Researchers Nada S. Al-Kadhi, Ehab A. Abdelrahman, and Saad A. Aljlil prepared the materials using the Pechini sol-gel route, a technique in which tartaric acid chelates metal cations and polyethylene glycol 400 helps polymerize the mixture into a homogeneous resin. Magnesium nitrate hexahydrate, strontium nitrate, and barium nitrate were dissolved together, combined with the tartaric acid solution, and stirred at 200 degrees Celsius until the solvents evaporated and a dry xerogel formed. The ground powder was then calcined in air for three hours, at 600 degrees Celsius to yield a sample labeled BSM600 and at 800 degrees Celsius to yield BSM800. The Pechini method matters because it distributes cations homogeneously at the molecular scale and allows several cations to be incorporated in a single step, producing fine oxide particles with controlled stoichiometry.

X-ray diffraction confirmed that each product contains five crystalline constituents: magnesium oxide, barium strontium carbonate (Sr0.5Ba0.5CO3), barium carbonate, strontium hydroxide monohydrate, and a mixed barium-strontium oxide, (Ba0.504Sr0.496)O. The mean crystallite size was 63.5 nanometers for BSM600 and 71.2 nanometers for BSM800, with the larger size attributed to thermally driven grain growth, coalescence, and partial strain relaxation at the higher calcination temperature. Interestingly, the authors note that strontium hydroxide monohydrate cannot survive at 600 to 800 degrees Celsius; its presence is attributed to post-calcination rehydration of the highly hygroscopic SrO-rich oxide phase by atmospheric moisture during cooling, storage, and handling, a behavior consistent with earlier work showing that SrO hydrates so rapidly in ambient air that its diffraction pattern cannot be recorded before hydration products appear.

Complementary techniques mapped the composition and morphology. Energy-dispersive X-ray spectroscopy verified carbon, oxygen, magnesium, strontium, and barium in both products, and CHN analysis gave bulk carbon contents of 5.6 weight percent for BSM600 and 4.3 percent for BSM800, reflecting more extensive burnout of organic residues at the higher temperature. Because EDS is semi-quantitative and surface-localized, the bulk cation composition was verified by X-ray fluorescence, which gave Mg:Sr:Ba molar ratios of 29.2:38.9:31.9 for BSM600 and 29.6:38.5:31.9 for BSM800, close to the nominal precursor ratio and confirming that no cation loss occurred during calcination. Field-emission scanning electron microscopy showed granular to faceted grains with mean grain sizes of 0.43 micrometers for BSM600 and 0.78 micrometers for BSM800, while high-resolution transmission electron microscopy revealed near-spherical to polyhedral nanoparticles with mean diameters of 39.04 and 76.01 nanometers respectively. The authors explain that the XRD crystallite size of BSM600 exceeds its TEM particle size because XRD yields a volume-weighted average dominated by larger crystallites, whereas TEM gives a number-weighted average in which numerous small particles contribute equally.

Batch adsorption tests, performed in triplicate with dye-only controls to exclude irreversible alkaline degradation or carbinol precipitation, revealed a strong dependence on pH. Removal improved sharply as pH rose: at pH 2, only 2.25 percent of the dye was eliminated by BSM600 and 1.71 percent by BSM800, but at pH 10 the figures climbed to 96.18 percent and 78.78 percent. The explanation lies in the surface charge of the materials. The point of zero charge was 8.45 for BSM600 and 8.83 for BSM800. Below these values, protonated surfaces carry a net positive charge and electrostatically repel the cationic dye; above them, deprotonation generates negatively charged sites that attract it. At the working pH of 10, both adsorbents carried negative zeta potentials of −24.6 millivolts for BSM600 and −18.3 millivolts for BSM800, promoting attraction toward the cationic –N+(CH3)2 center of malachite green. The stronger performance of BSM600 follows from its lower point of zero charge, which makes its surface more negative at the same alkaline pH.

Infrared spectroscopy after adsorption supported several concurrent binding mechanisms. All parent bands of the pristine material were retained, showing the host framework remained intact, while new bands appeared that could be assigned to malachite green. Intensification and shifting of the metal-oxygen and carbonate bands indicated electrostatic attraction between the negatively charged M–O− and CO₃2− sites and the dye’s cationic center. New O–H bands pointed to hydrogen bonding between surface hydroxyls and the dye’s nitrogen atoms and aromatic pi-electrons. Shifts in carbonate combination bands and the appearance of new low-frequency bands indicated surface complexation, in which Mg2+, Sr2+, and Ba2+ sites coordinate with the dye and lower the symmetry of adjacent carbonate groups. Finally, a red-shift of the aromatic C=C band from about 1614 to 1587 wavenumbers was consistent with pi-pi stacking between the dye’s triphenylmethane rings and the sp2-hybridized domains of the carbon support. Post-adsorption electron microscopy showed a smoother, more rounded surface with reduced intergranular porosity, consistent with a continuous dye layer coating the external surface without structural degradation.

