Removal of nonylphenol polyethoxylates by raw lignite coal and activated carbon: materials characterization, adsorption studies, and modeling the adsorption isotherms

Abstract

The present work deals with the evaluation of raw lignite coal (LC) as a low-cost adsorbent to remove nonylphenol polyethoxylates (NPEOs) from wastewater. Commercial activated carbon (AC) was also studied for comparison purposes. Both adsorbents were characterized by scanning electron microscopy, dynamic light scattering, BET surface area, and Fourier transform infrared spectroscopy (FTIR). The AC surface was more irregular than LC due to the presence of cavities on the surface of AC particles. BET surface area of AC was two orders of magnitude higher than the corresponding to LC. FTIR spectra of both adsorbents revealed the presence of abundant surface functional groups. Adsorption assays demonstrated that NPEOs followed a Langmuir-type equation. However, Langmuir constants were a function of the average length of the ethoxylate chain (n) of the tested NPEOs samples. A mathematical model was developed to represent the obtained adsorption equilibrium isotherms of NPEOs. Using a single set of parameters, the developed model can describe the adsorption of different mixtures of NPEOs. The model can also predict the distribution of oligomers in equilibrium, which is crucial to evaluate the endocrine-disrupting potential of the treated wastewater.