Is the adsorption of soil organic matter to haematite (-Fe2O3) temperature dependent?

Soil organic matter interactions with minerals are a key determinant in the long-term protection and sequestration of carbon in soils and sediments. Climate warming could potentially alter the dynamics of the Earth's largest carbon pools through processes controlling the formation of mineral-organic associations (MOAs). We studied the adsorption of Elliott soil humic acid (HA) and fulvic acid (FA) to the iron oxide mineral haematite (-Fe2O3) at temperatures from 15 to 35 degrees C through batch sorption experiments. The G degrees, H degrees and S degrees of sorption were derived for HA and FA and suggested that formation of MOAs was spontaneous and exothermic. The Langmuir sorption coefficients for HA and FA decreased with warming, but the maximum sorption capacities were not temperature dependent. The decrease in sorption coefficient was less when the temperature was increased from 30 to 35 degrees C than from 15 to 30 degrees C. Our infrared, ultraviolet and visible spectroscopy analysis of the bound and unbound HA and FA revealed preferential adsorption of oxygen- and hydrogen-substituted aromatic and aliphatic carbon, which was independent of temperature. The adsorption of a hydrophobic probe molecule, Rose Bengal, to haematite increased with warming. Our results suggest that (i) MOAs in warmer climates might be less sensitive to temperature changes, (ii) the amount of iron oxide-associated organic matter might remain unchanged with warming and (iii) hydrophobic interactions with iron oxides may be temperature dependent.