The effects of water-flooding and wet-to-dry transition on phase transformation of schwertmannite chemically formed in constant pH condition

Schwertmannite formed under constant pH maintained by alkali additives exhibits great arsenic (As) adsorption capacities and has been used to remediate As-contaminated soil. However, variable soil moisture contents probably induce the phase transformation of schwertmannite, consequently affecting the environmental fate of contaminants in the mineral. This study investigates the effects of extreme water conditions such as water-flooding and wet-to-dry transition at 40 °C in paddy soil on the phase transformation of schwertmannite formed under constant pH (Sch-2.7) or uncontrolled pH (Sch) and their underlying mechanisms. Sch-2.7 exhibited significant phase transformation under the above conditions due to the lower crystallinity and more structural defects, whereas Sch remained stable. In water-flooding environments, goethite was the final transformation product within 120 h. Especially, Sch-2.7 more rapidly transformed into goethite in aerobic environments than in anaerobic environments. This was because Sch-2.7 catalyzed the oxidation of self-released Fe(II) by dissolved oxygen (DO) even under acidic (pH < 2.5) conditions, generating reactive oxygen species (ROS, such as ·OH radical and H₂O₂), which significantly expedited goethite formation. During the wet-to-dry transition, Sch-2.7 transformed within 4 h, surpassing the speed under water-flooding conditions. However, the extent of transformation was limited, predominantly on the surface. As water progressively evaporated, Fe and SO₄²⁻ released from Sch-2.7 enriched on its surface, forming a hydrophobic layer of FeOOH and parabutlerite. The surface layer effectively prevented the residual elements in Sch-2.7 from leaching, resulting in a significant leaching reduction of 92.8% for SO₄²⁻ and 99.7% for Fe, thus delaying further phase transformation. The transformation pathway variations of schwertmannite formed with alkali additives under different moisture contents have important implications for comprehensively understanding its behavior as an As remediation agent in paddy soil.