Batch and Continuous Fixed-Bed Lead Removal Using Himalayan Pine Needle Biochar: Isotherm and Kinetic Studies

Abstract

Pine needle litter in Himalayan forests leads to forest fires, ground water recharge inhibition, soil acidification and contamination, and stops the growth of grass and plants. This study provides a possible solution for pine needle litter problem by converting it to biochar. Pine needle litter lying on the ground for approximately a month was collected from the Himalayan region. The pine needle litter biochars were generated using slow pyrolysis (residence time, 30 min; heating rate, 10 °C/min) at 350, 450, 550, 650, and 750 °C. Finally, pine needle litter biochar prepared at 550 °C (PNBC550) was selected for sorptive removal of aqueous lead both in batch and column studies. The PNBC550 was characterized for proximate and elemental compositions, crystallinity, surface area, morphology, and functional groups. A BET surface area of 230.9 m²/g was obtained for PNBC550. Batch sorption studies were carried out to study (1) the adsorption versus pH studies (at pH 2 to 7), (2) isotherms (at 10, 25, and 35 °C) to evaluate the temperature effect on the sorption efficiency, and (3) kinetics to reveal the effect of time, adsorbent dose, and initial concentration on the reaction rate. Increasing pyrolysis temperature raised lead sorption up to 550 °C. Lead adsorption increased considerably as pH rose from 2 to a maximum adsorption around pH 5 and above. The sorption data were fitted using different isotherm models and kinetic equations. The Langmuir adsorption capacity increased from 22.93 mg/g at 10 °C to 40.43 mg/g at 35 °C, showing that adsorption was endothermic. Fixed-bed studies were conducted at room temperature with an initial lead concentration of 7.85 mg/L and 4.0 g of PNBC550 at initial pH 5.0 and a flow rate of 3 mL/min. Desorption studies conducted under the same experimental conditions found about 90–93% lead recovery. Development of high-efficiency biochars for lead remediation provides a sustainable solution for the Himalayan pine needle litter problem. The biochars also possess the possible potential for aqueous removal of other metal cations.

1. Introduction

The World Health Organization and Bureau of Indian standards have assigned 10 μg/L as the permissible limit of lead in drinking water.^1,2 However, Central Water Commission, India (CWC), has reported existing concentrations of lead exceeding 10 μg/L at 47 monitoring stations of 30 Indian rivers, with the highest concentration of 48.92 μg/L.³ Various treatment technologies⁴ including adsorption⁵⁻⁸ have been implemented to remediate lead from water. The search for sustainable, affordable, and efficient adsorbents for heavy metal remediation is still being studied. During the pyrolysis to biochar, the physical and chemical properties of biomass are altered, a wide range of functional groups, and surface area and pores are formed, thereby enhancing sorption ability.⁹ The sorption capacity of biochar depends on surface area, biochar surface properties, adsorbate concentration, pH, and temperature.^10,11 Most heavy metal sorption onto the biochar proceeds through metal–biochar bindings through chemical precipitation and cation exchanges.^6,12

Lead sorption by biochars produced via pyrolysis from pine and oak bark,¹² pine wood,¹³ manure,^14,15 digested biomass,⁵ coconut shell,¹⁶ and hickory wood¹⁷ has been studied widely. The use of pine needle litter biochar produced via slow pyrolysis for lead remediation from water has not been reported. This study aims at producing biochar from pine needle litter to serve as a forest and waste management tool. Pine needle litter is one of the largest forest wastes reported in Himalayan forests, with a productivity of 6.3 t ha^–1 year^–1 in India.^18,19 Pine needle decaying is a very slow process. Hence, thick layers of accumulated pine needles lead to forest fires, ground water recharge inhibition, soil acidification and contamination, and stop the growth of grass and plants.²⁰⁻²³ Large-scale forest fires due to thick pine needle layers contributes significantly toward air pollution and environmental degradations such as soil moisture, soil fertility, and seedling loss.^24,25 Therefore, benefits of using pine needle litter for biochar preparation include forest fire mitigation and enhanced ground clearance, resulting in enhanced groundwater recharge. Pine needle litter abundance in mountainous regions and its negative environmental and economic implications make it an excellent feedstock for biochar production. Biochars were prepared at five temperatures. Biochar with the highest surface area and adsorption capacity was used to further study the lead adsorption behavior versus concentration, time, dose, pH, and temperature. These data were fitted with different isotherm and kinetic models. Fixed-bed sorption and desorption studies were performed, column parameters were calculated, and the mathematical models were fitted to the experimental data.

2. Results and Discussion

2.1. Characterization

The elemental composition of PNBC550 before and after lead sorption is shown in Table 1. The elemental composition of the pristine biochar was carbon (67.2%), oxygen (22.3%), hydrogen (2.6%), and nitrogen (1.2%). Along with these, minor percentages of different minerals of silicon, calcium, iron, magnesium, and potassium were also present (Table 1). The presence of these minerals played a paramount role in the sorption capacity and mechanism. Lead uptake was 0.29% after sorption (Table 1) versus nil before adsorption. Similar observations have been observed in SEM–EDX elemental maps, suggesting successful sorption of Pb²⁺ (Figure 2).

