Effects of low-molecular-weight organic acids on the transformation and phosphate retention of iron (hydr)oxides.

The retention and mobilization of phosphate in soils are closely associated with the adsorption of iron (hydr)oxides and root exudation of low-molecular-weight organic acids (LMWOAs). This study investigated the role of LMWOAs in phosphate mobilization under incubation and field conditions. LMWOAs-mediated iron (hydr)oxide transformation and phosphate adsorption experiments revealed that the presence of LMWOAs decreased the phosphate adsorption capacity of iron (hydr)oxides by up to ~74 % due to the competition effect, while LMWOAs-induced iron mineral transformation resulted in an approximately six-fold increase in phosphate retention by decreasing the crystallinity and increasing the surface reactivity. Root simulation in rhizobox experiments demonstrated that LMWOAs can alter the contents of different extractable phosphate species and iron components, leading to 10 % ~ 30 % decreases in available phosphate in the near root region of two tested soils. Field experiments showed that crop covering between mango tree rows promoted the exudation of LMWOAs from mango roots. In addition, crop covering increased the contents of total phosphate and available phosphate by 9.08 % ~ 61.20 % and 34.33 % ~ 147.33 % in the rhizosphere soils of mango trees, respectively. These findings bridge the microscale and field scale to understand the delicate LMWOAs-mediated balance between the retention and mobilization of phosphate on iron (hydr)oxide surface, thereby providing important implications for mitigating the low utilization efficiency of phosphate in iron-rich soils.

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.

Generic CD-MUSIC-eSGC model parameters to predict the surface reactivity of iron (hydr)oxides.

The surface reactivity of iron (hydr)oxides plays a crucial role in controlling their interfacial reactions, for which various surface complexation models have been developed. The diversity of mineralogical properties of iron (hydr)oxides has resulted in a redundancy of model parameters, which hampers the modeling of iron (hydr)oxides in soils and sediments, where goethite, hematite and ferrihydrite dominate the iron (hydr)oxide mass fraction. To capture their combined surface reactivity, optimized generic protonation parameters of the Charge Distribution-Multisite Complexation (CD-MUSIC) extended-Stern-Gouy-Chapman (eSGC) model were derived by reanalyzing literature datasets and tested with some newly synthesized iron (hydr)oxides. It was observed that the proton and monovalent ion affinity constants of the different iron (hydr)oxides were located in a narrow range. For the singly- and triply-coordinated hydroxyl sites the obtained generic log(affinity constants) were 8.3 and 11.7 for the protonation reaction and -0.5 for the reaction with the monovalent background ions. Their combination with fixed site densities of singly-/triply-coordinated hydroxyl sites of 3.45/2.70, 5.00/2.50, and 5.80/1.40 sites/nm2 for goethite, hematite, and ferrihydrite, respectively, provided good results. The Stern layer capacitances of the inner and outer Stern layers were set equal and could be acquired by an empirical correlation with the sample specific surface area (SSA). The CD-MUSIC-eSGC model with the generic model parameters enables good quality predictions of the proton reactivity of iron (hydr)oxides in 1:1 electrolyte solutions regardless of the sample heterogeneity. The advantages of the generic CD-MUSIC-eSGC model are twofold: (1) protonation of iron (hydr)oxides can be described without making use of spectroscopic measurements and proton titrations, and (2) the model calculations are greatly simplified.

Complexation mechanism of Pb2+ at the ferrihydrite-water interface: The role of Al-substitution.

Ferrihydrite is a poorly crystalline iron (hydr)oxide and highly efficient adsorbent for heavy metals. Al-substitution in ferrihydrite is ubiquitous in nature. However, the effect of Al-substitution on the surface reactivity of ferrihydrite remains unclear due to its low crystallinity. The present study aims to clarify the microstructure and interfacial reaction of Al-substituted ferrihydrite. Al-substitution had little effect on the morphology and surface site density of ferrihydrite, while the presence of ≡AlOH-0.5 sites resulted in higher proton affinity and surface positive charge of ferrihydrite. Besides, the affinity constant of Pb2+ adsorption on the surface of ferrihydrite decreased at higher Al content, which further decreased the adsorption performance of ferrihydrite for Pb2+. The modeling results revealed that bidentate complex was the dominant Pb complexation species on the surface of ferrihydrite, which was less affected by Al-substitution. The present study provides important insights into the effect of Al-substitution on the interfacial reaction at the ferrihydrite-water interface. The obtained parameters may facilitate the future advance of surface complexation model.

Microstructure of Al-substituted goethite and its adsorption performance for Pb(II) and As(V).

Naturally occurring goethite commonly undergoes Al-substitution, while how changes in microstructure induced by Al-substitution affect the interactive reaction of Pb(II) or As(V) at the goethite-water interface remains poorly understood. This study reveals the structural properties of Al-substituted goethite and its adsorption behavior for Pb(II) and As(V) by multiple characterization techniques and Charge Distribution-Multisite Surface Complexation (CD-MUSIC) modeling. Al-substitution caused an obvious decrease in the length-to-width ratio in goethite particles and a slight decrease in the proportion of (110) facets. The presence of Al-O sites and higher surface roughness induced by Al-substitution contributed to a higher inner Stern layer capacitance (C1) and surface charge density of goethite. CD-MUSIC modeling results further revealed that the affinity constant of Pb(II) complex (log KPb) at the goethite-water interface and the adsorption capacity of goethite for Pb(II) decreased with increasing amount of Al-substitution, while an opposite tendency was observed for As(V) adsorption. The dominant species of both Pb(II) and As(V) on goethite were bidentate complexes, and Al-substitution had a minor impact on the abundance of Pb(II) and As(V) complexes on the surface of goethite. Overall, these experimental and modeling results provide new and important insights into the interfacial reactivity of Al-substituted goethite and facilitate the prediction of the environmental fate of heavy metals.

Insights into the improving mechanism of defect-mediated As(V) adsorption on hematite nanoplates.

The fate of As(V) in subsurface environments is strongly affected by ubiquitous iron oxides. Defects are commonly present in natural hematite, while the impacts of defects on the active sites and complexation mechanism of hematite for As(V) remain poorly understood. In this study, the defect-rich hematite was employed to investigate the surface charge characteristics and As(V) adsorption behavior using potentiometric acid-base titration and CD-MUSIC model in comparison with corresponding defect-poor hematite. The total arsenate-active site density (5.7 sites/nm2) on defective hematite includes 1.2 sites/nm2 of original sites and 4.5 sites/nm2 of Fe vacancy-induced sites. The result revealed that the vacant Fe3+ sites in defective hematite was compensated by the protons in solution, thus resulting in a considerable increase in site density as well as positive charge. The CD-MUSIC modeling results demonstrated that the presence of Fe vacancies in hematite is beneficial to the improvement in affinity constants for both monodentate and bidentate arsenate complexes. The high adsorption capacity of defective hematite (2.60 µmol/m2) compared to defect-free hematite (1.33 µmol/m2) is attributed to its large affinity constants as well as its more active surface sites, thereby playing a vital role in reducing the threats of heavy metals in the environment.