Acacia Biochar Reduces Arsenic Uptake and Enhances Growth of Lettuce (Lactuca sativa) in a Contaminated Hydroponic System

Hydroponic and soilless systems are increasingly adopted as low-cost, sustainable solutions for global food production, yet they remain highly susceptible to contamination by potential toxic elements (PTEs), particularly arsenic. While biochar is widely recognized as an effective amendment for mitigating PTE contamination in soil-based systems, its ability to alleviate PTE stress in hydroponic environments has been largely overlooked. The gap reveals a critical and underexplored frontier in controlled-environment agriculture, where extending biochar-based mitigation strategies could yield substantial benefits. Here, we evaluated whether Acacia auriculiformis wood biochar could alleviate arsenic (As) toxicity in lettuce (Lactuca sativa) grown in a continuous-flow hydroponic system. Using a completely randomized factorial design (arsenic species × dose × biochar) with three independent replicates per treatment, we tested biochar under 0.2 and 0.8 mg/L of As(III) and As(V). Arsenic significantly (p < 0.05) reduced lettuce growth, with As(III) being more toxic than As(V). Biochar significantly (p < 0.05) improved morphological traits (2.4–103%), cell membrane stability (5.5–12%), photosynthetic pigments (3–73%), and stress indicators proline (8–11%) and malondialdehyde (8–14%). Arsenic accumulated mainly in roots (1.7–2.63 mg/kg) and shoots (0.76–1.36 mg/kg), but biochar reduced accumulation by 28–47% in roots and 33–48% in shoots. Additionally, biochar enhanced nutrient uptake (K, P, Ca, Mg, B, Zn, Cu, S, Mn) at both arsenic levels. Overall, the results indicate that Acacia biochar can substantially reduce arsenic toxicity and improve plant physiological responses in continuous-flow hydroponics, highlighting its promise as a viable and scalable mitigation tool for safeguarding soilless food production systems against PTE contamination.