Sequential Extraction Results Research Articles

Iron and phosphate species regulates arsenic speciation and potential mobility in contaminated soils

Arsenic in soils poses a high environmental risk. The understanding of arsenic geochemical speciation, mobility, and other potential factors in contaminated soils is crucial for appropriate remediation strategy development and environmental assessment. The objective of this study was to investigate the arsenic oxidation state and its form in each step of sequential extraction applied to different types of contaminated soils, and to analyze the impact of sequential extraction forms of soil Fe and phosphate. Soil samples were collected from three agricultural regions: acid mine drainage (AMD)-impacted red soils (n = 5, 61.1–248.6 mg As/kg) and As-contaminated groundwater-impacted soil including yellow soils (n = 6, 23.2–32.1 mg As/kg) and chestnut soils (n = 5, 9.0–13.3 mg As/kg). The results of sequential extraction revealed that As was primarily associated with Fe(III) oxyhydroxides. The highest proportion of amorphous Fe(III) oxyhydroxide-bound As was observed in the southern red soils, which was attributable to the coprecipitation/immobilization of high Fe and As concentration levels in AMD during irrigation. The amount of adsorbed As (mass fraction) increased linearly with increasing amounts of As and is related to the presence of both amorphous and crystalline Fe phases in the soils. This demonstrates the immobilization role of reactive Fe phases in controlling the potential mobility of As in contaminated soils impacted by As-contaminated groundwater and AMD. Soil phosphate, with mass concentrations 2–4 orders of magnitude higher than those of As, occupied most of the Fe(III) oxyhydroxide reactive sites. Phosphate-extractable As was 4.3–80.7 mg/kg, accounting for 18.3–76.0 % (median of 33.5 %) of total As, indicating the competitive effect of phosphate on the desorptive release of As. The AMD-impacted paddy soil exhibited much higher proportions of phosphate-extractable As and a predominance of As(III) in the water-soluble extract, revealing the high potential mobility and toxicity of As in flooded soil. The dominant occupation of Fe adsorption sites by soil phosphate likely contributes to low efficiency of soil Fe in immobilizing As. To reduce As mobility, it is imperative to develop future strategies for phosphates used as sustainable fertilizer to support crop culture.

Metals geochemistry and ecological risk assessment in lateritic soils and their transfer to tea plants

ABSTRACT Tea plant cultivation has been demonstrated to change the physicochemical properties of soils and metal bioavailability. The present study reported the geochemistry of four selected metals (Fe, Mn, Cr, and Ni) in lateritic soil cores regarding various depths cultivated with tea plants of different ages (3, 6, and 10 years old). The ecological risk and bioaccumulation of these metals in tea plants were also evaluated. The results indicate the acidic property of tea plant cultivation soils (pH < 4.5) and highlight further soil acidification due to long-term cultivation. The soils were strongly affected by farming methods and showed high levels (average from all soil cores) of total organic carbon contents (4.0% w/w), total nitrogen (2806 mg kg−1), and total phosphorus (1730 mg kg−1). We suggest that the selected metals originated naturally, supported by a high percentage in residual fractions from the results of soil sequential extraction (over 95% for Mn, Fe, and Ni and ∼ 90% for Cr). These metals exhibited no ecological risk to the adjacent environment. Mn showed the highest bioavailability, possibly due to the acidic soils and nitrogen fertiliser applications, increasing the release of Mn from soils. Mn also exhibited the highest bioconcentration factor (BCF) among four metals and 2.4 ÷ 6.5-folds higher BCFs in tea leaves than in roots. Despite the dominant content of Fe in soils (average of 20% w/w), Fe indicated lower amounts in tea leaves than Mn (average of 513 mg kg−1 vs 1354 mg kg−1). This could be attributed to the formation of Fe-rich root coatings on the tea plant root surface as a barrier to prevent Fe from translocating to other higher tea plant parts. Ni and Cr, as potentially toxic metals, were found at the lowest contents in tea leaves with low BCFs, indicating consumer safety in terms of exposure by these metals.

