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Environ Eng Res > Volume 27(3); 2022 > Article
Bao, Di, Yang, Wang, Sun, and Dong: Experimental study on adsorption characteristics of Cu2+ and Zn2+ by datong lignite

Abstract

In view of the high content of Cu2+ and Zn2+ in acid mine drainage (AMD), the adsorption properties of lignite for Cu2+ and Zn2+ were studied. The adsorption performance of lignite for Cu2+ and Zn2+ was revealed by combining with FT-IR, XPS and EDS. The results showed that the adsorption kinetic model of lignite for Cu2+ and Zn2+ conformed to the quasi-first-order kinetic model. The isothermal adsorption line fitting models of lignite for Cu2+ and Zn2+ were in accordance with the Langmuir model and the Freundlich model, respectively. The maximum equilibrium adsorption capacities of lignite for Cu and Zn were 67.84 mg/g and 55.5 mg/g. The adsorption process of Cu2+ and Zn2+ by lignite involved electrostatic, coordination and ion exchange. Under the condition of coexistence of two kinds of ion, the adsorption site of lignite had stronger binding ability to copper ions.

1. Introduction

Mineral resources are indispensable means of production for human development. Due to unreasonable development and lack of effective supervision, a large number of mine wastewater discharges, which accounts for about 10% of China’s industrial wastewater discharge [1]. Mine wastewater is the general term of natural leaching water, mineral processing wastewater, overflow water of mineral processing waste dam and leaching water of slag accumulation field [2]. Mine wastewater can be divided into AMD [3] and Alkaline Mine Wastewater [4] according to acidity and alkalinity. Direct discharge of mine wastewater without treatment will not only cause serious pollution to the surrounding groundwater and surface water [5], but also cause serious damage to the local ecosystem, especially the damage of AMD to the environment [6]. AMD has a low pH value and contains high concentration of sulfate and many kinds of toxic heavy metal ions (such as Fe, Al, Mn, Pb, Zn, Cu, Cd, Ni, Hg, As, Cr and other heavy metal ions) [79]. At present, the common treatment methods for AMD include the physical method, the chemical method and the biological method [10]. Physical methods include ion exchange, adsorption, membrane filtration, coagulation and flocculation, flotation and electrochemical methods. Chemical methods include alkali neutralization precipitation and sulfide precipitation. Biological methods include constructed wetland method and microbial method [11]. In the actual treatment process, the cost of physical and chemical treatment of AMD is high, and it is extremely easy to cause secondary pollution [12]. Constructed wetland method covers an large area, and is vulnerable to seasonal climate [8]. As a result of its harmless and economic characteristics, microbial method is currently the most promising and concerned treatment method for AMD treatment [12]. Microbial method uses organic matter as carbon source and energy through sulfate-reducing bacteria (SRB) in anaerobic conditions to reduce SO42− to S2−. S2− and metal ions in wastewater generate sulfide precipitation to remove sulfate and heavy metal ions in AMD [13]. However, in the actual treatment process, the acid conditions of AMD inhibit the growth of SRB and the heavy metal ions in AMD have toxic effects on SRB [14], resulting in low growth activity of SRB and low treatment efficiency of AMD. Therefore, it is the key to look for a material that can improve AMD pH and adsorb heavy metal ions to repair SRB efficiently.
Lignite reserves are abundant in China. It has been proved that the reserves are about 130 billion tons, accounting for 13% of the national coal resources. Most of them are located in the shallow surface and are easy to be mined [15]. Lignite is a kind of coal whose coalification degree is between peat and bituminous coal. It is rich in humic acid, and its humic acid content is generally between 10% and 80% [16]. Humic acid in lignite is a kind of polyelectrolyte with negative charge, which can adsorb heavy metal ions in AMD by electrostatic attraction. The developed pore structure and specific surface area of lignite provide abundant active sites for heavy metal ions [17]. At the same time, lignite has natural porous properties [18] and large specific surface area. The surface contains active groups such as carboxyl, alcohol hydroxyl, phenol hydroxyl, quinone, carbonyl and methoxy groups [17]. The active groups on the surface of lignite can adsorb heavy metal ions on the surface of lignite by ion exchange and coordination, which is negatively charged [19, 20] and has good affinity for H+ and heavy metal ions. It can improve the pH of AMD and remove heavy metal ions in AMD. H. Polat et al. [21] showed that the adsorption capacity per unit surface area of lignite was at least four times that of activated charcoal. M. Uçurum [22] used lignite to remove heavy metal Pb in water. The results showed that 80 mesh lignite had the best adsorption effect on Pb, and the best adsorption capacity was 29.92 mg/g. Qiongqiong He [23, 24] used lignite as an effective low-cost adsorbent to remove organic pollutants from wastewater. Mohan [25] used lignite to adsorb and remove Fe2+, Fe3+ and Mn2+ heavy metal ions in AMD. The recovered heavy metal ions were resolved by 0.1 mol/L nitric acid, and the recovery rate of heavy metals was 100%. The adsorption capacity of the lignite can be recovered after the analysis. Fethiye et al. [26] studied the kinetic and thermodynamic parameters of Cr(III) adsorption on lignite from Turkey and Anatolia. The experimental results showed that the adsorption rate of Cr(III) reached 90% at pH = 4.5. However, the water quality report of AMD in a coal mine in Datong, Shanxi Province, China showed that the content of Cu2+ and Zn2+ in the wastewater was high (pH = 3~5, Cu2+ = 55~63 mg/L and Zn2+ = 77~80 mg/L). The local lignite has the characteristics of large reserves and convenient utilization. Therefore, it is convenient to use local lignite to treat AMD for large-scale application in mining area.
In view of the high content of Cu and Zn in AMD, this experiment used lignite as raw material to explore the adsorption capacity of lignite for Cu2+ and Zn2+ in AMD. The adsorption characteristics of Cu2+ and Zn2+ on lignite were analyzed by adsorption kinetics and isothermal adsorption experiments. The competitive adsorption mechanism of Cu2+ and Zn2+ on lignite was revealed by the adsorption effect of copper and zinc on lignite in single metal and binary metal systems. The lignite before and after AMD adsorption was characterized and analyzed to reveal the internal mechanism of adsorption of Cu2+ and Zn2+ by lignite, by combining with FT-IR and XPS, so as to provide new methods for the efficient repair of AMD by lignite-assisted SRB.

