Phosphate removal in constructed wetland with rapid cooled basic oxygen furnace slag

- RC-BOFS was an effective adsorbent for phosphate.
- Contact time, initial concentration and pH value affect the adsorption capacity.
- Adsorption mechanisms were well described by SEM-EDS, XRD and FTIR.
- High phosphate adsorption capacity and near-neutral pH was observed in CW with sand and RC-BOFS.

The objective of this study was to evaluate adsorption characteristics of phosphate by rapid cooled basic oxygen furnace slag (RC-BOFS) through various conditions and removal rate of phosphate in small-scale constructed wetland with RC-BOFS as filter material. The phosphate adsorption by RC-BOFS was rapid in the first 0.5 h and the pseudo-second-order kinetic model fit the data better than the pseudo-first-order kinetic model. The maximum phosphate adsorption capacities of RC-BOFS under different pH were in the following order: 3.57 mg P g−1 (pH 5) > 2.47 mg P g−1 (pH 7) > 1.46 mg P g−1 (pH 9). Small-size RC-BOFS (0.8-2.3 mm) was more efficient with 23% higher phosphate adsorption than big-size RC-BOFS (2.3-4.6 mm). Characterization of RC-BOFS before and after phosphate adsorption by XRD, FTIR and SEM-EDS indicated that phosphate adsorption by RC-BOFS was dominated by metal oxide and precipitation by calcium and was closely related to the slag chemical properties. The phosphate saturation time in constructed wetland with coarse sand was predicted about 292 days, whereas the longevity of constructed wetland with adding about 25% RC-BOFS to the coarse sand can significantly increase up to 1349 days. It was concluded that the horizontal flow constructed wetland with sand 75%:RC-BOFS 25% ratio could achieve high phosphate removal rate and near-neutral pH for meeting the acceptable water quality discharge standard from water treatment plant.

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