Abstract
Amorphous calcium silicate hydrates (A-CSHs) were synthesized using soluble silicates extracted from a natural siliceous shale (M-rite) and Ca(OH)2. Simultaneous thermogravimetry and differential thermal analysis confirmed that the synthesized A-CSHs contained no detectable amount of free Ca(OH)2. Their performance on phosphate (Pi) recovery from aqueous solutions was examined using a 3.0-L bacth reactor. A-CSHs possessed a greater ability to recover Pi from a synthetic anaerobic sludge digestion liquor than did CaCl2 and Ca(OH)2. 29Si magic-angle-spinning NMR analysis suggested that wet A-CSHs consisted of silicate polymers (average chain length of 3.5) that are linked to each other through ion binding with Ca2+. Based on Ca2+ release and settleability experiments, it was speculated that Ca–Pi–silicates aggregates were formed by the ionic association of Pi, Ca2+, and negatively charged silicates. This hypothesis could reasonably explain the high settleability of Pi removed by A-CSHs. Powder X-ray diffraction analysis showed that recovered products had an amorphous structure similar to that of A-CSHs. In this study, we suggest that A-CSHs have a unique mechanism for recovering Pi, thereby enabling their high reactivity and settleability.
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