2. Materials and Methods

2.1. Experimental Materials and Instruments

Experimental materials: lignite raw materials used in the experiment were from Shanxi Fuhong Mineral Products Co., Ltd. The lignite included 52% of humic acid. After crushing lignite with high-speed crusher (HD-ZK), lignite with particle size of 80 mesh was selected for standby. Dry lignite samples were taken for SEM and BET detection. Among them, SEM was detected by ZEISS Sigma 300 instrument, and BET was detected by MIKE ASAP 2460 instrument. SEM detection was shown in Fig. 1. BET results showed that the Langmuir surface area, BET surface area, pore area and average pore diameter of lignite were 48.60, 6.72, 5.64 and 6.72 m2/g, respectively.
Water quality detection methods and instruments: PHS-3C pH meter was used for pH measurement. Copper and zinc ion concentrations were determined by flame atomic spectrophotometer Z-2000. Turbidity was measured by V-1600PC visible spectrophotometer. Zeta potential measurement was measured by ZS90 Zeta potentiometer. EDS detection was measured by ZEISS Sigma 300. The lignite before and after the experiment was dried in a 105°C blast dryer for 24 h, and then grinded to 200 mesh with quartz bowl. The grinded lignite was evenly mixed with KBr at the ratio of 1%, and the mixture was pressed. The FTIR surface functional groups of the tablet were detected by Nicolet iS50 and Nicolet Nexus410 FT-IR Fourier transform infrared spectrometer. The detection range of FTIR is 4,000~400 cm−1. The crushed lignite was pressed and placed in Bruker D8 Advance X-ray photoelectron spectrometer for XPS detection.

2.2. Experimental Method

2.2.1. Adsorption characteristics of lignite in single metal system

Adsorption kinetics experiment of lignite on Cu2+ and Zn2+: Analytically reagent CuSO4·5H2O and ZnSO4·7H2O were used to prepare Cu2+ standard solution and Zn2+ standard solution. The Cu2+ standard solution (Zn2+ standard solution) was taken with a liquid gun (JOANLAB) and filtered by 0.45 μm microporous membrane. The concentrations of Cu2+ and Zn2+ in Cu2+ standard solution and Zn2+ standard solution were determined to be 400 mg/L, and the pH was adjusted to 4 with 1 mol/L diluted HNO3. 2 g of lignite was added respectively, after sealing, placed in 150 r/min constant temperature gas bath shaker at 25°C to oscillate. Samples were taken in the minute of 15, 30, 60, 120, 180, 240, 300, 360, 480, 1,320, 1,380, 1,440, 1,500 with a liquid-shift gun, respectively. The samples were filtered by 0.45 μm microporous membrane, and the pH value and the concentrations of Ca2+, Cu2+, Zn2+ were determined.
Isothermal adsorption experiment of lignite for Cu2+ and Zn2+: Cu2+ standard solution and Zn2+ standard solution were prepared by analytical pure CuSO4·5H2O and ZnSO4·7H2O. The Cu2+ standard solution (Zn2+ standard solution) was taken with a liquid transfer gun and filtered by 0.45 μm microporous membrane. The concentrations were determined to be 1600 mg/L. The above standard solutions were diluted to 800, 640, 320, 160, 80, 40, 20, 10 and 5 mg/L and the pH was adjusted to 4 of all solutions. 1 g of lignite was added to 100 mL of Cu2+ and Zn2+ solutions with concentrations of 1,600, 800, 640, 320, 160, 80, 40, 20, 10 and 5 mg/L, respectively. After sealing, it was placed in a constant temperature gas bath shaker at 25°C and 150 r/min for 24 h. At 24 h, the samples were sampled with a liquid gun and filtered through a 0.45 μm microporous membrane to measure the concentrations of Ca2+, Cu2+ and Zn2+. After the reaction was terminated, the pH value was measured, and the turbidity was measured after standing for 2.5 h.