Table 1. Physiochemical Characteristics of Biochar (PNBC550) Produced from Pine Needle Litter at Pyrolysis Temperature of 550 °C.

proximate and ultimate analysis
biochar	yield (%)	C (%)	H (%)	N (%)	S (%)	O (%)	ash (%)	moisture content (%)	volatile matter (%)	fixed carbon (%)	bulk density (kg/m3)	pH	pHpzc	S.A. (m2/g)
pine needle biomass		47.35	5.98	1.55	0.2	33.97	1.58	9.37	65.75	35.83
PNBC550	30.7	67.23	2.58	1.22	0.00	22.30	6.62	7.45	25.54	78.99	264.99	6.75	∼2	230.9

mineral composition (%)
biochar	SiO2	Al2O3	Fe2O3	MgO	CaO	Na2O	K2O	MnO	TiO2	P2O5	Pb2O3
pristine biochar	0.25	0.22	0.73	0.44	8.85	0.00	0.53	0.17	0.05	0.53	nil
biochar after lead adsorptiona	0.37	0.05	0.77	0.05	8.88	0.00	0.35	0.19	0.04	0.64	0.29

^aBiochar after lead adsorption was prepared at an adsorbent dose of 2 g/L, pH 5.0, temperature of 25 °C, and initial lead concentration of 50 mg/L.

Figure 2.

SEM–EDX elemental mappings of pine needle biochar (PNBC550) before and after lead adsorption.

The BET surface area and ash content of PNBC550 were determined as 230.9 m²/g and 6.6%, respectively. The point of zero charge (pH_pzc) of all the PNBCs was ∼2. Thus, at a solution pH value below 2, the surface is positively charged, increasing the electrostatic repulsion between the sorbent surface and heavy metal ions.³⁹ PNBC550 has a bulk density of 264.99 kg/m³ (Table 1).

The SEM and TEM micrographs of the PNBC550 are shown in Figure 1. The SEM magnifications of 1000× and 3000× appear in Figure 1A,B, respectively. They illustrate the availability of large pore spaces of varying sizes, categorized as macropores, mesopores, and micropores. This agrees with the 230.9 m²/g BET surface area value. The low-magnification biochar image shows macro- and mesopores of irregular shapes with intact plant cell wall structures (Figure 1A). However, at high magnifications, the presence of micropores is clearly visible along with small biochar fragments (Figure 1B). TEM images in Figure 1C,D have 10,000× and 100,000× magnifications, respectively. The TEM images reveal the existence of thin sheet-like structures and grain boundaries. A typical biochar particle with irregular shape and edges along with fused mineral phases is clearly visible in the TEM spectra (Figure 1C,D).

Figure 1.

Scanning electron micrographs at (A) 1000× and (B) 3000× magnifications and transmission electron micrographs at (C) 10000× and (D) 100,000× magnifications of the pine needle biochar (PNBC550).

Figure 2 shows SEM–EDX elemental maps and EDX graphs of PNBC550 before and after lead sorption. In addition to the main elements, Ca is abundant, which agrees well with the physiochemical characterization and XRD. Figure 2 confirms that lead is widely distributed over the biochar surface. The distinct lead peaks in the EDX spectra of PNBC550 after lead sorption confirm its sorption onto biochar. Surface elemental composition shown by SEM–EDX shows similar composition properties as in bulk biochar elemental composition, as provided in Table 1.

The FTIR spectra (4000–500 cm^–1) of PNBC550 before and after lead sorption appear in Figure 3. The 669 cm^–1 peak in lead-loaded and pristine biochars corresponds to out-of-plane graphene bending.⁴⁰ The 885 cm^–1 peak belongs to out-of-plane C–H bending for aromatic rings. Both peaks indicate aromatization in the pyrolyzed feedstock.⁴¹ A peak at 1560 cm^–1 might be attributed to quinone functions.^42,43 A sharp peak at 2360 cm^–1 is associated with CO₂ stretching vibration.⁴⁴ O–H stretching vibrations were found at 3600–3800 cm^–1. The FTIR spectra of PNBC550 before and after lead sorption are the same except for some intensity loss in the C=O and OH stretching vibrations. Thus, involvement of these functional groups in the sorption process is confirmed.

Figure 3.

FTIR spectra of pine needle biochar (PNBC550) before and after lead sorption.

The XRD patterns of the PNBC550 before and after lead sorption are shown in Figure 4. The broad peak centered on the 2θ value of 22° in both biochars corresponds to the crystal plane (002), which reflects the amorphous structure of carbonized carbon.⁴⁵ A sharp peak at 26.5° also corresponds to (002) planes of graphitized carbon (JCPDS card no. 41-1487). These observations are compatible with the FTIR spectra. The 26.5° peak corresponds to layered sp²-hybridized carbon sheets with a d-spacing of 3.35 Å.⁴⁶ A strong peak at 29.4° indicates the presence of calcite.^47,48 The distinctive peak at 24.63° for PNBC350 shows the presence of calcium sulfate. These distinct peaks are correlated with the significant fraction of calcium minerals found in elemental analysis. A peak at 43.5° corresponds to the formation of turbostratic carbon crystallites within the biochar’s structure, which is a characteristic feature of biochar produced at 550 °C.^49,50 The peak at 72.5° corresponds to MgO.⁵¹

Figure 4.

XRD spectra of pine needle biochar (PNBC550) before and after lead sorption.

2.2. Sorption Studies

2.2.1. Effect of pH on Lead Removal

Lead removal was studied in the initial pH range of 2 to 7 for all five biochars (PNBC350, PNBC450, PNBC550, PNBC650, and PNBC750). The results are shown in Figure 5A. Change in pH with an increase in pyrolysis temperature and difference in the equilibrium pH and initial pH (ΔpH) for PNBC550 were determined and are depicted in Figure 5B,C, respectively. At pH values greater than 7, lead hydroxide/oxide species form since Pb²⁺ ions suffer from solvation, hydrolysis, and lead oxide precipitates. Thus, precipitation competes with adsorption and absorption; hence, it cannot be independently studied.¹² Pb²⁺ removal increased as the solution pH increased and became fairly constant around pH 5. Similar results have also been reported earlier.^5,6,12 The minimum Pb²⁺ removal was ∼10% at pH 2 for all five biochars. Lead removal increased >80% as pH approached 5, except for PNBC350. In the low pH range, the Pb²⁺ ions compete with the acidic (H₃O⁺) ion for binding sites. Most lead sorbed as reversibly held Pb²⁺ over pH 2−7. Protonation of carboxylate groups at low pH prevents carboxylate anion formation and coordination of Pb²⁺. As pH rises, surface oxygen groups increasingly deprotonate, progressively attracting and sorbing Pb²⁺ ions. As pH rises, electrostatic adsorbate and adsorbent attraction dominates, increasing Pb²⁺ adsorption. Thus, adsorption increases as pH rises from 2 to 7.

