Cr incorporation, aggregation and phase transformation of Fe/Cr-organic coprecipitates: Roles of organic functional groups.

Coprecipitation of chromium and organic matter (OM) with Fe(III) hydroxide nanoparticles (NPs) controls Fe/Cr/C cycling. However, how OM's functional groups control composition (C/Fe and Cr/Fe ratios) and stability (aggregation and phase transformation) of Fe/Cr-OM coprecipitates remains unclear. Humic acid (HA), sodium alginate (SA), bovine serum albumin (BSA), and dextran (DET) were selected as model OMs. Combined macroscale and nanoscale analyses revealed that Cr was associated with both Fe hydroxide phases and OM-rich domains. Spatially Cr was strongly associated with Fe hydroxide phase through lattice substitution, whereas bulk Cr/Fe differences arose primarily from OM functional group-dependent Cr complexation, with ∼5% of aqueous Cr complexed by HA/SA compared with < 1% by BSA/DET. OM chemistry also controlled aggregation pathways and phase transformation. Carboxyl-rich HA and SA promoted fast random aggregation through carboxyl deprotonation and surface charge neutralization, and their strong HA/SA adsorption inhibited ferrihydrite transformation. Unexpectedly, BSA, containing both carboxyl and amine groups, induced oriented aggregation and accelerated ferrihydrite transformation to nano-lepidocrocite. DET affected neither colloidal stability nor phase transformation, owing to weaker binding and deprotonation of hydroxyl over carboxyl. This study identified functional-group-specific controls on Fe/Cr-OM coprecipitate composition and stability, improving understanding of Fe, Cr, and C cycling.