Comparative investigation on removal of thallium(I) from wastewater using low-grade pyrolusite and pyrolysis residue derived from oily sludge: performance, mechanism and application

The pyrolysis residue (PR) and low-grade pyrolusite (LGP) were developed as adsorbents for Tl(I) removal from aqueous solution. The Tl(I) removal performance and mechanisms of PR and LGP were investigated. At initial solution pH of 4, Tl(I) concentration of 1.6 mg.L-1 and adsorbent dosage of 3 g.L-1, the Tl(I) removal efficiency achieved 93.44% for PR and 98.25% for LGP. Both the adsorption of PR and LGP for Tl(I) follow the pseudo.second.order kinetic model. The fitting results of intraparticle diffusion model show that the rate.controlling step of Tl(I) adsorption on PR and LGP include film diffusion and intraparticle diffusion. The adsorption isotherm data for PR are fitting better by Temkin model, while the adsorption of Tl(I) on LGP is described well by the Freundlich model. The XRD and XPS analysis reveal that the Tl(I) removal by PR is achieved via surface complexation and chemical precipitation in the form of TlFe2S3 and Tl2S, while oxidationinduced surface precipitation of Tl2O3 and the enhanced surface complexation are deduced as the main mechanisms for Tl(I) removal by LGP. The treatment of real industrial wastewater implies that PR has a comprehensive removal capacity to heavy metals (Zn, Cd, Tl) and adding LGP after the adsorption by PR could remarkably reduce Tl concentration up to the emission standards of pollution for inorganic chemical industry.