Evaluation of Removal Efficiency of Ni(II) and 2,4-DCP Using in Situ Nitrogen-Doped Biochar Modified with Aquatic Animal Waste.

Currently, biochar (BC) has shown promising potential in groundwater and surface-water remediation. In this work, Trapa natans husks based biochar (TBC) was prepared and modified with aquatic animal waste (shrimp and crab) to produce shrimp-modified biochar (SBC) and crab-modified biochar (CBC), respectively. The as-prepared BCs (TBC, SBC, and CBC) were characterized by X-ray diffraction, scanning electron microscopy, elemental analysis, Boehm titration, Fourier transform infrared, and X-ray photoelectron spectroscopy. SBC and CBC had more developed nitrogen-containing functional groups than TBC, which indicates that the crude proteins in shrimp and crab have successfully achieved in situ nitrogen doping. Results of batch experiments showed that SBC and CBC had larger groundwater pollutants (2,4-dichlorophenol (2,4-DCP) and Ni(II)) adsorption capacities than TBC. According to batch adsorption experiment and characterization analysis results, the proposed adsorption mechanism of 2,4-DCP includes hydrogen bonding and π-π electron-donor-acceptor interaction, while the mechanism for Ni(II) adsorption are proposed to be surface complexation, ion exchange, and electrostatic attraction.

Introduction

The increasingly developed industrial and agricultural production has not only brought a more convenient life to human beings but also led to worldwide environmental pollution, among which water pollution has become a major concern. The rapid growth of population, along with the worldwide water crisis, has made the preservation of groundwater and surface-water vital issue. Ni(II) and 2,4-dichlorophenol (2,4-DCP) are both typical pollutants in groundwater and surface-water. As one of the most widely distributed heavy metal pollutants in the environment, Ni(II) mainly comes from industries including machine manufacturing, ceramic pigment, and electronic remote control. The compounds of Ni(II) are carcinogenic to humans, so the international agency for research on cancer of the World Health Organization has added Ni(II) compounds to its list of carcinogens in 2017.[1,2] 2,4-Dichlorophenol (2,4-DCP), as herbicide synthetic precursor, has been widely used in agricultural production. It can cause the disorder of bilayer phospholipid structure of human cells, further leading to cancer and mutagenesis, so the maximum allowable concentration of phenol in water source is 0.093 mg/L.[3,4] Herein, the development of a highly efficient adsorption technology for both pollutants has become a focus of groundwater and surface-water remediation research.

Biochar (BC) is a kind of porous carbonaceous solid produced by thermal or hydrothermal pyrolysis of biomass (natural plant materials, industrial and agricultural waste, etc) at anaerobic atmosphere and relatively low temperatures (<700 °C) for environmental applications.[5,6] BC could be used as soil conditioner, reductant, carrier of slow release fertilizer, carbon dioxide fixing agent, etc.[7,8] Some studies have been done to evaluate the application of biochar as an adsorbent in water treatment to remove various pollutants, including heavy metals and organic pollutants.[9] However, the high cost remains a major obstacle to the large-scale application of biochar in wastewater treatment. Thus, it is necessary to develop an affordable and green absorbent with excellent physicochemical properties. Biochar contains plenty of polar functional groups, such as hydroxyl, carboxylic, and amino groups,[10] thus providing a good potential for its modification. So far, the discussed modification methods of biochar include (1) the loading of metal ions, oxides, or ions;[11,12] (2) mixing with reducing or oxidizing salt;[13] and (3) modification with other specific organic functional groups.[14] Modification of plant-based biochar by aquatic animal waste could change the number and types of oxygen-containing and nitrogen-containing surface functional groups of the biochar, leading to improvements of its performance for the adsorption of pollutants. Moreover, the utilization of aquatic animal waste as an economical and green material will provide an alternative way for the large-scale production of modified biochar.[15]

Trapa natans is a widely distributed aquatic plant in China, whose husks contain a large amount of lignin and cellulose. Tons of T. natans husk were burned or discarded annually just in the Taihu Lake area in China, causing great threat to the growth of the local aquatic organisms.[16] The carbon content of T. natans husk is approximately 50%, offering an ideal basis for biochar preparation. T. natans husks based biochar (TBC) with developed specific surface area, pore structure, and surface functional groups has been extensively studied.[17,18] By this means, the abundantly existing biomass material could be better utilized and the relative environmental impacts caused by burning could be avoided as well.

