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Ferrihydrite has been prevalently used in soil remediation as an effective amendment for in situ immobilization of arsenic (As). However, its poorly crystalline structure is unstable and can pose the risk of re-releasing As into soil. In this study, sequential extraction was coupled with a newly-established method, chelex-metsorb diffusive gradient in thin films (DGT), to study the ferrihydrite transformation/dissolution process and its effect on the mobility of arsenic in soil. Experiments in this work found that high soil moisture (70% SWHC) with a low soil redox potential (Eh) can significantly increase the rate of the ferrihydrite transformation/dissolution process compared to 30% SWHC. Soils with low pH and high available iron (Fe) content may also accelerate ferrihydrite transformation/dissolution, while soils with high clay fraction and high soil total organic matter (STOM) may inhibit the process. The amount of arsenic adsorption can also affect ferrihydrite transformation, even exceeding the effect of soil pH and dissolved Fe. Arsenic release was clearly observed in all three soils across all treatments, and it was also affected by changes in soil redox potential. More arsenic was released at high soil moisture (70% SWHC) roughly 7–15 d after the release of Fe. In addition, a partial arsenic fraction was transformed, along with ferrihydrite, from a combined As (F1 As) amorphous phase to combined As (F2 As) well-crystallized phase. These results suggested that ferrihydrite transformation/dissolution can affect the mobility of arsenic and that this phenomenon is more extreme at higher soil moisture levels. Unlabelled Image • Soil redox condition significantly regulated the ferrihydrite transformation/dissolution process. • Ferrihydrite transformation rate in soils was also controlled by the soil properties and amount of arsenic adsorption. • Ferrihydrite transformation/dissolution may lead to a short time release of the labile As. • Labile As release had a strong correlation with Fe(II) release. • Arsenic fractionation was also transformed with transformation of ferrihydite. [ABSTRACT FROM AUTHOR]

Ferrihydrite is commonly used as an amendment to remove or stabilize arsenic (As) in water, soils, and sediments due to its high specific surface area (>200 m 2 /g). However, its instability under oxic or anoxic conditions sometimes limits its efficiency for arsenic adsorption. This study employed a two-step sequential extraction method and the diffusive gradients in thin films (DGT) technique to investigate the effect of soil properties and water management on ferrihydrite transformation in soils. The results of sequential extraction and DGT indicated that ferrihydrite transformation and dissolution occurred within all treatments in three red soils derived from plate shale (PS), sandstone (SS) and quaternary red clay (QR). Ferrihydrite transformation and dissolution rate under 30% soil water holding capacity (SWHC) treatments were the fastest in PS followed by SS and QR, while in 70% SWHC treatments the order was SS > PS > QR. Ferrihydrite in 70% SWHC was transformed about 2–7 times faster than in 30% SWHC for the various treatments. The soil pH may be the main factor controlling ferrihydrite transformation in 30% SHWC, and in 70% SHWC it may work synergistically with Fe(II) as a result of continuous, partial anaerobic conditions formed after about 22 days. Variation among the three soils was controlled by different amounts of soil total organic matter, pH, available Fe, and the content of clay fraction (particulate size <0.02 mm). In the present study, SWHC was expected to be the direct factor controlling ferrihydrite transformation and dissolution process. However, the factor controlling these processes was actually variation in soil moisture, which promotes the dissolution of H + solutes and formation of Fe(II) in partial anaerobic conditions. The results obtained in this study have demonstrated the potential of using ferrihydrite on contaminated farmland as an amendment for arsenic stabilization. [ABSTRACT FROM AUTHOR]

