Preparation of CuCl₂ modified biochar for levofloxacin removal from municipal wastewater

3.3. LFX adsorption mechanism

FT-IR spectra of Cu-BC before and after LFX adsorption are also analyzed. Some peaks have changed after LFX adsorption (Figure 3) [24]. The -CH bending vibration appears at 874 cm^-1 [25]. However, the peak location and peak intensity have changed after LFX adsorption. This result proves that the π-π interaction contributes to LFX adsorption. Besides, the peak location and peak intensity of the C=C group have also changed after LFX adsorption, indicating that π-π interaction occurs during the LFX adsorption process. Stretching vibration of O-H at 3427 cm^-1 is migrated to 3431 cm^-1 after LFX adsorption. It can be explained by the formation of H-bonding between Cu-BC and LFX in the adsorption process. The peak intensity of metal-O at 567 cm^-1 changes after LFX adsorption. This result shows that Cu compounds may be involved in the adsorption process of LFX [26]. Besides, the peak intensity and peak location of the C-O (1189 cm-1) group are moved to 1115 cm^-1 after LFX adsorption [27]. This outcome implies that the C-O group on the Cu-BC forms H-bonding with atoms in LFX, which promotes LFX adsorption by Cu-BC via the formation of H-bonding with functional groups in LFX [28]. The peak intensity and peak location of the C=O/C=C (1609 cm^-1) group shifted to 1617 cm^-1 after LFX adsorption. The drop in and vibration of the C=O/C=C peaks in the stable aromatic or graphitic structures of Cu-BC after LFX adsorption benefits LFX adsorption via π-π interaction [29].

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Figure 3.

FTIR analysis of Cu-BC before and after LFX adsorption.

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3.4.1. Effect of initial LFX concentration

The LFX degradation performance of the CuO-BC activation PDS under three concentration gradients (10-20 mg/L) is explored. As shown in Figure 4(a), when the LFX concentration is 10 mg/L, the LFX degradation removal by CuO-BC activation PDS reaches 94.33%. With an increase in the LFX concentration, the LFX degradation removal declines. The reason is that the limited activators CuO-BC and PDS could only provide a certain amount of ROSs. The generated ROSs are insufficient to completely degrade the high-concentration LFX wastewater, resulting in a reduction in the LFX degradation removal. When LFX concentration reaches 20 mg/L, the LFX degradation removal decreases from 94.33% to 85.37%. When LFX concentration increases from 15 to 20 mg/L, LFX degradation removal undergoes only a marginal decrease. The reason for this phenomenon is that many intermediates are produced during the LFX degradation process. The generated intermediates are adsorbed on Cu-BC. While the adsorbed intermediates occupy the PDS reaction site. Meanwhile, these intermediates will further be degraded, which consumes the ROSs generated from Cu-BC activation PDS process.

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Figure 4.

(a) The LFX degradation capability of CuO-BC activation PDS at different concentration gradients, (b) Quenching tests by employing quenching agents in the Cu-BC/PDS system, (c-d) Spectra of Cu 2p of Cu-BC before and after use.