Shrimp and crab powder are both common aquatic animal wastes with a large output. China produces about 60,000 tons of shrimp annually,[19,20] of which chitin and crude protein are the main components. To the best of our knowledge, no study has been conducted so far using shrimp bran and crab powder to modify T. natans husks based biochar to explore the performance and removal mechanism of typical groundwater pollutants.

The overall objectives of this study was to prepare modified T. natans husks based biochar from aquatic animal waste (shrimp and crab) and evaluate its efficiency in removing 2,4-DCP and Ni(II) from aqueous solution. The specific objectives are as follows: (1) to prepare and characterize SBC (shrimp modification), CBC (crab modification), and TBC (pure T. natans husks based biochar); (2) to determine the effects of pH, contact time, and initial pollutant concentration on pollutant removal; and (3) to further discuss the pollutants removal mechanisms of SBC and CBC through X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR), and batch adsorption experiment.

Results

Doping Ratio Optimization of Animal Waste

As displayed in Figure , for both biochar samples, the adsorption capacities on 2,4-DCP first increased and then decreased with the increasing doping ratio of shrimp bran and crab. The biochar samples for experiments were selected according to the maximum adsorption capacity of 2,4-DCP in this experiment, so the TBC, SBC-1.0 (referred to as SBC in the following discussion), and CBC-1.0 (referred to as CBC in the following discussion) were selected for subsequent experiments.

Figure 1

Optimization of doping ratio (animal wastes: TH).

Textural Characteristics of Biochar

The scanning electron microscopy (SEM) images of BCs are shown in Figure S1. According to Figure S1, the surfaces of BCs were rough and had developed pores of different sizes and irregular structures, which is conducive to improving their adsorption characteristics.[21] The pore density of SBC and CBC is higher than that of TBC, resulting in a higher SBET and Vtot (cm3/g) of the two samples, which is in accordance with the data in Table . Table also shows that Smic/Stot (%) and Vmic/Vtot (%) of TBC were higher than that of SBC and CBC, which is possible because animal waste calcination and collapse resulted in smaller micropores ratio in SBC and CBC.[22]Figure a shows that the main distribution range of the pore size was 1–5 nm. For N2 adsorption–desorption isotherm in Figure a, inset, capillary condensation and hysteresis of isotherms (in higher P0/Ps) shows that the isotherm is of I and IV hybrid according to the IUPAC, so the pore size distribution was dominated by the microporous structure.[23] In general, doping with a small amount of animal waste did not change the physical properties of biochar in a large area but slightly improved its physical adsorption properties.

Table 1

Textural Parameters of the Carbons

samples	SBET (m2/g)	Smic (m2/g)	Smic/Stot (%)	Vtot (cm3/g)	Vmic (cm3/g)	Vmic/Vtot (%)
TBC	1367.57	475.99	34.81	1.421	0.189	13.30
SBC	1401.02	406.34	29.00	1.518	0.158	10.41
CBC	1415.43	417.86	29.52	1.541	0.162	10.51

Figure 2

(a) Pore size distribution and N2 adsorption/desorption isotherms (inset) of BCs. (b) X-ray diffraction (XRD) spectra of BCs.

The XRD patterns of the BC samples are shown in Figure b. No obvious diffraction peak is detected in the whole range, indicating that the BCs is of amorphous structure. Additionally, the greater peak strength of SBC and CBC in the 0–25° range was attributed a higher crystallinity than that of TBC.[24] In general, the physical characteristics of SBC and CBC were similar to those of TBC, implying that the doping of animal wastes did not lead to a remarkable change in biochar.

