A description of the aggregation properties of aquatic pedogenic fulvic acids: Combining physico-chemical data and microscopical observations

Abstract

The size and aggregation properties of water fulvic acids of pedogenic origin (the pedogenic refractory organic matter or PROM) is discussed on the basis of: (1) physico-chemical data (molecular weight, diffusion coefficient, molecular size both in solution and while adsorbed at air-water and mercury water interfaces); and (2) photomicrographic data using transmission electron microscopy, or TEM, applied to ultrathin sections of embedded PROM. The usefulness of combining these two types of technologies is demonstrated. The former indicates that aggregation occurs at PROM concentrations > 200 mg l −1 and that the size of dissolved non-aggregated molecules is close to 1 nm. The size of molecules adsorbed at some interfaces is smaller, possibly as a result of dehydration. Using TEM, only particles with a size > 2–3 nm are clearly resolved, a result which may be due to (a) the limited resolution of the TEM technique employed and/or (b) possible aggregation processes which occur during the preparation of samples. Microscopical observations show that most of the PROM fraction is composed of 3 nm granules, either isolated or aggregated together into much larger entities. The granules are similar to those observed by others studying soil fulvic acids. The aggregates must be loose ones, as they can pass ultrafiltration membranes, even when the aggregate size is much larger than the pore size. An integrated interpretation of the results from both types of technologies suggests that, in fresh surface waters, PROM can form a continuum of aggregated particles with widely varying size, in rather fast equilibrium. At surfaces, dehydration may occur, making the aggregation a less readily reversible process. This finding leads to a conclusion of particular interest to practical water scientists and engineers; ultrafiltration is not always a simple process when applied to waters rich in fulvic acids and it could become an unreliable one if induced colloid formation is not controlled. The implications of the results are discussed with respect to: the behaviour of PROM in natural waters; the interpretation of electron microscopical observations; the interpretation of ultrafiltration fractionation results.