Interactions of dissolved organic matter with short-range ordered aluminosilicates by adsorption and co-precipitation

Abstract

Retention of dissolved organic matter (DOM) by short-range ordered aluminosilicates (SROAS) by adsorption and co-precipitation contributes to carbon accrual in soils and sediments. In this study, we investigated effects of SROAS composition on DOM adsorption, partitioning of carbon moieties by adsorption and co-precipitation, and the mineral structure of co-precipitates. We used four types of sorptive solutions, representing DOM collected in situ from topsoil and subsoil horizons of a Dystric Cambisol, and water-extracted DOM from beech and fir litter. We studied adsorption of soil DOM on three SROAS that structurally resemble proto-imogolites and Si-rich allophanes as a function of contact time (1–168 h) at initial pH 5. Co-precipitation of soil and litter-extracted DOM was quantified as a function of the molar Al:C ratio (0.3–1.4) and at two levels of Si concentration (molar Al:Si = 1 and 2). To resolve the impact of DOM on mineral structure, we first examined time-dependent structural evolution of SROAS within 1 to 72 h and subsequently investigated the effects of DOM interference in crystallization processes. Mineral structure of SROAS and co-precipitates was resolved by infrared, solid-state 27Al- and 29Si-NMR spectroscopy. Chemical composition of DOM prior to reaction with SROAS and in co-precipitates was analysed by solid-state 13C-NMR spectroscopy. Maximal C contents of adsorption complexes were 7.1 mg C g−1 for Al-rich and 20.4 mg C g−1 for Si-rich SROAS. We found selective adsorption of aromatic C and preferential exclusion of polysaccharide and alkyl C for both topsoil and subsoil DOM. Adsorption was larger for a Si-rich SROAS, since it exhibited a greater accessibility of aluminol groups. As a function of aromatic C content in initial DOM, 39.9 to 81% of C was retained by co-precipitation. Composition of co-precipitated organic matter was determined by C speciation in DOM supply, involving marked uptake of polysaccharides. In the absence of DOM, up to 50% of Si was present in imogolite-like configuration after 72 h, evidencing rapid development of short-range order. Complexation of Al by DOM during formation of SROAS caused partial exclusion of Si and slowed structural evolution, consequently enhancing abundance of ill-defied Si species in co-precipitates. Interactions of DOM with SROAS may cause selective accumulation of organic compounds and promote Si mobility in Andosols and Podzols.