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Paddy soils undergo wet-dry cycles that greatly influence the behaviour and availability of nutrients, but also of potentially toxic elements (PTEs). This study assessed the quality of paddy soils (actively cultivated and abandoned) and rice (white, brown, and wild) produced in the Baixo Vouga Lagunar (BVL) region, central-north Portugal. Surface soils were analysed for physicochemical parameters and chemical compositions, alongside sequential selective chemical extraction to evaluate metal(loid) availability. Chemical analyses were also performed on interstitial- and irrigation waters, and rice grains. The BVL soils are very strongly to moderately acidic (pH = 4.4–5.8), with organic matter contents reaching up to 34%, and exhibit a wide range of electrical conductivity values. Abandoned rice fields generally show higher values of these parameters and evidence of saline water intrusion. Several sites showed As, Cu, Pb, and U concentrations exceeding Portuguese thresholds for agricultural soils. While Cu levels were similar in both cultivated and abandoned fields, the latter had higher contents of As, Pb, and U. A geogenic origin is envisaged for these metal(loid)s, though anthropogenic contributions cannot be excluded. Sequential selective chemical extraction showed that Pb and U are strongly associated with available fractions, whereas amorphous Fe-oxyhydroxides primarily support As and Cu. Nevertheless, porewaters and irrigation waters showed low concentrations of these PTEs, suggesting minimal mobilisation to water. Furthermore, translocation to rice grains was low, with concentrations well below European Commission limits, indicating that elevated PTEs in soils do not necessarily lead to toxic levels in rice, providing reassurance regarding food safety.

The characterization and leaching mechanism of REEs from phosphogypsum (PG) in HCl was studied in-depth. REEs contained in the PG were 208 ppm, of which Y, La, Ce, and Nd were the four most abundant elements. The modes of occurrence of rare earth elements (REEs) in the PG were quantified using the sequential chemical extraction (SCE) method. Among the five REE occurrence species, the metal oxide form accounted for the largest proportion, followed by the residual, organic matter, and ion-exchangeable fractions, and REEs bound to carbonates were the least. From the comparison of the distributions of REEs and calcium in different occurrence states, it can be determined that REEs contained in the PG were mainly present in the residue state (existed in the gypsum lattice) and the metal oxide state (easily leached). The leaching results show that the suitable leaching conditions were acid concentration of 1.65 mol/L, S/L ratio of 1/10, and reaction temperature of 60 °C. At the condition, the maximum leaching efficiency for ∑REE was 65.6%, of which the yttrium leaching rate was the highest and reached 73.8%. Importantly, A new kinetic equation based on the cylindrical shrinking core model (SCM) was deduced and could well describe REE leaching process from PG. The apparent activation energy for ∑REE leaching was determined to be 20.65 kJ·mol−1.

The wastewater treatment process generates large amounts of P-rich organic waste (sewage sludge (SS)). The direct application of SS in agriculture, being controversial, is gradually being replaced by incineration, leading to the concentration of both P and heavy metals in the solid residual-sewage sludge ash (SSA). The novel closed-loop, cradle-to-cradle (C2C) approach leads to maintaining P production at current levels and counteracts its depletion in the future. The aim of this review is the presentation of the implementation of the C2C approach for P recovery. The paper focuses on steps that comprise P C2C, starting from the SS properties, being a derivative of wastewater type and treatment processes, to SS pre-treatment and finally leading to certified P-fertilizers production from SSA by application thermochemical or wet chemical extraction technologies. Examples of SSA treatment technologies and the final products are provided. It has been summarized that future research should focus on the production of SSA-based fertilizers aligning with the C2C concept and determining its effect on the various agriculture and horticulture crops.

