A Biosorption-Pyrolysis Process for Removal of Pb from Aqueous Solution and Subsequent Immobilization of Pb in the Char

Abstract

The application of biosorption in the removal of heavy metals from water faces a challenge of safe disposal of contaminated biomass. In this study, a potential solution for this problem was proposed by using a biosorption-pyrolysis process featured by pretreatment of biomass with phosphoric acid (PA). The PA pretreatment of biomass increased the removal efficiency of heavy metal Pb from water by sorption, and subsequent pyrolysis helped immobilize Pb in the residual char. The results indicate that most (>95%) of the Pb adsorbed by the PA-pretreated biomass was retained in the char, and that the lower pyrolysis temperature (350 °C) is more favorable for Pb immobilization. In this way, the bioavailable Pb in the char was hardly detected, while the Pb leachable in acidic solution decreased to <3% of total Pb in the char. However, higher pyrolysis temperature (450 °C) is unfavorable for Pb immobilization, as both the leachable and bioavailable Pb increased to >28%. The reason should be related to the formation of elemental Pb and unstable Pb compounds during pyrolysis at 450 °C, according to the X-ray diffraction study.

1. Introduction

Water pollution by heavy metals, such as Pb, is one of the most serious environmental issues in the world. In comparison with other technologies for removing heavy metals from water, biosorption has the advantages of low operation cost, abundant biomass availability, and easy acceptance by the public [1,2,3,4,5]. Many kinds of plant-derived biomass wastes have been used for sorption of heavy metals, such as Pb, Cd, and Cu [6,7,8]. However, the practical applications of biosorption in the removal of heavy metals face two challenges. The first is that the sorption capacity of many indigenous biosorbents needs to be enhanced. Except for some biosorbents derived from algae and wheat bran, the maximum sorption of Pb by many lignocellulosic materials (seed husks, hulls and leaves, etc.) has shown to be <50 mg/g [6,9]. For seeking the solution of this problem, chemical modifications of biomass have been used for enhancing the sorption capacity [10,11,12]. The chemical modifying agents, including inorganic bases (e.g., NaOH), mineral acids (e.g., HNO₃), and organic compounds (e.g., urea) can enhance the ion exchange, and increase the functional groups and metal holding capacity of biosorbents. Among these modifying agents, phosphoric acid (PA) has shown to be a good candidate. PA has been used to modify olive pomace [13], corncob [14], rice husk [15], bagasse [16], wheat straw biochar [17] and so forth, so that their sorption to heavy metals was greatly enhanced. The second and more significant challenge is related to the safe disposal of the contaminated biomass after sorption of heavy metals. As the biomass is readily decomposed, the adsorbed heavy metals would be released or leached from the contaminated biomass if it was not properly disposed.

The proposed methods for disposal of heavy metal-contaminated biomass include incineration [18], composting [19], and pyrolysis [20], among others. Both incineration and composting have the problem of secondary pollution because the heavy metals will be dispersed into surroundings. In comparison, pyrolysis is operated in a closed system, and dispersion of heavy metals is controllable. Thus, pyrolysis has been used more frequently for post-treatment of heavy metal-contaminated biomass [21,22,23,24,25,26]. Pyrolysis is useful for valorization of contaminated biomass by transforming it into bio-fuel [27,28], and has also been used for preparing biochars that are good adsorbents to many pollutants [29,30,31]. The distribution of heavy metals in various fractions of pyrolysis product (e.g., oil or char) depends on the specific pyrolysis technique and heavy metal species [32,33,34]. For example, higher pyrolysis temperatures resulted in more emission of heavy metals into volatile fractions (e.g., oil) [27,35]. In contrast, slow pyrolysis at relatively low temperatures is more efficient for retention of heavy metals in the char [36,37]. Furthermore, at the same pyrolysis conditions, the heavy metals Cu, Zn, and Pb are more readily retained in the char than Cd [38]. As the char is more resistant to degradation than the biomass, pyrolysis is a feasible solution for disposal of heavy metal-contaminated biomass if most of the heavy metals are retained in the char and their stability in the char is guaranteed. In our previous studies, we investigated the stability of Pb and Cd in the char, and found that addition of phosphates in the contaminated biomass before pyrolysis enhanced the retention and stability of heavy metals [39,40,41].

