Remediation of Cr(VI)-contaminated soil using self-sustaining smoldering.

Self-sustaining smoldering is an emerging technology for nonaqueous-phase liquid remediation; however, it is rarely applied for Cr(VI)-contaminated soil treatment. In this study, self-sustaining smoldering using rice straw (RS) as a surrogate fuel was applied to remediate Cr(VI)-contaminated soil for the first time. Thirteen one-dimensional vertical smoldering experiments were conducted to investigate the effectiveness of the smoldering method and the effects of key experimental parameters on smoldering remediation performance. Smoldering was observed to be self-sustaining within the range of RS particle size from <0.16 to 2.00-4.00 mm, airflow from 0.2 to 1 m/h, and Cr(VI)-impacted soil/RS ratios from 2:1 to 6:1. The Cr(VI)-contaminated soil was effectively remediated, which was confirmed by lowered Cr(VI) contents in the treated samples (decreased by 52.22-86.57%) and the elevated fraction of Cr oxidizable and residual form (increased by 1.14-3.30 and 2.97-4.00 times, respectively), compared to the control. The reducing gases (CO and CH) generated during the smoldering played a crucial role in the remediation process. The contents of available P and K in the remediated soil containing the remaining biochar and ash increased, thus improving soil reusability. Hence, this study shows that smoldering with RS as supplemental fuel is a promising Cr(VI)-contaminated soil management technique without supplying substantial external energy.