From Rice Waste to Water Cleaner: Porous Silica Ceramics Tackle Dyes and Industrial Coolant

Every year, the global rice industry burns millions of tonnes of husks to power mills and dry grain, leaving behind an ash that is more than 80 percent silica by weight. For decades this residue has been treated as a disposal problem, piling up around processing facilities across Southeast Asia and beyond. Now a team of materials scientists in Thailand has shown that this agricultural by-product, together with a humble sedimentary rock called diatomite, can be transformed into robust, porous ceramic granules capable of scrubbing industrial dyes from water and even treating the oily coolant waste generated by metal casting plants. The work, published in the journal Results in Chemistry, offers a rare example of a low-cost adsorbent that has been carried all the way from laboratory beakers to a 20-cubic-metre industrial treatment tank.

The research, led by Pat Sooksaen and colleagues at Silpakorn University, addresses a stubborn gap in water treatment technology. Activated carbon remains the workhorse adsorbent for removing organic contaminants from industrial effluents, prized for its enormous surface area and broad affinity for organic molecules. Yet its high cost of production and, crucially, its expensive regeneration have kept many industries from adopting adsorption-based treatment at scale, particularly for high-volume, low-value waste streams such as metalworking coolant. Meanwhile, a vast literature on cheap natural adsorbents, from agricultural biochars to modified clays, has largely remained confined to powdered materials tested under idealized conditions with synthetic dye solutions, rarely confronting the granulation, mechanical strength and thermal processing demands of real industrial deployment.

The Thai team’s approach was deliberately simple. They combined two silica-rich precursors, rice husk ash supplied by a working rice mill in Nakhon Pathom and diatomite from a Bangkok supplier, with ordinary terracotta clay acting as a binder. Rice husk ash is the residue of combustion at 500 to 600 degrees Celsius, and it consists of fine, irregular grains riddled with nanoscale mesopores. Diatomite, by contrast, is composed of the fossilized cell walls of aquatic diatoms, preserving an intricate honeycomb architecture with pores generally smaller than five micrometres. X-ray fluorescence confirmed the raw materials’ credentials: the ash contained 84.37 weight percent silica and the diatomite 90.13 percent, while the terracotta contributed alumina and, importantly, iron oxide, which acts as a flux during firing.

Four formulations were prepared by ball milling, mixing either silica source with terracotta at volume ratios of 60:40 and 80:20, then shaping the blends into spherical granules using an in-house granulator rotating at 50 to 100 revolutions per minute with water as the binding agent. The granules were sieved to a narrow fraction of 5 to 9 millimetres, a size chosen for practical handling and separation rather than optimized kinetics, and then fired for one hour at 800, 1000 or 1200 degrees Celsius. This firing step proved to be the decisive variable in the entire study, governing whether the granules emerged as open, adsorbent-rich sponges or as dense, glassy pebbles with little capacity to hold contaminants.

The characterization data told a striking and somewhat counterintuitive story about the two silica sources. In its raw state, rice husk ash boasted a specific surface area of 33.45 square metres per gram, roughly twenty times that of raw diatomite, reflecting its finely mesoporous amorphous structure. But firing at just 800 degrees Celsius collapsed that area by more than 90 percent, to 2.66 square metres per gram, revealing that the ash’s delicate pore network is thermally metastable and begins to sinter well below the temperatures used in ceramic processing. Diatomite behaved in the opposite manner: heating to 800 degrees actually increased its surface area by about 47 percent and its pore volume by 56 percent, apparently because the burnout of residual organic matter and moisture unblocked channels in the diatom cell walls that had been obstructed in the raw rock.

X-ray diffraction explained the mechanism behind this degradation. Both raw materials were dominated by cristobalite, a crystalline form of silica, with alpha-quartz as a secondary phase. As firing temperature climbed from 800 to 1200 degrees, the cristobalite reflections grew sharper and more intense, marking the devitrification of the amorphous silica fraction into a thermodynamically ordered crystal lattice. This recrystallization goes hand in hand with pore collapse. Scanning electron micrographs showed that at 1000 to 1200 degrees, liquid-phase sintering driven by the iron oxide and alkali fluxes contributed by the terracotta fused particles together, eliminated open pores and produced smooth, dense surfaces that would severely restrict the diffusion of dye molecules into the granule interior. The characteristic cylindrical frustules of the diatomite partially collapsed as well.