In this work, a novel biosorption-pyrolysis process using PA-pretreated biomass was used to attempt to remove Pb from aqueous solution and subsequently immobilize Pb in the char. The specific objectives were to test two hypotheses: the first is that the PA pretreatment of biomass will enhance its sorption to Pb, and the second and more significant is that the subsequent pyrolysis of the contaminated biomass will increase the stability of Pb in the char. Different from our previous studies using phosphates as the additive for pyrolysis of the contaminated biomass [39], the biomass derived from an aquatic plant was firstly treated with PA, and then used for sorption of Pb in aqueous solution. The Pb-contaminated biomass was pyrolyzed, and the retention and stability of Pb in the char was evaluated. The comparison with the biosorption-pyrolysis process using un-pretreated biomass was also performed, and the immobilization mechanism of Pb in the char was investigated by using X-ray diffraction (XRD) and infrared spectra (IR).

2. Materials and Methods

2.1. Biomass and PA Pretreatment

The plant biomass was derived from the Hydrocotyle verticillata species, an aquatic plant that is widely distributed in tropical and subtropical zones. Its great adaptability and vitality makes it a good choice for phytoremediation of polluted or eutrophic water. The floating part (leaves and stems) of the Hydrocotyle plant was collected from a local river at Shaoxing, Zhejiang Province of China. The biomass was dried in an oven at 100 °C to a constant weight, and pulverized to a size of <0.15 mm. Elemental analysis using the EA3000 elemental analyser (Euro Vector, Italy) indicates that the pristine biomass (Mass) is composed of C 40.7%, O 29.7%, H 5.9%, N 1.5%, and S 0.6% by weight. The ash content of the biomass measured at 750 °C is 15.7% by weight. Phosphoric acid (H₃PO₄, PA) of analytical grade (Sinopharm Chemical Reagent Co. Ltd., China) was used for pre-treating the biomass. The biomass powder was soaked in 30 wt% PA solution for 24 h, then washed by water until the pH of the effluent reached 6.0. Two PA dosages with the mass ratio of PA/biomass = 0.5 or 1.0 were chosen according to the preliminary experiments. The dry solid residue obtained by PA pretreatment is labeled hereafter as PA_0.5Mass or PA_1.0Mass, respectively, according to the PA dosage (0.5 or 1.0) used for pretreatment.

2.2. Pb Removal and Sorption Experiments

Pb(NO₃)₂ of analytical grade (Sinopharm Chemical Reagent Co. Ltd., Shanghai, China) was used to prepare Pb solutions for removal and sorption experiments. Of the biosorbent (un-pretreated biomass (Mass) or PA-pretreated biomass (PA_0.5Mass or PA_1.0Mass)), 50 mg was added to 20 mL of aqueous Pb solution with initial concentrations (C₀, mg/L) ranging from 5 to 50 mg/L. The slurry was adjusted to an initial pH of 5.0 with diluted HNO₃ solution, and placed in a thermostatic shaker bath at 25.0 ± 0.2 °C and shaken for 6 h when the sorption reached equilibrium according to our preliminary experiments. The supernatant solution was sampled and the Pb concentration (C_e) in solution was analyzed by the flame atomic absorption spectrometry (AAS) (AA-7000, Shimadzu, Japan). The removal efficiency (%) was calculated by Equation (1).

$$\mathrm{Removal}\mathrm{efficiency}(\%)=\frac{C_0-C_e}{C_0}\times 100\%$$

(1)

The isothermal sorption experiments were conducted in a way similar to the removal experiments, except that a higher C₀ range (50–300 mg/L) was used, so as to leave a certain equilibrium Pb concentration (C_e) in the solution. The quantity adsorbed (Q_e, mg/g) was calculated by Equation (2):

$$Q_e=(C_0-C_e)\times \frac{V}{m}$$

(2)

where V is the volume of aqueous Pb solution (0.02 L) and m is the mass of biomass used for sorption experiments (0.05 g). The isothermal sorption data were further analyzed with the Langmuir and Freundlich sorption isotherm models. The Langmuir model has been expressed in Equation (3):

$$Q_{\mathrm{e}}=\frac{Q_{\mathrm{m}}b{C}_{\mathrm{e}}}{1+b{C}_{\mathrm{e}}}$$

(3)

where Q_m gives the maximum sorption capacity (mg/g) based on the monolayer adsorption model, and b (L/mg) is the Langmuir constant. The empirical Freundlich model has been expressed in Equation (4):

$$Q_{\mathrm{e}}=k_{\mathrm{F}}{C}_{\mathrm{e}}{}^{1/n}$$

(4)

where k_F is the Freundlich sorption constant, and n gives the sorption intensity of biosorbents.

