Effects of aging processes on spent mushroom Substrate-Derived Biochar: Adsorption characteristics of Cd(II) and Cr(VI)

3.1.1 Changes in surface morphology

The feedstock (consists of 85 % wood sawdust) for biochar preparation was selected for surface morphology observation. As shown in Fig. 1, PB exhibits clear fiber textures with sharp edges and larger fragments. NB also shows clear textures, but its edges are smoother compared to PB. The surface texture of AB is severely damaged, with no distinct edges. FB fibers are significantly fractured. MB fibers are covered with impurities.

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Fig. 1

Surface morphology of different biochar.

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3.1.2 Changes in elemental composition

Elemental composition and content of biochars before and after aging were displayed in Table 1. Compared with AB, NB, FB and PB, MB had a similar decrease in the contents of C and H and an increase in the contents of O and N. Among these five adsorbents, MB changed greatest in element content, for instance, the contents of C, H and H/C decreased by 8.63 %, 35.13 % and 29.01 %, respectively, and the content of O, N, O/C and (O+N)/C increased by 15.05 %, 38.92 %, 25.92 % and 27.53 %, respectively. Under the condition of acidification, the surface of biochar is easy to break and be oxidized quickly, while natural aging is a relatively mild and slow oxidation process. The ratio of O/C reflects the content of hydrophilic functional groups, while (O+N)/C represents the content of polar functional groups (Tan et al., 2015). The results showed that the number of oxygen-containing functional groups increased after the aging treatments, as well as the ratio of O/C (Lin et al., 2012). In addition, the hydrophilicity and the polarity of biochar were enhanced. The significant increase in N content of AB might be the result of HNO₃ oxidation. The change in element content of FB was contrary to that of other three aged biochars, for example, the O and N contents decreased, while the C and H contents increased. The contents of C, H and H/C in MB increased by 3.49 %, 5.75 % and 2.17 %, respectively, while the content of O, N, O/C and (O+N)/C decreased by 5.40 %, 15.27 %, 8.59 % and 9.17 % respectively. The decrease in O/C and (O+N)/C of FB indicated that the destruction of hydrophilicity and polarity of biochar by freezing-thawing treatment. From the elemental composition analyses, the unstable C on the surface of biochar might be decomposed and depleted due to the action of temperature, moisture or microorganisms, resulting in a decrease in the C element. At the same time, the temperature and oxidation environment cause more oxygen-containing functional groups formed on the surface of the biochar, resulting in an increase in the O element and an increase in the O/C ratio (Salinas et al., 2000).

3.1.3 Changes in mineral structure

The XRD spectra of all samples are shown in Fig. 2. The five biochars (Fig. 2a) exhibit significant differences in the cellulose peaks between 20°-30° (2θ) (Hu et al., 2014; Liu et al., 2018), indicating that different aging conditions profoundly influence the cellulose of biochar. Comparing the spectra of each biochar with and without Cd and Cr (Fig. 2b, c, d, e, and f, respectively), it is found that no obvious new peaks are generated in the spectra after metal ions loaded, as shown in Fig. S1. Table 2 lists the top three Cd and Cr-containing crystals with the highest matching degree for each sample. Most of these crystals have high FOM values. Only Ca0.67Cd0.33CO3, CdSO3, Cd(S2O3)(H2O)2, and C6H12CrN3O12!3H2O in MB and PB have FOM values less than 10. This result indicates that the adsorbed Cd and Cr are difficult to form a large number of crystals. Therefore, in terms of crystals, the differences in the adsorption mechanism of these biochars cannot be compared.

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Fig. 2

XRD spectra of various biochars (a), comparison of spectra of various biochars with their spectra after adsorption of two ions (b, c, d, e, and f).

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3.1.4 Changes in electrical nature

The surface electrical nature of biochars can be reflected by Zeta potential. As shown in Fig. 3, the Zeta potentials on the biochars changed with the variation of pH. At pH 2, the Zeta potentials of AB, FB, MB and PB were 26.30, 5.42, 24.0, and 4.51 mV, respectively, indicating all these biochars had a positively charged surface. On the contrary, the Zeta potential value of − 9.06 mV illustrated that NB was negatively charged. The reduction in AB might be due to the introduction of a large amount of H⁺ in acidification process (Chingombe et al., 2005).

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Fig. 3

Zeta potentials of the different biochars.

