Sorption of naphthalene and its hydroxyl substitutes onto biochars in single-solute and bi-solute systems with propranolol as the co-solute

- OH and NO2 enhanced naphthalene substituents sorption on low-temperature biochar.
- Hydrophobicity and molecular volume determined the sorption on high-temperature biochar.
- The ion-pair reaction increased the sorption of anionic 4-nitro-1-naphthol.
- pH changed sorption mechanisms in both single-solute and bi-solute systems.

The sorption of naphthalene and its hydroxyl-substituted compounds onto two biochars that were produced at different temperatures was studied. The influences of propranolol as a co-solute and solution pH on the sorption were investigated. The sorption capacities of 1-naphthol and 4-nitro-1-naphthol on the biochar with a pyrolytic temperature of 200 °C (BC200) were greater compared to naphthalene. This occurred because the substitution of hydroxyl- and nitro-groups on the aromatic ring enhanced surface adsorption, e.g. hydrogen bond and π-π electron donor-acceptor interaction, while the hydroxyl in 1-naphthalenemethanol did not show the promoting effect. The sorption on the biochar with a pyrolytic temperature of 700 °C (BC700) followed a different sequence of 4-nitro-1-naphthol < 1-naphthalenemethanol < 1-naphthol < naphthalene, which was determined by the hydrophobicity and molecular volume of the sorbates, indicating the dominance of hydrophobic partition and pore-filling in the adsorption on BC700. The co-existence of propranolol promoted the surface hydrophobicity of BC200, which led to an enhancement in the sorption of naphthalene and 1-naphthalenemethanol. Propranolol prohibited the sorption of 1-naphthol by competing for the mutual available surface adsorption sites. The ion-pair reaction between anionic 4-nitro-1-naphthol and cationic propranolol increased the sorption of 4-nitro-1-naphthol. On BC700, propranolol generally prohibited the sorption of the tested sorbates mainly due to competition and pore-blocking. The influence of pH was complicated in both single-solute and bi-solute systems. It simultaneously affected the surface charge of biochars and the form of the tested compounds, which led to the shifting of sorption mechanisms. The finding will shed light on the suitable application of biochar technology.

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