Keywords: adsorption, calcite, fluoride removal, optoelectronic wastewater, phosphate removal, TFT-LCD, wastewater treatment, thin-film transistor LCDs, liquid crystal displays, precipitation
Removal of phosphate and fluoride from optoelectronic wastewater by calcite
The objective of this study is to study the removal of phosphate (PO43− ) and fluoride (F−) by using calcite from the thin-film transistor liquid crystal display (TFT-LCD) manufacturing wastewater. The highest phosphate adsorption capacity of 40.65 mg/g calcite was found at pH 6.5. It is proposed that both adsorption and precipitation were involved in the removal of phosphate and fluoride. Solid characterisation inferred the evidence of calcium phosphate precipitations, mainly brushite (CaHPO4 • 2H2O) and HAP (Ca10(PO4)6(OH) 2) at pH around 5. The presence of fluoride tends to inhibit the removal of phosphate. Fluoride removal, 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. Results show that calcite is an effective adsorbent for phosphate and fluoride removal from TFT-LCD wastewater.