Abstract

Coconut shell biochar (CSB) was selected as raw material to obtain two kinds of modified biochars by pickling and iron modification. The pickling coconut shell biochar (PCSB) and pickling-iron modified coconut shell biochar (PICSB) were used as adsorbents to remove NO3-N in alkaline rare earth industry effluent. The results showed that pickling smoothed the surface of CSB, and α-FeOOH was formed on the surface of PCSB because of FeCl3 solution modification. Suitable adsorbent dosages of PCSB and PICSB were both 2.0 g/L. The NO3-N adsorption process by PCSB and PICSB both reached equilibrium at 30 min. The quasi-first-order kinetic model shows good fit to the NO3-N adsorption by PCSB. Whereas, the quasi-second-order kinetic model is more suitable for PICSB adsorbing NO3-N. The adsorption mechanisms of PICSB for NO3-N removal were ligand exchange and electrostatic attraction, and that of PCSB for NO3-N removal was electrostatic attraction. The NO3-N adsorption amounts of PCSB and PICSB decreased with increasing adsorption temperature and pH. The maximum NO3-N adsorption amounts of PCSB and PICSB were 15.14 mg/L and 10.75 mg/L respectively with adsorbent dosage of 2.0 g/L, adsorption time of 30 min, adsorption temperature of 25 ± 1 °C, and initial solution pH of 2.01.

INSTRUCTION

With the rise of the rare earth industry, rare earth elements have been widely used in military, chemical, glass ceramics, agriculture and new materials due to their irreplaceable optical and electromagnetic properties (Jha et al. 2016; Chen et al. 2018). But a large amount of NH4NO3 effluent is produced in the process of rare earth saponification and separation, which makes the N element contamination in the water environment serious enough to exceed the standard (Yin et al. 2018). According to the Emission Standards of Pollutants from Rare Earths Industry (GB26451-2011) in China, the TN concentration of wastewater must be lower than 50 mg/L. NH4-N could be almost completely removed through mature processes such as membrane treatment, tripe effect evaporation, and precipitation (Ahmadiannamini et al. 2017). However, the NO3-N removal process is not mature due to its later development, and the presence of residual NO3-N in effluent could cause humans to suffer from methemoglobin and make the TN concentration excessive (Belkada et al. 2018; Jalili et al. 2018), which requires the NO3-N to be further treated. Different methods have been applied for the removal of NO3-N, such as electrodialysis, denitrification, membrane biofilm reactor, and so on (Kalaruban et al. 2017; Liu et al. 2018; Luo et al. 2018; Wang et al. 2018). But these methods could not be widely used due to the high operating cost, long denitrification time and low efficiency. Therefore, it is necessary to find a simple and effective method to remove NO3-N in alkaline rare earth industry effluent.

The adsorption method has been widely used due to its simple and convenient operation. In recent years, biochar has become more and more recognized as a new type of adsorbent in many environmental applications because of its low price and wide application. Agricultural residuals such as coconut shell, straw, fruit peel, beetroot and bagasse can be utilized as biochar by high-temperature cracking under anaerobic or anoxic conditions (Nunell et al. 2015; Demiral & Güngör 2016; Hafshejani et al. 2016; Wei et al. 2018). Previous studies showed that the removal rate of NO3-N in water by unmodified biochar was lower than that by modified (e.g. iron modification) biochar. For example, the NO3-N removal rate in groundwater treated by bagasse biochar loaded with nano-zero-valent iron was nearly 80%, and the removal rate of NO3-N by unloaded biochar was less than 5% (Hafshejani et al. 2016). The removal rate of NO3-N in steel pickling wastewater by 4 g/L bimetallic Fe/Ni nanoparticles biochar was 99.55% (Li et al. 2017). For iron impregnated coconut shell biochar (CSB), maximum dye removal efficiency of 99.1% was obtained with 4 g/L of dosage, 50 mg/L of initial dye concentration and pH of 3.00 (Rubeena et al. 2018). It can be seen that the contaminants in the different types of acidic industry effluent mentioned above were almost completely removed by the iron-modified biochar. However, there are few studies on the treatment of alkaline effluent such as rare earth industrial efflunet by iron-modified biochar. It is not clear whether the removal effect of iron-modified biochar in alkaline effluent is as good as that in acidic effluent. Previous experimental research has no clear discussion on this aspect. Thus, the effect of iron-modified biochar on the treatment of alkaline rare earth industry effluent is worth studying.

To explore the adsorption effect of iron-modified biochar on NO3-N in rare earth industrial effluent, CSB was used as the raw material for pickling and iron modification, and the effects of adsorbent dosage, adsorption time, solution temperature and intial solution pH were also considered, using pickling coconut shell biochar (PCSB) and pickling-iron modified coconut shell biochar (PICSB). Moreover, to understand the NO3-N adsorption process and evaluating its performances, the experimental data were fitted with quasi-first-order and quasi-second-order kinetic models; combining with the charaterization, the possible mechanisms of NO3-N adsorption are also proposed. Through experimental research, the NO3-N adsorption effect by PCSB and PICSB in alkaline rare earth industry effluent will be examined.

