Introduction

Recently, the energy is very limited, but the demand for it is on the increase (Li et al. 2021a; Wang et al. 2021a). As a low-carbon and great-density energy, nuclear energy is becoming increasingly important, which can fully meet the energy demand in the contemporary world and lessen environmental destruction (Willauer et al. 2015). With the booming advancement of nuclear energy, uranium (U(VI)) requirements are also on the rise globally (Yan et al. 2011; Lin et al. 2022). While U(VI) plays a vital role in the nuclear energy, it is baleful to human health and can cause varied environmental problems as a result of strong toxicity and radioactivity (El-Din et al. 2018; Singh et al. 2021). Thereby, the U(VI) recovery in wastewater is great importance to environmental protection, and it is also very essential to seek novel adsorbents to efficiently recover U(VI) in wastewater.

Nowadays, numbers of technologies were employed in recovering U(VI) ions out of wastewater (Khedr 2015; Li et al. 2017a, 2020a; Tolkou et al. 2020; Bi et al. 2021; Liu et al. 2021; Chen et al. 2022; Xue et al. 2022). Among the many existing technologies, the adsorption has been acknowledged as a very efficient means for convenient operation, security, practicability, and ecological protection (Anirudhan and Jalajamony 2013; Arica and Bayramoglu 2016). Undoubtedly, the key to adsorption method is to exploit new and effectual adsorbents to ameliorate adsorption performance (Bi et al. 2022). U(VI) adsorbents of sorts such as organic polymers, inorganic materials, biological materials, covalent organic frameworks (COFs), and MOFs were utilized in wastewater (Yang et al. 2019; Ma et al. 2020; Li et al. 2020b, 2021b; Liang et al. 2021). Among them, MOFs attracted widespread interest for its glorious framework stability, specific surface area, and regular porousness (Carboni et al. 2013; Wang et al. 2015; Li et al. 2017b). Quite a few scholars have designed and synthesized various MOF-based adsorbents with magnificent U(VI) adsorption performances from wastewater (Li et al. 2018; Zhang et al. 2019a; Wang et al. 2021b). However, many microorganisms in wastewater can exert significant detrimental impacts upon adsorption capacity for MOF-based adsorbents (Park et al. 2016). The reason for the greatly reduced adsorption performance is that microorganisms can be attached to the adsorbent’s surface and multiply in its internal pores, thus blocking the pores and covering the active sites of U(VI) adsorption (Mei et al. 2022; Wang et al. 2022). Consequently, it is very necessary to devise an innovative adsorbent with conspicuous adsorption capacity and antimicrobial activity for authentic wastewater treatment.

A Zr-based MOF with the name of PCN-222 was thought to be an optimistical MOF to design enormously efficacious U(VI) adsorbents because of subsequent superiorities (Feng et al. 2012). Firstly, the synthesis procedure of PCN-222 is uncomplicated and its preparation process is non-polluting to the environment (Liu et al. 2020a). Secondly, PCN-222 can be easily modified by the introduction of solvent-assisted ligands (Li et al. 2019). Thirdly, in comparison with other MOFs, PCN-222 has more lasting biggish channels, preferred metal combining competence and acid–base-resisting peculiarities, which can exhibit a good U(VI) adsorption performance in a broad range of pH values (Yang et al. 2021). Fourthly, PCN-222 possessed a certain antimicrobial activity at the presence of Zr6(μ3-O)4(μ3-OH)4(H2O)4(OH)4 clusters, reducing the damaging impact of microorganism on U(VI) adsorption performance (Aguado et al. 2014; Zheng et al. 2020). Thus, PCN-222 is a promising MOF with regard to high-efficiency U(VI) adsorbent exploitation from wastewater. Moreover, highly efficient chelating groups are also important as to the construction of efficient U(VI) adsorbents (Bai et al. 2020a; Zeng et al. 2021; Liu et al. 2022). It is well-known that the amidoxime group has excellent U(VI) selectivity and has been widely applied to adsorption, but the synthesis of amidoxime groups required highly poisonous cyanide, incurring significant risks for environment and human health (Paz et al. 2010; Kandre et al. 2013; Rahman et al. 2016). In comparison with amidoxime groups, the oxime-group raw materials are non-toxic and harmless (Damljanovic et al. 2006). More importantly, the oxime group also has exhibited outstanding selective adsorption performances to U(VI) ions (Tian et al. 2011).

