Study on the performance of sewage sludge biochar modified by nZVI to remove Cu(II) and Cr(VI) in water

With the vigorous development of industry, various pollution problems emerged one after another. Among them, heavy metals played an important role in many fields, but this also made heavy metal pollution increasingly serious (Xiang et al. 2021). The most widely used and polluted heavy metals were copper and chromium, which were often highly toxic, bioaccumulative, and difficult to be biodegraded. Improper disposal will cause serious threats to the biological environment (Niu et al. 2021), such as damage to human nerves and kidneys (Wang et al. 2022). Therefore, the removal of copper and chromium ions from the environment was significant for governing the global environment. Now commonly used heavy metal treatment methods include membrane filtration, ion exchange, precipitation, and adsorption (Almasian et al. 2018; Mandal et al. 2021). Among them, adsorption was widely used because of its high efficiency and low price (Zamora-Ledezma et al. 2021).

In recent years, biochar has been regarded as excellent heavy metal adsorption material due to its developed pores, surface functional groups galore, and dirt-cheap cost (Yenisoy-Karakaş et al. 2004). Straw, pineapple leaves, feces, graphene, and carbon nanotubes were all widely used to prepare biochar to remove heavy metals (Li et al. 2019b). However, with plants that can be used as high value-added products such as feed and fertilizer, calcination into biochar is not the most valuable way. Graphene and carbon nanotubes are limited to large scale use due to their high price. Therefore, there is a need to find a carbon rich waste biomass to prepare biochar to achieve the best use of resources. With the increase in population and domestic water consumption in recent years, the output of sewage sludge has also increased sharply. About more than 6 million tons of dry sewage sludge were generated in China in 2013 (Yang et al. 2015). Not only that, the sludge contained many pathogenic bacteria, eggs, and toxic and harmful substances (Tunçal et al. 2015; Yu et al. 2022), which lead to the inability of the sludge to be fully utilized as resources. Therefore, the disposal of municipal sludge has received extensive attention. Recently, municipal sludge has been considered as a good premise for the preparation of biochar for heavy metal adsorption due to its complex pore structure, richness in functional groups, small amount of magnetic metals and high mineral concentration (Li et al. 2019a; Zhao et al. 2019). Besides, as a result of the complex pore structure of biochar, it can also be used as a dispersant for nanomaterials (Ning et al. 2022).

However, in general, the removal ability of raw biochar to heavy metals was still limited. Therefore, the modification of raw biochar has become an important way to upgrade the adsorption ability of biochar (Li et al. 2021). For example, nano-iron oxide (FeO_x)-modified carbon nanotube materials can adsorb Sb(III) up to 172 mg/g, which was about 43 times that of pristine carbon nanotube materials (Cheng et al. 2022). In addition, conventional adsorbent materials almost always use their own physical properties for adsorption, but this often leads to unstable adsorption and heavy metals may be re-released into the environment. Moreover, for example, Cr(VI), a heavy metal that is toxic in the high valence state and less toxic in the low valence state, simple adsorption cannot solve the toxicity problem, so it is more effective to reduce it and adsorb it for immobilization on the adsorbent surface. Recently, nano zero-valent-iron (nZVI) has been extensively researched in the reduction of Cr(VI), Pb(II), and Cu(II) in view of its large specific surface area and strong reducing ability (Liang et al. 2021). Unfortunately, nZVI was prone to aggregation and oxidation during the preparation and loading process. The iron oxide film produced after oxidation not only reduced the activity of nZVI, but also resulted in the inability of nZVI to encounter with heavy metals in water thus leading to a decrease in removal performance, which greatly limited its use (Zhao et al. 2016). Therefore, combining nZVI with a base material with dispersing ability was one of the important ways to break through the bottleneck of its use. In addition, there were often a variety of heavy metals in the environment, which caused them to compete for adsorption during removal by adsorbent materials, resulting in poor adsorption. Therefore, it is important to investigate the competitive relationship between heavy metals for the application of heavy metals in complex water bodies.

