L. W. de Jonge

Colloids and Colloid-Facilitated Transport of Contaminants in Soils: An Introduction

Until some two decades ago, it was believed that only the soil liquid and gaseous phases were mobile and could facilitate the transport of chemicals and nutrients through the vadose zone. It is now generally accepted that also part of the soil solid phase is mobile, and that mobile organic and inorganic soil colloids may facilitate chemical transport. However, the magnitude and significance of these colloidal transport processes are yet to be determined. It is essential to examine whether current models for transport and fate of chemicals in soil and groundwater need to be revised. The collection of papers in this special section of Vadose Zone Journal mainly take their origin, but not exclusively, from an international workshop “Colloids and Colloid-Facilitated Transport of Contaminants in Soil and Sediments” held at the Danish Institute of Agricultural Sciences, Denmark, 19–20 Sept. 2002. The workshop was organized to review our present knowledge of colloid behavior and transport in porous media and the possibility of colloid-bound transport of contaminants and nutrients in soil and groundwater. Here we will first give a brief introduction to the topic of mobilization and transport of colloids in the vadose zone, and highlight previous evidence of colloid-facilitated transport. We then introduce the review and technical papers in the special section. We hope that the information provided in this special section will lead to improvements in our understanding and associated conceptual models of contaminant transport and fate in soil.

Particle leaching and particle-facilitated transport of phosphorus at field scale

Strongly sorbing compounds such as P, pesticides, and heavy metals can be transported through soils while being adsorbed to mobile colloidal particles. While the rapid leaching of nonadsorbing chemicals is relatively well understood, the particle-facilitated transport of highly sorbing chemicals such as P requires further investigation. The aim of this work was to study spatial variations in particle-facilitated transport of P at the field scale, and investigate which soil-physical or chemical parameters relate to the observed variations. Leaching experiments were performed in the laboratory on 42 undisturbed soil columns sampled in a grid covering 25 by 30 m of an agricultural field. The columns were equilibrated in the laboratory to a pressure head of −20 cm and irrigated at a rate of 10 mm h −1 with an artificial rainwater solution. The experiments exhibited considerable variation among the columns in the accumulated mass of particles and P leached during the 3.5 h of irrigation. Columns taken from the lower part of the field showed the highest mass of leached particles. These columns had higher clay contents and contained more continuous macropores. The mass of particles was negatively correlated to the average electrical conductivity of the effluent, and positively correlated to the macropore flow velocity. The accumulated masses of particulate organic and inorganic P were linearly related to the accumulated mass of particles leached. About 75% of the leached P was transported in a particle-facilitated manner. Overall, soil structure controlled to a large extent the leaching of particles and particle bound P.

Glyphosate sorption in soils of different pH and phosphorus content

The sorption mechanism of glyphosate, one of the most frequently used herbicides in the world, resembles that of phosphate. This study quantifies the variation in glyphosate sorption and desorption to a coarse sandy soil and to a sandy loam soil with varying phosphorus content and pH. Using batch experiments, glyphosate adsorption and desorption isotherms were determined on soil samples taken from long-term field experiments that received different additions of phosphorus and lime during 60-year (coarse sand) and 100-year (sandy loam) periods. Sorption isotherms were non-linear and manifested adsorption desorption non-singularity. The isotherms were best fitted with an extended Freundlich model, which had earlier been shown to describe phosphate sorption data well. The phosphate content in the soils had a significant influence on the sorption of glyphosate. With 0.5 M bicarbonate extractable P (pH 8.5) increasing from 6.2 to 58.7 in the loamy sand and 9.1 to 87.4 in the coarse sand, the extended Freundlich adsorption coefficient (Kf,MF,ads) decreased from 214.7 to 106 and from 154.0 to 83.5, respectively. Liming of the coarse sandy soil resulted in stronger glyphosate sorption because of an increase of reactive amorphous aluminum and iron hydrous oxides with increasing pH values. Glyphosate competes with phosphate for sorption sites, a quality that might result in glyphosate being sorbed more weakly in soils with high phosphorus levels.

Leaching of colloids and dissolved organic matter from columns packed with natural soil aggregates

Transport of pollutants by colloids and dissolved organic matter (DOM) may increase the leaching of strongly sorbing pollutants (e.g., PAHs, heavy metals, radionuclides, and certain pesticides). A prerequisite for colloid-and DOM-facilitated transport is the release of colloids and DOM from soil. In the present study, the leaching of colloids and DOM from columns packed with natural soil aggregates (2-4 mm) was investigated. Aggregates with different organic matter content were used: Aggregates from Soil 1, with 3.6% organic matter content, and aggregates from Soil 2, with 2.5%. The leaching experiments showed that colloid leaching increased with higher organic matter content. Colloid leaching was strongly affected by the ionic strength of the infiltrating water but less so by the specific type of cation. However, prolonged leaching (20 h) with KCl increased the leaching of colloids, probably because of the ion exchange of naturally occurring polyvalent ions with K+. The accumulated amount of colloids leached during 20-h period was orders of magnitude lower than the amount of dispersible colloids (determined by rotation of soil water mixtures), and the organic carbon fraction (foc) of the leached colloids was 3 to 4 times higher than the foc of the dispersible colloids. The leaching of DOM from Soil 1 was greater, but, relative to the soil organic matter content, it was similar for the two soils. The leaching of DOM was not significantly affected by the chemistry of the irrigation water. Irrigation with solutions of KCl and deionized water increased the hydrodynamic dispersion coefficient on Soil 2, due to swelling of the clay minerals and closure of the soil pores. Irrigation with CaCl2 led to lower hydrodynamic dispersion, because of shrinking clay minerals. Soil 1 was less sensitive to shrinking and swelling of clay minerals because of its higher organic matter content.