In view of this, a novel adsorbent (PCN-222-OM) with outstanding U(VI) adsorption capacity and antimicrobial activity was synthesized through PCN-222 oxime functionalization founded on the directed molecular structure design to recover U(VI) from wastewater for the environmental protection and pollution control. PCN-222-OM with plenteous oxime groups possessed glorious U(VI) adsorption capacity and selectivity under weak acid condition, which also had high selective U(VI) removal rate, and the residual U(VI) concentration after adsorption could meet discharge standards. The potential adsorption mechanism was considered to be a comprehensive consequence of coordination and electrostatic interaction; the N–H and oxime groups fulfilled a momentous function in adsorption process of PCN-222-OM. Simultaneously, PCN-222-OM also possessed outstanding antimicrobial activities against E. coli and Staphylococcus aureus, which effectively reduced negative impacts upon the adsorption performances. All in all, PCN-222-OM was an auspicious adsorbent to recover U(VI) from wastewater for the environmental protection and pollution control, which also provides a fresh mind as for the construction of efficient adsorbent.

Experimental

PCN-222-AD and intermediate synthesization

The activated PCN-222 and PCN-222-NH2 were synthesized throughout solvothermal method based on previous literatures (He et al. 2018; Li et al. 2019). The course was documented in supporting documents (Section S3). Thereupon, PCN-222-NH2 (0.05 g) was allocated to mixed toluene solution (35 mL) containing racemic-2,2′-bis(diphenylphosphino)-1,1′-binaphthyl (BINAP) and tris(dibenzylideneacetone)dipalladium (Pd2(dba)3) under N2 condition to acquire an even mixed solution. Whereafter, 5 mL toluene containing 3-bromopropanal (0.3 g) was injected into the mixed solution mentioned above, which was warmed to 100 ℃ and reacted under N2 condition for 48 h. The suspension was under centrifugation and cleansed by toluene, methanol, and deionized water ordinally; PCN-222-AD was attained after resulting deposition dried out at 50 °C.

PCN-222-OM synthesization

Synthesized PCN-222-AD (0.5 g) had been dispersed to methanol/deionized water (1:1, v:v, 30 mL), NH2OH·HCl (2.0 g), and Na2CO3 (0.9 g) continuously added with stirring. After 24 h of refluxing at 70 °C, the suspension was separated and cleansed in methanol and deionized water ordinally. After drying up the clean deposition at 60 °C overnight, PCN-222-OM was achieved.

Batch adsorption and regeneration experiments

The influences of various factors on adsorption performances were investigated, which mainly included pH value, initial U(VI) concentration, adsorbent dosage, contact time, and temperature. Relevant detail was displayed under supporting information file (Section S4). Furthermore, the desorption and regeneration experiments were also done and the procedures are demonstrated in Section S5 of supporting information file.

Adsorption in simulated nuclear industry wastewater

The simulated nuclear industry wastewater samples with different U(VI) concentrations were prepared in accordance with a shop inspection report about nuclear fuel-element-plant wastewater, to evaluate the selective adsorption capacity of PCN-222-OM to U(VI) ions. The specific steps of adsorption experiments in simulated nuclear industry wastewater are displayed in Section S6 of the supporting information file.

Antimicrobial activity experiments

PCN-222-OM as well as intermediate antimicrobial activity about E. coli and S. aureus were measured in accordance with our group’s previous researches (Bi et al. 2023). The related experiments conducted in this study mainly included minimum inhibitory concentration (MIC), inhibition zone test, and inhibition of bacterial growth curve. The specific steps of antimicrobial activity experiments are displayed in Section S7 of the supporting information file.

Result and discussions

Synthetic process of PCN-222-OM

A schematic diagram about PCN-222-OM synthesis is exhibited in Fig. 1. First of all, Zr6(μ3-O)4(μ3-OH)4(H2O)4(OH)4 clusters within PCN-222 were modified by aminomethylphosphonic acid to obtain PCN-222-NH2 with abundant amino groups. Secondly, 3-bromopropanal was used as a modifying agent to convert amino groups into aldehyde groups and PCN-222-AD was obtained. Finally, in order to produce oxime groups, PCN-222-AD was ulteriorly converted through the oximation reaction and PCN-222-OM was triumphantly obtained.

