The process at the heart of the review is dark fermentation, a biotechnology in which anaerobic bacteria dismantle organic matter, sugars, and waste streams such as food waste, palm oil mill effluent, and agricultural residues, in the absence of light and oxygen. During this microbial dismantling, hydrogen gas is released as a byproduct of the bacteria’s energy metabolism. The appeal is obvious: the feedstock is often waste that would otherwise rot in landfills, the reaction proceeds at ambient to moderately warm temperatures without light energy, and the gas produced is pure energy carrier rather than a carbon-containing fuel. Yet the technology has struggled to escape the laboratory. Yields remain stubbornly low relative to the theoretical maximum, and the microbial communities that do the work are notoriously sensitive to disturbance, swinging between productive and unproductive metabolic states as conditions drift.

The review’s first pillar is a systematic account of the physicochemical and operational variables that govern microbial performance. pH emerges as perhaps the most decisive lever. Fermentative hydrogen producers operate best in a mildly acidic window, typically around pH 5 to 6, where the hydrogenase enzymes responsible for releasing hydrogen gas remain active and where competing microbes that channel carbon into methane or other reduced products are suppressed. Let the pH drift too low and acid accumulation crashes the culture; let it rise and methanogens invade, consuming the hydrogen that the fermenters worked to produce. Temperature is the second master variable, with mesophilic and thermophilic regimes each selecting for distinct microbial consortia and distinct metabolic end products. Thermophilic operation, the authors note, often supports higher hydrogen yields and faster kinetics, but demands more energy input and careful reactor management.

Beyond pH and temperature, the review examines the characteristics of the inoculum itself, the starting microbial community seeded into the reactor. Heat treatment of seed sludge is a common strategy to kill off hydrogen-consuming methanogens while sparing spore-forming hydrogen producers such as Clostridium species. Organic loading rate, the amount of substrate fed per unit reactor volume per day, must be balanced carefully: push it too high and volatile fatty acids accumulate faster than the microbes can process them, acidifying the reactor and stalling hydrogen production; run it too low and the process becomes economically unattractive. Reactor configuration adds another layer of control. Immobilized-cell systems, upflow anaerobic sludge blanket reactors, packed bed biofilm reactors, and membrane bioreactors all retain biomass more effectively than simple suspended-growth tanks, allowing higher cell densities, faster throughput, and greater resilience to washout during shock loads.

Into this well-mapped landscape, the review introduces its genuinely novel perspective: multifunctional green materials, with hydroxyapatite as the star. Hydroxyapatite, with the chemical formula reflecting its calcium, phosphate, and hydroxide components, is the mineral that gives bones and teeth their rigidity. In a fermenter, it performs several jobs at once. Its surfaces serve as attachment sites for bacteria, promoting cell immobilization and the formation of dense, stable granules that resist washout in continuous operation. Its mineral chemistry buffers pH, absorbing and releasing ions in ways that damp the acidification that so often kills hydrogen-producing cultures. And through the controlled release of calcium and phosphate ions, it appears to support microbial metabolic stability, keeping the community locked into hydrogen-producing pathways for longer stretches of operation.

The evidence for this approach is not merely theoretical. The review’s own authors have published experimental work showing that eggshell-derived hydroxyapatite enhanced thermophilic hydrogen production, with the material promoting the formation of biogranules, compact microbial aggregates in which cells packed closely together exchange metabolites efficiently and maintain favorable microenvironments. Other studies cited in the review report that hydroxyapatite fabrication improved hydrogen output from glucose dark fermentation, and that graphene-hydroxyapatite composites boosted productivity from duckweed biomass. The mechanism is elegantly circular: the material that stabilizes the microbes is itself made from waste, so a discarded eggshell becomes the scaffold on which bacteria convert discarded food into fuel.

How the hydroxyapatite is made matters enormously, and the review devotes detailed attention to synthesis routes. Physical methods include pyrolysis and spray techniques that produce fine, well-defined particles. Chemical routes encompass wet chemical precipitation, sol-gel synthesis, hydrothermal treatment, sonochemical processing, and microwave-assisted reactions, each offering control over crystal size, morphology, and surface area, properties that directly determine how well the material buffers pH and hosts bacterial attachment. Biological and green synthesis routes use plant extracts, microbial activity, or biogenic templates to direct mineral formation under mild conditions, avoiding harsh reagents and high energy demands. Most compelling from a sustainability standpoint are the waste-based routes: eggshells, cow and fish bones, mussel and oyster shells, sea urchin spines, calcified algae, and even industrial byproducts such as phosphogypsum and carbide slag have all been converted into hydroxyapatite. Calcination temperature during processing, the review notes, strongly influences the crystallinity and purity of the final product, and higher specific surface area enhances the release of calcium ions, the very property that makes the material useful in fermentation.

This waste-to-material-to-energy chain is where the review’s conceptual contribution becomes clearest. By linking bioprocess engineering with materials science, the authors construct a framework for what they describe as more efficient, resilient, and environmentally friendly biohydrogen systems. The circular economy logic runs in both directions: organic waste feeds the fermenters, and inorganic waste supplies the additives that make the fermenters work better. Nothing in the chain requires virgin mined minerals or energy-intensive synthetic chemistry if biogenic sources are chosen. The review also situates hydroxyapatite within a broader family of additives, from carbonaceous materials and magnetite nanoparticles to nickel and cobalt ferrites, that have been explored to enhance fermentative hydrogen yields, but argues that hydroxyapatite’s combination of buffering capacity, immobilization support, biocompatibility, and waste-derived availability gives it a distinctive edge.

Challenges remain before the vision reaches industrial scale. The review is candid that low yield and process instability still limit the scalability of dark fermentation, and the integration of a solid additive introduces new variables: optimal dosing, particle size selection, long-term material stability, and the economics of recovering or disposing of spent mineral. Techno-economic assessments of dark fermentation systems generally conclude that profitability hinges on cheap feedstock, high productivity, and valorization of the fermentation effluent, which can itself be converted to biomethane, bioplastics, or microbial lipids in cascading processes. Hydroxyapatite strengthens two of those three pillars, feedstock flexibility and productivity, but the full chain must be demonstrated at pilot scale. The review’s framework, mapping process parameters onto material design, gives researchers a structured way to run those demonstrations, and it signals a broader shift in bioenergy thinking: the microbes may be the engine, but increasingly, the materials around them are the engineering that keeps the engine running.

Subject of Research: Enhancement of dark fermentative biohydrogen production using waste-derived hydroxyapatite additives

Article Title: Integrating process parameters and green hydroxyapatite (HAP) strategies for sustainable dark fermentative biohydrogen production

Article References: Zisha, S. Z., Che Man, H., Omar, R., Abdul Rashid, S., Tan, J. P., Abdul Manaf, S. F., Mumtaz, T., & Jamali, N. S. (2026). Integrating process parameters and green hydroxyapatite (HAP) strategies for sustainable dark fermentative biohydrogen production. Environmental Science and Pollution Research. https://doi.org/10.1007/s11356-026-38274-x

Image Credits: AI Generated

DOI: 10.1007/s11356-026-38274-x

Keywords: biohydrogen, dark fermentation, hydroxyapatite, waste valorization, circular economy, bioprocess optimization, cell immobilization, pH buffering, anaerobic digestion, sustainable energy, green synthesis, microbial consortium