Removal of Pb(II), Cu(II), and Ag(I) from aqueous solutions using biochar derived by P-enriched water hyacinth

ABSTRACT

A new synthesis was developed to prepare engineered biochar derived from phosphorous-rich water hyacinth. The statistical analysis indicated a significant correlation between P content and the removal rates of Pb (II), Cu (II), and Ag (I), revealing the pivotal role of enrich P in the biochar for heavy metals (HM) removal. SEM-EDX, XRD and FTIR analyses validated the presence of P functional groups on the surface of P-biochar. Batch experiments showed that the strong acidic initial solution negatively affected the adsorption of HM ions, and a similar negative impact was observed during Cu (II) adsorption under the high ionic strength conditions. Competitive adsorption experiment confirmed P-biochar’s resilience against competing metal ions during the adsorption of HM ions. Pb (II) and Ag (I) adsorption were primarily governed by precipitation/reduction, along with complexation reactions involving P functional groups. Cu (II) adsorption was dominated by complexation between Cu (II) and P functional groups.

KEYWORDS:

• Plant biomass
• biochar
• phosphorous-enriched feedstock
• heavy metals removal
• adsorption mechanism

1. Introduction

As society and the economy continue to advance, environmental contamination persists as a consequence of various pollutants. Heavy metals (HM), typical pollutants, not only compromise environmental quality but also pose significant risks to humans and organisms even at low concentrations. This is attributed to the non-biodegradability, bioaccumulation, and poor mobility of HM [1–3]. Aquatic environments, particularly rivers, lakes, and groundwater, play a dual role as both sink and primary transport mediums for HM [4,5], with even precious metals being lost through contaminated water release [6]. Consequently, there is a growing interest in the scientific and engineering communities to address the treatment of aqueous HM contaminants before their release into environments [7–9]. Traditional treatment approaches for HM decontamination can be classified as physical, chemical and biological treatments, such as membrane filtration, chemical precipitation, electrochemical recovery, bioremediation, and photoreduction [10–12]. While these approaches have been extensively evaluated in aqueous solutions, their large-scale application is hindered by high costs and complex operations [13]. Compared with these approaches, adsorption, known for its simplicity and cost-effectiveness, stands out as a preferred option [11,14]. Activated carbon (AC), the most commonly used adsorbent, exhibits exceptional efficiency due to its large surface area, porous structure, and adsorption sites. However, its widespread application is limited by high production costs, extreme processing temperatures (>800°C), and dependence on fossil-based resources [15]. Consequently, there is a need to explore alternative adsorbents derived from low-cost and renewable sources through straightforward processes.

In recent decades, biochar derived from various biomass sources has emerged as a potential substitute for HM removal from aqueous solutions, offering advantages such as low cost and abundant feedstock [16] [17] [18]. While most pristine biochars exhibit limited HM adsorption compared to traditional adsorbents like AC, various activation approaches have been developed to enhance their efficiency [19–22]. Among these activation approaches, the method focusing on the inherent high common elements contents of original feedstock, instead of biochar or its precursor modified with chemical compounds is recently highlighted. Yao et al. [23] and Ahmad et al. [24] demonstrated high removal efficiency of phosphate, Cu(II), Cd(II) and Pb(II) by Mg/K-enriched biochar prepared from feedstocks enriched with Mg/K through bioaccumulation during slow pyrolysis.

Recent studies suggested that biochars supported by phosphorous functional groups (P-biochars) exhibit superior HM removal ability compared to their original biochars due to the interaction between metal ions and phosphorous functional groups (P-containing groups) [13,25–27]. These interactions include processes such as precipitation and complexation. Among these studies, chemical activation remains a prevalent method for preparing P-biochar. Phosphorous (P), like Mg and K, is also a common element in plants as essential nutrients. A great number of aquatic plants have high P content, especially those used for eutrophication remediation [28]. This suggests that P-biochar might be synthesized directly from plants that are rich in P through slow pyrolysis. If feasible, this novel approach may not only create an easier production approach for P-biochar (pretreatment is unrequired), but also make it possible to produce substitutive options for the plant nutrient enrichment and even phytoremediation technologies to create valuable by-products. However, very few literatures regarding P-biochar directly synthesized from plant biomass residues without pretreatment have been reported to date.

In this study, a novel approach was used to prepare P-biochar to adsorb Pb(II), Cu(II), and Ag(I) from aqueous solutions. P-biochar adsorbents were synthesized directly from P-enriched water hyacinth (Eichnornia crassipes (Mart.) Solms), an aquatic plant used for eutrophication remediation due its strong ability to adsorb and accumulate P [29]. The main objective of this study was to evaluate whether P-biochar could be prepared by directly pyrolysis of P-enriched water hyacinth. Physicochemical properties of the resulting P-biochars before and after HM ions adsorption were characterized. Batch adsorption experiments were carried out to evaluate HM removal capability of the P-biochars and the relationship between biochar’s HM removal ability and its P content was explored. Specifically, the influencing factors for HM ions adsorption onto P-biochar and reusability, as well as the mechanisms of HM ions adsorption were investigated.

2. Materials and methods

2.1. Materials and P-biochar preparation

All chemical reagents used in this study including Pb(NO₃)₂, Cu(NO₃)₂·3 H₂O, AgNO₃, Cd(NO₃)₂·4 H₂O, Ni(NO₃)₂·6 H₂O, Co(NO₃)₂·6 H₂O, Mg(NO₃)₂·6 H₂O, NaNO₃, KH₂PO₄, Hoagland’s nutrient reagent (minus phosphate), 65% HNO₃ and NaOH were analytical grade, and purchased from Tianjin Chemical Works Co. Ltd. (Tianjin, China). All chemical solutions were prepared with deionized water (DI, ρ = 8 MΩ cm).