2.2.2. Adsorption characteristics of lignite in binary metal system

The analytical pure CuSO4·5H2O and ZnSO4·7H2O were used to prepare the standard solution with Cu2+ and Zn2+ concentrations of 1,600 mg/L. The above standard solutions were diluted to 800, 640, 320, 160, 80, 40, 20, 10 and 5 mg/L and the pH was adjusted to 4 of all solutions. 1 g of lignite was added to 100 mL Cu2+ and Zn2+ solutions of 1,600, 800, 640, 320, 160, 80, 40, 20, 10 and 5 mg/L, respectively. After sealing, the lignite was placed in a constant temperature gas bath shaker at 25°C and 150 r/min for 24 h. At 24 h, the samples were taken with a liquid gun and filtered through a 0.45 μm microporous membrane to determine the concentrations of Ca2+, Cu2+ and Zn2+. After the end of the reaction, pH was determined, and turbidity was determined after standing for 2.5 h.

2.2.3. Pollutant leaching experiment of lignite after adsorption of Cu2+ and Zn2+

The lignite samples after adsorption of 800 mg/L Cu2+ and Zn2+ were dried in an air dryer at 105°C for 24 h. 0.5 g of lignite after adsorption of Cu2+ and Zn2+ was placed in 100 mL deionized water, at 200 r/min and 25 °C oscillation for 24 h. The pH value of leaching solution, the concentration of Cu2+ and Zn2+ were determined, and the feasibility of secondary utilization of lignite after adsorption of Cu2+ and Zn2+ was analyzed.

3. Results and Discussion

3.1. Adsorption Kinetics of Lignite on Cu2+ and Zn2+

In order to further explore and analyze the process of lignite adsorption on Cu2+ and Zn2+, the adsorption kinetics experiments of lignite adsorbing Cu2+ and Zn2+ were carried out in single metal system. The experimental results were fitted by using quasi-first-order kinetics and quasi-second-order kinetics model [27]. The fitting results are shown in Fig. 2(a) and Table 1.
The quasi-first-order dynamic model is:
(1)
dqtdt=K1(qe-qt)
Where,
  • qt: adsorption time is the adsorption quantity at time t, mg/g

  • qe: adsorption amount at adsorption equilibrium, mg/g

  • t: adsorption time, min

  • K1: quasi-first-order kinetic reaction rate constant, 1/min

The quasi-second-order dynamic model is:
(2)
dqtdt=K2(qe-qt)2
Where,
  • qt: adsorption time is the adsorption quantity at time t, mg/g

  • qe: adsorption amount at adsorption equilibrium, mg/g

  • t: adsorption time, min

  • K2: quasi-second-order kinetic reaction rate constant, 1/min

As shown in Fig. 2(a), the adsorption of Cu2+ by lignite reached the adsorption equilibrium at 24 h, and the adsorption of Zn2+ reached the adsorption equilibrium at 6 h. The adsorption of Cu2+ and Zn2+ by lignite were basically divided into three stages: rapid adsorption stage, slow adsorption stage and equilibrium adsorption stage. In the first 30 min, because of the rich adsorption sites on the surface of lignite, lignite adsorbed Cu2+ and Zn2+ rapidly, which was called a rapid adsorption stage. With the progress of adsorption, the adsorption sites on lignite surface gradually decreased, and the adsorption rate gradually decreased. But it was still greater than the desorption rate. The adsorption amount slowly increased with time, and the adsorption entered the slow adsorption stage. At 24h and 6h, the adsorption rate of Cu2+ and Zn2+ by lignite was equal to the desorption rate, and the adsorption capacity was basically stable. The adsorption was in a dynamic equilibrium, and the adsorption reached the equilibrium adsorption stage. The equilibrium adsorption capacity of lignite for Cu2+ and Zn2+ were 67.84 mg/g and 55.5 mg/g, respectively. The higher specific surface area and developed pore structure were the main reasons for the better adsorption performance of lignite.
From Table 1, the correlation coefficient R2 of the pseudo-first-order kinetic equation of Cu2+ and Zn2+ adsorption by lignite was large, and the theoretical equilibrium adsorption qe of Cu2+ and Zn2+ by lignite calculated by the pseudo-first-order kinetic model was close to the experimental equilibrium adsorption. The correlation coefficient R2 of the pseudo-second-order kinetic equation was low, and the theoretical equilibrium adsorption qe was much lower than the experimental equilibrium adsorption. Therefore, the adsorption kinetics of lignite for Cu2+ and Zn2+ were more in line with the quasi-first-order kinetic model. The pseudo-first-order kinetic model assumes that the adsorbate was physical adsorption, so physical adsorption was the main mechanism of lignite adsorption of Cu2+ and Zn2+. The equilibrium adsorption capacities of lignite for Cu2+ and Zn2+ were 67.84 mg/g and 55.5 mg/g, respectively. The adsorption rates of lignite for Cu2+ and Zn2+ were 0.033 min−1 and 0.041 min−1, respectively. From Fig. 2(b), the calcium ion concentration and solution alkalinity released by lignite were significantly higher when copper is adsorbed. It showed that Cu2+ was easier to ion exchange with calcium ions in lignite. The equilibrium adsorption capacity of lignite for Cu2+ was higher than that for Zn2+, but the adsorption rate of lignite for Zn2+ was higher than that for Cu2+, indicating that the selectivity of lignite for Cu2+ was better than that of Zn2+. Covelo et al. [28] found that the adsorption of Cu by humus conformed to the pseudo-first-order kinetic model, and the equilibrium adsorption amount of Cu was about 6~7 μmol/g, and that of Zn was about 0~2 μmol/g. Runhu Zhang[29] showed that the maximum adsorption capacity of lignite for Cr6+ was 24.96 mg/g. Sohan Shrestha[30] and other studies have shown that the maximum saturated adsorption capacity of activated carbon fiber (lignite and coconut shell-based activated carbon fiber) prepared by lignite combined with coconut shell for Zn2+ was 9.43 mg/g. Compared with the above studies, the adsorption of copper and zinc by lignite was significantly better than that of activated carbon fiber prepared by humus and lignite combined with coconut shell.

