Towards a better understanding on adsorption mechanism of various heavy metal with phosphorus rich hydrochar

3.2.1 Adsorption kinetics

There were three different types of heavy metal with divalent used to investigate the adsorption effect of CMHCC: Pb(II), Cd(II), and Hg(II). A mixture of 20 mg CMHCC and 20 mL of 100 mg·L⁻¹ Pb(NO₃)₂, Cd(NO₃)₂, or HgCl₂ solution was added to a 25 mL glass bottle at room temperature (25 °C), and the initial pH of the mixture was adjusted to 6.0 by HNO₃ or NaOH solutions. Concentration of the adsorbent was maintained at 1 g·L⁻¹ in all the treatments. The glass bottles containing CMHCC and heavy metal’ solution were shaken in a thermostatic reciprocating shaker at 200 rpm·min⁻¹. After oscillation (200 rpm·min⁻¹, 25 °C), 5 mL solution was filtered (0.22 μm) at predetermined times (0.5, 1, 2, 4, 6, 12, 18, and 24 h), and the filtrate obtained from each bottle by immediate filtration of the suspension through a nylon membrane filter with a pore size of 0.22 μm was stored at 4 °C. The contents of phosphorus and heavy metal were determined via ICP. The adsorption capacities of Pb(II), Cd(II), and Hg(II) were calculated using Eq. (1) and the experimental results were fitted using the adsorption dynamic model (Eqs. (2) and (3).

In the equations, Q_e represented the equilibrium adsorption capacity in mg·g⁻¹; C₀ and C_e represent the mass concentration of Pb(II), Cd(II) or Hg(II) at the initial and adsorption equilibrium in mg·g⁻¹, respectively; V₀ was the volume of the heavy metal’ solution in L; and m represented the dosage of hydrochar in g; Q_t represented the adsorption capacity at time t in mg·L⁻¹; k₁ represented the reaction rate constant of the pseudo − first − order model in min⁻¹; k₂ was the reaction rate constant of the pseudo − second − order model in g·(mg·min)⁻¹; and a and b were the initial adsorption rate and activation energy − related constants in mg·(g·min)⁻¹ and g·mg⁻¹, respectively.

3.2.2 Adsorption isotherms

The adsorption isotherm experiments were carried out to determine the adsorption capacity of CMHCC for Pb(II), Cd(II), and Hg(II). 20 mg of CMHCC was weighed and mixed with 20 mL of three kinds of heavy metal’ solution containing various concentrations (100, 200, 300, 400, 500, and 600 mg·L⁻¹) in a 25 mL glass bottle and the initial pH of the mixture was adjusted to 6.0 by HNO₃ or NaOH solutions. After 24 h of oscillation (200 rpm·min⁻¹, 25 °C), the filtrate from each glass bottle was collected after filtering the suspension and was stored at 4 °C. The contents of phosphorus and heavy metal were determined via ICP, respectively. The adsorption capacities of Pb(II), Cd(II), and Hg(II) were calculated using Eq. (1), and the experimental results were fitted using and the isothermal adsorption model (Eqs. (4) and (5)).

In the equations, Q_e represented the equilibrium adsorption capacity in mg·g⁻¹; C₀ and C_e represent the mass concentration of Pb(II), Cd(II) or Hg(II) at the initial and adsorption equilibrium in mg·g⁻¹, respectively; Q_m represented the theoretical maximum adsorption capacity in mg·g⁻¹; K_L represented the equilibrium constant of the Langmuir model; K_F was the Freundlich constant, and n was the empirical constant.

3.2.3 Effect of initial pH on adsorption

The initial solution pH value affects the degree of protonation of functional groups on the adsorbent surface. It also changes the existing forms of heavy metal ions in solution [31]. At low pH values (pH less than 3), the protonation of surface functional groups made the surface of CMHCC positively charged, which had a repulsion effect on heavy metal cations. In addition, a large number of H3O⁺ competed with Pb(II), Cd(II), and Hg(II) to occupy the adsorption site (Su et al., 2022).