The adsorption tests bore out these microstructural observations with almost brutal clarity. When 100 grams of granules were immersed in 200 millilitres of a 25 milligram-per-litre methylene blue solution, a standard cationic dye used to benchmark adsorbents, and aerated for four hours at room temperature, the granules fired at 800 degrees performed dramatically best. The 60RHA:40T formulation removed 97.7 percent of the dye, corresponding to an uptake of 0.0487 milligrams per gram under these screening conditions, while the diatomite-based 80D:20T granules achieved 93.0 percent. At 1000 degrees, removal fell to between 51.6 and 86.8 percent, and at 1200 degrees it collapsed to as little as zero for the 60D:40T formulation, a direct mirror of the pore closure documented by electron microscopy and porosimetry.

Infrared spectroscopy provided a molecular-level view of how the dye binds. Before adsorption, the granules displayed the signature of a silanol-terminated silica surface: a broad hydroxyl stretching band near 3450 wavenumbers, strong silicon-oxygen-silicon stretching modes between 1140 and 1080 wavenumbers, and bending modes near 800 and 460 wavenumbers. After dye uptake, two new features appeared, an aromatic ring-stretching band near 1600 wavenumbers and a carbon-sulfur-carbon stretch at 615 to 625 wavenumbers, both characteristic of methylene blue. Their appearance alongside the silica framework bands supports electrostatic binding of the cationic dye to deprotonated silanol sites on the ceramic surface, although the authors are careful to note that surface charge and pH effects were not established in this study.

Perhaps the most industrially significant result concerned reuse. Granules saturated with dye were heated in air at 600 degrees Celsius for one hour, decomposing the organic dye trapped within their pores. Infrared analysis showed the dye-associated bands almost completely vanished, with only a weak residual aromatic feature remaining in one formulation. When the regenerated granules were tested again, removal efficiencies of 85.4 to 93.8 percent were retained, only modestly below the original performance. This one-step thermal regeneration, achievable at temperatures far lower than those needed to reactivate spent activated carbon, is precisely the kind of economics-friendly feature that could make ceramic adsorbents competitive, although the authors emphasize that multi-cycle durability, mechanical attrition and long-term capacity retention remain untested.

Finally, the team selected the 80RHA:20T formulation, chosen for its higher proportion of cheap agricultural waste and lower binder demand rather than its top laboratory performance, for a qualitative demonstration at an aluminum ingot casting facility. Granules fired in a gas-fired refractory kiln at 900 to 1100 degrees were loaded into a 20-cubic-metre batch tank at an adsorbent-to-wastewater ratio of 1:5 and used to treat metalworking coolant effluent over a 30-hour cycle. The authors are candid that no quantitative influent and effluent analyses or activated-carbon baseline were obtained, so the trial demonstrates deployment feasibility rather than validated treatment performance. Even so, the study charts a credible path from rice mill waste to industrial water remediation, and points toward the analytical work, isotherms, pH studies, mechanical testing and repeated reuse trials, that will determine whether these humble ceramic granules can genuinely challenge activated carbon in the factories that need them most.

Subject of Research: Valorization of rice husk ash and diatomite into porous silica ceramic adsorbents for wastewater treatment

Article Title: Valorization of rice husk ash and diatomite into porous silica ceramics for industrial wastewater remediation: from dye adsorption to coolant treatment

Article References: Sooksaen, P., Thongyoug, P., Thipsupanimit, W., Sreearunothai, P., & Puathawee, P. (2026). Valorization of rice husk ash and diatomite into porous silica ceramics for industrial wastewater remediation: from dye adsorption to coolant treatment. Results in Chemistry, 31, Article 103921. https://doi.org/10.1016/j.rechem.2026.103921

Image Credits: AI Generated

DOI: 10.1016/j.rechem.2026.103921

Keywords: rice husk ash, diatomite, porous ceramics, silica, wastewater treatment, methylene blue, adsorption, coolant wastewater, terracotta, sintering, regeneration, industrial waste valorization