Transferring red mud to efficient adsorbent for the adsorption and immobilization of Ni(Ⅱ) from aqueous solution

Red mud, a solid waste produced in large quantities, possesses a high specific surface area and is rich in metal oxides, making it a promising material for adsorbent preparation. However, its practical application is constrained by its relatively low adsorption capacity and the potential environmental risks it poses. This study focused on the preparation of modified red mud (MRM) with enhanced adsorption performance for Ni2+ using a hydrothermal method involving sodium hydroxide and colloidal silica. The maximum adsorption capacity of MRM for Ni2+ reached 8.22 mmol·g–1, a substantial improvement compared to raw red mud (0.28 mmol·g–1) and sulfuric acid-activated red mud (0.46 mmol·g–1). The pseudo-second-order kinetic and Langmuir isotherm models accurately described the monolayer chemical adsorption process of Ni2+ on MRM. Additionally, leaching tests with simulated rainwater demonstrated that Ni-loaded MRM exhibited high stability, suggesting its potential for safe stockpiling or repurposing, such as in construction materials. Sequential extraction, XRD, FT-IR, and XPS results revealed that cation exchange was the primary mechanism in the adsorption process, with Ni2+ being immobilized within the zeolite framework structure of MRM, contributing to its strong adsorption stability. Inner-sphere complex formation also played a role in Ni2+ adsorption. In conclusion, this method offers an effective approach to both red mud utilization and heavy metal removal from wastewater, presenting a practical solution for waste management through resource recovery and environmental remediation.

Characterization of the Bioavailability of Per- and Polyfluoroalkyl Substances in Farmland Soils and the Factors Impacting Their Translocation to Edible Plant Tissues.

In this study, various crops and farmland soils were collected from the Fen-Wei Plain, China, to investigate the bioavailability of perfluoroalkyl substances (PFAS), their accumulation in edible plant tissues, and the factors impacting their accumulation. PFAS were frequently detected in all of the crops, with total concentrations ranging from 0.61 to 35.8 ng/g. The results of sequential extractions with water, basic methanol, and acidic methanol indicate that water extraction enables to characterize the bioavailability of PFAS in soil to edible plant tissues more accurately, especially for the shorter-chain homologues. The bioavailability of PFAS was remarkably enhanced in the rhizosphere (RS) soil, with the strongest effect observed for leafy vegetables. The water-extracted Σ16PFAS in RS soil was strongly correlated with the content of dissolved organic carbon in the soil. Tannins and lignin, identified as the main components of plant root exudates by Fourier transform-ion cyclotron resonance mass spectrometry, were found to enhance the bioavailability of PFAS significantly. Redundancy analysis provided strong evidence that the lipid and protein contents in edible plant tissues play important roles in the accumulation of short- and long-chain PFAS, respectively. Additionally, the high water demand of these tissues during the growth stage greatly facilitated the translocation of PFAS, particularly for the short-chain homologues and perfluorooctanoic acid.

Remediation effect of walnut shell biochar on Cu and Pb co-contaminated soils in different utilization types

Biochar, with its dual roles of soil remediation and carbon sequestration, is gradually demonstrating great potential for sustainability in agricultural and ecological aspects. In this study, a porous biochar derived from walnut shell wastes was prepared via a facile pyrolysis coupling with in-situ alkali etching method. An incubation study was conducted to investigate its performance in stabilizing copper (Cu) and lead (Pb) co-contaminated soils under different utilization types. The biochar effectively decreased the bioavailable Cu (8.5–91.68%) and Pb (5.03–88.54%), while increasing the pH, CEC, and SOM contents in both soils. Additionally, the results of sequential extraction confirmed that biochar promoted the transformation of the labile fraction of Cu and Pb to stable fractions. The mechanisms of Cu and Pb stabilization were found to be greatly dependent on the soil types. For tea plantation yellow soil, the main approach for stabilization was the complexation of heavy metals with abundant organic functional groups and deprotonation structure. Surface electrostatic adsorption and cation exchange contributed to the immobilization of Cu and Pb in vegetable-cultivated purple soil. This research provides valuable information for the stabilization of Cu and Pb co-contaminated soils for different utilization types using environmentally-friendly biochar.

