Key Takeaways & Executive Findings
- •• • Adding 3% AMRM increased cumulative methane yield to 633.9 mL/g VS, a 175.1% improvement over control and 55.2% over raw RM, with VS removal reaching 75.1%, demonstrating significant enhancement in AD efficiency. • • Acid modification increased AMRM's specific surface area by 347% relative to RM, enabling rapid adsorption of VFAs, especially propionic acid, as evidenced by adsorption kinetics (Qe of 40.234 mg/g for AMRM vs. 25.416 mg/g for RM in pseudo-first-order model). • • AMRM enriched hematite (Fe2O3) content, which elevated ETS activity and coenzyme F420 levels, indicating enhanced DIET between syntrophic bacteria and methanogens, a critical mechanism for stable methane production. • • The dual-pathway synergy—physical adsorption in the initial phase and electron transfer in the long term—effectively mitigated acid stress and sustained high-rate methanogenesis, offering a robust solution for high-organic-load FW treatment.
Abstract
Anaerobic digestion (AD) is a viable route for energy recovery from food waste (FW), yet it often suffers from process instability due to volatile fatty acids (VFAs) accumulation and subsequent pH drop. This study modified red mud (RM) with hydrochloric acid to produce acid-modified red mud (AMRM), aiming to optimize its alkalinity and physicochemical properties. The efficacy of AMRM as an additive in FW anaerobic digestion was systematically evaluated. Results demonstrated that adding 3% AMRM significantly enhanced the buffering capacity and controlled VFAs accumulation, particularly propionic acid. This was attributed to the developed pore structure of AMRM, whose specific surface area increased by 347% compared to raw RM, facilitating rapid VFAs adsorption. Furthermore, AMRM enriched hematite (Fe2O3), which elevated electron transport system (ETS) activity and coenzyme F420 content, suggesting its role as an electron carrier promoting direct interspecies electron transfer (DIET) between syntrophic bacteria and methanogens. Consequently, the cumulative methane yield reached 633.9 mL/g VS, which was 175.1% and 55.2% higher than the control and raw RM groups, respectively. The VS removal efficiency was 75.1%. This study provides a dual-pathway mechanism—adsorption and electron transfer—for enhancing AD performance, offering a cost-effective and sustainable strategy for FW treatment and RM valorization.
1. Introduction
Anaerobic digestion (AD) of food waste (FW) is a cornerstone technology for renewable energy generation, yet its commercial viability is often undermined by process instability arising from volatile fatty acids (VFAs) accumulation. When the organic loading rate is high, VFAs—particularly propionic acid—accumulate, causing a sharp pH decline that inhibits methanogenic activity and can lead to irreversible acidification. Conventional countermeasures, such as adding alkaline chemicals (e.g., NaOH, CaO) or conductive materials (e.g., biochar, magnetite), have shown promise but suffer from high costs, secondary pollution, or inconsistent performance. For instance, while conductive materials like magnetite can enhance direct interspecies electron transfer (DIET), their synthesis is energy-intensive, and their dosage optimization remains empirical. Alkaline pretreatment, on the other hand, may introduce inhibitory ions (e.g., Na+) that suppress methanogens, negating the intended benefits.
Red mud (RM), a highly alkaline bauxite residue from alumina production, presents a dual opportunity: it is an abundant industrial waste with high iron oxide content (Fe2O3 30–60%) and porous structure, making it a potential low-cost additive for AD. However, its strong alkalinity and potential sodium toxicity can inhibit methanogens. This study addresses this bottleneck by acid-modifying RM with hydrochloric acid to neutralize excess alkalinity and leach out sodalite and calcite phases, thereby enhancing its specific surface area and enriching hematite. The resulting acid-modified red mud (AMRM) is hypothesized to act as both a pH buffer and an electron shuttle, synergistically adsorbing VFAs and promoting DIET. This work systematically evaluates the performance of AMRM in FW anaerobic digestion, providing mechanistic insights into its dual role and demonstrating a sustainable pathway for RM valorization.
Loading authentic research manuscript (Pages 1–5)...
LIU Liang, KANG Xiangjing, QING Mengxia, ZHANG Xinrui (2026). Acid-Modified Red Mud Enhances Anaerobic Digestion of Food Waste via Synergistic Adsorption and Electron Transfer: Performance and Mechanism. Journal of Environmental Engineering Technology. https://doi.org/10.13205/j.hjgc.202607011
Research & Educational Purpose Only: The translations, structured abstracts, analytical annotations, and data reports provided by SinoGreenTechare intended exclusively for academic research, internal corporate R&D, and educational benchmarking. They do not constitute formal engineering, chemical safety, legal, or professional advice.
