Green-brick preparation method for the resource utilization of sewage sludge and phosphogypsum with a low heavy-metal pollution risk

It is an extremely challenging to dispose of sewage sludge (SS) and phosphogypsum (PG) from wastewater treatment and the phosphorus chemical industry because of the risk of heavy-metal pollution. This research revealed that by using SS and PG to prepare green brick (GB), not only could these resources be effectively utilized but also the prepared GB could immobilize heavy metals with relatively high efficiency; the heavy-metal immobilization mechanisms during production were analyzed via a series of analytical techniques. The results showed that when the resource utilization rate of PG and SS was above 60%, the compressive strength of GB reached 7.67 MPa, and the permeability coefficient reached 0.02 cm/s. These parameters met the requirements for “grass bricks”. In contrast to SS and PG, the immobilization rate of heavy metals was greater than 90%. The leaching concentrations of Ni, Cr, Pb, Cd and As ranged from 34.4, 26.89, 246.63, 5.46 and 4.43 mg/kg to 0.05, 2.42, 0.56, 0.05 and 0.21 mg/kg, respectively. These values complied with the “GB 5085.3–2007”. XRD, FTIR, and SEM analyses showed that these results were mainly related to the passivation of PG and SS, the formation of hydration products, and the porous structure of GB.