Iron occupies a paradoxical place in biology. It is the fourth most abundant element in Earth’s crust, and virtually every living organism depends on it: hemoglobin cannot carry oxygen without it, mitochondria cannot generate energy without it, and DNA replication and repair rely on iron-dependent enzymes. Yet the same redox chemistry that makes iron indispensable also makes it dangerous. In aerobic conditions, iron readily catalyzes the production of hydroxyl and other reactive radicals through Fenton and Haber-Weiss chemistry, attacking proteins, peroxidizing membrane lipids, and damaging nucleic acids. Excess iron in the human body can cause corrosive injury to the gastrointestinal tract, hemorrhagic necrosis, and interference with the heart, liver, and central nervous system. The American Association of Poison Control Centres recorded more than 4,000 single-exposure cases of iron poisoning in 2015 alone, underscoring that even an essential nutrient becomes a hazard at the wrong concentration.

The World Health Organization has set a maximum allowable limit of 0.3 milligrams of iron per liter in drinking water, but industrial activity routinely pushes concentrations far higher. Mining, agriculture, and manufacturing discharge iron-laden effluents into waterways, and conventional removal methods carry their own burdens. Chemical oxidation and physical techniques such as adsorption on synthetic media and ion exchange can be effective, but they tend to generate secondary pollutants and hazardous byproducts, and they are often expensive to operate at scale. Iron is also notoriously stable in aqueous solution, making it a stubborn target. Bioremediation, the use of living or dead biological material to transform and detoxify contaminants, has long been touted as a sustainable alternative: it is inexpensive, avoids toxic residues, and can even allow recovery of the captured metal for reuse.

Bacteria are the workhorses of this approach. Their enormous diversity, rapid growth, and ability to thrive in hostile environments make them ideal candidates for metal cleanup. Many species deploy a layered arsenal against toxic metals: biosorption, in which metal ions passively bind to negatively charged carboxyl, phosphoryl, amino, and sulfo groups on the cell wall; bioaccumulation, in which living cells actively internalize metals using exopolymers and polysaccharides; bioprecipitation, where metabolites convert dissolved metals into insoluble solids; and bioleaching. Thermophilic bacteria, adapted to the scalding conditions of hot springs, bring an extra advantage. Their enzymes and cell structures remain stable at temperatures that would denature the proteins of ordinary microbes, which is precisely the kind of resilience needed to treat hot industrial effluents without costly cooling.

To find such a microbe, the researchers collected water from the Gauri Kund spring in Rudraprayag district, a geothermal site with a temperature of about 52 degrees Celsius and a near-neutral pH of 6.3. Samples were transported in sterile thermos flasks to preserve temperature, and bacteria were grown on iron-supplemented agar at 55 degrees Celsius. Three metal-resistant isolates emerged, designated GA4, GA5, and GA6, and all could grow in the presence of iron ions. Genetic fingerprinting of the most promising strain, GA6, using 16S rRNA gene sequencing and BLAST analysis revealed a 98.14 percent similarity to Bacillus thermophilus HS-BTL2, a species previously described from compost and only rarely studied. Notably, the team reports that this is the first time B. thermophilus has been documented in a hot spring in the Indian Himalayas, adding a new entry to the region’s microbial catalogue a decade after the devastating Kedarnath floods.

Laboratory characterization showed that all three isolates grew across a striking range of temperatures, from 45 to 80 degrees Celsius, with optimal growth at 55 degrees Celsius and pH 7.0, conditions that mirror the spring itself. The GA6 strain proved to be a rod-shaped, spore-forming, Gram-positive bacterium, and biochemical profiling with a 35-test carbohydrate kit showed it could metabolize a wide array of sugars, including trehalose, maltose, and glycerol, alongside positive reactions for esculin hydrolysis and citrate utilization. Crucially, the minimum inhibitory concentration test, which measures the highest iron concentration the bacterium can tolerate, came in at 450 micrograms per liter, confirming that GA6 is highly resistant to iron toxicity and well suited to survive in contaminated environments.

The bioremediation experiments delivered the study’s headline numbers. When live B. thermophilus cells were incubated with iron-contaminated medium for 48 hours at 55 degrees Celsius, inductively coupled plasma mass spectrometry showed iron concentrations falling from 41.36 to 5.72 micrograms per liter, a biosorption rate of 86.17 percent. The dead biomass performed even better. Cells grown in bulk, killed by autoclaving, dried, and ground into powder, reduced iron from 22.05 to just 1.81 micrograms per liter, removing 91.79 percent of the metal. This pattern, dead biomass outperforming living cells, is consistent with earlier studies on Bacillus subtilis and Acinetobacter strains, and the researchers attribute it to the larger effective surface area of dead cells and their freedom from the metabolic constraints that govern living organisms.