MATERIALS AND METHODS

Materials and reagents

CSB was purchased from the Gongyi Wanjiajing Environmental Protection Material Co., Ltd., Henan, China, and the composition can be seen in Table 1. All agents were analytical grade and deionized water was used in all experimental procedures. FeCl3·6H2O, KNO3, NaOH, ZnSO4, HCl, sulfamic acid and were all obtained from Sinopharm Chemical Reagent Beijing Co., Ltd., Beijing, China.

Table 1

The composition of CSB

Sample C % H % O % N % S % Mg % Al % Fe % Ca % 
CSB 89.40 0.62 6.88 2.17 0.08 0.03 0.10 0.04 0.68 
Sample C % H % O % N % S % Mg % Al % Fe % Ca % 
CSB 89.40 0.62 6.88 2.17 0.08 0.03 0.10 0.04 0.68 

Modified biochar preparation

CSB was ground with a mortar, passed through a 0.33 mm sieve, and mixed with HCl solution (400 mL, 1 M). Shaking the mixture (150 rpm, 1 h) in a constant temperature oscillator at 25 ± 1 °C, the mixture was centrifuged (4,000 rpm, 5 min) to obtain the solid product. Then the solid product was washed repeatedly with deionized water until the supernatant was nearly neutral. Finally, the product was taken out and dried to constant weight in an oven at 105 ± 1 °C for 2 h, and PCSB was obtained; the PCSB was added into FeCl3 solution (250 mL, 0.6 M), shaken and centrifuged in the same way as above, then the solid product was taken out and washed with deionized water 3 times, then the solid product was dried to constant weight in an oven at 105 ± 1 °C for 2 h, and PICSB was obtained.

Modified biochar characterization

The morphology and element analysis of CSB, PCSB and PICSB were examined by scanning electron microscopy (SEM) (Quanta 520, Thermo Fisher Scientific, America) and Energy Dispersive Spectrometer (EDS) (INCA, Oxford Instruments, UK), with voltage and current settings of 5 kV and 10 mA; the specific surface area and total pore volume of CSB, PCSB and PICSB were examined by Brunauer-Emmett-Teller (BET) measurements (D8-02, Germany); the types of CSB, PCSB and PICSB functional groups were detected by Fourier transform infrared spectroscopy (FTIR) (Nicolet 5700, Thermo Fisher Scientific, America) with the spectra recording from 4,000 to 400 cm−1; And the surface phases of CSB, PCSB and PICSB were detected by X-ray diffraction (XRD) (D8-02, Bruker, Germany).

Rare earth industry effluent

The experimental rare earth industry effluent was taken from a rare earth high-tech company in Zibo, Shandong, China. The properties of rare earth industry effluent were determined by Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES). The properties of rare earth industry effluent are shown in Table 2. It can be seen that the main pollutant in the effluent was NO3-N, and its concentration was 32.10 mg/L. The pH of the rare earth industry effluent was 9.0, which shows the effluent is alkaline. Besides NO3-N, the effluent also contained heavy metal elements such as Zn, Cu, Cr, Ce, Pb, La, Nd, Cd and Pr. However, heavy metals were not considered because their concentrations were much lower than that of NO3-N.

Table 2

The solution pH and concentrations of contaminants of rare earth industry effluent

Solution pH The concentrations of contaminants (mg/L)
 
NO3-N Zn Cu Cr Ce Pb La Nd Cd Pr 
9.01 3.2 × 101 3.5 × 10–2 3.6 × 10–3 9.1 × 10–4 8.6 × 10–4 1.6 × 10–4 1.0 × 10–4 5.9 × 10–5 5.4 × 10–5 1.7 × 10–5 
Solution pH The concentrations of contaminants (mg/L)
 
NO3-N Zn Cu Cr Ce Pb La Nd Cd Pr 
9.01 3.2 × 101 3.5 × 10–2 3.6 × 10–3 9.1 × 10–4 8.6 × 10–4 1.6 × 10–4 1.0 × 10–4 5.9 × 10–5 5.4 × 10–5 1.7 × 10–5 

Batch adsorption experiments

The effects of NO3-N adsorption by PCSB and PICSB were studied in an initial solution pH range of 2.02–12.01, an adsorbent dosage range of 0.4–10.0 g/L at predetermined time intervals (10, 20, 30, 40, 50 and 60 min) with a speed of 150 rpm at different adsorption temperatures (25 ± 1 °C, 35 ± 1 °C and 45 ± 1 °C). The pH was adjusted to the desired values with 0.1 M HCl or 0.1 M NaOH. Then, the mixtures were separated by centrifugation (4,000 rpm, 10 min). The residual concentration of NO3-N in the supernatant was determined by ultraviolet spectrophotometry.

