1. Introduction
Lignin, as the second largest natural polymer in nature after cellulose, has a high carbon content (30–40 wt.%), abundant surface functional groups, and resistance to biochemical degradation, making it widely applicable in many fields [
1]. However, lignin is a by-product of the papermaking industry, with an annual global production exceeding 50 million tons, leading to severe environmental issues [
2,
3]. Currently, lignin is mainly used as a low-value fuel for power generation, with only about 2% being developed into high-value functional materials such as water reducers, adhesives, rubber, and carbon fibers [
1]. The resource utilization of lignin is primarily constrained by its unique physical and chemical properties, such as its richness in organic and inorganic impurities and complex structure [
4]. The development of green, efficient technologies for the resource utilization of lignin is urgent and of significant importance.
Accompanied by the development of socio-economic conditions, the extensive use of heavy metal products has brought about severe heavy metal pollution [
5]. These pollutants can gradually accumulate and intensify through the food chain, ultimately posing a threat to human health [
6]. For example, the intake of lead can adversely affect the human brain and nervous system, as well as damage the hematopoietic system and cause irreversible harm to internal organs such as the kidneys and liver [
7,
8]. Consequently, there has been widespread attention paid towards developing targeted technologies to address heavy metal pollution [
9]. Currently, the primary methods for treating heavy metal-containing wastewater include chemical precipitation, electrochemical treatment, ion exchange, membrane filtration, phytoremediation, and adsorption [
10,
11,
12]. Among these, adsorption is widely promoted due to its simplicity, high removal efficiency, strong applicability, reusability, and low cost [
10,
13]. Biochar, which is produced by the pyrolysis of biomass materials under limited or oxygen-free conditions, has a high carbon content, well-developed porous structure, and abundant surface functional groups [
14]. Extensive research has demonstrated that biochar exhibits an excellent adsorption performance for heavy metal ions in water, making it a highly promising adsorbent [
9].
Notably, biochar is constrained by its inherent surface charge, pore structure, and active adsorption sites, which result in an overall inferior heavy metal adsorption performance compared to other adsorbents like activated carbon, molecular sieves, and MOFs [
10,
13]. For example, research by Tan et al. found that biochar prepared from apple tree branches had a theoretical maximum adsorption capacity for Pb(II) of 53.90 mg/g [
15]. In contrast, Zhang et al. reported that activated carbon derived from rape straw had a theoretical maximum adsorption capacity for Pb(II) of 253.20 mg/g, which is about five times that of apple tree branch biochar [
16]. Studies have shown that functionalized biochar with metal salts can significantly enhance its heavy metal adsorption performance, garnering widespread attention [
17,
18]. Typically, metal salts perform a variety of functions: they serve as activators to facilitate the evolution of biochar’s pore architecture, offer plentiful adsorption sites by acting as active centers, and function as modifiers to alter the physicochemical attributes of biochar, such as the surface charge and pH [
9,
19]. For example, Chen et al. discovered that pomelo peel biochar modified with FeCl
3 and Na
2CO
3 achieved a theoretical maximum adsorption capacity for Pb(II) of 205.39 mg/g, over two times that of pristine biochar [
17]. While metal salt treatment improves the adsorption performance of biochar, it also increases production costs and poses secondary environmental issues. Interestingly, certain biomass sources, being agro-industrial by-products, are naturally rich in metal minerals and can directly produce high-performance biochar, such as industrial alkaline lignin [
20].
In this study, industrial alkali lignin (IALG), rich in metal minerals, was used as a raw material to produce biochar (IALG-BC) through direct pyrolysis at 700 °C using a one-pot method. The surface chemical characteristics, pore features, surface functional groups, and surface morphology of IALG-BC were analyzed, and its effectiveness in immobilizing Pb(II) in wastewater was examined. Additionally, biochar derived from acid-treated IALG, which had its ash content removed (acid-treated IALG-based biochar (A-IALG-BC)), was used as a comparison to elucidate the role of ash (metal minerals) in the preparation and application process of biochar. This research aims to provide new perspectives and technical support for the resourceful utilization of industrial alkali lignin.
3. Materials and Methods
3.1. Raw Materials and Reagents
IALG, lead chloride (PbCl2), sodium hydroxide (NaOH), and hydrogen chloride (HCl) were supplied by Sigma-Aldrich. Deionized water, generated via a Milli-Q water purification system (Millipore, Inc., Bedford, MA, USA), was utilized for the preparation of all solutions. To regulate the pH of the solution, adjustments were made using 0.1 M NaOH and HCl. The ash content of IALG is 20.16 wt.%, with sodium being the main component (14,059 mmol/kg). The ash in IALG was treated using a mixture of nitric acid and hydrofluoric acid. After multiple washes with deionized water, it was dried to obtain ash-free lignin (A-IALG).
