Solid-liquid Distribution Coefficients Research Articles

Modeling phosphorus transport in soil columns amended with polyaluminum chloride and anionic polyacrylamide water treatment residuals: implications for stormwater bioretention systems

ABSTRACT Understanding phosphorus transport in soil columns amended with polyaluminum chloride and anionic polyacrylamide water treatment residuals (PAC-APAM WTRs) is crucial for the effective recycling of PAC-APAM WTRs into traditional soil-based stormwater bioretention systems. Phosphorus transport in columns containing three distinct soil types amended with PAC-APAM WTRs under saturated steady-state flow conditions was effectively modeled using three different models: the convection-dispersion equation (CDE) model with linear isotherm, the CDE model with Langmuir isotherm, and the chemical non-equilibrium two-site model (TSM). In Soils 1 and 2, amended with PAC-APAM WTRs, the primary mechanism governing phosphorus transport transitioned from instantaneous adsorption to two-site adsorption as flow rate increased. In contrast, Soil 3 amended with PAC-APAM WTRs was predominantly governed by two-site adsorption throughout the experiments. An increase in flow rate reduced the solid–liquid distribution coefficient and the fraction of equilibrium adsorption sites, resulting in decreased phosphorus adsorption and increased phosphorus mobility. This study strongly recommends the selection of soil amended with PAC-APAM WTRs that exhibits higher instantaneous phosphorus adsorption. Additionally, this study emphasizes the importance of appropriately designing the depth of the ponding layer, utilizing the TSM model, and refining modeling techniques to optimize phosphorus retention and mitigate pollution risks in stormwater management.

Risk quick sketch: Soil captured most anatoxin-a and microcystin-RR rather than cylindrospermopsin and microcystin-LA/-LY

Cyanobacteria proliferate in warm, nutrient-rich environments and release toxic secondary metabolites into natural waters. Using cyanotoxin-contaminated water to irrigate crops could expose humans and biota, but the risk may be low if agricultural soils can sorb and retain cyanotoxins. In this report, we compared the sorption and desorption capacities of multi-class cyanotoxins/anabaenopeptins in soils of variable properties with a batch sorption procedure. The target compounds were anabaenopeptin-A, anabaenopeptin-B, anatoxin-a, cylindrospermopsin, and microcystins -LR, -RR, -LA, -LY, -LW, and -LF. Based on solid-liquid distribution coefficients (Kd), we classified cylindrospermopsin and microcystin-LA/-LY as “very low sorptivity”, anabaenopeptin-A, -B and microcystin-LR, -LF, and -LW as “low sorptivity”, and anatoxin-a and microcystin-RR as “medium sorptivity”. We remain concerned about irrigating agricultural land with water contaminated with high levels of CYN and MC-LA/-LY because of their relatively low affinity and high desorption proportion in soils. The results also suggest that soil sorption can be an effective immobilization pathway for anatoxin-a and microcystin-RR. The generated data will be useful for prioritizing research on the most bioavailable cyanotoxins/anabaenopeptins that are likely to be released by the soil matrix, for environmental risk assessment.

Cesium stability on the interlayers of K- or Rb-fixing micaceous minerals investigated by both experimental and numerical simulation methods

The frayed edge site (FES) of micas, a partially weathered interlayer site, selectively adsorbs Cs radioisotopes. Despite extensive research on Cs+ adsorption, the interactive dynamics of FES elements remain unclear. This study employs experimental and computational methods to examine how interlayer cations at the FES affect Cs stability. We measured the solid–liquid distribution coefficients of Cs+ for partially expanded K- and Rb-fixed biotite using chemical extraction and adsorption methods. We evaluated the standard Gibbs free energy for the Cs exchange reaction between the FESs of K- and Rb-fixed muscovite models and bulk water, expanding the d001 spacing from collapsed to fully expanded conditions. Our results reveal that the interlayer cation significantly influences Cs+ affinity for FES, with the substitution of K+ with Rb+ largely reducing Cs+ stability. The computational approach further disclosed that the K+ to Rb+ replacement only at the wedge-shaped part of the FES contributed to the decrease in Cs+ stability whereas the replacement at other interlayer sites caused little impact. Our studies offer microscopic structural insights into FES, highlighting the critical role of the wedge-shaped part of FES in Cs+ stability.