2.3. Pyrolysis of Pb-Contaminated Biomass

To obtain an adequate amount of contaminated biomass for subsequent pyrolysis, the biomass sample (1.5 g) was mixed with 500 mL of Pb solution with C₀ = 100 mg/L at initial pH = 5.0, and the mixture was stirred magnetically at room temperature for 6 h. Then the solid residue was filtered out and dried at 100 °C. The Pb-contaminated biomass powder was marked as Pb@Mass, Pb@PA_0.5Mass, or Pb@PA_1.0Mass according to the biomass sample used for biosorption. Pyrolysis of the Pb-contaminated biomass was carried out at 350 or 450 °C in a quartz tube furnace filled with N₂ gas. The relatively lower pyrolysis temperature was used because a higher temperature will lead to less retention of Pb in the char, according to a previous report [39]. The resulting Pb-concentrated chars are referred to hereafter as Pb@Char350, Pb@PA_0.5Char350, Pb@PA_1.0Char350, Pb@Char450, Pb@PA_0.5Char450, and Pb@PA_1.0Char450, respectively, depending on the temperature and feedstock biomass used for pyrolysis. The amount of Pb loaded in the biomass (Q_mass, mg/g) and then concentrated in the chars (Q_char, mg/g) was determined using a microwave digestion method [40], and the Pb concentration in the dilute digestion solution was determined using AAS. Retention efficiency of Pb (%) in the char after pyrolysis was calculated by Equation (5).

$$\mathrm{Retention}\mathrm{efficiency}\mathrm{in}\mathrm{the}\mathrm{char}(\%)=\frac{Q_{\mathrm{char}}}{Q_{\mathrm{mass}}}\times \mathrm{Yield}\mathrm{of}\mathrm{char}(\%)$$

(5)

2.4. Evaluation of the Stability of Pb in the Solid Samples

The bioavailable Pb, namely that available for utilization by living organisms, was evaluated with the DTPA extraction method using a buffer DTPA (diethylene triamine pentacetate acid) solution according to the ISO standard (ISO 14870-2001). The leachable Pb in the samples was evaluated based on the amount of extractable Pb with the acid solution used in the Toxicity Characteristic Leaching Procedure (TCLP, EPA Test Method 1311). The details for the DTPA extraction and the TCLP test follow the procedures reported previously [40,42].

2.5. Sample Analysis

Surface composition of the biomass samples was determined with a JSM-6360LV scanning electron microscope (SEM) (JEOL, Tokyo, Japan) equipped with an X-act energy dispersive X-ray spectrometer (EDS) (Oxford, UK). Infrared spectra (IR) were recorded in the 4000–400 cm⁻¹ region on a Nexus FT-IR spectrophotometer (Nicolet, Glendale, WI, USA) using a KBr pellet. The crystalline species in the biomass and chars were identified by X-ray diffraction (XRD) using D/MAX3A (Rigaku, Tokyo, Japan) equipment with CuKα radiation and a goniometer rate of 4°/min.

3. Results and Discussion

3.1. Influence of PA Pretreatment on the Removal of Pb by Biomass

The results in Figure 1a indicate that the removal efficiency of Pb by the Hydrocotyle biomass (Mass) is enhanced after PA treatment, despite that the sample (PA_1.0Mass) pretreated with more PA performed worse than the counterpart using less PA (PA_0.5Mass). At the dosage of biomass used in this study (2.5 g/L), Pb is almost completely removed by PA_0.5Mass when the initial concentration (C₀) of Pb (II) ranged from 5~50 mg/L. In contrast, the removal efficiency of Pb by the un-pretreated biomass dropped to <85% with the C₀ increase to 50 mg/L. The enhanced removal efficiency corresponds well to the improved sorption of Pb by biomass after PA pretreatment (Figure 1b). The maximum sorption (Q_m) calculated with the Langmuir model (Equation (3)) increased to 78.7 mg/g for PA_0.5Mass from 69.6 mg/g for the indigenous biomass (Mass) (

Table 1. Fitting results of the sorption data with the Langmuir and the Freundlich models.

