Enhanced lead accumulation in cassia bark linked to lichen and algae: A case study from Northern Vietnam.

High lead (Pb) levels in cassia products pose a significant threat to both the global spice supply chain and public health. Addressing this critical issue, our study investigates Pb accumulation within the soil-cassia system in Northern Vietnam, a major cassia-producing region. Soil and cassia samples were collected from six key cassia-growing regions along the Red River, and their compositions and properties were systematically evaluated. Although soil Pb concentrations were low, significant enrichment of Pb was detected in the cassia bark skin (1.6-3.3 mg kg). Notably, the enhanced accumulation of Pb in cassia bark shows a clear correlation with the presence of epiphytic lichen and algae. We observed that Pb concentrations in these lower plant species were approx. one order of magnitude higher than in the bark itself. Our findings strongly suggest that lichen and algae likely act as adhesion agents, significantly contributing to increased Pb accumulation on the bark. Despite low soil Pb concentrations indicating it is not the primary source, evidence points strongly towards airborne Pb deposition; therefore, the removal of epiphytic, surface-dwelling lower plants should be prioritized as a key mitigation strategy. We strongly recommend expanding research efforts to develop proactive and preventive measures.