Abstract
Chromate, Cr(VI), contamination in soil and groundwater poses serious threat to living organisms and environmental health worldwide. Sulphate green rust (GRSO4), a naturally occurring mixed-valent iron layered double hydroxide has shown to be highly effective in the reduction of Cr(VI) to poorly soluble Cr(III), giving promise for its use as reactant for in situ remedial applications. However, little is known about its immobilization efficiency inside porous geological media, such as soils and sediments, where this reactant would ultimately be applied. In this study, we tested the removal of Cr(VI) by GRSO4 in quartz sand fixed-bed column systems (diameter × length = 1.4 cm × 11 cm), under anoxic conditions. Cr(VI) removal efficiency (relative to the available reducing equivalents in the added GRSO4) was determined by evaluating breakthrough curves performed at different inlet Cr(VI) concentrations (0.125–1 mM) which are representative of Cr(VI) concentrations found at contaminated sites, different flow rates (0.25–3 ml/min) and solution pH (4.5, 7 and 9.5). Results showed that (i) increasing Cr(VI) inlet concentration substantially decreased Cr(VI) removal efficiency of GRSO4, (ii) flow rates had a lower impact on removal efficiencies, although values tended to be lower at higher flow rates, and (iii) Cr(VI) removal was enhanced at acidic pH conditions compared to neutral and alkaline conditions. For comparison, Cr(VI) removal by sulphidized nanoscale zerovalent iron (S-nZVI) in identical column experiments was substantially lower, indicating that S-nZVI reactivity with Cr(VI) is much slower compared to GRSO4. Overall, GRSO4 performed reasonably well, even at the highest tested flow rate, showing its versatility and suitability for Cr(VI) remediation applications in high flow environments.
Highlights
Chromium (Cr) is one of the most common heavy metals found in the biosphere and a key micronutrient, but it is a frequent contaminant in soils and groundwater worldwide, due to the uncontrolled release of Cr contaminated waters from various industries
This study demonstrated the high effectiveness of Sulphate green rust (GRSO4) to immobilise mobile Cr(VI) inside porous sand media
Cr(VI) removal efficiencies in sand columns under dynamic flow conditions were substantially lower than in batch studies, at higher inlet Cr(VI) concentrations. This is likely explained by the constant influx of Cr(VI) solution which keeps reaction rates high on green rusts (GRs) particle surfaces likely promoting the formation of passivating rims on GR surfaces, as observed before in batch experiments with high initial Cr(VI) concentrations
Summary
Chromium (Cr) is one of the most common heavy metals found in the biosphere and a key micronutrient, but it is a frequent contaminant in soils and groundwater worldwide, due to the uncontrolled release of Cr contaminated waters from various industries (e.g., wood treatment, electroplating operations, leather tanning or metal plating solutions). The dominant Cr(VI) species at pH > 7 is chromate (CrO42−), whilst at pH values between 2 and 7, Cr(VI) may be present as dichromate (Cr2O72−) and hydrogen chromate (HCrO4−). These negatively charged Cr(VI) oxyanions act as strong oxidants, are highly soluble (i.e., Digiacomo et al Geochem Trans (2020) 21:8 the dominant species in industry waste spills), are known to be highly toxic, mutagenic and carcinogenic and pose severe threats to ecosystems and human health. It is a key mandate to clean-up Cr contaminated sites
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