3.2. Isothermal Adsorption Lines of Lignite on Cu2+ and Zn2+

In order to further explore the equilibrium mechanism of lignite adsorption on Cu2+ and Zn2+, the isothermal adsorption experiments of lignite adsorption on Cu2+ and Zn2+ were carried out in single metal and binary metal systems, respectively. The Langmuir isotherm model and Freundlich isotherm model [31] were used to fit the isothermal adsorption line. The adsorption isotherms of Cu2+ and Zn2+ of lignite were fitted by Langmuir isotherm model and Freundlich isotherm model. The fitting results are shown in Fig. 2(c)~(e) and Table 2.
Langmuir isotherm model:
(3)
qe=KLqmCe1+KLCe
Where,
  • qe: adsorption capacity at equilibrium, mg/g

  • qm: theoretical maximum adsorption capacity, mg/g

  • Ce: solution adsorbate concentration at adsorption equilibrium, mg/L

  • KL: Langmuir constant, L/mg

Freundlich isotherm model:
(4)
qe=KFCe1n
Where,
  • qe: adsorption capacity at equilibrium, mg/g

  • Ce: solution adsorbate concentration at adsorption equilibrium, mg/L

  • KF: adsorption constant, (mg/g)(1/mg)1/n

  • n: Adsorption strength constant

From Fig. 2(c), according to the isotherm classification system [32], the isothermal adsorption lines of lignite adsorbing Cu2+ and Zn2+ in single metal and binary metal systems were all L-2 curves in typical L-type isotherms. Junting Sun [33] explored the adsorption of Pb, Cu, Zn, Cd, and Cr by porous carbon (HPC), and the results showed that the adsorption process of the five metals conformed to the Langmuir isotherm. The maximum adsorption capacity of HPC for Cu was 56.4 mg/g. Lijun Wang et al. [34] explored the adsorption of Cu, Pb and Cd by bagasse. The results showed that Langmuir isotherm could describe the adsorption of metal ions well by bagasse. Meanwhile, the results showed that the maximum adsorption capacity of bagasse for Cu2+ was 21.98 mg/g. Compared with the above studies, the equilibrium adsorption capacity of Cu2+ and Zn2+ by lignite was 67.84 mg/g and 55.5 mg/g. The adsorption effect of lignite was significantly better than that of porous carbon and bagasse, and it was more convenient to obtain lignite in the mining area when treating AMD. The isothermal adsorption lines of Cu2+ and Zn2+ adsorbed by lignite in single metal and binary metal systems did not reach an obvious equilibrium, indicating that at the initial concentration set in the experiment, lignite did not show a limited adsorption capacity for Cu2+ and Zn2+. Therefore, lignite in single metal and binary metal systems had good adsorption properties for Cu2+ and Zn2+. With the increase of the initial concentrations of Cu2+ and Zn2+, the equilibrium adsorption capacity of lignite on Cu2+ and Zn2+ gradually increased. The main reason is that the larger the initial concentrations of Cu2+ and Zn2+ in the system are, the larger the concentration difference of Cu2+ and Zn2+ in the solid phase and liquid phase is. Thus the driving force for the migration of Cu2+ and Zn2+ to the surface of lignite increased. Therefore, the equilibrium adsorption capacity became larger with the enhanced adsorption capacity of lignite.
The equilibrium adsorption capacity of lignite on Cu2+ and Zn2+ in single metal system was higher than that of lignite on Cu2+ and Zn2+ in binary metal system, indicating that there was competition between Cu2+ and Zn2+ in binary metal system. Mahamadi C et al. [35] showed that the adsorption characteristics of single-element system and multi-element system were different. Sejin Oh [36] and other studies showed that the adsorption effect of activated carbon on Co, Ni and Cu in single metal system was better than that in multi-metal system. This was due to the coexistence of components in the system competitive adsorption sites of adsorbent. The affinity order of lignite on Cu2+ and Zn2+ was: Cu2+ > Zn2+. At the same time, the surface charge of lignite was negative. The zeta potential measurement results showed that the surface potential of lignite was −18.4 at pH = 7. K.B. Thapa [19] measured the zeta potential of lignite about −15. The