Driving factors for distribution and transformation of heavy metals speciation in a zinc smelting site

Heavy metal pollution at an abandoned smelter pose a significant risk to environmental health. However, remediation strategies are constrained by inadequate knowledge of the polymetallic distribution, speciation patterns, and transformation factors at these sites. This study investigates the influence of soil minerals, heavy metal occurrence forms, and environmental factors on heavy metal migration behaviors and speciation transformations. X-ray diffraction analysis revealed that the minerals associated with heavy metals are mainly hematite, franklinite, sphalerite, and galena. Sequential extraction results suggest that lead and zinc are primarily present in the organic-sulfide fractions (F4) and residual form (F5) in the soil, accounting for over 70% of the total heavy metal content. Zinc displayed greater instability in carbonate-bound (16%) and exchangeable (2%) forms. The migration and diffusion patterns of heavy metals in the subsurface environment were visualized through the simulation of labile state heavy metals, demonstrating high congruence with groundwater pollution distribution patterns. The key environmental factors influencing heavy metal stable states (F4 and F5) were assessed by integrating random forest models and redundancy analysis. Primary factors facilitating Pb transformation into stable states were available phosphorus, clay content, depth, and soil organic matter. For Zn, the principal drivers were Mn oxides, soil organic matter, clay content, and inorganic sulfur ions. These findings enhance understanding of the distribution and transformation of heavy metal speciation and can provide valuable insights into controlling heavy metal pollution at non-ferrous smelting sites.

Performance of an integrated sediment interceptor in removing phosphorus from agricultural drainage water

Reducing phosphorus (P) loss from agricultural drainage water is challenging. In this study, we aimed to remove P from agricultural drainage water by developing an integrated sediment interceptor with adsorbent modules filled with Zr/Zn nanocomposite-modified ceramsite (ZMC-interceptor). The results of sequential chemical extraction and 31P NMR showed that the contents of H2O-P (1.15 % of total P), NaHCO3-Pi (10.48 % of total P), and ortho-P (orthophosphate, 90.6 % of total P) in the sediments of the ZMC-interceptors were higher than those in nearby field soils. The average enrichment ratios of particulate P (PP, >450 nm), medium-colloidal P (MCP, 220–450 nm), fine-colloidal P (FCP, 1–220 nm), and truly dissolved P (Truly DP, <1 nm) in the sediment over the field soil were 1.37, 1.21, 1.70, and 3.01, respectively. No significant differences were found in the sediment P-trapping function with and without ZMC integrated sediment interceptors. However, the ZMC-interceptors remarkably reduced total P (39.7 % for influent concentrations of 0.19–0.68 mg L−1) from agricultural drainage water compared to those unmodified ceramsite-interceptors (21.7 % for influent concentrations of 0.17–0.66 mg L−1) during the drainage ‘window period’ (June–August 2022). This was mainly due to the higher removal efficacies of MCP (19.7 %), FCP (23.3 %), and Truly DP (34.8 %) of the ZMC-interceptors. This study highlighted that the ZMC-interceptor not only trapped P in the sediment but also facilitated the removal of different-sized P fractionated from agricultural drainage water.

