Preparation of multi-amine-grafted mesoporous silicas and their application to heavy metal ions adsorption

Single amine-, double amine- and trinal amine-grafted mesoporous silica SBA-15 (denoted as 1N-SBA-15, 2N-SBA-15 and 3N-SBA-15) have been synthesized by refluxing SBA-15 and γ-aminopropyltriethoxysilane (APTS), N-[3-(trimethoxysilyl)-propylethylene]diamine (TPED) and trimethoxysilyl propyl diethylenetriamine (TPDT), respectively, in dry toluene under nitrogen atmosphere. These adsorbents could remove the metal ions of Pb²⁺, Cu²⁺, Cd²⁺, Zn²⁺ as well as Hg²⁺ rather completely. Thus these adsorbents will be a better choice when more than one kind of metal ions needs to be removed. From 1N-SBA-15 to 3N-SBA-15, their adsorption capacities for Hg²⁺ increase from 382 mg/g to 726 mg/g, which can be attributed to not only the large content of amino groups but also the rather increased steric freedom and accessibility of the functional groups in the large pore channels of SBA-15. The Hg/N ratio decreases with the increase of N numbers. A decrease about 20-47% of adsorption capacity of regenerated samples has been obtained and could be attributed to the N loss in concentrated acid. In addition, the fresh dry samples were found to show much higher adsorption capacity than the aged ones due to the competitive adsorption between atmosphere moisture and the metal ions on the amino groups.