1. Introduction
Environmental pollution caused by heavy metals or their compounds is mainly driven by mining and dressing, electroplating, smelting, and a variety of uses of heavy metal products [
1,
2]. Some industrial enterprises discharge, indirectly or directly, untreated heavy metal-contained wastewater into the environment, leading to severe water pollution [
3,
4,
5]. Manganese is associated with an abundant supply of mineral ores in Hunan Province (China), causing the generation of many Mn tailings wasteland in the long-term mining and smelting process. Due to poor management, some heavy metals (e.g., Zn, Cu, Cd, and Pb) present in the left lands around the manganese ore area enter the surrounding water (surface water and groundwater), or reach soil, resulting from washing and dissolution by rainwater. These facts suppose high risks of damaging the safety of agriculture and animal husbandry production, as well as for human and environmental health [
6,
7].
Cu, Pb, and Cd are the most common heavy metals presenting toxicity. Once they enter the human body via drinking, eating, breathing or via direct contact, they will deposit and accumulate, with high risks of causing significant damage to the normal function of the body [
8,
9]. And once threshold levels are exceeded, it is easy to cause gene mutations and affect cell inheritance. In some cases, it could produce teratosis or induce cancer [
10,
11].
At the same time, these heavy metals can penetrate the soil environment under the action of water flow. The roots of plants absorb nutrients from the soil, as well as the heavy metals contained in the ground. The increased levels of heavy metals will affect the growth and development of plants [
12].
Overall, it is clear that there is a need for performing an appropriate treatment of wastewater containing heavy metals before being discharged into the environment.
The methods for treating heavy metal in wastewater include flocculation/precipitation, oxidation/reduction, ion exchange, membrane separation, and adsorption. However, some of these methods are not always suitable for removing heavy metals from wastewater due to expensive equipment, high operating costs, and high maintenance costs [
13,
14,
15].
The adsorption method has attracted the attention of researchers all over the world because of its simple operation, relatively low cost, and high efficiency and selectivity [
16,
17]. When adsorption is used to remove heavy metals from wastewater, its efficiency mainly depends on the choice of the adsorbents, such as biosorbents, polymer fiber, oxide minerals, activated carbon [
18,
19], and biochar [
20,
21], which need to have a capacity to effectively adsorb the heavy metals present in wastewater.
Biochar is a relatively novel, low-cost, and carbon-rich porous material, prepared by pyrolysis of biomass in oxygen-deficient environments [
22,
23]. It is featured with a large specific surface area, strong adsorption capacity, and does caus secondary pollution [
24,
25]. Biochar has high capacity for the adsorption of heavy metal ions in the environment. The metallic mineral components in biomass (such as calcium, potassium, and other) facilitate the production of biochar and the formation of void structure, thereby enhancing its physical adsorption potential [
26]. Especially in recent years, biomass containing heavy metals has been converted into biochar by pyrolysis under anaerobic conditions in an increasing number of research experiments. It is to be noted that the biomass containing heavy metals can be reduced to the maximum extent, effectively reducing the leaching and bioavailability of these heavy metals [
27]. Also relevant, the production of toxic and harmful substances (polycyclic aromatic hydrocarbons, dioxins, etc.), as well as greenhouse gases generated in the traditional incineration process, can be effectively avoided. In fact, this is a common method used for the disposal of plant wastes containing heavy metals [
28,
29].
To achieve a higher adsorption capacity of biochar, different modification methods have been considered. For example, Ying et al. [
30] used magnesium (Mg) nutrient solution to cultivate tomato plants for preparing a new type of magnesium-rich biochar. The experimental results showed that the obtained biochar had a Mg(OH)
2 and MgO particles-rich surface, which significantly improved its capacity for adsorption of phosphorus (P) in wastewater. According to Zhou et al. [
31], a ferromanganese binary oxide-biochar was prepared by means of an impregnation/sintering method for adsorption of Cu
2+ and Cd
2+. A manganese salt solution (MnCl
2, KMnO
4, etc.) with potential for modifying biomass could be used to load MnO particles on the surface of biochar materials through high-temperature pyrolysis. This would help to achieve a significant improvement of biochar composite materials in terms of the number of functional groups, void structure, specific surface area, and adsorption capacity [
32,
33]. These studies showed that the adsorption capacity of these biochars prepared by modification methods dramatically improved, thereby clearly increasing the efficiency in the removal of heavy metals.
One of the plants with potential for being used in the elaboration of biochar is
Phytolacca acinosa, which in fact was used as the raw material to prepare biochar in the current study. It is a perennial stout herbaceous plant with a height of 0.5–1.5 m and a strong adaptability. Numerous studies have shown that this plant has strong enrichment and tolerance to manganese in soil media [
34]. Mn accumulation in its leaves can exceed 19 g·kg
−1, with an average of 14 g·kg
−1. It was the first Mn hyperaccumulator discovered in China [
35].
