Effects of sewage sludge immobilized by composite phosphorus-bearing materials on speciations of heavy metals and growth of ryegrass

Sewage sludge treatment and disposal has become a pressing issue in China. Sewage sludge was passivated by composite phosphorus-bearing materials to decrease heavy metals bioavailability. The optimum immobilizing conditions that were 5 g of phosphate rock, 3 g of calcium superphosphate for 100 g sludge and 14 d of passivation time were determined by an orthogonal test. The physico-chemical properties and nutritional index of the passivated sludge complied with Chinese national standard GB/T23486-2009. In the pot experiment, 10% (wt) of passivated sludge (non-passivated sludge) was added into the background soil and labelled as passivated treatment (non-passivated treatment), respectively. Plant height, tiller count, chlorophyll content, and biomass all increased in the passivated treatment when compared to the control treatment, but they all dropped in the non-passivated treatment. Due to good passivation impact on Cu and Pb, their bioaccumulation factors did not significantly differ between the control and the passivated treatments, but were significantly greater in the non-passivated treatment. The bioaccumulation factors of Zn in the passivated and non-passivated treatments were much higher than those in the control treatment. These findings could provide a theoretical foundation for land usage of sewage sludge immobilized by composite phosphorus-bearing materials.

Automated summary

Sewage sludge treatment and disposal is a pressing issue in China. The passivation of sewage sludge using composite phosphorus-bearing materials effectively decreases the bioavailability of heavy metals. The optimal immobilizing conditions were determined as 5 g of phosphate rock, 3 g of calcium superphosphate for 100 g of sludge, and a passivation time of 14 days. The passivated sludge met the physico-chemical properties and nutritional index required by Chinese national standards. The pot experiment showed that the passivated treatment resulted in increased plant growth parameters compared to the control treatment, while the non-passivated treatment led to a decrease in these parameters. Bioaccumulation factors of copper and lead did not significantly differ between the control and passivated treatments, but were significantly higher in the non-passivated treatment. Zinc bioaccumulation factors were significantly higher in both the passivated and non-passivated treatments compared to the control treatment. These findings provide a theoretical foundation for the land usage of sewage sludge immobilized by composite phosphorus-bearing materials.