Impact and migration behavior of triclosan on waste-activated sludge anaerobic digestion

Triclosan (TCS), a hydrophobic antibacterial agent, is extensive application in daily life. Despite a low biodegradability rate, its hydrophobicity results in its accumulation in waste-activated sludge (WAS) during domestic and industrial wastewater treatment. While anaerobic digestion is the foremost strategy for WAS treatment, limited research has explored the interphase migration behavior and impacts of TCS on WAS degradation during anaerobic digestion. This study revealed TCS migration between solid- and liquid-phase in WAS digestion. The solid–liquid distribution coefficients of TCS were negative for proteins and polysaccharides and positive for ammonium. High TCS levels promoted volatile-fatty-acid accumulation and reduced methane production. Enzyme activity tests and functional prediction indicated that TCS increased enzyme activity associated with acid production, in contrast to the inhibition of key methanogenic enzymes. The findings of the TCS migration behavior and its impacts on WAS anaerobic digestion provide an in-depth understanding of the evolution of enhanced TCS-removing technology.

Construction and validation of parametric models to predict radium sorption in soils

Elucidating the factors affecting the transfer of naturally occurring radionuclides (NOR) between environmental compartments is a key part of the assessment of ecosystem's exposure to naturally occurring radionuclide materials (NORM). For that, the sorption and desorption solid-liquid distribution coefficients (Kd) of radium (Ra) were quantified in a collection of 31 soil samples with contrasting edaphic properties under controlled conditions in laboratory batch experiments. Ra sorption was demonstrated to be moderate to high, with Kd (Ra) values ranging from 102 to 103 L kg−1. Ra sorbed was mostly irreversible, as evidenced by desorption percentages lower than 2 %. An exploratory analysis with partial least squares (PLS) regression identified the soil properties that correlated with Kd (Ra) and discarded those that were not relevant for describing Kd variability. A dataset of the sorption Kd (Ra) values and associated soil properties was built from our own data and from the literature after performing an in-depth review of similar Ra sorption studies. For the first time, Kd (Ra) parametric prediction models were constructed using univariate linear regression (ULR) and multivariate linear regression (MLR). Ra sorption in soils was mostly explained by the soil properties directly or indirectly related to the available exchange sites, such as the levels of water-soluble and exchangeable Ca and Mg as well as the pH of the contact solution. The most promising models explained around 80 % of the Kd (Ra) data variance, only needing Kd (Ca + Mg) or additional soil descriptors such as pH, Mn content, and the specific surface area. The validation of the proposed models confirmed that Kd (Ra) can be predicted with only a few soil properties that can be characterised in routine analysis. Thus, the proposed models could be used to estimate the interaction of Ra in soils in risk assessment.

Adsorption-solubilization controlled extraction of pyrene from surfactant solutions by graphene oxide

Graphene and its derivatives such as graphene oxide (GO) has attracted increasing attention in water treatment because of its unique physical and chemical properties. Herein, the adsorption performance of GO for a typical polycyclic aromatic hydrocarbon (PAH), pyrene from solutions of either a non-ionic surfactant Tergitol 15-S-7 or an anionic surfactant sodium dodecyl benzene sulphonate (SDBS) is investigated. It demonstrates that the high affinity of GO to pyrene leads to multi-layer adsorption of pyrene, with an adsorption capacity 2 to 3 times of its own weight at a low GO concentration of 3.9 mg/L. Linear two-zone pyrene adsorption isotherms are identified, which should be due to competitive adsorption-solubilization processes of pyrene. Although Tergitol 15-S-7 has a considerably higher solubilization capacity (affinity) for pyrene compared to SDBS, faster adsorption kinetics, larger solid-liquid distribution coefficients, and higher recovery of pyrene from Tergitol 15-S-7 solutions are achieved. The higher affinity of SDBS to GO leads to its competitive adsorption that impairs the adsorption of pyrene by GO. Furthermore, the GO demonstrates excellent adsorption capacity for pyrene even after 5 recycling cycles.