zeta potential of lignite measured by Yanan Tu [20] was about −29. The zeta potential of lignite used in this study was different from that reported above, but lignite showed negative charge. The electronegativity of Cu2+ was greater than that of Zn2+. Under the electrostatic effect, Cu2+ was more likely to form a tight layer on the surface of lignite than Zn2+, and continuously entered the dense layer from the diffusion layer until the charge balance on the surface of lignite. Therefore, the affinity of lignite to Cu2+ was higher than that of Zn2+. So that the adsorption effect of lignite on copper ions was better than that of zinc.
From Fig. 2(d), 2(e) and Table 2, the isothermal adsorption line of lignite on Cu2+ was fitted, and the fitting effect of Langmuir model was better than that of Freundlich model. According to the Langmuir model fitting, the theoretical maximum adsorption capacity of lignite for Cu2+ was 228.59 mg/g. The isothermal adsorption line of Zn2+ adsorbed by lignite was used for fitting parameters, and the fitting effect of Freundlich model was better than that of Langmuir model. Lignite adsorption Zn2+ was a multi-molecular layer adsorption. Adsorption isotherm of Zn2+ on lignite Freundlich model fitting parameters 1/n was in the range of 0.5 ~ 1, indicating that the adsorption of Zn2+ by lignite was prone to occurrence. The value of KF reflects the adsorption capacity of adsorbent. The larger the KF value was, the better the adsorption performance of adsorbent was. The KF value of Freundlich model of Cu2+ adsorption by lignite in binary metal system was higher than that of Freundlich model of Cu2+ adsorption by lignite in single Cu2+ system. Therefore, the adsorption performance of lignite on Cu2+ was better than that on Zn2+ in the binary system. Cu2+ was more competitive when lignite adsorbed Cu2+ and Zn2+ simultaneously.
From Fig. 2(f), after lignite adsorbing Cu2+ and Zn2+, the pH of the system improved significantly. The pH improvement effect of lignite after Zn2+ adsorption was better than that of lignite after Cu2+ adsorption. The surface of lignite was charged negatively. In the process of adsorption of Cu2+ and Zn2+, H+ competes for the active sites adsorbed on the surface of lignite [37], resulting in a gradual increase in the pH of the system. At the same initial concentration, the equilibrium adsorption amount of Cu2+ by lignite was much higher than that of Zn2+ by lignite, indicating that Cu2+ was more easily adsorbed on the active sites on the surface of lignite than Zn2+. Therefore, the pH improvement effect of Zn2+ system was more obvious than that of lignite adsorption Cu2+. From Fig. 2(g), Ca2+ were dissolved in lignite when Cu2+ and Zn2+ were adsorbed. At the same concentration, the calcium concentration after adsorbing cooper was higher than that after adsorbing zinc. It showed that Cu2+ was easier to ion exchange with Ca2+ in lignite than Zn2+. After lignite adsorbing Cu2+ and Zn2+, the turbidity of the system was below 50 NTU. After lignite adsorbing Cu2+ and Zn2+, it could precipitate completely after standing for 2.5 h. As adsorbent lignite is easy to recover, and not easy to produce secondary pollution.

3.3. Leaching of Pollutants from Lignite after Adsorption of Cu2+ and Zn2+

Fig. 2(h) showed that the concentrations of copper and zinc ions in lignite leaching solution were 0.0007 mg/L and 0.0009 mg/L, respectively. The concentrations of copper and zinc ions in lignite leaching solution after adsorbing cooper were 0.0460 mg/L and 0.0010 mg/L, respectively. The concentrations of copper and zinc ions in lignite leaching solution after adsorbing zinc were 0.0004 mg/L and 0.0460 mg/L, respectively. The concentrations of copper and zinc ions in lignite leaching solution after adsorbing cooper and zinc were 0.0066 mg/L and 0.0420 mg/L, respectively. The leaching concentration of Cu2+ met the China National Surface Water Quality Standard (GB/T14848-9) type II water, and Zn2+ met the type I water. So lignite after adsorption of copper and zinc does not release secondary pollution.

