Reaction of forest floor organic matter at goethite, birnessite and smectite surfaces

Experiments were conducted to compare the affinity and reactivity of three different minerals for natural organic matter (NOM) in forest floor leachat e (FFL) from hardwood and pine forests. The FFLs were acidic (pH 4) with io nic strengths of 1.4 mM (hardwood) and 1.1 mM (pine), and they contained la rger organic molecules (weight average molecular weights [Mw] = 5-6 kDa) th an has: been reported recently for surface waters using similar methods. A synthetic diluent solution was prepared to match the inorganic chemistry of the FFL and to provide a range of initial dissolved organic carbon (DOC) c oncentrations (0-140 g C m(-3)) for reaction with goethite (alpha -FeOOH), birnessite (delta -MnO2) and smectite (montmorillonite, SWy-2) in suspensio n, and in corresponding blanks. A variety of macroscopic and spectroscopic methods were employed to show th at reaction with the three minerals resulted in distinctly different NOM ad sorption, fractionation and transformation patterns. Goethite exhibited a s teep initial slope in the adsorption isotherm and a maximum retention of 10 .5 g C kg(-1). The isotherm for montmorillonite was more linear, but equal amounts of C were adsorbed to goethite and montmorillonite (per unit sorben t mass) at maximum DOG. Whereas preferential uptake of high Mw, aromatic co nstituents via ligand exchange was observed for goethite, compounds of lowe r than average Mw were retained on montmorillonite: and no preference fur a romatic moieties was observed. Birnessite, which has an isoelectric point o f pH < 2, retained low amounts of organic C (<2 g C kg(-1)) but exhibited t he highest propensity for oxidative transformation of the NOM. The data ind icate that fractionation behavior of NOM is dependent on mineral surface ch emistry in addition to sorbent affinity fur organic C. This work also empha sizes the fact that abiotic transformation reactions must be considered in studies of NOM interaction with Fe(III) and Mn(IV) containing solid phases. Copyright (C) 2001 Elsevier Science Ltd.