Key Takeaways & Executive Findings
- •• • CG@PVA-SA composite filler achieved a water retention of 358 mg·g−1, a 27.9% improvement over the control (280 mg·g−1), directly enhancing the moisture environment critical for microbial activity in biotrickling filters. • • The filler exhibited a saturated adsorption capacity of 201.02 mg·kg−1 for isohexane, 76% higher than the control (114.24 mg·kg−1), which accelerates pollutant capture and mitigates mass transfer limitations. • • Rhodococcus ruber ZYH-ZY survival on CG@PVA-SA reached 79.0%, more than double the control (37.2%), demonstrating superior biocompatibility and biofilm support, essential for sustained biodegradation. • • Static degradation tests achieved 96.59% removal of 10 μL isohexane within 24 h, confirming the synergistic adsorption-biodegradation efficiency of the composite, which is pivotal for industrial BTF applications.
Abstract
Biotrickling filtration (BTF) is a promising technology for treating volatile organic compounds (VOCs), but its application to hydrophobic alkanes like isohexane is hindered by mass transfer limitations, low degradation efficiency, and high operational costs. To address these bottlenecks, this study developed composite fillers by incorporating biochars derived from coffee grounds (CG), coconut shells (CS), corn cobs (CC), and activated carbon (AC) into a polyvinyl alcohol-sodium alginate (PVA-SA) hydrogel matrix. The fillers were systematically characterized for water retention, pore structure, surface functional groups, crystalline phase, and acid-base resistance. Adsorption capacity, biofilm formation, and isohexane degradation were evaluated using the strain Rhodococcus ruber ZYH-ZY. Among the composites, CG@PVA-SA exhibited superior performance: water retention of 358 mg·g−1 (vs. 280 mg·g−1 for control), enhanced mesoporosity (specific surface area 4.77 m2·g−1, pore volume 11.46 cm3·kg−1, 10–30% higher than control), and robust acid-base stability (mass loss 21.37% at pH 2 and 31.98% at pH 10). Its saturated adsorption capacity reached 201.02 mg·kg−1 (vs. 114.24 mg·kg−1 for control), and it promoted bacterial colonization with a survival rate of 79.0% (vs. 37.2% for control). Static degradation tests showed 96.59% removal of 10 μL isohexane within 24 h. The abundant polar functional groups and suitable mesoporous structure of coffee ground biochar synergized with the PVA-SA matrix, enhancing water retention, mass transfer, and microbial colonization, thereby significantly improving isohexane purification. CG@PVA-SA is an ideal filler for BTF treatment of alkane VOCs, offering a cost-effective and efficient solution for industrial VOC control.
1. Introduction
Industrial emissions of volatile organic compounds (VOCs), particularly hydrophobic alkanes such as isohexane, pose significant environmental and health risks due to their toxicity and persistence. Biotrickling filtration (BTF) offers a cost-effective and environmentally benign solution, yet its performance is often constrained by poor mass transfer of hydrophobic pollutants and suboptimal biofilm formation on conventional packing materials. Traditional fillers lack the necessary hydrophobicity and surface chemistry to efficiently capture and degrade such compounds, leading to low removal efficiencies and high operational costs.
This study addresses these bottlenecks by engineering composite fillers that integrate biochar—a carbonaceous material rich in pores and polar functional groups—into a hydrophilic PVA-SA hydrogel matrix. The hypothesis is that the synergistic combination of biochar's adsorption capacity and the hydrogel's biocompatibility will enhance pollutant mass transfer, microbial colonization, and overall degradation performance. By systematically comparing biochars from different biomass sources, this work identifies the optimal filler formulation that maximizes water retention, structural stability, and bacterial viability, thereby providing a practical pathway to improve BTF efficiency for alkane VOCs.
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QU Miaomiao, ZHENG Yi, ZHOU Renlei, LUO Qiwen, FENG Yuqi, CHENG Zhuowei, SHI Yun, WANG Wenjun, YOU Juping, CHEN Jianmeng (2026). Mechanistic Insights into Biochar@PVA-SA Composite Fillers for Enhanced Biopurification of Isohexane in Biotrickling Filters. Chinese Journal of Environmental Engineering. https://doi.org/10.12030/j.cjee.202509095
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Frequently Asked Questions
What is the long-term mechanical stability of the CG@PVA-SA filler under continuous biotrickling operation, and how does it compare to conventional plastic or ceramic packings?
The study reports acid-base resistance with mass losses of 21.37% at pH 2 and 31.98% at pH 10, indicating chemical robustness. However, long-term mechanical stability under hydrodynamic shear and biofilm growth was not explicitly tested. The PVA-SA matrix is known for its elasticity, but its durability in industrial BTF columns over months requires further pilot-scale validation. Compared to rigid plastic packings, the composite may have lower compressive strength, but its superior water retention and biocompatibility could offset this in low-pressure systems.
How does the adsorption capacity of CG@PVA-SA for isohexane translate into actual mass transfer enhancement in a continuous BTF, given that adsorption is only a transient sink?
The saturated adsorption capacity of 201.02 mg·kg−1 is significantly higher than the control, which enhances the concentration gradient at the biofilm interface, thereby improving mass transfer of hydrophobic isohexane from the gas phase to the aqueous biofilm. In continuous operation, this adsorption capacity acts as a buffer, smoothing concentration fluctuations and providing a sustained substrate supply to the biofilm, which is critical for maintaining high degradation rates. The 96.59% degradation within 24 h in static tests suggests that the adsorbed isohexane is bioavailable and readily metabolized.
What is the cost comparison between CG@PVA-SA and conventional fillers, and does the use of waste-derived biochar offer a significant economic advantage?
The study highlights the cost-effectiveness of CG@PVA-SA, as coffee grounds are abundant and low-cost waste materials. The preparation process involves simple pyrolysis and gelation, which is scalable. While exact cost figures are not provided, the use of waste biomass reduces raw material costs compared to synthetic or activated carbon-based fillers. Additionally, the enhanced degradation efficiency and lower energy requirements (due to improved mass transfer) could reduce operational costs, making it economically attractive for industrial applications.
How does the presence of polar functional groups on the biochar surface influence the adhesion of Rhodococcus ruber ZYH-ZY, and what is the mechanism behind the high survival rate?
The biochar surface contains C–O and C=O polar groups, which enhance hydrophilicity and provide binding sites for microbial adhesion. These groups also facilitate hydrogen bonding and electrostatic interactions with bacterial cell surfaces, promoting initial attachment and biofilm formation. The high survival rate of 79.0% indicates that the composite provides a favorable microenvironment, possibly due to improved water retention and nutrient availability, which supports bacterial viability and activity.
What are the potential limitations of using CG@PVA-SA for treating high concentrations of isohexane or in the presence of other VOCs, and how does the filler perform under fluctuating loading conditions?
The study tested a single concentration (10 μL) in static conditions, so performance under high or fluctuating loads is not fully characterized. However, the high adsorption capacity and rapid degradation suggest that the filler can handle moderate loads. For high concentrations, the adsorption capacity may become saturated, leading to breakthrough, but the biofilm can degrade the adsorbed pollutant, providing a continuous removal mechanism. In mixed VOC streams, competitive adsorption may occur, but the polar functional groups and mesoporous structure could selectively adsorb alkanes. Further dynamic studies are needed to assess long-term stability and performance under realistic industrial conditions.
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