For P enrichment, small water hyacinth plants (length 3–4 cm) were selected and grown in a large container with 0.5-strength Hoagland nutrient solution at 20 mg/L phosphate until their length was ~10–12 cm (about 6 weeks). The solution was refreshed weekly and pH was maintained around 5.50 by adding 1 mol/L NaOH as needed. After harvest, the whole water hyacinth plants were washed several times, oven dried, and grounded into ~2 mm pieces as feedstock for biochar production. Then, the biomass feedstock was heated through slow pyrolysis in a tube furnace (MXG1200-40S, China) in N₂ atmosphere. The controller of the tube furnace was programmed to drive the internal chamber temperature to 300°C, 400°C, 600°C, and 800°C at a rate of 10°C/min and maintained at the peak temperature for 1 h [24,30]. All biochar samples were sieved through 0.5 mm, washed with DI water until the pH reached neutral, oven-dried, and sealed in an airtight bottle before use. The resulting P-biochar samples were referred to as PBC3, PBC4, PBC6 and PBC8.

Besides the four P-biochars prepared, an additional 18 biochars were used to analyze the relationship between their Pb(II)/Cu(II)/Ag(I) removal rate and respective P and other essential inorganic elements contents. Ten biochars were prepared using banana peel, bamboo, chicken manure, and peanut hulls using the same method. Additionally, eight more biochars were produced from bagasse, corn stalks, tea and coffee waste under various conditions as detailed in previously published studies [31,32].

2.2. Biochar characterization

In addition to P content, all four P-biochar samples were examined for other inorganic elements and C. According to the relationship between P content and HM removal rate, the physicochemical properties of pre- and post-adsorbed PBC4 including crystallographic structures, surface functional groups, surface morphology, and their surface elemental compositions, were further investigated in this study. Total inorganic elements were measured using inductively coupled plasma atomic emission spectroscopy (ICP-OES, PE Optima8000, PerkinElmer, U.S.A.) after acid digestion. Elemental C was obtained using an Elemental Analyzer (Perkin Elemer 2400, U.S.A.). X-ray diffraction (XRD) pattern was performed with a computer-controlled X-ray diffractometer (Panalytical X’Pert3 Powder, Malvern Panalytical, Netherland) equipped with a stepping motor and graphite crystal monochromator to identify the crystallographic structures. To determine the surface functional groups, fourier transform infrared (FTIR) spectra were reported by FTIR spectrometer (Nicolet 6700, Thermo Fisher Scientific, U.S.A.). The surface morphology and their compositions of elements at the same surface locations were also carried out through a scanning electron microscope (SEM, Tescan Mira LMS, Czech) equipped with an energy dispersive X-ray spectroscopy (EDX, Xford 30, Britain).

2.3. Adsorption experiments

2.3.1. Heavy metals adsorption ability

Pb(II), Cu(II), and Ag(I) solutions were prepared by dissolving Pb(NO₃)₂, Cu(NO₃)₂·3 H₂O and AgNO₃ in DI water, respectively, and HM ions solutions were used in all the adsorption experiments. Pb(II), Cu(II), and Ag(I) adsorption ability of all 22 biochars were identified using 100 ml digestion vessels with a 1: 500 biochar/solution ratio (0.05 g biochar in 25 ml solution) at room temperature (25 ± 0.5°C) for 48 h (sufficient for adsorption equilibrium). The concentrations of Pb(II)/Ag(I), and Cu(II) in the solution were 500 and 100 mg/L, respectively. The mixtures were immediately filtered, and the concentration of HM ions was measured by ICP-OES.

2.3.2. Adsorption kinetics and isotherm

PBC4 was selected to investigate both adsorption kinetics and isotherm in this study. In the kinetics tests, 500 mg/L Pb(II)/Ag(I), and 100 mg/L Cu(II) solutions were prepared, and appropriate time intervals ranged between 0.5 and 48 h. In the isotherm tests, the concentration of Cu(II) solution was in the range from 10 to 500 mg/L, while Pb(II) and Ag(I) solutions were ranged from 20 to 2000 mg/L, and 50 to 1000 mg/L, respectively. The initial Pb(II), Cu(II), and Ag(I) solution pH were roughly 5.75 ± 0.10, 5.80 ± 0.10 and 6.85 ± 0.10, which was natural pH with initial concentrations of 500 mg/L Pb(II)/Ag(I), and 100 mg/L Cu(II). The pH value was used in all solutions in the adsorption experiments except for the pH effect study.

In addition to the commonly used pseudo-first-order and pseudo-second-order models, the Elovich model and intraparticle diffusion model were also examined. Based on mononuclear and binuclear adsorption, the kinetics of the solid–liquid system was described in first- and second-order [33]. The models of Elovich and intraparticle diffusion describe the contribution of desorption and a diffusion process for adsorbate on the adsorbent, respectively [31,32]. Detailed information about the four kinetics fitting equations and the meaning of their relevant parameters can be found in Liu et al.’s [32].

Langmuir, Freundlich, Langmuir-Freundlich, and Redlich-Peterson, four common models, were used to fit the isotherm experimental data to explore how the adsorbate is distributed from the liquid to the solid phases under an equilibrium condition. Monolayer adsorption onto a homogeneous surface was described by the Langmuir model []. In contrast, the other three models, as empirical or semiempirical models, usually describe heterogeneous adsorption processes [34]. Detailed information about the four isotherm fitting equations and the meaning of their relevant parameters can be found in Liu et al.’s [32].

