Metal-modified silica adsorbents for removal of humic substances in water

Abstract

As novel adsorbents for humic substances, Fe-, Mg-, and Ca-modified silica gels SiO 2 Fe, SiO 2 Mg, and SiO 2 Ca were prepared, and their adsorbabilities to humic and fulvic acids were evaluated in water at 25 °C for 20 h. Among these adsorbents, SiO 2 Fe indicated the highest adsorbability, in which removing humic substances in water was accomplished to 80–97%. By calcination at 600 °C and washing with water, adsorbabilities of the silica adsorbents deteriorated except for the case of calcination of SiO 2 Ca. This is due to changing metal modification structures and losing chlorine, judging from elemental analysis, TG-DTA, and XPS. Especially, the modifier Fe was tightly fixed on the silica surface of SiO 2 Fe, since the metal content was almost constant even after the calcination and water washing unlike the other adsorbents. Therefore, we found that SiO 2 Fe is the most useful adsorbent among the silica adsorbents and that its modification structure is composed of Si O Fe, Fe Cl, and Fe OH. In addition, the adsorption mechanism is explained by an interaction between Fe and humic substance molecule having carboxylate and phenolate groups, accompanied with anion exchange of chloride ion Cl − for the humic substance molecule via the silica pores. SiO 2 Fe may be applied to an adsorbent alternative for charcoal in water purification plants, and the used SiO 2 Fe may be further reused as a fertilizer since humic substances have plant-glowing ability and silica strengthens plant parts.