Copyright & Intellectual Property Notice: Original copyright of the underlying source articles and experimental data remains with the respective authors, institutions, and original publishing journals. SinoGreenTech claims intellectual property only over its proprietary translations, analytical syntheses, and AEO structured enhancements in accordance with international fair use and academic citation principles.
Frequently Asked Questions
What is the optimal dosage of AMRM for enhancing methane production, and how does it compare to raw RM in terms of buffering capacity?
The optimal dosage was 3% (w/w) based on the study. At this dosage, the cumulative methane yield reached 633.9 mL/g VS, which was 175.1% higher than the control (no additive) and 55.2% higher than the raw RM group. The buffering capacity was significantly improved, as evidenced by controlled VFAs accumulation and stable pH, whereas raw RM exhibited strong alkalinity that could inhibit methanogens.
How does AMRM's adsorption capacity for propionic acid compare to raw RM, and what kinetic model best describes the adsorption process?
AMRM exhibited a higher adsorption capacity for propionic acid, with an equilibrium adsorption capacity (Qe) of 40.234 mg/g in the pseudo-first-order model, compared to 25.416 mg/g for raw RM. The pseudo-first-order model provided a better fit for AMRM (R² = 0.999) than the pseudo-second-order model (R² = 0.968), indicating that the adsorption process is physisorption-dominated, likely due to the increased specific surface area (347% higher).
What is the mechanistic role of hematite in AMRM for promoting direct interspecies electron transfer (DIET), and how is it experimentally evidenced?
Hematite (Fe2O3) in AMRM acts as an electron conduit, facilitating DIET between syntrophic bacteria and methanogens. This is evidenced by increased electron transport system (ETS) activity and higher coenzyme F420 content in the digestion system. Coenzyme F420 is a key methanogenic cofactor, and its elevation indicates enhanced methanogenic activity. The conductive hematite particles likely bridge the physical gap between species, enabling efficient electron transfer.
What are the long-term stability implications of using AMRM in continuous anaerobic digesters, particularly regarding the risk of iron accumulation or secondary pollution?
While this study was conducted in batch mode, the use of AMRM at 3% dosage introduces iron oxides into the digestate. Iron is an essential micronutrient for microbial growth, but excessive accumulation could pose environmental risks if digestate is applied to land. However, the study did not report iron leaching or toxicity. Future work should assess long-term iron fate and potential regulatory compliance. The dual benefits of enhanced methane yield and waste valorization suggest a favorable trade-off, but pilot-scale studies are needed to evaluate operational stability.
How does the performance of AMRM compare to other conductive additives like biochar or magnetite in terms of methane yield enhancement and cost-effectiveness?
In this study, AMRM achieved a methane yield of 633.9 mL/g VS, which is comparable or superior to reported values for biochar and magnetite. For instance, Wu et al. reported 524.4 mL/g VS with magnetite addition. AMRM's advantage lies in its low cost as an industrial waste, whereas biochar and magnetite require energy-intensive production. The 347% increase in surface area and hematite enrichment provide a dual mechanism that may outperform single-mechanism additives. However, direct comparative studies under identical conditions are necessary for definitive conclusions.
Related Chinese Research & Cross-Citations
Synergistic Regulation by Long- and Short-Chain Quorum Sensing Signaling Molecules Enhances Sulfamethoxazole Metabolism in Electroactive Biofilms within a Microbial Electrolysis Cell Coupled Anaerobic Digestion System
High-strength sulfamethoxazole (SMX) wastewater severely inhibits anaerobic microorganisms, reducing organic degradation and methane yield. This study investigated the effects of short-chain (C6-HSL) and long-chain (C12-HSL) N-acyl-homoserine lactone (AHL) signaling molecules, individually and in combination, on the construction, performance, and antibiotic resistance gene (ARG) profiles of anaerobic electroactive biofilms within a microbial electrolysis cell coupled anaerobic digestion (MEC-AD) system. Compared to the control (no AHLs), SMX removal efficiency increased by 9.26%, 7.44%, and 10.67% for C6-HSL (T1), C12-HSL (T2), and combined (T3) treatments, respectively. Methane production rates rose by 20.4%, 16.9%, and 23.1% for T1, T2, and T3, respectively. AHLs promoted extracellular polymeric substance secretion, enhancing electroactive microbe attachment to the anode. Microbial community analysis revealed increased diversity and modulated key functional genera. Notably, Georgenia abundance increased by 16.77% (T1) and 36.47% (T3) but decreased by 15.99% (T2). ARG analysis showed that single AHLs elevated intI1, sul1, and sul2 abundances, whereas combined AHLs (T3) exhibited a milder response, with sul2 abundance reduced by 4.92% relative to control. This suggests synergistic AHLs suppress ARG host proliferation. This study first demonstrates that combined short- and long-chain AHLs enhance electroactive biofilm formation, maintain microbial community stability, and modulate ARG dissemination risk, offering a quorum sensing-based strategy for antibiotic wastewater treatment and risk management.
