.2-0.063 mm 67 ND bulk density, g/cm3 1.6 1.6 0.40 Porosity 0.38 5.7 pH (CaCUt 6.2 5 Fe, kg kg-'§ Al, kg kg-'§
3.0 x 10'5 4.0 X ID'
3.5 X 10~< ND
t ND = not detectable. t Extraction ratio soil/0.01 M CaCl2:1:2.5. § Dithionite-citrate-bicarbonate extractable. Fig. 1. Schematic representation of the experimental setup. to Leenheer (1981), revealing =62% hydrophobic moieties and =32% hydrophilic moieties. To protect the percolation solutions from microbial transformation, AgNO3 was added at a concentration of 2 x 10~6 M. The extract was filtered twice through SUPUR-102 0.45 |xm membrane filters (Gelman Sciences, Ann Arbor, MI) and then diluted to an inflow concentration of =4 X 1CT3 M on a carbon basis. As a conservative tracer, 10~3 M Nad was added to the adsorption-step solutions (Table 2). All solutions were prepared with deionized and degassed water. All chemicals were Merck nanograde.
Experimental Methods Soil column experiments were performed using a computercontrolled repacked soil column system (7 cm in height, 9 cm in diameter, Fig. 1). A stainless steel porous plate (Krebssoge, Germany) was used as the bottom capping of the soil column, with suction applied by means of a hanging water column. For continuous feed of the percolation solution to the sprinkling unit and transport of column effluent to the fraction collector, a peristaltic pump (Gilson Minipuls 3, Gilson Co., Worthington, OH) was used with Technicon Tygon R3607 tubes (i.d. 1.49 mm). A fraction collector (Foxy, Isco Inc., Lincoln, NE) was used to separate distinct effluent fractions at time intervals corresponding to about 0.3 pore volumes (PVs). Prior to the experiments, soil columns were saturated with water from bottom to top at low flow rates (3 mL/h) to prevent air entrapment and guarantee a uniform flow domain. Steady-state unsaturated flow conditions were established while irrigating the system with a tracer-free solution after the suction had been applied. Based on the soil moisture characteristic of the materials (data not shown), suction heads of 25 and 55 cm were used in the QS and GS columns, respectively. This procedure guaranteed similar and time-constant water contents in the experiments (Table 3). As macroscopic pore water velocity is given by the ratio of Darcy velocity and water contents, variations were achieved by changing the volumetric flow rates. After the effluent flow rates proved constant with time, consecutive adsorption and desorption steps were carried out. Constant feed of the percolation solutions was maintained until breakthrough was completed (typiTable 2. Composition of percolation solutions. Solution A: adsorption tracer 3 Dissolved organic C, mol C/L 4.2 X 10 NaCl, mol/L io-3 2 X 10-' AgNO3, mol/L 10 ' KCIO4, mol/L PH
4.5
Solution B: stationarity desorption
