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TECHNICAL REPORTS

Heavy Metals in the Environment

Hydrophobicity of Soil Colloids and Heavy Metal Mobilization

Effects of Drying

Berlin Univ. of Technology, Dep. of Soil Science, Salzufer 11-12, D-10587 Berlin, Germany

^* Corresponding author (sondra.klitzke{at}tu-berlin.de)

Received for publication October 4, 2006.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSION REFERENCES

 
Drying of soil may increase the hydrophobicity of soil and affect the mobilization of colloids after re-wetting. Results of previous research suggest that colloid hydrophobicity is an important parameter in controlling the retention of colloids and colloid-associated substances in soils. We tested the hypothesis that air-drying of soil samples increases the hydrophobicity of water-dispersible colloids and whether air-drying affects the mobilization of colloid-associated heavy metals. We performed batch experiments with field-moist and air-dried (25°C) soils from a former sewage farm (sandy loam), a municipal park (loamy sand), and a shooting range site (loamy sand with 25% C_org). The filtered suspensions (<1.2 µm) were analyzed for concentrations of dissolved and colloidal organic C and heavy metals (Cu, Cd, Pb, Zn), average colloid size, zeta potential, and turbidity. The hydrophobicity of colloids was determined by their partitioning between a hydrophobic solid and a hydrophilic aqueous phase. Drying increased hydrophobicity of the solid phase but did not affect the hydrophobicity of the dispersed colloids. Drying decreased the amount of mobilized mineral and (organo-)mineral colloids in the sewage farm soils but increased the mobilization of organic colloids in the C-rich shooting range soil. Dried samples released less colloid-bound Cd and Zn than field-moist samples. Drying-induced mobilization of dissolved organic C caused a redistribution of Cu from the colloidal to the dissolved phase. We conclude that drying-induced colloid mobilization is not caused by a change in the physicochemical properties of the colloids. Therefore, it is likely that the mobilization of colloids in the field is caused by increasing shear forces or the disintegration of aggregates.

Abbreviations: COC, colloidal organic carbon • DOC, dissolved organic carbon • DOM, dissolved organic matter • TOC, total organic carbon

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSION REFERENCES

 
THE INFLUENCE of drying on the physicochemical properties of the soil solution and the solid phase has been well researched. Dried soil samples showed a drastic increase in the concentration of dissolved organic matter (DOM) after rewetting (Bartlett and James, 1980; Baskaran et al., 1994; Courchesne et al., 1995; Münch et al., 2002; Kjærgaard et al., 2004b), leading to a decrease in the pH of the soil solution (Courchesne et al., 1995). The observed increase in dissolved organic carbon (DOC) concentration may be attributed to the disruption of microbial biomass (Christ and David, 1996) and the breakdown of aggregate bonds (Raveh and Avnimelech, 1978). Because DOC contributes to an enhanced mobilization of dissolved heavy metal species by forming soluble metal complexes (Brümmer et al., 1986), drying and rewetting may be conducive to enhancing metal leaching. Drying was found to increase water repellency of the solid phase (Dekker et al., 2001) due to increasing hydrophobicity.

Numerous recent studies have shown that colloid-bound heavy metal transport plays a crucial role in soils (Keller and Domergue, 1996; Jensen et al., 1999; Denaix et al., 2001) and is found to be of greater importance than transport as dissolved ions (Egli et al., 1999; Jensen et al., 1999). Several authors describe the influence of drying and rewetting as important factors controlling colloid release. El-Farhan et al. (2000) observed the highest peak of particle mass recovery after the infiltration on an initially dry field soil. Similarly, field studies of Jann et al. (2002) revealed increasing mobilization of colloids after dry periods. The authors attributed this phenomenon to microerosion and abrasion induced by shear forces. Likewise, Denaix et al. (2001) found markedly increased concentrations of mobilizable Pb-containing colloids in the soil water after dry periods. However, the drying-induced mobilization of colloids seems to be limited to the initial phase of re-wetting: In the frame of column studies using dried soil (soil-water potential: 15 500 hPa), Kjærgaard et al. (2004b) observed an initial increase in colloid release followed by a constant decrease as the pore volume increases. This initial increase may be explained by slaking of aggregates due to the compression by trapped air during the wetting phase (Le Bissonnais, 1996). In the same study, Kjærgaard et al. (2004b) investigated the effect of initial soil matrix potential on water-dispersible colloids, revealing that as a result of enhanced interparticle bonding or cementation of colloids drying leads to a decrease of dispersible colloids in the soil suspension.

