Magnetic recovery reveals sequential extraction bias and supports mass-conservative assessment of lanthanum-based sediment phosphorus inactivation.

Lanthanum-based phosphorus inactivation agents (PIAs) are widely used to mitigate internal phosphorus (P) loading in eutrophic lakes, yet their performance is commonly assessed by sequential extraction of PIA-amended sediments. When reactive PIA particles remain in the sediment matrix, phosphate released during extraction may re-adsorb onto remaining reactive PIA surfaces, leading to overestimation of apparent P immobilization. Here, we synthesized magnetically recoverable magnetite/lanthanum hydroxide beads (MLaB) as a model La-based PIA to isolate and quantify this bias in a controlled anoxic sediment-water system using homogenized sediment. Magnetic recovery allowed sediment fractionation to be performed with and without prior MLaB removal and enabled direct quantification of P accumulated on the recovered MLaB. Compared with separated samples, non-separated fractionation overestimated depletion of releasable sediment P by 8.42% and 13.06% at the low and high dosages, respectively, with the bias mainly expressed in BD-P and NaOH-P. Across all sampling points, P measured on recovered MLaB closely matched system-scale reactive P depletion, clustering near the 1:1 relationship with a mean absolute deviation of 5.23%. La L3-edge X-ray absorption spectroscopy further showed that immobilized P was associated with rhabdophane-like LaPO4·nH2O phases. These results demonstrate that magnetic recovery of a model La-based inactivation agent can identify extraction-induced overestimation and provide a direct, mass-conservative endpoint for assessing P immobilization by the MLaB phase.