2.3.3. Influence factors of heavy metals adsorption ability

The effect of initial pH on the interaction between PBC4 and adsorbate was evaluated by adjusting the initial pH ranging from 1 to 5.75 and 1 to 5.80 with 500 mg/L Pb(II) and 100 mg/L Cu(II) solutions, respectively, as well as ranging from 1 to 6.80 with 500 mg/L Ag(I) solution. Varying initial pH was adjusted with 0.01 mol/L NaOH/HNO₃. The impact of ionic strength (IS) on HM ions adsorption by PBC4 was also investigated by adding NaNO₃ into 500 mg/L Pb(II)/Ag(I), and 100 mg/L Cu(II) solutions. The Na(I) concentration was in the range of 0.001 to 0.1 mol/L in Pb(II)/Cu(II) solution and 0.001 to 0.01 mol/L in Ag(I) solution. Besides, the competitive adsorption tests were conducted by a series of dual-metal solution of Pb(II), Cu(II), Ag(I), Cd(II), Ni(II), Co(II), and Mg(II), which consisted of Pb(II)/Ag(I) and one coexistent metal ions with 500 mg/L concentration, as well as Cu(II) and one coexistent metal ions with 100 mg/L concentration. Other procedures were the same as the one described above.

2.4. Regeneration experiment

To examine the reusability of PBC4, a series of adsorption and desorption experiments were performed by using the saturated PBC4 collected from isotherm experiments after discarding the supernatant Pb(II)/Cu(II)/Ag(I) solution. This adsorption-desorption test was repeated four times. In the desorption experiment, the saturated PBC4 was washed multiple times with DI water to remove non-adsorbed HM ions, and agitated with 0.1 mol/L HNO₃ for 1 h. After filtering, the recovered biochar was washed with DI water several times until the pH was neutral, then oven-dried at 80°C for further adsorption test with the same procedure described above.

2.5. Statistics

In this study, the least squares method was used to study the relationship of Pb(II), Cu(II), and Ag(I) removal ability and P content of all 22 biochars by various approaches from current and previous research. All adsorption and regeneration experiments were carried out in triplicate, and the means data were reported. Differences between the means were analyzed with one-way ANOVA followed by Tukey’s HSD test at a significant level of p < 0.05. Error bars represent a standard deviations (SD) of triplicate determinations. Origin 9.0 software was used to conduct all statistics (coefficient of determination (R²), p and SD) and perform all figures. Additionally, the data of FITR were fitting by Peakfit 4.12 before testifying the presence of surface functional groups.

3. Results and discussion

3.1. Elemental analysis in P-biochar

As shown in Table 1, carbonization through slow pyrolysis at different temperatures concentrated P in all four resulting biochar samples. The content of P varied from 0.84% to 1.96% in P-biochars. which is higher than that of other biochars (generally below 0.50%) previously studied [32,35]. In comparison, the contents of other essential inorganic elements ranging 4.32–7.29% for Ca, 1.89–3.76% for Mg, 0.11–0.36% for Na and K were similar to that in most of the biochars reported in literatures [13,32,36–38]. The contents of minor inorganic elements and HM, such as Fe, Al, Cu, Zn, Pb, Ag, Mn, Cd, and As were very low even under the detection limit. Additionally, the content of C indicated that all four resulting biochar samples were carbon-rich with carbon compositions varying from 53.18% to 80.09%, which is typical of pyrolyzed biomass [32,39]. These results revealed that the new approach could successfully prepare carbon-nonmetal biochar composite from plant feedstock enriched with anionic nutrient elements through bioaccumulation.

Table 1. Elemental analysis of P-biochars used in this study (dry mass %). (PBC3 = P-biochar prepared at 300°C, PBC4 = P-biochar prepared at 400°C, PBC6 = P-biochar prepared at 600°C, and PBC8 = P-biochar prepared at 800°C).

Biochars		Mass based (%)^b
	C	P	Ca	Mg	K	Na	Al	Fe	Cu	Zn	Pb	Ag	Mn	Cd	As
PBC3	53.18	0.84	4.32	1.89	0.36	0.11	0.005	0.01	nd^a	0.003	nd	nd	0.002	nd	nd
PBC4	62.72	1.23	5.11	2.45	0.30	0.15	0.006	0.03	nd	0.004	nd	nd	0.002	nd	nd
PBC6	72.77	1.72	5.62	2.12	0.31	0.21	0.004	0.05	0.005	0.004	nd	nd	0.005	nd	nd
PBC8	80.09	1.96	7.29	3.76	0.28	0.23	0.004	0.05	0.01	0.007	nd	nd	0.006	nd	nd

^and: Below the detection limit.

^bThe data was the average of the three replicates.

3.2. Effect of P enrichment on heavy metals removal by biochar

The adsorption ability of Pb(II), Cu(II), and Ag(I) ions for all four P-biochars were evaluated. According to data in Figure 1, the HM removal rate significantly changed among biochar samples. The correlations between HM removal rate and P content of P-biochars were examined statistically. Comparison of Pb(II), Cu(II), and Ag(I) adsorption onto four P-biochars created in this study demonstrated that biochar with high P content removed a high percentage of Pb(II), Cu(II), and Ag(I) ions from the solutions, which were more than 50% removal rate, except the removal rate of Ag(I) ions by PBC3 was only ~37% Figure 1(c). The finding is consistent with results from previous literatures that P-biochars have great adsorption ability to aqueous metal ions [40–42]. Meanwhile, Figure 2 shows the correlations between HM removal rate of a total of 22 biochar samples prepared by different approaches from current and previous studies and their P content. Based on the results, statistical report showed that there was a great and statistically significant correlation between P content and Pb(II), Cu(II), and Ag(I) removal rates Figure 2(a), R² = 0.7781, Figure 2(b), R² = 0.7812, Figure 2(c), R² = 0.791, and p < 0.05, respectively). Conversely, the other essential inorganic elements (Ca, Mg, and K) are not important to Pb(II), Cu(II), and Ag(I) removal by the biochars and no significant correlations were found (Table S1). So it can draw a conclusion that Pb(II), Cu(II), and Ag(I) adsorption onto the resulting P-biochars is dominated by the presence of P in the carbon matrix. This also provides a reasonable explanation why the new approach that plant enriched with P element was successful in synthesizing biochar for HM removal.