The equilibrium amount (qe) of NO3-N adsorbed is calculated by Equation (1) (Abdel-Rahman et al. 2018): 
formula
(1)
The removal rate (% Removal) of NO3-N is calculated by Equation (2) (Abdel-Rahman et al. 2018): 
formula
(2)
where C0 and Ce are initial and equilibrium concentrations of NO3-N (mg/L), respectively. m is the mass of biochar (g) and V is the volume of the solution (L).

Kinetics study

Batch kinetic experiments were carried out by adding 2.0 g/L PCSB and PICSB to 50 mL effluent sample solution with the NO3-N concentration of 32.10 mg/L, and intial solution pH of 9.01. The mixtures were oscillated at different predetermined times (10, 20, 30, 40, 50 and 60 min) respectively at 25 ± 1 °C. Afterward, the mixtures were separated by centrifugation (4,000 rpm, 10 min). The residual concentration of NO3-N in the supernatant was determined by ultraviolet spectrophotometry. All the experiments were repeated three times. The experimental results were fitted by quasi-first-order (Equation (3)) and quasi-second-order kinetic model (Equation (4)).

Quasi-first-order 
formula
(3)
Quasi-second-order 
formula
(4)
where qe is the equilibrium adsorption capacity, mg/g; qt is the adsorption capacity at time t, mg/g; t is adsorption time, min; k1 is the rate constant of the quasi-first-order kinetic model, min−1; k2 is the rate constant of the quasi-second-order kinetic model, g/(mg·min)−1.

RESULTS AND DISCUSSION

Characterizations of biochar

CSB, PCSB and PICSB were characterized using SEM, EDS, BET, FTIR and XRD. Figure 1 shows the SEM-EDS images of CSB, PCSB and PICSB. The SEM images show that CSB possesses a rugged surface with a few pores. Large amount of tiny particles may be the ash that appears in the pores and on the surface of CSB. After pickling, the surface of PCSB is smoother than that of CSB, which shows a high internal porosity. After iron modification, the appearance of many small particles of iron-containing substances makes the surface of PICSB rough and uneven, and the pores are reduced. The results of EDS shows that the composition of CSB is single, mainly C (mass fraction 89.60%) and O (mass fraction 10.40%). A small amount of Cl (mass fraction 1.30%) is loaded on the surface of CSB after HCl pickling, while a small amount of Fe (mass fraction 1.20%) is loaded on the surface of PCSB after iron modification. Table 3 shows the specific surface area and total pore volume of CSB, PCSB and PICSB. The specific surface area and total pore volume of PCSB are both larger than those of CSB after pickling. However, after iron modification, some pores of PCSB are blocked by iron-containing particles, resulting in a small decrease of specific surface area and pore volume, which is consistent with the SEM results. However, the specific surface area of PICSB can still reach 814.62 m2/g, which can provide sufficient adsorption sites for NO3-N. Figure 2 shows that the N2 adsorption/desorption isotherms and pore size distribution curves of CSB and PICSB are approximately the same. From the N2 adsorption desorption curve, the adsorption amount rises sharply in the low pressure region (0 < P/P0 < 0.1), the amount of adsorption achieves equilibrium in the medium pressure region (0.3 < P/P0 < 0.8) and the high pressure region (0.9 < P/P0 < 1.0), which indicates that the number of micropores in CSB and PICSB is large, and the number of mesopores and macropores is small. The maximum accumulative pore volume V of CSB and PICSB appears at pore diameters (pd) of 1.14 nm and 1.22 nm, respectively. And there are smaller peaks at 2 nm < pd < 35 nm, which also indicates CSB and PICSB have a large number of micropores and a small number of mesopores. The N2 adsorption/desorption isotherm of PCSB shows a large peak at pd of 1.79 nm and 11.44 nm, a small peak at pd of 25.24 nm, and a maximum peak at pd of 4.52 nm, this indicates that there are abundant mesopores and a few micropores in PCSB. This may be because the pickling washes away the ash particles inside the CSB pores. Average pore diameters of PCSB and PICSB were found to be 3.30 and 2.28 nm, respectively. The radius of NO3 is 0.129 nm, which makes it possible for NO3-N to penetrate into the pores of both PCSB and PICSB. Figure 3(a) shows the FTIR curves of CSB, PCSB, and PICSB. A broad band at 3,410 cm−1 is attributed to the overlap of O-H and N-H stretching vibration of CSB (Xiong et al. 2018). Bands at 1,550 cm−1 and 1,090 cm−1 are related to C-O and C-C stretching vibration (Bekiaris et al. 2016). A weak peak at 625 cm−1 only for PICSB illustrates that the Fe-O stretching vibration occurs after iron modification of PCSB (Zhang et al. 2016). An apparent decrease of the area under the -OH stretching and bending vibration modes was observed after pickling and iron modification, revealing that the number of -OH groups on the PICSB surface decreases (Cheng et al. 2016), which may be due to pickling. Also, red shifted bands at about 3,390 cm−1 are observed, which suggests that the interactions between iron ions and -OH lead to the weakness of the O-H vibration of PCSB and PICSB (Ma et al. 2014). The above shows that iron is likely to exist in the form of Fe(OH)X or Fe-O-OH. Figure 3(b) shows the XRD curves of CSB, PCSB, and PICSB. CSB corresponds to three diffraction peaks at 24.520°, 26.420°, and 43.900° respectively, which are assigned to the characteristic diffraction peaks of CSB (Sun et al. 2017). There are no significant differences in peak intensity and angle between CSB, PCSB and PICSB, indicating that the property and surface phase of CSB don't change significantly after pickling and iron modification. A peak of PICSB appears at 33.320 °, indicating that the Fe element is determined to be loaded onto the PCSB surface in the form of α-FeOOH (Budimirović et al. 2017).