The most consequential test came when the bacterium was unleashed on real-world pollution. Effluent was collected from the drainage area of the State Infrastructure and Industrial Development Corporation of Uttarakhand Limited (SIDCUL) in Haridwar, adjusted to neutral pH, and inoculated with the bacterium. After 48 hours at 55 degrees Celsius, iron in the effluent had dropped from 58.30 to 21.02 micrograms per liter, a removal rate of 63.94 percent. That figure trails the laboratory results, as expected in a chemically complex industrial matrix, but it still compares favorably with earlier work in which Escherichia coli, Bacillus subtilis, and Pseudomonas putida removed 56, 62, and 80 percent of iron from distillery effluent respectively. The result demonstrates that a thermophile harvested from a Himalayan pilgrimage site can function in the unglamorous reality of industrial wastewater.

The researchers argue that dead biomass offers particular practical advantages for deployment: it requires no nutrients or environmental control, poses minimal risk of introducing living organisms into sensitive ecosystems, avoids competition with native microbes, and can be stored and reused as a stable biosorbent. At the same time, live cells bring metabolic versatility, including the production of siderophores, chelating agents, and extracellular polymeric substances that bind and sequester metals, and the team notes that synthetic biology could eventually engineer B. thermophilus for enhanced degradation of specific contaminants. Either way, the thermophile’s thermostable enzymes, which remain active at temperatures up to 80 degrees Celsius, mean treatment plants could process hot effluent streams directly rather than cooling them first, a significant energy saving.

Beyond the engineering promise, the study carries a conservation message. The hot springs of the Garhwal Himalaya, shaped by the same tectonic forces that raised the mountains, harbor microbial communities that remain largely unexplored, and sites like Gauri Kund face pressure from development, tourism, and natural disasters. The authors frame their work as both a technological proof of concept and a case for protecting these geothermal habitats as reservoirs of biodiversity with direct value to human welfare. If a single spore-forming bacterium scooped from a sacred spring can strip more than nine-tenths of the iron from contaminated water at 55 degrees Celsius, the argument goes, the untapped microbial wealth of the Himalayan geothermal belt may hold solutions to pollution problems that conventional chemistry has struggled to solve, affordably, sustainably, and without creating new hazards in the process.

Subject of Research: Thermophilic bacterial bioremediation of iron from industrial wastewater

Article Title: Thermophilic remediation of iron using Bacillus thermophilus: effect of variables and microbiological characteristics

Article References: Patil, A., Rajamohan, N., Rohilla, V., & Arya, M. (2025). Thermophilic remediation of iron using Bacillus thermophilus: effect of variables and microbiological characteristics. Discover Biotechnology, 2(1), Article 10. https://doi.org/10.1007/s44340-025-00019-w

Image Credits: AI Generated

DOI: 10.1007/s44340-025-00019-w

Keywords: bioremediation, Bacillus thermophilus, thermophiles, heavy metals, iron, wastewater, industrial effluent, hot springs, biosorption, Himalaya, ICP-MS, water treatment

The wastewater at the heart of the study came straight from the dyeing process outlet of a textile factory in Isfahan, in central Iran, and it was as dirty as the researchers expected. Chemical oxygen demand, a measure of organic pollution, stood at 8100 milligrams per liter, roughly 135 times the permissible limit for discharge to surface water in Iran. Color intensity registered 23,480 platinum-cobalt units, turbidity 620 nephelometric turbidity units, and total suspended solids 3890 milligrams per liter. Elemental analysis added another layer of concern. The effluent carried 5500 micrograms per liter of aluminum, 5300 of iron, an extraordinary 4200 of the toxic metal thallium, along with antimony, manganese, zinc, copper and lead at levels far above natural background. In Iran only about 20 percent of industrial effluents receive proper treatment before discharge, so the stakes for finding an affordable, effective recipe are high.

Polyaluminum chloride, or PACl, has become a favored coagulant in water treatment because it works across a wide pH range, forms dense and fast-settling flocs and produces less sludge than traditional alum. It is sold in both powdered and liquid form, and the two differ in ways that matter to plant operators. The powdered grade used here contained 28.05 percent aluminum oxide, more than double the 10.48 percent of the liquid grade, but liquid PACl arrives ready to dose, skipping on-site dissolution. The researchers optimized each coagulant independently through standard jar tests on 1-liter samples, defining the optimum as the minimum dose that pushed residual turbidity to 20 NTU or below with the smallest settled sludge volume. Powdered PACl settled on 0.7 grams per liter while liquid PACl required 3.0 milliliters per liter. Every scenario also received sodium hypochlorite at 75 milliliters per liter, chosen because color removal plateaued beyond that dose, and calcium oxide at 1.0 gram per liter, enough to push pH above 10.5 so that dissolved metals could precipitate as hydroxides.