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The point of zero charge (pH_PZC) is the pH value at which the net charge on the particle is zero or the particle possess equal number of positive and negative charges. Once this pH is reached, the particles do not move towards either the anode or the cathode in the electric field. In Fig. 3, the pH_PZC values were approximately 3.74 (AB), 2.40 (FB), 3.21 (MB) and 2.16 (PB). When pH<pH_PZC, the biochar combines with excess proton H⁺ in the solution to exhibit positive polarity, which is beneficial to anion adsorption, while at pH>pH_PZC, the Zeta potential become negative which is conducive to cationic adsorption (Yuan et al., 2024).

3.1.5 Changes in oxygen-containing functional groups and CEC

During the aging process, the unsaturated aliphatic hydrocarbons and aromatic rings on the surface of the biochar are destroyed, and the oxygen-containing functional groups are introduced to change the CEC. Oxygen-containing functional groups are essential to participate in ion exchange and complexation during the adsorption of metal ions (Jin et al., 2020). Aging can change the type and amount of functional groups on the surface of biochar, affecting its adsorption efficiency. From Table 3, the content of oxygen-containing functional groups of PB was 1.83 mmol/g, that of AB and MB increased to 1.94 and 2.03 mmol/g, respectively, and that of NB and FB reduced to 1.58 and 1.75 mmol/g, respectively. Apparently, the content of phenolic hydroxyl groups was much less than that of carboxyl groups and lactone groups, because both microbial and acid treatment led to an oxidation reaction that increased the number of oxygen-containing functional groups. In addition, freezing-thawing cycles can reduce the number of oxygen-containing functional groups on biochar because the mechanical stress from the repeated expansion and contraction can disrupt the biochar's surface integrity, leading to the detachment or degradation of these functional groups.

CEC is an indicator of positive ion exchange capacity. The degree of development of CEC depends on the level of aging. The change trend of CEC among five adsorbents was similar to that of oxygen-containing functional groups. Compared with the CEC of PB, that of AB and MB increased by 13.48 % and 20.87 %, respectively, probably due to oxidation processes creating hydroxy and carboxylic oxygen-containing functional groups, while NB and FB decreased by 31.78 % and 34.03 %, respectively, which could be explained that oxygen-containing functional groups were difficult to dissociate, resulting in lower CEC. At the same time, during the freezing-thawing process there are loss of volatile components, which are found to have an acidic function, leading to the decrease of CEC. Since the O/C ratio can also characterize CEC of biochar, the smaller the O/C, the smaller the CEC (Shaaban et al., 2014). The same conclusion could be clearly seen from Table 3.

3.1.6 Changes in surface functional groups

FTIR analysis can explore the changes of surface functional groups. The Fig. 4 (a) shows the differences in functional groups of PB and various aged biochars from each other. The peak at 1610 cm⁻¹ represented C=C stretching vibration of the aromatic and –NH (Wang et al., 2017). After four aging treatments, the absorption peak intensity decreased, which indicated that the content of substances with benzene ring decreased after aging treatment. Near 1570 cm⁻¹, a new peak appeared in the spectrum of AB, which represented the vibration peak of –NO₂ (Ding et al., 2014). This indicated that –NO₂ formed on the surface of biochar during acidification and aging caused by the substitution of hydrogen ions by nitrate ions during the HNO₃/H₂SO₄ treatment. The intensity of peak near 1410 cm⁻¹ representing CO₃^2– wagging vibration was weaken in AB and NB while strengthened in FB and MB (Cao et al., 2010; Rajapaksha et al., 2014). Peaks near 1320 cm⁻¹ represented aromatic O-H oscillation in lignin (Chen et al., 2008), as seen in Fig. 4 (a), the intensities of these peaks in aged biochars were all weakened, indicating that the lignin content in these biochars was reduced after aging. The peaks around 1020 cm⁻¹ corresponded P-OH (Kalinke et al., 2016), after aging treatment, the peaks of silicon-bearing groups fluctuated greatly, and that in NB changed most obviously, indicating that silicon-bearing groups were exposed by aging treatment. The stretching vibration peak at 780 cm⁻¹ in AB, representing the C-H of the benzene ring, was weakened, indicating that the content of benzene ring decreased after acidification, which was consistent with the result of the decrease in benzene ring content at 1610 cm⁻¹ (Kalinke et al., 2016).

