Abstract
The TFT-LCD manufacturing plants generate a large amount of wastewater with high concentrations of phosphate (PO43-). The main focus in this study is to investigate the removal of this PO43--P by calcite. Before dealing with industrial wastewater, synthetic wastewater was used for comparison purpose. The initial PO43- concentrations of 30 and 300 mg/L were used for synthetic wastewater. The industrial wastewater was found to contain 332 mg/l of phosphate, 19.52 mg/l of fluoride, and 10.5 mg/l of calcium. Effects of pH and F- concentration were examined, and then the data were compared with model predictions by PHREEQC. The optimum pH for phosphate removal was found to be 5.5 to 7.0, 7.0 to 12.0, and 5.5 to 8 for industrial, low P, and high P synthetic wastewater, respectively. The highest PO43- adsorption capacity of 40.65 mg/g calcite was found at pH 6.5 for industrial wastewater and 4.18 mg/g calcite at pH 7.9 for low P synthetic wastewater. The data fits both Langmuir and Freundlich adsorption isotherms for industrial wastewater and low P wastewater, but the data for high P wastewater does not fit with isotherms. Judging from the PO43- adsorption capacity, it can be concluded that
both adsorption and precipitation were involved for removal of PO43-P when high
concentration of PO43- was present. Solid characterization (SEM and X-RD) inferred the evidence of calcium phosphate precipitations, mainly brushite (CaHPO4.2H2O) and HAP (Ca10(PO4)6(OH)2) at pH around 6. However, for low P wastewater, there was no clear evidence of any calcium phosphate precipitations. The presence of F- tends to inhibit the removal of PO4
3-P to a certain extent. The PO43- adsorption capacity decreased from 33.9
mg/g to 16.1 mg/g with the increase of fluoride concentration from 19.52 mg/L to 85.77 mg/L for the same amount of adsorbent and pH. The inhibitory effect is mainly caused by the competition upon active sites for adsorption and Ca2+ ions to form precipitates. Fremoval, as high as 85.2 %, was obtained at pH 6.5 and calcite dosage of 10 g/l, and the main mechanism was through fluorite (CaF2) precipitation and adsorption onto brushite and HAP. These results showed that calcite is an effective adsorbent for PO43-P removal
from TFT-LCD wastewater.

Abstract
The TFT-LCD manufacturing plants generate a large amount of wastewater with high concentrations of phosphate (PO43-). The main focus in this study is to investigate the removal of this PO43--P by calcite. Before dealing with industrial wastewater, synthetic wastewater was used for comparison purpose. The initial PO43- concentrations of 30 and 300 mg/L were used for synthetic wastewater. The industrial wastewater was found to contain 332 mg/l of phosphate, 19.52 mg/l of fluoride, and 10.5 mg/l of calcium. Effects of pH and F- concentration were examined, and then the data were compared with model predictions by PHREEQC. The optimum pH for phosphate removal was found to be 5.5 to 7.0, 7.0 to 12.0, and 5.5 to 8 for industrial, low P, and high P synthetic wastewater, respectively. The highest PO43- adsorption capacity of 40.65 mg/g calcite was found at pH 6.5 for industrial wastewater and 4.18 mg/g calcite at pH 7.9 for low P synthetic wastewater. The data fits both Langmuir and Freundlich adsorption isotherms for industrial wastewater and low P wastewater, but the data for high P wastewater does not fit with isotherms. Judging from the PO43- adsorption capacity, it can be concluded that
both adsorption and precipitation were involved for removal of PO43-P when high
concentration of PO43- was present. Solid characterization (SEM and X-RD) inferred the evidence of calcium phosphate precipitations, mainly brushite (CaHPO4.2H2O) and HAP (Ca10(PO4)6(OH)2) at pH around 6. However, for low P wastewater, there was no clear evidence of any calcium phosphate precipitations. The presence of F- tends to inhibit the removal of PO4
3-P to a certain extent. The PO43- adsorption capacity decreased from 33.9
mg/g to 16.1 mg/g with the increase of fluoride concentration from 19.52 mg/L to 85.77 mg/L for the same amount of adsorbent and pH. The inhibitory effect is mainly caused by the competition upon active sites for adsorption and Ca2+ ions to form precipitates. Fremoval, as high as 85.2 %, was obtained at pH 6.5 and calcite dosage of 10 g/l, and the main mechanism was through fluorite (CaF2) precipitation and adsorption onto brushite and HAP. These results showed that calcite is an effective adsorbent for PO43-P removal
from TFT-LCD wastewater.

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