Residual leachability of CCA-contaminated soil after treatment with biodegradable chelating agents and lignite-derived humic substances

The aim of this study was to enhance the soil remediation of timber treatment sites; the potential application of biodegradable chelating agents and humic substances as enhancing agents was assessed in terms of the residual leachability of chromium, copper and arsenic (CCA). This study applied four leachability tests on a field-contaminated soil after 48-h washing with ethylenediamine-N,N-disuccinic acid (EDDS), glutamic-N,N-diacetic acid, ethylenediaminetetraacetic acid and humic substances derived from lignite and two other sources. It was noteworthy that the reduction in the total metal concentrations after soil washing was not predictive of the leaching behaviour. When assessed by toxicity characteristic leaching procedure (TCLP) and waste extraction test (WET), Cu and As leachability was decreased as a result of their extraction by soil washing. By contrast, when assessed by synthetic precipitation leaching procedure (SPLP) and European Council Waste Acceptance Criteria (ECWAC) tests, Cu and As leachability was found to increase, probably because the effect of destabilization of residual metals during soil washing was more observable in unbuffered leaching solutions. On the other hand, Cr leachability was acceptably low in TCLP and WET but still exceeded drinking water standard in SPLP and ECWAC tests. The three chelating agents were able to meet the criteria for Cu in all leachability tests, while the limits of As concentrations could only be met by EDDS in TCLP test. The three humic substances reduced the leachate concentrations of Cu and As without destabilizing the residual metals; however, the reduction was insufficient to meet the required limits in all leachability tests considered.