Phosphate Removal in Relation to Structural Development of Humic Acid-Iron Coprecipitates

Precipitation of Fe-hydroxide (FH) critically influences the sequestration of PO₄ and organic matter (OM). While coatings of pre-sorbed OM block FH surfaces and decrease the PO₄ adsorption capacity, little is known about how OM/Fe coprecipitation influences the PO₄ adsorption. We aimed to determine the PO₄ adsorption behaviors on humic acid (HA)-Fe coprecipitates in relation to surface and structural characteristics as affected by HA types and C/(C + Fe) ratios using the Fe and P X-ray absorption spectroscopy. With increasing C/(C + Fe) ratios, the indiscernible changes in the proportion of near-surface C for coprecipitates containing HA enriched in polar functional groups implied a relatively homogeneous distribution between C and Fe domains. Wherein PO₄ adsorbed on FH dominated the P inventory on coprecipitates, yielding PO₄ sorption properties nearly equivalent to that of pure FH. Structural disruptions of FH caused by highly associations with polar functional groups of HA enhanced the C solubilisation. While polar functional groups were limited, coprecipitates consisted of core FH with surface outgrowth of HA. Although surface-attached HA that was vulnerable to solubilisation provided alternatively sites for PO₄ via ternary complex formation with Fe bridges, it also blocked FH surfaces, leading to a decrease in PO₄ adsorption.