Application of Cold Region Regenerable Biomass in Phosphorus Adsorption in Reclaimed Water

: In order to enhance the using e ﬃ ciency of the adsorbent and decrease production costs, reclaimed saturation Lanthanum modiﬁed pine needles (LH pine needles) have been studied as a possible solution. Pine needles gathered from the woods of Northeast China area were used as raw material for generating LH pine needles by alkali-isopropanol treatment and chemical precipitation. To explore the utilization of LH pine needles as a recycling adsorbent in wastewater treatment plants (WWTPs) and laboratory water distribution. Results show that removal e ﬀ ective of phosphorus (P) by LH pine needles in low concentration reclaimed water of WWTPs was 41% and up to more than 92% in its adding standard. In the wide pH range, LH pine has an e ﬀ ective adsorption capacity for phosphorus; pH can also interfere with the adsorption capacity of LH pine as there is a negative correlation between them. The adsorption of phosphorus by LH pine needles is divided into three stages with a pH ranging from 3~11. Ligand exchange reaction, electrostatic reaction and Lewis acid reaction are PO 43 − adsorption mechanism. The analysis of the recycling e ﬃ ciency of LH pine needles proved that LH pine needles have good regeneration performance. After being eluted by NaOH regeneration agent for more than 10 times, the adsorption e ﬃ ciency of phosphorus can still be stable at over 90% in seven cycles. into three show that for phosphorus of wide-area pH range in water, and the adsorption e ﬀ ect is stronger in the low pH range. The highest adsorption e ﬃ ciency can be achieved 92%, and the treatment capacity of low concentration raw water was also 41%. LH pine needles retain the characteristics of the original amorphous solid, which indicates the structural stability of the modiﬁed pine needle. The experimental study on recycling and reuse of LH pine needle showed that LH pine needle had good regeneration performance.


Introduction
As early as 2015, the United Nations Environment Programme addressed that waste water was an undervalued resource that should not be wasted, while reclaimed water had most added value [1,2]. Utilization of reclaimed water is receiving increased attention in many regions, countries and different industries around the world, such as the United States, Europe, Singapore and China [3][4][5]. In China, the applications of reclaimed water include groundwater recharge, supply for rivers and lakes, landscapes, irrigation of agricultural crops and industrial reuse, the mainstream of sewage is still discharged [2][3][4][5]. Due to the complexity of sewage and the limitation of treatment process, the regenerated water after deep treatment still contains plant nutrients such as nitrogen and phosphorus (P). Although reclaimed water of waste water treatment plants (WWTPs) contains low concentrations of pollutants, it may still threaten the environment by normal pollutant which supplied directly [6]. Phosphorus (P), a necessity of life while polluting the ecological environment in many ways, is also an important contaminant in reclaimed water that should be removed. P removal mechanisms in WWTPs include physical (sedimentation), chemical (adsorption, precipitation and complexation) and biological processes (vegetation and microbial) [7,8], as well as

Batch Adsorption Experiments
Batch experiments were conducted to investigate the phosphorus adsorption performances of LH pine needles. Samples were placed into multiple 150 mL glass stoppered conical flasks containing 100 mL phosphorus solutions at target concentrations. Flasks were then placed on a rotary shaker and stirred at 160 rpm at a temperature of 298 K. Samples of 0.1 g LH pine needles were added to a series of solutions (reclaimed water from west-north-south suburb WWTPs) with standard addition of phosphorus concentrations (10 mg/L) for 720 min at a temperature of 298 K. The effect of initial pH was studied by mixing 0.1 g LH pine needles with 10 mg/L phosphorus solution at pH values ranging from 3 to 10 for 720 min. Solution pH was adjusted with 0.1 mol/L HCl and NaOH.
The experiments of the recycling efficiency of LH pine needles were then conducted. The adsorption saturated LH pine needles were filtered by 0.45 μm glass fiber membrane and then added to 100 mL regenerative solvent (ammonia, NaCl, and NaOH) with a concentration of 0.1 mol/L. After further centrifugation for 30 min, the analytical LH pine needle was filtered through a vacuum extraction filter. The solution containing phosphate was determined by ion chromatography. The regenerated LH pine needles were washed 3 times with deionized water, and the adsorption and desorption experiments were conducted once again.
All of the experiments were carried out in triplicate.