Table 3

The specific surface area and total pore volume of biochar

Sample Surface area (m2/g) Pore volume (cm3/g) Average pore diameter (nm) 
CSB 1,001.08 0.47 1.87 
PCSB 1,058.60 0.83 3.30 
PICSB 814.62 0.50 2.28 
Sample Surface area (m2/g) Pore volume (cm3/g) Average pore diameter (nm) 
CSB 1,001.08 0.47 1.87 
PCSB 1,058.60 0.83 3.30 
PICSB 814.62 0.50 2.28 
Figure 1

The SEM-EDS images of CSB (a), PCSB (b), PICSB (c).

Figure 1

The SEM-EDS images of CSB (a), PCSB (b), PICSB (c).

Figure 2

N2 adsorption/desorption isotherm and pore diameter of CSB (a), PSCB (b) and PICSB (c).

Figure 2

N2 adsorption/desorption isotherm and pore diameter of CSB (a), PSCB (b) and PICSB (c).

Figure 3

The FTIR curves of CSB, PCSB, PICSB (a) and the XRD curves of CSB, PCSB, PICSB (b).

Figure 3

The FTIR curves of CSB, PCSB, PICSB (a) and the XRD curves of CSB, PCSB, PICSB (b).

Effect of adsorbent dosage

Under the conditions of adsorbent dosage of 2.0 g/L, adsorption time of 24 hours and adsorption temperature of 25 ± 1 °C, the adsorption amounts of CSB, PCSB and PICSB are shown in Figure 4(a). The adsorption capacity of CSB is much smaller than PCSB and PICSB. Therefore, we choose PCSB and PICSB for the futher experiments. Figure 4(b) shows the results of the NO3-N removal rate and adsorption capacity of PCSB and PICSB versus different dosages with adsorption time of 30 min, adsorption temperature of 25 ± 1 °C and initial solution pH of 9.01. When the dosage is less than 2.0 g/L, the adsorption capacity of NO3-N increases rapidly with the increase of PCSB and PICSB dosages. When the dosages of PCSB and PICSB are small, NO3-N quickly occupies the active sites on the surface of the biochar, which leads to the large adsorption capacities of PCSB and PICSB. When the dosages of PCSB and PICSB increase gradually, there are enough active sites on the surface of the modified biochars, which results in the small adsorption capacities of PCSB and PICSB. At the dosage of 2.0 g/L, the removal rates of PCSB and PICSB for NO3-N removal are 55.33% and 30.70%, the NO3-N adsorption amounts of PCSB and PICSB are 8.88 mg/L and 4.93 mg/L respectively. The removal rate of NO3-N doesn't increase significantly when PICSB and PCSB are overdosed. Therefore, based on the results, adsorbent dosage of 2.0 g/L was selected as the optimum dosage for further experiments.

Figure 4

Effect of dose (a) and comparsion of adsorption capacities of CSB, PCSB, PICSB (b) Effect of adsorption time (c) of NO3-N adsorption. Quasi-first/second-order kinetics curves (d, e) and the pHZPC of virgin and spent PICSB (f). (g) Effect of adsorption time of NO3-N adsorption. (h) Effect of initial solution pH of NO3-N adsorption.

Figure 4

Effect of dose (a) and comparsion of adsorption capacities of CSB, PCSB, PICSB (b) Effect of adsorption time (c) of NO3-N adsorption. Quasi-first/second-order kinetics curves (d, e) and the pHZPC of virgin and spent PICSB (f). (g) Effect of adsorption time of NO3-N adsorption. (h) Effect of initial solution pH of NO3-N adsorption.