Figure 1. Removal rates of (a) Pb (II), (b) Cu (II), and (c) Ag (I) by four P-biochars. Mean values among P-biochars are different using Tukey’s HSD test at p < 0.05. (initial concentrations of adsorbate: 500 mg/L Pb (II), 100 mg/L Cu (II), and 500 mg/L Ag (I), corresponding pH 5.75 ± 0.10, 5.80 ± 0.10, 6.85 ± 0.10, respectively; contact time: 48 h; temperature: 25 ± 0.5°C; adsorbent dose: 2 g/L; and adsorbate solution volume: 25 ml). PBC3=P-biochar prepared at 300°C, PBC4=P-biochar prepared at 400°C, PBC6=P-biochar prepared at 600°C, and PBC8=P-biochar prepared at 800°C. Error bars represent the standard deviations of triplicate samples.

Figure 2. Correlation between (a) Pb (II), (b) Cu (II), and (c) Ag (I) removal rates and P content of a total of 22 biochars. (initial concentrations of adsorbate: 500 mg/L Pb (II), 100 mg/L Cu (II), and 500 mg/L Ag (I), corresponding pH 5.75 ± 0.10, 5.80 ± 0.10, 6.85 ± 0.10, respectively; contact time: 48 h; temperature: 25 ± 0.5°C; adsorbent dose: 2 g/L; and adsorbate solution volume: 25 ml).

3.3. Characterization of P-biochar before and after adsorption

Like PBC6 and PBC8, PBC4 exhibited great HM adsorption ability (Figure 1). Importantly, it consumed less electrical power due to a relatively lower synthesis temperature compared to PBC6 and PBC8. The high temperature generally means considerable electricity consumption [16,43]. Thus, pre- and post-adsorbed PBC4 were selected to be further investigate. SEM images of pre- and post-adsorbed PBC4 with 3000× magnification are shown in Figure 3. Figure 3(a) exhibited the morphology of pre-adsorbed PBC4, an irregular shape with knaggy surface, which was also found in PBC4 after Cu(II) adsorption Figure 3(c), suggesting that precipitation might not play a role in Cu(II) adsorption. On the contrary, the images exposed a relatively rough surface of PBC4 after Pb(II) and Ag(I) adsorption due to a great number of tiny particles distributed onto the surface of PBC4 Figure 3(b,d), which could be resulted from Pb(II) and Ag(I) precipitated onto PBC4. Meanwhile, the EDX spectrum showed a relatively strong P peak in pre- and post-adsorbed PBC4 (Figure 3e-h). Strong signals of Pb, Cu, and Ag were also detected in post-adsorbed PBC4 in EDX data Figure 3(f-h). These results further confirmed that P was successfully implanted onto PBC4, and lead, copper and silver elements were presented on the surface of PBC4 after HM ions adsorption.

Figure 3. (Continued)

Figure 3. SEM image (a-d) and EDX spectrum (e-h) of pre- and post-adsorbed PBC4: (a) pre-PBC4, 3000×; (b) post-adsorbed PBC4 treated with Pb (II), 3000×; (c) post-adsorbed PBC4 treated with Cu (II), 3000×; and (d) post-adsorbed PBC4 treated with Ag (I), 3000 × . The EDS spectra (e-h) were obtained at the same location as shown in the SEM images. PBC4=P-biochar prepared at 400°C.

The wide angle XRD patterns of pre- and post-adsorbed PBC4 were conducted as shown in Figure 4. Compared with Pb- and Ag-loaded biochars, there were no special peaks detected in pre-adsorbed and Cu-loaded biochars except for peaks representing SiO₂ and CaCO₃, two common minerals within biochars. In contrast, the XRD patterns exposed the strong crystallinity of Pb- and Ag-loaded biochar. The data of XRD validated the existence of metallic Ag crystals on the surface of PBC4 after Ag(I) adsorption, while the strong signals, indexing Pb₅(PO₄)₃Cl and Pb₅(PO₄)₃OH, were found in XRD patterns of Pb-loaded biochar as well, which is consistent with Zhao et al. [13] and Zhou and Zhang et al.’s [44]. Therefore, the tiny particles in the SEM images of Pb- and Ag-loaded biochars are Pb₅(PO₄)₃Cl and Pb₅(PO₄)₃OH precipitation, as well as metallic silver particles, respectively, suggesting that Pb(II) ions from aqueous solution were precipitated onto the surface of PBC4, while Ag(I) adsorption onto PBC4 should be attributed to Ag(I) reduced by PBC4.

Figure 4. XRD patterns of PBC4 before and after Pb(II), Cu(II), and Ag(I) adsorption. PBC4=P-biochar prepared at 400°C.