Models	Langmuir			Freundlich
	Qm (mg/g)	b (L/mg)	R2	kF	n	R2
Mass	69.6	0.0788	0.992	20.6	4.21	0.855
PA0.5Mass	78.7	0.373	0.996	36.7	5.72	0.853
PA1.0Mass	76.9	0.242	0.993	32.8	5.39	0.839

). The Q_m values of the biosorbents in this study are close to those lignocellulosic materials with high sorption capacity to Pb reported in literature [6,7]. In addition, the n values calculated with the Freundlich model are much larger than 1 (Table 1), indicating that all the biosorbents favored the sorption of Pb [17]. The kinetic analysis indicates that the sorption of Pb by all three biosorbents (Mass, PA_0.5Mass, and PA_1.0Mass) fitted well to the pseudo-second-order model (R² > 0.99), and the sorption rate by the biomass was increased by PA-pretreatment according to the calculated rate constants (Table S1 in the Supplementary Materials). The quantity of Pb adsorbed by all three biosorbents (Mass, PA_0.5Mass and PA_1.0Mass) increased with the initial pH and the initial Pb concentrations in solution (Figures S2 and S3 in the Supplementary Materials). Therefore, the Hydrocotyle biomass and its PA-pretreated derivatives are potential sorbents for removal of heavy metals from water.

The relatively high sorption of Pb by the Hydrocotyle biomass and its PA-pretreated derivatives is related to their surface properties. Figure 2 shows the SEM images of the un-pretreated biomass (Figure 2a) and the PA-pretreated biomass (Figure 2b). The rough surface of PA_0.5Mass favors the sorption by providing more reactive sites. The specific surface area measured by the BET method is 7.24 m²/g for PA_0.5Mass, which is higher than that of the un-pretreated biomass (1.42 m²/g). Thus, PA-pretreatment increases the surface area of biomass and makes more reactive sites available for the sorption of Pb. The EDS analysis indicates that there are some inorganic minerals composed of Ca and K on the biomass surface, which are useful for sorption of Pb through cation exchange. For example, Pb can replace K in carbonate and sulfate to form precipitates, and this is a major pathway for sorption of Pb by those biomasses and biochars rich in inorganic minerals [43,44,45]. The sorption of Pb by biomass is also related to the surface functional groups. As can be seen in Figure 3, the bands at 1735, 1440, 1160–1060 cm⁻¹ indicate oxygen-containing functional groups (C=O, C–OH, and C–O) on the biomass surface, which facilitate the sorption of Pb through surface complexation that has also been reported to contribute much to biosorption of heavy metals [46,47]. Some previous studies have shown that PA treatment can strengthen the acid functional groups of biomass [13,16,48], due to the dissociation of these groups. In addition, PA pretreatment of the biomass introduces a new PO₄³⁻ group (at 1023 cm⁻¹) that can bind Pb through formation of phosphates, according to the IR spectra (Figure 3). However, pretreatment with a higher dosage of PA (1.0 g/g dry biomass) may result in over-acidification of biomass, which may inhibit the precipitation of Pb, so that the decreased removal efficiency and sorption capacity was observed on PA_1.0Mass in comparison with PA_0.5Mass (Figure 1). The over-acidification of biomass is confirmed by the change of pH before and after sorption of Pb. For example, at the same initial Pb concentration (e.g., C₀ = 250 mg/L) and same initial pH = 5.0, pH in the solution after sorption of Pb dropped to 4.8, 4.7, and 4.4, respectively for the sorption systems using Mass, PA_0.5Mass, and PA_1.0Mass. Namely, the pH in the PA_1.0Mass system is much lower than the other two systems, and lower pH favors dissolution of Pb and impacts its removal by biomass.