Effect of adsorption time

Figure 4(c) shows that the effect of adsorption time on NO3-N adsorption capacity of PCSB and PICSB was conducted with an adsorbent dosage of 2.0 g/L, adsorption temperature of 25 ± 1 °C and initial solution pH of 9.01. The adsorption rate of PCSB and PICSB for removing NO3-N are both large in the beginning stage (Hafshejani et al. 2016; Yin et al. 2018), the adsorption processes both reach equilibrium at 30 min. There are numerous available fresh adsorption sites on the surface of PCSB and PICSB when a certain amount of modified biochars are added at the initial stage of NO3-N adsorption (Olgun et al. 2013). And the high concentration gradient from the liquid to the surface of the biochar promotes the adsorption of NO3-N, the removal rate of NO3-N increases rapidly. But after a period of time, the active sites on the surface of PCSB and PICSB are gradually occupied, and the NO3-N adsorption rate slows down. According to the obtained results, adsorption time of 30 min was chosen for further experiments.

Adsorption kinetics

The kinetic model is one of the most important indicators to study the mechanism of the adsorption process. Figure 4 shows the quasi-first-order kinetic curves (d) and quasi-second-order kinetic curves (e) of the NO3-N adsorption by PCSB and PICSB. The quasi-first-order adsorption kinetic model assumes that the adsorption rate decreases linearly with the adsorption capacity (Hafshejani et al. 2016). The quasi-second-order adsorption kinetic model assumes that the adsorption rate is controlled by the chemical adsorption mechanism (Zhang et al. 2013). Table 4 shows kinetic parameters for the removal of NO3-N by PCSB and PICSB. The quasi-first-order kinetic model of NO3-N adsorption by PCSB fitted better compared with the quasi-second-order kinetic model, which indicates that the NO3-N adsorption by PCSB is controlled by physical adsorption. While the fitting effect of the quasi-second-order kinetic model is more superior to the NO3-N adsorption by PICSB. The quasi-second-order kinetic equation for NO3-N adsorption by PICSB with higher value of correlation coefficient (R2) and accurate qe,cal value appears to provide a better fit as compared to the quasi-first-order kinetic model. Therefore, PICSB adsorbs NO3-N by chemical adsorption. FTIR analysis has already confirmed that the functional groups of PICSB include N-H, OH and Fe-O. XRD analysis showed that iron exists on the surface of PICSB in the form of α-FeOOH. Figure 4(f) showed that the pHZPC values of the PICSB in rare earth effluent before and after adsorption were found as 4.98 and 5.80, respectively. This shift in pHZPC may be attributed to the replacement of -OH on the surface of PICSB by NO3 ions and the substituted -OH was released into the solution (Chaudhary et al. 2019). Combined both experimental results and literature, mechanisms (Ι. and II.) for the adsorption of NO3-N by PICSB and PCSB were proposed (Ma et al. 2014).

  • I.
    Ligand exchange between α-FeOOH of PICSB and NO3-N 
    formula
  • II.
    Electrostatic attraction between N-H and OH of PICSB and NO3-N 
    formula
     
    formula
Table 4

Kinetic parameters for the removal of NO3-N by modified coconut shell biochar

Sample qe,exp mg/g Quasi-first-order kinetics model
 
Quasi-second-order kinetics model
 
k1 min−1 qe,cal mg/g R2 k2 g/(mg·min) qe,cal mg/g R2 
PCSB 9.70 0.1557 9.68 0.9987 0.0146 11.66 0.9567 
PICSB 5.01 0.1339 2.30 0.8465 0.2095 5.12 0.9979 
Sample qe,exp mg/g Quasi-first-order kinetics model
 
Quasi-second-order kinetics model
 
k1 min−1 qe,cal mg/g R2 k2 g/(mg·min) qe,cal mg/g R2 
PCSB 9.70 0.1557 9.68 0.9987 0.0146 11.66 0.9567 
PICSB 5.01 0.1339 2.30 0.8465 0.2095 5.12 0.9979 

Effect of solution temperature

The adsorption experiments were evaluated at different temperatures. Figure 4(g) shows that 25 ± 1 °C, 35 ± 1 °C and 45 ± 1 °C were changed to investigate the effect of solution temperature on the NO3-N adsorption capacity of PCSB and PICSB with adsorbent dosage of 2.0 g/L, adsorption time of 30 min and initial solution pH of 9.01. The NO3-N adsorption value of PCSB and PICSB decreased from 8.88, 4.93 mg/g to 5.56, 2.63 mg/g respectively when the solution temperature increased from 25 ± 1 °C to 45 ± 1 °C, indicating the NO3-N adsorption by PCSB and PICSB decreases with increasing temperature. The reason is that adsorption is an exothermic process; when the temperature rises, the rising of temperature within a certain range is favorable for desorption, which is not conducive to NO3-N adsorption (Halajnia et al. 2013). Moreover, NO3-N could diffuse at a suitable temperature, and it is more likely to penetrate into the outer boundary layer and internal pores of the biochar. When the temperature rises, the adsorbate NO3-N tends to escape from the solid phase of PCSB and PICSB, which may be too active to be captured by interstitial and functional groups, resulting in a reduction of the NO3-N adsorption amount (Li et al. 2018).