This study focuses on the assessment of surface soils from industrially polluted region (El Tebbin) of southern Cairo, Egypt. The impact of agricultural, residential and industrial land use on soils developed from Nile river sediments has significantly compromised their function. Previous evidence has shown that the food chain is contaminated and enhances risk of contaminant exposure of the residential communities. This study investigates factors controlling potentially toxic element (PTE) distribution (Co, Ni, Pb, Cd, Zn, Cr and Cu) in El Tebbin soils and provide estimates of their mobility and bioavailability. The PTE concentrations are characterised by high variability as result of the variety of natural and anthropogenic influences. Highest spatial variability is found for Zn, Cd, Pb and Cu (C.V = 260.0%, 280.4%, 140.8% and 159.6% respectively) and enrichment factors indicate strong anthropogenic inputs. For Co and Ni, relatively low spatial variability (C.V = 65.8% and 45.0% respectively) with depletion in Ni suggests a relatively minor contribution from anthropogenic sources. For Cr, a more uniform distribution pattern showing depletion to minimal enrichment across the study area (C.V = 19.2%) reflects almost exclusive lithogenic control. Using principle component analysis (PCA) to explore concentration data reveals that the major inputs affecting PTE distribution are modified by primary soil properties (texture and pH). Their relative bioavailability (identified through sequential chemical extraction) relates strongly to local input sources. Those elements dominated by lithogenic input (Ni and Co) were found predominantly in soil residual fractions (95.6% and 90.5% respectively), while elements with stronger anthropogenic contributions (Cd, Zn, Pb and Cu) showed much higher portion in the more mobile and bioavailable fractions obtained from sequential chemical extraction, with average proportions of the totals being 62.6%, 57%, 40.7% and 39.2% respectively. Those PTEs with strong anthropogenic influence are potentially much more mobile for bioaccumulation in food chain with increased health risk for exposed residents and are confirmed by elevated concentrations of Cd, Zn, Pb and Cu recorded in local plant species. The main pollution sources were further highlighted by cluster analysis and showed vehicle traffic and specific industrial activities but which varied significantly from site to site. The identification of sources through the approach developed here allows prioritisation of monitoring and regulatory decisions by the local government to reduce further environmental exposure of the local population.

The rising generation of organic waste (OW) can be a concern, representing at the same time a valuable opportunity for the phosphorous (P) recycling; however, yet little is known about plant-available P release from this source. In this work, two anaerobic digestates, from agro- (AW D ) and bio-waste (BW D ), and their respective composts (AW C and BW C ), were selected to assess their P-release via sequential chemical extraction (SCE) and P species via solution 31 P-NMR in NaOH+EDTA extracts. These products were also tested for the relative-P efficiency (RPE soil ) in a soil incubation (30 mg P kg −1 ), in comparison with a chemical-P source and a reference compost. The organic products were also compared for the ryegrass relative-P efficiency (RPE tissue ) at the same P-rate (30 mg P kg −1 ), in a 112-day pot experiment in an Olsen-P poor soil (<3 mg kg −1 ), under a non-limiting N environment. The NaOH+EDTA extractions showed that inorganic P prevailed in all samples as proven by solution 31 P-NMR. SCE showed very different labile-P (H 2 O+NaHCO 3 ) and AW doubling BW products (84 vs. 48%); this was also confirmed by the soil incubation test in which AW D and AW C attained the best RPE soil . Pot tests generally confirmed laboratory outcomes showing that AW D and AW C attained the best RPE tissue , from 3 to 4-folds of the BW D and BW C performance. The results showed that the feedstock mainly affects plant P availability from recycled OW and that their SCE are very informative in the description of plant-available P from this type of products to be used in rational fertilization plan.

In recent years, recovering critical elements from coal has attracted considerable interest due to their significant potential and resulting advantages. A prime example is the coal-hosted Al-Ga-Li-REE deposit within the Jungar Coalfield of Inner Mongolia, northern China, where lithium (Li), gallium (Ga), and aluminum (Al) are successfully extracted from coal ash. However, the specific forms in which these elements exist, crucial for developing effective extraction methods, remain unquantified. This research investigated the distribution of Li, Ga, Nb, Zr, and rare earth elements (REEs) within the coal. The study employed a combination of analytical techniques, including inductively coupled plasma mass spectrometry (ICP-MS), sequential chemical extraction (SCE), scanning electron microscopy coupled with energy-dispersive X-ray spectrometry (SEM-EDS), and X-ray powder diffraction analysis (XRD). The analyzed coals exhibited enriched levels of Li, Ga, Zr, Nb, and REEs. Kaolinite and boehmite were the primary mineral constituents, along with minor amounts of calcite, pyrite, rutile, goyazite, and chlorite. Sequential chemical extraction revealed that Li and Ga are primarily associated with aluminosilicate phases (71.84%–84.39%) and, to a lesser degree, organic matter (12.15%–25.09%). Zirconium and Nb were also predominantly found within aluminosilicates (68.53%–95.96%). REEs occur mainly in carbonate (28.28%–60.78%), aluminosilicate (11.6%–33.08%), and organic (22.04%–29.42%) fractions.