A mixture of 20 mg CMHCC and 20 mL of 100 mg·L⁻¹ Pb(NO₃)₂, Cd(NO₃)₂, or HgCl₂ solution was added to a 25 mL glass bottle at room temperature (25 °C). The initial pH of the mixture was adjusted to a fixed value within the range of 2 – 6 using HNO₃ or NaOH solution, and the mixtures were shaken in a thermostatic reciprocating shaker at 200 r·min⁻¹ for 24 h at room temperature (25 °C). At the end of shaking, final pH was measured again, and the contents of phosphorus and heavy metal were determined via ICP. The effect of pH on the adsorption by CMHCC was studied in solution varying pH from 2 to 6, because the gray precipitate would be produced in HgCl₂ solution when pH ≥ 7.

3.5.1 Contribution of the interaction with minerals

Contribution of the interaction with minerals (Q_min, mg·g⁻¹) was calculated as Eq. (12).

3.5.2 Contribution of the ion − exchange

The adsorption capacity attributed to ion − exchange (Q_exc, mg·g⁻¹) was calculated from the net release of these cations. Specifically, the net amounts of cations released were calculated by subtracting the specific cation in the control group (CMHCC in deionized water) from the corresponding experimental group (CMHCC in heavy metal’ solutions). Since the substances containing K⁺ and Na⁺ often existed in the form of soluble compounds, while Ca²⁺ and Mg²⁺ were generally composed of insoluble precipitates, this work optimized the previous research via measuring the contents of Ca²⁺ and Mg²⁺ to calculate (Eq. (13)).

Here, Q_Mg and Q_Ca were the net release amount of Ca²⁺ and Mg²⁺, respectively.

3.5.3 Contribution of the mineral precipitation

The contribution of mineral precipitation (Q_pre, mg·g⁻¹) can be calculated as Eq. (14).

3.5.4 Contribution of the complexation

The adsorption capacity resulting from complexation with the oxygenated functional groups (Q_com, mg·g⁻¹) was estimated from the difference in pH before and after adsorption (Eq. (15)).

where, Q_pH was the adsorbed heavy metal by complexation with oxygenated functional groups on demineralized CMHCC.

3.5.5 Contribution of other mechanisms

The amount of adsorption from other mechanisms (Q_oth, mg·g⁻¹) was calculated as Eq. (16).

4.2.1 Kinetics model

The fitting results of kinetic model of CMHCC were shown in Table 1 and Fig. 2. In the adsorption processes of Pb(II) via CMHCC (pH = 6), the R² values of pseudo − first − order kinetic model and pseudo − second − order kinetic model were 0.98 and 0.99, respectively, while those of adsorption processes of Hg(II) via CMHCC were 0.97 and 0.98, respectively. Thus, the adsorption processes of Pb(II) and Hg(II) were better fitted by pseudo − second − order kinetic model. This phenomenon may indicated that physisorption and chemisorption likely coexisted during adsorption of Pb(II) and Hg(II). On the contrary, the R² values of pseudo − first − order kinetic model and pseudo − second − order kinetic model in the adsorption of Cd(II) via CMHCC were 0.99 and 0.91, respectively, which indicated that physisorption dominated Cd(II) adsorption by CMHCC (Chowdhury et al., 2022).

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Fig. 2

Fitting parameters of kinetic model (a) and isotherm model (b) of absorption. Experiment condition: [dosage] = 1 g·L⁻¹, [temperature] = 25 ℃, [pH] = 6.0.

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The equilibrium time of absorption of three kinds of heavy metal by CMHCC was distinct. When pH = 6, the equilibrium time of Pb(II) absorption was about 3 h, and that of Hg(II) absorption was approximately 2 h, while that of Cd(II) absorption was about 6 h. The different adsorption equilibrium time may be due to the dominated adsorption, the physisorption was reversible and slow (Zhang et al., 2022a), which corresponded to the kinetic model fitting results mentioned above (Fig. 2). Furthermore, the equilibrium time of absorption was attributed to the same valency metal ions having different electronegativities and atomic radii. The smaller the hydrated radius and higher electronegativity is, the greater the affinity in the sorption process (Xiao et al., 2020). Pb(II) might have preferential sorption compared with Cd(II), for the electronegativity values and hydrated radius of Cd(II) are 0.69 and 4.26 Å, respectively, while those of Pb(II) are 2.33 and 4.01 Å, respectively.

