Nanoscale and molecular evidences for adsorptive fractionation of dissolved organic matter at the interfaces of Al-bearing ferrihydrite and water

Abstract

Ferrihydrite (Fh) usually exists in the form of Al-bearing Fh in soils and sediments, and Al substitution may have a remarkable influence on Fh characteristics, controlling its reaction with dissolved organic matter (DOM). Yet, little is known about the impacts of Al-bearing Fh on the fate of DOM. Here, selective binding and molecular fractionation of DOM was investigated at the interfaces of Al-bearing Fh and water. Al substitution altered surface properties such as point of zero charge (PZC), surface OH groups, and specific surface areas (SSA), structure, composition, and adsorption capacity of Fh. Specifically, within 30 mol% Al substitution, Al entered into ferrihydrite structure by isomorphous substitution to form Al-substituted ferrihydrite and immobilized DOM increased with Al substitution owing to increasing surface OH groups, SSA, and pore volumes. Once the amount of added Al exceeds 30 mol%, gibbsite was formed except for Al-substituted Fh, and adsorbed DOM decreased with Al addition. The coordination environment of Al-substituted Fh consisted of Fe-O and edge- and corner-sharing FeO₆ octahedral. Microscopic analysis at nanoscale disclosed that DOM was evenly distributed within Al-bearing Fh aggregates and on gibbsite surface, and DOM immobilized within Al-bearing Fh nanopores had a higher oxidation state. Mass spectrometry analysis at molecular scale revealed that compared with gibbsite, high molecular weight substances and substances containing more oxygenated groups or highly in unsaturation preferentially bound to Al-bearing Fh, and Fh with 30 mol% Al substitution induced most pronounced molecular fractionation. Collectively, these findings shed novel insights into the impact of Al substitution on interfacial adsorptive fractionation of DOM, contributing to in-depth understanding geochemical cycling of C and predicting organic C cycling across aquatic-terrestrial interfaces.