Wood-derived biochar as a potential sustainable alternative to lichens for monitoring airborne potentially toxic elements

In this study, we evaluated two wood-derived biochars (BCH) as passive monitoring matrices for atmospheric PTEs, benchmarking their performance against the epiphytic lichen Evernia prunastri (L.) Ach. The materials were first characterized in terms of elemental composition, surface chemistry (XPS), specific surface area (BET), and morphology (SEM). They were then exposed in outdoor and indoor campaigns, representative of combined wet and dry deposition outdoors and predominantly dry deposition indoors. Exposure settings were selected to capture contrasting deposition regimes and to enable an overall (setting-agnostic) comparison of matrix sensitivity, not to rank settings by contamination. The accumulation of As, Cd, Cu, Zn, Ni, Pb, Cr, and Sb was quantified by ICP-MS and interpreted using the Exposure-to-Control (EC) ratio and the Pollution Load Index (PLI). Calculated as the geometric mean of EC ratios, PLI summarized the relative multi-element accumulation response of each matrix. Accumulation patterns varied among elements and exposure settings. BCH2 showed the highest overall PLI, although BCH1 had the largest BET surface area (62.1 m2 g−1, compared with 37.8 for BCH2 and 30.1 for the lichen), indicating that surface area alone does not explain the observed accumulation patterns. Outdoor magnetic measurements showed a larger absolute increase in mass-specific susceptibility in BCH2 (64.0 × 10−9 m3 kg−1) than in the lichen (10.6 × 10−9 m3 kg−1). However, the high intrinsic magnetic background of BCH2 resulted in substantially lower relative magnetic contrast, limiting its suitability for magnetic biomonitoring. Overall, the results support the potential of biochar as a complementary matrix for chemical monitoring of atmospheric PTEs, including indoor settings.