Enhancing Lead Passivation in Contaminated Soil: Impacts of the Chemical Modification of Oxygen-Containing Groups (OCGs) on the Surface of Hydrochar

ABSTRACT In this study, we selected walnut shell-derived pristine HC (pHC) and two modified derivatives obtained by oxidative and alkali treatments (designated mHC-O and mHC-A, respectively) to investigate the passivation performance of Pb(II) in contaminated soil. The yields of mHC-A and mHC-O were 70.58% and 95.38%, respectively, after posttreatment. According to the XPS, FTIR, and acidity titration results, the amounts of the three major OCGs (i.e. C−O, C=O, and O−C=O) all increased after H2O2 oxidation. Moreover, chemical modification fundamentally changed the structural characteristics of pHC. DTPA extraction tests indicated that the extractable Pb content after 30 days of incubation decreased by 13.60%, 19.32% and 14.53% at 7% (w/w) doses of HC, mHC-O and mHC-A, respectively, compared to that of the control. Furthermore, mHC-O was superior to pHC and mHC-A in terms of Pb passivation efficiency, which was consistent with the results of BCR sequential extraction and leaching tests using three root exudates. For mHC-O at the 7% dose, the contents of bioavailable Pb in the contaminated soil were 19.85 and 8.83 mg/kg based on DTPA leaching and BCR analysis, respectively, while they were 0.616, 0.564 and 0.07 mg/kg in the root exudate leaching tests of Ophiopogon japonicus, radish and spinach, respectively, indicating that the root exudate leaching method lowered the bioavailable Pb content compared to the DTPA and BCR methods. Overall, Oxidation posttreatment with HC could improve the passivation of Pb in contaminated soils.

Enhancing soil redox dynamics: Comparative effects of Fe-modified biochar (N–Fe and S–Fe) on Fe oxide transformation and Cd immobilization

Biochar and modified biochar have gained wide attention for Cd-contaminated soil remediation. This study investigates the effects of rape straw biochar (RSB), sulfur-iron modified biochar (S–FeBC), and nitrogen-iron modified biochar (N–FeBC) on soil Fe oxide transformation and Cd immobilization. The mediated electrochemical analysis results showed that Fe modification effectively enhanced the electron exchange capacity (EEC) of biochar. After 40 days of anaerobic incubation, compared to the treatment without biochar (CK), the concentrations of CaCl2-extractable Cd in N–FeBC, S–FeBC, and RSB treatments decreased by 79%, 53%, and 23%, respectively. Compared with S–FeBC, N–FeBC significantly decreased the soil Eh and increased soil pH within the first 15 days, which could be attributed to its higher EEC and alkalinity. There is a negative correlation between the concentration of CaCl2-extractable Cd and soil pH (p < 0.01). The sequential extraction results showed that both N–FeBC and S–FeBC promoted Cd transfer from acid-soluble to Fe/Mn oxides bound fraction (Fe/Mn–Cd). N–FeBC significantly increased the concentration of amorphous Fe oxides (amFeox) from 4.0 g kg−1 in day 1 to 4.6 g kg−1 in day 15 by promoting the NO3−-reducing Fe(II) oxidation process, while S–FeBC significantly increased amFeox from 4.0 g kg−1 in day 15 to 4.8 g kg−1 in day 40 by promoting the Fe(II) recrystallization. There is a positive correlation between the concentration of amFeox and Fe/Mn–Cd (p < 0.01). The scanning electron microscopy analysis showed that Cd was bound to the amFeox coating on the surface of Fe-modified biochar. By acting as an electron shuttle, the active surface of Fe-modified biochar may serve as a hotspot for Fe transformation, which promotes amFeox formation and Cd immobilization. This study highlights the potential of Fe-modified biochar for the remediation of Cd-contaminated soils and provides valuable insights into the development of effective remediation approaches for Cd-contaminated soils.

A mechanistic approach to arsenic adsorption and immobilization in aqueous solution, groundwater, and contaminated paddy soil using pine-cone magnetic biochar