3.2. Influence of Pyrolysis on Pb Retention in the Char

Pyrolysis of the contaminated biomass at high temperatures will increase the risk of emission of Pb with volatile matters, and decrease the retention of Pb in the char, according to the previous studies [40,41,42]. Thus, two relatively low temperatures (350 and 450 °C) were used for pyrolysis of the Pb-contaminated biomass in this work, and the retention efficiency of Pb in various chars is shown in Figure 4. As can be seen, the Pb retentions in the two chars derived from the un-pretreated biomass were both less than 85%, and higher pyrolysis temperature (450 °C) led to lower retention efficiency due to the emission of Pb with volatile matters [27,35]. In comparison, the Pb retentions in the four chars (PA_0.5char350, PA_1.0char350, PA_0.5char450, and PA_1.0char450) derived from the PA-pretreated biomass were all >95%, and a higher dosage of PA used for pretreatment favors retention of more Pb in the char, indicating that PA pretreatment helped inhibit the emission of Pb during pyrolysis. PA is often used for activation of carbonization of biomass [49,50], and has shown to be effective for increasing the yield of char according to the previous reports [51,52], which is in consistency with that observed in this study (Figure 4a). The higher yield of char means less production of volatile matters, and introduction of PO₄³⁻ may also strengthen the binding of Pb to the solid char, which will be investigated in-depth in the following Section 3.4. Therefore, PA pretreatment of the biomass and subsequent pyrolysis enhanced the retention of Pb in the char, which is beneficial for reducing the hazard of secondary pollution due to pyrolysis of heavy metal-contaminated biomass.

3.3. Influence of Pyrolysis on Stability of Pb in the Char

The stability of Pb retained in the chars was first evaluated with the DTPA extraction method. Figure 5a shows the bioavailability of Pb in six chars, and the three Pb-contaminated biomass samples were also included for comparison. As can be seen, Pb adsorbed by biomass is readily bioavailable, whether the un-pretreated biomass or the PA-pretreated biomass was used as the sorbent. The DTPA-extractable Pb from the three biomass are all over 85%, so the sole PA pretreatment could not ensure the stability of Pb, although its sorption was enhanced as that shown in Figure 1b. Pyrolysis of the contaminated biomass sharply reduced the bioavailability of Pb in the char; in particular, the DTPA-extractable Pb is lower than 5% for the three chars obtained by pyrolysis at a lower temperature (Pb@Char350, Pb@PA_0.5Char350, and Pb@PA_1.0Char350). The DTPA-extractable Pb in the last two chars derived from the PA-pretreated biomass is close to zero. The results show that pyrolysis significantly enhanced the stability of Pb, and PA pretreatment can further reduce the bioavailability of Pb in the char. However, the DTPA-extractable Pb from the chars was lifted to >28% as the pyrolysis temperature increased to 450 °C, indicating that a higher pyrolysis temperature is unfavorable for immobilization of Pb. The similar results about the influence of pyrolysis temperature on stability of heavy metals in the char have been reported by Shi et al. [40]. The reason may be related to the different binding states between Pb and the chars obtained at different pyrolysis temperatures, which will be further discussed in the following Section 3.4.