4.2.2 Isothermal model

The fitting results of isothermal adsorption model were presented in Fig. 2. The R² values of the Langmuir model and Freundlich model for Pb(II) adsorption by CMHCC were 0.97 and 0.96, respectively, and those for Cd(II) adsorption were 0.97 and 0.94, respectively. Clearly, the Langmuir model fit the adsorption of Pb(II) and Cd(II) better. Moreover, for Hg(II) adsorption, the R² values of the Langmuir model and Freundlich model were both 0.98, thus, the Langmuir model fit the adsorption of Hg(II) better. Therefore, the adsorption of Pb(II), Cd(II), and Hg(II) by CMHCC were monolayer adsorption, indicating that single − layer surface adsorption might be the dominant process governing the sorption process (Xu et al., 2022). The corresponding Q_e of Pb(II), Cd(II), and Hg(II) on CMHCC were 466.45, 339.48, and 264.19 mg·g⁻¹, respectively. CMHCC exhibited excellent sorption properties than many reported sorbents including hydrochars and some carbon − based materials (Table S3).

4.2.3 Effect of pH

The adsorptions of CMHCC on three kinds of heavy metal were affected by the pH (Fig. 3a). In general, under acidic conditions (pH from 2 to 6), the adsorption effect of CMHCC on three kinds of heavy metal enhanced with the increasing of solution pH. As shown in Figure S1, the surface of CMHCC was positively charged when pH = 2, while that was relatively negative charged at pH = 4. The more acidic the solution, the more positive charge on the surface of adsorbent, so that the adsorbent was easier to be protonated, which weakened the adsorption effect. As the pH continued to increase to 6, the adsorption sites became less positively charged, and more sorption sites could be deprotonated, enabling complexation with more Pb(II), Cd(II), and Hg(II).

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Fig. 3

Effect of pH on adsorption (a) and endogenous phosphorus leaching performance (b). Experiment condition: [dosage] = 1 g·L⁻¹, [temperature] = 25 ℃, [time] = 24 h.

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When the pH increased from 2 to 6, the Q_e enhanced 31 times (3.20 vs. 99.49 mg·L⁻¹), thus, the adsorption of Pb(II) by CMHCC was significantly affected by pH. From this we can inferred that electrostatic attraction made a great contribution to adsorption on Pb(II).

4.4.1 Analysis of phosphorus morphology

The phosphorus in solution was denoted as Rp, and the phosphorus on CMHCC@HM was denoted as BCp, which included the endogenous phosphorus undissolved and the phosphorus that binded with heavy metal and then attached to the surface of hydrochar. The morphology of BCp was analyzed (Fig. 4a), which including L − P, RSP, Fe/Al − P, and Ca − P according to four − step continuous extraction method mentioned above.

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Fig. 4

Analysis of different forms of phosphorus and heavy metal in CMHCC@HM. a: analysis of phosphorus morphology; b: analysis of heavy metal’ morphology.

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Phosphorus in the form of L − P was exchangeable, and L − P was easily diffused into solution and used by organisms. Phosphorus in the form of RSP was considered as occluded phosphate. RSP generally coated by Fe₂O₃, and was biologically unavailable (Tang et al., 2018). Phosphorus in the form of Fe/Al − P can be bioavailable under certain conditions, so Fe/Al − P was potential activated. Phosphorus in the form of Ca − P was an insoluble substance and difficult to be used by organisms, and Ca − P was mostly phosphate of calcium such as apatite phosphorus (Zhang et al., 2020b).