Because drying of soil may induce changes in the solid phase (e.g., increasing hydrophobicity), it is postulated that this may affect colloid dispersibility. Few studies provide first hints on colloid hydrophobicity as an important parameter influencing colloid retention in soils. Wan and Wilson (1994a) demonstrated an increased retention of colloidal latex particles and bacteria with increasing particle hydrophobicity in an unsaturated sand column experiment. Similarly, under saturated conditions, hydrophobic colloids showed a lower recovery than hydrophilic colloids. Thus, the authors concluded that hydrophilic colloids are more mobile than hydrophobic ones (Wan and Wilson, 1994a,b) because they sorb less strongly to the gas–water and solid–water interfaces. These findings, together with the observation that drying can increase the hydrophobicity of the bulk soil, are in conflict with the concept of colloid mobilization after soil drying. In addition to having an impact on colloid mobilization, colloid hydrophobicity plays a key role in metal bioavailability. Carvalho et al. (1999) reported that the relative hydrophobicity of metal–colloid complexes may affect their bioavailability by enhancing their transport across membrane lipid bilayers.

In addition to directly affecting colloid properties, drying and rewetting of soils may affect the dispersibility of colloids by altering the physicochemical properties of the soil solution; for example, increasing DOC concentration of the soil solution enhances colloid stability (Kretzschmar et al., 1999). Moreover, air-dried soils are commonly used for various soil analyses. Although it is well documented that heavy metal and organic C concentrations in the extracts of air-dried and moist soils differ from each other (Wang et al., 2002; Tom-Petersen et al., 2004), studies of the effect on the colloid-bound fractions are absent. Thus, the results of this study may provide important information for the design of analytical procedures.

The effect of drying-induced hydrophobization on the amount and properties of dispersible colloids has not been investigated, and the underlying mechanisms of colloid release after drying are unknown. Therefore, the aim of this study was to investigate whether (i) drying of soil samples increases not only the hydrophobicity of the solid phase but also the hydrophobicity of dispersible colloids and (ii) drying of soil samples increases the colloid-bound and dissolved metal fractions.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSION REFERENCES

 
Soil Samples
The experiments were conducted with soil samples taken from the A_h horizons of the Berlin sewage farm (Buch soil) classified as Regosol (FAO, 1998) with sandy clay loam texture and the Berlin municipal park Tiergarten (TG) classified as Cambisol with sandy loam. They are both developed on quartenary fluvial sands; however, the sediment structure of the Tiergarten park was changed at the beginning of the last century by adding building rubble and dredged lake sediment. The sites were chosen because they were contaminated by heavy metals due to the impact of sewage water (Buch soil) and building rubble (TG soil). The experimental site on the sewage farm has been divided into three subplots of similar texture (labeled B1, B2, and B3). The entire site is characterized by a high small-scale heterogeneity. Additionally, we investigated the top soil (0–10 cm) of a former shooting range site near Gütersloh (GS), Lower Saxony, Germany, where oak trees have been growing for the past 30 yr. The soil is a podzolic Cambisol with a texture of loamy sand. It developed on fluvial sandy stream deposits of the Pleistocene. This soil was included in the study due to its high contamination by Pb, which is known to have a high affinity to colloids (Egli et al., 1999; Jensen et al., 1999; Denaix et al., 2001) and because of its high content of organic matter. We sampled the A_eh horizon, which was interrupted by the forest floor O_h horizon. For the analyses we mixed both horizons.