Arsenic (As) poisoning in groundwater and rice paddy soil has increased globally, impacting human health and food security. There is an urgent need to deal with As-contaminated groundwater and soil. Biochar can be a useful remedy for toxic contaminants. This study explains the synthesis of pinecone-magnetic biochar (PC-MBC) by engineering the pinecone-pristine biochar with iron salts (FeCl3.6H2O and FeSO4.7H2O) to investigate its effects on As(V) adsorption and immobilization in water and soil, respectively. The results indicated that PC-MBC can remediate As(V)-contaminated water, with an adsorption capacity of 12.14 mg g−1 in water. Isotherm and kinetic modeling showed that the adsorption mechanism involved multilayer, monolayer, and diffusional processes, with chemisorption operating as the primary interface between As(V) and biochar. Post-adsorption analysis of PC-MBC, using FTIR and XRD, further revealed chemical fixing and outer-sphere complexation between As(V) and Fe, O, NH, and OH as the main reasons for As(V) adsorption onto PC-MBC. Recycling of PC-MBC also had excellent adsorption even after several regeneration cycles. Similarly, PC-MBC successfully immobilized As in paddy soil. Single and sequential extraction results showed the transformation of mobile forms of As to a more stable form, confirmed by non-destructive analysis using SEM, EDX, and elemental dot mapping. Thus, Fe-modified pine-cone biochar could be a suitable and cheap adsorbent for As-contaminated water and soil.

Susceptibility of Cd availability in microplastics contaminated paddy soil: Influence of ferric minerals and sulfate reduction

The combined contamination of cadmium (Cd) and microplastics (MPs) in paddy soil always occurred, while its influence on Cd availability remained unclear. This study investigated the Cd availability in Cd-MPs co-contaminated paddy soil in consideration of both ferric minerals and sulfate reduction under flooding conditions. The presence of MPs resulted in a higher Cd releasing risk, as represented by the increase in the available Cd and decrease in Fe-Mn oxide-bound Cd contents, especially on the 7th and 14th days based on the sequential extraction results. MPs facilitated the formation of Fe-organic ligands, which accelerated the reductive dissolution of iron minerals but decreased the amounts of amorphous iron minerals due to the release of dissolved organic substances into pore water. Furthermore, MPs promoted the relative abundance of sulfate-reducing bacteria (such as Streptomyces and Desulfovibrio genera), thus increasing the contents of reductive S species, which was advantageous to the co-precipitation of Fe, S, and Cd on the surface of MPs based on our experimental and statistical results. Taken together, both iron and sulfate reduction under anaerobic conditions played a critical role in Cd mobilization in Cd-MPs co-contaminated paddy fields.

Mineralogical fingerprint and human health risk from potentially toxic elements of Fe mining tailings from the Fundão dam

In 2015, >50 million cubic meters of Fe mining tailings were released into the Doce River basin from the Fundão dam, raising the question of its consequences on the affected ecosystems. This study aimed to establish a mineralogical-(geo)chemical association of potentially toxic elements (PTEs) from Fe mining tailings from the Fundão dam, collected seven days after the failure, through a multidisciplinary approach combining assessment of the risk to human health, environmental geochemistry, and mineralogy. Thus, eleven tailings samples were collected with the support of the Brazilian Military Police Fire Department. Granulometry, magnetic measurements, optical microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, and sequential chemical extraction of PTEs analyses were performed. Contamination indexes, assessment of risk to human health, and Pearson correlation were calculated using the results of sequential chemical extraction of PTEs. The predominance of goethite in Fe oxyhydroxide concentrates from the mud indicates that the major source of hematite may not be from tailings, but from pre-existing soils and sediments, and/or preferential dissolution of hematite in deep flooded zones of the tailings column of the Fundão dam. Moreover, the high correlation of most carcinogenic PTEs with their crystallographic variables indicates that goethite is the primary source of contaminants. Goethites from Fe mining tailings showed high specific surface area and Al-substitution, and due to their greater stability and reactivity, the impacts on PTE sorption phenomena and bioavailability may be maintained for long periods. However, their lower dissolution rate, and the consequent release of heavy metals would promote greater resilience for affected ecosystems, preventing significant PTE inputs under periodic reduction conditions. More specific studies, involving the crystallographic characteristics of Fe oxyhydroxides should be developed since they may provide another critical component of this set of complex and dynamic variables that interfere with the bioavailability of metals in ecosystems.