The microporous structure of organic and mineral soil materials

Microporous properties of soil materials are considered important to the physical sequestration processes of contaminants and the influence on risk assessment for chemicals in the environment. We studied the microporous properties of five organic soil materials and two agricultural topsoils and thei

Pyrene Sorption to Water-Dispersible Colloids: Effect of Solution Chemistry and Organic Matter

Chemical sorption to mobile soil colloids is a controlling factor for colloid-facilitated chemical transport in the vadose zone and groundwater. We investigated sorption of pyrene to soil colloid suspensions originating from soils differing in organic matter content for different solution chemistries. Colloids were obtained from two soils with different organic matter contents but similar geological histories by three different methods: (i) chemical dispersion, (ii) mechanical dispersion in water, and (iii) spontaneous release in water. Batch sorption experiments were conducted at five pyrene concentrations, in either pure water or at two different concentrations of K + and Ca 2+ . Generally, K + addition enhanced pyrene sorption, whereas Ca 2+ addition decreased sorption. The chemically dispersed colloids exhibited the highest pyrene sorption capacity and had the most nonlinear sorption isotherms, whereas whole soil had the most linear isotherm. Model calculations of the potential amounts of leachable pyrene illustrated the importance of including both colloid- and dissolved organic matter (DOM)-facilitated transport in risk assessment models when dealing with pyrene transport. The leaching potential of dissolved pyrene (with no DOM- and colloid-facilitated transport) was 5% of the leaching potential when both DOM- and colloid-sorbed pyrene was included.

Coupling transport and biodegradation of toluene and trichloroethylene in unsaturated soils

Trichloroethylene (TCE), a common groundwater pollutant generally resistant to aerobic biodegradation, can be cometabolized in the presence of another compound such as toluene. The coupled transport and biodegradation of TCE and toluene was investigated and modeled in laboratory soil columns. Toluene biodegradation was linked to microbial growth using Monod kinetics, while TCE degradation was described using Michaelis-Menten kinetics modified to account for changing enzyme levels. Biodegradation of TCE was modeled as a mass fraction of the toluene degradation rate. Both growth and decay were incorporated into the equations to model microbial population dynamics. With the exception of the initial biomass, a single set of parameters to describe both degradation functions was obtained from independent soil batch experiments. Physical parameters were obtained from sterile soil columns. The initial biomass declined from the inlet to the outlet side of the chamber. Toluene was fully degraded in the soil column with the majority occurring closest to the inlet chamber. A substantial amount of TCE was not degraded because it diffused faster and was transformed at a lower rate than toluene.

Colloids and Colloid-Facilitated Transport of Contaminants in Soils

Until some two decades ago, it was believed that only the soil liquid and gaseous phases were mobile and could facilitate the transport of chemicals and nutrients through the vadose zone. It is now generally accepted that also part of the soil solid phase is mobile, and that mobile organic and inorganic soil colloids may facilitate chemical transport. However, the magnitude and significance of these colloidal transport processes are yet to be determined. It is essential to examine whether current models for transport and fate of chemicals in soil and groundwater need to be revised. The collection of papers in this special section of Vadose Zone Journal mainly take their origin, but not exclusively, from an international workshop “Colloids and Colloid-Facilitated Transport of Contaminants in Soil and Sediments” held at the Danish Institute of Agricultural Sciences, Denmark, 19–20 Sept. 2002. The workshop was organized to review our present knowledge of colloid behavior and transport in porous media and the possibility of colloid-bound transport of contaminants and nutrients in soil and groundwater. Here we will first give a brief introduction to the topic of mobilization and transport of colloids in the vadose zone, and highlight previous evidence of colloid-facilitated transport. We then introduce the review and technical papers in the special section. We hope that the information provided in this special section will lead to improvements in our understanding and associated conceptual models of contaminant transport and fate in soil.

Mobilization and transport of natural colloids in a macroporous soil

Abstract Colloid-mediated transport has been suggested as an important process in the leaching of adsorbed contaminants. Mobilization and transport of in situ colloids was quantified in intact soil columns (18.3 cm diameter, 20 cm length) of a sandy loam soil. In the experiments irrigation intensities of either 11 or 30 mm per hour were used on samples from 2–22 cm and from or 42–62 cm depth. Furthermore, the effect of flow interruptions of 1 2 hour, 1 day or 7 days duration were investigated. The stop-flow experiments resulted in significantly higher initial particle concentrations after longer flow interruptions, which supports the theory that colloid mobilization is governed by a time-dependent, desorption/diffusion-type release process at the macropore walls and that the hydraulic forces from introducing macropore flow are of minor importance. There was no significant effect of irrigation intensity on the amount of particles mobilized when normalized to volume outflow. This suggest that the effect from time-dependent mobilization is counterbalanced by a flow-dependent effect of hydraulic shear stress or thickness of the immobile water film and thereby a reduced effective diffusion length of the mobilized colloids at higher flow rates.

Colloid-facilitated transport of pesticide in undisturbed soil columns

Abstract The purpose of this work was to verify whether facilitated transport enhances the vertical movement of a relatively strongly adsorbing pesticide, and to study whether ionic strength and pH affects the pesticide and particle transport. Experiments were carried out with 20*20 cm undisturbed soil columns taken from the topsoil (sandy loam, typic Hapludalf) from a field under normal cultivation near Rogen, Denmark. The selected pesticide, prochloraz, was applied to the surface as a pulse in solution. Facilitated transport was significant, but was not dominating the transport of the pesticide: about 10% of the pesticide was bound to particles with diameter d > 0.24 μm. Preferential flow and particle transport were the two most important factors determining the amount of pesticide leached. Decreasing ionic strength and increasing pH promoted leaching of particles and pesticide.