Phosphate removal and recovery through crystallization of hydroxyapatiteusing xonotlite as seed crystal
CHEN Xuechu , KONG Hainan , WU Deyi , WANG Xinze , LIN Yongyong
DOI:
Received May 19, 2008,Revised July 08, 2008, Accepted , Available online
Volume 21,2009,Pages 575-580
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Xonotlite was synthesized and tested for phosphate removal and recovery from synthetic solution in a batch mode. The e ects of pH,
initial calcium concentration, bicarbonate concentration on phosphate removal through crystallization were examined. The morphology
and X-ray di raction (XRD) pattern of xonotlite before and after crystallization confirmed the formation of crystalline hydroxyapatite.
The results indicated that the crystallization product had a very high P content (> 10%), which is comparable to phosphate rock at
the dosage of 50–200 mg xonotlite per liter, with a maximum P content of 16.7%. The kinetics of phosphate removal followed the
second-order reaction equation. The phosphate removal ability increased with increasing pH. The precipitation of calcium phosphate
took place when pH was higher than 7.2, whereas the crystallization occurred at pH 6.0. A high calcium concentration could promote
the removal of phosphate via crystallization, while a high bicarbonate concentration also enhanced phosphate removal, through that
the pH was increased and thus induced the precipitation process. When xonotlite was used to remove phosphate from wastewater, the
removal e ciency could reach 91.3% after 24 h reaction, with removal capacity 137 mg/g. The results indicated that xonotlite might
be used as an e ective crystal seed for the removal and recovery of phosphate from aqueous solution.