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3.4.2. The influence of pH

The pH can influence the LFX degradation reaction by regulating the adsorption capacity of Cu-BC. The ionization constants (pKa) of LFX are pK_a1= 6.02 and pK_a2 = 8.15 [30]. There are three ionization forms of LFX at different pH values. When pH < pKa1, the LFX exists in the form of a cation (LFX ⁺). When pKa1 < pH < pKa2, the LFX exists in the form of a zwitterion (LFX^±) [31]. The pH_pzc of the Cu-BC is 6.2 (Figure S2). When the pH ranges from 3 to 5, the solution is acidic with a considerable amount of H⁺. The surface of Cu-BC is protonated with positively charged groups, which makes it more prone to attack S₂O₈^2- and activate it to generate the ROSs like SO₄•^-. The protonated LFX ⁺ is more likely to be attacked by SO4•^-. When pH is within 7 to 9, the Cu-BC surface is negatively charged and LFX is in the LFX^± state, which is also prone to being attacked by ROSs [16]. When pH is 11, both the Cu-BC and LFX surfaces are negatively charged. Due to electrostatic repulsion, LFX adsorption on Cu-BC is inhibited (Figure S3a). Moreover, a large quantity of OH^- in the solution may react with Cu²⁺ to form precipitates, resulting in the loss of active sites and pore blockage. This is not conducive to Cu-BC activation PDS, reducing LFX degradation efficiency. Evidently, both strongly acidic and strongly alkaline environments are not favorable for LFX degradation. Firstly, the electron transfer rate from the Cu-BC surface to PDS is relatively low in a strongly acidic environment, and the generation of ROSs is restricted. Secondly, the SO₄•^- may react with H₂O or OH^- to generate ·OH with low activity and short lifetime, leading to a decrease in degradation efficiency in a strongly alkaline environment.

3.4.3. Effect of PDS dosage on LFX degradation

PDS is a precursor of oxidizing active substances, and its dosage is a key influencing factor in the LFX degradation process. When the dosage of the PDS increases from 5 to 10 mM, the LFX degradation removal gradually increases (56.92%-94.33%) (Figure S3b). The active sites on Cu-BC could rapidly activate low-concentration PDS to generate ROSs for LFX degradation. However, the generated limited number of ROSs is not enough for LFX degradation. When the dosage of PDS is further increased to 15 mM, the degradation removal of LFX is 99.87%. The increase in PDS concentration enables the Cu-BC to activate PDS to produce more ROSs for LFX degradation. When the dosage of PDS increases from 15 to 25 mM, the LEX degradation removal decreases. It can be explained that PDS would compete for the active sites of Cu-BC, and the excessive PDS would also consume SO₄•^- and ·OH, resulting in the decrease of LEX degradation removal.

3.4.4. Effect of Co-existing anions and cations

Figure S3(c) shows the influence of Na⁺, Ca²⁺, Cl^-, SO₄^2-, and HCO₃^- on LFX degradation in the Cu-BC/PDS system. The influence of Na⁺, Ca²⁺, Cl^-, and SO₄^2- on LFX degradation is not obvious. However, the existence of HCO₃^- significantly inhibits the LFX degradation. It can be explained that the pH of the Cu-BC/PDS system increases after the addition of HCO₃^-. Simultaneously, HCO₃^- can react with some ROSs in the Cu-BC/PDS system. The quenching reaction occurs, which suppresses the LFX degradation. The above analysis indicates that the Cu-BC can be used for activation of PDS for LFX degradation in the actual wastewater.

3.4.5. Stability of Cu-BC/PDS system

The recycling performance of the Cu-BC/PDS system for LFX degradation is shown in Figure S3(d). As Figure S3(d) shows, the ability of Cu-BC to activate PDS for LFX degradation decreases as cycle time increases. The reason can be explained that the LFX degradation products remaining on the surface of Cu-BC will impede the interaction between ROSs and LFX molecules, competing for the active sites. However, LFX degradation removal is still larger than 80% after four cycles, indicating that the Cu-BC/PDS system has strong recycling stability for LFX degradation.

3.4.6. PDS activation degradation mechanism analysis

The quenching tests are carried out by employing quenching agents to identify the main active species in the Cu-BC/PDS system. Methanol (MeOH), tert-butanol (TBA), furfuryl alcohol (FFA), and p-benzoquinone (BQ) are employed to detect the ROSs of the SO₄•^-, ·OH, ¹O₂, and O₂^•-, respectively. As shown in Figure 4(b), when TBA and BQ are added to the Cu-BC/PDS system, no significant effect on the degradation of LFX is observed. This result indicates that •OH and O₂^•- aren’t the main ROSs for LFX degradation in the Cu-BC/PDS system. However, when MeOH and FFA are added to the Cu-BC/PDS system, the degradation and removal of LFX are significantly hindered. This result demonstrates that SO₄•^- and ¹O₂ are the main ROSs for LFX degradation in the Cu-BC/PDS system.

