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. 2025 Jun 9;10(24):25479-25488.
doi: 10.1021/acsomega.5c00705. eCollection 2025 Jun 24.

Stabilization Mechanism of Highly Toxic Hexavalent Chromium during Carbothermal Reactions in Lettuce

Qiong Wang  1   2 , Hugang Li  1   3
Affiliations

Stabilization Mechanism of Highly Toxic Hexavalent Chromium during Carbothermal Reactions in Lettuce

Qiong Wang et al. ACS Omega. .

Abstract

As a remediation plant for remediating cadmium-contaminated soil, highly toxic Cr-(VI) is enriched in lettuce, making the rational treatment of this biomass a challenge to be solved. In this work, CaO, which is cheap and available, was used as a stabilizer for the pyrolysis of polluted lettuce. Results revealed that the Cr-(VI):Cr ratio in lettuce-derived biochar obtained at 800 °C decreased by 93.6% in comparison to that at 400 °C due to the immobilization effects. Spearman correlation analysis also revealed that Cr-(VI) was negatively correlated with CaO dosage (R 2 = -0.81, p ≤ 0.01) and pyrolysis temperature (R 2 = -0.86, p ≤ 0.01). Furthermore, Cr-(VI) was reduced to Cr-(V) and Cr-(III) in the presence of CaO during the pyrolysis. XRD analysis and SEM-EDS analysis further confirmed that the Cr-(V) compound refers to Ca5(CrO4)3OH as well as Ca5(CrO4)3Cl. The intensity of the peak of the aromatic ring and heteroaromatic compounds on the surface of biochar presents a significant increase when the CaO dosage was higher than 10%. Furthermore, these redox-active functional groups act as electron donors in the reduction of Cr-(VI). Meanwhile, Cr-(VI) was partially converted preferentially to Ca2CrO4Cl in the copresence of CaO and Fe2O3, resulting in the immobilization of Cr-(VI). This study clarifies the influence of CaO dosage and temperature on the Cr-(VI) transformation and characteristic of biochar and provides a new strategy for Cr-(VI) remediation of the Cr-contaminated biomass.

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Figures

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1
Effect of CaO dosage and pyrolysis temperature on the biochar yield.
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2
Elemental distribution of RL and biochar derived under different conditions.
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Content of Cr­(III) and Cr­(VI) in biochar samples (a); the Cr­(VI):Cr ratio of RL and biochar from different treatments (b). The content of Cr­(III) in biochar was calculated by the difference between the total content of Cr and the content of Cr­(VI).
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XRD spectra of the RL, biochar derived from 10% CaO and 700 °C treatments, respectively.
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SEM images of RL (a); biochar obtained from 10%-CaO treatment (b) and 700 °C treatment (c).
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SEM-EDS of biochar from 10%-CaO treatment (a–d) and 700 °C treatment (e–h).
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FTIR spectrum of biochar. Effect of reaction temperature (a) and CaO dosage (b).
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Spearman correlation heatmap of operation conditions, Cr speciation, organic/inorganic element distribution, and BET characteristics (*p ≤ 0.01).
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Potential mechanism of the Cr­(VI) reduction during the copyrolysis process.
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