To further confirm P contribution to Pb(II), Cu(II), and Ag(I) adsorption onto PBC4, FTIR measurements of pre- and post-adsorbed PBC4 were performed. Figure 5(a) presents the adsorption peaks at around 766, 990, 1110, 1382, 1595, 2722, 2910 and 3424 cm⁻¹ for pre-adsorbed PBC4, which are corresponding to =C-H, P⁺-O, P-OH/P-O-C/P-O-P, -CH₃, carbonyl C=O/aromatic C=C, H-C=O, aliphatic-CH₂/aliphatic -CH₃ and –OH, respectively [18,22,31,44–47];. Among these functional groups, other functional groups usually can be found in various biochars, except for P-containing groups. The P-containing groups occurred in biochars with high P content only. These abundant P-containing groups theoretically might be responsible for HM ions adsorption onto PBC4. According to the data of FTIR spectra of post-adsorbed PBC4 Figure 5(b-d), although there were no new functional groups formed compared to pre-adsorbed PBC4, peaks assigning to original P-containing groups covering the pre-adsorbed PBC4 obviously changed after Pb(II), Cu(II), and Ag(II) adsorption. For Pb- and Cu-loaded biochars, peak indexing P-OH/P-O-C/P-O-P in polyphosphates was detected at 1126 and 1050 cm⁻¹, while peak indexing to P⁺-O in hypophosphite ester changed from 990 cm⁻¹ to 970 and 953 cm⁻¹, suggesting that P-containing groups did play a key role in Pb (II) and Cu (II) adsorption. This observation might be explained by three reasons: (1) crystal compounds containing P-O were formed during Pb(II) adsorption process, which is consistent with the data of SEM and XRD Figures 2(b) and 3(b); (2) Pb(II) were complexed by P⁺-O; and (3) both P-OH/P-O-C/P-O-P and P⁺-O on the surface of PBC4 participated in Cu(II) adsorption due to the complexation reactions. Moreover, FTIR analysis of Ag-loaded biochar exhibited the fact that the peak of P⁺-O at 990 cm⁻¹ disappeared, but the peak intensity of P-OH/P-O-C/P-O-P at around 1110 cm⁻¹ increased, meaning that more phosphate occurred after Ag(I) adsorption, which could ascribe to Ag(I) reduced as metallic Ag by P⁺-O [48]. Besides, Ag(I) might complexed with P-OH/P-O-C/P-O-P. Based on these findings, we can further speculate that the amount of P-containing groups had been successfully implanted onto the surface of biochar and served as active adsorption sites for Pb(II), Cu(II), and Ag(I) adsorption by different mechanisms.

Figure 5. FTIR spectra of PBC4 before (a) and after (b) Pb(II), (c) Cu(II), and (d) Ag(I) adsorption. PBC4=P-biochar prepared at 400°C.

3.4. Adsorption kinetics and isotherm

To better explore the processes governing the adsorption of Pb(II), Cu(II), and Ag(I) ions onto PBC4, studies including both kinetics and isotherm were performed and adsorption models were applied. The data and models of HM adsorption by PBC4 can be found in Figures 6 and 7.

Figure 6. Kinetics data and modeling for heavy metals adsorption by PBC4: (a) Pb (II), (b) Cu (II), and (c) Ag (I) full adsorption; (d) Pb (II), (e) Cu (II), and (f) Ag (I) pre-equilibrium adsorption versus square root of time. (initial concentrations of adsorbate: 500 mg/L Pb (II), 100 mg/L Cu (II), and 500 mg/L Ag (I), corresponding pH 5.75 ± 0.10, 5.80 ± 0.10, 6.85 ± 0.10, respectively; temperature: 25 ± 0.5°C; adsorbent dose: 2 g/L; and adsorbate solution volume: 25 ml). PBC4=P-biochar prepared at 400°C. Symbols are experimental data, and lines are model results.

Figure 7. Isotherm data and modeling for heavy metals adsorption by PBC4: (a) Pb (II), (b) Cu (II), and (c) Ag (I) adsorption. (contact time: 48 h; temperature 25 ± 0.5°C; adsorbent dose: 2 g/L, adsorbate solution volume: 25 ml; and pH 5.75 ± 0.10 for Pb (II) solution, 5.80 ± 0.10 for Cu (II) solution, and 6.85 ± 0.10 for Ag (I) solution). PBC4=P-biochar prepared at 400°C. Symbols are experimental data, and lines are model results.

3.4.1. Adsorption kinetics

In Figure 6, the data of Pb(II), Cu(II), and Ag(I) adsorption kinetics are shown. Obviously, the adsorption equilibriums of HM onto PBC4 were achieved more than 24 h. In addition, PBC4 presented nearly similar trends during Pb(II) and Ag(I) adsorption processes, which including two phases: a rapid adsorption initial phase during the first initial time of adsorption and a slow phase to arrive equilibrium Figure 6(a,c), while only slow adsorption phase was exhibited during Cu(II) adsorption process Figure 6(b). The rapid adsorption phase is mainly controlled by physisorption, and chemisorption is the dominant force governing the slow adsorption phase until reaching equilibrium. These findings revealed that Pb(II) and Ag(I) adsorption rates onto PBC4 May be controlled by physisorption in the initial 8 h, then chemisorption played an important role after 8 h. However, the whole Cu(II) adsorption process was merely governed by chemisorption. The best-fit parameters are shown in Table 2, and the Elovich model was the best model describing the adsorption of Pb(II), Cu(II), and Ag(I) ions onto PBC4 with R² > 0.97. Elovich model assumes that the chemisorption attended in the process of pollutants adsorption onto the heterogeneous surfaces. Hence, HM ions adsorption onto PBC4 in this study could be mainly governed by chemisorption via complexation and/or precipitation involving surface functional groups of PBC4, which was supported by the results of characterization of PBC4 as well. Detailed information regarding the dominating mechanisms of HM ions adsorption onto PBC4 will be discussed later. Additionally, several previous studies on kinetic behavior have shown that intraparticle diffusion is crucial to the adsorption process [34,44,49]. In this study, the diffusion limitation also made a great contribution to the adsorption of Pb(II), Cu(II), and Ag(I) ions onto PBC4. As shown in Figure 6(d-f), the data of the pre-equilibrium (i.e. before 24) Pb(II), Cu(II) and Ag(I) adsorption demonstrated a linear dependency (R² = 0.9276, 0.9765 and 0.8945, respectively) with huge intercept, meaning that both film and intraparticle diffusion mechanisms may play an important role in governing the rate-limiting of HM ions adsorption for PBC4.

Table 2. Best-fit models parameter of Pb(II), Cu(II), and Ag(I) adsorption kinetics and isotherms by PBC4. (PBC4 = P-biochar prepared at 400°C).