Consequently, in this work, municipal sludge was used to prepare sludge biochar and nZVI-loaded sludge biochar was prepared by modifying the sludge biochar by liquid phase reduction. More importantly, two kinds of biochar were successfully used for the removal of single Cu(II) and Cr(VI), as well as mixed Cu(II) and Cr(VI) from water. The main research of this paper is as follows: (1) Unmodified sludge biochar (BC700) and nZVI supported sludge biochar (nBC700) were prepared and characterized; (2) The adsorption capacities of BC700 and nBC700 for Cu(II) and Cr(VI) under different initial pH and reaction time conditions were also investigated; (3) The kinetics and isotherms of the modified biochar and unmodified biochar in the adsorption process were explored; (4) The competition between Cr(VI) and Cu(II) in the adsorption process was investigated; (5) The adsorption mechanism of the modified biochar was analyzed in detail by X-ray diffraction (XRD), Fourier transform-infrared (FT-IR), and X-ray photoelectron spectroscopy (XPS).

Copper ion standard solution (Cu(II), for AAS) and chromium ion standard solution (Cr(VI), for AAS) were purchased from Sigma-Aldrich (Shanghai, China). Ferric chloride hexahydrate (FeCl₃·6H₂O), cetyltrimethylammonium bromide (CTAB), hydrochloric acid (HCl), sodium hydroxide (NaOH), potassium borohydride (KBH₄), and absolute ethanol (C₂H₅OH) were bought from China Sinopharm Group. The above drugs are of analytical grade and do not require further purification. All experimental water was obtained from ultrapure water prepared by the Milli-Q system. In this experiment, dewatered sludge (from Wunan Wastewater Treatment Plant in Wujin District, Jiangsu Province, China) was used as the biochar precursor.

First, place the sludge in a drying oven at 105 °C until the sludge weight remains unchanged. After cooling, the sludge passed through a 100-mesh sieve. Then, the obtained sludge powder was placed in a tube furnace and heated up to 700 °C at a rate of 10 °C/min under a nitrogen atmosphere and held for 2 h. The sludge biochar was obtained after the tube furnace was cooled (BC700). In this study, liquid-phase reduction method was used to prepare sludge biochar loaded by nZVI. In brief, liquid A was prepared by adding 2 g BC700 to 50 mL of ethanol; liquid B was prepared by adding 20.75 g FeCl₃·6H₂O and 2 g cetyltrimethylammonium bromide (CTAB) to 50 mL water. Add the two solutions of A and B to the three-necked flask and stir for 30 min, then pass N₂ into the three-necked flask to prevent ZVI oxidation, add 150 mL of 0.6 mol/L KBH₄ dropwise from the other port, keep stirring at 25 °C for 6 h, and leave to age for 3 d. The above mixed solution was centrifuged. The solid obtained by centrifugation was washed five times with 200 mL ethanol, and finally freeze-dried to obtain nZVI modified biochar (nBC700).

To investigate the adsorption performance and mechanism of BC700 and nBC700 on Cu(II) and Cr(VI), biochar adsorption experiments on Cu(II) or Cr(VI) alone were conducted. All experiments were performed in 50 mL brown vials placed in a constant temperature shaker (25 °C). To explore the effect of pH on the adsorption process, 15 mg BC700 or nBC700 was added to solutions of Cu(II) or Cr(VI) (30 mL, 200 mg/L) with pH between 2 and 6 (8) to initiate adsorption experiments. To probe the relationship between adsorption time and adsorption capacity, 24-h adsorption experiments of BC700 and nBC700 on Cu or Cr were carried out at pH = 4. Afterward, an aliquot of the supernatant was taken at the given time interval and filtered through a 0.45 μm membrane. The obtained filtrate was assayed for the remaining heavy metals using atomic absorption spectrophotometer (AA300).

The competitive adsorption behavior of Cu(II) and Cr(VI) on BC700 or nBC700 was also investigated when Cu(II) and Cr(VI) were mixed (30 mL). In brief, the effect of changing Cr(VI) concentration on Cu(II) adsorption by BC700 or nBC700 was examined by fixing the concentration of Cu(II) (200 mg/L) and changing the concentration of Cr(VI) (0, 50, 100, 200 mg/L), and vice versa.

The characterization of biochar and experimental analysis methods are detailed in supplementary material document (Text S1 and Table S1).

