In a recent study published in Advanced Composites and Hybrid Materials, Ayoub Chaoui, Abdelaziz Imgharn, Ana C. Estrada, and their colleagues investigated a novel approach to water treatment: a biochar@polyaniline (BC@PANI) composite for remediating highly toxic hexavalent chromium (Cr(VI)) from aqueous solutions. Cr(VI) is a global environmental threat due to its carcinogenic properties and severe health impacts, necessitating effective removal and reduction strategies. This research focuses on utilizing biogas residue digestate, a sustainable waste product, to create advanced adsorbent technologies.

The biocharBiochar is a carbon-rich material created from biomass decomposition in low-oxygen conditions. It has important applications in environmental remediation, soil improvement, agriculture, carbon sequestration, energy storage, and sustainable materials, promoting efficiency and reducing waste in various contexts while addressing climate change challenges. More (BC) was prepared from poultry by-product digestate, undergoing drying, washing, grinding, pyrolysisPyrolysis is a thermochemical process that converts waste biomass into bio-char, bio-oil, and pyro-gas. It offers significant advantages in waste valorization, turning low-value materials into economically valuable resources. Its versatility allows for tailored products based on operational conditions, presenting itself as a cost-effective and efficient More at 500°C, and acid activation. The BC was then modified with polyaniline (PANI) through an in-situ chemical polymerization method, creating composites with 25%, 50%, and 75% (w/w) PANI. The synthesized materials were thoroughly characterized to understand their structural changes and functionalities. Powder X-ray Diffraction (XRD) analysis indicated that PANI modification introduced an amorphous structure, decreasing the crystallinity of the biochar. Fourier Transform Infrared (FTIR) spectroscopy confirmed the successful incorporation of PANI by identifying characteristic vibrational bands of its quinonoid and benzenoid units. Raman spectroscopy further substantiated the successful modification of biochar by polyaniline. BET surface area measurements showed a significant reduction from 39.138 m²/g for raw BC to 7.933 m²/g for the PANI-coated material, indicating successful pore blockage and polyaniline loading.

The adsorption performance of the BC@PANI composite was systematically evaluated through batch experiments. The optimal PANI content was found to be 50%, achieving an 80.62% Cr(VI) removal rate. This demonstrated a fivefold improvement over standalone biochar (15.75% removal), highlighting a strong synergistic effect between PANI and activated biochar. The solution’s pHpH is a measure of how acidic or alkaline a substance is. A pH of 7 is neutral, while lower pH values indicate acidity and higher values indicate alkalinity. Biochars are normally alkaline and can influence soil pH, often increasing it, which can be beneficial More significantly influenced the adsorption efficiency. The BC@PANI composite exhibited its highest efficacy (over 95% removal, reaching a maximum adsorption capacity of 125 mg/g) under low pH conditions (pH 2), specifically below its Point of Zero Charge (PZC) of 2.76. At low pH, the protonation of amino groups on the BC@PANI surface facilitates electrostatic interactions with anionic Cr(VI) species (HCrO₄⁻ and Cr₂O₇²⁻). Following this adsorption, Cr(VI) undergoes in-situ chemical reduction to less toxic Cr(III), which is then immobilized through chelation with nitrogen-containing groups in PANI.

Adsorbent dosage studies revealed that increasing BC@PANI from 0.1 g/L to 0.75 g/L rapidly increased removal efficiency to 94.12%, correlating with an increase in available active sites. Beyond this, removal efficiency plateaued, indicating site saturation. Conversely, the adsorption capacity decreased at higher dosages due to particle agglomeration. The contact time study showed a rapid increase in Cr(VI) removal efficiency to 84% within the initial 5 minutes, attributed to the abundance of active sites and strong driving force. This was followed by a slower increase until equilibrium was reached after approximately 90 minutes.

Initial Cr(VI) concentration positively influenced adsorption capacity. At 298 K, the capacity reached 760.10 mg/g for an initial concentration of 1000 ppm. Temperature also positively impacted adsorption, increasing capacity from 732.67 mg/g at 298 K to 838.35 mg/g at 318 K. This endothermic nature of the process was confirmed by thermodynamic analysis, with positive ΔH° (34.30 kJ/mol) and ΔS° (180.46 J/mol/K) values, and negative ΔG° values across temperatures, indicating spontaneity. The ΔH° value exceeding 20 kJ/mol further affirms that the adsorption is predominantly governed by chemical interactions.

Co-ion studies demonstrated that common anions (Cl⁻, NO₃⁻, SO₄²⁻, CO₃²⁻) had minimal influence on removal efficiency, though NO₃⁻ showed the most negative impact (9% decrease) due to competitive interaction. This notable selectivity is advantageous for treating real-world wastewater. Kinetic analysis revealed that the pseudo-second-order model best described the adsorption process, suggesting a chemisorptive nature. The Elovich model implied a heterogeneous or multi-mechanism phenomenon. Intra-particle diffusion showed a three-stage process, indicating that it was not the sole rate-limiting factor. Finally, regeneration experiments showcased the composite’s reusability. After four adsorption/desorption cycles, the Cr(VI) removal efficiency marginally decreased by about 5% (from 94.75% to 89.13%). Even after five cycles, the composite retained an impressive 82.5% Cr(VI) removal efficiency, demonstrating its exceptional reusability and stability.

In conclusion, this study establishes BC@PANI as a promising, practical, and sustainable solution for removing Cr(VI) ions from water, offering a valuable avenue for integrating such sorbents into water treatment engineering practices.

Source: Chaoui, A., Imgharn, A., Estrada, A. C., Ben Hamou, A., Farsad, S., Nouj, N., Ez-zahery, M., Trindade, T., Albourine, A., & El Alem, N. (2025). Chromium (VI) remediation via biochar@polyaniline composite: advancing water treatment using biogas residue digestate. Advanced Composites and Hybrid Materials, 8(1), 312.