Chemical Characteristics of Biochar

The infrared spectra (400–4000 cm–1) of BCs are presented in Figure a. All the three samples exhibited 5 similar peaks: peak (I) represents the characteristic peak related to aromatic structure (600–800 cm–1); peak (II) is related to C–O (epoxy, hydroxyl and carboxylic groups), C–N, and C–C (1020–1300 cm–1); peak (III) is the typical characteristic peak of C=O (conjugated carbonyl group) and C=N (amide group) (1420–1480 cm–1); peak (IV) corresponds to C=O (carboxyl or lactone groups) (1510–1615 cm–1); and peak (V) is related to −OH (hydroxyl groups) and −NH2 (3415–3442 cm–1).[25,26] The transmittance corresponding to different peaks indicated that the doping of shrimp bran and crab obviously changed the chemical properties of biochar.

Figure 3

FTIR spectra of the BCs before (a) and after (b) 2,4-DCP adsorption.

Boehm titration was used for further quantitative analysis of functional groups, and the results are shown in Table . The total base amount of SBC and CBC were higher than that of TBC, indicating that the doping of shrimp bran and crab improved the chemical properties of biochar.[27] The results of elemental analysis (Table ) further confirmed that the doping of animal crude protein increased the N content of SBC and CBC. Table also shows that the total amounts of acid groups in BCs were higher than that of the base groups, which agreed well with the results of isoelectric point analysis that the isoelectric points of SBC and CBC were lower than that of TBC (Figure a), suggesting that SBC and CBC are more suitable for ion-exchange adsorption than TBC.[28]

Table 2

Boehm’s Titration Results and Element Composition of the SBC

samples	carboxyl (mmol/g)	lactone (mmol/g)	phenolic (mmol/g)	acidic (mmol/g)	basic (mmol/g)	total (mmol/g)	C %	O %	N %	H %
TBC	1.135	0.214	0.833	2.182	1.506	3.688	73.53	20.72	0.49	5.26
SBC	1.724	0.698	1.208	3.631	1.753	5.383	75.64	19.18	1.47	3.71
CBC	1.651	0.728	1.109	3.488	1.781	5.269	76.89	17.99	1.38	3.74

Figure 4

(a) Points of zero charge (PZC) onto BCs. (b) Effect of initial pH on the removal of 2,4-DCP. (c) Effect of initial pH on the removal of Ni(II).

XPS was employed to identify the surface functional groups of BCs as shown in Figure S2. C 1s (284 eV) and O 1s (532 eV) were major peaks of BCs, and N 1s (400.52 eV) was also introduced to the surface of SBC and CBC. C 1s, O 1s, and N 1s were fitted to the peaks by XPS-PEAK software; the peak numbers and relative content of the surface functional groups determined by C 1s, O 1s, and N 1s spectra from XPS for BCs before and after Ni(II) and 2,4-DCP adsorption are shown in Table . The major C 1s peak could be fit to three curves from the following groups (Figure S3a,c,e): C-I represents the sp2 imaging carbon peak (C–C or C–H) (284.6–285.1 eV); C-II represents phenolic hydroxyl, alcoholic hydroxyl, and ether groups (C–O) (286.31–286.5 eV); and C-III represents −COOH (288.6–289.42 eV). The major O 1s peak could be fitted to three curves from the following groups (Figure a,c,e): O-I represents carbonyl and quinone (C=O) (531.1 ± 0.3 eV); O-II represents hydroxyl, ether, ester, and anhydride (532.3 ± 0.3 eV); and O-III represents carboxyl (−COOH) (535.1 ± 0.3 eV). The major N 1s peak could be fitted to two curves from the following groups (Figure a,c): N5 represents pyrrole-like N (C-NH2, 400.3 ± 0.3 eV) and NX represents pyridine-N-oxynitride (C-NO2, 402.5–402.6 eV).[29]