The FT-IR spectra of BC700 and nBC700 were depicted in Figure 1(g). The peaks of BC700 and nBC700 around 3,422, 1,458, 1,105, 1,039 cm⁻¹ were corresponding to the vibration of -OH in biochar, aromatic C-H, C-O, and Si-O-Si, respectively (Wan et al. 2021). The peaks near 2,922 cm⁻¹, 2,853 cm⁻¹ and 1,630 cm⁻¹ were related to the C-H vibration of alkanes, C = O or C = C on biochar (Meng et al. 2022), which indicated that the unmodified sludge biochar was rich in oxygen-containing functional groups; the peak around 875 cm⁻¹ and 778 cm⁻¹ was the carbonate and C-H vibration in the aromatic ring in the sludge (Dong et al. 2013). The absorption peaks of BC700 near 565 cm⁻¹ and 420 cm⁻¹ were the vibrations of Fe-O and Fe-OH (Xu et al. 2021), which indicated that the sludge biochar had trace Fe elements, which was consistent with the performance of SEM-EDX. In addition, the stretching vibration of Fe-O and Fe-OH on nBC700 was stronger, proclaiming that iron oxide films were produced at the surface of nZVI during loading and sample preservation, which may hinder the contact of nZVI with heavy metal ions, thus weakening the adsorption capacity of nBC700 (Bae et al. 2018). The absorption peak around 462 cm⁻¹ was caused by O-Si-O bending vibration (Shi et al. 2018). Infrared analysis showed that the loading of nZVI did not change the type and strength of the original functional groups, the surface of the modified biochar was still rich in oxygen-containing functional groups, which provided possible action sites for the removal of heavy metals (Wu et al. 2019).

XRD characterized the crystallization of biochar surface before and after nano zero-valent-iron loading. Figure 1(k) evinced that the main components in BC700 were SiO₂ (PDF-#99-0088), CaCO₃ (PDF-#05-0586), and a small number of Fe₂O₃ (PDF-#40-1139). The main components in nBC700 were similar to BC700, but the absorption peaks of Fe⁰ (PDF-#50-1275) at 42.99°, 44.70°, and 56.6° were additionally found in nBC700. In addition, the SiO₂ diffraction peak at 26.64° degrees was reduced obviously, which may be because of the destruction of some of the original SiO₂ crystal structures because of the loading of nZVI particles. The above showed the nZVI was stuck on the sludge biochar successfully, which was consistent with the performance observed by SEM-EDX.

With a gradual increase in pH from 2 to 4, the adsorption capacities of BC700 and nBC700 for Cu(II) also increased gradually. However, under this condition, the removal efficiency of Cu was not satisfactory, and the adsorption capacities were all below 200 mg/g, which may be because H⁺ contested with Cu(II) for the active sites on the biochar under low pH conditions, resulting in a poor adsorption effect of Cu(II) on biochar. In addition, when the pH was less than 4, since the zeta potential of nBC700 was positive, the electrostatic repulsion also hindered the contact between Cu²⁺ and the biochar surface. When pH = 4, BC700, and nBC700 exhibited the largest adsorption capacities for Cu(II), which were 72.82 and 221.01 mg/g, respectively. When pH was greater than 4, the adsorption capacity of Cu(II) decreases slightly, which may be due to that the oxyhydroxides of Fe(II), Fe(III), and Cu(II) increased and deposited on the surfaces of BC700 and nBC700 at the same time as the pH increases, hindering the progress of reduction and complexation reactions (Zahedifar et al. 2021). In addition, it can be seen that nBC700 had a higher adsorption capacity than BC700 even at pH = 2, which may be due to the adsorption or surface complexation of Cu(II) by the existence of Fe-O and Fe-OH (Mandal et al. 2020). What's more, after the pH was greater than 4.3, the surface zeta potential of nBC700 was negative, which happened to form an electrostatic attraction with Cu²⁺, which was more favorable to the contact between Cu(II) and nBC700 surface and thus enhanced the removal of Cu(II). The above results showed that the removal of Cu(II) was more favorable at a slightly higher pH, and the prepared nBC700 can greatly improve the removal of Cu(II).