2- Line ferrihydrite (Fh) is widely used as a robust amendment for rapid arsenic removal or remediation in water or soil. However, the poorly crystalline phase of Fh is unstable and leads to arsenic leaching after long-term submergence in reductive aquatic and soil environments. In this study, the synthesized As(V)-bound Fh was characterized by various spectral approaches to investigate the factors that may affect the variation in As(V)-Fh in long-term continuously submerged soil suspensions. The X-ray diffraction (XRD) results showed that hematite was the main product and that goethite was the byproduct after 360 d of incubation. Approximately 12–17% and 4–5% Fh were transformed at As/Fe mole ratios of 0.005 and 0.05, respectively. After 360 d of incubation, the hematite morphology was clearly observed by scanning electron microscopy (SEM), and the As(V)-Fh surface areas were also decreased by 17.3–27.6% and 11.9–16.6% for As/Fe mole ratios of 0.005 and 0.05, respectively. In a comparison of the two tested soils (soils sampled in Sichuan Province (SC) and Hunan Province (HN)), As(V)-Fh transformed faster in HN soil suspensions, and more hematite and goethite were formed. Furthermore, during the incubation period, As(V) was transformed to As(III), and both species were released into the suspension from the As(V)-Fh surface. It was suggested that soil pH and Fe(II) concentration were key factors controlling the As(V)-Fh transformation process, and the differences between the two soils were due to the different soil pH values and contents of available Fe. Arsenic release was mainly caused by Fh transformation and ligand competition with soil organic matter (SOM). [Display omitted] • As(V)-ferrihydrite transformed to hematite and goethite after 360 d of incubation. • As(V) and As(III) leaching was observed during the incubation. • Fe(II) and pH were the main factors leading to the As(V)-Fh transformation. • Transformation of As(V)-Fh and competition with SOM were the reasons for As release. [ABSTRACT FROM AUTHOR]

2- Line ferrihydrite (Fh) is widely used as a robust amendment for rapid arsenic removal or remediation in water or soil. However, the poorly crystalline phase of Fh is unstable and leads to arsenic leaching after long-term submergence in reductive aquatic and soil environments. In this study, the synthesized As(V)-bound Fh was characterized by various spectral approaches to investigate the factors that may affect the variation in As(V)-Fh in long-term continuously submerged soil suspensions. The X-ray diffraction (XRD) results showed that hematite was the main product and that goethite was the byproduct after 360 d of incubation. Approximately 12–17% and 4–5% Fh were transformed at As/Fe mole ratios of 0.005 and 0.05, respectively. After 360 d of incubation, the hematite morphology was clearly observed by scanning electron microscopy (SEM), and the As(V)-Fh surface areas were also decreased by 17.3–27.6% and 11.9–16.6% for As/Fe mole ratios of 0.005 and 0.05, respectively. In a comparison of the two tested soils (soils sampled in Sichuan Province (SC) and Hunan Province (HN)), As(V)-Fh transformed faster in HN soil suspensions, and more hematite and goethite were formed. Furthermore, during the incubation period, As(V) was transformed to As(III), and both species were released into the suspension from the As(V)-Fh surface. It was suggested that soil pH and Fe(II) concentration were key factors controlling the As(V)-Fh transformation process, and the differences between the two soils were due to the different soil pH values and contents of available Fe. Arsenic release was mainly caused by Fh transformation and ligand competition with soil organic matter (SOM). [Display omitted] • As(V)-ferrihydrite transformed to hematite and goethite after 360 d of incubation. • As(V) and As(III) leaching was observed during the incubation. • Fe(II) and pH were the main factors leading to the As(V)-Fh transformation. • Transformation of As(V)-Fh and competition with SOM were the reasons for As release. [ABSTRACT FROM AUTHOR]

The effect of exogenous phosphate (P, 200 mg⋅kg −1 soil) on the lability and phytoavailability of arsenic (As) was studied using the diffusive gradients in thin films (DGT) technique. Lettuce were grown on the As-amended soils following the stabilization of soil labile As after 90 days incubation. Phosphate (P) application generally facilitated plant growth except one grown on P-sufficient soil. Soil labile As concentration increased in all the soils after P application due to a competition effect. Plant As concentration increased in red soils collected from Hunan Province, while decreases were observed in the other soils. Even though, an overall trend of decrease was obtained in As phytoavailability along with the increase of DGT-measured soil labile P/As molar ratio. The functional equation between P/As and As phytoavailability provided a critical value of 1.7, which could be used as a guidance for rational P fertilization, thus avoiding overfertilization. [ABSTRACT FROM AUTHOR]