The samples were homogenized and sieved (2 mm) after the removal of roots and stones. The soils did not contain any significant amounts of aggregates before homogenization and sieving. The field-moist soil samples were put in plastic bags and stored in a refrigerator. An aliquot of the samples was dried at 25°C until constant weight and kept in air-tight plastic containers at room temperature until use.

To characterize the soil samples, the pH was determined in deionized water and 0.01 M CaCl₂ solution using 10 g of air-dried soil and 25 mL of the respective solution. The suspension was left to stand overnight before the pH was measured using a WTW inoLab pH meter. Concentrations of organic C, N, and S (C_org, N_org, and S_org) were measured by a C/N/S-analyzer (vario EL III; Elementar, Hanau, Germany) with soil dried at 105°C. From these parameters, the C/N-ratio was derived. For the determination of the water content, field-moist samples were dried at 25°C and 105°C until constant weight. Table 1 shows the analyzed properties of the sampled soil horizons. Total metal concentrations of the soil were determined by nitric acid-assisted digestion in closed vessels (180°C, 6 h). After cooling, the digested samples were filtered (Ø 150 mm, type 0790 1/2, ref. no. 10301645; Schleicher & Schuell, Dassel, Germany), transferred into 25-mL volumetric flasks, and filled to the mark with deionized water. The solutions were analyzed for Pb, Cu, Cd, and Zn concentrations. For measurement details, refer to the Analyses section.

Experimental Setup
Based on the procedure of Curtin et al. (1994), who used a soil to water ratio of 1:10 to determine clay dispersibility, we chose a soil to water ratio of 15 g field-moist soil to 150 mL total solution volume in combination with turbidity measurements (see below). The weight of the dried soil samples was corrected for the loss of water. Samples were shaken for 16 h using an end-over-end-shaker at 18 rpm (GFL 3040, Burgwedel, Germany).

At the end of the experiment, pH and conductivity were measured in the suspensions before filtration (1.2-µm cellulose-nitrate filter, Type 11303-047N; Sartorius, Göttingen, Germany). We determined total organic carbon (TOC) concentrations and total concentrations of Cd, Cu, Pb, and Zn. We determined the turbidity, hydrophobicity, particle size, and zeta potential of the filtrate. An aliquot of the suspension was ultracentrifuged at 300 000 g for 1 h at 10°C (Beckman Optima TL, Krefeld, Germany) to separate colloids larger than 14 nm and with a density >1.2 g cm³ (based on Stoke's law) from the solution. The supernatant was transferred into plastic vessels, acidified with nitric acid, and analyzed for Cd, Cu, Pb, Zn, and DOC concentrations (considered as truly dissolved). The difference between concentrations in ultracentrifuged and not ultracentrifuged samples accounts for colloidal fractions (operationally defined) of the above-mentioned elements.

Analyses
Cadmium, Cu, Pb, and Zn concentrations of the colloidal suspensions were determined after microwave-assisted nitric acid digestion (CEM, MARS Xpress) according to the procedure described in USEPA Method 3015 (USEPA, 1994). The Zn measurement was performed using a flame atomic absorption spectrophotometer (1100; PerkinElmer, Milano, Italy) at a wavelength of 213.7 nm. Copper, Pb, and Cd analyses were determined using a graphite furnance atomic absorption spectrophotometer (Spectra AA 880Z; Varian, Darmstadt, Germany) (wavelength: 327.4, 283.3, and 228.8 nm, respectively). The organic C concentration of the suspensions and solutions was measured by a total organic carbon analyzer (TOC-5050 A; Shimadzu, Duisburg, Germany). The turbidity was determined by a turbidimeter (2100P ISO; Hach, Düsseldorf, Germany). Hydrophobicity of the colloids was measured as a partitioning coefficient between the aqueous suspension and a solid hydrophobic phase (C₁₈ Prep LC Packing, diameter: 40 µm; Bakerbond Type 7025-00; JT Baker, Phillipsburg, NJ). Ten milliliters of filtered suspension were added to 30-mg C₁₈ spheres and shaken for 2 h at 100 rpm (KS501 digital; IKA Labortechnik, Staufen, Germany) to allow for hydrophobic colloids to sorb onto the solids. Afterward, the suspensions were filtered over a 15 µm nylon gauze to remove the C₁₈ spheres. The turbidity (T2) of the filtrate obtained was remeasured. Hydrophobicity (H) was calculated according to the following formula:

