Evaluation of fire retardant leaching and redistribution in unmodified and thermally modified Scots pine ( Pinus sylvestris L.) after one-sided water exposure

Abstract Monoammonium phosphate (MAP) is a commonly used fire retardant for wood protection. While effective at appropriate concentrations, exposure to moisture can cause redistribution and leaching, reducing the long-term fire performance. In this study, the uptake, distribution, leaching and redistribution of MAP was investigated in unmodified and thermally modified Scots pine ( Pinus sylvestris L.) subjected to one-sided water exposure. Fire retardant distribution was evaluated using X-ray fluorescence, while inductively coupled plasma mass spectrometry was used for quantitative analysis. Higher MAP uptake was observed in unmodified specimens compared to thermally modified, while similar initial distribution patterns were identified within both material groups. Following one-sided water exposure, unmodified MAP-treated specimens exhibited greater MAP depletion and a deeper affected region than thermally modified MAP-treated specimens. In both groups, depletion was most pronounced near the exposed surface. These findings highlight that redistribution, and surface depletion should be considered when evaluating the long-term performance of fire retardant treated wood.