Element		Parameter1	Parameter2	Parameter3	R^2
	Adsorption kinetics
	First-order	k1 = 2.088 × 10^−1 h^−1	Qe = 178.5 mg/g		0.8949
Pb	Second-order	k2 = 1.140 × 10^−3 g/(mg h)	Qe = 208.6 mg/g		0.9444
	Elovich	ß = 2.272 × 10^−2 g/mg	ɑ = 95.24 mg/(g h)		0.9789
	First-order	k1 = 4.555 × 10^−2 h^−1	Qe = 40.69 mg/g		0.9765
Cu	Second-order	k2 = 6.749 × 10^−4 g/(mg h)	Qe = 56.00 mg/g		0.9805
	Elovich	ß = 5.785 × 10^−2 g/mg	ɑ = 2.556 mg/(g h)		0.9846
	First-order	k1 = 9.780 × 10^−1 h^−1	Qe = 146.5 mg/g		0.8749
Ag	Second-order	k2 = 8.300 × 10^−3 g/(mg h)	Qe = 158.75 mg/g		0.9390
	Elovich	ß = 4.646 × 10^−2 g/mg	ɑ = 1453 mg/(g h)		0.9901
	Adsorption isotherms
	Langmuir	K = 4.685 × 10^−1 L/mg	Q = 253.9 mg/g		0.9525
Pb	Freundlich	Kf = 119.3 mg^(1-^n) L^n/g	n = 1.196 × 10^−1		0.9407
	Langmuir-Freundlich	Klf = 4.641 × 10^−1 L^n/mg^n	Q = 284.4 mg/g	n = 4.974 × 10^−1	0.9619
	Redlich-Peterson	Kr = 158.0 L/g	a = 8.418 × 10^−1 L^n/mg^n	n = 9.500 × 10^−1	0.9600
	Langmuir	K = 2.137 × 10^−2 L/mg	Q = 92.95 mg/g		0.9645
Cu	Freundlich	Kf = 9.588 mg^(1-^n) L^n/g	n = 3.790 × 10^−1		0.9967
	Langmuir-Freundlich	Klf = 1.740 × 10^−2 L^n/mg^n	Q = 522.3 mg/g	n = 4.182 × 10^−1	0.9970
	Redlich-Peterson	Kr = 85.40 L/g	a = 8.412 L^n/mg^n	n = 6.309 × 10^−1	0.9969
	Langmuir	K = 9.158 × 10^−2 L/mg	Q = 209.2 mg/g		0.9390
Ag	Freundlich	Kf = 72.72 mg^(1-^n) L^n/g	n = 1.753 × 10^−1		0.9592
	Langmuir-Freundlich	Klf = 2.858 × 10^−1 L^n/mg^n	Q = 271.9 mg/g	n = 4.002 × 10^−1	0.9726
	Redlich-Peterson	Kr = 300.8 L/g	a = 3.345 L^n/mg^n	n = 8.614 × 10^−1	0.9664

3.4.2. Adsorption isotherms

As shown in Figure 7, the extent of HM ions adsorption onto PBC4 increased rapidly as the initial concentration increased in the adsorbate solution, especially Pb(II) and Ag(I) adsorption. Afterwards, the rapid adsorption step became slow until adsorption equilibrium was achieved. The rapid adsorption occurred at low adsorbate can be attributed to the abundant adsorption sites on the surface of PBC4. With the increase of the initial concentration of HM, adsorption sites are occupied until the saturation of all adsorption sites are reached at equilibrium step. Table 2 lists a range of parameters of the isotherms models. All the models nearly reproduced the isotherm data fairly well with R² > 0.94. However, adsorption of Pb(II), Cu(II), and Ag(I) ions onto PBC4 fitted better with Langmuir-Freundlich model (R² = 0.9619, 0.9970 and 0.9726, respectively) compared with other three models. In general, Langmuir-Freundlich model are used to describe heterogeneous adsorption process. Hence, findings revealed that the interaction HM ions and PBC4 could be affected by both the Langmuir and Freundlich models, and the adsorption of HM ions should be multilayer onto a heterogeneous surface, which is consistent with the data of PBC4 characterization. Furthermore, based on the data of Langmuir model, the maximum adsorption capacities of Pb(II), Cu(II), and Ag(I) ions onto PBC4 were 253.9, 92.95 and 209.2 mg/g, respectively, which were much greater than that of many biochars reported in previous literatures [13,18,24,41,50].

3.5. Influence factors of adsorption ability

3.5.1. Effect of pH

In general, the pH value of adsorbate solution is one of the primary factors impacting on adsorption ability. The relationship between the initial pH and Pb(II), Cu(II), Ag(I) adsorption ability of PBC4 is shown in Figure 8. Under the strong acidic condition, PBC4 exhibited very low adsorption ability to Pb(II) and Cu(II) Figure 8(a,b), and even no Ag(I) adsorption onto PBC4 was detected Figure 8(c). Then, with the increase of initial pH, especially from the strong acidic condition to the weak acidic condition, the adsorption ability of HM ions improved. Pb(II), Cu(II), and Ag(I) adsorption ability jumped into 218.29, 27.57 and 176.87 mg/g at pH 4, pH 3 and pH 5, respectively, which account for more than 98%, 75% and 96% of the adsorbed HM ions onto PBC4 at nature pH with an initial concentration of 500 mg/L Pb(II)/Ag(I), and 100 mg/L Cu(II). However, the trend was not the completely same with Pb(II) and Ag(I) as there were no noticeable effects of changing pH from 4 to 5.75 and from 5 to 6.85 on adsorption ability, respectively. The pH-dependent adsorption is mainly associated with the existence of H⁺ in the solution, since plenty of H⁺ at lower initial pH can compete for the adsorption sites, resulting in prevention of the formation of precipitation containing Pb(II) and metallic Ag, as well as Pb(II)/Cu(II)/Ag(I) complexation [18,51,52]. Also, it was clearly found that Ag(I) adsorption onto PBC4 did not occur at pH between 1 and 3, revealing that PBC4 made it impossible to reduce and complex Ag(I) in the strong acidic condition, since amounts of H⁺ can hinder electrons donated from P⁺-O and occupy sites used to complex between Ag(I) and P-OH/P-O-C/P-O-P. Conversely, as pH is over the weak acidic condition, H⁺ in the solutions markedly reduced, favoring an increment in adsorption sites and electrons from electronating agent due to deprotonation reaction which are contributed to the reactions of chemical adsorption [13]. In a word, despite limited Pb(II), Cu(II), and Ag(I) adsorption onto PBC4 under the strong acidic conditions (pH < 4), there was an efficient and steady adsorption ability of HM ions onto PBC4 when pH is under the weak acidic conditions.