Effect of initial solution pH

Initial solution pH is the most important factor to detect the effect of NO3-N adsorption by PCSB and PICSB in alkaline rare earth industry effluent. Figure 4(h) shows the effect of initial solution pH within the range of 2.01–12.02 on NO3-N adsorption by PCSB and PICSB with adsorbent dosage of 2.0 g/L, adsorption time of 30 min, and adsorption temperature of 25 ± 1 °C. The adsorption amount of NO3-N by PCSB and PICSB decreased with the increase of initial solution pH. PICSB and PCSB reached a maximum value of 15.14 mg/L and 10.75 mg/L respectively at initial solution pH of 2.01. When NO3-N was adsorbed by PCSB in the pH range of 2.01–7.99, the amount of adsorption decreased slowly but had a dramatic decline when pH > 7.99. The electrostatic attraction between PCSB and NO3 is strong under acidic condition (Hafshejani et al. 2016; Li et al. 2017). But when the pH rises, the surface electrical charge of PCSB changes, electrostatic repulsion appears on the negatively charged PCSB and NO3, competition between NO3 and increasing OH also occurs for the same sites on the surface of PCSB, which could lead to a decrease in NO3-N adsorption. However, the adsorption capacity of PICSB was higher than that of PCSB in the pH range of 2.01–5.01, and when pH > 5.01, the adsorption capacity of PICSB was lower than that of PCSB, demonstrating that a strong acidic condition is more favorable for NO3-N removal by PICSB. This is because the coordination of α-FeOOH with water forms a hydroxylated surface under a strong acidic condition, and the dissolved -OH groups could exchange anions with NO3 (Cheung et al. 2007). Figure 4(f) shows that the pHZPC is 4.98. When the pH is greater than 5.01, the surface of PICSB is negatively charged. The coordination of α-FeOOH with water may be weakened. The stable α-FeOOH covering the surface of PICSB could lead to PICSB passivation (Li et al. 2017), resulting in a sharp decrease in the adsorption capacity of NO3-N and to lower than that of PCSB. Therefore, the removal rate of NO3-N in the alkaline rare earth industrial effluent can be improved by adjusting the acidity and alkalinity of the effluent. The NO3-N removal rate is maximized by using PICSB at the condition of pH 2.01, adsorbent dosage of 2.0 g/L, adsorption time of 30 min, and adsorption temperature of 25 ± 1 °C.

Comparison of PCSB and PICSB with other materials

The adsorption performances of PCSB and PICSB were compared with other materials such as La modified biochar (La-biochar), oxidized biochar, biochar/montmorillonite composite, biochar/MgFe-LDH composite (Table 5); PCSB and PICSB here were found to be much superior.

Table 5

Comparison of NO3-N adsorption capacity of PCSB and PICSB with other materials

Adsorbents Maximum adsorption Capacity (mg/g) References 
La-biochar 8.71 Wang et al., 2015  
Oxidized biochar 3.97 Sanford et al., 2019  
Biochar/montmorillonite composite 9.00 Viglašová et al., 2018  
Biochar 4.00 Viglašová et al., 2018  
Biochar/MgFe-LDH composite 7.22 Xue et al., 2016  
PCSB 10.75 Present study 
PICSB 15.14 Present study 
Adsorbents Maximum adsorption Capacity (mg/g) References 
La-biochar 8.71 Wang et al., 2015  
Oxidized biochar 3.97 Sanford et al., 2019  
Biochar/montmorillonite composite 9.00 Viglašová et al., 2018  
Biochar 4.00 Viglašová et al., 2018  
Biochar/MgFe-LDH composite 7.22 Xue et al., 2016  
PCSB 10.75 Present study 
PICSB 15.14 Present study 

CONCLUSIONS

Iron modification and pickling can effectively promote the adsorption of NO3-N in rare earth industrial effluent by CSB. The surface of PCSB was made smooth and flat by pickling, the surface of PICSB was loaded with α-FeOOH after iron modification. Combining the characterization and kinetic models, the adsorption mechanisms of PICSB for NO3-N removal were ligand exchange and electrostatic attraction, and that of PCSB for NO3-N removal is electrostatic attraction. Under the condition of adsorbent dosage of 2.0 g/L, adsorption time of 30 min, NO3-N concentration of 32.10 mg/L, adsorption temperature of 25 ± 1 °C and initial solution pH of 2.01, NO3-N adsorption amounts of PCSB and PICSB could reach the maximum values of 15.14 mg/L and 10.75 mg/L respectively, which could best remove NO3-N from rare earth industrial effluent.