Demonstrating sorption analogy of lanthanides in environmental matrices for effective decision-making: The case of carbon-rich materials, clay minerals, and soils

Examining the effect of lanthanide-contaminated wastes, which have the potential to impact to other environmental compartments, requires conducting interaction studies with soils, as feasible first receptors of lanthanide leachates, and, if necessary, with sorbent materials, such as clay minerals and carbon-rich materials, which can serve as natural barriers and immobilisation agents used in remediation strategies. In this context, it is relevant to have available and reliable data on solid–liquid distribution coefficients (Kd) to understand the lanthanide sorption in these environmental matrices. Moreover, confirming lanthanide sorption analogies permits filling data gaps and data extrapolation among different contaminated scenarios, and thus facilitate to have available input data for decision-making related to the impact of a contaminated site. In this study, we demonstrate for the first time an analogous sorption of La, Sm, and Lu in carbon-rich materials (i.e., biochar and activated charcoal), clay minerals and soils, through laboratory batch experiments. The obtained sorption Kd values revealed similar sorption patterns among the three lanthanides for each matrix tested, even at different initial lanthanide concentrations. In all matrices, the maximum Kd values exceeded 104 L kg−1, with a significant decrease when testing high lanthanide concentrations. The analogy was first confirmed by examining the Kd correlations for the La-Sm, Lu-Sm, and La-Lu pairs within each matrix, for which strong linear correlations were obtained in all cases. Data compilations were built with own and literature data, and derived cumulative distribution functions revealed statistically equal lanthanide distributions and Kd best estimates. In addition to this, Kd variability decreased when grouping the data according to significant material properties. For the first time, Kd (Ln) best-estimates for different scenarios and materials were proposed as input data for risk assessment models.

Screening redox stability of iron rich by-products for effective phosphate immobilisation in freshwater sediments

Iron (Fe) rich by-products can be added to lake or river sediments to immobilise phosphate (PO4) and lower eutrophication risks. These Fe materials differ in mineralogy and specific surface area, hence differing in PO4 sorption capacity and stability under reducing conditions. This study was set up to identify key properties of these amendments in their capacity to immobilise PO4 in sediments. Eleven Fe rich by-products, collected from drinking water treatment plants and acid mine drainage, were characterised. The PO4 adsorption to these by-products was first determined under aerobic conditions and the solid-liquid distribution coefficient KD for PO4 correlated strongly to oxalate extractable Fe content. A static sediment-water incubation test was subsequently used to evaluate the redox stability of these by-products. The reductive processes gradually released Fe to solution and more Fe was release from the amended than from the control sediments. The total Fe release to solution was positively related to ascorbate reducible Fe fractions in the by-products, suggesting that such fractions indicate potential loss of P retention capacity on the long term. The final PO4 concentration in the overlying water was 5.6 mg P L−1 in the control and was successfully lowered by factor 30–420 depending on the by-product. The factor by which solution PO4 was reduced in Fe treatments increased with increasing KD determined under aerobic conditions. This study suggests that efficient by-products to trap P in sediments are characterised by a high oxalate Fe content and a low reducible Fe fraction.

Using a batch test to derive sorption data of fluoroquinolone antibiotics in humic acids

Fluoroquinolone antibiotics (FQs) are of concern due to their disrupting effects on environmental bacterial communities. Evaluating FQ sorption by soil components is important to understand their interaction with soils and to address their environmental (bio)availability. However, data in soil organic components, especially humic acids, are scarce. Batch experiments following OECD guidelines are suitable for testing the sorption of pollutants in solid matrices. Here, we applied this methodology, with specific changes in the experimental setup, to derive sorption data and to identify the factors affecting sorption of four common FQs in seven humic acids with contrasting properties. The effect of shaking time, pH, calcium concentration in solution and dissolved organic carbon (DOC) content on the quantification of the solid-liquid distribution coefficient (Kd) of norfloxacin in three reference humic acids was evaluated. Sorption reversibility and sorption analogy of four FQs were additionally assessed in these three reference materials, whereas the effect of initial norfloxacin concentration was evaluated in the overall set of seven humic acids. Sorption was fast, strong, non-linear, irreversible and affected by changes in the pH and calcium levels in solution. The bell-shaped sorption trend at varying pH values confirmed the key role of FQ speciation in sorption and the high Kd values indicated a positive effect of soil organic matter components on FQ sorption in bulk soils at environmentally relevant pH values.•Relevant factors affecting sorption of pollutants in environmental matrices must be considered to derive Kd values with low variability and high representativeness.•In this article we modify the experimental conditions of standard batch tests to identify the factors that affect the sorption of FQs in humic acids.•The methodological approach followed can be extrapolated to the evaluation of other combinations of pollutant and environmental matrix.

