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Competitive mechanism and influencing factors for the simultaneous removal of Cr(III) and Zn(II) in acidic aqueous solutions using steel slag: Batch and column experiments

Chen, Guoliang, Yang, Liyun, Chen, Jun, Miki, Takahiro, Li, Shuwu, Bai, Hao, Nagasaka, Tetsuya
Journal of cleaner production 2019 v.230 pp. 69-79
active sites, adsorption, aqueous solutions, calcium silicate, carbonates, chemical composition, chromium, hydrolysis, metal ions, pH, slags, solubility, steel, wastewater, zinc
Steel slag is a waste produced in the steelmaking process. Because of its physical structure and chemical composition, it can be used as a metal ion removal agent in wastewater. In this study, column and batch experiments were carried out in order to determine the removal efficiency, competitive adsorption mechanism and influencing factors of the Cr3+ and Zn2+ by slag in acidic condition. A single column experiment showed that the removal rates of Cr3+ and Zn2+ by slag were 85.79% and 76.52%, respectively. However, when the two ions coexisted, the presence of Cr3+ could promote slag hydrolysis and produce more active adsorption sites since the solubility product of Cr(OH)3 was 6.3 × 10−31, which was smaller than that of Zn(OH)2 (3.0 × 10−17) at 25 °C. Moreover, the kinetic adsorption constant of Zn2+ was larger than that of Cr3+, and the hydration radius of Zn2+was smaller than that of Cr3+; thus Zn2+ could rapidly occupy more active sites on slag than Cr3+, which might increase the removal efficiency of Zn2+ and decrease the removal efficiency of Cr3+ in mixed column. The Cr3+ or Zn2+ and OH− released by the slag hydrolysis formed precipitation when the pH value of the solution was greater than 4.6 or 6.8, respectively. In addition, amorphous carbonate, calcium silicate hydrate deposit, and ferrate or silicate could also be formed after the Cr3+ and Zn2+ were removed by the slag. However, the hydroxide and carbonate would rerelease metal ions and reduce the removal efficiency of the slag in an acidic flow column.