ACKNOWLEDGEMENTS

This work was supported by the National Natural Science Foundation of China (No.41402208), the Shandong University of Technology Young Teachers Development Support Program (No.4072-114017), and the Shandong Natural Science Fund Project (No.ZR2016EEM19).

REFERENCES

REFERENCES
Abdel-Rahman
L. H.
,
Abu-Dief
A. M.
,
Abd-El Sayed
M. A.
&
Zikry
M. M.
2018
Disposal of heavy transition Cd2+ ions from aqueous solution utilizing nanosized flamboyant pod (Delonix regia)
.
Journal of Transition Metal Complexes
1
,
1
10
.
Ahmadiannamini
P.
,
Eswaranandam
S.
,
Wickramasinghe
R.
&
Qian
X. H.
2017
Mixed-matrix membranes for efficient ammonium removal from wastewaters
.
Journal of Membrane Science
526
,
147
155
.
Bekiaris
G.
,
Peltre
C.
,
Jensen
L. S.
&
Bruun
S.
2016
Using FTIR-photoacoustic spectroscopy for phosphorus speciation analysis of biochars
.
Spectrochim Acta A Mol Biomol Spectrosc
168
,
29
36
.
Belkada
F. D.
,
Kitous
O.
,
Drouiche
N.
,
Aoudj
S.
,
Bouchelaghem
O.
,
Abdi
N.
,
Hocine
G.
&
Mameri
N.
2018
Electrodialysis for fluoride and nitrate removal from synthesized photovoltaic industry wastewater
.
Separation & Purification Technology
204
,
108
115
.
Budimirović
D.
,
Veličković
Z. S.
,
Djokić
V.
,
Milosavljević
M. M.
,
Markovski
J. S.
,
Lević
S.
&
Marinkovic
A.
2017
Efficient As(V) removal by α-FeOOH and α-FeOOH/α-MnO2 embedded PEG-6-arm functionalized multiwall carbon nanotubes
.
Chemical Engineering Research and Design
119
,
75
86
.
Chaudhary
M.
,
Rawat
S.
,
Jain
N.
,
Bhatnagar
A.
&
Maiti
A.
2019
Chitosan-Fe-Al-Mn metal oxyhydroxides composite as highly efficient fluoride scavenger for aqueous medium
.
Carbohydrate Polymers
216
,
140
148
.
Chen
L.
,
Wu
Y. L.
,
Dong
H. J.
,
Meng
M. J.
,
Li
C. X.
,
Yan
Y. S.
&
Chen
J.
2018
An overview on membrane strategies for rare earths extraction and separation
.
Separation & Purification Technology
197
,
70
85
.
Cheung
W.
,
Szeto
Y.
&
Mckay
G.
2007
Intraparticle diffusion processes during acid dye adsorption onto chitosan
.
Bioresource Technology
98
(
15
),
2897
2904
.
Hafshejani
L. D.
,
Hooshmand
A.
,
Naseri
A. A.
,
Mohammadi
A. S.
,
Abbasi
F.
&
Bhatnagar
A.
2016
Removal of nitrate from aqueous solution by modified sugarcane bagasse biochar
.
Ecological Engineering
95
,
101
111
.
Halajnia
A.
,
Oustan
S.
,
Najafi
N.
,
Khataee
A. R.
&
Lakzian
A.
2013
Adsorption–desorption characteristics of nitrate, phosphate and sulfate on Mg-Al layered double hydroxide
.
Applied Clay Science
81
(
4
),
305
312
.
Jalili
D.
,
RadFard
M.
,
Soleimani
H.
,
Akbari
H.
,
Kavosi
A.
&
Abasnia
A.
2018
Data on Nitrate-Nitrite pollution in the groundwater resources a Sonqor plain in Iran
.
Data in Brief
20
,
394
401
.
Jha
M. K.
,
Kumari
A.
,
Panda
R.
,
Kumar
J. R.
,
Yoo
K. K.
&
Lee
J. Y.
2016
Review on hydrometallurgical recovery of rare earth metals
.
Hydrometallurgy
161
,
77
101
.
Kalaruban
M.
,
Loganathan
P.
,
Kandasamy
J.
,
Naidu
R.
&
Vigneswaran
S.
2017
Enhanced removal of nitrate in an integrated electrochemical-adsorption system
.
Separation & Purification Technology
189
,
260
266
.
Li
P.
,
Lin
K.
,
Fang
Z.
&
Wang
K. M.
2017
Enhanced nitrate removal by novel bimetallic Fe/Ni nanoparticles supported on biochar
.
Journal of Cleaner Production
151
,
21
33
.
Li
J.
,
Yu
G. W.
,
Pan
L. J.
,
Li
C. X.
,
You
F. T.
,
Xie
S. Y.
,
Wang