[1]

where T1 is the turbidity of the suspension before C18 treatment. Further details about the method are described in Klitzke and Lang (2007).

The contact angle of the bulk soil samples as a measure for hydrophobicity of the solid phase was determined by indirect measurements. The field-moist samples were measured by the capillary rise method as described by Adamson (1990), and air-dried samples were measured by the Wilhelmy plate method (Bachmann et al., 2003). The use of two methods was considered essential due to the different wettabilities of the samples caused by the drying process. Figure 1 depicts the relation of the following parameters: (i) contact angle, (ii) hydrophobicity as determined by C₁₈ spheres, and (iii) wettability.

View larger version (8K): [in this window] [in a new window]   Fig. 1. Relation between contact angle, hydrophobicity, and wettability.

For each parameter of the complete sample set, a paired t test was conducted to determine significant differences between results from field-moist and air-dried samples. Because the properties of the Gütersloh soil differ greatly from the other soils included in the study, a second t test was performed based on the Buch and Tiergarten soils only. Statistically significant differences between the triplicates of field-moist and air-dried samples of individual soils were determined by an unpaired t test. We used a level of significance of 95% (P < 0.05) for both tests.

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSION REFERENCES

 
Statistically significant differences for each parameter of the complete sample set between field-moist and air-dried samples are displayed in Table 2. The results of the t test are helpful only if there are no inverse effects and if all soils show the same tendency (i.e., increasing or decreasing concentrations of a parameter after drying). If this condition is not met, opposing effects cancel each other out, and the result of the t test is distorted. The significance of effects for individual soils can be indirectly determined by the error bars displayed in the graphs, representing twice the SD at a confidence level of 95%.

View larger version (27K): [in this window] [in a new window]   Fig. 2. Dissolved and colloidal Corg concentrations in suspensions of field-moist and air-dried samples (error bars depict 1 SD).

View larger version (48K): [in this window] [in a new window]   Fig. 3. (a) Dissolved and colloidal Cd concentrations in suspensions of field-moist and air-dried samples (error bars depict 1 SD). (b) Dissolved and colloidal Zn concentrations in suspensions of field-moist and air-dried samples (values of Gütersloh soil are below quantification limit; error bars depict 1 SD). (c) Dissolved and colloidal Cu concentrations in suspensions of field-moist and air-dried samples (error bars depict 1 SD; air-dried colloidal concentrations of the B1 sample not determined). (d) Dissolved and colloidal Pb concentrations in suspensions of field-moist and air-dried samples (error bars depict 1 SD).

In contrast to Cu, the effect of drying on the mobilization of dissolved Pb varied for the different samples. The Buch samples were constant, and the Tiergarten and the Gütersloh soils (according to the t test) significantly increased dissolved Pb concentrations after drying (Fig. 3d). In the Gütersloh soil, the increase can be ascribed to the extremely high concentration of DOC, leading to the mobilization of dissolved organic Pb complexes (McBride, 1994). In the Buch soils, we did not observe any increase in dissolved Pb concentrations despite increasing DOC concentrations. The higher affinity of Cu to soil organic matter (McBride, 1994) and the higher stability constants of Cu for humic acids (Lubal et al., 1998) and Cu-EDTA complexes (Lindsay, 1979) could explain why the drying-induced release of organic C does not lead to a mobilization of Pb in the Buch soils but to a preferred complexation of Cu. In the Gütersloh soil, however, the extremely high Pb concentrations exceed the Cu concentrations; therefore, despite the higher affinity of Cu for organic matter, it is displaced by Pb from the organic complex.