Fig. 1
figure 1

Schematic diagram of synthetic procedures of PCN-222-OM

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Morphology

TEM images and TEM-EDS mappings for PCN-222 and PCN-222-OM are evinced in Fig. 2; PCN-222 had a tubular structure at 2.3 μm length and obvious crystal lattices at more enormous magnification. After oxime modification, PCN-222-OM was much coarser than the initial PCN-222, yet its framework structure, crystal lattice, and dimension essentially stayed unchanged. TEM-EDS mappings demonstrated the well-proportioned distribution for C, N, O, Zr, and P elements inside PCN-222-OM, and the uniformed P element distribution proved PCN-222 oximation modification was possibly successful.

Fig. 2
figure 2

a TEM image and TEM-EDS mappings of PCN-222 (O, N, Zr, C). b TEM image and TEM-EDS mappings of PCN-222-OM (C, O, N, Zr, P)

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Structures

FT-IR analysis

Figure 3a illustrated the FT-IR spectra about PCN-222-OM and its intermediates; PCN-222 characteristic peaks were stationed at 2923, 1709, 1641, 1604, 1550, 1403, 965, 800, and 720 cm−1 severally, identical to literature data (Jia et al. 2019). After PCN-222 amination modification, two fresh peaks appeared at 1148 and 992 cm−1 on account of -NH2 and P-O group stretching vibrations apart, evincing the -NH2 groups which were triumphantly transplanted on PCN-222 (Bi et al. 2023). As far as PCN-222-AD was concerned, the peak intensity of -NH2 stretching vibration at 1148 cm−1 dropped and the one of C = O groups localized at 1603 cm−1 increased, and a fresh peak formed at 1101 cm−1 because of -C-N–C group stretching vibration, betokening a total transition from -C-NH2 to -C-N–C, and PCN-222-AD was prepared (Sharma et al. 2005; Yamauchi et al. 2013). After the oximation modification, two fresh peaks arose at 3328 and 1020 cm−1 in PCN-222-OM compared to PCN-222, owing to the stretching vibration of the associating -OH and N–O in C = N–OH groups (Mei et al. 2021).

Fig. 3
figure 3

The chemical structure characterizations of PCN-222-OM and its intermediates. a FT-IR. b XRD. c XPS. d N2 adsorption–desorption isotherm

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XRD analysis

In Fig. 3b, several diffraction peaks were located at 2Theta of 4.8°, 7.1°, 8.2°, 9.2°, and 14.4° in PCN-222 XRD pattern, basically in accord with the literature data, proving the PCN-222 was successfully synthesized (Zhang et al. 2016). In comparison with PCN-222, the diffraction peak positions of PCN-222-OM and its intermediates were basically unchanged, demonstrating PCN-222 crystal structure was undamaged in multistep modifications.

XPS analysis

In order to prove that the oximation modification on PCN-222 was successful, the XPS analysis was conducted and the results are demonstrated in Fig. 3c. PCN-222 mainly comprised of C 1s, N 1s, O 1s, and Zr 3d. After oximation modification, PCN-222-OM mainly comprised of C 1s, O 1s, N 1s, P 2p, and Zr 3d. P element appearance was due to the use of aminomethylphosphonic acid modifier in the modification process. After the oxime modifications, the main introduced functional groups were C = N–OH groups, so the N 1s spectra were detailedly analyzed. As indicated in Fig. S1, three peaks in PCN-222 N 1s high-resolution spectrum originated from C = N groups (397.2 eV), C-N groups (399.4 eV), and -NH groups (401.3 eV) respectively (Yi et al. 2019; Guo et al. 2021). A novel peak at 399.3 eV after the oximation modification was the cause of newly generated C = N–OH groups (Zhu et al. 2022).

BET analysis

The evident hysteresis phenomena were observed in the adsorption/desorption isotherms of PCN-222 and PCN-222-OM (Fig. 3d), matching with archetypal type IV, demonstrating mesoporous structure came into existence in PCN-222 and PCN-222-OM (Liu et al. 2015; Bai et al. 2020b). The related parameters about adsorption/desorption isotherms are also summarized in the Table 1 and Fig. S2. Specific surface area and main pore size of PCN-222-OM were 1160 m2·g−1 and 3.5 nm apart, which were smaller than those of PCN-222 (2546 m2·g−1 and 3.7 nm) now that generated oxime groups occupied and blocked the pore space of PCN-222 (Wu et al. 2018; Amini et al. 2021). Although the introduction of oxime groups caused a fall for PCN-222 specific surface area and pore size, it was valuable for powerful adsorption.