The pseudo-first-order and pseudo-second-order kinetic models simulated the adsorption data of BC700 and nBC700, respectively (Figure 3(b)). For Cr(VI) and Cu(II), whether BC700 or nBC700, the correlation coefficients of the pseudo-second-order model were greater than those of the pseudo-first-order models (Table S2). This indicated the adsorption way of Cr(VI) and Cu(II) by sludge biochar was mainly chemical adsorption, and it generated surface complexation reaction with certain adsorption sites, rather than pure physical adsorption (Wan et al. 2019).

In addition, as shown in Figure 3(c), the intraparticle diffusion model described the adsorption process. By plotting adsorption capacity and the square root of adsorption time, a three-stage curve was obtained, which represented the three steps of heavy metal ion adsorption: S1: surface diffusion, S2: intraparticle diffusion, S3: equilibrium adsorption stage. The relevant fit data were given in Table S3. After modification, nBC700 significantly improved the adsorption capacity in both S1 and S2 stages, which meant that the loading of nano zero-valent-iron effectively changed the original surface structure of biochar and also helped the spread of heavy metal ions on the surface of biochar (Lyu et al. 2018). The rate of the S1 stage was the largest, followed by S2, and the rate of S3 was the lowest (Figure 3(c)). In the S1 stage, the boundary diffusion rates of Cr(VI) and Cu(II) on nBC700 were much higher than those on BC700, which partly explained the higher removal efficiency of nBC700 than BC700 (Cai et al. 2021). This result was consistent with the kinetic results (Wang et al. 2021). The straight line obtained by the linear fitting of the three stages did not intersect the coordinate origin, which indicated intraparticle diffusion was not the step holding biochar adsorption absolutely but was determined by both surface diffusion and intraparticle diffusion processes (Yi et al. 2020).

The Sips model also described the isothermal adsorption of heavy metals by biochar (Figure 4). From the perspective of R² (Table S5), the Sips model fitted better than the Langmuir isotherm adsorption model, which indicated Cu(II) and Cr(VI) formed a single-layer covering on the heterogeneous BC700 and nBC700 surfaces with uneven energy in the process of adsorption (Qu et al. 2021).

In Figure 5(b), when there was only Cu(II), the adsorption capacities of BC700 and nBC700 for Cu(II) were 73.42 and 220.88 mg/g; when Cr(VI) with the concentration of 50 mg/L was added and the concentration of Cu(II) was fixed, the adsorption capacities of BC700 and nBC700 for Cu(II) decreased to 26.33 and 96.91 mg/g, which were significantly lower than those of single heavy metal. This indicated that there was competitiveness between Cu(II) and Cr(VI) and the presence of Cr(VI) will significantly affect the adsorption of biochar on Cu(II). While the concentration of Cr(VI) continued to increase, the adsorption capacities of BC700 and nBC700 for Cu(II) continued to decrease gradually. The reason for the above results may be that after the introduction of high concentration Cu(II), Cu(II) occupied some active sites and formed cation bridges, which made more Cr(VI) contact nZVI or functional groups through electrostatic attraction, thus triggering a strong reduction or complexation reaction (Qiu et al. 2020).

Cycling experiments were used to demonstrate the stability of the material (Fig. S2 (a)). The experimental results showed that the adsorption capacity of nBC700 decreased from 158.01 mg/g to 99.32 mg/g for Cr(VI) and from 221.01 mg/g to 160.57 mg/g for Cu(II) after five cycles of experiments. The above results showed that nBC700 was stable for the removal of Cu(II) and Cr(VI), and still maintained high adsorption capacities after five cycles of experiments. To investigate the risk of leaching of heavy metal ions from the sludge biochar itself in the environment during the use of the nBC700, we burned the raw sludge and tested the remaining ash from the burn with an X-ray fluorescence spectrometer to investigate the composition and content of the sludge. A total of 21 oxides were detected (Table S6). Referring to the discharge standard of water pollutants for chemical industry (DB32/939-2020) and discharge standard of water pollutants for iron and steel industry (GB13456-2012), in which Cr, Cu and Ni were classified as heavy metals. BC700 was impregnated at pH = 4 and tested for the above three heavy metals. The results showed that no leaching of the above three heavy metals was detected. In addition, we examined whether nBC700 leached Fe under the condition of adsorption of Cr(VI) (Fig. S2 (b)). The results showed that the maximum concentration of Fe ion leaching was only 1.151 mg/L; this amount was in accordance with the discharge standard of water pollutants for iron and steel industry (GB13456-2012) of less than 10 mg/L. This also demonstrated that nZVI was almost not separated from the sludge biochar but was directly involved in the reduction and adsorption of Cr(VI).