Influence of Drying on the Composition and Properties of Dispersible Colloids
All analyzed soils had substantial amounts of colloids in the suspension, except for the Tiergarten soil, which releases few if any colloids.

Based on turbidity measurements, the statistical analysis of all samples does not show any significant effect of drying on the mobilization of colloids (Table 2). This is because drying shows inverse effects for different soil samples: The Gütersloh soil shows increasing colloid mobilization after drying, whereas the Buch soils show decreased colloid mobilization and the Tiergarten soil shows constant colloid mobilization after drying. Therefore, in the statistical analysis, two opposing effects cancel each other out. The decrease and constancy in colloid mobilization are in contrast to results obtained from field studies reported by Denaix et al. (2001), who conducted their study on soil of similar texture and pH, and by Jann et al. (2002), who conducted their study on calcareous gravel, indicating colloid mobilization after re-wetting of dry soil. However, decreasing colloid concentrations are supportive of findings in sandy soils of different clay content as reported by Kjærgaard et al. (2004a, 2004b).

The decreasing amount of mobilized colloids may be explained by the following three processes:

1. Enhanced aggregation during the drying process as described by Thill and Spalla (2003).
   
2. Drying-induced disintegration of organomineral complexes as suggested by Peltovuori and Soinne, 2005. The break-up of weak bonds between organic matter and hydroxides (Haynes and Swift, 1989) may reduce the stability of mineral colloids, resulting in a decrease in colloid mobilization.
   
3. Hydrophobization of colloids (see below).
   

The difference in changes of turbidity was most pronounced for the samples B1 and Gütersloh. Although soil B1 demonstrated a decrease in turbidity after drying, the Gütersloh soil showed the opposite (Fig. 4). The high content of organic matter of the latter creates the potential for a three-dimensional network forming under field-moist conditions (Schaumann et al., 2000). Such a cross-linked gel structure of organic material could impede the release of colloids into the solution. In previously dried soil, however, any such restrictive network that existed would have likely been destroyed and would take time to redevelop. Thus, on rewetting, colloids would initially be able to move freely into solution, resulting in an enhanced release of colloids. An additional factor that may have contributed to the increased release of colloids from the organic-rich Gütersloh soil is thought to be the disruption of biomass (Christ and David, 1996) and possible release of organic matter associated with microbial death and cell lysing on drying. The higher initial water content of the Gütersloh soil may also have been a factor.

View larger version (9K): [in this window] [in a new window]   Fig. 4. Turbidity in the soil suspensions of field-moist and air-dried samples (error bars depict 1 SD).

View larger version (11K): [in this window] [in a new window]   Fig. 5. Contact angle of field-moist and air-dried samples (error bars depict 1 SD).

View larger version (10K): [in this window] [in a new window]   Fig. 6. Hydrophobicity of colloidal suspensions of field-moist and air-dried soil (error bars depict 1 SD).

Concentrations of colloidal organic C (COC) remained constant or showed an increase (Fig. 2) for most soils, with the differences being most pronounced and statistically significant (t test) for the Tiergarten and Gütersloh soils. Our results support the findings of Kjærgaard et al. (2004a, 2004b), who observed an increase in COC after drying. We explain this phenomenon by the disintegration of organomineral complexes (Peltovuori and Soinne, 2005) and the disruption of microbial biomass (Christ and David, 1996) in the C-rich Gütersloh soil.

Our results reveal no increase in the colloid-bound metal fraction (data not shown), as would be expected from suggestions in the literature (Denaix et al., 2001). We observed a clear decline in the concentrations and fractions of colloid-bound Cd and Zn (Table 2). Our data suggest a preferential cementation of inorganic particles of the soil matrix induced by the drying process, leading to a lower degree of colloid dispersibility after rewetting.