Figure 8. Effect of pH on heavy metals adsorption by PBC4: (a) Pb (II), (b) Cu (II), and (c) Ag (I) adsorption. (initial concentrations of adsorbate: 500 mg/L Pb (II), 100 mg/L Cu (II), and 500 mg/L Ag (I); contact time: 48 h; temperature: 25 ± 0.5°C; adsorbent dose: 2 g/L; and adsorbate solution volume: 25 ml). PBC4=P-biochar prepared at 400°C. Error bars represent the standard deviations of triplicate samples.

3.5.2. Effects of ionic strength and competing metal ions

In addition to pH, other metal ions can possibly interfere with the adsorption ability of Pb(II), Cu(II), and Ag(I) ions. To investigate the resilience against ions and selectivity of PBC4, the effects of IS and competing metal ions on were analyzed. As shown in Figure 9(a,c), Pb(II) and Ag(I) adsorption performance were best when IS is over 0.01 mol/L and 0.001 mo/L, respectively, indicating that IS had positive effect on two HM ions even under the low concentration IS conditions. Whereas adsorption ability of Cu(II) ions suffered a significant drop as IS was higher than or equal to 0.1 mol/L, suggesting the adverse effect of high IS on Cu(II) adsorption onto PBC4 Figure 9(b). Findings from the experiment with IS illustrated that PBC4 has the resilience to counter the negative effect of IS when adsorbing Pb(II), Cu(II), and Ag(I) in solutions with IS interference, especially under the high IS conditions.

Figure 9. Effect of ionic strength (IS) on heavy metals adsorption by PBC4: (a) Pb (II), (b) Cu (II), and (c) Ag (I) adsorption. (initial concentrations of adsorbate: 500 mg/L Pb (II), 100 mg/L Cu (II), and 500 mg/L Ag (I), corresponding pH 5.75 ± 0.10, 5.80 ± 0.10, 6.85 ± 0.10, respectively; contact time: 48 h; temperature: 25 ± 0.5°C; adsorbent dose: 2 g/L; and adsorbate solution volume: 25 ml). PBC4=P-biochar prepared at 400°C. Error bars represent the standard deviations of triplicate samples.

Meanwhile, with 500 mg/L Pb(II) as the background solution, a dramatic competitive relationship existed between Pb(II) and Cu(II). Pb(II) adsorption ability dropped into 86.27 mg/g Figure 10(a), while the presence of Ag(I), Cd(II), Ni(II), Co(II), and Mg(II) showed no obvious effect on Pb(II) adsorption. The adsorption ability of Cd(II), Ni(II), Co(II), and Mg(II) ions were very low, and similar results were found in dual-metals (Ag(I) mixed with Cd(II), Ni(II), Co(II), and Mg(II), respectively) solutions (Table S2). Also, a relatively fierce competing relationship between Ag(I) and Pb(II), Cu(II) were exhibited with 500 mg/L Ag(I) as the background solution Figure 10(c). With 100 mg/L Cu(II) as the background solution, Cu(II) adsorption suffered a distinct reduce in the presence of Pb(II) and Ag(I) Figure 10(b), and the adsorption performance of Cd(II), Ni(II), Co(II), and Mg(II) were very low (Table S2). The results suggested PBC4’s resilience against common competing metal ions (e.g. Cd(II), Ni(II), Co(II), and Mg(II)) in wastewater when adsorbing Pb(II), Cu(II), and Ag(I) ions, though the concentrations of these competing ions were as high as that of Pb(II), Cu(II), and Ag(I) ions. The limited adsorption of ions could be attributed to their low competition or blocking for the adsorption sites [13,24]. In other words, PBC4 is selective to adsorb metal ions. The obvious differences between Pb(II), Cu(II), and Ag(I) adsorption onto PBC4 in the presence of three HM ions (Figure 10 and Table S2) revealed that Pb(II) was easier to be adsorbed and had highest adsorption priority followed by Ag(I) and Cu(II). Shi et al.’s [53] study suggested that numerous factors, such as radii of the hydrated ions, metal electronegativity, and the charge they carry, usually have a noticeable impact on competitive adsorption of metal ions onto adsorbents. In this study, adsorption of Pb(II), Cu(II), and Ag(I) ions may be greatly influenced by their electronegativity and atomic radii. The electronegativity and atomic radii followed the order of Pb(II) > Ag(I) > Cu(II) (2.33, 1.93 vs 1.90; ionic radius = 122, 126 vs 72 p.m.). On one hand, the higher electronegativity can have stronger adsorption potential. On the other hand, the higher atomic radii commonly means the lower hydrated radius which might maximize the Pb(II), and Ag(I) competitive adsorption [18,24].

Figure 10. Effect of competing ions on heavy metals adsorption by PBC4: (a) Pb (II), (b) Cu (II), and (c) Ag (I) adsorption. (initial concentrations of adsorbate: 500 mg/L Pb (II), 100 mg/L Cu (II), and 500 mg/L Ag (I), corresponding pH 5.75 ± 0.10, 5.80 ± 0.10, 6.85 ± 0.10, respectively; contact time: 48 h; temperature: 25 ± 0.5°C; adsorbent dose: 2 g/L; and adsorbate solution volume: 25 ml). PBC4=P-biochar prepared at 400°C. Error bars represent the standard deviations of triplicate samples.