Solid-liquid distribution coefficients of Sr(II) at a proposed nuclear power plant site in China and their relations to cation exchange capacity and pH

Solid-liquid distribution coefficients of Sr(II) at a proposed nuclear power plant site in China and their relations to cation exchange capacity and pH

Deriving parametric and probabilistic Kd values for fluoroquinolones in soils

The evaluation of the sorption affinity of fluoroquinolone antibiotics (FQs) in soils, by means of the derivation of solid-liquid distribution coefficients (Kd), is a valuable information for assessing their environmental mobility. Aiming to develop Kd (FQ) prediction tools in soils, in the first stage of this study we constructed a Kd (FQ) sorption dataset using current literature data. Furthermore, additional sorption and desorption data for norfloxacin were obtained in seven different soils of contrasting properties. Sorption isotherms of norfloxacin were linear under the experimental conditions tested and desorption percentages increased for scenarios in which low sorption was noted. Sorption tests in the same soils were then extended to ciprofloxacin, enrofloxacin and ofloxacin and pooled in the dataset, revealing comparable Kd (FQ) values among the FQ tested after analyzing the overall dataset consisting in 312 entries of Kd (FQ). A partial least square (PLS) regression model was then developed to predict values of Kd (FQ) based on specific relevant soil properties (i.e., pH, cation exchange capacity and organic carbon and texture information), and, for the first time, FQ properties (fraction of cationic FQ species) affecting sorption. Additionally, probabilistic, Kd (FQ) best estimates in soils were derived through cumulative distribution functions (CDFs) for the overall and for partial datasets created by grouping Kd (FQ) values according to key soil properties affecting FQ sorption (i.e., pH, organic carbon content and texture information). This latter approach permitted to derive more representative Kd (FQ) best estimates for the soils to be assessed, and with a lower related variability than that derived from the overall dataset. Best estimates Kd (FQ) values were > 1000 L kg−1 for most acidic to neutral soils, suggesting strong sorption, although lower sorption and thus higher environmental mobility may be expected in scenarios with soils with alkaline pH, low OC and high sand contents. SynopsisThis study aims to derive parametric and probabilistic Kd values for fluoroquinolone antibiotics in soils on the basis of a few relevant soil physicochemical properties.

A radioecological model with moisture-dependent Kd: Application to 129I and 79Se natural release to a grassland

A radioecological model previously developed to simulate chlorine cycling in a Scots pine forest was modified to examine the effect of soil hydrochemical conditions on the fate of 129I and 79Se released to a grassland through natural discharge of contaminated groundwater. To this end, the constant solid-liquid distribution coefficient (Kd) in the original model was replaced by a parametric equation to estimate 129I and 79Se Kd values from soil saturation — as a proxy for soil redox potential — and a set of Kd values determined experimentally under oxic and anoxic conditions. Additionally, the multi-compartment Scots pine tree module was replaced by a two-compartment module to represent 129I and 79Se cycling in grass. Simulations undertaken with the model indicated a considerable effect of soil redox conditions on 129I and 79Se accumulation in the soil column, especially in the saturated subsoil above the water table. The constant Kd overestimated 129I accumulation in the soil in relation to the parametric Kd. In contrast, the constant Kd underestimated 79Se accumulation in the soil. These results have implications for radiological impact assessments, specifically regarding the degree of conservatism in the Kd used in the assessment. In respect of bioavailability to grass, the simulated soil-to-plant transfer factors of 129I and 79Se compared favourably with values reported in the literature for similar soils and plant species, giving confidence in the model performance. The model presented here is a step forward in radioecological modelling as it includes the key processes that drive radionuclide transfers in soil-plant systems and the effect of soil redox conditions on sorption. The model can be readily extended to other cultivated lands and release scenarios to predict radionuclide transfer up the food chain.

