Use of recycled waste materials for the stabilization of Cr(VI) in a sandy soil

Hexavalent chromium (Cr(VI)) is one of the most dangerous metals in polluted soils because of its carcinogenicity and high bioavailability. Conversely, its reduced form Cr(III) is low-toxic and very stable in the soil, since it occurs mainly as precipitated hydroxides. Different strategies can be adopted to remediate Cr(VI)-polluted soils and, among these, solidification/stabilization (S/S) is very efficient, being usually a cost-effective and rapid technology. It consists on the soil treatment with materials or reagents able to decrease the solubility and toxicity of heavy metals and/or entrap them in stable solid phases. In this study, glass and aluminium recycled from municipal solid wastes were used to treat, under alkaline hydrothermal conditions, a Cr(VI)-spiked soil with the aim of reducing Cr(VI) to Cr(III) and immobilizing Cr(III) in insoluble newly formed solid structures. A sandy soil was spiked with a solution of Cr(VI) to obtain, after 3 months of aging, a stable concentration of 580 mg kg-1. The soil was then added with a mixture (MIX) of pulverized glass and aluminium (1/10 and 1/20 MIX/soil, w/w). The samples were then added with KOH and deionised water (1:2 w/v), and stored at 90°C in closed HDPE bottles. Two controls without MIX, one containing KOH and the other with water alone, were also prepared. After 1, 7, 30, 60 and 90 days, soil aliquots were sampled and analyzed for Cr(VI) concentration (methods USEPA 3060A and 7196A) and Cr fractionation (BCR sequential extractions). Chromium stabilization was further investigated by analyzing the soil samples for elemental distribution (micro X-ray fluorescence – μXRF), mineralogical composition (X-ray powder diffraction – XRPD), and 3D structure of the solid phases of new formation (high resolution micro X-ray computed tomography - μCT). The reduction of Cr(VI) to Cr(III) was accelerated and intensified by the S/S treatment. In fact, about 98% Cr(VI) was reduced in only 7 days of treatment with MIX 1/20. To obtain a similar result, 30 days of treatment with MIX 1/10 and 90 days with KOH alone were needed. Prolonging the S/S process, Cr(VI) concentration was further decreased proving that Cr(III), once reduced, is not re-oxidised. Chromium mobility decreased considerably and continuously during the treatment with MIX. After 7 days, the amount of Cr associated to the more recalcitrant soil fractions was higher than 85% in the presence of MIX (at both ratios), and equal to 55% and 38% in the controls with KOH and water, respectively. Soil structure was significantly modified by the alkaline hydrothermal treatment, as proved by the formation of large soil aggregates containing Cr along with Si and Al. In fact, the addition of MIX, enriching the soil with Si and Al, accelerated the formation of soil aggregates characterized by a high micro-porosity, as revealed by μCT analysis. In most of the samples, XRPD analysis revealed the formation of a K-zeolite (edingtonite) and a strong decrease of quartz and illite. However, the stabilization of Cr was not related to zeolite formation. The innovative S/S treatment tested in this study efficiently reduces Cr(VI) to Cr(III) in the soil and stabilises Cr in newly formed soil aggregates. Scanning electron microscopy analysis of soil aggregates is also going to be performed to better understand the mechanisms of Cr stabilization within the aggregates.