The phosphorus on CMHCC mainly existed in the form of L − P, and the rest almost can be classified into two categories: RSP and Ca − P. This indicated that phosphorus on CMHCC existed in an unstable state and easily entered the water or be used. For CMHCC@Pb, the phosphorus mostly existed in the form of Ca − P. Phosphorus of CMHCC was converted from L − P to Ca − P during the Pb(II) adsorption. Phosphate combined with Pb²⁺ to form insoluble precipitates, such as Pb₅(PO₄)Cl and Pb₃(PO₄)₂. The significant transformation of phosphorus morphology during the adsorptions of Cd(II) and Hg(II) was the increase of Fe/Al − P, for Fe/Al − P contents of CMHCC@Cd and CMHCC@Hg were 175 and 174 times higher than that of CMHCC, respectively. This indicated that REDOX reaction was involved in the removal of heavy metal. Besides, Ca − P content of CMHCC@Cd was increase by 224 %, indicating that co–precipitation was involved in removal of Cd(II), but the proportion was not as large as that in Pb(II) removal. The removal of three heavy metal by CMHCC had a commonality, that is, the proportion of bioavailable phosphorus decreased. This may indicate that using CMHCC to remove heavy metal not only had great adsorption effect, but also little harm of eutrophication caused by endogenous phosphorus release.

4.4.2 Analysis of heavy metal’ morphology

The environmental behavior and ecological effect were not evaluated by total contents of heavy metal in hydrochar, but heavy metal’ morphology were the key factors to determine the impact on the environmental ecosystems. As shown in Fig. 4b, heavy metal’ morphologies of CMHCC@HM were extracted, and the HM_ex, HM_ac, and HM_ge represented the exchangeable, acid soluble, and generally stable heavy metal, respectively. The HM_ex was sensitive to environment, easy to migrate and transform, and can be absorbed by plants. The HM_ac was harmful to the environment, and its release activity was enhanced when the environmental conditions become acidic. The HM_ge was insoluble and had an impact on organisms only through chemical reactions into soluble substances.

After adsorption of heavy metal on CMHCC, three kinds of heavy metal mainly existed in the form of HM_ge. For CMHCC@Pb, although HM_ge had the largest proportion, accounting for 74.83 %, HM_ex also accounted for 24.08 %. This indicated that the heavy metal biological risk of CMHCC@Pb was high. For CMHCC@Cd, HM_ge had the largest proportion accounted for 93.25 %, indicating that the heavy metal biological risk of CMHCC@Cd was low. Though HM_ge of CMHCC@Cd had the largest proportion, Ca − P did not, which was speculated that the insoluble and stable precipitation formed may was mainly carbonate rather than phosphate.

4.4.3 Characterization

To clarify the interaction mechanisms between CMHCC and three kinds of heavy metal, a series of characterizations were performed and the results were discussed in detail. The SEM and SEM − EDS images of CMHCC@HM were shown in Fig. 5. The SEM analysis clearly showed that the difference in physical microstructure of the CMHCC and CMHCC@HM (Subhashish et al., 2022). Compared with CMHCC, the pore structures of CMHCC@HM were less. The surface plane of CMHCC exhibited heterogeneous textural properties, with a strong porous structure. And the presence of these cavities gave the CMHCC capacity for biosorptions of heavy metal (Subhashish et al., 2023). In fact, these heavy metal salts filled the holes. The surface of CMHCC@Pb was rough and had many small crystalline materials attached to it (Fig. 5a). On the contrary, the surface of CMHCC@Hg had obvious massive crystalline material. The EDS images showed that the Pb and P were evenly distributed on the surface of on CMHCC@Pb (Fig. 5b − c), and the distribution of Pb was similar to that of P. This phenomenon was similar to that of CMHCC@Cd (Fig. 5e − f). However, on the surface of CMHCC@Hg, the P still distributed dispersedly, the Hg concentrated on crystal particles, which was not similar to that of P (Fig. 5h − i). According to this phenomenon, we speculated that in adsorption of Hg(II) by CMHCC, endogenous phosphorus was scarcely any involved, and the crystal particles formed after adsorption were not phosphate substances.

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Fig. 5

SEM and EDS images of CMHCC@HM. a: SEM images of CMHCC@Pb; b–c: EDS images for Pb and P of CMHCC@Pb; d: SEM images of CMHCC@Cd; e–f: EDS images for Cd and P of CMHCC@Cd; g: SEM images of CMHCC@Hg; h–i: EDS images for Hg and P of CMHCC@Hg.