Table 1 N2 adsorption/desorption parameters of PCN-222 and PCN-222-OM
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Uranium (VI) adsorption assay

pH effect

As seen from Fig. 4a, the pH values were highly influential in adsorption performance over PCN-222-OM and intermediates. PCN-222 adsorption capacity cumulated fleetly at pH = 2.0–4.0 and fell at pH = 4.0–10.0. PCN-222 maximum adsorption capacity was 361.4 mg·g−1. Adsorption capacities of PCN-222-NH2 and PCN-222-AD registered the same trends while pH values made changes. Their adsorption capacities increased largely at pH = 2.0–5.0 and afterwards fell throughout pH = 5.0–10.0. The maximum adsorption capacity of PCN-222-NH2 and PCN-222-AD was 314.5 and 306.1 mg·g−1 at pH = 5.0 apart. PCN-222-OM maximum adsorption capacity was 403.4 mg·g−1 at pH = 6.0, much larger than its intermediates. Moreover, the main U(VI) existent forms within pH = 1.0–14.0 were investigated. In Fig. 4b, major species in U(VI) solution were UO22+, UO2(OH)+, and (UO2)3(OH)5+ with positive charge at pH ≤ 7.0. With further increase of pH (7.0–10.0), the predominant U(VI) forms were (UO2)3(OH)7−, UO2(OH)3−, and uranyl carbonates. Simultaneously, PCN-222 and PCN-222-OM zeta potential at pH = 2.0–10.0 was tested. PCN-222 had a positive surface charge below pH = 3.4 and a negative one at higher pH values (Fig. S3). The surface of PCN-222-OM was positively charged under pH = 5.2; nevertheless, its surface charge turned negative with pH value further rise. Therefore, there appeared a forceful repulsion between protonated PCN-222-OM and positive-charge U(VI) species in low pH range, resulting in poor adsorption capacity under low pH (Zhang et al. 2005). Progressive deprotonation of the oxime groups with increasing pH values accounted for increasing adsorption capacity. Yet, PCN-222-OM adsorption capacity dwindled as pH value appreciated to larger degrees as PCN-222-OM surface charge turned positive into negative, creating a big electrostatic repulsion with electronegative U(VI) species at high pH (Li et al. 2015). Briefly, PCN-222-OM superlative adsorption pH value was 6.0 and maximum adsorption capacity was 403.4 mg·g−1.

Fig. 4
figure 4

a Effect of pH value on the U(VI) adsorption capacities of PCN-222-OM and its intermediates (T = 25 ℃, t = 6 h, m/V = 0.2 g·L−1, C0 = 100 mg·L.−1). b The distribution of U(VI) hydroxide species under different pH values (1–14)

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Adsorbent dosage effect

Considering U(VI) removal rate and cost performance, it was crucial to find the best adsorbent dosage. In our exploration, various PCN-222-OM dosages (m/V = 0.05–0.3 g·L−1) were placed inside 50 mL solution with 100 mg·L−1 original concentration. In Fig. 5, adsorption capacity was eroded with a constant increase in PCN-222-OM dosage. PCN-222-OM adsorption capacities were 596.6 and 273.2 mg·g−1 at m/V = 0.05 and 0.3 g·L−1 apart. Additionally, removal rate boosted swiftly with PCN-222-OM dosage increase and almost remained unchanged at m/V = 0.2 g·L−1. This reason for changing trends was that adsorption site quantities rose with constant increase in PCN-222-OM dosage when the m/V value was under 0.2 g·L−1. However, a big competitive effect and spatial site resistance between adsorbents caused difficulty binding PCN-222-OM and U(VI) ions together at m/V > 0.2 g·L−1, so further increasing PCN-222-OM dosage could not help to significantly improve its U(VI) removal rate (Wang and Guo 2020; Zhu et al. 2021a). Fig. S4 illustrates the influence of PCN-222 dosage upon its adsorption ability and removal rate, which had identical trend with PCN-222-OM. Therefore, m/V = 0.2 g·L−1 was nominated as adsorbent dosage during the following tests.

