Chitosan-lignin hydrogels enriched with biochar and Se/Cu nanoparticles for the mitigation of cadmium and drought stress in maize

Contamination of agricultural soils with heavy metals poses a critical threat to global food security and human health due to their high mobility and long biological half-life. Consequently, there is an urgent need for innovative remediation strategies, such as the development of safe multi-functional soil amendments, that do not disrupt food production. Among various additives, biochar (BC), obtained through the pyrolysis of organic wastes (including agricultural residues), is one of the most extensively studied. BCs can immobilize pollutants due to their developed surface area and abundance of functional groups. Hydrogels (HG) are another type of modifier that can simultaneously immobilize metals and improve the water-holding capacity of soils. Special attention is given to biopolymer-based HGs due to their biocompatibility and biodegradability. Furthermore, nanoparticles (NPs) have been reported to decrease heavy metal toxicity to plants. Thus, in this study, a series of hybrid chitosan-lignin HGs enriched with wheat straw-derived BC and selenium (Se) or copper (Cu) NPs were developed, and their effect on maize germination under water-deficit and cadmium contamination stress was evaluated.

During the study, agricultural soil was modified using 1% (w/w) of the developed HGs: (1) HG filled with BC; (2) HG filled with BC and SeNPs; (3) HG filled with BC and CuNPs; and (4) a combination of HGs (2) and (3). The plant growth experiment was conducted in a growth chamber and included soils contaminated with cadmium (35 mg/kg) and uncontaminated controls. Two weeks after sowing, watering was stopped, to simulate water-deficit conditions, and water evapotranspiration was monitored gravimetrically. After one week, seedlings were collected, and their fresh/dry mass and length were determined.

A decrease in evapotranspiration rates was observed for the soil modified with HGs. For example, the control soil lost 68 g/pot of water during 7 days, while the soils modified with HG/BC and HG/BC/SeNPs lost 59 and 57 g, respectively. Additionally, these HGs demonstrated a stimulatory effect on maize growth. The average shoot height increased from 18.3 cm in the control to 20.9 cm, and dry mass rose from 0.029 g to 0.037 g for the soils modified with HG/BC and HG/BC/SeNPs. The root dry mass also increased in both cases. Moreover, under cadmium contamination, both hydrogels neutralized the negative impact of the heavy metal on shoot growth. In contrast, HG filled with BC and CuNPs had an inhibitory effect on plant biomass growth. The mixture of hydrogels demonstrated a moderated effect on plant germination.

Acknowledgements: The research was funded by the Polish National Agency for Academic Exchanges under the Strategic Partnerships Program (BNI/PST/2023/1/00108) and the National Science Centre (2024/08/X/NZ9/00561).