In this study, a redox precipitation method was used to load manganese dioxide (MnO₂) nanoparticles on biochar (BC) (BC@MnO₂) pyrolyzed from the invasive water hyacinth, and the adsorption of Cd(II),Cu(II), Zn(II), and Pb(II) was investigated. Several techniques were used to characterize the adsorbents. The results revealed that the BC surface was covered by many intertwined thin amorphous MnO₂ nanosheets, which significantly increased its specific surface area and pore volume. The adsorption of heavy metal ions by BC was negligible, whereas the MnO₂-containing adsorbents exhibited a high capacity for adsorbing heavy metal ions. However, the MnO₂-normalized adsorption amount decreased with increasing MnO₂ load and was largely unchanged at MnO₂ loads of 26.6% to 30.2%. The capacity for adsorbing heavy metal ions of BC@MnO₂ was pH-dependent, but the adsorption affinity was unaffected by coexisting ions. Column tests revealed that BC@MnO₂ with a load of 26.6% had a high capacity for removing heavy metal ions from simulated and real electroplating wastewater. Therefore, BC@MnO₂ with a load of 26.6% shows promise as a regenerable adsorbent for removing heavy metal ions from water/wastewater. This study could lay an essential foundation to develop a win-win strategy for heavy metal ions removal from wastewater using biochar derived from water hyacinth.

Zinc oxide nanoparticles (ZnO NPs) have been applied widely in agriculture, and many studies were conducted to evaluate the effect of ZnO NPs on plant growth. So far, few studies have been investigated with regard to the potential effect of ZnO NPs on cereal yield formation or Zn content in grains. Herein, we used a pot experiment, which was conducted involving five dosages of Zn (0.3, 0.6, 1.2, 2.4, and 4.8 g Zn pot⁻¹), to evaluate the impacts which ZnO NPs made in rice yield, dry matter accumulation, rice quality and grain Zn contents. The results demonstrated that ZnO NPs increased the production of grain, dry matter accumulation and particulate Zn content. Compared with control treatment, ZnO NPs application presented higher rice yield with more panicle number (4.83–13.14%), spikelets per panicle (4.81–10.69%), 1000-grain weight (3.82–6.62%) and filled grain rate (0.28–2.36%). Additionally, the dry jointing, heading and mature periods, LAI, SPAD values, and photosynthetic potentials of ZnO NPs were all significantly higher relative to treatment without ZnO NPs. The more photosynthetic substances and higher dry matter accumulated in the whole rice growing stage resulted in higher rice grain yield. Furthermore, ZnO NPs increased brown rice rate, milled rice rate, head rice rate, chalkiness size, chalkiness grain rate, chalkiness degree, amylose content and protein content, improving rice processing and appearance qualities. For the Zn nutrition in rice grain, ZnO NPs application significantly increased the Zn content of edible polished rice and promoted the relocation of Zn from the aleurone layer. This study effectively demonstrated that ZnO NPs could be a potential high-performed fertiliser for enhancing rice yield and quality.

Zinc (Zn) is a critical co-factor for enzymes involved in photosynthesis, delaying leaf senescence and enhancing photosynthetic efficiency. Supplementing rice leaves with Zn can improve yield, quality, and Zn content in edible parts, addressing food security and micronutrient deficiencies. In this study, we evaluated the effects of spraying Zn oxide nanoparticles (ZnO NPs) (0, 5, 10, and 20 mg L⁻¹) at the rice panicle initiation stage on photosynthesis, yield, and grain quality through a two-year field experiment. Results showed that foliar application of ZnO NPs at the panicle initiation stage increased the leaf area index, net photosynthetic rate, and photosynthetic potential, leading to a 1.5–6.4% increase in grain yield through a higher grain filling rate and 1000-grain weight. ZnO NPs also delayed leaf senescence and prolonged the duration of active photosynthesis, which significantly contributed to higher biomass production and improved grain filling, further enhancing yield. Additionally, the enhancement in photosynthetic efficiency and delayed senescence promoted the production of high-quality grains. ZnO NPs improved rice appearance quality by reducing the chalkiness grain rate and degree. The rice tasting value increased by 3.3–7.0%, reflecting improvements in appearance, viscosity, and balance, along with reductions in hardness. ZnO NPs raised peak viscosity and breakdown values while lowering setback values. Furthermore, ZnO NPs significantly increased Zn content in brown and milled rice by 13.8–56.0% and 20.1–78.6%, respectively, and improved Zn bioavailability by reducing the phytate-to-zinc molar ratio. These findings highlight the potential of ZnO NPs as a sustainable nanotechnology-based approach to simultaneously improve rice productivity, quality, and nutritional value, offering a promising solution for addressing food security and micronutrient deficiency in rice-based diets.

Improving rice yield, eating quality, and zinc (Zn) nutrition is crucial to meet the growing demand for high-quality and nutritious food, while zinc itself plays a pivotal role in rice growth and quality formation. In this two-year field study, we investigated the effects of zinc oxide nanoparticles (ZnO NPs) one-time foliar application at 5, 10, and 20 mg L⁻¹ during the gestation stage on grain yield, rice quality, and zinc biofortification. Although some year-to-year variations and year × treatment interaction in the magnitude of these effects were observed, the positive responses to ZnO NPs were consistent across the two years. Results showed that ZnO NPs application increased grain yield by 1.40–4.62%, mainly supported by enhanced net photosynthetic rate and SPAD values after heading. Meanwhile, ZnO NPs significantly enhanced taste value (1.61–7.22%) and breakdown value (5.36–15.63%), while reducing chalkiness rate (5.73–18.62%), chalkiness degree (11.57–27.18%), amylose content (3.72–6.76%), and setback value (8.98–24.53%). Additionally, ZnO NPs markedly increased the zinc content (23.73–85.10% in brown rice; 29.07–103.07% in polished rice) and reduced the phytic acid to zinc molar ratio by 18.46–48.39%, improving zinc enrichment and bioavailability. These findings suggest that ZnO NPs foliar application is effective to simultaneously enhance grain productivity, rice quality, and zinc density.