Fig. 5
figure 5

Effect of PCN-222-OM dosage on the U(VI) adsorption capacity and removal rate (T = 25 ℃, t = 6 h, V = 50 mL, C0 = 100 mg·L.−1, pH = 6.0)

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Adsorption isotherms

The adsorption capacities and removal rates of PCN-222 and PCN-222-OM at divergent initial U(VI) concentrations (C0) were explored. In Fig. 6, adsorption capacities increased rapidly with C0 rise. The growth rate of adsorption capacity was greater below C0 = 100 mg·L−1. Afterwards, it decreased above C0 = 100 mg·L−1, which proved adsorption processes of PCN-222 and PCN-222-OM achieved balances. Additionally, the maximum removal rates over PCN-222 and PCN-222-OM were 56.8% and 92.0% at C0 = 0.5 mg·L−1 accordingly, which progressively fell with C0 further increase. The removal rate dropped dramatically in C0 = 100–200 mg·L−1, presumably because PCN-222 and PCN-222-OM adsorption processes reached saturation (Dan et al. 2016).

Fig. 6
figure 6

Effect of initial concentration on the U(VI) adsorption capacity and removal rate of PCN-222 and PCN-222-OM (T = 25 ℃, t = 6 h, m/V = 0.2 g·L.−1, pH = 6.0)

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Langmuir and Freundlich models, illustrated as Eqs. (S3–S4), were also applied to clarify adsorption processes of PCN-222 and PCN-222-OM. Involved constants are summarized in Table 2. As for PCN-222-OM, the R2 values of Langmuir and Freundlich models were 0.991 and 0.984 at 25 ℃ singly. Langmuir model’s R2 value was bigger than Freundlich model’s, denoting monolayer adsorption brought U(VI) adsorption process under control. Furthermore, the Langmuir model (R2 = 0.995) of PCN-222 was more fit than the Freundlich model (R2 = 0.985), saying the PCN-222 adsorption process was also a monolayer adsorption (Chen et al. 2021).

Table 2 Langmuir and Freundlich isotherm parameters for U(VI) adsorption on PCN-222 and PCN-222-OM (C0 = 100 mg·L−1, T = 25 ℃, m/V = 0.2 g·L−1, pH = 6.0)
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Adsorption kinetics

With an advance of contact time, adsorption capacities for PCN-222 and PCN-222-OM considerably escalated and attained equilibrium at 360 min (Fig. 7a). The maximum adsorption capacities for PCN-222 and PCN-222-OM were 221.5 and 403.4 mg/g separately. Pseudo-first-order and pseudo-second-order models (Eqs. (S8–S9)) were put into practice to explore mechanisms. Simultaneously, correlative coefficients are also generalized in Table 3, which demonstrated pseudo-second-order model (R2PCN-222 = 0.965, R2PCN-222-OM = 0.992) more precisely abided by the data than the other model (R2PCN-222 = 0.819, R2PCN-222-OM = 0.876), proving the chemical adsorption was a controllable step in PCN-222 and PCN-222-OM adsorption processes (Shi et al. 2020).

Fig. 7
figure 7

a Effect of contact time on the U(VI) adsorption capacities of PCN-222 and PCN-222-OM. b Pseudo-first-order model fitting curve. c Pseudo-second-order model fitting curve (T = 25 ℃, C0 = 100 mg·L−1, m/V = 0.2 g·L.−1, pH = 6.0)

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Table 3 Pseudo-first-order kinetic and pseudo-second-order kinetic coefficients for U(VI) adsorption on PCN-222 and PCN-222-OM (C0 = 100 mg·L−1, m/V = 0.2 g·L−1, pH = 6.0, T = 25 ℃)
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Adsorption thermodynamics

In Fig. 8, adsorption capacities of PCN-222 and PCN-222-OM progressively aggrandized as the temperature went up, proving the high temperature was valuable for PCN-222-OM adsorption (Xie et al. 2017). Thermodynamic parameters are numerated by Eqs. (S5–S7) and recorded in Table S1. Positive ΔH and ΔS values indicated the process was endothermic and randomness increased within solid-solution interface (Liu et al. 2018). When temperatures increased, the ΔG values turned more negative, demonstrating a spontaneous process (Akkaya 2013).

