Sediment-derived humic fractions regulate phosphate adsorption and retention by calcined modified red clay

Calcined modified red clay produced at 700 ° C (MRC-700) has been widely applied for phosphate removal in eutrophic waters. However, humic substances released from sediments may interfere with phosphate fixation and reduce its retention stability. This study employed sediment-derived humic acid (HA) and fulvic acid (FA) as representative humic fractions to examine their effects, together with critical water chemistry parameters, on phosphate adsorption, competition, and release behaviors related to MRC-700. Both HA and FA inhibited phosphate adsorption by MRC-700, although the magnitude of inhibition differed substantially between the two fractions. At the highest humic-fraction concentration tested, HA and FA reduced phosphate adsorption at 480 min by 74.1% and 40.8%, respectively. Preloading experiments further demonstrated that HA was more effective at destabilizing previously fixed phosphate. At a humic-fraction concentration of 100 mg·L^-1, phosphate desorption induced by HA and FA reached 17.9% and 7.5%, respectively. Water chemistry conditions further regulated these competitive interactions. Lower pH and higher temperature enhanced phosphate adsorption by MRC-700, whereas the inhibitory effects of humic fractions persisted across these conditions. Increasing ionic strength mitigated the adverse influence of humic fractions on phosphate adsorption. Overall, the results indicate that sediment-derived HA can substantially constrain both the phosphate adsorption capacity and retention stability of MRC-700. These findings highlight the importance of considering humic-fraction composition and water chemistry when evaluating the effectiveness of mineral-based phosphorus control materials in eutrophic aquatic systems.