An interpretation to Cr(Ⅵ) leaching concentration rebound phenomenon with time in ferrous-reduced Cr(Ⅵ)-bearing solid matrices

Toxicity characteristic leaching procedure (TCLP) is a prevalent way to evaluate the treatment effectiveness for Cr(Ⅵ)-bearing solid matrices (CBSM). But when a certain amount of residual reductants are present in the treated CBSM, Cr(Ⅵ) leaching concentration rebound phenomenon (CLCRP) occurs, which invalidates the TCLP. This study explores the microstructure of ferrous-reduced CBSM and proves that the residual Cr(Ⅵ), Fe_xCr_1-x(OH)₃ precipitate and residual ferrous are separately distributed in a three-layer structure. In natural scenarios, the residual ferrous in the out-layer is firstly flushed away by rainfall and groundwater or oxidized by dissolved oxygen, resulting in the decrease of ferrous with time. Residual Cr(Ⅵ), due to being blocked by precipitate layer, is less flushed away. While in TCLP, all of released residual ferrous and Cr(Ⅵ) are in the leachate and react till one of them is almost exhausted, resulting in the underestimation of Cr(Ⅵ) leaching concentrations. The longer the samples experience the natural scenarios, the less of the residual ferrous, resulting in the decline of underestimation of Cr(Ⅵ) leaching concentrations with time. This study also provides a pretreatment which can effectively reduce the residual ferrous, achieving more accurate Cr(Ⅵ) leaching concentrations and eliminating CLCRP.