Figure 5.

Effect of lead sorption using pine needle biochars: (A) biochars prepared at pyrolysis temperatures of 350, 450, 550, 650, and 750 °C; (B) initial pH vs equilibrium pH of the biochars [initial Pb²⁺ concentration, 10 mg/L; adsorbent dose, 2 g/L; temperature, 25 °C]; (C) change in pH in PNBC550 with respect to different initial pH values.

The filtrate’s equilibrium pH was higher than the initial pH for all five biochars. The difference in the equilibrium pH and initial pH (ΔpH) was determined and is depicted in Figure 5C. The ΔpH increased initially and then decreased, indicating precipitation and ion exchange as possible mechanisms for lead adsorption.¹⁵ The equilibrium pH was affected by the pyrolytic temperature at which biochars were formed. Pyrolysis temperatures above 500 °C increasingly promote decarboxylation and dehydration, altering the surface functional groups present and decreasing surface acidity, consequently lowering negative charge on the surface.⁵² Higher pyrolysis temperatures produce more alkaline biochars (Figure 5B). Higher pyrolytic temperatures also generate more insoluble carbonates of magnesium and calcium, which enhance basicity. This can also raise Pb²⁺ adsorption capacity. The biochar prepared at temperatures from 550 to 750 °C removed >90% Pb²⁺. The cost to produce PNBC550 was lower since less energy was required and higher yields occur. Thus, it was selected for further sorption experiments. The optimal pH value of 5 was selected to achieve maximum lead removal capacity by true adsorption without any possible precipitation.

Changes in pyrolysis temperature lead to changes in surface properties. Surface and bulk adsorbent properties play an important role in sorption. Relations between key biochar properties and Pb²⁺ sorption at pH 5 onto pine needle biochars prepared at different temperatures are shown in Figure 6. Significant correlation between BET surface area, ash content, and % calcium oxide content is shown in Figure 6A,B,D, respectively. Initially, a correlation is observed between equilibrium pH and pyrolysis temperature and Pb²⁺ sorption, but sorption stabilizes as pyrolysis temperature reaches 550 °C (Figure 6C). The relation of sorption with ash content and % calcium oxide suggests the role of divalent cations such as calcium in Pb²⁺ sorption via the cation exchange mechanism, while relation with the BET surface area suggests the role of surface area in the sorption process. Thus, surface area, surface active sites, and the presence of divalent cations play an important role in sorption.

Figure 6.

Relation between sorption capacity of biochars produced at different temperatures and (A) % ash, (B) BET surface area, (C) equilibrium pH, and (D) % calcium oxide.

2.2.2. Sorption Dynamics

The contact time to reach equilibrium was investigated at different Pb²⁺ concentrations and different adsorbent doses. Sorption was rapid during the initial phase followed by a slow increase as it approached equilibrium. The adsorption of lead onto the biochar is the function of contact time. The change in Pb²⁺ concentration versus contact time on PNBC550 at 25 °C was investigated at 10, 25, and 50 mg/L Pb²⁺ using a fixed 2 g/L adsorbent dose for 48 h (Figure S1A). A substantial increase in the adsorption capacity of PNBC550 was found as the concentration of Pb²⁺ increased from 10 to 50 mg/L. The lead sorption capacity increased from 4.42 to 21.4 mg/g for 10 and 50 mg/L of Pb²⁺, respectively. The ability of Pb²⁺ to bind to the internal pore structure of biochar might explain this increase. Equilibrium was reached in ∼8 h with a removal capacity of ∼12 mg/g. About 60–90% removal occurred in about 12 h.

The effect of adsorbent dose was studied at 25 °C for a Pb²⁺ concentration fixed at 25 mg/L using three gradient adsorbent doses 1, 2, and 4 g/L (Figure S1B). Dose optimization was necessary to find the maximum number of sites available for Pb²⁺ binding. The lead removal increased with more adsorbent dose because the number of adsorption sites for Pb²⁺ increased as the dose increased.⁹ Nearly 50 to 90% of adsorption occurred in the first 1 h of contact time. The sorption capacity varied from 6 to 21.3 mg/g for adsorbent doses 1 to 4 g/L. The adsorption equilibrium was reached in around 8 h with >95% removal by the dose of 4 g/L. Pb²⁺ removal was complete in about 12 h. The high adsorption capacities at a relatively low biochar doses make PNBC550 a desirable adsorbent for Pb²⁺ removal.

The experimental results were modeled to nonlinear pseudo-first-order,⁵³ pseudo-second-order,⁵⁴ Elovich equation,⁵⁵ and intraparticle diffusion⁵⁶ models (using OriginPro 8 from OriginLab, USA) to help elucidate the sorption mechanism and determine the rate process and kinetic parameters for fixed-bed adsorption design. These simulations are shown in Figure S2, Figure 7, and Table 2.

Figure 7.

Kinetic data fitted to (A) pseudo-first-order model, (B) pseudo-second-order model, (C) Elovich model, and (D) intraparticle diffusion model to determine the effect of initial Pb²⁺ concentration using PNBC550 [Pb²⁺ concentrations, 10, 25, and 50 mg/L; adsorbent dose, 2 g/L; pH 5.0; temperature, 25 °C].