Phytolacca acinosa does not show the biological defects of many hyperaccumulators (small size, low biomass, slow growth rate, long growth cycle). It has many advantages such as good adaptability, strong fecundity, and wide geographical distribution. Besides, it should be noted that the specific plant specimens used in this experiment came from a manganese mining area, which is interesting in order to investigate if the overall procedure could aid to solve the problem of treatment and disposal of
Phytolacca acinosa containing heavy metals. In addition, converting these plants into biochar could be useful to achieve the adsorption of heavy metals present in wastewater.
In view of that, in this work, manganese-rich pokeweed biomass was used as the raw material for producing novel biochar materials at different temperatures (300, 400, and 500 °C) under anaerobic pyrolysis conditions. Then, the manganese-rich pokeweed biochars were investigated as regards their potential for removing Cu2+, Pb2+, and Cd2+ from wastewater, this is in terms of adsorption performance, recyclability, and environmental pollution risks. This was carried out performing experiments varying pH, time, temperature, dosage, concentration, and circulation, also focusing on potential risk analysis. Moreover, parameters relevant for kinetics and adsorption isotherms models were also investigated. Finally, the characterization of the biochars before and after adsorption, as well as the adsorption mechanism, were also studied by means of FTIR, SEM-EDS, BET, elemental analysis, and XRD and XPS techniques. The results of the study could be of environmental relevance, especially for recycling of biomass and removal of heavy metals from polluted media.
2. Materials and Methods
2.1. Chemicals and Raw Materials
The raw materials used for manufacturing biochars in this study were pokeweed plants from the manganese mining area of Xiangxi Prefecture, Hunan province, China. The chemicals used in the experiments, including NaOH (analytical reagent), MgCl2 (analytical reagent), CH3COONH4 (analytical reagent), HONH3CI (analytical reagent), and CH3COOH (analytical reagent) were obtained from Guangdong Xilong Scientific Co. Ltd. (China). Pb(NO3)2 (analytical reagent), Cu(NO3)2·3H2O (analytical reagent) Cd(NO3)2·4H2O (analytical reagent), HNO3 (analytical reagent), NaOAc (analytical reagent), and 30% H2O2 (analytical reagent) were obtained from Chengdu Jinshan Chemical Reagent Co., Ltd. (China). All the solutions including reagents were prepared with ultrapure water.
2.2. Preparation of the Biochar
Peeled manganese-rich pokeweed was used as biomass raw material, which was washed, dried, crushed, and passed through a 100-mesh sieve. A specific amount of sieved biomass was weighed and then placed in a vacuum tube furnace and heated at a rate of 5 °C min−1 under N2. Afterwards, it was heated to 300 °C, 400 °C, or 500 °C and kept for 2 h. After being cooled to room temperature, it was washed with ultrapure water and dried to obtain the manganese-rich pokeweed biochar materials (designed as BC300, BC400, and BC500). The number (300–500) indicates the pyrolysis temperature during the preparation.
2.3. Characterization
The surface morphology changes of the BC300, BC400, and BC500 biochar materials used in this study were observed by means of a Scanning electron microscopy-energy spectrometer (Quanta FEG 250, USA). The specific surface area and pore structure of the BC300, BC400, and BC500 materials were determined by using Brunauer–Emmett–Teller methodology (NOVA4200e, USA). The C, N, H, S, and O elemental contents for the BC300, BC400, and BC500 biochar samples were measured by using an elemental analyzer (EAICE-440, USA). The changes in the surface functional groups of the BC300, BC400, and BC500 materials before and after adsorption were determined by using Fourier transform infrared spectroscopy (Nicolet 5700 Spectrometer, USA). The changes in the crystallinity of the BC300, BC400, and BC500 biochars before and after the adsorption of the heavy metals were determined by using X-ray diffraction (XRD-6100, Japan). The chemical composition of the surfaces of the BC300, BC400, and BC500 biochars was determined by using X-ray photoelectron spectroscopy (ESCALAB 250Xi, USA).
2.4. Adsorption Experiments
We performed every and each adsorption experiment in a constant temperature oscillation incubator at 160 rpm, with triplicate samples in each group, where the average values were obtained for analysis and RSD of less than 5%. The concentration of heavy metals before and after adsorption was determined by ICP-OES (Agilent 5800, USA). The calculation formulas of the adsorption capacities and removal rate of biochars towards to heavy metals are as follows:
where
Qe is adsorption capacity of biochar (mg·g
−1);
C0 and
Ce are the initial and residual concentration of heavy metal remaining in solution (mg·L
−1), respectively;
V is the solution volume (mL);
M is the mass of biochar (g); and
R (%) is the heavy metal removal rate from solution.
Amounts of 15 mg of BC300, BC400, and BC500, as well as of NM500 and activated carbon, were added to 50 mL Erlenmeyer flask, and each of them received 50 mL of Cu2+, Pb2+, or Cd2+ solutions (100 mg·L−1), respectively. Under the same adsorption parameters, we compared the difference in the adsorption capacity among the tested materials.
In the range of pH 2–6, we added 5 mg of BC300, BC400, and BC500 and 50 mL of Cu2+, Pb2+, and Cd2+ solutions (100 mg·L−1) to a 50 mL Erlenmeyer flask. Then, a small amount of NaOH or HNO3 (0.1 mol·L−1) was dropped to adjust the pH.
