Inorganic flocculant for sludge treatment: Characterization, sludge properties, interaction mechanisms and heavy metals variations

As an industrial waste, phosphogypsum was modified to produce flocculant for sludge dewatering. In this paper, characteristics of flocculant, properties of treated sludge, and interactions of sludge and flocculant were investigated. Results suggested that after modification, flocculant showed a positive electrical property and a porous structure. Besides, larger sludge flocs formed in treated sludge showed a higher settleability and filterability. Flocculant could narrow sludge colloid network by compressing its Electrical double-layer due to the presence of CaSO4. With potential change, the electronegative colloidal network cracked quickly and released sludge particles, active groups, unstable heavy metals and 82.91% of bound water. Moreover, porous adsorption between sludge particles and flocculant was found under molecular electrostatic potential and Van Der Waals force caused by flocculant addition. After modification, shear modulus of CaSO4, SiO2 and Al2O3 in modified phosphogypsum increased by 21%, 23% and 17%, respectively. This provided a strong skeleton support for sludge particles, which is significant to sludge dewatering. Particularly, through chelation, adsorption and rolling-sweeping process, risk level of unstable heavy metals excepting Cu in sludge filter cake was largely weakened. Immobilized rate of risky heavy metals was 23.96% (Cd-F1/F2), 39.92% (Cr-F1), 11.11% (Pb-F1/F2), 21.21% (Zn-F1), 35.49% (Ni-F1/F2), and 78.61% (As-F1/F2), respectively. Therefore, this study provided significant insight for developing efficient method to promote bound water removal from sludge, and to stabilize risky heavy metals in sludge.