Table 2. Pseudo-First-Order, Pseudo-Second-Order, Elovich, and Intraparticle Diffusion Model Parameters for Pb²⁺ Adsorption at Different PNBC550 Doses and Pb²⁺ Concentrations.

		pseudo-first-order model			pseudo-second-order model			Elovich model			intraparticle diffusion model
		at different PNBC550 doses [pH 5; Pb2+ concentration, 25 mg/L; PNBC550 doses, 1.0, 2.0, and 4.0 g/L]
PNBC550 dose (g/L)	amount adsorbed as calc. from experiments, qe (mg/g)	qe (mg/g)	k1 (h–1)	R2	qe (mg/g)	K2 (g/mg·h)	R2	α (mg/g·min)	Β (g/mg)	R2	kd1 (mg/g·h0.5)	C1	R2	kd2 (mg/g·h0.5)	C2	R2
1.0	21.34	18.47	0.35	0.78	20.60	0.02	0.91	39.63	0.29	0.97	4.41	3.61	0.97	1.21	13.1	0.98
2.0	12.31	10.92	0.62	0.78	12.13	0.07	0.93	47.98	0.54	0.97	2.70	2.96	0.99	0.41	9.70	0.72
4.0	6.16	5.99	1.40	0.90	6.316	0.36	0.99	961.0	1.65	0.80	2.31	1.90	0.99	0.06	5.83	0.39

		at different Pb2+ conc. (mg/L) [pH 5; PNBC550 dose, 2 g/L; Pb2+ concentrations, 10, 25, and 50 mg/L]
Pb2+ conc. (mg/L)	amount adsorbed as calc. from experiments, qe (mg/g)	qe (mg/g)	k1 (h–1)	R2	qe (mg/g)	K2 (g/mg·h)	R2	α (mg/g·min)	Β (g/mg)	R2	kd1 (mg/g·h0.5)	C1	R2	kd2 (mg/g·h0.5)	C2
10.00	4.42	96.30	4.33	0.75	4.48	2.94	0.98	19.69	0.26	0.98	5.61	0.41	0.99	1.55	11.03
25.00	13.60	98.24	12.77	0.74	13.67	0.83	0.91	41.5 × 106	4.97	0.72	3.66	3.79	0.98	0.18	12.46
50.00	21.38	88.28	19.82	0.91	22.09	0.23	0.97	178.00	0.58	0.90	0.87	2.89	0.97	0.01	4.40

The kinetic data fit best to the pseudo-second-order model (R² varying from 0.908 to 0.977 for the Pb²⁺ concentration study and R² = 0.909–0.988 for the biochar dose study). The theoretical q_e (22.09 mg/g) matched well with the experimental q_e (21.38 mg/g) (Table 2). The model assumes the binuclear sorption process with chemisorption as a rate-limiting step. At low PNBC550 doses (1 and 2 g/L), the Elovich equation gave a better fit than the pseudo-second-order model (R² = 0.973 and 0.965). Similar observations were reported in a concentration kinetic study with R² = 0.983 at 10 mg/L of Pb²⁺ followed by a big drop at high concentration.³⁷ Multilinear plots were found when applying the intraparticle diffusion (IPD) model to the experimental data. Thus, adsorption is controlled by more than one step. Figure S2D and Figure 7D confirm that Pb²⁺ adsorption proceeds in two steps, first, mass transfer of the adsorbate molecule to the PNBC550 boundary layer followed by adsorbate diffusion into PNBC550. Thus, a variety of chemisorption reactions including ion exchange and surface adsorbate complexation seem to play a large role in rate control of sorption.

2.2.3. Adsorption Isotherm/Equilibrium Studies

An adsorption isotherm expresses the relation between adsorbate concentration and adsorption capacity at a fixed adsorbent dose and temperature. Studies were conducted at 10, 25, and 35 °C using lead concentrations from 5 to 100 mg/L on PNBC550 (dose, 2 g/L) (Figure S3). The isotherm plots obtained were positive, concave, and S-shaped and reached a plateau at fairly high equilibrium concentrations, suggesting favorable Pb²⁺ sorption. The capacity increased with rising temperature, consistent with endothermic adsorption. The fitted isotherm plots are shown in Figure 8A–F. Table 3 contains the isotherm parameters and regression coefficients (R²) for each model. All the isotherm models fit well, with R² values exceeding 0.921 except for the Freundlich isotherm at 10 and 25 °C, where R² = 0.849 and R² = 0.882, respectively. The data best fitted the Redlich and Peterson model (R² value range of 0.997–0.999). Thus, Pb²⁺ sorption follows Freundlich multilayer behavior at high concentration and Langmuir homogeneous site-specific monolayer behavior at low adsorbate concentration.^12,44 These conclusions are consistent with kinetic studies where Pb²⁺ sorption onto PNBC550 was governed by multiple processes.

Figure 8.

Lead sorption equilibrium date fitted to (A) Freundlich, (B) Langmuir, (C) Redlich–Peterson, (D) Toth, (E) Sips, and (F) Radke–Prausnitz using PNBC550 at 10, 25, and 35 °C [pH 5.0; initial Pb²⁺ concentration range, 2–100 mg/L; adsorbent dose, 2 g/L].