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Fig. 4

FTIR spectra of adsorbent (a), adsorbent-Cd (b), and adsorbent-Cr (c).

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From the spectra in Fig. 4 (b) and (c), it was found that functional groups appeared after aging treatment played an very important role in Cd(II) and Cr(VI) adsorption. The functional groups including CO₃^2– at 1410 cm⁻¹, Si-O-Si at 1020 cm⁻¹ and PO₄^3- at 870 cm⁻¹ (Liu et al., 2016) in MB changed greatly after the adsorption of two metal ions. For FB, the CO₃^2– at 1410 cm⁻¹ and Si-O-Si at 1030 cm⁻¹ were significantly weakened after adsorbing Cd(II) and Cr(VI). For PB, the intensity of the peaks represented C=C and –NH near 1620 cm⁻¹ was weakened after adsorbing Cd(II) and Cr(VI). At the same time, after adsorbing Cr(VI), the peak near 1320 cm⁻¹ which represented O-H disappeared, indicating this functional group played an important role in the adsorption. After aging, all biochars had a significant weakness in ester Si-O-Si stretching vibration at 1020 cm⁻¹ after adsorbing Cd(II) and Cr(VI). Oxygen-containing functional groups were very important electron-donating groups on the surface of biochar, and could participate in redox reactions (Hale et al., 2011; Zhang et al., 2020). In addition, protonated oxygen-containing functional groups could electrostatically attract Cr(VI) oxyanions under acidic conditions (Zhang et al., 2018). Great change of CO₃^2– and PO₄^3- after adsorption could be indicated that them could cause precipitation with metal ions, enhancing the adsorption capacity of BC for Cd(II) and Cr(VI) (Wang et al., 2019). In conclusion, the aging treatment greatly affects the variety and quantity of the surface functional groups involved in adsorption including Si-O-Si, CO₃^2–, PO₄^3-, –NH, and O-H. Mechanisms such as redox, electrostatic attraction, coprecipitation could be occurred in the adsorption process.

3.2.1 Effect of pH

The solution pH has a great influence on the adsorption for metal ions. From Fig. 5 (a), it could be seen that with the increase of pH from 2 to 5, Q values of five biochars for Cd(II) presented a ascendant trend. Obviously, at tested pH, the Q value of MB was the higher than other four biochars, and it was the maximum of 24.2 mg/g when the pH was 5, but when the pH was 2 the Q value was only 20.3 mg/g. AB had the lowest adsorption capacity, the Q value increased from 15.0 to 22.0 mg/g when pH was increased from 2 to 5, but it was still lower than other four biochars. The overall adsorption capacity for Cd(II) was in the following order: MB>PB>FB>NB>AB.

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Fig. 5

Adsorption performance of biochars for Cd(II) (a) and Cr(VI) (b) at different solution pH (30 °C, 160 r/min, adsorbent dosage of 1 g/L, adsorption time of 120 min, initial concentration of 25 mg/L).

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Fig. 5 (b) showed the trend of adsorption capacity of five biochars for Cr(VI) at different pH. It could be seen clearly that with the increased of pH, the adsorption capacity of the five biochars showed a downward trend. MB still perform best under different pH condition. The Q of MB reached 24.1 mg/g at pH 2, and decreased to 19.2 mg/g when the pH increased to 5. The lowest adsorption capacity for Cr(VI) was found in NB. When the pH increased from 2 to 5, the Q of NB decreased from 22.7 to 12.2 mg/g. The overall adsorption capacity for Cr(VI) was in the following order: MB>AB>FB>PB>NB.

It was also evident that MB exhibited good performance in both Cd(II) and Cr(VI) adsorptions at all tested pH values than other four biochars. Microorganisms can increase the activity of biochar by supplementing unstable organic compounds (such as glucose or fresh biomass), thereby increasing the adsorption of heavy metal ions (Keith et al., 2011; Pulsawat et al., 2003). Polysaccharides, nucleic acids and polypeptides formed by microbial bacteria metabolism also have HPO₄^2-, OH^– and COO^– groups, which are conducive to electrostatic adsorption for metal ions (Pulsawat et al., 2003).

