Highly efficient boron/sulfur-modified activated biochar for removal of reactive dyes from water: Kinetics, isotherms, thermodynamics, and regeneration studies

Water pollutants such as synthetic dyes can cause significant problems for human health and ecosystems due to their chemical properties and environmental interactions. Contamination of surface and underground water caused by the discharge of synthetic dyes is a widespread problem that arises primarily from industrial activities such as textile manufacturing, leather processing, paper production, and plastics industries. Since adsorption is one of the most efficient and reliable methods to remove pollutants from water, in this work, pine tree logging residues (LR) were used to produce boron/sulfur chemically modified biochars with superior adsorption performance and recyclability. The biochars were produced using a two-step pyrolysis procedure with potassium hydroxide as a chemical activator. The specific surface areas (B.E.T.) of the biochars were 2645 m ² g ⁻¹ for the boron-treated biochar (LR-Boron), 2524 m ² g ⁻¹ for the sulfur-treated (LR-Sulfur), and 3141 m ² g ⁻¹ for the control biochar (LR-Control, without boron or sulfur), respectively. The LR-Boron biochar showed an exceptional degree of graphitization of (I _D/I _G=0.45), while the LR-Sulfur biochar displayed an I _D/I _G= 1.02; for comparison, the LR-Control exhibited an I _D/I _G= 0.81, showing that the sample subjected to boron treatment created carbon-rich in graphitic structures. The three biochars were evaluated as adsorbents for removing reactive black-5 azo dye (RB-5) from water and mixtures of several dyes in synthetic aqueous effluents. The adsorption data showed that all carbons exhibited outstanding RB-5 removal performance. Kinetic measurements were well fitted by the Avrami fractional order model, and the LR-sulfur carbon displayed the fastest adsorption kinetics. Isotherm measurements were well fitted by the Liu model, with a theoretical Q _max of around 1419 mg g ⁻¹ (LR-Control), 1586 mg g ⁻¹ (LR-Boron), and 1766 mg g ⁻¹ (LR-Sulfur) at 316 K. The presence of sulfur-functional groups on the LR-Sulfur biochar surface was probably the reason for the superior adsorption performance of this biochar. Both sulfur and boron-treated biochars exhibited higher regeneration potentials, maintaining around 60–67 % removal capacity after 7 cycles compared to 35 % for the LR-Control biochar. Thermodynamic adsorption studies showed that the adsorption process was endothermic, favorable, and compatible with physical adsorption. All produced biochars were highly efficient for removal of pollutants from concentrated synthetic effluents.