Understanding the effects of sludge characteristics on the biosorption of triclosan

On account of hydrophobic nature, the adsorption process on solids is considered as the major pathway for triclosan (TCS) removal in wastewater treatment plants. In this work, four sludge sources (primary sludge, thickened sludge, dewatered sludge, and anaerobic digested sludge) were collected to evaluate the adsorption performance of TCS. The solid-liquid distribution coefficients of TCS were increased with total solids increasing of primary sludge, thickened sludge, and dewatered sludge, whereas decreased in anaerobic digested sludge. Results further revealed differences in sludge floc sub-structures of TCS adsorption. The residues contained most of adsorbed TCS in all sub-structures, while distinguished in various extracellular polymeric substances (EPS). The major contributor of EPS sub-fractions to TCS adsorption was identified as tightly bound EPS in thickened sludge and soluble EPS in anaerobic digested sludge. Based on the excitation-emission matrix spectra and Fourier infrared spectrum results, the protein-like and humic acid-like substances were closely related to the TCS adsorption, and hydrogen bond, hydrophobic interaction, and electrostatic interaction were considered as the dominant mechanisms. This study comprehensively reveals the effects of sludge sources and sub-structures on TCS adsorption, which improves the understanding of interaction and migration processes between TCS and sludge.

New insight into mechanism of Cr(VI) migration and transformation in typical soils of chromite ore processing residue (COPR) contaminated sites

Chromite ore processing residue (COPR) storage sites are widely distributed all over the world, causing serious soil and groundwater pollution. However, the differences in soil constituents and properties in different regions are significant, and the dynamic migration and transformation of Cr(VI) in different types of soil under alkaline condition of the COPR site is still unclear. In this study, typical black soil, red soil and loess in different regions of China were chosen to investigate the adsorption kinetics and thermodynamics of Cr(VI) under the original pH conditions of the soil, and then the alkaline Cr(VI) solution was introduced into the soil column to simulate the dynamic migration and transformation process of Cr(VI) at COPR sites. According to the results, the Cr(VI) breakthrough curve predicted by the solid–liquid distribution coefficient Kd based on the static isotherm adsorption experiments significantly underestimated and overestimated the retention effect of black soil and red soil on Cr(VI) dynamic migration, respectively. For the black soil, the retention of Cr(VI) was dominated by Cr(VI) reduction, which was a slow reaction compared with Cr(VI) adsorption. Therefore, the reduction kinetics process during the column experiment cannot be neglected. With respect to the red soil, the outlet Cr(VI) concentration turned to be higher than the inlet concentration with the soil alkalization, which indicated that the adsorbed Cr(VI) desorbed again, and this was the main reason for the overestimation of Cr(VI) retention effect by the red soil. This study shows that the environmental risks of Cr in different types of soil are quite different, mainly related to the valence and occurrence form of Cr governed by the soil constituents and properties. In addition, the stable form of Cr in the black soil column after the reaction indicates that the soil organic matter can be used as a potential environmentally friendly remediation material for Cr(VI) contaminated soils at COPR sites.

Modelling the sorption behaviour of perfluoroalkyl carboxylates and perfluoroalkane sulfonates in soils

A simple parametric model was developed to predict the sorption of perfluoroalkyl substances (PFASs) in soils. Initially, sorption and desorption solid-liquid distribution coefficients (Kd and Kd,des respectively) of eight PFASs (five perfluoroalkyl carboxylates, PFCAs, and three perfluoroalkane sulfonates, PFSAs) in seven soils with organic carbon (OC) content ranging from 1.6 to 41% were quantified using batch experiments. The information obtained helped to fill the gaps in a literature-based database of Kd values of PFASs, which was lacking data on soils with high OC content. The overall dataset finally comprised 435 entries. Normalized sorption coefficients for the soil OC and mineral fraction contents (KOC and KMIN respectively) were deduced for each PFAS by correlating the corresponding Kd values obtained under a wide range of experimental conditions with the fraction of organic carbon (fOC) of the soils. Furthermore, the sorption mechanisms in each phase were shown to depend mainly on PFAS chain length. The dependence of KOC and KMIN values on PFAS chain length defined the basic equations to construct the model for predicting PFAS sorption, applicable to both PFCAs and PFSAs with chain lengths ranging from 3 to 11 fluorinated carbons. The validation of the proposed model confirmed its ability to predict the Kd of PFASs based only on the soil OC and silt+clay contents and PFAS chain length. Therefore, it can be used in the first stages of a risk assessment process aiming at estimating the potential mobility of PFASs in soils after a contamination event. SynopsisThis study develops a new parametric model to predict the sorption of perfluoroalkyl substances (PFASs) in soils.

