Effect of oxygen deprivation on soil hydrophobicity during heating

Abstract

Previous studies of the effects of heating on soil hydrophobicity have been conducted under free availability of oxygen. Under fire, however, soils may be deprived of oxygen due to its consumption at the heat source and inadequate replenishment in the soil. In the present study, effects of heating on soil hydrophobicity are examined for three initially hydrophobic Australian eucalypt forest soils under standard and oxygen-deprived atmospheres for temperatures (T) of 250–600°C and durations (t_E) 2–180 min. Hydrophobicity assessments using water droplet penetration time (WDPT) tests indicate substantial differences between the absence and presence of oxygen. Heating to 250–300°C enhanced hydrophobicity from initial respective WDPTs of 2029 s, 361 s and 15 s to > 18 000 s for all samples under both atmospheres. Depending on heating duration, hydrophobicity was eliminated (WDPTs ~0 s) in air between 210 and 340°C, but under oxygen-deprived conditions between 400 and 510°C. Relationships between the destruction temperature for hydrophobicity T_D and t_E provide temperature–duration thresholds below which hydrophobicity persists under oxygen concentrations <21%. As established temperature–duration thresholds for hydrophobicity destruction are based on the free availability of oxygen, caution is advised in their applicability to field situations where heating under burning may occur in oxygen-depleted conditions.