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

As novel adsorbents for humic substances, Fe-, Mg-, and Ca-modified silica gels SiO2Fe, SiO2Mg, and SiO2Ca were prepared, and their adsorbabilities to humic and fulvic acids were evaluated in water at 25 °C for 20 h. Among these adsorbents, SiO2Fe 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 SiO2Ca. 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 SiO2Fe, since the metal content was almost constant even after the calcination and water washing unlike the other adsorbents. Therefore, we found that SiO2Fe is the most useful adsorbent among the silica adsorbents and that its modification structure is composed of SiOFe, FeCl, and FeOH. 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. SiO2Fe may be applied to an adsorbent alternative for charcoal in water purification plants, and the used SiO2Fe may be further reused as a fertilizer since humic substances have plant-glowing ability and silica strengthens plant parts.