Based on the above analysis, a possible LFX degradation mechanism was proposed. Cu-BC contacts with PDS and subsequently activates the PDS to generate ROSs for LFX degradation. The Cu⁺ is converted into Cu²⁺ in the activation PDS process. Besides, the electrons of the LFX and Cu-BC layer can convert Cu²⁺ into Cu⁺. This result indicates that the interconversion occurs between Cu²⁺ and Cu⁺, and the electron transfer expedites the LFX degradation during the transformation process. The surface of the Cu-BC with rich electronic can supply electrons for activation of PDS. Owing to the interconversion of Cu²⁺/Cu⁺, the transferred electrons can be effectively transferred and activate PDS via the carbon network structure and Cu on BC. Subsequently, the O-O bond of PDS is broken, and a variety of ROSs are generated to attack LFX after a series of reactions, realizing LFX removal.

The spectra of Cu 2p of Cu-BC before and after use have been shown in Figures 4(c) and (d). The peaks at 933.99 and 953.43 eV could correspond to the Cu(0)/Cu⁺ [32]. The existence of Cu²⁺ is proved by the strong satellite peaks at 962.63 eV and 943.97 eV [33]. However, the peak location and peak area of the Cu(0)/Cu⁺ have changed after use. Besides, the corresponded peak area Cu(0)/Cu⁺ increases from 39.95% to 50.11% after activation PDS reaction. While the Cu²⁺ species correspond to the peaks at 954.89 eV and 934.05 eV. Meanwhile, the binding energy of Cu²⁺ also changes after the reaction. Besides, the corresponded peak area decreases from 60.05% to 48.89% after activation PDS reaction. The above results demonstrate that Cu²⁺ in Cu-BC is reduced to Cu⁺ after activation of the PDS reaction for degradation of LFX. This result also indicates that Cu(I)/Cu(II) cycling occurs on Cu-BC. It can provide many e^- to generate ROSs for the degradation of LFX during valence interconversion.

3.6. Photocatalytic mechanism analysis

The active substances in LFX degradation process were analyzed using the active substance quencher experiment. Methanol (MeOH), isopropanol (IPA), and p-benzoquinone (BQ) are used as scavengers for holes (h⁺), hydroxyl radicals (•OH), and superoxide radicals (•O₂^–), respectively. The experimental results of the active substance quencher experiment are shown in Figure S4(c). The experiment results reveal that the LFX degradation removal decreases from 99.10% to 50.42% and 41.79% after the addition of MeOH and IPA during the degradation process, respectively. This result indicates that the h⁺ and •OH play a vital role in the LFX degradation process. However, when BQ is added into the LFX degradation, the LFX degradation removal decreases from 99.10% to 62.37%, indicating that the •O₂^– involves the LFX degradation process. However, the •O₂^– is not the key reactive species for LFX degradation.

As Figure 5(b) shows, the Cu₂O on Cu-BC can be photoexcited, which generates a huge number of holes and electrons under visible light irradiation. Subsequently, the hole and electron are separated to generate the photo-generated electrons (e^-) and photo-generated holes (h⁺) (Figure 5b). The active species of the •O2- is produced by the reaction of the e^- and adsorbed O₂ on Cu-BC. While the reaction of h⁺ and adsorbed H₂O produces the •OH. Firstly, LFX is adsorbed on Cu-BC, owing to the developed pore structure in the dark condition, and LFX is accumulated onto Cu-BC for LFX photocatalytic degradation in the photocatalytic process. The generated photodegradation reactive species, such as h⁺, •OH, and •O₂^-, involve the LFX removal, which finally degrades into CO₂ and H₂O.