Key factors in the adsorption of natural organic matter to metal (hydr)oxides: Fractionation and conformational change

Assessment of the sorptive fractionation of organic matter at the goethite-water interface

The carboxylic (COOH) and phenolic (PhOH) functional groups of natural organic matter (NOM) are the major binding sites for environmental compounds such as minerals and contaminants. This study focusses on the sorptive fractionation of a humic acid onto goethite (α-FeOOH) at various pH, [NOM], [NaCl], and under static (batch) and dynamic flow (column) conditions. During batch experiments at low pH and [NOM], adsorption behaviors of COOH- and PhOH-rich compounds were similar. At low [NaCl] and high pH or [NOM], the adsorption of NOM decreased, with a preferential adsorption of PhOH over COOH-rich compounds. This can be ascribed to the surface site saturation and competition between COOH, PhOH and OH- for surface binding, combined with electrostatic effects. Higher [NaCl], which was previously shown to favor NOM-NOM interactions at mineral surface, led to increased adsorption of COOH, while the adsorption of PhOH was not affected. Under flow-through conditions, multiple-binding-site mechanism was shown to control the fractionation of PhOH- and COOH-rich compounds. These results provide insights into the molecular interactions between the NOM and soil components, which control the fate and behavior of contaminants and nutrients in the environment.

Review of Fe/Mn-based chemical stabilizers for remediating arsenic and antimony co-contaminated soil

Arsenic (As) and antimony (Sb) frequently co-occur in soil contaminated by mining, smelting and traffic emissions, creating an urgent need for effectively simultaneous remediation strategies. Although chemical stabilization has garnered significant attention for its high remediation efficiency, a systematic comparison of stabilization effects and mechanisms for As and Sb in co-contaminated soil remained unexplored. Iron-based materials are widely recognized as the most effective stabilizers for As and Sb in soil. Meanwhile, manganese-based materials, owing to their superior oxidizing capacity that maintains As and Sb in the less toxic pentavalent species, have also attracted considerable interest. Iron-manganese-based materials provide an efficient solution for stabilizing As and Sb in soil by synergistically combining the advantages of both iron and manganese components. This review therefore elaborated on the core stabilizers, including iron-, manganese-, and iron-manganese-based materials. The stabilization efficiencies and underlying mechanisms of As and Sb in soil were comprehensively examined, with key environmental factors also discussed in relation to their stabilization performance. As widely used soil amendments, raw biochar and clay materials demonstrate limited efficacy in stabilizing As and Sb in soil. However, they can be employed as functional modifiers to enhance the dispersion of iron or manganese particles, thereby improving stabilization performance. While current progress is systematically evaluated, the development of stabilizers enabling simultaneous immobilization of As and Sb remains a critical research priority for future research.

pH-Dependent preferential adsorption and stability of humic substances on goethite: The dual role of aromatic and aliphatic moieties

Preferential adsorption occurs during interaction between humic substances (HS) and metal (hydr)oxides, however, the preferential adsorption and thermal stability mechanism remains poorly understood. Here we show that solution pH can modify the configuration of humic acid (HA) and fulvic acid (FA), and thereby altering their fractionation and thermal stability in relation to goethite. At pH 5-7, particles with a molar mass of 3.5-15 kDa were preferentially adsorbed for both fulvic acid (FA) and humic acid (HA). These particles enriched in aromatic moieties and carboxylic groups (-COOH), which enhanced the stability of HS by facilitating the formation of inner-sphere complexes. Conversely, at pH levels exceeding 9.2, large particles (> 50 kDa) and those rich in aliphatic moieties were preferentially adsorbed for both HA and FA, attributed to the increased significance of hydrophobic interactions. However, this stabilization effect on HS was limited under these conditions. FA particles exhibit a lower degree of chemical heterogeneity compared to HA particles. For FA, the variations in aromatic structures (-COOH) and aliphatic moieties were coupled, following a consistent trend in preferential adsorption as pH changed. In contrast, HA demonstrated an opposite pH dependency, with aromatic (-COOH) and aliphatic moieties displaying divergent patterns of preferential adsorption. These innovative discoveries clarify the behavior of organic matter in various environmental conditions and its interactions with minerals surfaces, thereby establishing a critical theoretical foundation for understanding the sequestration of soil organic carbon, emphasizing the dual roles of aromatic and aliphatic moieties in the preferential adsorption and stability of humic substances on goethite.

Natural Organic Matter Stabilizes Pristine Nanoplastics but Destabilizes Photochemical Weathered Nanoplastics in Monovalent Electrolyte Solutions

Photochemical weathering and eco-corona formation through natural organic matter (NOM) adsorption play vital roles in the aggregation tendencies of nanoplastics (NPs) in aquatic environments. However, it remains unclear how photochemical weathering alters the adsorption patterns of NOM and the conformation of the eco-corona, subsequently affecting the aggregation tendencies of NPs. This study examined the effect of Suwannee River NOM adsorption on the aggregation kinetics of pristine and photoaged polystyrene (PS) NPs in monovalent electrolyte solutions. The results showed that photochemical weathering influenced the conformation of the eco-corona, which, in turn, determined NP stability in the presence of NOM. Hydrophobic components of NOM predominantly bound to pristine NPs through hydrophobic and π-π interactions, and extended hydrophilic segments in water hindered NP aggregation via steric repulsion. Conversely, hydrogen bonding facilitated the binding of these hydrophilic segments to multiple photoaged NPs, thereby destabilizing them through polymer bridging. Additionally, the stabilization and destabilization capacities of NOM increased with its concentration and molecular weight. These findings shed light on the destabilizing role of NOM in weathered NPs, offering new perspectives on environmental colloidal chemistry and the fate of NPs in complex aquatic environments.

