Simultaneous ε-Keggin Al13 chloride salt and anionic polyacrylamide coagulation-flocculation system for agglomeration of trimetallic Cr-Ni-Fe doped core-shell microspheres of graphitized coke

Abstract

Core-shell microspheres of graphitized carbon consisting of a solid carbonaceous core surrounded by a porous Cr-Ni-Fe shell are produced in ultra-high temperature ethane steam crackers. Considerable volumes of these microspheres are suspended in quench water and are directed to oceans while they have not been detected as pollutants so far. The importance of this work is studying and development of an environmental remediation process for extraction of coke microspheres from aqueous solution by ε-Keggin Al13 chloride salt as coagulant and anionic polyacrylamide (PAM) as flocculant in a stirred tank reactor equipped with silt density index meter and particle size analyzer. The effect of dissolved ions of Hofmeister series on zeta potential and electrical double layer compression was investigated. The role of charge density and chain length of PAM were also examined in favor of patch mechanism. The results revealed that the surface charge of suspended coke microspheres is controlled by the solid carbonaceous core while the porous metallic shell is ineffective. The aggregation of coke:Al13 flocs follows a multiple mechanism including point of zero charge, electrical double layer compression, charge neutralization, hydrophobic interaction, sweep floc, inter-particle polymer bridging and electrostatic patch flocculation. Surface adsorption of ε-Keggin Al13 chloride salt coagulants on the coke microspheres is limited to the critical level of 8 ppm because of the steric effect of ε-Keggin Al13 chloride salt clusters and the restricted bare surface of carbonaceous core matrix under the porous metallic shell. The sphericity parameter determined by surface to volume ratio of coke:Al13 flocs and it was used to quantify the degree of polymer bridging between the flocs.