Enhanced arsenic immobilization from contaminated soil to crops, carbon sequestration, and soil fertility using laterite Biochar composites

Overview

This study evaluated a novel laterite biochar composite (LBC), a material combining laterite (a natural iron-rich soil) with hemp-stalk-derived biochar, for its capacity to immobilize arsenic in arsenic-contaminated paddy soil and reduce arsenic transfer from soil to rice grain. The research combined batch adsorption experiments, fixed-bed column studies, and pot experiments growing rice (cultivar Pusa 1847) in soil spiked to 106 mg arsenic (As)/kg. Three LBC dosages (0.5%, 1%, and 2% wt/wt in soil) were tested against a no-amendment control. The study also characterized the mechanism of arsenic immobilization via iron plaque formation on rice roots and organometallic complex formation in soil using SEM-EDS, XPS, FTIR, and XRD.

This is a soil amendment/pot study with grain-level arsenic data. It provides context for uptake pathways and immobilization mechanisms rather than finished-food concentration survey data.

Key numbers

All values are reported in the paper’s own units (mg/kg).

Grain arsenic concentration:

• Control (0% LBC): 17.79 mg/kg
• 0.5% LBC: 6.5 mg/kg (63.5% reduction)
• 1% LBC: not separately reported for grain (intermediate reduction stated)
• 2% LBC: 0.1 mg/kg (99.4% reduction), described as below the WHO recommended safe limit of 0.3 mg/kg

Root arsenic concentration (digestion):

• Control: 904.25 mg/kg
• 2% LBC: 122.6 mg/kg

Leaf arsenic concentration:

• Control: 52.27 mg/kg
• 2% LBC: 14.43 mg/kg

Iron plaque arsenic and iron concentrations:

• As in iron plaque decreased from 666 mg/kg (control) to 102 mg/kg (2% LBC)
• Fe in iron plaque increased from 612 mg/kg (control) to 28,550 mg/kg (2% LBC), a 365.6% increase over control

Arsenic reduction in rice seeds (from batch adsorption framing):

• LBC decreased arsenic levels in rice seeds by 99.5% (2% LBC), 76% (1% LBC), and 63.5% (0.5% LBC) during pot experiments

Soil carbon effects:

• Total carbon increased by 34.5% (2% LBC), 20.7% (1% LBC), and 13.8% (0.5% LBC) compared to control
• Soil organic carbon (SOC) content increased by 110% (2% LBC), 75.7% (1% LBC), and 40% (0.5% LBC)

Rice plant growth:

• Dry matter: 99 g/pot (2% LBC), 51.3 g/pot (1% LBC), 23 g/pot (0.5% LBC), vs. 10.0 g/pot (control)
• Grain yield increased by 801% using 2% LBC vs. control
• Plant height increased by 78% with 2% LBC treatment at 105 days after transplanting

Evidence fitness

EF-2: The study provides grain-level arsenic data across multiple LBC dosages in a pot experiment using arsenic-contaminated field soil (106 mg As/kg, a heavily contaminated soil). Analytical methods include ICP-MS for elemental analysis and XPS, SEM-EDS, FTIR, and XRD for mechanistic characterization. The pot design does not replicate field conditions but uses field-collected contaminated soil. The heavily contaminated starting soil (106 mg/kg) is far above typical agricultural paddy soil arsenic concentrations, which limits direct extrapolation to low-contamination rice production regions.

Methods (brief)

Biochar produced from hemp stalks by pyrolysis at 400 °C, sieved to 500 microns, combined with acid-leached laterite at 50:50 weight ratio to form LBC. Arsenic-contaminated soil (0 to 20 cm depth) collected from Saharanpur district, Uttar Pradesh, India. Soil spiked with sodium arsenate heptahydrate and sodium arsenite at 50 mg As/kg soil concentration (total soil As approximately 106 mg/kg including background). Rice cultivar Pusa 1847 grown in pots (1.5 kg soil per pot) under continuous flooding from July to November. LBC incubated in soil before sowing. Three replicates per treatment. Plant height measured at 15, 45, 75, and 105 days after transplanting. After harvest: ICP-MS for arsenic in roots, leaves, and seeds; DCB extraction for iron plaque analysis; SOC by Walkley-Black method; dehydrogenase enzyme activity (DHA) by TTC reduction test. Statistical analysis by ANOVA with Tukey post hoc test (p < 0.005) using SPSS ver.27.

Implications

This study is relevant as upstream mitigation context for arsenic in rice. The laterite biochar composite represents a novel soil amendment approach combining iron-based adsorption with biochar carbon sequestration. The dual mechanism (organometallic complex formation in soil plus enhanced iron plaque on roots as a mass transfer barrier) is mechanistically distinct from pure iron or pure biochar amendments. The extreme soil contamination level (106 mg As/kg) used in this study is substantially higher than typical paddy soils, so the percentage reductions reported should not be directly extrapolated to lower-contamination agricultural settings. The finding that 2% LBC reduced grain arsenic to 0.1 mg/kg from a starting soil of 106 mg/kg is notable for remediation of contaminated sites.

Provenance notes

• Open access under CC BY-NC-ND 4.0.
• Received 29 October 2024, accepted 12 May 2025, published 23 May 2025.
• LBC synthesis is covered by an Indian patent (details referenced in the paper).
• Soil collected from paddy field in Saharanpur district, India.
• Hemp stalk sourced from M/S Gohemp Agroventures Pvt. Ltd., Roorkee, India; plant identified as Cannabis Sativa L. with herbarium voucher.

Related evidence

• Rice
• Arsenic

Update history

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