Table 3

Peak Numbers and Relative Content of the Surface Functional Groups Determined by C 1s, O 1s, and N 1s Spectra from XPS for AC and ACF before and after Ni(II) and 2,4-DCP Adsorptiona

		peak C 1s			peak O 1s			peak N 1s
samples		C-I	C-II	C-III	O-I	O-II	O-III	N-I	N-II
TBC	BE (eV)	284.44	285.50	288.90	530.90	532.81	535.01
	content (%)	42.75	24.17	33.08	16.97	62.76	20.27
SBC	BE (eV)	284.44	285.86	288.98	531.18	532.97	535.60	400.01	403.30
	content (%)	42.00	25.35	32.65	15.48	64.22	20.30	83.64	16.36
CBC	BE (eV)	284.44	285.85	288.99	531.09	532.96	535.53	400.03	403.15
	content (%)	43.83	25.58	30.59	17.35	65.63	17.02	83.15	16.85
TBC-Ni(II)	BE (eV)	284.44	285.63	288.99	530.90	533.11	535.01
	content (%)	46.18	23.23	30.59	22.82	58.20	18.98
SBC-Ni(II)	BE (eV)	284.44	286.25	289.29	531.18	533.37	535.80	400.05	403.30
	content (%)	45.60	23.32	31.08	26.79	55.87	17.35	73.28	26.72
CBC- Ni(II)	BE (eV)	284.44	285.57	289.18	531.09	533.28	535.66	404.06	404.15
	content (%)	47.84	22.25	29.91	25.80	57.95	16.25	75.42	24.58

BE: binding energy; C-I: sp2 C = CC–C/C–H; C-II: phenolic hydroxyl group, alcoholic hydroxyl group, and ether group; C-III: −COOH; O-I = carbonyl and quinone; O-II = hydroxyl, ether, ester, and anhydride; O-II = carboxyl; N-I = N5; and N-II = NX.

Figure 5

XPS spectra for BCs before and after Ni(II) adsorption: O 1s spectrum of TBC (a), TBC-Ni (b), TBC (c), TBC-Ni (d), CBC (e), and CBC-Ni (f).

Figure 6

XPS spectra for BCs before and after Ni(II) adsorption: N 1s spectrum of SBC (a), SBC-Ni(II) (b), CBC (c), and CBC-Ni(II) (d).

For the TBC sample, the doping of shrimp bran and crab improved its chemical properties, namely, the introduction of nitrogen functional groups, the increase in the total functional groups, and the decrease of the isoelectric point value.

Adsorption Kinetics

Figure S4 shows that the adsorption capacity of 2,4-DCP and Ni(II) on BCs increased with the extension of time. The adsorption capacity increased rapidly in the initial stage of reaction, then the increase became slow, and finally reached a dynamic equilibrium. Figure S4 and Table S1 show that the R2 for the pseudo-second-order equation were larger than those for the pseudo-first-order equation, and Qcal is basically consistent with Qexp for the pseudo-second-order equation. Therefore, the pseudo-second-order equation was more suitable to simulate the adsorption of pollutants by BCs, indicating that the adsorption was a chemical process.[30]

Adsorption Isotherms

For the 2,4-DCP adsorption in Figure S5a–c and Table S2, the R2 for the Freundlich isotherm model were larger than those for the Langmuir isothermal model, and the Qexp was more consistent with Qcal. The better fit of the Freundlich isotherm model implied that the adsorption of 2,4-DCP by the BCs was mainly a heterogeneous adsorption of multimolecular layers. Additionally, Qm, K1, and K2 all decreased with the increasing temperature, which indicated that the adsorption of 2,4-DCP by BCs was an exothermic process, and the increase of temperature hindered the decomposition of organics by oxygen-containing functional groups.[31]

For Ni(II) adsorption, the Langmuir isothermal model was more suitable for describing the thermodynamic properties of Ni(II) adsorption, as displayed in Figure S5d–f and Table S2. The result showed that the Ni(II) adsorption on BCs was mainly a monolayer specific site adsorption. In addition, adsorption of Ni(II) by BCs was an endothermic process.[32]

Table S2 displays the maximum adsorption capacities of BCs, among which SBC (863.24 mg/g for 2,4-DCP and 44.78 mg/g for Ni(II)) has a higher adsorption capacity than CBC (728.69 mg/g for 2,4-DCP and 39.40 mg/g for Ni(II)) and TBC (589.35 mg/g for 2,4-DCP and 26.269 mg/g for Ni(II)) for both Ni(II) and 2,4-DCP.

