Investigation of multiple adsorption mechanisms for efficient removal of ofloxacin from water using lignin-based adsorbents

Abstract

Two series of lignin (LN)-based adsorbents, namely, cross-linked lignin (LNEs) with different crosslinking densities and carboxymethyl cross-linked lignin (LNECs) with various degrees of carboxymethyl substitution, were prepared to remove ofloxacin (OFL), a popular fluoroquinolone (FQ) antibiotic, from water. LNEs and LNECs exhibited satisfactory performance in OFL adsorption. Both of them had high adsorption capacity (the maximum contribution of 0.828 mmol/g), good anti-interference to some inorganic salts, and efficient regeneration and reuse performance. The crosslinking density and degree of carboxymethyl substitution strongly affected the content and distribution of oxygen-containing groups in these LN-based adsorbents, which played important roles in OFL adsorption. The pH dependencies of the adsorption performance of LNEs and LNECs indicated the involvement of multiple adsorption mechanisms, including hydrogen bond, electrostatic attraction, π-π electron–donor–acceptor interactions, and negative charge-assisted hydrogen bond. Different mechanisms were dominant under various pH levels, in a near neutral pH, the synergistic effect of electrostatic attraction and π-π interaction allows LINEs and LINECs to reach maximum adsorption capacity. Five FQs with similar structures and their two sub structural analogs were compared in terms of adsorption behavior and electrostatic potential by density functional theory using quantum chemical calculation. FQs with secondary amino groups and low π electron cloud density readily bound to LN-based adsorbents. Hence, LNEs and LNECs were efficient and environment-friendly adsorbents.

Polarity of Cationic Lignin Polymers: Physicochemical Behavior in Aqueous Solutions and Suspensions

“…A similar assumption was also considered in a previous report on analyzing the adsorption behavior of lignin-based polymers. [29] The dipole moment is an important physical property of a molecule that is vital in determining molecular polarity and is a key factor for understanding and quantifying the intermolecular interactions. [30,31] Dipole moments occur when there is a charge separation between two ions in an ionic bond or between atoms in a covalent bond due to the differences in their electronegativity, which tends to orient a molecule in a polarized dielectric.…”

Influence of water washing treatment on Ulva prolifera-derived biochar properties and sorption characteristics of ofloxacin

“…Here, the OFL had a good hydrophilicity (i.e. log K ow = − 0.39) and the highest sorption values did not all appear at the lowest solubility, which suggested that the sorption of OFL by U.P- biochars was not entirely dependent on the hydrophobic interaction 38 , 39 .…”

“…Thus, the sorption capacity might reduce through the weakening ionic exchange and cationic–π interactions between biochar and OFL. However, the sorption capacity of U.P- biochars also exhibited a relatively high level at neutrality, which was probably due to H-bond, π–π and hydrophobic interactions being involved in the sorption process 38 . Meanwhile, the positive surface charge of U.P- biochars decreased due to the deprotonation effect, leading to significant reduction in the electrostatic repulsion between U.P- biochars and OFL.…”

“…It was noted that the sorption capacity of BCW600 continually decreased when the pH varied 3 to 11. These variation tendencies in a pH-dependent fashion were mainly due to the fact that the pH would strikingly shift the surface charge properties of biochar and the species distribution of OFL (OFL + , OFL 0 and OFL − ) 38 . However, the present literature has reported that the sorption of OFL by biochars expressed no significant change for an initial pH ranging from 3 to 9 7 .…”

“…The electrostatic repulsion was maintained between OFL and U.P- biochars obtained at a pyrolysis temperature ≤ 400 °C, while the electrostatic attraction was generated between OFL and U.P- biochars obtained at a pyrolysis temperature ≥ 500 °C, because these two types of U.P- biochars were negatively charged (≤ 400 °C) and positively charged (≥ 500 °C) at pH 9 34 . The negative charge-assisted H-bond (( − )CAHB) may serve as another important mechanism for the sorption of the anionic species of a hydrophilic compoundon a negatively charged carbon surface, which was easily generated as the H-bond donor and H-bond acceptor had comparable pK a values 38 . The difference between the pK a2 (8.28) of OFL and the pK a (strong carboxylic acid ( pK a 5–6.4), moderate acid/lactone ( pK a 6.4–10.3) and weak phenolic acid ( pK a 10.3–13)) 41 of U.P- biochars generated at a pyrolysis temperature ≤ 400 °C was less than 4, which may have been beneficial to produce (−)CAHB at pH 9.…”