Preparation of Solid-Phase Carbon Sources with Different Ratios and Their Carbon Release Properties
Low C/N ratios in wastewater treatment plant effluent necessitate external carbon sources for denitrification, but conventional liquid carbon sources are costly and unstable. This study prepared nine composite solid-phase carbon sources by combining PHBV with natural cellulose materials (straw, sawdust, corncob) at different mass ratios. Dynamic release experiments, DOC analysis, UV-Vis spectroscopy, and EEM fluorescence were employed to characterize carbon release. Results showed that increasing cellulose proportion in corncob-based sources led to release patterns opposite to those of straw- and sawdust-based sources. For straw and sawdust, higher cellulose ratios accelerated release rates, increased total release and duration, reduced aromaticity and molecular weight of released DOM, and promoted protein-like components (tryptophan, tyrosine), indicating enhanced bioavailability. Under identical ratios, corncob-based sources exhibited moderate total release, release durations exceeding 134 h, lower DOM aromaticity and molecular weight, and lower humic substance proportion, indicating superior bioavailability and engineering potential. Among the nine sources, JG5, MX5, and CC4 (PHBV:cellulose mass ratios of 4:5, 4:5, and 1:1, respectively) showed optimal comprehensive performance with low theoretical maximum release, long release periods, and high mass transfer coefficients. EEM-PARAFAC identified three DOM components (protein-like C1, C2; humic-like C3), with protein-like components dominating. This study validates the relationship between cellulose proportion and release kinetics and reveals synergistic regulation of DOM components, offering guidance for designing effective solid-phase carbon sources.
Enhancement of Anaerobic Digestion Operational Efficiency for Guar Gum Production Wastewater Using a Microaerobic-Biochar Coupled System
Guar gum production wastewater contains 1,2-propanediol, which in conventional anaerobic treatment causes propionate accumulation and microbial inhibition. Microaerobic conditions foster fermentative bacterial metabolism, enhancing organic substrate conversion, while biochar promotes anaerobic microbial aggregation and oxygen tolerance. This study treated actual guar gum wastewater using three configurations: blank control, anaerobic, and microaerobic-biochar (O2/BC) coupled systems. Under mesophilic conditions (37 °C), with micro-aeration at 0.2 mL/(g VS·d) and biochar dosage of 15 g/L, the O2/BC system achieved a COD removal efficiency of 90%, 10.6 percentage points higher than the anaerobic control. Effluent COD and propionate concentrations dropped to 3800 mg/L and 0.15 g/L, respectively, representing reductions of 49.6% and 98.4% versus the control. Biogas production was 1.64 times that of the control, with a maximum methane concentration of 77.2%. Fourier transform infrared spectroscopy (FT-IR) indicated increased abundance of –OH, –CH2–, and C–O functional groups on sludge surfaces, revealing biochar's adsorption enhancement. Scanning electron microscopy (SEM) showed dense microbial aggregates dominated by long bacilli, distinct from conventional anaerobic sludge. Microbial community analysis revealed increased abundance of Clostridium and Comamonas, modulating the propionate-to-acetate ratio and optimizing acidification efficiency, thereby promoting complex organic degradation. This study provides a novel technical pathway for biological treatment of alcohol-rich organic wastewater.
