Enhanced phosphorus removal from wastewater using virgin and modified slags : performance, speciation and mechanisms

Abstract: Argon oxygen decarburization slag (AOD) was tested in batch and column experiments to investigate its phosphorus (P) removal performance. The effects of factors such as AOD dose, initial P concentration of the feeding solution, and aging on the P removal ability of the slags were analyzed. In a column experiment, electric arc furnace slag (EAF), blast furnace slag (BFS) and AOD were combined in five different ways to determine optimal conditions for P removal. In another column experiment, the three types of slag were modified with polyethylene glycol (PEG) and NaOH to adjust their dissolution properties and the effect on P removal performance was examined. In the batch experiments, AOD exhibited very promising P removal ability. It removed 94.8% of P from 6.5 mg P L-1 synthetic solution in 4 hours with a dose of 5 g L-1. Maximum P removal capacity of 27.5 mg P g-1 was achieved. In the dual-filter column experiment, the column packed with only EAF had the best P removal performance (consistently above 93%). Amorphous calcium phosphate (ACP) was identified as the main P species in the five slag samples collected from the outlet chambers. The contributions from crystalline calcium phosphate (Ca-P) and P adsorbed on iron/aluminum (hydr)oxides were greater in samples from the inlet chambers. The P speciation results revealed that P was predominantly removed by the slags through formation of ACP. The second column experiment showed that modification with PEG and NaOH solution only enhanced short-term P removal by the slags. However, exhaustion of the modified slags occurred much earlier, indicating that the modification process had shortened the lifespan of the slags. Untreated AOD showed better P removal than untreated EAF until pore volume 244, probably due to faster dissolution rate of gamma dicalcium silicate (dominating in AOD according to the XRD results) than of beta dicalcium silicate (dominating in EAF).