Table 3. Nonlinear Freundlich, Langmuir, Redlich–Peterson, Toth, Sips, and Radke–Prausnitz Adsorption Isotherm Parameters Obtained for Pb²⁺ Adsorption on Pine Needle Biochar (PNBC550).

	temperature
isotherm parameter	35 °C	25 °C	10 °C
Frendulich
KF (mg/g)	1.356	1.669	1.994
n	1.545	1.731	2.012
R2	0.928	0.882	0.849
Langmuir
Q° (mg/g)	40.43	31.81	22.93
b	0.016	0.023	0.034
R2	0.967	0.945	0.961
Redlich–Peterson
KRP (L/g)	0.450	0.460	0.479
aRP (L/mg)	1.206 × 10–6	1.786 × 10–6	7.854 × 10–5
β	2.993	2.979	2.234
R2	0.997	0.999	0.999
Toth
KT (mg/g)	0.491	0.512	0.545
B (L/mg)	0.003	0.004	0.009
β	–2.698	–2.028	–0.705
R2	0.995	0.999	0.998
Sips
KLF (L/g)	0.147	0.124	0.134
αLF (L/mg)	0.005	0.005	0.007
βS	1.542	1.666	1.708
R2	0.980	0.966	0.970
Radke–Prausnitz
a	0.651	0.726	0.7767
b	40.27	31.82	22.93
β	2.178 × 10–15	1.491 × 10–15	4.073 × 10–16
R2	0.957	0.9274	0.919

PNBC550’s Langmuir adsorption capacities were 22.93, 31.81, and 40.43 mg/g at 10, 25, and 35 °C, respectively. Thus, PNBC550 can be used for Pb²⁺ sorption from warm process waters. Separation factors (R_L) were determined at 10, 25, and 35 °C over a broad concentration range (Table 4). R_L values increased with higher Pb²⁺ concentration, as previously reported.¹² The values of R_L varied between 0.384 and 0.227. These values are less than 1, indicating favorable adsorption.⁵⁷

Table 4. Thermodynamic Parameters and Separation Factors for Lead Adsorption onto PNBC550 at Temperatures 10, 25, and 35 °C.

	thermodynamic parameter
temperature (°C)	ΔG° (kJ/mol)	ΔH° (kJ/mol)	ΔS° (kJ/mol)	separation factor (RL)
10	–9.72	17.34	0.026	0.384
25	–9.35	29.87	0.068	0.303
35	–8.66	21.88	0.043	0.227

2.2.4. Thermodynamic Studies

The thermodynamic parameters ΔG°, ΔH°, and ΔS° were calculated (Table 4). The Pb²⁺ sorption on PNBC550 was spontaneous with negative ΔG° values (−8.6 to −9.7 kJ/mol), consistent with the ion exchange mechanism where ΔG° values lie between −10 and −20 kJ/mol.³⁹ When the ΔG° magnitude is ∼20 kJ/mol, physisorption dominates due to adsorbate/adsorbent electrostatic interactions.³⁹ However, ΔG° values from −80 to −400 kJ/mol belong exclusively to chemisorption.³⁹ A large negative ΔG° value indicates chemical bond formation between the adsorbate and adsorbent.¹⁷ The ΔG° values for Pb²⁺ sorption here lie between −8.6 and −9.7 kJ/mol, indicating favorable Pb²⁺ sorption onto PNBC550. The positive ΔH° ( +21.88 kJ/mol) and negative ΔS° (−43 J/mol·K) values at 35 °C illustrate the endothermic and more ordered nature of the sorption products.

2.3. Fixed-Bed Column

Packed-bed columns provide a continuous adsorption system in contrast to batch sorption studies, allowing metal removal at a lower cost. The effectiveness of adsorption is described by the breakthrough curve (BTC), obtained by plotting effluent concentration versus time. The breakthrough time and the shape of breakthrough curve allow the determination of the column’s performance. Typically, the BTC is S-shaped (Figure 9). Initially, the bed has no Pb²⁺. Over the time, the primary adsorption zone (PAZ) is established. Necessary fixed-bed column parameters including concentration (C_x) required for establishing of PAZ, concentration (Cb) required for initial PAZ formation, volume (V_x) required for establishing of PAZ, volume (V_b) required for initial PAZ formation, time (t_x) to establish the PAZ, time (δ) for the axial movement of PAZ and length of the PAZ were calculated. Fractional capacity (f), bed depth (D), time (t_b) required for initial PAZ formation, mass flow rate to the absorber (F_m), the % column saturation at breakthrough, the bed volume, the empty-bed-contact-time (EBCT), and the biochar usage rate were also determined. These parameters were estimated using the equations defined previously,⁵⁸ and they are shown in Table 5.

Figure 9.

PNBC550 breakthrough curve (Pb²⁺ concentration, 8 mg/L; initial pH 5.0) [effluent pH curve for Pb²⁺ adsorption by PNBC550 is provided in the Supporting Information].

Table 5. Continuous Fixed-Bed Column Parameters Obtained Using the Breakthrough Curve.

parameter	PNBC550
Cο (mg/mL)	7.85 × 10–4
Cx (mg/mL)	7.82 × 10–4
Cb (mg/mL)	6.80 × 10–4
Vx (mg/cm2)	24.81
Vb (mg/cm2)	7.78
(Vx – Vb) (mg/cm2)	17.03
Fm (mg/cm2/min)	7.4 × 0–4
D (cm)	6.50
tx (min)	3.35 × 103
tb (min)	8.51 × 102
tδ (min)	22.96 × 102
F	62.94 × 10–2
δ (cm)	4.64
empty-bed-contact-time (EBCT) (min)	6.80
percent saturation	72.37
biochar usage rate (g/L)	1.29

The fixed-bed column Pb²⁺ capacity (10.52 mg/g) was lower than the batch sorption capacity (31.81 mg/g) due to the presence of a wide mass transfer zone. This resulted in the reduction of actual bed capacity (Figure 9). The pH variations of the influent and effluent solution during the column studies were evaluated (Figure S4). The equilibrium pH initially rose to 7.72 and then dropped gradually to 6.53 as the column approached the breakpoint. The high initial effluent pH is due to the basic nature of biochar. At the exhaustion point, the equilibrium pH (6.10) and initial pH (5.23) were comparable. This similarity in pH corresponds to no Pb²⁺ adsorption occurring onto the column. Thus, the column has reached the maximum adsorption capacity and is exhausted. Similar observations were reported.⁵⁹