Plant uptake of perfluoroalkyl substances in freshwater environments (Dongzhulong and Xiaoqing Rivers, China)

This study provides new knowledge on the mobility, behavior, and partitioning of 17 perfluoroalkyl substances (PFASs) in the water-sediment-plant system along the Dongzhulong and Xiaoqing Rivers. The fate of PFASs in these rivers is also discussed. The study area is affected by the industrial production of perfluorooctanoic acid (PFOA). The ∑PFASs in water and sediments close to the industrial discharge were 84,000 ± 2000 ng/L and 2300 ± 200 ng/g dw, respectively, with the concentrations decreasing along the river due to dilution. PFOA was the dominant compound (74–97% of the ∑PFASs), although other PFASs were identified close to urban areas. Principal component analysis and solid-liquid distribution coefficients revealed that long-chain PFASs accumulated in the sediment whereas short-chain PFASs remained in the water all along the river. PFASs were taken up by plants and remobilized to different plant compartments according to shoot concentration factors (SCFs), root concentration factors (RCF), and transfer factors (TFs). Among the four plant species studied, floating plants absorbed high levels of PFASs, while rooted species translocated short-chain PFASs from the roots to the shoots. Therefore, floating species, due to their high uptake capacity and large proliferation rate, could eventually be used for phytoremediation.

Distribution and behaviour of naturally occurring radionuclides within a Scots pine forest grown on a CaF2 waste deposit related to the Belgian phosphate industry

The distribution and behaviour of naturally occurring radionuclides within a vegetated part of a CaF2 sludge heap from the Belgian phosphate industry was studied. A Scots pine forest plot was selected as study area. Trees were approximately 20 years old and showed a disturbed health state. Seasonal sampling campaigns of soil, roots, wood, inner and outer bark, needles and twigs gave insight on 238U, 226Ra, 210Pb and 210Po transfer and distribution between pine tree compartments. Soil samples were analysed for their texture, total organic and inorganic carbon, field capacity, pH and radionuclide content. Solid-liquid distribution coefficients (Kd) were experimentally determined for 238U, 226Ra (using Ba as analogue) and 210Pb based on adsorption-desorption batch tests.Results indicated higher 238U, 232Th, 226Ra, 210Pb and 210Po activity concentrations in the deeper soil layers while the first 20 cm contained less radionuclides but had a higher level of organic carbon. Additionally, results indicated no seasonal changes in the 238U:226Ra ratio in the soil while the 226Ra:210Pb ratio was significantly higher in spring compared to winter in the 20–60 cm soil layer. Pine tree roots served as natural translocation barrier for all radionuclides with high retention in the roots and low translocation to the above ground tree compartments. When considering the above ground compartments, 210Pb and 210Po were mostly present in the bark, needles and twigs. Furthermore, 238U and its progeny were highly accumulated in mosses. These results allowed us to establish more realistic soil-to-plant transfer factors. In addition, experimentally mimicking pore water acidification in the root zone resulted in lower 238U and 210Pb Kd values compared to using a standard CaCl2 solution.This study provides an integrated radioecological picture of knowledge and site specific data needed to study the long-term influence of vegetation on radionuclide dispersion in forest ecosystems.

Mechanisms of antimony ageing in soils: An XAS study

Antimony (Sb) retention in soils increases in the long term, resulting in lower soluble Sb concentrations and thus a decreased environmental risk of Sb. However, the mechanisms responsible for Sb ageing in soils are not clear, although this is necessary to understand, describe and predict Sb ageing processes in soils. To do so, the long-term retention of added Sb(V) to nine soil samples with contrasting soil properties and to a model iron (Fe; hydrous ferric oxide or HFO) and aluminium (Al; hydrous aluminium oxide or HAO) hydroxide under aerobic conditions was studied with X-ray absorption spectroscopy (XAS). The solid-liquid distribution coefficients (KD) of the soils increased by a factor of 2–4 between 2 weeks and 6 months. In addition, ageing of Sb was more pronounced in the Sb(V)-spiked HFO than in the soils, while no ageing of Sb(V) in the HAO was noted. The added Sb(V) did not change redox speciation throughout six months of aerobic incubation and no indications of antimonate precipitates with Fe or calcium (Ca) were found. Conversely, it was found that with time, Sb(V) was increasingly retained by the HFO and by the soils, likely through micropore diffusion and/or through substitution of Sb(V) into surface Fe octahedra in ferrihydrite. In addition, in the soils, the Sb(V) surface speciation on Fe hydroxides changed with time leading to higher proportion of edge-sharing bidentate Sb⋯Fe complexes. These results suggest that Sb(V) ageing in soils is controlled by different mechanisms simultaneously, i.e., by slow micropore diffusion and/or substitution processes that remove Sb(V) from the reactive surface of the Fe hydroxides and by slow surface processes, with slow sorption reactions more prevalent for edge-sharing bidentate Sb(V) than for corner-sharing bidentate Sb(V) surface complexes.

