Numerous studies have reported the potential of silica as a biochar (BC) modifier. However, despite its high silica content, fly ash is rarely used for BC modification. Herein, modified BCs were produced by co-pyrolysis of corn stalks with alkali-fused fly ash (AFFA) at 200 and 600 °C (denoted as AFFA/BC). The Pb(II) adsorption mechanism and adsorption performance were investigated. The AFFA/BC had larger specific surface areas than the pure BC samples (2.54–137 vs. 0.50 m² g⁻¹) owing to their stable carbon structure. The Pb(II) adsorption capacity of AFFA/BC in water was approximately 6% higher than that of BC owing to the increased cation (Na⁺) exchange and new bonding sites, such as C–O and Si–O. AFFA/BC exhibited good Pb(II) adsorption performance in high-concentration simulated wastewater (pH 4–6), with a maximum adsorption capacity of 110.29 mg g⁻¹. The Pb(II) adsorption mechanism was in accordance with the pseudo-second-order kinetic and Langmuir isotherm models. At 25 °C and pH 5, the theoretical Pb(II) adsorption capacities of AFFA₂₀₀/BC and AFFA₆₀₀/BC were 201.66 and 186.81 mg g⁻¹, respectively, compared to 145.98 mg g⁻¹ of BC. Physical adsorption, precipitation, cation exchange, and complexation were identified as the main Pb(II) adsorption mechanisms through X-ray photoelectron spectrometry. Graphical Abstract

The oxidation of biochar occurs due to both natural and human influences during the soil carbon sequestration process. Therefore, it is crucial to produce high-stability biochar to achieve carbon neutrality. Fly ash-doped biochar was obtained from fly ash and corn stalks by employing hydrothermal/pyrolysis treatment, along with alkali impregnation at different temperatures. The microstructural characteristics and carbon sequestration potentials were studied as an essential performance parameter that was influenced by mineral doping and treatment temperature. The yield and carbon retention of P500-1:2 improved by 54.15% and 6.81%, respectively, and the carbon loss following H₂O₂ oxidation was only 9.93% as depicted by the results. In comparison with hydrothermal biochar, pyrolysis biochar is superior in terms of its carbon sequestration potential. SiO₂, Al₂O₃ and other components in fly ash continue to dissolve at high temperatures and react with carbon in biochar, promoting the formation of aromatic carbon and generating a physical protective layer to prevent biochar from oxidation, hence improving the chemical and thermal stability of biochar. High temperature and mineral interaction also contribute to high aromatic structure (H:C < 0.4) formation, significantly improving the specific surface area and thermal stability of biochar. Graphical Abstract

The concentration, spatial distribution characteristics, river–lake relationship analysis and source apportionment of heavy metals in the sediments of the Yangtze River, Poyang Lake and its tributaries were studied in this work. Heavy metals were detected more frequently in the sediments of the Yangtze River compared with the sediments of Poyang Lake and its tributaries. V, Cr, Pb and Ni were the dominant heavy metals in Poyang lake, with V being the most abundant in the lower Yangtze River, Poyang Lake and its tributaries. As, Cu, Ni and V showed similar distribution patterns, with a fan-shaped increasing trend in the southwestern area of Poyang Lake. The spatial distribution of Cr, Hg, Pb and Cd showed a large spatial variability with a decreasing distribution from the northwest to the southeast of the lake. The heavy metals in the sediments of Poyang Lake are related to those in its tributaries. The organic matter, oxidation-reduction potential and depth of sediments are correlated with the heavy metals in sediments. Mining, industrial and road traffic sources were the main sources of heavy metals in the study area. Except for Cd and Hg, most heavy metals in Poyang Lake exhibited a low ecological risk in an environmental evaluation. The results of this study might guide future studies on heavy metals in the sediments of Poyang Lake.