Purpose: The study examines the effectiveness of red mud, blast furnace (BF) slag, and alum-derived water treatment sludge as immobilizing agents for excessive soluble P that had accumulated in three green waste-based composts.
Methods: The three wastes were applied at 0%, 5%, 10%, and 20% w/w to three different composts, all containing extremely high concentrations of extractable P, and were incubated for 60 days. Water-soluble P was measured regularly throughout the incubation period, and at the end, P extractable with resin, 0. 05 M NaHCO 3, and 0. 005 M H 2SO 4 were also measured.
Results: In the water extracts, inorganic P made up more than 85% of the total P present. All three materials had the ability to adsorb P and thus lowered water-soluble P concentrations. Water treatment sludge was clearly the most effective material, and this was attributed to its amorphous nature (thus, large Brunauer-Emmett-Teller surface area) and its acid pH (6. 8) compared with the alkaline pH (10-11) of the other two materials. Water treatment sludge was also the most effective at lowering resin- and NaHCO 3-extractable P. When H 2SO 4 was used as the extractant, BF slag tended to be the most effective material at lowering extractable P, followed by water treatment sludge, and red mud. That is, the P immobilized by water treatment sludge was extractable with acid but not with water, resin, or NaHCO 3.
Conclusions: Water treatment sludge has the potential to be used as an effective immobilizing agent for soluble P in composts, and it should be trialed under field conditions.