Five treatment configurations were tested in triplicate. The first used powdered PACl, the second liquid PACl, the third a blend of the two, and the fourth and fifth replaced commercial coagulant entirely, or nearly so, with clarifier sludge collected from a local drinking water plant that itself uses PACl. The idea behind the sludge experiments was elegantly circular: the sedimentation basin sludge contains residual polyaluminum chloride and amorphous aluminum hydroxides, so it might act as a free, waste-derived coagulant that simultaneously reduces chemical purchases and diverts waste from landfills. The sludge was mixed into wastewater at a ratio of one part sludge to five parts effluent, the ratio that preliminary trials showed produced maximum floc formation. For the fifth scenario, a modest half-milliliter dose of liquid PACl was added on top of the sludge to test whether virgin and recycled coagulants could work synergistically.

The results split the metals into two camps. Zinc and chromium were the consistent success stories, removed at better than 96 percent in every configuration, with residual zinc concentrations falling below 0.2 micrograms per liter in most scenarios. Thallium, iron, manganese and silicon also dropped by more than 98 percent in the liquid and mixed PACl scenarios. Barium and strontium, by contrast, proved stubbornly recalcitrant, with strontium removal never exceeding 33 percent anywhere in the study, marking these two elements as priority targets for future work. Most striking was the fate of aluminum itself. Clarifier sludge alone removed 99.8 percent of aluminum, the best figure of any scenario, and near-complete removal of lithium and barium besides. But in the fifth scenario, where a small PACl dose was layered onto the sludge, aluminum removal collapsed to zero, and the treated water actually carried more aluminum than the mixed inlet, 4800 micrograms per liter against 4567 going in.

That counterintuitive collapse points to a phenomenon known as overdosing or charge reversal. Coagulants work by neutralizing the negative charges that keep colloidal particles suspended, and beyond the optimum dose the excess positive charge can flip particle surfaces back to a stable, restabilized state, re-suspending material that had already clumped. The sludge, already laden with residual aluminum hydroxides, plus the added PACl apparently tipped the system past that threshold. The authors caution that they did not measure zeta potential, so the mechanism remains a hypothesis, but the practical lessons are unambiguous: more coagulant is not better, combining waste-derived and virgin coagulants without re-optimization can backfire, and fixed-dose recipes are inadequate for wastewater whose composition shifts from batch to batch.

On the conventional pollutants, the combined PACl scenario proved the most robust all-rounder, cutting chemical oxygen demand by 95 percent to a residual 380 milligrams per liter and turbidity by 95 percent, while removing nearly 97 percent of suspended solids. Clarifier sludge alone, despite its dazzling color removal of 99.6 percent, managed only 19.8 percent turbidity removal and 79.9 percent for chemical oxygen demand, confirming that recycled sludge cannot substitute for commercial coagulant on high-strength textile effluent. Electrical conductivity rose in every scenario because calcium oxide and PACl both add dissolved ions, and final pH ranged from 10.74 to 12.51, values that would require neutralization before any discharge or reuse.

The study’s most distinctive move was its scoring system. Rather than ranking treatments by individual removal percentages, the team calculated the grey water footprint, an indicator that translates each pollutant load into the volume of freshwater needed to dilute it to regulatory limits. The critical pollutant for the raw effluent was chemical oxygen demand, driving a footprint of 121,500 cubic meters per month at the facility’s assumed discharge of 750 cubic meters per month, meaning the factory’s pollution load would demand dilution water more than 160 times its own flow. Liquid PACl delivered the best outcome: a 92.96 percent reduction in the footprint, statistically the top performance by analysis of variance, with the combined PACl scenario statistically comparable at 90.85 percent and powdered PACl at 88.27 percent. The sludge-only scenario ranked last at 83.33 percent, despite its standout aluminum and color numbers, because its weak performance on organic matter and turbidity dominated the overall environmental burden.

