Cadmium immobilization in lake sediment using different crystallographic manganese oxides: Performance and mechanism

Cd pollution in sediments poses severe threats to environmental safety and human health. Mn oxides have potential merit for the remediation of Cd pollution in sediment but have not received enough attention. Although Mn oxides have proven effective as adsorbents for removing heavy metals from water/wastewater, the performance and the underlying mechanism of Cd immobilization in sediments by Mn oxides remain unclear. Here, three crystallographic Mn oxides δ-MnO₂, γ-MnOOH, and Mn₃O₄ were used as amendments to investigate their potential for the in situ immobilization of Cd in lake sediment. Experimental data showed that when the sediment samples were treated with synthesized Mn oxides at dosages of 2% and 6% (w/w) for 56 days, the TCLP (toxicity characteristic leaching procedure) leachable Cd in the sediment decreased by 43.9-66.81%, and the PBET (physiologically based extraction test) extractable Cd decreased by 45.16-99.40%. Additionally, the acid-soluble fraction of Cd was partially transformed to a residual fraction, resulting in a 27.55-35.49% decrease in acid-soluble Cd and a 25.16-30.36% increase in the residual Cd fraction. Sediment pH and oxidation-reduction potential were important factors affecting the bioavailability of Cd in the remediation process. Furthermore, scanning electron microscopy, X-ray diffractometer, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy analysis illustrated that the interaction between the amendment and Cd mainly involved complexation with O-containing groups, ion-exchange as > OCd⁺, and precipitation with carbonate. The efficient remediation capacity and associated mechanism for Mn oxides provide insights for the improved restoration of heavy metal-contaminated sediment.

Uptake of vegetable and soft drink affected transformation and bioaccessibility of lead in gastrointestinal track exposed to lead-contaminated soil particles

Accidental ingestion of Pb-contaminated soil particles by direct hand-to-mouth activity or by swallowing airborne dust particles is important pathway of human exposure to Pb. Appropriate evaluation of Pb risk to human is important in determining whether the soil needs remediation or not, however, there is paucity of data about the dietary influences on Pb bioaccessibility (Pb-BA) and transformation in humans. This study chose two typical foods, spinach and cola, representing vegetable and soft drink, respectively, and investigated their effects on Pb species in gastrointestinal tract using the physiologically based extraction test. Results showed that ingestion of spinach and cola decreased the Pb-BA by 52%-94% in the gastric phase and by 38%-95% in the intestinal phase, respectively. The reduction of Pb-BA by spinach was attributed to the precipitation of Pb with phosphorus in spinach and the sorption of Pb by the generated hydrolysate and un-hydrolysate from spinach in gastrointestinal tract. Cola decreased Pb-BA mainly via formation of insoluble Pb phosphates precipitates. Analysis of X-ray diffraction and MINTEQ modeling demonstrated that the dissolved Pb was transformed to precipitated or sorbed Pb with intake of cola or spinach. Our findings suggest that dietary habit greatly influence the speciation and subsequent Pb-BA in the gastrointestinal tract, which should be incorporated into human health risk assessment of Pb-contaminated soil.

Transformation and bioaccessibility of lead induced by steamed bread feed in the gastrointestinal tract

Accidental ingestion of contaminated soil has been recognized as an important pathway of human exposure to lead (Pb), especially for children through hand-to-mouth activities. Intake of food following the soil ingestion may affect the bioaccessibility of Pb in the gastrointestinal tract. In this study, the effect of steamed bread on the transformation and subsequent bioaccessibility of Pb in two soils was determined by the physiologically based extraction test (PBET). Two compounds, Pb(NO₃)₂ and PbCO₃, were included in the evaluation for comparison. In the gastric phase, Pb bioaccessibility decreased as the steamed bread increased due to the sorption of Pb on the undissolved steamed bread, especially in PbCO₃, Pb bioaccessibility decreased from 95.03% to 85.40%. Whereas in the intestinal phase, Pb bioaccessibility increased from 1.85% to 5.66% and from 0.89% to 1.80% for Pb(NO₃)₂ and PbCO₃, respectively. The increase was attributed to the transformation of formed Pb carbonates into soluble organic-Pb complexes induced by the dissolved steamed bread at neutral pH as indicated by MINTEQ modeling. For the PbCO₃-contaminated soil, the change in Pb bioaccessibility in both gastric and intestinal phases behaved like that in the pure PbCO₃ compound, the steamed bread increased the bioaccessibility of Pb in the intestinal phase, but the decreased bioaccessibility of Pb was observed in the gastric phase after the steamed bread was added. However, in the soil contaminated with free Pb²⁺ or sorbed Pb forms, the steamed bread increased the Pb bioaccessibility in both gastric and intestinal phases. This was probably due to the higher dissolved organic carbon induced transformation of sorbed Pb (Pb sorbed by Fe/Mn oxides) into soluble Pb-organic complex. Results from this study indicated that steamed bread had an influence on the Pb speciation transformation, correspondingly affecting Pb bioaccessibility in the gastrointestinal tract.