Colloid-bound Cu concentrations decreased in two soils after drying despite increasing COC concentrations. This implies that there is no correlation between these two parameters, although Cu has a high affinity to organic matter (McBride, 1994). These findings suggest that (i) colloidal Cu in these soil suspensions may be mainly bound to inorganic particles, which may have an organic surface coating, and (ii) because the suspensions of air-dried samples showed elevated concentrations of DOC, one might hypothesize an equilibrium between colloidal Cu and dissolved organically complexed Cu (i.e., high DOC concentrations could lead to a desorption of Cu from the colloids). This assumption would be in line with increasing concentrations of dissolved Cu in the suspensions of the air-dried samples.

Although the drying process leads to a significant decrease in colloidal Pb concentrations for the Buch and the Tiergarten soils, concentrations remain constant in the Gütersloh sample. If the Gütersloh soil is left out in the t test (Table 2), changes for the Buch and Tiergarten soils are significant. This difference could be ascribed to the lower pH_H2O and the high organic C content of the Gütersloh soil, allowing for the formation of Pb-organic precipitates (Lang et al., 2005). However, the fractions of colloidal Pb for all soils (data not shown) remain unchanged by the drying process. For the Buch soils colloidal Pb accounts for between 70 and 96% of total Pb in suspension, whereas in the Gütersloh soil dissolved Pb is predominant. Similarly to Cu, the data set does not reveal any correlation between COC and colloidal Pb. These findings lead to the conclusion that colloidal Pb in the Buch soils is bound predominantly to inorganic colloids.

	CONCLUSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSION REFERENCES

 
Our batch experiments showed that drying of the studied soils does not lead to a uniform increase in the concentration of water-dispersible colloids. We conclude that the influence of drying on the dispersibility of pedogenic colloids is likely to depend on their composition. In a C-rich matrix, mainly organic colloids are mobilized by soil drying, whereas in a more mineral matrix, (organo)-mineral colloids are immobilized, possibly due to enhanced cementation during the drying process. Drying-induced (im)mobilization of colloids does not always go along with the change in concentration of colloid-associated heavy metals. This indicates that drying does not only affect the mobilization of colloids but also the equilibrium between colloidal and dissolved species. Absolute concentrations of the investigated colloid-bound heavy metals were found to decrease in almost all soil samples for Cd and Zn and, in some soils, for Cu and Pb. This decrease may be attributed to the immobilization of colloids (as observed for Zn and Cd and, in some samples, for Pb) or to the mobilization of heavy-metal sorbing DOM leading to a redistribution of the metal between colloidal and dissolved phase (as observed for Cu). Because drying did not influence the physicochemical properties of the colloids, our results suggest that drying-induced colloid mobilization in the field is most likely due to shear forces (as explained by Jann et al., 2002) or the dispersion of macroaggregates (Kjærgaard et al., 2004a) rather than by a major change in physicochemical properties of the colloids. Future studies should assess whether the presented results are applicable to intact cores.

	ACKNOWLEDGMENTS

 
We thank Claudia Kuntz for her help with laboratory analyses, Gabriele Schaumann for fruitful discussion, David Meredith for proofreading the manuscript, and the reviewers for helpful and constructive criticism and comments. This work was supported by the German Research Foundation (La 1398/2).

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSION REFERENCES

 