Effect of pH on 2,4-DCP/ Ni(II) Adsorption

To explore the adsorption performance of pollutants on biochar at different pH values, a certain amount of biochar samples was added into a conical flask (150 mL) containing 50 mL of the pollutant solution (2,4-DCP (200 mg/L); Ni(II) (30 mg/L)) with different pH values (2,4-DCP (pH = 2–12); and Ni(II) (pH = 2–8)).

The ionization degree of 2,4-DCP under different pH values conforms to eq (33,34)where C0 is the concentration of unionized 2,4-DCP, CT is the total concentration of 2,4-DCP, pH is the pH value after adsorption equilibrium, and the value of pKa is 7.85.

Ni(II) can potentially exist as the species of Ni2+ (pH < 8), Ni(OH)+, Ni(OH)20, Ni(OH)3–, and Ni(OH)42–, depending on the pH value.[35]

The solution pH is one of the most important parameters influencing the adsorption process, as pH not only affects the adsorbent surface charge, functionalities, and adsorption site but also the ionization degree and the existing form of pollutants.

For 2,4-DCP adsorption in Figure b, the removal efficiency of 2,4-DCP by BCs decreased with the increasing pH value and the same results have been reported in other literatures refs (31, 36). When the pH value of the solution was lower than pKa, most of the adsorption sites on the surface of BCs were protonated and exhibited positive charges. In this case, 2,4-DCP was neutral and there was attraction between the surface of biochar and 2,4-DCP, thus the removal rate of 2,4-DCP was relatively high.[37] As the pH of the solution increased, the ionization degree of 2,4-DCP and 2,4-DCP anions increased, and the surface of biochar was neutral or negatively charged, so the electrostatic repulsion reduced the removal rate of 2,4-DCP. In addition, 2,4-DCP anions are more water-soluble and likely to form stronger bonds with water molecules in solution, so the removal efficiency of 2,4-DCP by biochar in an alkaline medium was lower than that in the acidic medium.[38]

As shown in Figure c, the adsorption capacity for Ni(II) of BCs increased rapidly in the pH value range of 2.5–4.0 and then slowly increased in the range of 3.5–8.0. In a medium of low pH value, the positive charge of biochar is not conducive to the adsorption of Ni(II). On the contrary, when the pH was high, biochar with a higher degree of dissociation had a higher negative charge on the surface, and electrostatic attraction promoted the adsorption process.[39]

Pollutant Adsorption Mechanism

Based on the above results, although the textural performances of SBC and CBC are not much different from that of TBC, the former two have better adsorption capacities for Ni(II) and 2,4-DCP in comparison of TBC, suggesting that the chemical properties of biochar (including surface charges, functionalities, and adsorption sites) played an important role in the adsorption process.

Figure illustrates the FTIR spectra of the BCs (TBC, SBC, and CBC), 2,4-DCP-adsorbed BCs (TBC-2,4-DCP, SBC-2,4-DCP, and CBC-2,4-DCP). The appearance of peak (new peak) and changes in transmittance indicate that 2,4-DCP has been loaded on the surface of 2,4-DCP-adsorbed BCs. The transmittance corresponding to peaks (II, III) of SBC-2,4-DCP and CBC-2,4-DCP slightly reduced as compared with those on BC-2,4-DCP, which indicated that C–O (epoxy, hydroxyl, and carboxylic groups) and C=O (conjugated carbonyl group) participated in the adsorption reaction of biochar to 2,4-DCP. The change of peak (V) also explained that the amino group may increase the removal of 2,4-DCP. Possible reasons are proposed to explain this phenomenon: (1) hydrogen bonding (H-bonding existed between 2,4-DCP and O/N groups) and (2) π–π electron–donor–acceptor (EDA) interaction (2,4-DCP and protonated C–O/C=O).[40,41]