The grey water footprint also exposed a hidden bottleneck that conventional single-parameter assessment would have missed. In the combined PACl scenario, chemical oxygen demand was cut so effectively that antimony, with only 49 percent removal, quietly became the limiting pollutant, its footprint climbing above the organic load’s. Optimizing one pollutant, in other words, can mask the factor that actually constrains environmental performance. The researchers argue that the metric offers an objective common currency for comparing heterogeneous treatment trains, and note that their chemically enhanced primary treatment approached the footprint reductions typically associated with secondary biological processes such as activated sludge.

For plant operators, the study lands on a concrete protocol: liquid PACl at 3.0 milliliters per liter, sodium hypochlorite at 3750 milligrams per liter of active chlorine, and calcium oxide at 1.0 gram per liter, a combination that cut the grey water footprint by 93 percent while stripping more than 99 percent of thallium and zinc. Clarifier sludge, meanwhile, earns a narrower but genuine role as a free, selective coagulant aid for specific targets such as aluminum and color, and as a landfill-diversion strategy, so long as its dose is optimized independently and its reactive aluminum content is properly characterized. The authors flag open questions, including chlorinated byproducts from hypochlorite oxidation, the economics of full-scale deployment, and the mechanistic differences between PACl formulations, but the headline conclusion stands: when environmental impact is tallied honestly across every pollutant, the liquid form of a familiar coagulant is the strongest tool yet tested for taming textile wastewater.

Subject of Research: Comparative evaluation of PACl coagulant variants and recycled clarifier sludge for treating real textile wastewater using the grey water footprint metric

Article Title: Comparative assessment of PACl variants and clarifier sludge for textile wastewater treatment: A grey water footprint approach

Article References: Tabandeh, L., Arastou, K., & Ebrahimi, A. (2026). Comparative assessment of PACl variants and clarifier sludge for textile wastewater treatment: A grey water footprint approach. Cleaner Engineering and Technology, 34, Article 101319. https://doi.org/10.1016/j.clet.2026.101319

Image Credits: AI Generated

DOI: 10.1016/j.clet.2026.101319

Keywords: textile wastewater, polyaluminum chloride, grey water footprint, coagulation-flocculation, clarifier sludge, heavy metal removal, chemical oxygen demand, sodium hypochlorite, calcium oxide, circular economy, water treatment, industrial effluent

The research team, led by Prayitno with Sri Rulianah, Wahyuni Ningsih, and Wahyu Widyananda, all affiliated with Indonesian institutions collaborating through the State Polytechnic of Malang, set out to build a quantitative model of thermal demulsification, the process by which emulsified oil droplets are coaxed into merging and separating from water. Their wastewater came directly from a turpentine plant in Trenggalek, where the production of turpentine oil and gum rosin leaves behind a cocktail of cellulose fibers, resin acids, carboxylic acids, and dissolved turpentine oil. These natural surfactants form rigid interfacial films around oil droplets, creating emulsions so stable that conventional treatment methods like flotation, neutralization, and sedimentation struggle to meet regulatory thresholds, particularly for oil and grease.

The experimental setup was elegantly simple. Two liters of filtered wastewater, held at its naturally acidic pH of 1 to 2, was placed in a beaker inside a temperature-controlled water bath. A commercial polymeric demulsifier, Nalco 14750, was dosed at 50, 100, or 150 milligrams per liter, while heating temperatures ranged from 30 to 50 degrees Celsius. Half the experiments ran with vigorous agitation at 5,000 revolutions per minute; the other half proceeded in complete stillness. Each five-minute treatment was then assessed for four regulated parameters: turbidity, total suspended solids, biochemical oxygen demand, and oil and grease. The removal data were fitted to second-order polynomial regression models, producing response surfaces that map exactly how temperature and dose interact to drive pollutant removal.

The mechanism underlying the process reads like a microscopic tug-of-war at the oil-water interface. The non-ionic polymeric demulsifier carries both hydrophilic and hydrophobic segments, allowing it to migrate to the droplet surface, penetrate the rigid film formed by gum rosin and carboxylic acids, and displace the natural emulsifiers stabilizing the emulsion. Meanwhile, moderate heat thins the interfacial film, lowers the zeta potential of the droplets, compresses the electrical double layer, and reduces the viscosity of the surrounding water, all effects that encourage droplets to collide, coalesce, and rise. The wastewater’s extreme acidity adds another lever: at pH 1 to 2, weakly acidic groups on the natural emulsifiers become protonated, reducing the negative surface charge on droplets and weakening electrostatic repulsion before the demulsifier even arrives.

