Fish Scale Waste Transformed into Carbon-Apatite Adsorbent That Captures Ciprofloxacin from Water

The team, publishing in Environmental Science and Pollution Research, prepared a carbon/apatite composite from Nile tilapia scales and showed that it can hold nearly 80 milligrams of the drug per gram of material under near-neutral conditions, a range common in natural water bodies.

The work matters because antibiotics that survive conventional wastewater treatment slip into rivers and surface waters, where their presence can select for fluoroquinolone-resistant bacteria and disturb aquatic microbial communities. Adsorption is attractive for capturing such micropollutants because it is versatile, efficient, and simple to operate, especially in the highly diluted systems typical of emerging contaminants. What sets this study apart is that the researchers did not treat their fish-scale-derived material as a single black box. Instead, they separated its carbonaceous and inorganic phases and quantified how much each contributes to capturing the drug.

Fish scales are a naturally biphasic biomaterial, combining an organic collagen fraction with an inorganic hydroxyapatite fraction. To convert them into an adsorbent, the team followed a preparation route established in their earlier work on 2,4-D adsorption: the scales were pre-carbonized at 330 degrees Celsius in an oxidative atmosphere, chemically activated with potassium hydroxide in a one-to-one ratio, heated to 950 degrees Celsius under nitrogen, washed with hydrochloric acid and water, and dried. The overall yield of the composite, after all stages including washing to constant pH, was 38.27 percent, with material loss during washing the main factor reducing yield.

Before any adsorption test, the researchers mapped the geometry and chemistry of the target molecule. Ciprofloxacin has minimum and maximum projected molecular areas of 43.14 and 96.6 square angstroms and molecular diameters of 5.02 and 7.48 angstroms. Because the molecule carries two ionizable groups, it exists as cationic, anionic, zwitterionic, or neutral microspecies depending on pH, with pKa values of 5.09 and 8.89. The composite’s point of zero charge was measured at 7.5, meaning its surface is positively charged below that pH and negatively charged above it.

That charge balance explains the pH behavior the team observed. Testing initial pH values of 4, 7, and 10 with 50 milligrams per liter of ciprofloxacin, they measured adsorption capacities of 10.63, 16.01, and 10.28 milligrams per gram respectively, with the pH 7 experiment settling near a final pH of 8. At acidic pH, electrostatic repulsion arises between the positively charged adsorbent surface and cationic ciprofloxacin; at basic pH, the negatively charged surface repels anionic molecules. Around pH 8, close to the point of zero charge, repulsion is minimized and the drug coexists as multiple species, including the zwitterionic form, which can bind through non-electrostatic mechanisms. The researchers also noted a practical complication: the adsorbent itself shifts solution pH, apparently by releasing ions such as phosphate, so continuous pH monitoring proved essential for accurate analysis.

Microscopy and spectroscopy revealed why the two phases behave so differently. Scanning electron microscopy showed the inorganic phase as dense, less porous, and rougher, while the carbonaceous phase, derived largely from collagen, was highly porous and irregular. X-ray diffraction confirmed sharp crystalline hydroxyapatite peaks in the inorganic phase alongside a broad amorphous halo from collagen-derived carbon, though the manual phase separation was incomplete, with cross-contamination visible in both patterns. Fourier-transform infrared spectroscopy identified hydroxyl, phosphate, and carbonate bands characteristic of hydroxyapatite, plus aromatic and carboxylic features on the carbon surface, and changes in the 1600 to 1400 per centimeter region after adsorption indicated that these groups participate in binding the drug.

The phase-separation experiment delivered the study’s most striking quantitative result. Under identical conditions of 30 degrees Celsius, pH 8, a 200 milligram per liter ciprofloxacin solution, and 48 hours of contact, the carbonaceous phase adsorbed 87.6 milligrams per gram while the inorganic phase managed only 43.9, a capacity 50.1 percent lower. The mixed composite reached 54.65 milligrams per gram in that test. The researchers attribute the carbon’s advantage to its aromatic structure, which supports pi-pi stacking interactions with the drug’s ring system, whereas the predominantly ionic hydroxyapatite surface offers fewer favorable sites for an organic molecule. Because separating the phases completely is not practical, the composite itself represents the realistic application.

Equilibrium experiments across pH 7 to 10 showed that the Langmuir-Dubinin-Radushkevich model, which combines monolayer surface adsorption with micropore filling, fit the data best, with R-squared values from 0.97 to 0.99. The model predicted total capacities of 84.01 milligrams per gram at pH 7, 83.09 at pH 8, 72.46 at pH 9, and 67.90 at pH 10, while the highest experimental capacity was 79.09 milligrams per gram at pH 8. Notably, the composite achieves this uptake with only a moderate BET surface area of 72.13 square meters per gram, suggesting that capacity depends on the accessibility and chemical affinity of adsorption sites and on cooperation between texture and surface chemistry rather than on total area alone. The characteristic pore diameter of 22.36 angstroms is roughly three times the drug’s maximum molecular diameter, a ratio of 2.99 that falls within the range where pore-size effects produce a pore-filling retention mechanism.

Kinetic experiments at initial concentrations of 100 and 200 milligrams per liter reinforced this picture. Equilibrium was reached in 15 hours at the lower concentration with a capacity of 55.0 milligrams per gram, and faster, in 12 hours, at the higher concentration with 64.7 milligrams per gram, as the larger concentration gradient increased the mass-transfer driving force. A pseudo-first-order model with two sites fit the 100 milligram per liter data best, consistent with fast adsorption on accessible carbonaceous sites followed by slower uptake at less accessible inorganic-phase sites. At 200 milligrams per liter, the pore-diffusion model prevailed, with effective diffusivities of 1.75 times ten to the minus thirteen and 3.24 times ten to the minus thirteen square meters per second, indicating that configurational diffusion within micropores is the rate-limiting step.

The authors are careful about the limits of their findings. All experiments used batch systems with synthetic aqueous solutions, and the relatively high ciprofloxacin concentrations were chosen to saturate the material and support reliable model fitting rather than to reproduce the trace levels found in environmental waters. The study did not evaluate regeneration cycles, competing species, real wastewater matrices, or continuous-flow operation, and the environmental and economic costs of potassium hydroxide consumption, 950-degree activation, inert gas demand, and acid washing still require assessment. Related hydroxyapatite-carbon composites have shown regeneration behavior that depends strongly on the target contaminant, so dedicated reuse experiments are needed. Still, by demonstrating that fish scales, an underutilized residue of the fish-processing industry, can yield a biphasic adsorbent whose carbon and apatite fractions cooperate chemically and texturally, the study offers a concrete route for turning aquaculture waste into a tool against pharmaceutical pollution.

Subject of Research: Adsorption of the antibiotic ciprofloxacin from water using a carbon/apatite composite adsorbent produced from fish scales

Article Title: Heterogeneous carbon/apatite composite adsorbent: cooperative ciprofloxacin adsorption

Article References: Alberti, S. W., Scheufele, F. B., Kruger, G. F., da Silva, L. C., & da Silva, E. A. (2026). Heterogeneous carbon/apatite composite adsorbent: cooperative ciprofloxacin adsorption. Environmental Science and Pollution Research. https://doi.org/10.1007/s11356-026-38338-y

Image Credits: AI Generated

DOI: 10.1007/s11356-026-38338-y

Keywords: ciprofloxacin, adsorption, fish scales, carbon/apatite composite, water treatment, hydroxyapatite, micropore filling, antibiotic pollution, Nile tilapia, isotherm modeling, kinetics, wastewater