XRD was also used to understand the changes in biochar crystallization before and after the reaction (Figure 1(b)). The diffraction peaks of CuFe₂O₄ (PDF-#25-0283), CuO (PDF-#45-0937), and C₄H₆CuO₄ (PDF-#46-0859) were found after the adsorption of Cu(II) on modified biochar (nBC700-Cu). The peaks indicated that nBC700 successfully complexed Cu(II) on the surface of biochar. These results were consistent with the results of FT-IR (Figure 1(a)). In addition, the produced Cu₂O (PDF-#05-0667) indicated that a reduction reaction also occurred on the biochar and successfully converted some Cu(II) into Cu(I), which also made Cu²⁺ more stable. After the modified biochar adsorbed Cr(VI) (nBC700-Cr), the diffraction peak of FeCr₂O₄ (PDF-#34-0140) was found (Li et al. 2018), indicating that nBC700 successfully complexed Cr(VI) on its surface.

The FT-IR spectra of the fresh and used modified biochar were shown in Figure 1(a). After nBC700 adsorbed Cu(II) (nBC700-Cu), the peaks of O-Si-O, Fe-O, C-H bond of the aromatic ring, carbonate, Si-O-Si, and C-O functional groups at 460, 565, 778, 875, 1,039 and 1,458 cm⁻¹, respectively, decreased significantly, indicating the above functional groups played a part in the process of adsorption of Cu(II). Moreover, the absorption peak diverted from 1,630 cm⁻¹ to 1,625 cm⁻¹ after Cu(II) was adsorbed, indicating that C = O or C = C worked during adsorption. The sharp peak at 1,384 cm⁻¹ corresponded to C-H after carboxyl complex Cu(II) (Zhou et al. 2019). The above studies showed that the abundant functional groups generated on sludge biochar played a role in the removal of Cu(II) (Jin et al. 2021).

Furthermore, the peak at 1,630 cm⁻¹ offset slightly to the right after adsorption of Cr(VI) by nBC700 (nBC700-Cr), which may be because C = O or C = C had a complexation reaction with Cr(VI) or Cr(III). The peak at 420 cm⁻¹ became weaker, which indicated that Fe-OH was complexed with Cr. Compared with the change in functional groups caused by the adsorption of Cu(II), the change in those of Cr(VI) was not obvious, which may be because the removal of Cr(VI) mainly rested with the reduction rather than the complexation.

The C1s spectra before and after modification are shown in Fig. S3. The presence of C-O and C = O in BC700 before modification indicated that the sludge biochar was rich in oxygen-containing functional groups. The binding energy of C-O and C = O did not change significantly after the modification, which indicated that the functional groups hardly reacted during the loading process. There was a slight increase in the relative area of C-O after loading, while the relative area of C = O decreased by a small amount. This may be because during the loading of nZVI, C = O was involved in the formation of iron oxide films from nZVI and was converted to C-O. In general, nBC700 still retained the oxygen-containing functional groups of BC700. The above results were consistent with the performance of FT-IR results.

The fine scan spectra of Fe 2p of BC700, nBC700, and nBC700 adsorbed Cu(II) and Cr(VI) (nBC700-Cu and nBC700-Cr) were shown in Figure 6(b). Compared with BC700, a Fe⁰ signal at 706.5 eV was found in the nBC700, which meant that nZVI was successfully loaded on BC700. This result accorded with the results of XRD. After loading nZVI, the peak area ratios of Fe(II) and Fe(III) changed from 55.6% and 44.3% to 61.53% and 37.83%, respectively. These changes may be due to the formation of oxides of FeOOH, Fe₂O₃, and Fe₃O₄ on the shell of the nZVI after the introduction, and the decrease in the percentage of Fe(III) may be due to the increase in the total iron content, the relative fraction of Fe(III) decrease.

