Role of sorbent surface functionalities and microporosity in 2,2′,4,4′-tetrabromodiphenyl ether sorption onto biochars

The study provides insight into the combined effect of sorbent surface functionalities and microporosity on 2,2′,4,4′-tetrabromodiphenyl ether (BDE-47) sorption onto biochars. A series of biochars prepared under different conditions were used to test their sorption behaviors with BDE-47. The extents of sorption behaviors were parameterized in terms of the single-point adsorption equilibrium constant (Koc) at three equilibrium concentration (Ce) levels (0.001Sw (solubility), 0.005Sw, and 0.05Sw) which was determined using the Freundlich model. To elucidate the concentration-dependent dominant mechanisms for BDE-47 sorption onto biochars, Koc was correlated with four major parameters using multiple parameter linear analysis accompanied with significance testing. The results indicated that at low concentration (Ce = 0.001Sw), the surface microporosity term, which represented a pore-filling mechanism, contributed significantly to this relationship, while as concentration was increased to higher levels, surface functionality related to surface adsorption began to take the dominant role, which was further confirmed by the results of Polanyi-based modeling. Given the above results, a dual mode model based on Dubinin-Radushkevich and de Boer-Zwikker equations was adopted to quantitatively assess the changes of significance of surface adsorption as well as that of pore filling with sorption process development. In addition, UV spectra of four typical aromatic compounds which represented the key structural fragments of biochars before and after interactions with BDE-47 were analyzed to determine the active functional groups and supply complementary evidence for the dominant interaction force for surface adsorption, based on which π–π electron-donor-acceptor interaction was proposed to contribute greatly to surface adsorption.