Fig. 8
figure 8

Effect of adsorption temperature on U(VI) adsorption capacities of PCN-222 and PCN-222-OM (t = 6 h, C0 = 100 mg·L−1, m/V = 0.2 g·L.−1, pH = 6.0)

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Selectivity of PCN-222-OM

U(VI) selectivity of PCN-222-OM was explored in complex multiple-metal-ion environment. The primary concentrations of applied co-existing cations were all 10 mg·L−1. PCN-222-OM possessed very high U(VI) selectivity when other cations were co-existing (Fig. 9). PCN-222-OM owned still higher removal rate for U(VI) ions (80.9%) than for other cations, proving PCN-222-OM had the highest U(VI) selectivity. Meanwhile, the influences of various co-existing anions (NO3−, CO32−, PO43−, and SO42−) on U(VI) removal rate were also under investigation and results are evinced in Fig. S5. The presence of anions brought about little impact upon U(VI) removal rate for PCN-222-OM, which also reflected the excellent U(VI) selectivity of PCN-222-OM.

Fig. 9
figure 9

Effect of co-existing cations on the U(VI) removal rate of PCN-222-OM (T = 25 ℃, C single cation = 10 mg·L−1, m/V = 0.2 g·L.−1, pH = 6.0, t = 6 h)

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Recyclability of PCN-222-OM

For continuable development, the recyclability of PCN-222-OM was explored. The experimental procedures about recyclability are itemized in supplemental file. In Fig. 10, the adsorption capacity declined only marginally in the fifth cycle in comparison with the first cycle. PCN-222-OM adsorption capacity still remained 391.8 mg·g−1 in the fifth cycle, demonstrating the eminent U(VI) recyclability of PCN-222-OM. Meanwhile, XRD and TEM results (Figs. S6 and S7) revealed that PCN-222-OM-U crystalline and morphology were well maintained through adsorption, reflecting marvelous stability and recyclability of PCN-222-OM.

Fig. 10
figure 10

The recyclability of PCN-222-OM in the U(VI) adsorption (T = 25 ℃, C0 = 100 mg·L−1, m/V = 0.2 g·L.−1, pH = 6.0, t = 6 h)

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Simulated nuclear industry wastewater test

For evaluating U(VI) selectivity of PCN-222-OM, simulated nuclear industrial waste samples of different U(VI) concentrations were prepared in accordance with a report about fuel-element-plant wastewater (Table S2). As illustrated in Fig. 11, PCN-222-OM possessed excellent U(VI) adsorption performances in the simulated fuel-element-plant wastewater samples of dissimilar U(VI) concentrations (10, 50, and 500 μg/L) and removal rates accomplished 48.0%, 54.2%, and 78.6% respectively, which proved the PCN-222-OM was prospective U(VI) adsorbent in complex nuclear industry wastewater.

Fig. 11
figure 11

The U(VI) adsorption performance of PCN-222-OM from simulated nuclear industry wastewater samples with different U(VI) concentrations (10, 50, and 500 μg·L.−1)

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Adsorption mechanism

Various characterizations were used for reconnoitering PCN-222-OM adsorption mechanism. In Fig. S6, PCN-222-OM XRD diffraction pattern remained essentially unchanged through U(VI) adsorption, manifesting crystalline structure did not show any considerable change in adsorption. As described in the FT-IR spectra (Fig. S8), compared with the original PCN-222-OM, the PCN-222-OM-U showed a characteristic vibration of O = U = O at 927 cm−1, probably because of the interaction between the adsorption active sites and U(VI) ions. Furthermore, the XPS spectra and adsorption energy simulation calculations were carried out for elucidation adsorption mechanism of PCN-222-OM in depth. Figure 12a emerged the U 4f peaks arose through PCN-222-OM U(VI) adsorption, which could be farther diverged into miscellaneous peaks. Two peaks in the U 4f high-resolution spectrum for PCN-222-OM-U were ascribed to U 4f5/2 (392.9 eV) and U 4f7/2 (382.1 eV), demonstrating PCN-222-OM successfully sorbed U(VI) ions (Perry 2015). Meanwhile, four peaks manifested in N 1s high-resolution spectrum for PCN-222-OM (Fig. 12c) were C = N (H2TCPP) (397.2 eV), C-N (399.3 eV), C = N–OH (399.4 eV), and -NH (401.6 eV). After adsorption equilibrium, binding energy position for C = N–OH and -NH migrated approximately 1.6 eV and 0.3 eV apart, indicating C = N–OH and -NH groups were essential within adsorption process.