Y.
&
Ma
J. L.
2018
Study of ciprofloxacin removal by biochar obtained from used tea leaves
.
Journal of Environmental Sciences
73
,
20
30
.
Luo
J. H.
,
Chen
H.
,
Yuan
Z. G.
&
Guo
J. H.
2018
Methane-supported nitrate removal from groundwater in a membrane biofilm reactor
.
Water Research
132
,
71
78
.
Ma
J. Q.
,
Shen
Y.
&
Shen
C. S.
2014
Al-doping chitosan-Fe (III) hydrogel for the removal of fluoride from aqueous solutions
.
Chemical Engineering Journal
248
,
98
106
.
Nunell
G. V.
,
Fernandez
M. E.
,
Bonelli
P. R.
&
Cukierman
A. R.
2015
Nitrate uptake improvement by modified activated carbons developed from two species of pine cones
.
Journal of Colloid and Interface Science
440
,
102
108
.
Rubeena
K. K.
,
Reddy
P. P.
,
Laiju
A. R.
&
Nidheesh
P. V.
2018
Iron impregnated biochars as heterogeneous Fenton catalyst for the degradation of acid red 1 dye
.
Journal of Environmental Management
226
,
320
328
.
Sanford
J. R.
,
Larson
R. A.
&
Runge
T.
2019
Nitrate sorption to biochar following chemical oxidation
.
Science of The Total Environment
669
,
938
947
.
Viglašová
E.
,
Galamboš
M.
,
Danková
Z.
,
Krivosudský
L.
,
Lengauer
C. L.
,
Hood-Nowotny
R.
,
Soja
G.
,
Rompel
A.
,
Matík
M.
&
Briančin
J.
2018
Production, characterization and adsorption studies of bamboo-based biochar/montmorillonite composite for nitrate removal
.
Waste Management
79
,
385
394
.
Wang
Z. H.
,
Guo
H. Y.
,
Shen
F.
,
Yang
G.
,
Zhang
Y. Z.
,
Zeng
Y. M.
,
Wang
L. L.
,
Xiao
H.
&
Deng
S. H.
2015
Biochar produced from oak sawdust by Lanthanum (La)-involved pyrolysis for adsorption of ammonium(NH4+), nitrate(NO3−), and phosphate(PO43−)
.
Chemosphere
119
,
646
653
.
Wang
Z.
,
Jiang
Y. H.
,
Awasthi
M. K.
,
Wang
J.
,
Yang
X. G.
,
Amjad
A.
,
Wang
Q.
,
Lahori
A. H.
&
Zhang
Z. Q.
2018
Nitrate removal by combined heterotrophic and autotrophic denitrification processes: impact of coexistent ions
.
Bioresource Technology
250
,
838
845
.
Wei
X. T.
,
Liu
Q.
,
Zhang
H. S.
,
Liu
J. Y.
,
Chen
R. R.
&
Li
R. M.
2018
Rapid and efficient uranium (VI) capture by phytic acid/polyaniline/FeOOH composites
.
Journal of Colloid and Interface Science
511
,
1
11
.
Xiong
X. N.
,
Yu
I. K. M.
,
Chen
S.
,
Tsang
D.
,
Cao
L. C.
,
Song
H.
,
Kwon
E. E.
,
Ok
Y. S.
,
Zhang
S. C.
&
Poon
C. S.
2018
Sulfonated biochar as acid catalyst for sugar hydrolysis and dehydration
.
Catalysis Today
314
,
52
61
.
Xue
L.
,
Gao
B.
,
Wan
Y.
,
Fang
J. N.
,
Wang
S. S.
,
Li
Y. C.
,
Muñoz-Carpena
R.
&
Yang
L. Z.
2016
High efficiency and selectivity of MgFe-LDH modified wheat-straw biochar in the removal of nitrate from aqueous solutions
.
Journal of the Taiwan Institute of Chemical Engineers
63
,
312
317
.
Yin
S. H.
,
Pei
J. N.
,
Jiang
F.
,
Li
S. W.
,
Peng
J. H.
,
Zhang
L. B.
,
Ju
S. H.
&
Srinivasakannan
C.
2018
Ultrasound-assisted leaching of rare earths from the weathered crust elution-deposited ore using magnesium sulfate without ammonia-nitrogen pollution
.
Ultrasonics Sonochemistry
41
,
156
162
.
Zhang
F.
,
Lan
J.
,
Yang
Y.
,
Wei
T. F.
,
Tai
Y. Q.
&
Song
W. J.
2013
Adsorption behavior and mechanism of methyl blue on zinc oxide nanoparticles
.
Journal of Nanoparticle Research
15
(
11
),
1
10
.
Zhang
F.
,
Wang
X.
,
Ji
X. H.
&
Ma
L. J.
2016
Efficient arsenate removal by magnetite-modified water hyacinth biochar
.
Environmental Pollution
216
,
575
583
.