3.6. Regeneration of P-biochar

The regeneration of PBC4 for the removal of Pb(II), Cu(II), and Ag(I) ions was evaluated by performing multiple cycle adsorption tests. 0.1 mol/L HNO₃ solution was selected as the desorption agent to desorb HM ions, since H⁺ generally makes it easy for most of the adsorbates onto biochars to desorb. At first, the Pb(II), Cu(II), and Ag(I) adsorption capacities of PBC4 were 272.18, 85.95 and 199.87 mg/g, respectively. Multiple cycle Pb(II), Cu(II), and Ag(I) adsorption revealed that the regenerated PBC4 undesirably adsorb HM ions after each adsorption-desorption cycle, only accounting for approximately 10–20% of the initial capacity (Figure 11), further authenticating that chemisorption is the main adsorption mechanism for PBC4 and the process is nonreversible for acidic solution served as desorption agent. Hence, P-biochar has high efficiency in removing Pb(II), Cu(II), and Ag(I) from aqueous solutions; however, its potential to be recycled and reused still needed to be deeply investigate, especially the development of desorption agent, which will be conducted in the follow-up study.

Figure 11. Regeneration and cycle performance of PBC4 for heavy metals adsorption: (a) Pb (II), (b) Cu (II), and (c) Ag (I) adsorption. (initial concentrations of adsorbate: 500 mg/L Pb (II), 100 mg/L Cu (II), and 500 mg/L Ag (I), corresponding pH 5.75 ± 0.10, 5.80 ± 0.10, 6.85 ± 0.10, respectively; contact time: 48 h; temperature: 25 ± 0.5°C; adsorbent dose: 2 g/L; and adsorbate solution volume: 25 ml). PBC4=P-biochar prepared at 400°C. Error bars represent the standard deviations of triplicate samples.

3.7. Adsorption mechanisms

Typically, the physisorption depending on porous and surface energy has effects on HM ions’ adsorption performance of biochars. In this study, the adsorption process can be separated into two phases according to the previous discussions (Section 3.3 and 3.4). The first phase was dominated by physisorption mechanisms in which Pb(II), Cu(II), and Ag(I) were adsorbed onto the surface of PBC4 mainly via Van der Waals forces. Then, adsorbed metal ions reacted with P-containing groups on the surface of PBC4 by different chemical mechanisms in the following phase, which were the primary mechanisms of HM ions adsorption. Specifically, Pb(II), and Ag(I) ions in the solution were adsorbed as respective Pb₅(PO₄)₃Cl/Pb₅(PO₄)₃OH precipitation and metallic Ag, along with complexation. In contrast, Cu(II) adsorption was attributed to the complexation between Cu(II) and P-containing groups. Also, adsorption kinetic and isotherms further confirmed that adsorption of HM ions onto PBC4 should be chemical adsorption on a heterogeneous surface, which is agree with the mechanisms analysis from characterization. Based on PBC4 containing plenty of P-containing groups (e.g. phosphate and hypophosphite) and findings from the correlations between P content and three HM ions removal ability, P-containing groups on the PBC4 participated in the adsorption and played a predominant role in the adsorption process. Detailed explanations are as follows: (1) Pb₅(PO₄)₃Cl/Pb₅(PO₄)₃OH precipitation and complexation were formed due to reaction between Pb(II) and phosphate, and hypophosphite, respectively; (2) Cu(II) adsorption was controlled by the surface deposition resulting from complexation reactions involving both phosphate and hypophosphite; and (3) Ag(I) can be reduced as metallic Ag by hypophosphite because of Ag(I) with a high standard potential (+0.799 V) which could normally be reduced by the electronating agents [54], along with the formation of complexation between Ag(I) and phosphate on the surface of PBC4.

4. Conclusion

A novel and straightforward approach has been devised to generate P-biochar directly from the naturally P-enriched plant material, employing slow pyrolysis without any prior pretreatment. The resulting P-biochar exhibited remarkable adsorption capacities for Pb(II), Cu(II), and Ag(I). However, a notable reduction in the adsorption ability for HM ions was observed under strongly acidic conditions. Meanwhile, P-biochar demonstrated resilience against the adverse effects of IS when adsorbing Pb(II), and Ag(I) in solutions with IS interference, especially high IS conditions. The results from competitive adsorption test confirmed that P-biochar has the resilience to counter competing metal ions when adsorbing Pb(II), Cu(II), and Ag(I) ions in dual-metals solutions, with a discernible adsorption priority order of Pb(II) > Ag(I) > Cu(II). The main mechanisms of Pb(II) and Ag(I) adsorption on P-biochar were precipitation as Pb₅(PO₄)₃Cl and Pb₅(PO₄)₃OH/reduction as metallic Ag, along with complexation reactions involving P functional groups. In contrast, Cu(II) adsorption was dominated by complexation between Cu(II) and P functional groups. The findings indicate that the P-biochar produced in this study serves as an eco-friendly and low-cost adsorbent. Considering its stability and efficiency, the P-biochar holds potential as a highly effective adsorbent to remove heavy metals from aqueous solutions. Nevertheless, further investigations are warranted to enhance the regeneration process of the P-biochar. And the effect of low temperature at air atmosphere on properties and HM adsorption capacities of the P-biochar will be explored in the future study.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Supplementary material

Supplemental data for this article can be accessed online at https://doi.org/10.1080/26395940.2024.2322491

Additional information

Funding

The work was supported by the Basic Research Program of Science and Technology Department of Guizhou Province [Qiankehejichu-ZK[2021]Yiban286] and Open Foundation of Collaborative Innovation Center for Wetland Conservation and Green Department of Hebei Province [2023XTCX032].