Fig. 12
figure 12

a The full-scale XPS spectra of PCN-222-OM before and after U(VI) adsorption. b U 4f spectrum of PCN-222-OM after U(VI) adsorption. (c) N 1s spectrum of PCN-222-OM before U(VI) adsorption. (d) N 1s spectrum of PCN-222-OM after U(VI) adsorption

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Meanwhile, adsorption energy calculations were carried out to explore interactions between functional groups within PCN-222-OM and U(VI) ions. Grounded on XPS analysis, C = N–OH and -NH groups were studied. In Fig. 13, their adsorption energies to U(VI) ions were − 9.527 eV and − 4.674 eV severally. In comparison with the -NH groups, the C = N–OH groups had stronger adsorption energy, proving they were dominating adsorption sites to U(VI) ions in PCN-222-OM. The -NH groups were secondary adsorption sites in the adsorption process of PCN-222-OM.

Fig. 13
figure 13

Adsorption energy calculations between U(VI) ions and multifarious functional groups in PCN-222-OM (a) C = N–OH groups and (b) -NH groups

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Antimicrobial activity appraisement

Minimum inhibitory concentration (MIC)

The antimicrobial activities for PCN-222-OM against E. coli and S. aureus were assessed via MIC tests. Bacterial suspension and solvent of the selfsame concentration were regarded as positive and negative control correspondingly. Figure 14 demonstrates that PCN-222-OM had evidence about great antimicrobial activity against E. coli and S. aureus. MIC values of PCN-222-OM against E. coli and S. aureus were 39.1 and 19.6 μg·mL−1 accordingly, evincing the outstanding antimicrobial activities of PCN-222-OM, providing assurance for efficient U(VI) extraction from complex environments.

Fig. 14
figure 14

The MIC values of PCN-222-OM and its intermediates against a E. coli and b S. aureus

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Inhibition zone test

As displayed in Fig. 15, most samples possessed significant inhibition zones against E. coli and S. aureus after a 24-h incubation. PCN-222-OM zone diameters for E. coli and S. aureus were 15.9 and 14.1 mm apart. By contrast, PCN-222-NH2 and PCN-222-AD had larger inhibiting zones against E. coli and S. aureus than PCN-222, demonstrating introduced functional groups could improve the antimicrobial activity and permeability to a certain extent. Simultaneously, PCN-222-OM possessed the largest inhibition zone diameters compared with other intermediates, demonstrating its eminent permeabilities and antimicrobial activities over E. coli and S. aureus.

Fig. 15
figure 15

The inhibition zone determinations of PCN-222-OM and its intermediates against E. coli and S. aureus. a Images of punch well agar diffusion test. (b, c) Zone of inhibition diameters against E. coli and S. aureus apart (sample 1 = PCN-222, sample 2 = PCN-222-NH2, sample 3 = PCN-222-AD, and sample 4 = PCN-222-OM)

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Inhibition of bacterial growth

PCN-222-OM inhibitions against E. coli and S. aureus multiplication were evaluated via OD600 values within different dosage concentrations (20–320 μg·mL−1). Lower OD600 values indicated lower bacterial concentrations and better antimicrobial activity. As seen from Fig. 16, there were no significant inhibitions against E. coli and S. aureus build-up while the PCN-222-OM concentration was below 20 μg·mL−1. PCN-222-OM did not produce significant inhibitions against E. coli and S. aureus multiplication until concentration range further increased to 40–160 μg·mL−1. Subsequently, they nearly stopped growing until the concentration was increased to 320 μg·mL−1. Bacteria growth curves proved that the PCN-222-OM had good antibacterial property as the concentration was high enough. Meanwhile, Fig. S9 was the SEM image of E. coli and S. aureus around PCN-222-OM treating E. coli and S. aureus. Their surfaces were partially damaged throughout PCN-222-OM treating, possibly because of the interaction between the Zr6(μ3-O)4(μ3-OH)4(H2O)4(OH)4 clusters and bacterial membrane.

Fig. 16
figure 16

The bacterial growth curves treated with PCN-222-OM against a E. coli and b S. aureus

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Environmental safety and antibiofouling activity

Environmental safety

It is very momentous for environmental safety of adsorbents in practical adsorption. The minnow viability experiments were made to appraise the environmental and biological hazards from PCN-222-OM. As illustrated in Fig. S10, all minnows kept excellent zoological activities through 48-h PCN-222-OM treatment, proving PCN-222-OM had splendid environmental safety.

