Adsorption of organic matter at mineral/water interfaces: 5. Effects of adsorbed natural organic matter analogues on mineral dissolution

摘要

The effects of the adsorption of pyromellitate, an analogue for natural organic matter, on the dissolution behavior of corundum (alpha-Al2O3) have been examined over a wide range of pyromellitate concentrations (0-2.5 mM) and pH conditions (2-10). The adsorption modes of pyromellitate on corundum have first been examined using in situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy and are shown to be dominated by a fully deprotonated, outer-sphere pyromellitate species (equivalent to A1OH(2)(+)center dot center dot center dot Pyr(4-)) at pH >= 5.0. At lower pH conditions, however, an additional protonated outer-sphere species (equivalent to A1OH(2)(+)center dot center dot center dot H(2)Pyr(2-)) and an inner-sphere species are also evident. In accordance with the ATR-FTIR findings, modeling of macroscopic pyromellitate adsorption data using an extended constant capacitance treatment was possible using two outer-sphere (equivalent to A1OH(2)(+)center dot center dot center dot Pyr(4-) and equivalent to A1OH(2)(+)center dot center dot center dot H(2)Pyr(2-)) and one inner-sphere (A1Pyr(3-)) adsorbed pyromellitate species. The presence of adsorbed pyromellitate strongly inhibited the dissolution of corundum under acidic (pH < 5) conditions, consistent with a mechanism previously proposed by Johnson et al.(I) whereby outer-spherically adsorbed Pyr(4-) species sterically protect dissolution-active surface sites from attack by dissolution-promoting species such as protons. A reduction in the protolytic dissolution rate of corundum results. A reference Suwannee River fulvic acid, which also adsorbs to aluminum (oxyhydr)oxide surfaces in a predominantly outer-sphere manner, was similarly shown to strongly inhibit the dissolution of corundum at pH = 3.