Occurrence Characteristics, Source Apportionment, and Ecological Risk Assessment of Pesticides in Plateau Lakes: A Case Study of Dianchi Lake
This study systematically investigated the occurrence, spatial distribution, sources, and ecological risks of 160 pesticides in Dianchi Lake, a typical plateau lake impacted by agricultural activities. A total of 37 pesticides were detected in the water, with total concentrations ranging from 64.2 to 1132.8 ng/L (average 610.0 ng/L). Fungicides, including boscalid (BOS), fluopicolide (FPC), and dimethomorph (DMM), were dominant, contributing up to 65.0% of the total concentration. Spatially, the southern lake region exhibited significantly higher concentrations (672.5 ng/L) than the north, attributed to intensive facility agriculture. Highly hydrophobic pesticides, such as penconazole (PEN), showed a tendency to enrich in bottom layers. Source apportionment identified inflowing rivers and wastewater treatment plant effluents as primary input sources, with average concentrations 7 and 9 times higher than lake water, respectively. Ecological risk assessment revealed that pesticides posed the highest risk to algae, followed by daphnia and fish. Prometryn (PMT) was identified as a high-risk factor for algae, while profenofos (PFF) and carbendazim (CBD) posed potential threats to higher trophic levels. These findings provide fundamental data and technical support for understanding pesticide pollution in plateau lake ecosystems.
Microbiome Mechanisms of Composite Carbon Sources for Enhancing Denitrification and Reducing N2O Emissions
Biological nitrogen removal in wastewater treatment plants (WWTPs) is often limited by insufficient influent carbon sources, necessitating external carbon addition to enhance denitrification. Conventional single carbon sources, such as sodium acetate, frequently fail to meet the metabolic demands of complex microbial communities, compromising nitrogen removal efficiency and stability. Composite carbon sources, by providing multiple electron donors, can improve metabolic cooperation among microorganisms, yet their underlying microbial mechanisms remain insufficiently understood. In this study, activated sludge from a municipal WWTP was used to investigate the microbial mechanisms of composite carbon sources during denitrification. Batch denitrification experiments were conducted in combination with metagenomic and metatranscriptomic analyses to systematically characterize microbial community structure and functional gene expression under different carbon source conditions. Results showed that, compared with sodium acetate as the single carbon source, the composite carbon source system (sodium acetate: sodium succinate: ethanol = 2:1:3) increased the denitrification rate from (6.822 ± 0.141) mg/(L·h) to (8.370 ± 0.186) mg/(L·h), representing a 22.7% improvement, while reducing N2O accumulation by approximately 55%. Metagenomic analysis revealed that Ottowia, Rubrivivax, Thauera, and Zoogloea were the dominant denitrifying genera. Metatranscriptomic results further demonstrated that the composite carbon sources significantly upregulated the transcription of key denitrification genes, with nirS, norB, and nosZ increasing by 37.8%, 27.4%, and 48.6%, respectively. In addition, the composite carbon sources promoted complementary carbon metabolic strategies among different microbial communities, enhancing electron donor supply and improving denitrification efficiency. These findings indicate that composite carbon sources synergistically enhance denitrification performance through regulation of functional gene transcription in complex microbial communities, providing a theoretical basis for carbon source optimization in WWTPs.
Key Environmental Behaviors and Pollution Control Strategies of Tire Wear Particles in Aquatic Environments
Tire wear particles (TWPs) are emerging pollutants and constitute the dominant type of microplastics (MPs) in urban stormwater runoff, accounting for up to 90% of MPs in some cases. They are characterized by small size, high mobility, complex composition, and significant toxicity. Current research on TWPs remains fragmented, lacking a comprehensive understanding of their environmental behaviors and pollution control in aquatic systems. This review systematically analyzes the enrichment and vectoring roles of TWPs for coexisting pollutants, and their environmental fate, including ecotoxicological impacts, detection methodologies, release of intrinsic additives, and aggregation and sedimentation behaviors. Drawing on insights from other microplastic studies, the paper explores control technologies across the pollution pathway—source, transport, and terminal treatment—and proposes feasible management strategies. Key findings indicate that TWPs can adsorb heavy metals and organic contaminants, with adsorption capacities influenced by aging processes. Their aggregation is governed by solution chemistry, with critical coagulation concentrations varying with ionic strength and pH. The release of additives such as zinc and benzothiazoles is significant, posing ecological risks. Future research should focus on real-water aggregation mechanisms, additive release under natural conditions, long-term performance of treatment facilities like constructed wetlands under TWPs stress, enzymatic degradation pathways, and integration of AI, big data, and IoT for cost-effective detection and risk modeling. This review provides a scientific basis for developing targeted pollution control measures for TWPs in aquatic environments.