Fractionation of W in the topsoil of a meadow soil above the oxidation zone of the W deposit: results of sequential extraction and LA-ICP-MS analysis of soil from the Grantcharitsa deposit, Bulgaria

A four-step sequential extraction procedure (SEP) was used to assess the distribution of W fractionation in the topsoil (0–20 cm) of the soil above the oxidation zone of the W deposit. Data on the extraction of W and other elements were obtained using LA-ICP-MS and SEM-EDX analyses of the dried, sieved and finely grinded original soil and residual samples after each step of the procedure. When processing LA-ICP-MS data, the SiO2 content of the original soil was used as an internal standard for all samples in the sequential extraction procedure. It was found that of the approximately 90 ppm of total W content in the soil, about 25% is associated with organic matter, 40% with amorphous hydrous Fe oxides (ferrihydrite), 25% with crystalline hydrous Fe oxides (goethite), and about 10% with residual material (scheelite, silicates).

Spatial variation of soil phosphorus in the water level fluctuation zone of the Three Gorges Reservoir: Coupling effects of elevation and artificial restoration

The water level fluctuation zone (WLFZ) is a distinctive and important component of the reservoir ecosystem. Due to periodic inundation, the fraction, spatial distribution, and chemical reactivity of soil phosphorus (P) within the WLFZ can potentially impact the loading of P into reservoir waters. However, a detailed study of this subject is lacking. In this study, the soil P in the WLFZ of the Three Gorges Reservoir, China, was examined using a combination of chemical sequential extraction, 31P NMR, and adsorption experiments. The results of chemical sequential extraction showed that HCl-Pi constituted the largest P pool among all P forms, with a mean concentration of 338 mg/kg. The content of HCl-Pi decreased significantly toward the dam, while the content of Res-P decreased in the opposite direction. The highest contents of most P forms and total P were observed at an elevation of 160 m. 31P NMR measurements showed that NaOH-EDTA Pi detectable in WLFZ soils at 145 m, 160 m, and 175 m elevation consisted mainly of orthophosphate and pyrophosphate, while NaOH-EDTA Po contained phosphate monoesters and phosphate diesters, accounting for 1.4 % to 46.2 % of NaOH-EDTA TP. Adsorption experiments showed that soil P in the WLFZ was a potential P source for reservoir waters, with chemisorption being the dominant mechanism of P sequestration. The adsorption equilibrium concentration of WLFZ soil was lower at higher elevations (>170 m) compared to lower elevations (<150 m), exhibiting a decrease in the average maximum adsorption from 271 mg/kg to 192 mg/kg. Statistical analysis suggested that Ca and Fe content, particle size, elevation, and artificial restoration were key factors affecting the fraction and content of soil P in the WLFZ. Our findings contribute to an improved understanding of the behavior of soil P in the WLFZ of large reservoirs and its potential contribution to the reservoir waters.

Fluorine soil stabilization using hydroxyapatite synthesized from minerals wastes

The objective of this study was to present a new approach to mineral waste valorization, based on the synthesis of hydroxyapatite (HAP) as an efficient and cost-effective adsorbent for the stabilization of fluorides (F-) in soil. Hydroxyapatites were synthesized from the reaction of potassium dihydrogen phosphate (KH2PO4) and waste paper fly ash (WPFA), fine limestone clay (FLC) and limestone filler (LF) rich in calcite. X-ray diffraction characterization results showed that for HAPFLC and HAPFL the main resulting phases were brushite (CaHPO5·2H2O) and for HAPWPFA was hydroxyapatite (Ca5(PO4)3OH). The FTIR spectra showed similar patterns to natural HAP containing orthophosphate groups (PO43-), hydroxylated groups (OH–) and both types A/B of carbonate apatite. SEM-EDS analysis of the individual HAP revealed a morphology consistent with phosphocalcic hydroxyapatite crystals. EDS analysis revealed a Ca/P atomic ratio equal to 1.92, 1.85 and 1.7 for HAPFLC, HAPLF and HAPWPFA respectively, which is similar to the stoichiometry of hydroxyapatites (Ca/P = 1.67). The use of HAP as an amendment to stabilize fluorides (F-) in the soil was demonstrated to be effective, the addition of 1% of the different HAP allowed the decrease of the concentration of F in the raw soil (73.8 mg/kg) to concentrations below the IWSI threshold (10 mg/kg), to 4.68 mg/kg, 5.63 mg/kg, and 0.8 mg/kg for HAPFLC, HAPFL and HAPWPFA respectively. Fluoride (F) sequential extraction results showed that it was extracted from the residual fraction (Fraction 4) after soil treatment, and was generally trapped on the hydroxyapatites (Ca5(PO4)3(OH)) by anion exchange with the hydroxides (OH–) to form the stable and insoluble fluorapatite ((Ca5(PO4)3F).

