Adsorption behavior regularity of bone char for ionic organic compounds in printing and dyeing wastewater

Ionic organic compounds represented by synthetic dyes are structurally complex. Existing studies mostly focus on individual, newly selected target pollutants under different test conditions, and rarely conduct comparative investigations of different ionic organics using bone char (BC) within a unified experimental framework. This makes it difficult to clarify the general adsorption rules of BC for printing and dyeing wastewater. In this study, slaughterhouse bone waste from livestock and poultry was pyrolyzed at 700 ℃ under N₂ to prepare BC, which was then subjected to detailed physicochemical characterization. On this basis, the adsorption behavior of BC towards typical ionic organic compounds with different types and structures was systematically compared under various environmental conditions, including cationic dyes (Malachite Green, triphenylmethane; Methylene Blue, heterocyclic thiazine; Rhodamine B, heterocyclic xanthene) and anionic dyes (Sunset Yellow, azo; Reactive Blue 19, anthraquinone; Acid Fuchsin, triphenylmethane). The prepared BC showed a well-developed mesoporous structure, with a specific surface area about 1709 times that of raw bone meal. After pyrolysis, the ash content, electrical conductivity, pH value and cation exchange capacity related to inorganic mineral components increased by 61.8%, 79.3%, 53.2% and 71.8%, respectively; characteristic peaks of surface functional groups were enhanced and the crystal structure became more ordered. These features indicate that the BC is a suitable adsorbent for ionic organic pollutants. With increasing BC dosage, both the removal efficiency and the equilibrium adsorption capacity increased, and the variation could be described by logarithmic, linear or power-function relationships depending on the pollutant. For cationic organics, the removal efficiency increased or remained nearly constant with increasing initial dye pH value, and the maximum removal of triphenylmethane cations increased. Anionic dyes were more effectively removed under acidic conditions. Increases in initial dye concentration and contact time led to a rapid rise in adsorption capacity, followed by a gradual approach to equilibrium. Adsorption of triphenylmethane dyes conformed to the Langmuir isotherm model, indicating predominantly monolayer adsorption on relatively uniform sites, whereas adsorption of heterocyclic, azo and anthraquinone dyes followed the Freundlich isotherm model, indicating multilayer adsorption on heterogeneous surfaces. For all target compounds, the adsorption kinetics fitted the pseudo-second-order model, suggesting that chemisorption plays a dominant role. The Gibbs free energy change of adsorption was negative, indicating that the processes occurred spontaneously. Except for anionic triphenylmethane dyes, the adsorption processes were endothermic and accompanied by an increase in entropy. Malachite Green, which showed the highest adsorption performance among the tested compounds, was selected as a model pollutant to analyse the adsorption mechanism. According to the proportion of surface element content, functional groups and crystal structure of BC before and after adsorption indicate that pore filling, electrostatic attraction, cation-π interaction, π-π interaction, ion exchange and complexation with mineral components jointly contribute to the adsorption process. In the two-component system, ionic organics exhibited a certain degree of synergistic interaction, and the adsorption capacity of BC for each component was simultaneously enhanced. These results help to clarify the adsorption behavior and mechanism of BC towards ionic organic pollutants in printing and dyeing wastewater, and provide theoretical support for its engineering application in wastewater treatment.