Figures and 6 illustrate the XPS spectra of the BCs (TBC, SBC, and CBC) and Ni(II)-adsorbed BCs (TBC-Ni(II), SBC-Ni(II), and CBC-Ni(II)). Figure shows that the binding energy of peaks (C-II, C-III, O-II, and O-III) of TBC- Ni(II), SBC-Ni(II), and CBC-Ni(II) had larger values than those on original BCs, and the decrease of the relative content of the peaks after Ni(II) adsorption, which indicated that O atom acted as an electron donor for Ni(II) adsorption.[42,43] This result can be explained by the following equation reactions (2)–(5)[44]As shown in Figure , the binding energies for the photoelectron peaks appearance of N5 and NX increased and the relative content decreased, which indicated that N acted as an electron donor during the Ni(II) adsorption. This conclusion can be explained by the following equations (6)–(8)[32,45]The adsorption mechanism mainly involves (1) surface complexation (O/N atoms and Ni(II)); (2) ion-exchange (O/N functional group protons and Ni(II)); and (3) electrostatic attraction (deprotonated carboxylic and phenolic and Ni(II)) and chemical interactions.[46]

The maximum Ni(II) and 2,4-DCP adsorption capacity of different adsorbents found in the literature are shown in Table . Generally, SBC exhibited higher Qmax-Ni(II) and Qmax–2,4-DCP than other adsorbents, which reflected that SBC could be a promising biochar for Ni(II) and 2,4-DCP removal. The cost of preparation of BCs is shown in Table S3.

Table 4

Maximum Ni and 2,4-DCP Adsorption Capacity of Different Adsorbents Found in the Literature

precursor	modifying agent	Qmax-Ni(II) (mg/g)	Qmax-2,4-DCP (mg/g)	reference
pomegranate peel			65.71	(47)
polygonum orientale Linn	Mn		244.00	(30)
phenolic resin	Fe2O3	13.83		(48)
phragmites australis	(NH3)2HPO4	31.81		(49)
TH		26.27	589.35	this work
TH	shrimp	44.78	863.24	this work
TH	crab	39.40	728.69	this work

Conclusions

In this work, the adsorption capacities (2,4-DCP and Ni(II)) of the three biochars derived from T. natans husks are compared and the adsorption mechanism is proposed. The SBET and quantity of total functional groups increased upon modification by shrimp and crab. The kinetics data demonstrated that modification on BC by animal waste could enhance the pollutant removal. For the three biochar samples, the Freundlich model was the best to describe the equilibrium data. The adsorption mechanism of 2,4-DCP mainly includes hydrogen bonding and π–π EDA interaction, and the adsorption mechanism of Ni(II) mainly lies in surface complexation, ion exchange, and electrostatic attraction.

Methods

Materials

Plant waste (T. natans husks, TH) was collected from the Mata Lake Constructed Wetland (Zibo, Shandong Province, China). Aquatic animal waste (shrimp and crab) was collected from a seafood booth in Shanghai, China. The plant and animal wastes were thoroughly cleaned with distilled water and then dried at the oven (80 °C) to constant weight. The dried wastes were crushed to 100 mesh. All the chemical reagents used in this work were of analytical grade and purchased from Guangdong Xilong Chemical Co., Ltd (China). The standard 2,4-DCP and nickel chloride stock solution of 10 mmol/L were prepared by dissolving the specific quality pollutants (2,4-DCP and nickel chloride) in distilled water. Experimental solutions of desired concentration were configured by diluting the stock solution with distilled water.

Preparation and Modification of Biochar

The preparation method of biochar has been described in our previous work. The TH (20.00 g) was mixed with aquatic animal waste (shrimp (0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6 g) and crab powder (0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6 g)) at a specific doping ratio (0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0%) and used as precursor samples for biochar production. The precursor samples were impregnated in H3PO4 solution (85 wt.%) in a 500 ml vessel at a mass ratio of 1: 2.2 (precursor samples: H3PO4) for 12 h, where H3PO4 was used as an activator. Then, the mixed samples were calcined at 500 °C for 1 h in a N2 atmosphere (heating rate = 10 °C /min, flow rate =12 L /min).[50] The obtained samples were subsequently soaked in tap water (80–90 °C) and rinsed with purified water to neutral (pH = 7). Finally, the samples were dried at 120 °C for 6 h and crushed into powder (160–200 mesh). The unmodified biochar samples and biochar samples modified with shrimp bran or crab are denoted as TBC, SBC-r, or CBC-r, respectively, where r represents the doping ratio (%).