Occurrence Mode of Sodium in Zhundong Coal, China: Relationship to Maceral Groups

The occurrence and separation relationships of Na and maceral groups in Zhundong coals were investigated in this study. The sequential extraction results indicate that the total Na content of all samples decreased with increasing sampling depth, and the level in inertinite-enriched samples (inertinite content 81.0%–84.0%) was significantly higher than that in corresponding raw coals and vitrinite-enriched samples (vitrinite content 90%). Additionally, H2O-Na (soluble salt species) and insoluble Na (acid-insoluble residues) were found to be concentrated in the inertinite-enriched samples. In combined SEM–EDS and microscope observations, local Na enrichments were detected in all raw coal and inertinite-enriched samples except for the vitrinite-enriched samples, but only inertinite-enriched samples were found to generally have over 10 wt% of Na enrichments, all of which occurred as NaCl. Moreover, Na is mostly filled or associated with cell-filling minerals in cells of fusinite. The maceral separation and Na removal of Zhundong coal were simultaneously achieved using triboelectrostatic separation. The vitrinite content in concentrates increased up to 60%, along with a reduced Na level, while the inertinite and sodium levels were both evidently raised in tailings. The obvious positive occurrence and separation correlation between sodium and inertinite offers new insight into, and a technical reference for, the sodium removal and maceral processing of Zhundong coal.

New data on mobility of transuranium elements in sediments of the Yenisei River

The inflow of transuranium elements to the Yenisei River was previously associated with the production of weapons-grade plutonium at the Mining-and-Chemical Combine (MCC, Zheleznogorsk, Russia), but the source of transuranium elements in the River today is fabrication of MOX fuel that started recently at the MCC. The current study presents results of sequential chemical extraction of radionuclides from sediment samples collected in 2014 and 2020 in two areas near the MCC discharge site and compares these results with the data obtained previously by sequential extraction of sediments collected during 1999–2007. Over the study period, the strength of binding of 137Cs and 60Co in the Yenisei River sediments was high (up to 100%) and remained so, while the percentages of 241Am and 152Eu in residual solids after sequential extraction increased considerably and the percentage of 239,240Pu in residual solids decreased in samples from all study areas. In samples collected at the position located close to the MCC discharge site, the percentages of the strongly bound 241Am and 152Eu as well as 239,240Pu were lower than in the samples from the other positions. The study demonstrated an enormous increase in 239,240Pu activity concentration in the top sediment layers collected at all positions in 2020 relative to 2014. In the same period, as literature data suggest, 239,240Pu activity concentrations also increased in aquatic organisms of the Yenisei River, which can be indicative of the growing potential bioavailability of plutonium in the aquatic ecosystem, which could be caused by the presence of the mobile form of plutonium in the routine discharges from the MCC.