Distribution of 137Cs and 60Co in plough layer of farmland: Evidenced from a lysimeter experiment using undisturbed soil columns

ABSTRACT Radionuclide fallout during nuclear accidents on the land may impair the atmosphere, contaminate farmland soils and crops, and can even reach the groundwater. Previous research focused on the field distribution of deposited radionuclides in farmland soils, but details of the amounts of radionuclides in the plough layer and the changes in their proportional distribution in the soil profile with time are still inadequate. In this study, a lysimeter experiment was conducted to determine the vertical migration of 137Cs and 60Co in brown and aeolian sandy soils, collected from the farmlands adjoining Shidaowan Nuclear Power Plant (NPP) in eastern China, and to identify the factors influencing their migration depths in soil. At the end of the experiment (800 d), > 96% of added 137Cs and 60Co were retained in the top 0–20 cm soil layer of both soils; very little 137Cs or 60Co initially migrated to 20–30 cm, but their amounts at this depth increased with time. The migration depth of 137Cs was greater in the aeolian sandy soil than in the brown soil during 0–577 d, but at the end of the experiment, 137Cs migrated to the same depth (25 cm) in both soils. Three phases on the vertical migration rate (v) of 60Co in the aeolian sandy soil can be identified: an initial rapid movement (0–355 d, v = 219 ± 17 mm year–1), followed by a steady movement (355–577 d, v = 150 ± 24 mm year–1) and a very slow movement (577–800 d, v = 107 ± 7 mm year–1). In contrast, its migration rate in the brown soil (v = 133 ± 17 mm year–1) was steady throughout the 800-d experimental period. The migration of both 137Cs and 60Co in the two soils appears to be regulated by soil clay and silt fractions that provide most of the soil surface area, soil organic carbon (SOC), and soil pH, which were manifested by the solid-liquid distribution coefficient of 137Cs and 60Co. The results of this study suggest that most 137Cs and 60Co remained within the top layer (0–20 cm depth) of farmland soils following a simulated NPP accident, and little reached the subsurface (20–30 cm depth). Fixation of radionuclides onto clay minerals may limit their migration in soil, but some could be laterally distributed by soil erosion and taken up by crops, and migrate into groundwater in a high water table level area after several decades. Remediation measures, therefore, should focus on reducing their impact on the farmland soils, crops, and water.

Comparative study of strontium adsorption on muscovite, biotite and phlogopite

Micaceous minerals are the natural materials that can block radioactive strontium (Sr) released in the environment, and their adsorption capacity and mechanism are highly divergent owing to the different properties of micas. In this work, we comparatively studied the adsorption of Sr(II) on three typical micas, muscovite, biotite and phlogopite. The effects of pH, contact time, ionic strength, and background electrolyte were evaluated. It was found that phlogopite and muscovite had the largest solid-liquid distribution coefficient (Kd) for a reaction time of 48 h under acidic and alkaline conditions, respectively. Under alkaline conditions, as the reaction time increased to 44 days, phlogopite and muscovite showed the highest and lowest Kd, respectively. The Kd for Sr(II) adsorption on biotite and phlogopite increased with increasing pH but decreased with increasing pH for muscovite. X-ray diffraction analysis revealed that the interlayer weathering of phlogopite (a new diffraction peak appeared at 2-theta of ~6.1°) occurred along with the adsorption of Sr(II) below pH 9.0 under 0.01 mol/L NaCl. Furthermore, the adsorption of Sr(II) was significantly inhibited in the presence of 10−5 and 10−2 mol/L Cs+, resulting in similar adsorption capacity for phlogopite and muscovite at pH ~4.1. Consequently, the difference in Sr(II) adsorption on muscovite, biotite and phlogopite mainly came from the synergistic process of adsorption and weathering, which induced the differences in availability of interlayer sites among micas over a certain time.