3.4. Adsorption Mechanism

3.4.1. FT-IR analysis and EDS analysis

From Fig. 3(a), after the adsorption of Cu2+, the OH stretching vibration absorption peak of lignite at 3,400 cm−1 corresponding to the adsorption of free water increased. The reason is that the carboxyl and hydroxyl functional groups on the surface of lignite molecule provided multiple adsorption sites for water molecules. Water molecules were easily adsorbed on the surface of lignite molecule through hydrogen bonding, resulting in the enhancement of OH stretching vibration absorption peak of lignite adsorbed free water in infrared spectrum. After adsorption of Cu2+, the OH signal in the aromatic structure of lignite at 3,050, 1,600 and 1,450 cm−1 weakened, and the free water signal enhanced, indicating that a certain amount of hydroxyl water released and the crystal structure collapsed. This is mainly due to the coordination between oxygen atoms in aromatic structure and Cu2+, resulting in the removal of hydroxyl water. After adsorption of Cu2+, the change of lignite in the range of 650~900 cm−1 corresponded to the series of absorption peaks of complex C-H out-of-plane bending vibration, indicating that the crystal structure of lignite underwent a certain collapse. From Fig. 3(b), after Zn2+ adsorption, the OH stretching vibration signal in the corresponding structure of lignite enhanced near 3,692 and 3,620 cm−1. An isolated electron pair of oxygen atoms existed in the oxygen-containing functional group of lignite, which has strong electrophilicity and is easy to form hydrogen bonds with water molecules [38], resulting in the enhancement of OH stretching vibration signal in the lignite structure. The C-O stretching vibration peaks of ethers, alcohols and phenols near 1,200 and 1,050 cm−1 enhanced after Zn2+ adsorption, indicating that Zn2+ coordinated with oxygen atoms in lignite molecular structure. The change of C-H signal of lignite after Zn2+ adsorption was not obvious, and the crystal structure of surface lignite did not change greatly. From Fig. 3(c), the changes of lignite after adsorption of Cu2+ and Zn2+ were similar to those of lignite after adsorption of Cu2+. The OH stretching vibration absorption peak corresponding to the adsorption of free water enhanced at 3,400 cm−1. The OH signal in the aromatic structure weakened at 3050, 1,600 and 1,450 cm−1, and the free water signal enhanced. This phenomenon indicated that the oxygen atoms in the aromatic structure coordinated with Cu2+ and Zn2+, and there was removal of a certain amount of hydroxyl water. In the meantime, the carboxyl and hydroxyl functional groups on the surface of lignite molecule were adsorbed on the surface of lignite molecule through hydrogen bonding, resulting in the enhancement of OH stretching vibration absorption peak of lignite adsorbing free water in infrared spectrum. In the range of 650~900 cm−1, a series of absorption peaks corresponding to the complex C-H out-of-plane bending vibration changed, the crystal structure underwent certain collapse. But the overall change was not as obvious as lignite after Cu2+ adsorption.
EDS was used to detect lignite, lignite after adsorption of Cu2+, lignite after adsorption of Zn2+ and lignite after adsorption of Cu2+ and Zn2+, the results are shown in Fig. 3(d)~(g). It can be seen from EDS that lignite mainly contained C, O, Al, Si, Au, S and Ca. Their contents were 64.02%, 12.82%, 2.66%, 8.50%, 5.70%, 4.36% and 0.25%, respectively. After adsorption of Cu2+, 1.09% of Cu element appeared in lignite, 0.99% of Zn element appeared in lignite after adsorption of Zn2+, 0.90% of Cu element and 0.80% of Zn element appeared in lignite after adsorption of Cu and Zn. EDS results show that lignite can adsorb Cu2+ and Zn2+ on the surface of lignite, resulting in the occurrence of Cu and Zn elements on the surface of lignite, and the adsorption amount of Cu2+ on lignite was greater than that of Zn2+. The results are consistent with the adsorption test results.

