Oral bioaccessibility (BAc) is a surrogate for the bioavailability (BAv) of a broad range of substances, reflecting the value that the approach offers for assessing oral exposure and risk. BAc is generally considered to have been validated as a proxy for oral BAv for the important soil contaminants Pb, Cd, and As. Here, using literature data for Ni BAc and BAv, we confirmed that Ni BAc (gastric only, with HCl mimicking stomach conditions) is a conservative measure of BAv for the oral exposure pathway. Measured oral BAv of Ni in soil was shown to be 50–100 times less than the simplest oral BAc estimates (%BAv = 0.012(%BAc) - 0.023 (r = 0.701, 95%CI [0.456, 0.847], n = 30)) in rats, demonstrating a significant conservatism for exposure assessment. The relationship between the oral BAv and BAc of nickel sulfate hexahydrate (NSHH) was comparable to that of soil, with measured oral BAv of NSHH (1.94%) being a small fraction of NSHH gastric BAc (91.1%). BAc and BAv reflect the underlying Ni speciation of the sample, with the bioaccessible leaching limits being represented by the highly soluble Ni salts and the poorly soluble Ni monoxide, and the environmental (e.g. soil properties) or gastric (e.g. food present) conditions. BAc has potential utility for chemical classification purposes because pure Ni substances can be grouped by %BAc values(using standardized methodologies for the relevant exposure routes), these groupings reflecting the underlying chemistry and speciation of the samples of substances tested here, with 0.008% %BAc for alloys (SS304, SS316, Inconel, Monel), <1% in green NiO and Ni metal massives, 0.9–23.6% for Ni powders, 9.8–22.7% for Ni sulfides, 26.3–29.6% for black oxidic Ni, and 82–91% for the soluble Ni salts. Oral BAc provides realistic yet conservative estimates of BAv for the hazard classification and risk assessment of Ni substances.

Initial Cadmium (Cd) isotope fractionation studies in cereals ascribed the retention of Cd and its light isotopes to the binding of Cd to sulfur (S). To better understand the relation of Cd binding to S and Cd isotope fractionation in soils and plants, we combined isotope and XAS speciation analyses in soil-rice systems that were rich in Cd and S. The systems included distinct water management (flooded vs. non-flooded) and rice accessions with (excluder) and without (non-excluder) functional membrane transporter OsHMA3 that transports Cd into root vacuoles. Initially, 13% of Cd in the soil was bound to S. Through soil flooding, the proportion of Cd bound to S increased to 100%. Soil flooding enriched the rice plants towards heavy isotopes (δ¹¹⁴/¹¹⁰Cd = −0.37 to −0.39%) compared to the plants that grew on non-flooded soils (δ¹¹⁴/¹¹⁰Cd = −0.45 to −0.56%) suggesting that preferentially light Cd isotopes precipitated into Cd sulfides. Isotope compositions in CaCl₂ root extracts indicated that the root surface contributed to the isotope shift between soil and plant during soil flooding. In rice roots, Cd was fully bound to S in all treatments. The roots in the excluder rice strongly retained Cd and its lights isotopes while heavy isotopes were transported to the shoots (Δ¹¹⁴/¹¹⁰Cdₛₕₒₒₜ₋ᵣₒₒₜ 0.16–0.19‰). The non-excluder rice accumulated Cd in shoots and the apparent difference in isotope composition between roots and shoots was smaller than that of the excluder rice (Δ¹¹⁴/¹¹⁰Cdₛₕₒₒₜ₋ᵣₒₒₜ −0.02 to 0.08‰). We ascribe the retention of light Cd isotopes in the roots of the excluder rice to the membrane transport of Cd by OsHMA3 and/or chelating Cd–S complexes in the vacuole. Cd–S was the major binding form in flooded soils and rice roots and partly contributed to the immobilization of Cd and its light isotopes in soil-rice systems.

Arsenic (As) exposure is a global public health concern affecting millions worldwide and stems from drinking water and foods containing As. Here, we assessed how agronomic practices and postharvest fermentation techniques influence As concentrations in rice bran, and calculated health risks from consumption. A global suite of 53 rice brans were tested for total As and speciation. Targeted quantification of inorganic As (iAs) concentrations in rice bran were used to calculate Target Hazard Quotient (THQ) and Lifetime Cancer Risk (LCR) across the lifespan. Mean iAs was highest in Thailand rice bran samples (0.619 mg kg⁻¹) and lowest in Guatemala (0.017 mg kg⁻¹) rice bran samples. When comparing monosodium-methanearsonate (MSMA) treated and the Native-soil counterpart under the irrigation technique Alternate Wetting and Drying (AWD) management, the MSMA treatment had significantly higher total As (p = 0.022), and iAs (p = 0.016). No significant differences in As concentrations were found between conventional and organic production, nor between fermented and non-fermented rice bran. Health risk assessment calculations for the highest iAs-rice bran dosage scenario for adults, children and infants exceeded THQ and LCR thresholds, and LCR was above threshold for median iAs-rice bran. This environmental exposure investigation into rice bran provides novel information with food safety guidance for an emerging global ingredient.

