Removal of phosphate from etching wastewater by calcined alkaline residue: Batch and column studies

The present study explored the effectiveness of calcined alkaline residue in removing phosphate from etching wastewater. Alkaline residue was calcined at various temperatures between 400 and 1000 °C with 100 °C interval for 2 h, and the highest phosphate sorption capacity was achieved at 800 °C. The pH impact, sorption kinetics, isotherms and thermodynamics of phosphate adsorption on AR800 (alkaline residue calcined at 800 °C) were investigated by batch tests. The kinetics was evaluated with the Lagergren pseudo-first-order and pseudo-second-order models, and the experimental data fitted well with the pseudo-second-order model, indicating chemisorption was the rate-limiting step of sorption process. The equilibrium data were further analyzed by the Langmuir, Freundlich and Dubinin-Radushkevich models. Equilibrium sorption of phosphate onto AR800 fitted best with the Langmuir model, and the maximum sorption capacity was 139 mg/g. The thermodynamic parameters, such as ΔG°, ΔH° and ΔS°, were also calculated, which confirmed that the removal of phosphate from etching wastewater by AR800 was a spontaneous and endothermic process. Moreover, fixed-bed column experiments were carried out to optimize flow rate, bed height and initial phosphate concentration. It was found that the breakthrough curves were well described by Thomas model. The results above show AR800 has high potentialities to be used as an adsorbent for phosphate removal from etching wastewater in large-scale.