In addition, we added 5 mg of BC300, BC400, and BC500 and 50 mL of Cu
2+, Pb
2+, and Cd
2+ solutions (100 mg·L
−1) to a 50 mL Erlenmeyer flask, and oscillated at the optimal pH. Then we took aliquots at the corresponding reaction time (0.25, 0.5, 1, 2, 3, 6, 8, 10, 12, 18, 24, and 36 h). To analyze the adsorption mechanism of Cu
2+, Pb
2+, and Cd
2+ on biochars, we used the pseudo-first-order (PFO) and the pseudo-second-order (PSO) kinetic models. The model formulas are as follows:
where
qe and
qt are the adsorption capacities (mg·g
−1) of biochars at equilibrium and at time
t, respectively, and
k1 (min
−1) and
k2 (g mg
−1·min
−1) are the rate constants at PFO and PSO, respectively.
At the optimal pH and reaction time, we added 5 mg of BC300, BC400, and BC500, and 50 mL of Cu2+, Pb2+, and Cd2+ solutions (100 mg·L−1) to a 50 mL Erlenmeyer flask, and oscillated at different temperatures (15, 20, 25, 30, and 35 °C).
Under the optimum pH, reaction time and reaction temperature, we added 5, 10, 15, 20, 25, and 30 mg of BC300, BC400, and BC500 respectively, and 50 mL of Cu2+, Pb2+, and Cd2+ solutions (100 mg·L−1) to a 50 mL Erlenmeyer flask, and then oscillated.
Under the optimum pH, reaction time, reaction temperature, and dosage, we added 50 mL of Cu
2+, Pb
2+, and Cd
2+ solutions with concentrations of 25, 50, 100, 150, 200, 300, and 400 mg·L
−1 respectively, to a 50 mL Erlenmeyer flask, and then oscillated. The adsorption mechanism of Cu
2+, Pb
2+, and Cd
2+ on biochars was analyzed using the Langmuir model and the Freundlich model. The model formulas are as follows:
where
qe and
Ce are the adsorption capacity of biochar at equilibrium (mg·g
−1) and the solution concentration at equilibrium (mg·L
−1), respectively;
qm is the biochar maximum adsorption capacity (mg·g
−1);
KL (L·mg
−1) and
KF ((mg·g
−1) (mg·L
−1)
−n) are the adsorption constants of Langmuir and Freundlich, respectively; and
n is an indicator of the adsorption intensity and heterogenicity.
The manganese-rich pokeweed biochars with the best comprehensive performance were taken as the representative material. The study was carried out for four consecutive cycles under the best adsorption parameters. After each cycle, the adsorbent was regenerated by means of H2SO4 or NaOH (0.1 mol·L−1), washed repeatedly with deionized water, and dried for later use.
2.5. Potential Risk Analysis
The Tessier chemical sequence extraction method was used to analyze the presence of heavy metals in manganese-rich pokeweed biomass and manganese-rich pokeweed biochar [
36,
37]. And the content of Mn, Cu, Zn, and Pb in the sample was determined by ICP-OES analysis. Finally, we employed the Hakanson heavy metal risk assessment index in evaluating the potential pollution risk of the heavy metals in the manganese-rich pokeweed biomass and manganese-rich pokeweed biochars. The formula is as follows:
RI is the potential ecological risk index; Er represents the potential ecological index of a single heavy metal; Cf represents the pollution factor; and Ci and Cn are the transferable part and the stable part of the distribution of the heavy metals, respectively.
4. Conclusions
In this study, three manganese-rich pokeweed biochar materials (designed as BC300, BC400 and BC500) were prepared from manganese-rich pokeweed plants to study its performance as regards adsorption of Cu2+, Pb2+, and Cd2+. Under the optimal conditions and values of the adsorption parameters, one of the biochars (BC500) showed maximum adsorption capacity as high as 246, 326, and 310 mg·g−1 for Cu2+, Pb2+, and Cd2+, meaning 98.5, 97.5, and 92.7% in removing rates. After being used for four cycles, the adsorption capacity was maintained, and the potential pollution risk of the heavy metals remained low. Regarding adsorption kinetics and adsorption isotherms, the PSO and the Langmuir models fitted well to the experimental data. By means of different characterization analyses (FTIR, SEM-EDS, BET, XRD, and XPS), it was shown that the Mn in the biochars was mainly MnO2, with these sorbent materials having many functional groups on their surfaces. The mechanism involved in the sorption of Cu2+, Pb2+, and Cd2+ onto the three biochars (BC300, BC400 and BC500) included ion exchange, electrostatic attraction, chemical adsorption, and precipitation. All three biochars (BC300, BC400, and BC500) were featured with high adsorption capacity, recyclability, and low environmental pollution. The elaboration of these biochar materials could be also used for scientific treatment and disposal of the manganese-rich pokeweed plants. Overall, the results of the study can be considered of clear environmental relevance.