Under agitation, the best single result came at 100 milligrams per liter of demulsifier, where turbidity fell by 92.21 percent, total suspended solids by 96.36 percent, biochemical oxygen demand by 99.84 percent, and oil and grease by an impressive 99.29 percent. But the response surfaces revealed a clear ceiling: temperatures above roughly 40 degrees Celsius combined with high-speed stirring actively sabotaged the process. The researchers traced this deterioration to well-established fluid dynamics. At 5,000 rpm, raising the temperature lowers the water’s viscosity, pushing the impeller Reynolds number higher and intensifying turbulence. According to Kolmogorov-Hinze theory, greater turbulent energy dissipation shrinks the maximum stable droplet diameter, meaning the stirrer begins slicing coalesced oil droplets back into tiny, newly stabilized fragments faster than the demulsifier can disrupt their regenerated interfacial films. In effect, the mixer re-emulsifies what chemistry has just separated.

The quiescent experiments told a strikingly different story. Without agitation, oil and grease removal ranged from 98.05 to 99.49 percent, consistently outperforming the stirred condition at comparable temperature and dose combinations, with a strong model fit of R-squared equal to 0.9312. Once the demulsifier and moderate heat had destabilized the emulsion, undisturbed conditions allowed the coalesced droplets to rise continuously to the surface, exactly as Stokes’ law predicts for creaming in a low-viscosity, quiescent fluid. Total suspended solids removal still reached up to 96.11 percent, aided by a slightly higher optimum temperature of around 43 degrees Celsius that compensated for the absence of mechanical collision energy. Turbidity removal peaked near 32 degrees Celsius at doses of 100 to 113 milligrams per liter.

The models also exposed a subtle danger in overdosing. Beyond roughly 100 milligrams per liter, removal efficiencies for oil and grease and biochemical oxygen demand began to falter. The explanation lies in surfactant physics: when the demulsifier concentration approaches the critical micelle concentration, excess molecules spontaneously assemble into micelles, spherical structures with oily interiors that encapsulate hydrocarbons and keep them dispersed in the water phase. Simultaneously, surplus surfactant adsorbing onto droplet surfaces enhances steric stabilization, blocking the very collisions the treatment is meant to promote. The result is a secondary, self-inflicted emulsion that resists settling, flotation, and even analytical detection, since standard methods count micelle-trapped oil as part of the oil and grease load.

To distill all four responses into a single operating recipe, the team applied the Derringer-Suich composite desirability method, treating every parameter as a larger-the-better characteristic and weighting them equally because each is independently regulated. Evaluated over a grid of 90,000 points across the experimental domain, the optimization delivered an unambiguous verdict. Without agitation, the overall desirability reached 0.8258 at approximately 35.2 degrees Celsius and 150 milligrams per liter of demulsifier, with the predicted model delivering oil and grease removal above 99.4 percent, total suspended solids removal above 96 percent, turbidity removal above 93 percent, and near-complete biochemical oxygen demand removal above 99.7 percent. The stirred condition managed only 0.7264, with a sharper, more fragile optimum that would make industrial operation far less forgiving of small deviations in temperature or dosing.

The practical implications extend well beyond one factory in Trenggalek. Skipping agitation eliminates the energy cost of continuous mixing, removes the capital expense of mixer installations, and cuts long-term maintenance for full-scale treatment plants. Compared with the electrocoagulation-Fenton process the same group previously tested, which achieved 99 percent oil and grease removal but demanded constant electrical input, consumable electrodes, and complex sludge handling, moderate thermal-chemical demulsification in a still tank is dramatically simpler. The authors note that the optimal dose of 150 milligrams per liter sits at the boundary of their tested range, meaning the true global optimum may lie beyond it, and they call for future work on demulsifier type, heating duration, and agitation speed. For now, the message to engineers battling oily industrial wastewater is refreshingly concise: dose it, warm it to about 35 degrees Celsius, and then let physics do the quiet work of separation.

Subject of Research: Thermal demulsification modeling of turpentine industrial wastewater using a polymeric demulsifier with multi-response optimization

Article Title: Analysis model of the demulsification process by heating in turpentine industrial wastewater

Article References: Prayitno, Rulianah, S., Ningsih, W., & Widyananda, W. (2026). Analysis model of the demulsification process by heating in turpentine industrial wastewater. Case Studies in Chemical and Environmental Engineering, 14, Article 101477. https://doi.org/10.1016/j.cscee.2026.101477

Image Credits: AI Generated

DOI: 10.1016/j.cscee.2026.101477

Keywords: turpentine wastewater, demulsification, oil and grease removal, wastewater treatment, polymeric demulsifier, response surface methodology, composite desirability, thermal separation, industrial effluent, emulsion breaking, polynomial regression, BOD removal