This experimental column data were fitted to the theoretical column models of Thomas,⁶⁰ Bohart and Adam (BDST),⁶¹ Yoon and Nelson,⁶² and Clark⁶³ (Figure S5). Mathematically, the Yoon–Nelson model is equivalent to the Bohart–Adam model, while the Bohart–Adam model is a special case of Thomas model when the Langmuir equilibrium isotherm is favorable. Hence, the different parameters of each model represent equivalent parameters. The models were nicely correlated with the experimental data with regression coefficients from 0.912 to 0.933 (Table 6). Similar observations have been reported previously.⁶⁴⁻⁶⁶

Table 6. Linear Thomas, Bohart–Adams (BDST), Yoon–Nelson, and Clark Fixed-Bed Adsorption Parameters Obtained for Pb²⁺ Adsorption on Pine Needle Biochar (PNBC550) [Influent Concentration, 7.85 mg/L; Bed Height, 6.5 cm; Flow Rate, 3 mL/min].

name of the model	model parameter
Thomas	KTh (L/g·min)	0.0002
	q° (mg/g)	10.73
	R2	0.912
Bohart–Adams (BDST)	K (L/g·min)	0.0002
	N° (g/L)	6.536
	R2	0.912
Yoon–Nelson	kYN (min–1)	0.0019
	Τ (min)	1802.4
	R2	0.912
Clark	kc (L/mg·min)	0.0002
	N° (g/L)	6.160
	R2	0.933

2.4. Desorption Studies

Sorbent reusability makes the adsorption treatment more cost-effective. Desorption studies were conducted to define the biochar’s recyclable, technical, and economic feasibility. A cost-effective and nonpolluting eluent is required that can easily regenerate and does not damage the biochar. Initially, three eluting agents, 0.005 M HNO₃, 0.01 M HNO₃, and 0.01 M EDTA, were used in batch desorption studies. Desorption was restricted to acidic conditions, where the adsorbent’s surface is protonated, favoring Pb²⁺ desorption. However, H₂SO₄ was avoided due to the low PbSO₄ solubility. PbSO₄ can precipitate and lower the biochar pore volume.³⁹ The chelating agent EDTA is used in lead poisoning since it forms a stable Pb²⁺-chelating compound.⁶⁷ A high desorption efficiency (90–93%) was shown by all three eluting agents (Figure 10). Within the addition of 20 mL of eluent, nearly 75–90% of Pb²⁺ adsorbed on the biochar was stripped. The similar desorption efficiencies made 0.005 M HNO₃ the most cost-effective eluting agent. Using 0.005 M HNO₃, the exhausted column desorption was performed at the same flow rate (3 mL/min) and bed height (4 cm) as in the fixed-bed sorption experiment. Approximately 80% of lead desorption was achieved on treating lead-loaded PNBC550 with 100 mL of 0.005 M HNO₃.

Figure 10.

PNBC550 desorption curves for Pb²⁺ using 0.01 M HNO₃, 0.005 M HNO₃, and 0.01 M EDTA.

3. Sorption Mechanism

Representative Pb²⁺ sorption structures onto PNBC550 are suggested in Figure 11. The formation of chelation complexes to various surface functional groups (hydroxyl, carboxyl, etc.) captures Pb²⁺ in the micro- and macropores. Chelation binds most of the lead that is taken up and releases protons to the aqueous phase, so this represents a type of ion exchange reaction. Different functional groups such as carbonyl, hydroxyl, phenolic, and quinone functional groups are involved in both divalent and monovalent complexations with divalent lead ions. The shifts and intensity changes of FITR and XRD peaks provided evidence for complex formation. Lead ions coordinate with carbonyl groups, as shown in Figure 11. Pb²⁺ complexation with carboxylate groups is shown in Figure 11 together with other representative complexes.

Figure 11.

Possible sorption mechanisms for Pb²⁺ removal using PNBC550.

The abundant presence of calcium along with a small amount of magnesium and potassium (Table 1) significantly contributes to biochar’s alkalinity.⁶⁸ These cations can exchange with Pb²⁺ ions through cation exchange mechanisms. Cationic exchange of lead ions from the solution has also been reported previously.^5,69,70 The exchange of Pb²⁺ can be due to the displacement of monovalent and divalent cations such as K⁺, Ca²⁺, and Mg²⁺ available on the surface of biochar to the solution. Incomplete Pb²⁺ desorption with nitric acid and EDTA also confirms the involvement of surface complexation reactions.⁷¹

4. Conclusions

The pine needle litter biochar (PNBC550) was prepared and effectively used for aqueous Pb²⁺ removal. The PNBC550 was characterized for yield, ash content, elemental composition (C, H, N, S, and O), pH_pzc, FTIR, XRD, SEM, SEM–EDX, TEM, WDXRF, EDXRF, and BET surface area (230.9 m²/g). Batch kinetic and equilibrium sorption studies were carried out to determine the sorption efficiency and reaction rate. The Langmuir adsorption capacity increased from 22.93 mg/g at 10 °C to 40.43 mg/g at 35 °C, showing that adsorption was endothermic. The fitting of experimental data to various models indicated that multiple mechanisms (e.g., precipitation, complexation, cation exchange, and intraparticle diffusion) are responsible for Pb²⁺ sorption. Fixed-bed column studies were conducted to evaluate process scaleup. The fixed-bed column Pb²⁺ capacity (10.52 mg/g) was lower than the batch sorption capacity (31.81 mg/g) due to the presence of a wide mass transfer zone. Pine needle biochar shows comparable or higher Pb²⁺ sorption capacity versus biochar based adsorbents (Table 7). Three eluting agents, 0.005 M HNO₃, 0.01 M HNO₃, and 0.01 M EDTA, were used for batch Pb²⁺ desorption. A high desorption efficiency (90–93%) was shown by all three eluting agents. Similarly, desorption studies were carried out to recover Pb²⁺ and to regenerate biochar without dismantling the column. Approximately 80% of lead desorption was achieved on treating lead-loaded PNBC550 with 100 mL of 0.005 M HNO₃. This study provides a possible solution for using the excessive pine needle litter in the Himalayan region for generation of high-efficiency biochars for remediating aqueous lead and possibly other metal cations.

