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

Interaction of polystyrene nanoplastics with impurity-bearing ferrihydrite and implication on complex particle sedimentation

Nanoplastics (NPs) usually coexist with impurity-bearing ferrihydrite (ImFh), and their interaction is related to their environmental fate. In this study, the aggregation between ImFh (impurities: Al, Mn and Si) and polystyrene nanoplastics (PSNPs), as well as the sedimentation of ImFh-PSNP complex particles in the aqueous phase were investigated systematically with particle concentrations of 100 mg/L ImFh and 10 mg/L PSNPs. Our results revealed that the PSNP suspension was dispersive and stable under various pH values and low ion strength. After coexisting with ImFh, PSNPs aggregated with the positively charged ImFh to form ImFh-PSNP complex particles, which destroyed the stability of PSNPs. The increase in pH and Na+ concentration could inhibit their aggregation, but high Na+ concentration (>20 mM) caused the homoaggregation of PSNPs. The aggregation capacity of PSNPs with ImFh was in the order of Al-bearing Fh > Fh > Mn-bearing Fh > Si-bearing Fh. Zeta potential and Derjaguin-Landau-Verwey-Overbeek (DLVO) calculations indicated that Al-bearing Fh showed higher positive potential than pure Fh, which caused stronger electrostatic interactions with PSNPs. However, Mn and Si in ImFh decreased the positive potential and inhibited the electrostatic interaction with PSNPs, and the effect of Si was greater than that of Mn. The aggregation between ImFh and PSNPs inhibited the sedimentation of their complex particles, and the higher aggregation capacity appeared to have a greater inhibition degree. Due to the "electrostatic patches" effect of PSNPs, the energy barrier of the ImFh-PSNPs particles was higher than that of the ImFh particles. Our findings clarified the influence of impurities on the interaction between ImFh and PSNPs and provided insight regarding their fate in the environment.

Adsorption Mechanisms of Glyphosate on Ferrihydrite: Effects of Al Substitution and Aggregation State

Ferrihydrite is one of the most reactive iron (Fe) (oxyhydr)oxides in soils, but the adsorption mechanisms of glyphosate, the most widely used herbicide, on ferrihydrite remain unknown. Here, we determined the adsorption mechanisms of glyphosate on pristine and Al-substituted ferrihydrites with aggregated and dispersed states using macroscopic adsorption experiments, zeta potential, phosphorus K-edge X-ray absorption near-edge structure spectroscopy, in situ attenuated total reflectance Fourier transform infrared spectroscopy coupled with two-dimensional correlation spectroscopy, and multivariate curve resolution analyses. Aggregation of ferrihydrite decreases the glyphosate adsorption capacity. The partial substitution of Al in ferrihydrite inhibits glyphosate adsorption on aggregated ferrihydrite due to the decrease of external specific surface area, while it promotes glyphosate adsorption on dispersed ferrihydrite, which is ascribed to the increase of surface positive charge. Glyphosate predominately forms protonated and deprotonated, depending on the sorption pH, monodentate-mononuclear complexes (MMH1/MMH0, 77-90%) on ferrihydrites, besides minor deprotonated bidentate-binuclear complexes (BBH0, 23-10%). Both Al incorporation and a low pH favor the formation of the BB complex. The adsorbed glyphosate preferentially forms the MM complex on ferrihydrite and preferentially bonds with the Al-OH sites on Al-substituted ferrihydrite. These new insights are expected to be useful in predicting the environmental fate of glyphosate in ferrihydrite-rich environments.

Combined amendment and capping of sediment with ferrihydrite and magnetite to control internal phosphorus release

This study developed novel active capping systems with recycling convenience using ferrihydrite (Fh) combined with magnetite (Mag), and investigated the effectiveness and mechanism for the restriction of endogenous phosphorus movement from sediment into overlying water (OW) by the combined use of Fh and Mag. The Fh/Mag combined amendment effectively hindered endogenous phosphorus release from sediment to OW in dissolved oxygen (DO)-deficit environment, and the immobilization of diffusion gradient in thin film-labile phosphorus (LP_DGT) and mobile phosphorus in the sediment played a key role in the control of endogenous phosphorus liberation by the Fh/Mag combined amendment. Combined capping sediment with Fh and Mag effectively hindered endogenous phosphorus release from sediment to OW in anoxic environment, and the inactivation of LP_DGT in the upper sediment played a key part in the control of sediment phosphorus release by the Fh/Mag mixture capping. The stability of phosphorus immobilized by the Fh/Mag combined covering layer was related to its construction way, and the majority (around 90%) of P immobilized to the Fh/Mag mixture covering layer had low risk of release in common pH (5-9) and DO-deficit environments. The Fh/Mag mixture amendment or capping did not increase the risk of sediment iron release, and it also did not produce a large impact on the diversity and richness of bacterial community in the sediment. The combined utilization of Fh and Mag as a composite amendment or capping material to prevent the internal phosphorus from being moved to OW can make full use of their respective advantages. The Fh/Mag mixture capping wrapped by permeable fabric has high potential to reduce the risk of endogenous phosphorus from sediment into OW due to its advantages of high internal phosphorus release suppression efficiency, environmental friendliness, application convenience and sustainability.

Mobilization of arsenic from As-containing iron minerals under irrigation: Effects of exogenous substances, redox condition, and intermittent flow

Irrigation activities can cause strong geochemical and hydrological fluctuations in the unsaturated zone, and affect arsenic (As) migration and transformation. The As geochemical cycle in the unsaturated zone is coupled with that of iron minerals through sorption-desorption, coprecipitation and redox processes. Dynamic batch experiments and wetting-drying cycling column experiments were conducted to evaluate As mobilization behaviors under the effects of exogenous substances, redox condition and intermittent flow. Our results show that As release under exogenous substances carried by irrigation (e.g., phosphate, carbonate, fulvic acid, humic acid, etc.) followed three trends with the types of exogenous inputs. Inorganic anions and organic matter resulted in opposite trends of arsenate release in different redox conditions. In anoxic environments, As(V) release was favored by the addition of phosphate and carbonate, while in oxic environments, the mobilization of As(V) was promoted by the addition of fulvic acid (FA). Further, intermittent irrigation promoted the reductive dissolution of Fe oxides and the mobilization of As. The addition of humic acid (HA) resulted in the mobilization of arsenate as As-Fe-HA ternary complexes. The mechanism of arsenic mobilization under irrigation has importance for prevention of arsenic exposure through soil to food chain transfer in typical high arsenic farmland.