Adsorption of aromatic carboxylate compounds on the surface of synthesized iron oxide-coated sands

The complex mineral assemblages of silica and Fe minerals play a significant role in the transport of compounds in soils and sediments. Five coated sands including Goethite, Lepidocrocite, Ferrihydrite, Hematite and Magnetite were synthesized by a heterogeneous suspension method and characterized by FTIR spectroscopy, XRD, BET surface area and chemical analyses. The synthesis results showed that the degree of coating (mg Fe/g sand) varied with the mineralogy of Fe coating phases, which may have different affinities towards the silica surface. Batch experiments were conducted with two compounds (2,5-dihydroxybenzoic acid and 1-hydroxy-2-naphthoic acid) to quantify the contributions to adsorption from different oxide coatings and compare adsorption characteristics of selected organic acids. Sorption of these compounds to coated sands was examined versus a wide range of conditions (time, pH, ionic strength and sorbate concentration). Because of the attachment of Fe oxide, the coated sand had higher specific surface area, involving a better adsorption efficiency of organic compounds. Mineral surface charge and pH proved to be important for the adsorption of these compounds. The batch results indicated that the degree of coating was the most significant factor enhancing the sorption of aromatic compounds on the surface of sand and the mineralogy of the Fe phase was of less importance.