Acid–base and copper-binding properties of three organic matter fractions isolated from a forest floor soil solution

Abstract

ABSTRACT.Vast amounts of knowledge about the proton-and metal-binding properties of dissolved organic matter (DOM) in natural waters have been obtained in studies on isolated humic and fulvic (hydrophobic) acids. Although macromolecular hydrophilic acids normally make up about one-third of DOM, their proton-and metal-binding properties are poorly known. Here we investigated the acid-base and Cu-binding properties of the hydrophobic (fulvic) acid fraction and two hydrophilic fractions isolated from a soil solution. Proton titrations revealed a higher total charge for the hydrophilic acid fractions than for the hydrophobic acid fraction. The most hydrophilic fraction appeared to be dominated by weak acid sites, as evidenced by increased slope of the curve of surface charge versus pH at pH values above 6. The titration curves were poorly predicted by both Stockholm Humic Model (SHM) and NICA-Donnan model calculations using generic parameter values, but could be modelled accurately after optimisation of the proton-binding parameters (pH ≤ 9). Cu-binding isotherms for the three fractions were determined at pH values of 4, 6 and 9. With the optimised proton-binding parameters, the SHM model predictions for Cu binding improved, whereas the NICA-Donnan predictions deteriorated. After optimisation of Cu-binding parameters, both models described the experimental data satisfactorily. Iron(III) and aluminium competed strongly with Cu for binding sites at both pH 4 and pH 6. The SHM model predicted this competition reasonably well, but the NICA-Donnan model underestimated the effects significantly at pH 6. Overall, the Cu-binding behaviour of the two hydrophilic acid fractions was very similar to that of the hydrophobic acid fraction, despite the differences observed in 3 proton-binding characteristics. These results show that for modelling purposes, it is essential to include the hydrophilic acid fraction in the pool of 'active' humic substances.

Influence of Soil Chemistry and Plant Physiology in the Phytoremediation of Cu, Mn, and Zn

“…Generally, increasing the pH decreases the solubility of Cu in soil . Most common forms of Cu in soil solutions are soluble organic chelates of this metal (Kasmaei and Fekri, 2012;van Schaik et al, 2010). The two major parameters that influence the variability of total Cu concentration as well as the available Cu concentration in soil are the CEC and SOM (Bravin et al, 2012;Vega et al, 2010;Wu et al, 2010).…”

“…Therefore, the mineral composition (particularly the content of other metal cations) will influence the availability of Cu in soil (Merdy et al, 2009). Cu is not readily leached from the soil horizons due to its high affinity for OM, and it tends to accumulate in surface soils (van Schaik et al, 2010). Figure 4 addresses the main processes that control Cu distribution within the plant.…”

Increasing concentrations of arsenic and vanadium in (southern) Swedish streams

“…The SHM was needed to calculate the partitioning of Fe between colloidal ferrihydrite and the dissolved phase, as dissolved Fe(III)-organic complexes represent important Fe species in natural waters. In the SHM complexation constants for monomeric Fe(III)-organic complexes according to van Schaik et al (2010) were used. Furthermore the model also considers the complexation of V(IV) to DOM, using results obtained for V(IV) binding to soil organic matter .…”

Effect of dissolved organic matter composition on metal speciation in soil solutions

“…Moreover, the nonhumic, hydrophilic fractions, generally representing 40-70% of the DOM in soil solutions, have been found to be dominant ligands in complexing metals in some cases (Croué et al, 2002;van Schaik et al, 2010;Pernet-Coudrier et al, 2011). However, due to the difficulties of their isolation, little exploratory work has been reported in the literature regarding the metal binding properties of hydrophilic fractions and the range of variability of their quality and quantity in the environment, which is related to the source of DOM (i.e., autochthonous and anthropogenic) (Baken et al, 2011).…”