Antibiofouling activity

In general, the biological contamination showed significant impacts on U(VI) adsorbent capability. Halamphora was used to assess PCN-222-OM antibiofouling activity. In Fig. S11(a), PCN-222-OM had great inhibition against Halamphora growth and cell mortality nearly achieved 81.3% a week later, possibly because of the synergistic role of Zr6(μ3-O)4(μ3-OH)4(H2O)4(OH)4 clusters and affluent groups within PCN-222-OM. Biological colonization on the adsorbent surface was also a major contributor to biological contamination. Halamphora cell viabilities are also investigated in Fig. S11(b). The minority of viable cells were noticed on PCN-222-OM surface a week later, demonstrating the PCN-222-OM could kill Halamphora cells as well as inhibit the adhesion.

Contrast about adsorption capacity and antimicrobial activity between PCN-222-OM and reported adsorbents

As illustrated in Table 4, PCN-222-OM was in possession of conspicuously finer adsorption capacity (403.4 mg·g−1) in proportion to already reported adsorbents, such as ZIF-90-AO, MOF-5, UiO-66-NH2, UiO-66-AO, and MIL-101 PCN-222-OM was a heartening U(VI) adsorbent. Meanwhile, the antimicrobial activity was also vital to sufficiently utilise the porous adsorbent performance. In Table 5, MICs of PCN-222-OM against E. coli and S. aureus were 39.1 and 19.6 μg·mL−1 severally, smaller than other adsorbents, which certified the antimicrobial activities of PCN-222-OM against E. coli and S. aureus were superior to reported absorbents. To sum up, PCN-222-OM was one golden U(VI) adsorbent due to eminent adsorption capacity and antimicrobial activity.

Table 4 Contrast of U(VI) absorption capacity between PCN-222-OM and reported adsorbents
Full size table
Table 5 Contrast about antimicrobial activity between PCN-222-OM and reported adsorbents
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Conclusion

One newfangled and efficient U(VI) adsorbent named PCN-222-OM with glorious antimicrobial activity, environmental safety, and antibiofouling activity was designed and synthesized grounded on PCN-222 directed molecular structure design. PCN-222-OM possessed outstanding U(VI) adsorption capacity at a maximum of 403.4 mg·g−1 at pH = 6.0, owing to interactions between U(VI) ions and abundant porous structures and oxime groups in PCN-222-OM. Meanwhile, PCN-222-OM also displayed splendid recyclability and selectivity in U(VI) adsorption, whose selective removal rate was 78.6% in the simulated nuclear industry wastewater with 500 μg/L U(VI) concentration. Kinetic and isothermal constants attested that PCN-222-OM adsorption process was predominantly in the charge of chemisorption and monolayer adsorption. XPS spectra and adsorption energy calculation results proved C = N–OH and -NH groups occupied key roles during the adsorption process and adsorption mechanism was considered to have synergy function about coordination and electrostatic interaction. PCN-222-OM presented eminent antimicrobial activity against E. coli and S. aureus due to the interaction between Zr6(μ3-O)4(μ3-OH)4(H2O)4(OH)4 clusters and bacterial membrane, which effectively reduced the damaging impact from bacteria upon U(VI) adsorption capacity. Meanwhile, PCN-222-OM disclosed fine environmental safety and antibiofouling activity, avoiding side impact about biological contamination on U(VI) adsorption capacity and meeting non-toxicity request in applications. In summary, PCN-222-OM became an inspiriting adsorbent in wastewater U(VI) recovery and engendered one valuable thought for effective adsorbent advancement.

The recommendations for future work

In comparison with the reported U(VI) adsorbents, the synthesized PCN-222-OM has a number of characteristic advantages in adsorption performance. Although PCN-222-OM has the advantages in large adsorption capacity, high adsorption efficiency, strong selective adsorption ability, strong resistance to biological contamination, and easy recycling, it still has a lot of room for improvement in the future work. On the one hand, in order to advance the practical engineering application of U(VI) adsorbents, the cost consumption in the preparation of U(VI) adsorbents can be further reduced in future research work. The preparation of MOF-based U(VI) adsorbents can be conducted with simpler group modification methods in future work. On the other hand, it is best to use real nuclear industrial waste samples in the possible future adsorption performance test work to make the adsorption performance results closer to the actual application.