• Adamson, A.W. 1990. Physical chemistry of surfaces. John Wiley & Sons, New York.
• Bachmann, J., S.K. Woche, M.-O. Goebel, M.B. Kirkham, and R. Horton. 2003. Extended methodology for determining wetting properties of porous media. Water Resour. Res. 39, Art. No. 1353. 11-1–11-13.
• Bartlett, R., and B. James. 1980. Studying dried, stored soil samples: Some pitfalls. Soil Sci. Soc. Am. J. 44:721–724.[Web of Science]
• Baskaran, S., N.S. Bolan, A. Rahman, R.W. Tillman, and A.N. Macgregor. 1994. Effect of drying of soils on the adsorption and leaching of phosphate and 2,4-dichlorophenoxyacetic acid. Aust. J. Soil Res. 32:447–464.[CrossRef]
• Brümmer, G.W., J. Gerth, and U. Herms. 1986. Heavy metal species, mobility, and availability in soils. J. Plant Nutr. Soil Sci. 149:382–398.[CrossRef]
• Carvalho, R.A., M.C. Benfield, and P.H. Santschi. 1999. Comparative bioaccumulation studies of colloidally complexed and free-ionic heavy metals in juvenile brown shrimp Penaeus aztecus (Crustacea: Decapoda: Penaeidae). Limnol. Oceanogr. 44:403–414.
• Christ, M.J., and M.B. David. 1996. Temperature and moisture effects on the production of dissolved organic carbon in a Spodosol. Soil Biol. Biochem. 28:1191–1199.[CrossRef]
• Courchesne, F., S. Savoie, and A. Dufresne. 1995. Effects of air-drying on the measurement of soil pH in acidic forest soils of Quebec, Canada. Soil Sci. 160:56–68.
• Curtin, D., C.A. Campbell, R.P. Zentner, and G.P. Lafond. 1994. Long-term management and clay dispersibility in two Haploborolls in Saskatchewan. Soil Sci. Soc. Am. J. 58:962–967.[Web of Science]
• Dekker, L.W., S.H. Doerr, K. Oostinidie, A.K. Ziogas, and C.J. Ritsema. 2001. Water repellency and critical soil water content in a dune sand. Soil Sci. Soc. Am. J. 65:1667–1674.[Abstract/Free Full Text]
• Denaix, L., R.M. Semlali, and F. Douay. 2001. Dissolved and colloidal transport of Cd, Pb, and Zn in a silt loam soil affected by atmospheric industrial deposition. Environ. Pollut. 113:29–38.
• Egli, M., P. Fitze, and M. Oswald. 1999. Changes in heavy metal contents in an acidic forest soil affected by depletion of soil organic matter within the time span 1969–93. Environ. Pollut. 105:367–379.[CrossRef]
• El-Farhan, Y.H., N.M. Denovio, J.S. Herman, and G.M. Hornberger. 2000. Mobilization and transport of soil particles during infiltration experiments in an agricultural field, Shenandoah Valley, Virginia. Environ. Sci. Technol. 24:3555–3559.
• FAO. 1998. World reference base for soil resources. Food and Agriculture Organization of the United Nations, Rome.
• Goebel, M.-O., J. Bachmann, S.K. Woche, W.R. Fischer, and R. Horton. 2004. Water potential and aggregate size effects on contact angle and surface energy. Soil Sci. Soc. Am. J. 68:383–393.[Abstract/Free Full Text]
• Haynes, R.J., and R.S. Swift. 1989. The effects of pH and drying on adsorption of phosphate by aluminium-organic matter associations. J. Soil Sci. 40:773–781.[CrossRef]
• Hurraß, J., and G.E. Schaumann. 2006. Properties of soil organic matter and aqueous extracts of actually water repellent and wettable soil samples. Geoderma 132:222–239.[CrossRef][Web of Science]
• Jann, S., K.U. Totsche, P. Jaesche, and I. Kögel-Knabner. 2002. Release and transport of colloids from the unsaturated zone under natural conditions: A lysimeter study. p. 137–141. In Proceedings of colloids and colloid-facilitated transport of contaminants in soils and sediments. International Workshop. Foloum, Denmark. 19–20 Sept. 2002.
• Jensen, D.L., A. Ledin, and T.H. Christensen. 1999. Speciation of heavy metals in landfill-leachate polluted groundwater. Water Resour. Res. 33:2642–2650.[CrossRef]
