Hydrophobicity of Soil Colloids and Heavy Metal Mobilization: Effects of Drying Sondra Klitzke* and Friederike Lang ABSTRACT 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 con- trolling the retention of colloids and colloid-associated substances in soils. We tested the hypothesis that air-drying of soil samples in- creases 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 mm) 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 hydro- phobicity 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. T HE INFLUENCE of drying on the physicochemical prop- erties of the soil solution and the solid phase has been well researched. Dried soil samples showed a dras- tic increase in the concentration of dissolved organic matter (DOM) after rewetting (Bartlett and James, 1980; Baskaran et al., 1994; Courchesne et al., 1995; Mu ¨ nch et al., 2002; Kj&rgaard et al., 2004b), leading to a decrease in the pH of the soil solution (Courchesne et al., 1995). The ob- served increase in dissolved organic carbon (DOC) con- centration 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 dis- solved heavy metal species by forming soluble metal com- plexes (Bru ¨mmer 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 dry- ing and rewetting as important factors controlling col- loid 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 phenom- enon 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 lim- ited to the initial phase of re-wetting: In the frame of column studies using dried soil (soil-water potential: 15 500 hPa), Kj&rgaard et al. (2004b) observed an initial increase in colloid release followed by a constant de- crease 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, Kj&rgaard et al. (2004b) investigated the effect of initial soil matrix po- tential on water-dispersible colloids, revealing that as a result of enhanced interparticle bonding or cementation of colloids drying leads to a decrease of dispersible col- loids 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 im- portant parameter influencing colloid retention in soils. Wan and Wilson (1994a) demonstrated an increased re- tention of colloidal latex particles and bacteria with in- creasing particle hydrophobicity in an unsaturated sand column experiment. Similarly, under saturated condi- tions, hydrophobic colloids showed a lower recovery than hydrophilic colloids. Thus, the authors concluded that hydrophilic colloids are more mobile than hydro- phobic ones (Wan and Wilson, 1994a,b) because they sorb less strongly to the gas–water and solid–water in- terfaces. These findings, together with the observa- tion 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 hydrophobic- ity plays a key role in metal bioavailability. Carvalho et al. (1999) reported that the relative hydrophobicity of metal–colloid complexes may affect their bioavail- ability by enhancing their transport across membrane lipid bilayers. Berlin Univ. of Technology, Dep. of Soil Science, Salzufer 11-12, D- 10587 Berlin, Germany. Received 4 Oct. 2006. *Corresponding author (sondra.klitzke@tu-berlin.de). Published in J. Environ. Qual. 36:1187–1193 (2007). Technical Reports: Heavy Metals in the Environment doi:10.2134/jeq2006.0427 ª ASA, CSSA, SSSA 677 S. Segoe Rd., Madison, WI 53711 USA Abbreviations: COC, colloidal organic carbon; DOC, dissolved organic carbon; DOM, dissolved organic matter; TOC, total organic carbon. Reproduced from Journal of Environmental Quality. Published by ASA, CSSA, and SSSA. All copyrights reserved. 1187 Published online June 27, 2007