Effects of flocculant modified phosphogypsum on sludge treatment: operating parameters, variations of chemical groups and heavy metals in sludge

Under a modified phosphogypsum (MPG) treatment, the effective removal of bound water and risky heavy metals from sludge has been achieved. In this paper, the effects of the operating parameters on sludge treatment, variations of the active groups in the sludge, and the heavy metals' mobility risk were investigated. The results showed that adding 40–50% DS (mass of dry solid in sludge) of MPG could directly remove approximately 62% of bound water from the sludge. Moreover, about 23.96% of CdF1/F2, 39.92% of CrF1/F2, 21.21% of ZnF1/F2, 35.49% of NiF1/F2, and 78.61% of AsF1/F2 were removed from the sludge due to a transition of risky heavy metals (metal-F1/F2) to stable heavy metals (metal-F3/F4/F5). Simultaneously, organic chelation was more effective in stabilizing Cd, Cr, Zn, and As. Additionally, the flocs entrainment can significantly improve the stabilization of Ni and Pb. In addition, the risky model of heavy metals showed that a higher oxidization performance of the sludge and less loosely bound extracellular polymeric substances (LB-EPS) content was propitious to the immobilization of Cd, Cr, Zn, Ni, and As. Nevertheless, the environmental risk of Cu and Pb varied with the addition of MPG. These results indicate the potential of this method for the synergetic removal of bound water and risky heavy metals from sludge.