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<p>Hexavalent chromium (Cr(VI)) is highly mobile in groundwater (GW) and poses severe toxicological and carcinogenic risks to human health, ecosystems, and the environment. Various remediation techniques have been developed in the last few decades to efficiently counter contaminations by Cr(VI) in an acidic medium, however, effective removal of Cr(VI) in strongly alkaline (pH 8.5–9.0) and hyper-alkaline (pH &gt; 10.5) aqueous medium remains a significant challenge. This study investigated the synergistic effects of two different zero-valent iron (ZVI) brands, i.e., micro-ZVI (MZVI) and nano-ZVI (NZVI), combined with Ni and Cu catalyst precursors to achieve efficient Cr(VI) removal in strongly alkaline and hyper-alkaline medium. This study was prompted by multiple observations/findings at different sites globally, such as in India, that a Cr(VI) contamination in GW may be accompanied by significantly elevated/high pH values (&gt; pH = 7, alkaline conditions), that is, due to certain geological or other circumstances/reasons which probably may have contributed to the Cr(VI) pollution in the past while it was originated. The experimental framework focused on evaluating the influence of different elevated alkalinity levels, pH 9.0 (strongly alkaline medium) and 11.5 (hyper-alkaline medium), on Cr(VI) (initial concentration of 50&#xa0;mg/L) removal rates using MZVI and NZVI. Additionally, the effects of introducing catalyst precursors such as Cu and Ni in the form of Cu(II)-chloride, Cu(II)-nitrate, Ni(II)-chloride, and Ni(II)-nitrate were assessed in combination with MZVI and NZVI at both pH 9.0 and pH 11.5. The catalyst precursors were tested with five different dosages (250&#xa0;µmol/g ZVI, 50&#xa0;µmol/g ZVI, 30&#xa0;µmol/g ZVI, 20&#xa0;µmol/g ZVI, and 10&#xa0;µmol/g ZVI). The results indicated that increasing alkalinity significantly reduces the Cr(VI) removal efficacy of both MZVI and NZVI, necessitating prohibitively high ZVI dosages, particularly for full-scale applications. However, the integration of catalyst precursors, specifically Cu(II)/ZVI and Ni(II)/ZVI, proved instrumental in achieving 100% Cr(VI) removal within just 5&#xa0;min of initiating the experiments with both MZVI and NZVI. Among the tested catalysts, Ni(II)/ZVI at a dosage of 10&#xa0;µmol/g ZVI (0.064 wt. %) displayed lower performance in both strongly&#xa0;alkaline and hyper-alkaline medium, whereas Cu(II)/ZVI demonstrated relatively superior efficacy. This study underscores the crucial role of catalyst precursor incorporation in optimizing Cr(VI) remediation, especially in strongly&#xa0;alkaline and hyper-alkaline conditions. These findings could be of considerable relevance to advanced remediation strategies for persistent Cr(VI) contaminated sites worldwide.</p>

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Implications of bi-metal catalysts on micro- and nano-zero-valent iron brands for hexavalent chromium removal in strongly alkaline and hyper-alkaline medium

  • 期刊论文

  • 发表日期：

    2025年6月9日

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  • Rahul Singh,
  • Sumedha Chakma,
  • Volker Birke

摘要

Hexavalent chromium (Cr(VI)) is highly mobile in groundwater (GW) and poses severe toxicological and carcinogenic risks to human health, ecosystems, and the environment. Various remediation techniques have been developed in the last few decades to efficiently counter contaminations by Cr(VI) in an acidic medium, however, effective removal of Cr(VI) in strongly alkaline (pH 8.5–9.0) and hyper-alkaline (pH > 10.5) aqueous medium remains a significant challenge. This study investigated the synergistic effects of two different zero-valent iron (ZVI) brands, i.e., micro-ZVI (MZVI) and nano-ZVI (NZVI), combined with Ni and Cu catalyst precursors to achieve efficient Cr(VI) removal in strongly alkaline and hyper-alkaline medium. This study was prompted by multiple observations/findings at different sites globally, such as in India, that a Cr(VI) contamination in GW may be accompanied by significantly elevated/high pH values (> pH = 7, alkaline conditions), that is, due to certain geological or other circumstances/reasons which probably may have contributed to the Cr(VI) pollution in the past while it was originated. The experimental framework focused on evaluating the influence of different elevated alkalinity levels, pH 9.0 (strongly alkaline medium) and 11.5 (hyper-alkaline medium), on Cr(VI) (initial concentration of 50 mg/L) removal rates using MZVI and NZVI. Additionally, the effects of introducing catalyst precursors such as Cu and Ni in the form of Cu(II)-chloride, Cu(II)-nitrate, Ni(II)-chloride, and Ni(II)-nitrate were assessed in combination with MZVI and NZVI at both pH 9.0 and pH 11.5. The catalyst precursors were tested with five different dosages (250 µmol/g ZVI, 50 µmol/g ZVI, 30 µmol/g ZVI, 20 µmol/g ZVI, and 10 µmol/g ZVI). The results indicated that increasing alkalinity significantly reduces the Cr(VI) removal efficacy of both MZVI and NZVI, necessitating prohibitively high ZVI dosages, particularly for full-scale applications. However, the integration of catalyst precursors, specifically Cu(II)/ZVI and Ni(II)/ZVI, proved instrumental in achieving 100% Cr(VI) removal within just 5 min of initiating the experiments with both MZVI and NZVI. Among the tested catalysts, Ni(II)/ZVI at a dosage of 10 µmol/g ZVI (0.064 wt. %) displayed lower performance in both strongly alkaline and hyper-alkaline medium, whereas Cu(II)/ZVI demonstrated relatively superior efficacy. This study underscores the crucial role of catalyst precursor incorporation in optimizing Cr(VI) remediation, especially in strongly alkaline and hyper-alkaline conditions. These findings could be of considerable relevance to advanced remediation strategies for persistent Cr(VI) contaminated sites worldwide.