Use of inorganic wastes as immobilizing agents for soluble P in green waste-based composts

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.

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(2)SO(4) were also measured.

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(2)SO(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).

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.