Characterization of As-Prepared Biochar Samples (BCs)

The surface morphology of BCs was observed by SEM (Zeiss Supra 40, Germany). Automatic specific surface area and pore size analyzer (GEMINT VII 2390) was used to determine the Brunauer–Emmett–Teller (BET) surface area and pore structure of BCs. The crystal phase structure was presented by an XRD diffractometer (Bruker D8 Advance, Germany). The surface functional groups were analyzed and quantified by FTIR (Nicolet-460, Thermo Fisher) and Boehm titration (Supporting Information).[27] The elemental contents of C, N, O, and H in BCs were determined by elemental analysis (Flash 2000, Thermo Fisher).[51] Point of zero charge (PZC) is used to determine the surface charge of BCs.[28] XPS analysis (Nicolet 460, Thermo Fisher) was conducted to determine the binding energy between electrons and characterize the elements on the surface of BCs.

Adsorption Experiments

Solution Configuration and Measurement

The concentration of 2,4-DCP was determined by a UV–vis spectrophotometer (UV-5100, Shanghai) at the wavelength of 281 nm.[52] The concentration of Ni(II) was determined by an ICP-OES analyzer (iCAP6300, Thermo).[53] The adsorption capacity Qe (mg/g) and removal rate (%) of BCs were calculated bywhere C0 (mg/L) is the initial concentration of the pollutants, Ce (mg/L) is the equilibrium solution concentration, V is the volume of solution (L), and M is the mass of BCs (g) employed.

Adsorption Kinetics Experiments and Kinetic Model

Pollutant solutions with specific concentration (2,4-DCP (200 mg/L) and Ni(II) (30 mg/L)) were configured in a 2 L beaker and stirred with a magnetic stirrer (400 rpm). Certain amounts of BCs were added to the solution (2,4-DCP (0.60 gBCs) and Ni(II) (1.00 gBCs)) and the time was set as the starting point of the kinetic experiments. Then, 20 mL of solution samples were taken at a desired time interval within the set time range (0–24 h). The solution sample was filtered through a 0.45 μm membrane, and the residual concentration of pollutants was detected using the method described in Section .

The pseudo-first-order eq shows that the adsorption rate is affected by the concentration of the solution and the adsorption capacity, while mass transfer resistance is the limiting factor of adsorption.[54] The pseudo-second-order eq shows that the limiting factor of adsorption is the adsorption mechanism rather than the mass transfer within particles.[55]where qe and q are the amounts of pollutants adsorbed at equilibrium and time t (mg/g), respectively, and k1 (1/h) and k2 (g/(mg min)) are the rate constants of the pseudo-first-order equation and pseudo-second-order equation, respectively.

Adsorption Isotherm

A certain amount of BCs was added into a conical flask (150 mL) containing pollutant solution (50 mL) with different concentration gradients (2,4-DCP (80–130 mg/L), Ni(II) (80–130 mg/L)). The conical flask was then shaken for 24 h in the oscillator under different temperatures (288.15, 303.15, and 318.15 K). Finally, the concentration of the remaining pollutant solution was determined.

The Langmuir isotherm equation (13)[56,57] describes the monolayer adsorption of the adsorbent. The Freundlich isotherm equation (14)[58] considers that adsorption is multilayer adsorption and there are many interactions between adsorbents and adsorbentswhere qe (mg/g) and Ce (mg/L) are the solid-phase and aqueous-phase concentrations, respectively. At adsorption equilibrium, Q0 (mg/g) is the maximum adsorption capacity. KL (L/mg) is the Langmuir constant. KF (mg1–1/ L1/g–1) and n are the Freundlich constants related to the adsorption capacity and intensity, respectively.