Assessment of biochar filter application in improving chromium stress tolerance and plant physiology in Chinese cabbage (Brassica rapa) under a flow-through water setup.

Background: With the increasing use of industrial wastewater for irrigation and the growing prevalence of heavy metal contamination in soils, chromium (Cr) pollution poses a significant threat to crop safety, particularly in industrially concentrated regions. Although biochar has been widely applied in soil remediation, its potential use as a real-time filtration medium in dynamic hydroponic systems remains largely unexplored. To address this gap, the present study investigated the efficacy of different biochar concentrations (0.5, 1.25, and 2.5 g/L) in mitigating Cr-induced stress (20 mg/L Cr (VI)) in a hydroponic system using Brassica rapa L., a fast-growing, Cr-sensitive leafy vegetable, as a model crop. The study aimed to evaluate plant growth and physiological responses under Cr stress and provide innovative strategies for protected agriculture.

Results: Scanning Electron Microscopy (SEM) and Brunauer-Emmett-Teller (BET) analysis revealed a highly porous biochar structure, while Fourier Transform Infrared Spectroscopy (FTIR) confirmed the presence of key functional groups (-OH,-COOH) essential for Cr adsorption. X-ray Diffraction (XRD) indicated the presence of well-crystallized minerals such as quartz. Additionally, X-ray Photoelectron Spectroscopy (XPS) analysis verified the successful adsorption of chromium on the biochar surface and revealed the coexistence of Cr (VI) and Cr (III) species, indicating that partial reduction of Cr (VI) occurred during the adsorption process-likely facilitated by redox-active oxygen-containing groups. In the absence of biochar, Cr exposure significantly reduced plant biomass, chlorophyll content, and antioxidant enzyme activity while increasing oxidative stress markers and Cr accumulation in plant tissues. In contrast, biochar treatments-particularly at 2.5 g/L-significantly improved plant growth, enhanced chlorophyll and antioxidant activity, decreased Cr accumulation in roots and shoots, and alleviated oxidative stress. At this optimal dose, soluble sugar and protein contents increased by 52.8% and 114.4%, respectively. Correlation analysis showed a strong negative relationship between Cr accumulation and growth traits, and a positive correlation between antioxidant enzyme activity and stress mitigation. Hierarchical Cluster Analysis (HCA) and radar chart visualizations further confirmed the distinct physiological profiles induced by biochar, with 2.5 g/L treatment demonstrating the most balanced improvements across multiple traits.

Conclusions: This study is the first to explore the use of biochar as a dynamic filtration medium for Cr remediation in hydroponic systems, demonstrating its dual role in Cr adsorption and physiological stress alleviation. The 2.5 g/L dose was identified as optimal, reducing Cr accumulation in aerial tissues by 62.4% and increasing soluble protein content by 114%. These findings offer a practical and environmentally sustainable solution for managing heavy metal risks in hydroponic and urban agriculture, particularly in resource-limited settings. The proposed technology combines environmental and economic benefits, making it especially suitable for facility-based agricultural production systems.