Textural measurements helped explain why BSM600 outperformed BSM800. BSM600 exhibited a BET surface area of 3.04 square meters per gram and a total pore volume of 0.03521 cubic centimeters per gram, roughly 2.8- and 2.5-fold higher than BSM800’s 1.07 square meters per gram and 0.01432 cubic centimeters per gram. Both materials derive most of their porosity from large mesopores and macropores, but BSM600 retained roughly five to eight times more pore volume in the 2 to 20 nanometer mesopore region, providing short diffusion pathways toward internal active sites. The higher calcination temperature promotes sintering, grain-boundary migration, and Ostwald ripening, collapsing smaller pores and burning off more of the carbon support. Notably, the removal efficiency of BSM600 did not increase in proportion to its surface area, a 2.8-fold larger area yielding only about a 1.2-fold higher removal, indicating that texture acts in concert with surface chemistry, particularly the lower point of zero charge that intensifies electrostatic attraction.

Kinetic and equilibrium analyses quantified the process. Uptake was rapid at first, with BSM600 removing 43.90 percent of the dye within 10 minutes and reaching equilibrium at 60 minutes with 96.05 percent removal, while BSM800 removed 34.99 percent at 10 minutes and plateaued at 78.51 percent after 80 minutes. The data fit the pseudo-first-order kinetic model better than the pseudo-second-order model, with predicted equilibrium capacities agreeing more closely with experimental values and higher correlation coefficients. Equilibrium data followed the Langmuir isotherm, indicating monolayer adsorption on energetically uniform sites: the Langmuir correlation coefficient was 0.9997 for BSM600 and 0.9985 for BSM800, far exceeding the Freundlich fits. The maximum adsorption capacity reached 371.75 milligrams per gram for BSM600 and 312.50 milligrams per gram for BSM800, placing both among the top performers in the literature, well above activated charcoal at 27 milligrams per gram, amino-functionalized graphene oxide at 91.48, sulfur-doped biochar at 94.50, and Fe/Ni@ZIF-8 nanoparticles at 151.52.

Thermodynamics confirmed the physical nature of the uptake. Removal decreased as temperature rose from 298 to 328 kelvin, with BSM600 falling from 96.05 to 89.79 percent and BSM800 from 78.51 to 69.76 percent, consistent with an exothermic process. The magnitude of the enthalpy change was below 40 kilojoules per mole for both samples, indicating physical adsorption, while negative entropy changes indicated a more ordered arrangement of dye molecules at the solid-solution interface. Negative Gibbs free energy values at all investigated temperatures confirmed spontaneous, thermodynamically favorable adsorption that becomes less favorable at higher temperatures. Increasing the initial dye concentration from 100 to 350 milligrams per liter lowered removal from 98.87 to 69.56 percent for BSM600 and from 94.04 to 56.75 percent for BSM800, reflecting progressive saturation of adsorption sites.

Regeneration results point toward practical reuse. Hydrochloric acid proved highly effective: at 1 molar concentration, desorption reached 86.54 percent for BSM600 and 85.09 percent for BSM800, and at 2 molar, near-complete dye release occurred with 99.62 percent and 99.77 percent respectively, because protons saturate surface sites, break electrostatic binding, and competitively displace the adsorbed cationic dye. X-ray diffraction of BSM600 after desorption showed the characteristic diffraction features essentially preserved, with no new crystalline phases or substantial displacement of principal reflections, and inductively coupled plasma analysis of the filtrate detected no magnesium, strontium, or barium, indicating excellent chemical stability against leaching. Across five successive adsorption-desorption cycles, removal declined gradually from 96.05 to 88.33 percent for BSM600 and from 78.51 to 68.45 percent for BSM800, a slight loss of active sites and minor fouling during reuse. The authors attribute the performance of both materials to a synergistic effect among the multiple phases within each nanocomposite, which creates a high density of accessible, energetically uniform active sites and diverse binding environments while preserving structural stability during regeneration.

Subject of Research: Multiphase Sr-Mg-Ba oxide nanohybrids for adsorptive removal of malachite green dye from water

Article Title: Carbon supported oxide nanohybrids based on Sr, Mg, and Ba for efficient malachite green remediation

Article References: Al-Kadhi, N. S., Abdelrahman, E. A., & Aljlil, S. A. (2026). Carbon supported oxide nanohybrids based on Sr, Mg, and Ba for efficient malachite green remediation. Journal of Saudi Chemical Society, 30(6), Article 81. https://doi.org/10.1007/s44442-026-00133-2

Image Credits: AI Generated

DOI: 10.1007/s44442-026-00133-2

Keywords: malachite green, adsorption, nanocomposites, Pechini sol-gel, magnesium oxide, strontium carbonate, barium carbonate, wastewater treatment, Langmuir isotherm, dye removal, regeneration, carbon support