Oxyanion Adsorptions by Mono-, Di-, and Triamino-Functionalized MCM-48

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Abstract
MCM-48 is functionalized with mono-, di-, and triamino-silanes without destruction of the mesoporous structure in order to efficiently remove chromate and arsenate in aqueous solutions. The adsorption capacities achieved 160 and 182 mg/g-adsorbent for chromate and arsenate, respectively, when triamino-functionalized MCM-48 was applied. The diamino-functionalized MCM-48 adsorbed much larger amounts of chromate and arsenate: 119 and 122 mg/g-adsorbent, respectively, than did diamino-functionalized fumed silica: 79.3 and 90.7 mg/g-adsorbent. The superiority arose from the large surface area of functionalized MCM-48. At the adsorption saturation, no clear difference was found in oxyanion/N stoichiometries among the mono-, di-, and triamino-functionalized MCM-48. On the other hand, the distribution coefficient, Kd, depended on the kind of silane fixed on MCM-48. It exceeded 2 × 105 when the concentration of oxyanions was low. The adsorbent by triamino-silylation showed the largest Kd; it was three or ten times higher than that of the adsorbent by monoamino-silylation for arsenate and chromate adsorptions, respectively. Although the adsorption capacity of diamino-functionalized MCM-48 was reduced to be 47% at 50 °C, it still adsorbed a large amount of arsenate.