Table 7. Langmuir Adsorption Capacities of Different Adsorbents Used for Pb²⁺ at Ambient Temperature.

adsorbent	thermochemical conversion process	adsorption capacity (mg/g)	reference
pine needle biochar	slow pyrolysis (550 °C)	31.8	this study
hickory wood	slow pyrolysis (600 °C)	11.2	(72)
Douglas fir biochar	gasification (900–1000 °C)	37.7	(73)
magnetized Douglas fir biochar	gasification (900–1000 °C)	26.0
pig manure biochar	slow pyrolysis (400 °C)	6.5	(15)
oak bark	fast pyrolysis (400–450 °C)	13.1	(6)
energy cane biochar	fast pyrolysis (400 °C)	45.7	(51)
Salisbury biochar (British broadleaf hardwood)	slow pyrolysis (600 °C)	47.6	(74)
coconut fiber biochar	slow pyrolysis (300 °C)	38.0	(16)
sugarcane bagasse biochar (BC250)	slow pyrolysis (250 °C)	20.5	(69)
sugarcane bagasse biochar (BC600)	slow pyrolysis (600 °C)	6.0
manganese oxide-modified pine biochar	slow pyrolysis (600 °C)	4.9	(75)
plum stone biochar	slow pyrolysis (600 °C)	23.1	(76)
surfactant-dispersed CNT-modified hickory biochar	slow pyrolysis (600 °C)	15.2	(77)
surfactant-dispersed CNT-modified bagasse biochars	slow pyrolysis (600 °C)	13.7
hickory biochar (B350)	slow pyrolysis (350 °C)	16.3	(72)
hickory biochar (B450)	slow pyrolysis (450 °C)	15.1
hickory biochar (B600)	slow pyrolysis (600 °C)	12.2
sewage sludge biochar	slow pyrolysis (350 °C)	6.5	(78)
H2O2-treated sewage sludge biochar	slow pyrolysis (350 °C)	25

5. Materials and Methods

5.1. Reagents and Equipment

All chemicals used were either of AR or GR grade. Details about reagents, equipment, stock solutions, and analysis are provided in Section S1.

5.2. Production of Biochar

Pine needle litter lying on the ground for a month approximately was collected from the Himalayan region from the forested area of Tehri district, Uttarakhand (latitude, 30°, 21.548′N; longitude, 78°, 12.474′E; elevation, 6261 m). Biochars were produced by slow pyrolysis (residence time, 30 min; heating rate, 10 °C/min) of pine (Pinus roxburghii) needle litter at 350, 450, 550, 650, and 750 °C. The collected litter was washed and dried before performing slow pyrolysis in a muffle furnace (model no. F6000, Thermo Scientific). These biochars were sieved to obtain a homogeneous size fraction (30–50 B.S.S. mesh). The sieved fraction was stored in an air-tight container until characterization and sorption experiments were conducted. More details about pine needle collection and biochar development are provided in Section S2.

5.3. Characterization of Biochar

5.3.1. Proximate and Ultimate Analysis

American Society for Testing and Materials method (ASTM) D1762-84 was used to determine the moisture content, volatile matter, and fixed carbon and ash content of the pine needle litter and biochar.²⁶ The ash content of the biochar was determined by heating biochar samples in an open quartz crucible for 6 h at 750 °C. Total carbon, nitrogen, and sulfur contents were measured using a varioMicro V3.1.1, CHNS analyzer. The pH and point of zero charge (pH_pzc) were measured using a Thermo Scientific pH meter (model ORION 5 STAR). The pH was measured after adding biochar to deionized water (1:20) (w/v) and allowing it to stand for 30 min. The point of zero charge was measured using 0.01 M NaCl adjusted to pH 2, 4, 6, 8, and 10 by either a 0.1 M HCl or 0.1 M NaOH aqueous solution. The pH-adjusted solutions (20 mL) were contacted with 0.02 g of biochar for 48 h. The supernatant liquid was then decanted, and pH was measured. The pH_pzc value was determined from the plot of initial pH versus pH of the supernatant.

5.3.2. Surface Area and Morphological Studies

The surface area analysis of pine needle litter biochar prepared at 550 °C (PNBC550) was measured from its N₂ isotherm using a gas sorption analyzer (Monosorb rapid BET surface analyzer). The morphology of the unloaded PNBC550 and lead-loaded PNBC550 was examined using scanning electron microscopy (SEM) (model EVO 40, Zeiss) at different magnifications ranging from 200× to 10,000×. SEM/EDX analysis was carried out on biochar loaded onto carbon tape on stainless steel stubs using a Zeiss EVO 40 SEM employing a BRUKER EDX system to study the surface elemental composition. Transmission electron microscopy (TEM) analysis was carried out using a transmission electron microscope (model JEOL 2100F) at magnifications up to 100,000× for both loaded and unloaded biochars.

5.3.3. Elemental and Spectral Studies

Wavelength dispersive X-ray fluorescence and energy dispersive X-ray fluorescence spectrometers (PANalytical Epsilon 5) were used to determine the elemental composition in the PNBC550 before and after lead adsorption. An X-ray diffractometer (X’Pert PRO, PANalytical) with Cu-Kα radiation at 45 kV and 40 mA from 5° to 90° at a scanning speed of 2° min^–1 was used to study the crystalline structure in the biochars. Qualitative and quantitative analysis of organic functional groups present on the biochar surface was performed using FTIR spectroscopy (7000 FTIR, Varian). KBR spectroscopy grade pellets were made under 10 tons of hydraulic pressure (model CAP-15 T, Spectrochrom). The spectra of pristine biochar and lead-loaded biochar were recorded in the transmittance mode from 4000 to 400 cm^–1 using eight scans at 4 cm^–1 resolution.