3.4.2. XPS analysis

From Fig. 4(a)~(d), there were mainly C, O, Ca, Fe, N, S and Si elements in raw lignite. Cu2p3 peak appeared in lignite after adsorption of Cu2+. Zn2p peak appeared in lignite after adsorption of Zn2+. Cu2p3 peak and Zn2p peak appeared in lignite after adsorption of Cu2+ and Zn2+. It indicates that chemical adsorption existed in the adsorption of Cu2+ and Zn2+ by lignite. From Table 3, the content of Ca in lignite after adsorption of Cu2+, lignite after adsorption of Zn2+and lignite after adsorption of Cu2+ and Zn2+ had different degrees of reduction, indicating that Ca2+ in lignite had ion exchange with Cu2+ and Zn2+ in the process of lignite adsorption of Cu2+ or Zn2+. The content of O in lignite after Cu2+ adsorption, Zn2+ adsorption, and Cu2+ and Zn2+ adsorption increased to varying degrees, indicating that the carboxyl and hydroxyl functional groups on the surface of lignite molecules were adsorbed on the surface of lignite molecules by hydrogen bonds with water molecules, in the process of lignite adsorbing Cu2+ or Zn2+. The content of S in lignite after adsorption of Cu2+, adsorption of Zn2+ and adsorption of Cu2+ and Zn2+ increased in varying degrees, indicating that SO42− could react with some functional groups in lignite to adsorb on the surface of lignite.
In Fig. 4(e)~(h), the peak areas of C1s(CO32−) of lignite after adsorption of Cu2+, adsorption of Zn2+ and adsorption of Cu2+ and Zn2+ reduced compared with those of original lignite. The relative contents of C1s(CO32−) of lignite after adsorption of Cu2+, adsorption of Zn2+ and adsorption of Cu2+ and Zn2+ in Table 3 correspondingly reduced, indicating that CO32− generated CuCO3 and ZnCO3 precipitated with Cu2+ and Zn2+ in the process of lignite adsorbing Cu2+ and Zn2+. The relative content of C1sC-C in lignite after adsorption of Cu2+, Zn2+ and adsorption of Cu2+and Zn2+ decreased compared with raw lignite, and the relative content of C1sC-C in lignite after adsorption of Cu2+ and adsorption of Cu2+ and Zn2+ had same variable quantity, indicating that the structure of lignite changed in the process of adsorption of Cu2+ or Zn2+. The structure of lignite changed obviously after adsorption of Cu2+, which was consistent with the results of FT-IR analysis. The relative content of C1sC-O in lignite after adsorption of Cu2+, adsorption of Zn2+ and adsorption of Cu2+ and Zn2+ increased compared with the original lignite, indicating that oxygen atoms in lignite structure coordinated with Cu2+ and Zn2+ in the process of lignite adsorption. Thus the relative content of C1sC-O in lignite increased with removal of hydroxyl water.
Fig. 5(a)~(d) shows that the peak areas of Ca2p3CaCO3 of lignite after adsorption of Cu2+, adsorption of Zn2+ and adsorption of Cu2+ and Zn2+ were smaller than that of raw lignite. The change of Ca2p3CaCO3 peak of lignite after adsorption of Cu2+ was the same as that of lignite after adsorption of Cu2+ and Zn2+. It indicates that Cu2+ or Zn2+ exchanged ion with Ca2+ to generate CuCO3 or ZnCO3 in the process of adsorption of Cu2+ or Zn2+ by lignite in single metal system. Thus Cu2+ or Zn2+ was adsorbed on the surface of lignite. In the process of lignite adsorbing Cu2+ and Zn2+ in binary metal system, only Cu2+ and Ca2+ ion exchange occurred, and CuCO3 was generated. Zn2+ did not participate in ion exchange.
Table 3 shows the relative content changes of chemical states of Cu and Zn in lignite before and after adsorption. Fig. 5(e) and Fig. 5(g) show that Cu2+ was adsorbed on lignite surface in Cu2+, CuO and CuCO3 chemical states after adsorption, and Zn2+ was adsorbed on lignite surface in ZnO and ZnCO3 chemical states after adsorption. In the single metal system, the adsorption of Cu2+ by lignite involved electrostatic, coordination and ion exchange, and the adsorption process was mainly physical adsorption. The adsorption process of Zn2+ by lignite involved electrostatic, coordination and ion exchange, and the adsorption process was mainly chemical adsorption. From Fig. 5(e) and Fig. 5(f), Cu2+ was adsorbed on the surface of lignite in the chemical states of Cu2+, CuO and CuCO3 in the single metal system and binary metal system after adsorption, indicating that the mechanism of Cu2+ adsorption by lignite in the binary metal system was the same as that on lignite in the single metal system, with both chemical and physical adsorption. From Fig. 5(g) and Fig. 5(h), Zn2+ was adsorbed on the surface of lignite in the chemical state of ZnO and ZnCO3 after adsorption in the single metal system. In the binary metal system, Zn2+ was adsorbed on the surface of lignite by ZnO chemical state after adsorption, indicating that the adsorption process of Zn2+ on lignite in single metal system involved electrostatic, coordination and ion exchange. However in the binary metal system, Zn2+ was adsorbed on the surface of lignite only through coordination. It can be seen that lignite had selectivity on Cu2+ in the binary metal system, and the adsorption performance of Cu2+ was better than that of Zn2+.

4. Conclusions

  1. The adsorption kinetics of Cu2+ and Zn2+ fitted the pseudo-first-order kinetic model, and the equilibrium adsorption capacities of Cu2+ and Zn2+ were 67.84 mg/g and 55.5 mg/g, respectively. The adsorption of Cu2+ by lignite was more in line with Langmuir model. The adsorption of Zn2+ on lignite fitted Freundlich model better.

  2. Copper and zinc removal from lignite mainly depended on physical adsorption, coordination and ion exchange. There was competitive adsorption between Cu2+ and Zn2+, and lignite had better adsorption effect on copper. The adsorbed copper ions were mainly adsorbed on the surface of lignite in the chemical states of Cu2+, CuO and CuCO3, and Zn2+ was adsorbed on the surface of lignite in the chemical states of ZnO and ZnCO3.

  3. The leaching experiments show that the concentration of Cu2+ were 0.0007 mg/L, 0.0460 mg/L, 0.0004 mg/L and 0.0066 mg/L, the concentration of Zn2+ were 0.0009 mg/L, 0.0010 mg/L, 0.0460 mg/L and 0.0420 mg/L. The release concentration of lignite pollutants adsorbed copper and zinc meets the national water quality standard of China (GB/T 14848-9).

Acknowledgments

This work was supported by the National Natural Science Foundation of China under Grant (41672247), Liaoning Province’s “Program for Promoting Liaoning Talents” under Grant (XLYC1807159), discipline innovation team of Liaoning Technical University under Grant (LNTU20TD-21).

Notes

Author Contributions

J.D. (Professor) made the experiment plan and guided the experiment in the whole process. S.B. (Ph.D. student) was in charge of data analysis and paper writing. J.Y. (Senior Engineer) and D.W. (Professor) was in charge of experiment guidance and paper revision. J.S. (Master student) and Y.D. (Ph.D. student) was responsible for detection.