The Pb immobilization by pyrolysis was further assessed with the TCLP test, and the results of leachable Pb in the biomass and chars are shown in Figure 5b. Interestingly, Pb adsorbed in the biomass was not readily leachable, with the highest leachable ratio of 5.34% observed on the biomass without PA pretreatment, although most of the Pb adsorbed in the biomass was bioavailable (Figure 5a). Such a difference should be related to the interactions between Pb and biomass, which are dominated by surface complexation and cation exchange, because there are plentiful oxygen-containing functional groups and exchangeable cations (e.g., K⁺) on the biomass surface according to the IR and EDS analysis (Figure 2 and Figure 3). These interactions are readily broken down by a chelating agent, such as DTPA, but insensitive to acetic acid used in the TCLP test. Furthermore, pyrolysis increased the amount of leachable Pb in the Pb@Char350 and Pb@Char450 chars, and higher pyrolysis temperatures resulted in more leachable Pb (Figure 5b). The reasons should be mainly related to the change of solid’s properties during the pyrolysis. Pyrolysis makes biomass lose functional groups such as –OH and C–O that are essential for sorption of Pb through surface complexation. At the meantime, inorganic minerals in biomass will be transformed into alkaline matter, such as carbonates or (hydr)oxides [53]. Thus, the interactions between Pb and solid will be changed by formation of Pb oxide or carbonates that are readily dissolved in acid. Therefore, direct pyrolysis, especially at higher temperatures (450 °C), is not favorable for immobilization of Pb in an acidic environment, because nearly half of Pb in the Pb@Char450 sample is leachable (Figure 5b). Despite this result, the encouraging fact is that the two chars (Pb@PA_0.5Char350 and Pb@PA_1.0Char350) obtained at 350 °C and derived from the PA-pretreated biomass show good immobilization of Pb, with the ratio of leachable Pb accounting for less than 3%. In combination with the results shown in Figure 5a, we conclude that pyrolysis at 350 °C can immobilize Pb adsorbed by the PA-pretreated biomass. The similar immobilization of heavy metals in the char using phosphates as the additive for pyrolysis has been observed in our previous studies [39,40,41]. What makes this work differently is that the biomass was firstly pretreated with PA, which enhanced its efficiency for removal of Pb before it was immobilized by pyrolysis. In general, such a combined biosorption-pyrolysis will be a promising strategy for removal of heavy metals from aqueous solution and subsequent safe disposal of the contaminated biosorbents.

3.4. Investigations about the Immobilization Mechanisms

The X-ray diffraction (XRD) technique and IR spectra were used to examine the biomass and chars obtained by different treatments, so as to understand the mechanisms of Pb immobilization. The XRD patterns (Figure 6) indicate that there is no crystalline constituent other than cellulose in the Pb-contaminated biomass. However, the stable PbS crystal was found in all the chars, which should be one reason for immobilization of Pb. Furthermore, several strong peaks attributed to the lead phosphates (e.g., Ca₈Pb₂(PO₄)₆(OH)₂ at 2θ = 51.6°, and Pb₉(PO₄)₆ at 2θ = 26.8° and 33.2°) were observed in the Pb@PA_1.0Char350 sample. These lead phosphates contribute to the enhanced stability of Pb in this char, so the bioavailability and leachability of Pb were suppressed. However, as the pyrolysis temperature increased to 450 °C, the elemental Pb and unstable Pb compounds (PbO and Pb₃(CO₃)(OH)₂) were formed according to the XRD patterns (Figure 6), which can explain the results obtained in the TCLP test, in which more leachable Pb was observed in the chars obtained at a high pyrolysis temperature (450 °C) (Figure 5). Further, a new peak at 1156 cm⁻¹ was found in the IR spectra (Figure 7) of two chars derived from the PA-pretreated biomass (Pb@PA_1.0Char350 and Pb@PA_1.0Char450), which could be assigned to C–O stretching vibrations in P–O–C (aromatic) linkage [49]. Thus, Pb phosphates would be bound to the carbon matrix through this linkage, which made Pb be stably anchored on the char [52].

4. Conclusions

The combined biosorption-pyrolysis process can be used for efficient removal of Pb from aqueous solution and then immobilization of Pb in the char, when the Hydrocotyle biomass is pretreated with PA. The PA-pretreatment increased the surface area and introduced more functional groups in the biomass, and enhanced the removal efficiency of Pb by biosorption. The subsequent pyrolysis of the Pb-contaminated biomass at relatively low temperatures (350 and 450 °C) retained most of the Pb in the char, and use of the PA-pretreatment further increased the retention efficiency to >95%. Furthermore, both the bioavailability and leaching potential of Pb in the char were significantly reduced by pyrolysis with the aid of PA-pretreatment, and pyrolysis at 350 °C is more favorable for the immobilization of Pb than that at 450 °C. The immobilization mechanisms are related to the formation of Pb phosphates and their linkage to the carbon matrix, according to the investigations with XRD and IR spectra. Despite the promising results obtained in this study, the long-time stability of Pb in the char deserves further investigation by considering more environmental factors, such as pH, temperature, humidity, and microbial effect, before the safe disposal of the char.