2,4-D removal by fish scales-derived carbon/apatite composite adsorbent: Adsorption mechanism and modeling

The increasing use of herbicides has generated health and environmental problems, as their bioaccumulative characteristics affect water resources. Traditional water treatment methods are commonly not effective in removing these compounds. The adsorption technique shows potential for these pollutants' removal due to the possibility of application in low concentrations. Therefore, the objective of this work was to produce and evaluate the adsorption of 2,4-D herbicide by fish scales-derived carbon/apatite composite (CApC) adsorbent materials. The CApC material was produced by applying a previous thermal treatment of the fish scales (330 degrees C for 3 h) under an oxidative atmosphere, followed by KOH (1:1 m/m) one-pot carbonization/activation under N2 atmosphere (10 degrees C min-1 up to 950 degrees C for 1 h). The CApC was then preliminarily evaluated on the 2,4-D adsorption as a function of pH. Subsequently, at the optimal pH, in-depth adsorption experiments were car-ried out under different 2,4-D initial concentrations and temperatures for the CApC adsorbent to acquire kinetic, equilibrium, and thermodynamic data. Furthermore, CApC was characterized by thermogravimetric analysis, X-ray diffraction, particle density and size distribution, N2 physisorption, Fourier transform infrared spectroscopy, and zero-charge point (pHzpc). It was found that the CApC showed higher adsorptive capacity at pH 6.5 (without pH adjustment), despite showing a wide pH range adsorption potential. The experimental data was evaluated through a unified and phenomenological approach by a monolayer-multilayer (MMA) model, which was able to describe the kinetics, equilibrium, and thermodynamics mechanistically. The equilibrium data indicated multilayer for higher concentrations. Also, adsorption thermodynamics was spontaneous and physical process for both monolayer and multilayer steps, however, showing specific endothermic and exothermic character for each step, respectively. The modeling results agree with retention mechanisms via H-bonds and 7C-7C stacking, whereas negligible electrostatic forces. Overall, the synthesized CApC adsorbent material showed efficiency and desirable characteristics for herbicide-contaminated water treatment.

Automated summary

The increasing use of herbicides has led to health and environmental problems due to their bioaccumulative nature and impact on water resources. Traditional water treatment methods are often ineffective in removing these compounds. The adsorption technique has shown promise for removing these pollutants, particularly at low concentrations.