Sorption of hydrophobic organic contaminants on functionalized biochar: Protagonist role of π-π electron-donor-acceptor interactions and hydrogen bonds

The sorption of five potent endocrine disruptors as representative hydrophobic organic contaminants (HOCs) namely estrone (E1), 17 beta-estradiol (E2), estriol (E3), 17 alpha-ethynylestradiol (EE2) and bisphenol A (SPA) on functionalized biochar (fBC) was systematically examined, with a particular focus on the importance of n electron-donor (phenanthrene: PHEN) and pi-electron-acceptors (1,3-dinitrobenzene: DNB, p-amino benzoic acid: PABA) on sorption. Experimental results suggested that hydrogen-bond formation and pi-pi-electron-donor acceptor (EDA) interactions were the dominant sorption mechanisms. The sorption of HOCs decreased as E1 > E2 > EE2 > E3 > BPA based on the Freundlich and Polanyi-Mane-models. The comparison of adsorption coefficient (K-d) normalized against hexadecane-water partition coefficient (K-HW) between HOCs and PHEN indicated strong pi-pi-EDA interactions. pi-pi interactions among DNB, PHEN and HOCs were verified by the observed upfield frequency (Hz) shifts using proton nuclear magnetic resonance (H-1 NMR) which identified the specific direction of pi-pi interactions. UV-vis spectra showed charge-transfer bands for pi-donors (PHEN and HOCs) with the model pi-acceptor (DNB) also demonstrating the role of pi-pi EDA interactions. The role of n electron-donor and pi-electron-acceptor domains in fBC was identified at different solution pH.