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The fabrication of crosslinked polyaniline/Pterocladia capillacea-activated carbon composite (CrossPANI/P-AC) at different ratios (1:0, 1:0.2, 1:0.6, and 1:1) was studied. CrossPANI/P-AC composites were fabricated by the in situ polymerization of aniline using hydrogen chloride as an acidic dopant, and ammonium persulfate as initiator, while Pterocladia capillacea-activated carbon was synthesized by the chemical activation method and incorporated into the polymer matrix. The samples were characterized by the terms such as Fourier transform infrared (FTIR) spectroscopy, Brunauer–Emmett–Teller, X-ray diffraction (XRD), thermogravimetric analysis, scanning electron microscopy, and energy-dispersive X-ray spectroscopy (EDX). FTIR spectroscopy showed the main characteristic peak positions of CrossPANI/P-AC; XRD showed low crystallinity of CrossPANI/P-AC. A high specific surface area for CrossPANI/P-AC was achieved at a ratio of 1:0.2 where Brunauer–Emmett–Teller surface area, total pore volume, and mean pore diameter values were 166.10 m2/g, 0.0141 cm3/g, and 3.40 nm, respectively. The capability of CrossPANI/P-AC (1:0.2) composite as adsorbent for Direct blue-86 (DB-86) dye from aqueous solution was investigated. The impact of initial dye concentration, temperature, pH, and contact time on the DB-86 dye adsorption from its water solution was examined. The equilibrium adsorption data were well represented by the Langmuir isotherm achieving maximum monolayer capacity (Qm) of 163.93 mg/g at a dose of 0.5 g/L. In contrast, the kinetic adsorption data were well fit by the pseudo-second-order model. Thermodynamic analysis demonstrated that DB-86 dye adsorption occurs spontaneously, endothermically, and physically in nature. The results demonstrated that these composites effectively removed DB-86 dye from aqueous solutions and could be recycled. Graphical abstract

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Starch-g-poly(acrylic acid)/Pterocladia capillacea–derived activated carbon (St-g-P(AA)/P-AC) composites were prepared via aqueous solution graft copolymerization using starch, acrylic acid, and activated carbon of red alga Pterocladia capillacea (0–10%) with N,N′-methylenebisacrylamide crosslinker and ammonium persulfate (NH4)2S2O8 initiator. Fourier-transform infrared (FTIR) spectroscopy, Brunauer–Emmett–Teller (BET), X-ray diffraction (XRD), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDX) were used to characterize St-g-P(AA)/P-AC composites. Additionally, St-g-P(AA)/P-AC composites were investigated for methylene blue (MB) dye removal from water. The impact of the beginning concentration of MB dye, temperature, pH, and adsorption time on MB dye removal was examined. The maximum adsorption capacity obtained at pH 8 was 496.29 mg/g at 0.02 mg/L composites dose and 100 mg/L MB dye. The properties of adsorption were studied by the adsorption isotherm, kinetic, and thermodynamic models. The pseudo-first-order and Freundlich isotherm models demonstrated the kinetics and equilibrium adsorptions data, respectively. The maximum monolayer capacity (qm) was 1428.57 mg/g from Langmuir isotherm. Thermodynamic parameters indicated that the MB dye adsorption is exothermic physisorption and spontaneous. The results show that St-g-P(AA)/P-AC composites were effective for MB dye adsorption from water solution and could be recycled.