Chromium transformations during aging in contaminated soils

Abstract

Extensive industrial activities have led to Cr contamination in the environment, which poses threats to ecosystems and human health. Currently, very little is known regarding the Cr accumulation in soils during aging. The purpose of this dissertation was (i) to enhance our understanding of Cr transformations occurring over a long period of time; (ii) to investigate the reduction of toxic Cr(VI) to non-toxic Cr(III) by soil microorganisms for developing bioremediation strategies for Cr-contaminated sites. Soil samples were collected from a longterm (> 30 years) tannery waste contaminated area in Shuitou (China) for this research. Chemical extraction methods showed the Cr(III) form was dominant (> 96.7% of total Cr) in these aged Cr-contaminated soils, with toxic Cr(VI) up to 144 mg kg-1. Of the total Cr(VI) present, immobile Cr(VI) represented > 90%. Synchrotron-based X-ray near edge structure spectroscopy demonstrated Cr species present were CrFeO3, CrOOH, and CaCrO4. The occurrence of immobile Cr(VI) species in long-term contaminated soils was further verified by a spiking experiment over 240-day aerobic incubation. Available Cr(VI) in soils continually decreased during aging, with immobile Cr(VI) increasing by 4.5 – 31% and immobile Cr(III) increasing by 68 – 95% of total spiked Cr(VI). These findings reveal that Cr(VI) reduction and immobilization were occurring concurrently in soils. Cr(VI) reduction occurs in soils with low pH and high organic carbon content via both chemical and biological processes, while Cr(VI) immobilization occurs in soils with cations (such as Ca2+) and Fe oxides. Shotgun metagenomic sequencing was used to analyze the microbial community composition in the soils and a batch solution experiment was employed to determine the Cr(VI) reduction capacity by soil microbial consortia. The results demonstrated the accumulation of high levels of Cr in a soil (e.g., 3141 mg kg-1 in S3-2) led to the increased abundance of Cr resistant and reducing microorganisms: Proteobacteria (69.9%) at phylum level, Betaproteobacteria (39.1%) at class level, and Massilia (12.6%) and Bacilli (0.57%) at genus level. Batch experiment results showed the addition of 1.0 g Cr-contaminated soils reduced 10 – 20 mg L-1 Cr(VI) in 20 mL of K2Cr2O7 solution at the condition of 30 oC at pH 7.8 – 8.0 within 7 days anaerobically and aerobically, when supplied with 0.2 g L-1 of Na-acetate as carbon and electron sources. The amount of Cr(VI) removed was highest (29.0 mg L-1) at 40 mg Cr(VI) L-1 in the presence of soil S3-2. Therefore, prospective application of mixed microbial consortia from high Cr-contaminated soils for bioremediation of Cr(VI)-polluted environments could be expected.