Mechanisms of Dissolved Organic Carbon Adsorption on Soil

Abstract

The subsurface transport of inorganic and organic contaminants may be strongly related to the movement of dissolved organic carbon (DOC) through a soil profile. A variety of soil chemical and hydrologic factors control the mobility of the DOC, which may enhance or impede the transport of the associated contaminants. In this study, the sources of DOC adsorption on two proposed waste‐site soils are defined, and the chemical mechanisms operative during the adsorption process are specified. Adsorption isotherms for the two soils determined at constant pH, ionic strength (I), and temperature indicated that DOC adsorption increased with increasing soil profile depth. Different adsorption capacities were exhibited by the two soils, however, which was related to their contrasting indigenous organic matter contents and mineralogies. The adsorption of DOC by the soils was not a function of solution I (I = 0.001 to 0.1 mol L−1 using NaCl); however, DOC adsorption was dependent on solution pH, with maximum adsorption occurring at ≃4.5. Competitive ion‐exchange studies using Na2SO4 as an ionic‐strength adjuster suggested that a portion of the DOC was electrostatically bound to the soil via anion exchange. By using thermodynamic principles, the predominant mechanism of DOC retention by the soil was found to be physical adsorption driven by favorable entropy changes. This is supported by preferential adsorption of the hydrophobic organic solutes to the soil relative to the hydrophilic organic solutes.

Transport of soluble organic and inorganic carbon in sandy soils under nitrogen fertilization

“…Solution pH may influence leaching of SOC, because pH affects the adsorption of SOC by soil particles. The adsorption of SOC by soil particles was reported to be maximum at pH 4.5 for a Tennessee soil (loamy, thermic Typic Paleudult) (Jardine et al 1989). In the present study, the pH of the leachate decreased over time in the Wabasso sand, which may have influenced the reduction in SOC leaching (Figs.…”

“…Jardine et al (1989) observed in a Tennessee soil (loamy, thermic Typic Paleudult) that SOC adsorption by soil decreased as the concentration of added SO 4 -2 increased, suggesting that anion exchange was a mechanism of the SOC desorption. Qualls and Haines (1991) reported that many of the compounds separated from SOC in forest soils were amino acids or had carboxylic function groups -compounds usually negatively charged at moderate to high pH.…”

“…1 and 3). Although low ionic strength of a soil solution may increase SOC leaching by enhancing desorption of SOC from the soil (Evans and Sorensen 1986), Jardine et al (1989) observed that adsorption of SOC by soil was not a function of ionic strength of soil solution within a range of 1 to 100 mmol L -1 . The variation in ionic strength observed in this study was within this range (Fig.…”

Origin and properties of humus in the subsoil of irrigated rice paddies

“…The indigenous organic matter was reported to suppress the adsorption of DOC in the subsoil under forests (Jardine et al 1989). However, the amount of DOC retained was larger in the untreated subsoil column than in the subsoil columns without organic matter in the present experiment as described above (Tables 1 and 3).…”

Sources and Transformation of Dissolved Organic Carbon in the Harp Lake Forested Catchment: The Role of Soils

“…Sorption and desorption exchange equilibria are determined by many factors, including soil mineralogy, organic carbon content, solution pH and sorption kinetics (Nodvin, Driscoll & Likens 1986;Jardine, Weber & McCarthy 1989). If the source of organic matter in the B horizon of Harp Lake soils is primarily the accumulation of leachates from the A and litter horizons, we may estimate magnitude of the sink for DOC by sorption.…”