• Kaiser, K., M. Kaupenjohann, and W. Zech. 2001. Sorption of dissolved organic carbon in soils: Effects of soil sample storage, soil-to-solution ratio, and temperature. Geoderma 99:317–328.[CrossRef][Web of Science]
• Keller, C., and F.-L. Domergue. 1996. Soluble and particulate transfers of Cu, Cd, Al, Fe, and some major elements in gravitational waters of a Podzol. Geoderma 71:263–274.[CrossRef][Web of Science]
• Kjærgaard, C., L.W. de Jonge, P. Moldrup, and P. Schjønning. 2004a. Water-dispersible colloids: Effects of measurement method, clay content, initial soil matric potential, and wetting rate. Vadose Zone J. 3:403–412.[Abstract/Free Full Text]
• Kjærgaard, C., P. Moldrup, L.W. de Jonge, and O.H. Jacobsen. 2004b. Colloid mobilization and transport in undisturbed soil columns: II. The role of colloid dispersibility and preferential flow. Vadose Zone J. 3:423–433.
• Klitzke, S., and F. Lang. 2007. A method for the determination of hydrophobicity of suspended soil colloids. Colloid Surf., A. (in press).
• Kretzschmar, R., M. Borkovec, D. Grolimund, and M. Elimelech. 1999. Mobile subsurface colloids and their role in contaminant transport. Adv. Agron. 66:121–181.[CrossRef]
• Lang, F., H. Egger, and M. Kaupenjohann. 2005. Size and shape of lead-organic associations. Colloids Surf., A. 265:95–103.[CrossRef]
• Le Bissonnais, Y. 1996. Aggregate stability and assessment of soil crustability and erodibility: I. Theory and methodology. Eur. J. Soil Sci. 47:425–437.[CrossRef]
• Lindsay, W.L. 1979. Chemical equilibria in soils. John Wiley & Sons, New York.
• Lubal, P., D. Siroky, D. Fetsch, and J. Havel. 1998. The acidobasic and complexation properties of humic acids: Study of complexation of Czech humic acids with metal ions. Talanta 47:401–412.[Medline]
• McBride, M.B. 1994. Environmental chemistry of soils. Oxford Univ. Press, New York.
• Münch, J.-M., K.U. Totsche, and K. Kaiser. 2002. Physicochemical factors controlling the release of dissolved organic carbon from columns of forest subsoils. Eur. J. Soil Sci. 53:311–320.[CrossRef]
• Peltovuori, T., and H. Soinne. 2005. Phosphorous solubility and sorption in frozen, air-dried and field-moist soil. Eur. J. Soil Sci. 56:821–826.
• Raveh, A., and Y. Avnimelech. 1978. The effect of drying on the colloidal properties and stability of humic compounds. Plant Soil 50:545–552.[CrossRef][Web of Science]
• Schaumann, G.E., C. Siewert, and B. Marschner. 2000. Kinetics of the release of dissolved organic matter (DOM) from air-dried and pre-moistened soil material. J. Plant Nutr. Soil Sci. 163:1–5.[CrossRef]
• Tom-Petersen, A., H.C. Bruun Hansen, and O. Nybroe. 2004. Time and moisture effects on total bioavailable copper in soil water extracts. J. Environ. Qual. 33:505–512.[Abstract/Free Full Text]
• Thill, A., and O. Spalla. 2003. Aggregation due to capillary forces during drying of particle submonolayers. Colloid Surf., A. 217:143–151.[CrossRef]
• USEPA. 1994. Method 3015: Microwave-assisted digestion of aqueous samples and extracts. USEPA, Washington DC. Available at http://www.epa.gov/sw-846/pdfs/3015.pdf#search=%22EPA%20method%203015%22 (verified 2 Apr. 2007).
• Wan, J., and J.L. Wilson. 1994a. Visualization of the role of the gas-water interface on the fate and transport of colloids in porous media. Water Resour. Res. 30:11–23.[CrossRef]
• Wan, J., and J.L. Wilson. 1994b. Colloid transport in unsaturated porous media. Water Resour. Res. 30:857–864.[CrossRef]
• Wang, Z., X.-Q. Shan, and S. Zhang. 2002. Comparison between fractionation and bioavailability of trace elements in rhizosphere and bulk soils. Chemosphere 46:1163–1171.[Medline]

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