Enhancement of phosphorus removal with near-neutral pH utilizing steel and ferronickel slags for application of constructed wetlands

• Steel and ferronickel slags were an effective adsorbent for phosphorus (P).
• The use of most slags is limited due to high pH in constructed wetlands (CWs).
• Highest P adsorption capacity of basic oxygen furnace slag (BOFS) was observed.
• Highest alkali neutralization capacity of ferronickel slag (FNS) was observed.
• High P removal rate with near-neutral pH was observed in mixed slags (BOFS + FNS).

The objective of this study was to enhance phosphorus (P) removal while achieving near-neutral pH in aqueous solution utilizing steel slags (Blast furnace slag, BFS; Basic oxygen furnace slag-air cooling, BOFS-A; Basic oxygen furnace slag-rapid cooling, BOFS-R) and ferronickel slag (FNS) for application to constructed wetlands (CWs). The maximum P adsorption capacities (mg kg−1) of the slags were in the order BOFS-R (3233) > BOFS-A (2320) ≫ BFS (607) > FNS (260) based on the Langmuir adsorption isotherm. The pH of the slags were in the following order: BOFS-R (11.9) > BOFS-A (9.0) ≫ BFS (8.9) > FNS (7.5). The basic oxygen furnace slags had the highest P adsorption capacity, while the ferronickel slag had the lowest pH among the tested slags. To achieve high P removal efficiency with near-neutral pH, the BOF slags and ferronickel slag (BOFS-A + FNS and BOFS-R + FNS) were mixed in batch experiments. To achieve effective P removal with near-neutral pH utilizing steel and ferronickel slags, the optimum conditions were found to be the ratio of BOFS-R:FNS of 7:3. Therefore, when mixed slags at the BOFS-R:FNS ratio of 7:3 were applied to CWs, they could achieve high P removal efficiency and near-neutral pH for meeting the acceptable drinking water quality discharge standard.