The intrinsic micro-scale mechanisms governing the photocatalytic elimination of hexavalent chromium (Cr(VI)) from aqueous media using pristine sludge-derived biochar remain insufficiently explored. In this study, pristine sludge biochars (designated as BC300, BC500, and BC700) were fabricated via pyrolysis at varying temperatures (300, 500, and 700 °C), and their physicochemical characteristics were comprehensively investigated. The performance of these biochars in Cr(VI) removal under visible light was assessed, with a particular focus on the involvement of surface functional moieties. It was observed that biochar synthesized at the lowest temperature (BC300) retained a higher oxygen content, a greater abundance of surface functional groups, and a lower point of zero charge (pHpzc), despite having a relatively smaller specific surface area and pore volume. UV–visible diffuse reflectance spectroscopy (DRS) and photoluminescence (PL) analyses indicated enhanced photoinduced charge separation and transfer in BC300. This sample demonstrated the most effective Cr(VI) removal, achieving near-complete elimination (99.98%) of 15 mg L⁻1 Cr(VI) in 100 mL aqueous solution at a dosage of 0.1 g L−1 within 120 min under visible light. Importantly, approximately 99.5% of the removed Cr(VI) was converted to the less toxic Cr(III). Functional group elimination, chemical modification, and post-reaction characterization collectively underscored the essential role of surface functional groups in the removal process. Two-dimensional correlation spectroscopy (2D-COS) integrated with FTIR analysis further revealed the sequential participation of functional groups on BC300: 1455 cm−1 (–CH2) > 1401 cm−1 (–COO–) > 795 cm−1 (Si–O–Si) > 538 cm−1 (Si–O–Si) > 3443 cm−1 (–OH) > 1637 cm−1 (C=C/C=O) > 1031 cm−1 (C–O) > 2855 cm−1 (C–H). Both sp3-hybridized (–OH, C–O, C–H) and sp2-hybridized (–COO–, C=C/C=O) functional groups contributed synergistically to Cr(VI) removal through adsorption, reduction, and complexation pathways. Additionally, BC300 exhibited satisfactory reusability across multiple cycles. These findings provide mechanistic insights into the development of sludge-based photocatalysts for efficient and sustainable Cr(VI) decontamination in water environments.

Revealing the pivotal role of surface functional groups in Cr(VI) photocatalytic removal by pristine sludge biochar under visible light

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发表日期：

2026年1月16日

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Shisuo Fan,
Yushan Zhang,
Pengyu Ji,
Xinnan Hu,
Manquan Zhao,
Na Zhou

摘要

The intrinsic micro-scale mechanisms governing the photocatalytic elimination of hexavalent chromium (Cr(VI)) from aqueous media using pristine sludge-derived biochar remain insufficiently explored. In this study, pristine sludge biochars (designated as BC300, BC500, and BC700) were fabricated via pyrolysis at varying temperatures (300, 500, and 700 °C), and their physicochemical characteristics were comprehensively investigated. The performance of these biochars in Cr(VI) removal under visible light was assessed, with a particular focus on the involvement of surface functional moieties. It was observed that biochar synthesized at the lowest temperature (BC300) retained a higher oxygen content, a greater abundance of surface functional groups, and a lower point of zero charge (pHpzc), despite having a relatively smaller specific surface area and pore volume. UV–visible diffuse reflectance spectroscopy (DRS) and photoluminescence (PL) analyses indicated enhanced photoinduced charge separation and transfer in BC300. This sample demonstrated the most effective Cr(VI) removal, achieving near-complete elimination (99.98%) of 15 mg L⁻¹ Cr(VI) in 100 mL aqueous solution at a dosage of 0.1 g L⁻¹ within 120 min under visible light. Importantly, approximately 99.5% of the removed Cr(VI) was converted to the less toxic Cr(III). Functional group elimination, chemical modification, and post-reaction characterization collectively underscored the essential role of surface functional groups in the removal process. Two-dimensional correlation spectroscopy (2D-COS) integrated with FTIR analysis further revealed the sequential participation of functional groups on BC300: 1455 cm⁻¹ (–CH₂) > 1401 cm⁻¹ (–COO^–) > 795 cm⁻¹ (Si–O–Si) > 538 cm⁻¹ (Si–O–Si) > 3443 cm⁻¹ (–OH) > 1637 cm⁻¹ (C=C/C=O) > 1031 cm⁻¹ (C–O) > 2855 cm⁻¹ (C–H). Both sp³-hybridized (–OH, C–O, C–H) and sp²-hybridized (–COO^–, C=C/C=O) functional groups contributed synergistically to Cr(VI) removal through adsorption, reduction, and complexation pathways. Additionally, BC300 exhibited satisfactory reusability across multiple cycles. These findings provide mechanistic insights into the development of sludge-based photocatalysts for efficient and sustainable Cr(VI) decontamination in water environments.