Reductive Remediation of Cr(VI)-Contaminated Soils in the Presence of Zero-Valent Metals and Bimetals

The time-dependent efficiency of Zero-Valent Metals (ZVMs) including Al⁰ and Zn⁰ and their bimetals (Fe/Al and Fe/Zn, 0.1 g shell metal/g core metal) to reduce Cr(VI) in three contaminated soils (calcareous, non-calcareous near neutral and slightly acidic) was studied. The Cr(VI)-contaminated soils (100 and 500 mg/kg) were amended with the reductants (0, 5, and 10 g/kg) and the concentration of exchangeable Cr(VI) was determined after 0.5, 4, 24, 48 and 168 hours. It was found that the average reducing capacity of the bimetallic particles (11.4 mg Cr/g) was much higher than the ZVMs (3.3 mg Cr/g). The ZVMs showed rapid passivation within only a few minutes, while the bimetallic particles preserved their reactivity even up to one hour. In addition, the efficiency of ZVMs in the slightly acid soil was much higher than in two other soils. There was a good performance of Fe/Al in the calcareous soil with a higher hazard potential than the two other soils. The Cr(VI) reduction capacity of the bimetallic particles in non-calcareous near-neutral soil was two times more than in calcareous soil. The pseudo-first-order Cr(VI) reduction rate constant for the bimetals (0.248 h^-1 ) was on average higher than those of the ZVMs (0.074 h^-1).