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Fig. 1
SEM image of lignite.
/upload/thumbnails/eer-2021-037f1.gif
Fig. 2
Kinetic and isothermal fitting of Cu2+ and Zn2+ adsorption by lignite (a) Kinetic fitting of Cu2+ and Zn2+ adsorption by lignite (b) pH and Ca2+ released after adsorption by lignite (c) Isothermal adsorption lines of Cu2+ and Zn2+ adsorption by lignite in single metal and binary metal systems (d) Adsorption isotherm fitting of Cu2+ adsorption by lignite in single metal and binary metal systems (e) Adsorption isotherm fitting of Zn2+ adsorption by lignite in single metal and binary metal systems (f) Changes of pH after adsorption of Cu2+ and Zn2+ by lignite (g) Changes of Ca2+ and turbidity after adsorption of Cu2+ and Zn2+ by lignite (h) Leaching of pollutants from lignite after adsorption of Cu2+ and Zn2+.
/upload/thumbnails/eer-2021-037f2.gif
Fig. 3
FT-IR spectra and EDS results before and after lignite adsorption (a) FT-IR spectra of lignite adsorbing Cu2+ (b) FT-IR spectra of lignite adsorbing Zn2+ (c) FT-IR spectra of lignite adsorbing Cu2+ and Zn2+ (d) EDS image of raw lignite (e) EDS image of lignite adsorbing Cu2+ (f) EDS image of lignite adsorbing Zn2+ (g) EDS image of lignite adsorbing Cu2+ and Zn2+.
/upload/thumbnails/eer-2021-037f3.gif
Fig. 4
XPS full-spectrum scanning map before and after lignite adsorption and XPS map of lignite C1s (a) XPS full-spectrum scanning diagram of raw lignite (b) XPS full-spectrum scanning diagram of lignite after adsorption of Cu2+ (c) XPS full-spectrum scanning diagram of lignite after Zn2+ adsorption (d) XPS full-spectrum scanning diagram of lignite after Cu2+ and Zn2+ adsorption (e) XPS spectra of raw lignite (f) XPS spectra of lignite C1s (g) XPS spectra of lignite C1s after Zn2+ adsorption (h) XPS spectra of lignite C1s after Cu2+ and Zn2+ adsorption.
/upload/thumbnails/eer-2021-037f4.gif
Fig. 5
XPS spectra of Ca2p, Cu2p and Zn2p in lignite (a) Ca2p map of raw lignite (b) Ca2p map of lignite after Cu2+ adsorption (c) Ca2p map of lignite after Zn2+ adsorption (d) Ca2p map of lignite after Cu2+ and Zn2+ adsorption (e) Cu2p map of lignite after adsorption of Cu2+ (f) Cu2p map of lignite after adsorption of Cu2+ and Zn2+ (g) Zn2p map of lignite after Zn2+ adsorption (h) Zn2p map of lignite after Cu2+ and Zn2+ adsorption.
/upload/thumbnails/eer-2021-037f5.gif
Table 1
Kinetic Parameters for Adsorption of Cu2+ and Zn2+ by Lignite
Quasi-first-order kinetic fitting Quasi-second-order kinetic fitting


qe K1 R2 qe K2 R2
Cu2+ 63.33427 0.03309 0.94098 58.11306 4.12306E45 0.67693
Zn2+ 51.85517 0.04066 0.9503 48.29604 −3.30029E27 0.7272
Table 2
Fitting Parameters of Isothermal Adsorption Lines of Lignite for Cu2+ and Zn2+ under Single Metal and Binary Metal Systems
Langmuir model Freundlich model


qm KL R2 1/n KF R2
Cu2+ 228.59014 1.65055E-4 0.9995 0.80948 0.32402 0.99293
Zn2+ 163.91855 5.57445E-4 0.99619 0.70469 0.50496 0.98866
Cu2+ in binary metal system 200.8214 4.9791E-4 0.99979 0.71248 0.51993 0.99564
Zn2+ in binary metal system 5,603.84788 8.96106E-5 0.97123 0.58296 0.50632 0.97125
Table 3
Changes of Relative Content of Elements in Lignite Before and After Adsorption
Atomic/%

Raw lignite Lignite after Cu2+ adsorption Lignite after Zn2+ adsorption Lignite after adsorption of Cu2+ and Zn2+
C1s 66.89 51.3 50.71 55.41
Ca2p 2.03 0.59 1.11 1.06
Cu2p 0 5.39 0 4.23
Fe2p 1.09 3.02 1.66 2.59
N1s 2.42 1.97 1.54 1.8
O1s 24.55 32.83 34.42 30.33
S2p 0.74 2.93 1.85 2
Si2p 2.27 1.97 2.92 2.21
Zn2p 0 0 5.79 0.37
C1s(CO32−) 7.55 6.98 6.61 6.53
C1sC-C 82.38 70.85 74.85 75.32
C1sC-O 7.35 8.66 7.75 8.43
Cu2p3 CuO 0 0.78 0 0.71
Cu2p3 CuSO4/CuCO3 0 3.4 0 2.66
Cu2p3 satellite peak1 0 3.98 0 1.48
Cu2p3 satellite peak2 0 4.33 0 2.98
Zn2p3 ZnO 0 0 6.26 0.55
Zn2p3 ZnSO4/ZnCO3 0 0 3.32 0
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