For the Cd(II) adsorption, with the increase of solution pH, the carboxylate and hydroxyl groups on the biochars were deprotonated, and then the density of negative charges increased, which led to a significant electrostatic attraction between positive metal ions and biochars. Therefore, the adsorption capacity increased with the increase of the pH value. This also explained why the adsorption capacity of AB was low, during the acidification process, a large number of acid radical ions were introduced, resulting in the lowering of the bichar pH. It had been reported that the alkalinity of biochar decreased after freezing-thawing and natural aging, which explained why the adsorption capacity of NB and FB for Cd(II) was lower than that of PB (Uchimiya et al.,2012). The same situation goes for the adsorption of Cr(VI). Under the condition of low pH, a large amount of H⁺ in the solution made the surface of biochar covered with a large number of positive charges, which were more conducive to adsorbing Cr(VI) (Sumathi et al., 2005). The lower the pH was, the higher the adsorption capacity was. However, with the increase of pH, the amount of OH^– in the solution also increased, which neutralized the positive charges on the surface of biochar, and subsequently decreased the adsorption capacity of Cr(VI) (Mohan et al., 2006). Above Zeta potential and pH_PZC analyses also explained why the optimum pH condition for Cd(II) adsorption was 5 and the optimum pH condition for Cr(VI) adsorption was 2. Of course, under strongly acidic conditions, a portion of Cr(VI) will be reduced to Cr(III), which can form complexes with water molecules and other ligands, further influencing the adsorption process.

3.2.2 Effect of initial ions concentration

The adsorption performance of biochars at different initial Cd(II) and Cr(VI) concentration was shown in Fig. 6. It could be seen that the adsorption capacity increased with the increase of initial concentration, no matter which kind of biochar or ion was tested. It is obvious that the adsorption capacity of MB was the highest at any initial concentration than that of other four biochar. For Cd(II), with the initial concentration increasing from 25.0 to 125.0 mg/L, the adsorption capacity of MB increased from 24.2 to 105.0 mg/g and its adsorption rate decreased from 96.8 % to 84.0 %. AB was the worst performer in Cd(II) adsorption, because its adsorption capacity only increased from 22.0 to 92.0 mg/g with the initial concentration increased from 25 to 125 mg/L, but its adsorption rate decreased from 88 % to 73.6 %. As for Cr(VI), with the initial concentration changing from 25.0 to 125.0 mg/L, the adsorption capacity of MB increased from 24.1 to 103.0 mg/g and the adsorption rate decreased by 14.0 %. NB had the lowest adsorption capacity (22.7 to 93.0 mg/g) and rate (90.8 % to 74.4 %) at all tested concentrations. When the mass of the aged biochars is constant and the concentration of adsorbate is equivalent, the number of effective adsorption sites on biochars determines their adsorption capacity for the same adsorbate (Cieslak-Golonka et a., 1996). Obviously, MB has the best adsorption capacity for both cationic Cd(II) and anionic Cr(VI), which is closely related to its high CEC and oxygen-containing functional group content (Table 3).

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Fig. 6

Adsorption performance of biochars for Cd(II) (a) and Cr(VI) (b) at different initial concentration (25 °C, 160 r/min, adsorbent dosage of 1 g/L, adsorption time of 120 min, pH 5 (a) and 2 (b)).

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3.2.3 Effect of biosorbent dosage

As shown in Fig. 7, with the increase of adsorbent dosage, all adsorption capacities showed a downward trend. For Cd(II), the adsorption capacity of the five biochars was in the following order: MB>PB>FB>NB>AB. Their maximum Q values were 24.2 mg/g, 23.5 mg/g, 22.7 mg/g, 22.2 mg/g, and 22 mg/g, respectively. For Cr(VI), the adsorption capacity of the five biochars was in the following order: MB>AB>FB>PB>NB. Their maximum Q values were 24.1 mg/g, 23.7 mg/g, 23.5 mg/g, 23 mg/g, and 22.7 mg/g, respectively. As seen in Fig. 7, with the increase of the adsorbent dosage, the declining trend of Q slowed down. As the dosage increased, the R value increased and gradually approached 100 %. At this time, it became difficult for the adsorbent to capture ions, so the Q value decreased.

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Fig. 7

Adsorption performance of biochars for Cd(II) (a) and Cr(VI) (b) at different adsorbent dosage (25 °C, 160 r/min, adsorption time of 120 min, pH 5 (Cd) and 2 (Cr), initial concentration of 25 mg/L).

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