It has been proposed that non-protein thiols and organic acids play a major role in cadmium phytoavailability and distribution in plants. In the Cd-hyperaccumulator Solanum nigrum and non-accumulator Solanum melongena, the role of these organic ligands in the accumulation and detoxification mechanisms of Cd are debated. In this study, we used X-ray absorption spectroscopy to investigate Cd speciation in these plants (roots, stem, leaves) and in the soils used for their culture to unravel the plants responses to Cd exposure. The results show that Cd in the 100 mg kg⁻¹ Cd-doped clayey loam soil is sorbed onto iron oxyhydroxides. In both S. nigrum and S. melongena, Cd in roots and fresh leaves is mainly bound to thiol ligands, with a small contribution of inorganic S ligands in S. nigrum leaves. We interpret the Cd binding to sulfur ligands as detoxification mechanisms, possibly involving the sequestration of Cd complexed with glutathione or phytochelatins in the plant vacuoles. In the stems, results show an increase binding of Cd to –O ligands (>50% for S. nigrum). We suggest that Cd is partly complexed by organic acids for transportation in the sap.

Phosphorus (P) plays key role in phytoplankton primary production in coastal water. In this study, seawater samples collected within China's Leizhou Peninsula coastal waters from October 2017 to July 2018 were examined to determine the seasonal variation, speciation, and transport flux of total dissolved phosphorus (TDP) linked to hydrographic features. TDP concentration and speciation had significant seasonal variations (P < 0.01), and the annual mean TDP concentration was 0.42 ± 0.25 μmol·L⁻¹. High concentrations of TDP occurred in coastal waters adjacent to Zhanjiang Bay and Jianjiang River estuary, whereas low TDP concentrations were found across large offshore areas. Dissolved inorganic and organic P were the main TDP bulk species in different seasons, comprising up to 55.5 ± 7.9% and 46.5 ± 22.6%, respectively. The Beibu Gulf was annually subjected to 3.5 × 10⁹ mol flux of TDP through the Qiongzhou Strait. Coastal currents, river plumes, and human activities were responsible for the dynamic variations in P species.

How aquaculture impacts the coastal phosphorus (P) cycle remains poorly understood. Here we compared different P species from two sedimentary records off Nan'ao Island, South China, with core S1 collected in a large seaweed cultivation area and core S2 in a non-mariculture area. The results showed that the concentration of total P (TP) in sediment cores varied from 143.67 to 400.92 μg/g, and organic P (OP) was the dominant P species. The TOC/OP ratios in the two sediment cores were higher than the Redfield ratio in 26 samples (52%) from core S1 and 39 samples (78%) from core S2, suggesting that terrestrial organic matter was an important carbon source to Shen'ao Bay. The lack of change in Ex-P (exchangeable or loosely sorbed P) and OP in the area around core S1 since the 2000s may be due to the large-scale seaweed cultivation.

The behavior of lead species from Tagus estuarine water collected during winter (January), spring (April), and summer (June) seasons were evaluated. Water samples were titrated with Pb+2 followed by differential pulse anodic stripping voltammetry (DPASV). Experimental voltammetric values were interpreted assuming a macromolecular heterogeneous ligand described in a simple way by two types of binding sites, CL1 and CL2, where CL1 is related to stronger binding groups with lower concentration compared to CL2. Water quality parameters like dissolved organic matter (DOC), pH, salinity, temperature, and total lead concentration were measured during the period under study. The results pointed to a higher concentration of CL1 and CL2 sites in April probably due to the phytoplankton bloom. The decrease of KL1 with the increase of salinity from winter to summer may be caused by the increase of major cations (as Ca2+) in solution. The trend of KL2 followed the pH shift in all seasons since an increase of pH favors Pb2+ complexation with CL2 sites. Finally, the decrease of DOC in summer could be responsible for the decrease in the concentration of the different sites in solution from April to June, with a similar decrease of 35±3% for all of them.

Calcium polysulfide (CaS5) is widely used in stabilizing Cr(VI)-contaminated soils. The stability properties of Cr(VI) and Cr are the most important indexes for evaluating the effectiveness of stabilized soil. This study investigated the effect of CaS5 dosage and curing age on the stability properties of Cr(VI) and Cr in contaminated soils and the relationship between leachability and Cr speciation. Results show that increasing the CaS5 dosage and curing age could improve the stability properties of Cr in stabilized soil. The leachability and Cr(VI) content in the stabilized soil significantly decreased along with increasing CaS5 dosage and curing age. The changes in the leachability of the soil were attributed to the changes in the Cr speciation distribution and microstructure of the stabilized soil. The exchangeable fraction was mainly converted into an oxidizable fraction, and a dense structure (ettringite and elemental sulphur) was formed along with increasing CaS5 dosage and curing age. The exchangeable and reducible fractions depended on Cr leachability, and the Cr in the synthetic precipitation leaching procedures predominantly resulted from the exchangeable and reducible fractions.