Lanthanum-modified biochar reducing arsenic bioavailability in soil and accumulation in rice: Characterization, performance, and mechanism

Arsenic (As) is a common toxic metalloid, and in As-contaminated paddy soils, safe rice production is crucial for human health. In this study, lanthanum-modified biochar (BC-L) was developed to reduce soil As availability and limit As accumulation in rice. Characterization using SEM-EDS, FTIR, XRD, and XPS revealed that La modification increased the biochar surface roughness, introduced La-OH groups, and facilitated La-O-As complex formation, which is essential for As immobilization. Applying BC-L significantly increased soil pH, CEC, and OM, helping convert bioavailable As into more stable forms and decreasing TCLP- and NaHCO₃-extractable As by up to 30.7% and 21.4%, respectively. Simultaneously, BC-L reduced As buildup in rice, with inorganic As in brown rice decreasing by 31.7–40.8% (below 0.35 mg/kg), while biomass increased by 6.0–14.5%. Mechanistically, BC-L acted through La complexation and iron plaque regulation at low doses (0.5–1 g/kg), thereby enhancing iron plaque sequestration. In contrast, higher doses (2–4 g/kg) primarily relied on direct soil As immobilization. It also lowered As phytotoxicity and prevented As translocation to grains. From an economic and practical perspective, the optimal application rate of BC-L was 1 g/kg, making it an effective amendment for remediating As-contaminated soils. • La-O-As complexes are the main cause for the immobilization of As • BC-L promotes rice growth and iron plaque formation • BC-L regulates plant As speciation to less toxic forms • 1 g/kg BC-L reduces inorganic As in brown rice to levels below the standard