3.2. Preparation of Biochar
A sample of 50 g of IALG was weighed and placed in a lidded ceramic crucible, which was then introduced into a muffle furnace. The pyrolysis process was conducted at a temperature of 700 °C for a duration of 2 h. Upon completion of the pyrolysis, the muffle furnace was allowed to cool naturally to room temperature. The resulting solid residue was subsequently removed and ground to produce the target biochar, designated as IALG-BC. Following the same procedure, A-IALG-BC was also prepared.
3.3. Characterization of Samples
An amount of 10 g of IALG was precisely measured using a ceramic crucible and positioned within a muffle furnace. The sample was subjected to calcination at 600 °C for a duration of 4 h. Upon completion, the mass of the residual solid was determined to calculate the ash content of the IALG. Subsequently, the IALG was digested with aqua regia and inductively coupled plasma optical emission spectrometry was employed (720, Agilent, Inc., Palo Alto, CA, USA) to quantify the Na concentration in the resultant solution, thereby ascertaining the sodium content within the IALG. Approximately 5 mg of the sample was weighed and placed into an alumina crucible. The sample was subjected to a controlled temperature increase from ambient room temperature to 800 °C, with a heating rate of 10 °C/min. The changes in the sample’s mass as a function of the reaction temperature were analyzed using a thermogravimetric analyzer (TG209 F3, Netzsch, Inc., Bavaria Free State, Germany). The specific surface area and pore structure characteristics of the sample were determined using a nitrogen adsorption/desorption apparatus (Nova 2200e, Quantachrome Instruments, Boynton Beach, FL, USA). The specific surface area of the sample was measured using the BET method, utilizing adsorption data within the relative pressure range of 0.01 to 0.2 (P/P0). The total pore volume was derived from the amount of nitrogen adsorbed at a relative pressure of approximately 0.99 (P/P0). The mesopore specific surface area and mesopore volume were obtained by calculating the differences from the micropore surface area and micropore volume, which were determined through t-plot analysis. Functional groups present on the sample surface were characterized using Fourier transform infrared spectroscopy (FT-IR, Nicolet 6700, Thermo Fisher Scientific, Inc., Waltham, MA, USA). The microstructure of the sample was primarily examined via scanning electron microscopy at an acceleration voltage of 20 kV (SEM, Inspect F50, FEI, Thermo Fisher Scientific, Inc., Waltham, MA, USA). The crystalline phases present in the sample were identified using an X-ray diffractometer under the conditions of nickel-filtered Cu Kα radiation (λ = 0.15406 nm) at a current of 20 mA and a voltage of 30 kV (XRD, SmartLAB 3, Rigaku Corporation, Saitama Prefecture, Japan).
3.4. Pb(II) Adsorption
A quantity of 0.1 g of biochar was added to 50 mL of Pb(II) solution in a wide-mouth Erlenmeyer flask. The suspension was placed in a constant temperature incubator and agitated at a speed of 120 r/min. Upon completion of the reaction, the mixture was separated using a 0.45 µm filter membrane. The concentration of Pb(II) in the filtrate was determined using an atomic absorption spectrometer (FAAS-M6, Thermo Fisher Scientific, Inc., Waltham, MA, USA). The initial concentration of Pb(II), reaction time, reaction temperature, and solution pH were set to 50–1000 mg/L, 2–8 h, 15–45 °C, and 1–5, respectively. Each sample was set up in triplicate, and the data used are the average of the three sets of values.
4. Conclusions
The high ash content in IALG acted as an efficient catalyst, significantly reducing the activation energy required during the primary pyrolysis stage to 14.27 J/mol, thereby enhancing the development of the resultant biochar’s porous structure. Moreover, the abundant minerals in IALG facilitated the deoxygenation and dehydrogenation processes within the carbon materials, which resulted in the attenuation or disappearance of the characteristic peaks associated with the functional groups. Following pyrolysis, the Na-related minerals were transformed into particles ranging from 80 to 150 nm in size, including Na2SiO3, Na2CO3, and NaCl. IALG-BC demonstrated a markedly superior Pb(II) adsorption performance relative to that of A-IALG-BC. According to the Langmuir model, the theoretical maximum adsorption capacity of IALG-BC for Pb(II) was 809.09 mg/g, which was approximately 40 times that of A-IALG-BC. The Na-related minerals present in IALG-BC functioned as active adsorption sites, playing an essential role in the efficient adsorption of Pb(II) through precipitation. This study elucidates the potential of industrial alkali lignin as a cost-effective and efficient adsorbent for the remediation of wastewater containing heavy metals. Nonetheless, further research is warranted to explore the development of recovery methods for the spent adsorbents and to establish effective regeneration processes.