Soil Organic Matter Adsorption on Iron Oxides and Kaolinite: Sequential and Selective Chemical Extractions

ABSTRACT

Stabilization of organic matter (OM), and formation and of aggregates are some beneficial effects of organo‐mineral association in soils. The objective of this study was to quantify the contributions of outer‐ and inner‐spheres adsorptions on total OM/mineral association by sequential and selective chemical extractions. To establish a conceptual model of different adsorption stabilities, pure minerals (kaolinite—Kt, hematite—Hm and goethite—Gt), abundant on highly weathered soils, and concentrated organic matter fraction (HF‐treated Histosols: OM1, OM2, OM3 and OM4) were used. An additional treatment was planned to check the formation of the ternary complex OMCa ²⁺ Kt. The five steps of sequential OM extractions were: K ₂ SO ₄ ; sodium hexametaphosphate (HMP); sodium dithionite (DIT); sodium pyrophosphate (PYR) and; NaOH. The OM adsorbed by outer‐sphere on Hm, Gt and Kt were considered the sum of K ₂ SO ₄  + HMP (anion exchange) and adsorption by the inner‐sphere on Fe oxides the sum of DIT+PYR (extraction of ferrol groups) and on Kt the NaOH (extraction of aluminol group). The organic carbon contents of OM/mineral were high, indicating adequate saturation of minerals (OM/mineral ratio in the saturation step of 1:4) (g kg ⁻¹ ): OM/Hm—92.1; OM/Gt—98.6; OM/Kt—97.6. Goethite presented the highest decrease in specific surface area after OM association (OM4/Gt—reduction of 21.1%). There was no increase in OM‐mineral adsorption with Ca(NO ₃ ) ₂ solution to formation of ternary complexes OM(COO) ⁻ Ca ²⁺ Kt(SiO) ⁻ . In relation to the initial OM/mineral mixtures, the average percentages of residual OC at the end of sequential extractions (after last extraction with NaOH) were: Hm—61.5%; Gt—70.7%; KtH ₂ O—60.3%; KtCa—60.8%. The greater stability of OM1/Hm (high residual OC) can be attributed to the higher occurrence of aromatic groups and the higher specific surface area of this mixture. The study concluded that there is strong adsorption by the inner‐sphere of OM on Hm, Gt and Kt; more stable the organo‐mineral complexes, lower rate of OM decomposition and higher carbon sequestration.