A low crystalline Fe-oxyhydroxy sulfate (FeOS) was used to immobilize arsenic (As) in soils in this study. The effects of FeOS amount, treatment time and soil moisture on As immobilization were investigated. The results showed that water-soluble and NaHCO 3 -extractable As were immobilized by 53.4–99.8 and 13.8–73.3 % respectively, with 1–10 % of FeOS addition. The highest immobilization of water-soluble (98.5 %) and NaHCO 3 -extractable arsenic (47.2 %) was achieved under condition of 4 % of FeOS and 80 % of soil moisture. Further, more amounts of FeOS addition resulted in less time requirement for As immobilization. Sequential chemical extraction experiment revealed that easily mobile arsenic phase was transferred to less mobile phase. The FeOS-bonded As may play a significant role in arsenic immobilization. Under leaching with simulated acid rain at 60 times pore volumes, accumulation amount of As release from untreated soil and soil amended with FeOS were 98.4 and 1.2 mg, respectively, which correspond to 7.69 and 0.09 % of total As amounts in soil. The result showed that the low crystalline FeOS can be used as a suitable additive for arsenic immobilization in soils.

A sequential chemical extraction with a defined series of eluotropic organic solvents with an increasing polarity (trichloromethane < ethyl acetate < acetone < acetonitrile < n-propanol < methanol) was performed on peat-bog humic acid. Six organic fractions were obtained and subjected to a physicochemical characterization utilizing methods of structural and compositional analysis. Advanced spectroscopic techniques such as Attenuated Total Reflectance (ATR-FTIR), total luminescence, and liquid-state 13C NMR spectrometry were combined with elemental analysis of the organic fractions. In total, the procedure extracted about 57% (wt.) of the initial material; the individual fractions amounted from 1.1% to 19.7%. As expected, the apolar solvents preferentially released lipid-like components, while polar solvents provided organic fractions rich in oxygen-containing polar groups with structural parameters closer to the original humic material. The fraction extracted with acetonitrile shows distinct structural features with its lower aromaticity and high content of protein-like structural motifs. The last two—alcohol extracted—fractions show the higher content of carbohydrate residues and their specific (V-type) fluorescence suggests the presence of plant pigment residues. The extraction procedure is suggested for further studies as a simple but effective way to decrease the structural complexity of a humic material enabling its detail and more conclusive compositional characterization.

The aim of this paper is to explore whether the long-term bioavailability of Zn in different soils can be predicted using operational extraction procedures. Green peas and beetroot were grown in two soils with contrasting physicochemical characteristics. Two Zn sources of different sizes (ZnO-nano or ZnO-bulk) were applied 1 year earlier, at different Zn application rates. The amounts of available Zn were assessed using the diffusive gradients in thin films (DGT) technique and different chemical extraction procedures: water-soluble (WS), CaCl 2 , rhizosphere-based low-molecular-weight organic acid (LMWOAs), DTPA-TEA, and NH 4 Ac. The different correlation and regression studies showed that the estimation of availability is dependent on the soil categorical variable, especially in the beetroot crop. Zn-DGT could be used to estimate the Zn concentration of the aerial part of the green pea using a general model for both soil and ZnO sizes. The estimation of long-term Zn bioavailability was successful using either medium-strength extractive solutions or the DGT technique. The extraction methods involving complexing agents or buffered salt solution overestimated the amount of bioavailable Zn in calcareous soil. Further studies will be necessary to know the amounts of Zn associated with the different soil fractions. Graphical abstract

The interactions of nanoparticles (NPs) with different soil components have impact on their fate, transport, and behavior. The present study was conducted to find operational extraction procedures for effective assessment of both plant availability and leaching to groundwater of Zn applied to soils from zinc oxide (ZnO) NPs. A greenhouse experiment was performed in lysimeters with different ZnO-NPs amounts (3, 20, and 225 mg Zn kg −1 ) in two typical soils in Central Spain. The experiment was carried out spanning the whole plant life-cycle of cherry tomato and common bean. The soil extractable Zn amounts were measured using chemical extraction procedures (the rhizosphere-based low-molecular-weight organic acids—LMWOAs—, CaCl 2 , DTPA-TEA, water-soluble—WS—, and NH 4 Ac) and the diffuse gradients in thin films (DGT) technique. The regression studies between extractable Zn concentrations and Zn concentrations in plant tissues showed that LMWOAs, CaCl 2 , WS, and the DGT technique successfully predicted Zn concentrations in all the different parts of plants. The prediction capacity of the DTPA-TEA and NH 4 Ac approaches were strongly influenced by soil type and plant species. LMWOAs, CaCl 2 , WS, and the DGT technique were the most robust approaches for evaluation of Zn bioavailability from the soils treated with ZnO-NP. Although LMWOAs and WS reagents were also able to estimate the total quantities of Zn that were leached to groundwater, the correlation improved when soil type was included as a categorical variable. DTPA-TEA and NH 4 Ac were able to predict the Zn concentrations in the plant tissues only if soil type was considered as a categorical factor. Graphical Abstract