Contrasting effects of different types of organic material on surface area and microaggregation of goethite

In general, the specific surface area (SSA) of minerals decreases when covered with organic matter, but how different organic materials contribute to these effects is, at present, poorly understood. Our aim was to elucidate the impact of carbohydrate and phenolic polymers on the coverage efficiency, change in SSA and microaggregation of goethite (alpha-Fe-OOH). For this purpose we coated goethite with polygalacturonic (PGA) and tannic acid as model compounds and dissolved organic matter (DOM) from wheat straw as a natural reference. Changes in SSA and particle aggregation were monitored using N-2 adsorption, laser particle-size analysis and scanning electron microscopy (SEM). Our results showed that sorption of PGA and tannins failed to reach the decrease in SSA of the mineral as observed from the adsorption of DOM (-26% of initial SSA relative to -6% and -9% by PGA and tannins, respectively). The coating efficiency reached 0.18 in 2 mg(-1) C sorbed for DOM and 0.11-0.15 m(2) mg(-1) for the other compounds. Nevertheless, only DOM coatings resulted in particle aggregation. We conclude that, while soil organic matter can cause aggregation of goethite particles into larger clusters, some constituents of organic matter are ineffective as coating and aggregating agents. (c) 2007 Elsevier B.V. All rights reserved.