Regulation of the co-transport of toluene and dichloromethane by adsorbed phase humic acid under different hydro-chemical conditions

The transport behavior of combined organic pollutants in soil and groundwater has attracted significant attention in recent years. Research on the influence of humic acid (HA) on organic pollutant transport behavior mainly focuses on the study of the mobile phase HA, with less research on the adsorbed phase HA, especially regarding its interaction with combined pollutants. To enhance understanding of the regulation of co-transport and retention of combined pollutants by adsorbed phase HA, in this study, tests were conducted to investigate how toluene (TOL) and dichloromethane (DCM) are transported in the presence of adsorbed phase HA at different pH levels and ionic strengths. As the proportions of HA-coated sand increased, so did its adsorption capacity for TOL and DCM, which can be attributed to adsorbed phase HA providing more adsorption sites compared to plain sand, thereby reducing the transport potential of the pollutants. The presence of both TOL and DCM facilitated their mutual transportation due to competitive adsorption controlled by the adsorbed phase HA content in the porous medium. Furthermore, it was observed that pH levels influenced the transport behavior of TOL and DCM when adsorbed phase HA was present since adsorbed phase HA transformation into mobile phase was regulated by pH levels. The transport patterns can be effectively simulated using the chemical nonequilibrium two-site sorption model in HYDRUS-1D, accurately reflecting the retardation coefficients and transport distances based on model parameters. This work sheds new light on the regulatory role of adsorbed phase HA in TOL and DCM transport under diverse hydrochemical conditions, with implications for accurately depicting the behavior of combined pollutants, optimizing the remediation strategies and improving remediation efficiency in contaminated sites.

Modeling of phosphate speciation on goethite surface: Effects of humic acid

Iron (hydr)oxides and humic acid (HA) are important active components in soils and usually coexist in the environment. The effects of HA on the adsorption and subsequent immobilization of phosphate on iron (hydr)oxide surface are of great importance in studies of soil fertility and eutrophication. In this study, two types of goethite with different particle sizes were prepared to investigate the phosphate adsorption behaviors and complexation mechanisms in the absence or presence of HA by combining multiple characterization and modeling studies. The adsorption capacity of micro- (M-Goe) and nano-sized goethite (N-Goe) for phosphate was 2.02 and 2.04 μmol/m2, which decreased by ∼25% and ∼45% in the presence of 100 and 200 mg/L HA, respectively. Moreover, an increase in equilibrium phosphate concentration significantly decreased the adsorption amount of goethite for HA. Charge distribution-multisite surface complexation (CD-MUSIC) and natural organic matter-charge distribution (NOM-CD) modeling identified five phosphate complexes and their corresponding affinity constants (logKP). Among these phosphate complexes, FeOPO2OH, (FeO)2PO2, and (FeO)2POOH species were predominant complexes on the surface of both M-Goe and N-Goe across a wide range of pH and initial phosphate concentrations. The presence of HA had little effect on the coordination mode and logKP of phosphate on goethite surface. These results and the obtained model parameters shed new lights on the interfacial reactivity of phosphate at the goethite-water interface in the presence of HA, and may facilitate further prediction of the environmental fate of phosphate in soils and sediments.

A new consistent modeling framework for the competitive adsorption of humic nanoparticles and oxyanions to metal (hydr)oxides: Multiple modes of heterogeneity, fractionation, and conformational change

HYPOTHESIS

The competitive interaction of oxyanions and humic nanoparticles (HNPs) with metal (hydr)oxide surfaces can be used to trace the ligand and charge distribution of adsorbed HNPs in relation to heterogeneity, fractionation, and conformational change.

EXPERIMENTS

Batch adsorption experiments of HNPs on goethite were performed in the absence and presence of phosphate. The size of HNPs was measured with size exclusion chromatography. The Ligand and Charge Distribution (LCD) model framework was further developed to describe the simultaneous interaction of HNPs and phosphate with goethite.

FINDINGS

Preferential adsorption decreases the mean molar mass of adsorbed HNPs, independent of the phosphate presence, showing a linear dependency on the adsorbed HNPs fraction. Phosphate ion can be used as a probe to trace the distribution of functional groups and the variation in affinity of HNPs. The spatial distribution of adsorbed HNPs is driven by the potential gradients in the electrical double layer, which changes the conformation of the adsorbed HNPs. At the particle level, the adsorption of heterogeneous HNPs has an affinity distribution, which can be explained by the variation in molar mass (kDa) and density of the functional groups (mol kg-1) of the HNPs. The presented model can simultaneously describe the competitive adsorption of HNPs and phosphate in a consistent manner.