As shown in the Cr 2p fine spectrum of nBC700 adsorbing Cr(VI) (nBC700-Cr) (Figure 6(c)), after nBC700 adsorbed Cr(VI), there was not only Cr(VI) but also Cr(III) on the surface of biochar. This indicated that Cr(VI) had a reduction reaction on biochar, which was consistent with the FT-IR and the results of previous studies (Wang et al. 2020). At the same time, by scanning the Fe 2p signal of nBC700-Cr (Figure 6(b)), it was found that, compared with the unreacted nBC700, the content of Fe(II) reduced from 61.53% to 45.3%, while that of Fe (III) rose from 37.8% to 54.5%, and the signal peak of Fe⁰ disappeared obviously, indicating that while Cr(VI) was reduced, Fe(II) and Fe⁰ participated in electron transfer at the same time, and finally transform into Fe(III).

As shown in the Cu 2p fine spectrum of nBC700 adsorbing Cu(II) (nBC700-Cu) (Figure 6(d)), a small amount of Cu(I) also appeared on the surface of biochar after nBC700 adsorbed Cu(II), which indicated that there was also a reduction reaction on the surface of biochar during adsorbing Cu(II). This result was consistent with XRD characterization (Figure 1(b)). By scanning the Fe 2p signal of nBC700-Cu, it is found that the proportion of Fe(II) was reduced from 61.53% to 58.03%, the content of Fe(III) changed from 37.83% to 41.9%, and the peak of Fe⁰ also disappeared obviously, which indicating the reduction of Cu(II) resulted from the oxidation of Fe⁰ and Fe(II). However, it was obvious that there still was Cu(II) on the surface of nBC700 after adsorbing Cu(II) and the peak area of it was far larger than that of Cu(I), which manifested that the adsorption of Cu(II) was not reined by redox but the complexation precipitation of functional groups. The above results were consistent with the FT-IR.

To sum up, the mechanism of nBC700 removing Cr(VI) was divided into: (1) Surface adsorption of biochar to Cr(VI) through electrostatic attraction; (2) Reduction of Cr(VI) by the redox of Fe⁰ and Fe(II) (Equations (1)–(4)); (3) Complexation precipitation of Cr(VI) by functional groups on the surface of biochar (Equations (5) and (6)). The reduction of Cr(VI) via the redox of Fe⁰ and Fe(II) was the primal way for Cr(VI) removal.

In conclusion, we prepared sludge biochar using municipal sludge, and prepared sludge biochar loaded nZVI by liquid-phase reduction method using biochar as a dispersant. The heavy metal removal experiments showed that nBC700 was better than BC700 in the aspect of removing Cu(II) and Cr(VI), and the maximum adsorption capacities for Cu(II) and Cr(VI) reached 215.40 and 177.10 mg/g, respectively. FT-IR, XRD, and XPS analysis demonstrated removing Cu(II) mainly depended on the complexation of functional groups on biochar, followed by the reduction by Fe⁰. The way of removing Cr(VI) mainly depended on the reduction reaction of Fe⁰, followed by the reaction with biochar functional groups. The heavy metal interaction experiment showed that Cr(VI) and Cu(II) competed for active sites, but Cr(VI) was more easily absorbed by biochar than Cu(II) due to the cation bridging effect caused by the adsorption of Cu(II). The results of this research may provide a possible method for the utilization of municipal sludge and the removal of Cu(II) and Cr(VI).

This work was supported by the National Major Project of Science & Technology Ministry of China (No. 2008ZX07421-002), the International Scientific and Technological Cooperation in Changzhou (No. CZ20140017), and the Scientific Research Foundation of Jiangsu Provincial Education Department, China (No. 21KJB610007). The authors would like to thank the analysis and testing center of Changzhou University for the FT-IR and XRD tests.

The authors report there are no competing interests to declare.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Xudong Kang: Conceptualization, methodology, formal analysis, writing-original draft. Feiyu Xiao: Data curation, visualization. Sihai Zhou: Investigation, visualization. Qiuya Zhang: Visualization, formal analysis. Liwei Qiu: Resources. Liping Wang: Resources, writing-review and editing, supervision, project administration.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.