Solid-solution Partitioning of Organic Matter in Soils as Influenced by an Increase in pH or Ca Concentration

Abstract

Organic matter is an important component of soil with regard to the binding of contaminants. Hence, the partitioning of organic matter influences the partitioning of soil contaminants. The partitioning of organic matter is, among other factors, influenced by the ionic composition and ionic strength of the soil solution. This study focuses on the behavior of organic matter after a change in the ionic composition of the soil solution, particularly in Ca concentration and pH. Different amounts of Ca(NO3)2 and NaOH were added to soil suspensions. The dissolved organic carbon (DOC) concentration increased with increasing pH (addition of NaOH), whereas an increase in Ca (addition of Ca(NO3)2) had the opposite effect. A stronger increase in DOC was observed if a single dose of NaOH was added, compared to a gradual addition of the same amount of NaOH. Cation binding by organic matter in the supernatant was calculated using the NICA-Donnan model. The log DOC concentration appeared to be correlated to the Donnan potential, calculated under the assumption that all DOC equals humic acid. This correlation was found for all eight neutral to acidic soils used in this study, although the slopes and elevations of the regression lines varied. The slope varied by a factor of 2 and the elevation appeared to be strongly influenced by the DOC concentration in the untreated soils, which is related to the total organic matter in the soil. Finally, we predicted the Donnan potential on the basis of an extraction of untreated soil with 0.03 M NaNO3, and the total additions of Ca(NO3)2 and NaOH. Comparison of these predictions with speciation calculations in solution showed a good correlation, indicating that a combination of one batch experiment and the presented calculation procedure can provide good estimations of DOC concentrations after addition of chemicals.

Sorption interaction of phenanthrene with soil and sediment of different particle sizes and in various CaCl2 solutions

“…One explanation for these observations is that Ca 2+ can be adsorbed directly onto the particle interface and reduce the electrostatic repulsion between humic substances and negatively charged surface of solids. An increase in Ca 2+ concentration enhances the sorption of humics to solid particles and decreases DOC content [29,35]. Furthermore, the solution CaCl 2 concentration strongly influences the surface characters of Beizhai soil and Guanting sediment.…”

“…The presence of calcium significantly influences the configuration of dissolved humic substances, which may in turn affect the structure of organic matter on the surface of solid particles and the sorption of HOCs to sorbents [26][27][28][29][30][40][41][42]. Ca 2+ can neutralize the negative charge of humic molecules and reduce the electrostatic repulsion between humic molecules, which may induce aggregate formation and generate an expanded structure with hydrophobic interior voids [24,29,30],…”

“…The critical concentration was found at 7.2 mmol/l for Ca 2+ at pH 7 and at pH 4, 3 mmol/l Ca 2+ is enough to induce coagulation. The coagulated humics are more differently accessed due to their more condensed structure [29]. In addition, the configuration change of humic substances in solution subsequently influences the structure of humics on mineral surfaces.…”

“…Furthermore, the extent of HOC sorption shows a strong dependence on the solution parameters, such as pH [24][25][26][27][28][29], ionic strength (I ) [23,[26][27][28], electrolyte cation valence [26,29,30], humic concentration [26,31], cosolute [32], and others. The cation, such as H + and Ca 2+ , can interact with carboxyl/hydroxyl functional groups on dissolved humic substances and mineral surfaces and reduce the negative electrostatic repulsion between the humic molecules and mineral interfaces.…”

“…The cation, such as H + and Ca 2+ , can interact with carboxyl/hydroxyl functional groups on dissolved humic substances and mineral surfaces and reduce the negative electrostatic repulsion between the humic molecules and mineral interfaces. A decrease in the cation concentration will result in both dissolution and desorption of organic matter, which in turn affects the sorption of hydrophobic chemicals to natural solids [26,29]. A change in pH or ionic strength of the soil solution leads to different configuration of dissolved humics ("open" or "closed"), which may subsequently alter the structure of organic matter on the mineral surfaces [25,27,30].…”