5.4. Sorption Experiments

5.4.1. Batch Sorption Studies

The batch lead sorption experiments were carried out to optimize pyrolysis temperature, pH, adsorbent dose, time, initial concentration, and temperature.^27,28 Studies in the 2–7 pH range were performed at desired biochar doses and lead concentration at 25 °C. Test solutions were agitated for specific time intervals (maximum, 48 h). The adsorption capacity of Pb²⁺ onto PNBC550 was calculated using eq 1.

1

where q_e (mg/g) is the amount of lead adsorbed, C_o (mg/L) and C_e (mg/L) are the initial and equilibrium metal lead concentrations, W is the adsorbent weight (g), and V (L) is the volume of lead solution.

Isotherm and kinetic studies were conducted to study the effect of contact time, adsorbent dose, and temperature on the sorption capacity of biochar. Isotherm studies employed a fixed amount of adsorbent (2 g/L) added to 50 mL of lead solution with the concentration varying from 5 to 100 mg/L at 10, 25, and 35 °C. Kinetic studies used a fixed lead concentration (25 mg/L) at three doses of PNBC550 (1, 2, and 4 g/L) followed by 48 h of agitation at 25 °C. Later, the lead concentration was varied (10, 25, and 50 mg/L) at a fixed PNBC 550 dose (2 g/L) with 48 h of agitation at 25 °C. The lead remaining was analyzed using AAS.

5.4.2. Column Sorption Studies

Fixed-bed column sorption was conducted in a 2.0 cm-diameter and 40 cm-height acrylic column at room temperature (∼25 °C). Pine needle biochar (PNBC550) (4.0 g) was slurried with warm DDW and packed into the column supported by glass wool. The slurry was fed in slowly to avoid air entrapment, consequently forming a well-packed column. The overall bed height of the wet packed column was 6.5 cm. Initially, the column was flushed with distilled water to remove any adhering ash. An influent flow rate of 3 mL/min was maintained under gravity from the top of the column. The column experiment was conducted with the influent of ∼8 mg/L lead concentration at pH 5. Effluent samples from the column were collected at 15 min time intervals and analyzed for final pH and lead concentration until the effluent and influent concentrations became similar. The fixed-bed column parameters, including times to establish a primary adsorption zone, breakthrough, and saturation as well as bed depth, fraction capacity, and column capacity, were evaluated by plotting C_e/C₀ versus volume of the effluent (mL).²⁷⁻²⁹C_e is the equilibrium concentration and C₀ is the initial concentration of influent. The breakthrough (C_b) and exhaustion concentrations (C_e) were determined when the effluent lead concentration reached 10 and 90%, respectively, of the initial lead concentration. Desorption of the exhausted column was carried out using 0.005 M HNO₃ as the eluent.

5.5. Desorption Studies

Desorption requires finding a suitable eluent. EDTA (0.01 M), HNO₃ (0.005 M), and HNO₃ (0.01 M) were tested. PNBC550 (0.1 g) was added to 50 mL of Pb²⁺ solution (50 mg/L) and agitated at 25 °C for 24 h. The suspension was filtered and the loaded biochar was collected and dried. The adsorbed lead was stripped from loaded biochar by immersion and agitation in 60 mL of each eluent at 25 °C for 30 min. The solution was filtered and its Pb²⁺ concentration was determined. Later desorption studies were carried out in a fixed-bed column. The saturated column was desorbed using HNO₃ (0.005 M). Effluent samples from the column were collected at every 15 min time interval over a 2 h period and analyzed for lead concentration. The desorption efficiency was calculated using eq 2.

2

5.6. Adsorption Modeling

5.6.1. Sorption Isotherms

The adsorption isotherm models describe the adsorbate’s mobility from the aqueous phase to solid phase at a constant temperature. The saturation of the solid phase results in adsorption equilibrium. These models attempt to explain the capacity of the solid phase (adsorbent), the mechanism involved in sorption, and the effect of the thermodynamic parameters on adsorption.³⁰ The isotherm models (Langmuir,³¹ Freundlich,³² Radke and Prausnitz,³³ Redlich–Peterson,³⁴ Sips,³⁵ and Toth³⁶) applied to the sorption data are summarized in Table S1 with their equations.

5.6.2. Sorption Kinetics

Kinetic models assist in designing effective adsorption systems.^37,38 Different kinetic equations applied on the sorption data are summarized in Table S1.

5.6.3. Thermodynamic Behavior

The thermodynamic studies were at 10, 25 and 35 °C with the Pb²⁺ concentrations varying from 5 to 100 mg/L. The Gibb’s free energy change (ΔG°), enthalpy change (ΔH°), and entropy change (ΔS°) were calculated according to van’t Hoff eqs 3–5.

3

4

5

where b, b₁, and b₂ are the Langmuir constants at 10, 25, and 35 °C, respectively, R is the gas constant (8.314 J/mol·K), and T (K) is the absolute temperature.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.0c00216.

• Detailed reagents and equipment; biochar production; detailed batch sorption studies; table containing isotherm and kinetic models used to fit the sorption equilibrium and kinetic data; figures containing the effect of initial lead concentration; pseudo-first-order, pseudo-second-order, Elovich, and intraparticle diffusion graphs; sorption isotherm experimental curve; sorption dynamic effluent pH curve; column linear sorption models Thomas, BDST, and Yoon–Nelson model and Clark model; and references (PDF)

Author Present Address

^§ (V.C.) Environment and Water Resource Division, Department of Civil Engineering, IIT Madras, Chennai 600036, India.

The authors declare no competing financial interest.