Long-term missing role of small colloids and nanoparticles on the loading and speciation of phosphorus in catfish aquaculture ponds in west Alabama

Increasing loading of phosphorus (P) into freshwater systems is deemed as one of the key drivers triggering harmful algal blooms (HABs). However, conventional water quality monitoring of P normally uses the operational cutoff (e.g., 450-nm filter membrane) to separate particulate and dissolved phases (entities passing through the 450-nm membrane are regarded as dissolved phase), which completely neglects the roles of small colloids (450–100 nm) and nanoparticles (100–1 nm). Herein, a new particle size separation approach was used to separate water samples collected from catfish aquaculture ponds in west Alabama into six size fractions: large particles (>1000 nm), large colloids (1000–450 nm), small colloids (450–100 nm), large nanoparticles (100–50 nm), small nanoparticles (50–1 nm), and the truly dissolved phase (<1 nm). The speciation and concentration of P in these six size fractions were then investigated using Hedley's sequential extraction method. The new particle size separation results showed that particle loading (mass) followed the order: >1000 nm, 450–100 nm, 1000–450 nm, 100–50 nm, and 50–1 nm. This is mainly due to the abundance of large-sized (>1000 nm) zooplankton and phytoplankton such as algae and cyanobacteria in the catfish aquaculture ponds. Importantly, the small colloid (450–100 nm) and nanoparticle (100–1 nm) size fractions, which were previously regarded as the dissolved phase using the 450-nm membrane filtration operation, accounted for ∼41.8% of the total particle mass. The Hedley's sequential extraction results showed that sodium hydroxide (NaOH)-extracted P represented the largest P pool, followed by water (H2O)- and sodium bicarbonate (NaHCO3)-extracted P pools. Smaller particles exhibited a higher loading of P due to their large surface areas. These new findings suggest that the new particle size separation approach needs to be adopted for future water quality monitoring and mitigation of HABs in freshwater ecosystems.

Transformation kinetics of exogenous lead in an acidic soil during anoxic-oxic alteration: Important roles of phosphorus and organic matter

Lead (Pb) can enter soil environment during flooding events such as surface runoff and intensive rainfall. However, the key transformation processes of exogenous Pb during anoxic-oxic alteration remain poorly understood particularly how phosphorus and organic matter contribute to Pb immobilization/release. Here, a kinetic model was established to investigate the Pb transformation in an acidic soil with two levels of Pb contamination under alternating anoxic-oxic conditions, based on the results of seven-step sequential extraction, dissolved organic carbon, sulfate, iron, phosphorus, and surface sites. Results showed that the potentially available Pb, including dissolved, exchangeable, and specifically adsorbed fractions, was gradually transferred to the fulvic complex, Fe–Mn oxides bound, and sulfides bound Pb after 40-day incubation under anoxic conditions, while the fulvic complex Pb further increased after 20-day incubation under oxic conditions. The concentration of phosphorus that was extracted by 0.5 M HCl or 0.03 M NH4F in 0.025 M HCl increased under anoxic conditions and decreased under oxic conditions. When Pb-binding to phosphorus is considered during kinetic modeling, the simulated results of Pb transformation suggest that phosphorus is more important than organic matter for Pb immobilization under anoxic conditions, while the phosphates, Fe–Mn oxides, and sulfides immobilized Pb is slowly released and then complexed by fulvic acids during the re-immobilization of dissolved organic matter in soil under oxic conditions. The model established with low Pb level has been successfully applied to describe the Pb transformation with high Pb level. This study provides a comprehensive understanding of the roles of phosphorus and organic matter in controlling Pb transformation in soil from kinetic modeling.

Manganese Adsorption onto Permanganate-Modified Bamboo Biochars from Groundwater

Potassium permanganate-modified bamboo biochar (MBB) was used to adsorb manganese from simulated groundwater and its performance was compared to that of unmodified bamboo biochar (BB), activated carbon, and manganese greensand. The adsorption kinetics, adsorption isotherms, and manganese fractions were investigated. The Langmuir model was the best fit for manganese adsorption by MBB and BB at the maximum adsorption capacities of 21.277 and 0.803 mg g−1, respectively. The heat of adsorption from the Temkin model indicated that manganese adsorption occurs via an ion exchange process for MBB and a physical adsorption process for BB. The sequential extraction results revealed that manganese was strongly bound to the iron/manganese oxide fraction, in accordance with the chemical adsorption established in pseudo-second order kinetic data records.