Particle and structure characterization of fulvic acids from agricultural soils

Purpose

Structural studies on fulvic acids (FAs) are of significant importance since FAs are involved in many environmentally important processes, such as adsorption and transportation of nutrients, trace elements, and organic pollutants. Interactions between suspended and dissolved soil components are controlled by a variety of attractive and repulsive interparticle forces influenced partially by FA properties. The aim of this paper is a detailed characterization of FAs derived from agricultural soils varied with physicochemical properties.

Materials and methods

Forty topsoils (pH_KCL?=?3.8–7.8, clay content?=?0–6%, and TOC?=?7.0–187.2 g kg^?1) were collected from rural area. Fulvic acids (FAs) were isolated according to the IHSS method. The overall FA solutions were purified by nonionic macroporous acrylic ester resin (DAX-8), and the organic carbon content in FA fraction (FA-OC) was determined by a liquid C–N analyzer. The particle size diameter (PSD) and polydispersity (PDI) were analyzed by a dynamic light scattering technique, while the zeta potential (ZP) was measured using an electrophoretic light scattering method. Spectroscopic properties of FAs, including occurrence and distribution of functional groups, were investigated by near-IR spectroscopy.

Results and discussion

Agricultural soils differed substantially, with FA-OC content ranging from 0.6 to 8.8 g kg^?1 that accounted for 0.5 to 22.6% of TOC. The PSD exhibited wide range of particle size (0.2 to 69.6 nm) and was characterized by different polydispersity (14–183.1%). The ZP described the behavior, and surface charge of FA particles varied from ??1.7 to +?3.3 mV. Low ZP characterized 77% of FAs and indicated the ease of aggregate formation and intermolecular connections. The measured ZP also showed that suspended organic particles of FAs had both positive and negative charges, which was confirmed by the spectroscopic analysis. The presence of negative charges on FA particle surfaces was connected with the occurrence of phenol and carboxyl groups while positive charge with amine.

Conclusions

Detailed characterization of FAs from agricultural soils confirms their heterogeneous and complex nature. The results indicate that FAs mainly exist as small molecules that form molecular aggregates or associations in solutions. FA in a solution of a similar ionic strength may be positively or negatively charged due to its chemical structure and aggregate behavior which affects their properties in the soil.

     

Dissipation and sorption processes of polycyclic aromatic hydrocarbons (PAHs) to organic matter in soils amended by exogenous rich-carbon material

Purpose

The aim of the research was to assess the effect of biochar addition on aging, degradation, and sorption processes of polycyclic aromatic hydrocarbons (PAHs) to soil organic matter. The study was carried out as a sorption experiment in strictly controlled water and air conditions, which allowed for the accurate observation and prediction of PAH behavior in soils.

Materials and methods

Four soils were fortified with a PAH mixture (Fluorene-Flu, Anthracene-Ant, Phenanthrene-Phe, Pyrene-Pyr, Chrysene-Chry) at 20 mg kg^?1 of single-compound concentration level. The experiment was carried out in two trials: soils?+?5PAHs amended with biochar and soil?+?5PAHs without biochar addition with incubation times of 0, 1, 3, 6, and 9 months. After each interval time, the extractable (E-SOM) and stable organic matter (S-SOM) were measured as well as PAHs determined in two forms: total concentration (PAH-tot) and residual concentration (PAH-rest) after E-SOM extraction. The PAH loss and half-life times were estimated according to pseudo first-order kinetics equation.

Results and discussion

The amounts of PAH-tot in the soils without biochar decreased by an average of 92%, while in soil with biochar, this was 41% after 9 months of aging. The amount of PAHs-rest bounded with S-SOM after 9 months of incubation varied from 0.9 to 3.5% and 0.2 to 1.3% of the initial PAH concentration, respectively, for soils non-induced and induced by biochar. In soils without biochar, Flu, Ant, Phe, and Pyr exhibited similar T_1/2 (43–59 days), but Chry was characterized by a much higher and broader T_1/2 than other hydrocarbons (67–280 days). Biochar addition to the soils significantly influenced the half-life changes for all PAHs. The highest changes were noted for Phe (14-fold increase), and the lowest was for Flu (7-fold increase).

Conclusions

The addition of exogenous-rich carbon material such as biochar to the soil significantly changes the behavior and sorption potential of PAHs in the soil. Soils enriched with biochar are characterized by a higher persistence of PAHs, longer aging time, and lower affinity for sorption by native organic matter structures. Soils freshly polluted by PAH are mainly sorbed by E-SOM, which significantly increases their accessibility and reduces formation of bound-residues in the soil.