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The XRD spectra of CMHCC@HM was shown in Fig. 6. For CMHCC@Pb, the characteristic peaks at 20.8°, 27.28°, and 42° represented Pb₃(PO₄)₂ (PDF#73–0834), which confirm that endogenous phosphorus was involved in the removal of Pb(II) (Li et al., 2022). The characteristic peak at 26.6° and 40° represented PbSiO₃ (PDF#74–1101) and PbCO₃ (PDF#85–1088), respectively, confirming the role of precipitation. Besides, PbCO₃ (Ksp = 3.3 × 10⁻¹⁴) was less soluble than CaCO₃ (Ksp = 2.8 × 10⁻⁹) and MgCO₃ (Ksp = 2.6 × 10⁻⁵) (Liu et al., 2022), hence, the removal mechanism of Pb(II) may involve ion − exchange. Moreover, Pb₃(CO₃)₂(OH)₂ (PDF#72–1144) verified hydroxyl group participated in adsorption (Qu et al., 2022), which suggested that the functional groups on CMHCC may were complexed with Pb(II) for adsorption. The Pb₅(PO₄)₃Cl verified precipitation participated in adsorption (Ge et al., 2022). For CMHCC@Hg, the ion − exchange and physicochemical for the formation of Hg₂Cl₂ (Kong et al., 2011). For CMHCC@Cd, co–precipitation and complexation involved in the adsorption for the formation of Cd(OH)PO₃·H₂O and Cd₃(PO₄)₂ (Zhang et al., 2020a), and ion − exchange also involved in the adsorption possibly for Cd₃(PO₄)₂ existed (Wang et al., 2022).

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Fig. 6

Characterization of CMHCC@HM and CMHCC. a: XRD spectra, b: FTIR spectra.

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The FTIR spectra of CMHCC@HM was shown in Fig. 6b. For CMHCC@Pb, the peak value of Si–O–Si disappeared, possibly due to the cation–π bond interaction existed (Feng et al., 2023). The peaks of CO₃²⁻ and PO₄³⁻ existed, possibly due to precipitation Pb₃(CO₃)₂(OH)₂ and Pb₃(PO₄)₂ (Pei et al., 2021), which confirming the previous speculation (Fig. 6a). The Pb–P peak at 2300 – 2450 cm⁻¹ appeared, indicating the presence of phosphates of heavy metal (Xia et al., 2020). The band at 665 cm⁻¹ was indicative of SiO₃²⁻ (Tang et al., 2015).

For CMHCC@Hg, the –OH group peak shifted, presumably the hydroxyl group complexed with Hg(II), and the peak of –COOH disappeared, which possibly due to the complexation of Hg(II) with phenolic hydroxyl (CO⁻) and carboxylic (COO⁻) group (Xu et al., 2016). The COO⁻ peak at 1159 cm⁻¹ appeared, proving the formation of complexations. The C = C and C = O peaks shifted, which possibly due to the cation − π interaction (Liu et al., 2018). The Hg–S peak at 2550 – 2650 cm⁻¹ appeared, indicating the presence of heavy metal sulfides, such as HgS (Xia et al., 2020). The C–S bond formed on surface hydrochar led to formation of HgS by formation of complexations based on Hard − Soft and Acid Base (HSAB) theory. Without the addition of sulfur, Hg adsorption normally occurred at carboxyl and carbonyl groups, forming HgO and HgCl₂ (Fig. 6a) (Jeon et al., 2020).

For CMHCC@Cd, the –OH group peak shifted, which presumably due to the hydroxyl group complexed with Cd(II). The peaks of PO₃⁻, PO₄³⁻ and CO₃²⁻ shifted, which possibly due to the precipitation and formation of complexations for the formation of Cd(OH)PO₃·H₂O, Cd₃(PO₄)₂, and Cd₃CO₃ (Zhang et al., 2020a). The peak of Cd–P peak at 2300 – 2450 cm⁻¹ appeared, indicating the presence of phosphates of heavy metal. The shift of –CH₂/CH₃ group may due to the H⁺ provided from –CH₂/CH₃ had an ion − exchange reaction with Cd(II) (Ge et al., 2022). And the CMHCC adsorb Cd (II) via the role of O–containing groups, mainly by reactions of organic complexation (Zhang et al., 2022b).