Analytical Methods
Phosphorus concentrations were determined using a Metrohm 881 ion chromatograph coupled with a Metrosep A Supp 4 column. A solution with 1.8 mm of Na2CO3 and 1.7 mm of NaHCO3 was used as a mobile phase with a flow rate of 1.2 mL/min.

Batch Adsorption Experiments
Batch experiments were conducted to investigate the phosphorus adsorption performances of LH pine needles. Samples were placed into multiple 150 mL glass stoppered conical flasks containing 100 mL phosphorus solutions at target concentrations. Flasks were then placed on a rotary shaker and stirred at 160 rpm at a temperature of 298 K. Samples of 0.1 g LH pine needles were added to a series of solutions (reclaimed water from west-north-south suburb WWTPs) with standard addition of phosphorus concentrations (10 mg/L) for 720 min at a temperature of 298 K. The effect of initial pH was studied by mixing 0.1 g LH pine needles with 10 mg/L phosphorus solution at pH values ranging from 3 to 10 for 720 min. Solution pH was adjusted with 0.1 mol/L HCl and NaOH.
The experiments of the recycling efficiency of LH pine needles were then conducted. The adsorption saturated LH pine needles were filtered by 0.45 µm glass fiber membrane and then added to 100 mL regenerative solvent (ammonia, NaCl, and NaOH) with a concentration of 0.1 mol/L. After further centrifugation for 30 min, the analytical LH pine needle was filtered through a vacuum extraction filter. The solution containing phosphate was determined by ion chromatography. The regenerated LH pine needles were washed 3 times with deionized water, and the adsorption and desorption experiments were conducted once again.
All of the experiments were carried out in triplicate.

Analytical Methods
Phosphorus concentrations were determined using a Metrohm 881 ion chromatograph coupled with a Metrosep A Supp 4 column. A solution with 1.8 mm of Na 2 CO 3 and 1.7 mm of NaHCO 3 was used as a mobile phase with a flow rate of 1.2 mL/min.
The concentration of residual lanthanum ions in the solutions after the adsorption process was determined. Phosphorus adsorption capacities (q e , mg P/g) and removal efficiencies (%) were determined using the following equations: In these equations, q e (mg P/g) was the phosphorus adsorption capacity, V (mL) was the volume of the solution, C 0 and C e (mg/L) were respectively the concentrations before and after adsorption and m (g) was the adsorbent mass.

Characteristics of LH Pine Needles
The Fourier transform infrared spectra of AI pine needles and LH pine needles were obtained by Thermo FisherNicolet 6700 spectrometer. The sample was prepared by KBr tablet. The test range was 450 cm −1~4 000 cm −1 . The morphological structures of AI pine needles and LH pine needles were obtained by SEM (XL30-ESEM, FEI, Portland, OR, USA) and TEM (TECNAIF20, FEI, Portland, OR State, USA).
(1) Surface morphology analysis As shown in Figure 1, SEM images and TEM images of LH pine needles are shown. It can be seen from Figure 1a that the LH pine needle have many pleated outer surfaces, but the appearance is smooth without significant change, indicating that the hydroxide and isopropanol pretreatment does not destroy the lignified skin of the outer surface of the pine needle. However, significant changes can be seen on the inside as shown in Figure 1b, and many fine white spots can be seen by observing the inside of the LH pine needle, these spots being confirmed as lanthanum oxide according to the EDAX pattern, 11.84% (wt%) of La was obviously loaded on the pine needle. pretreatment destroyed the hydrogen bond between lignin and carbohydrate or cellulose molecules, and the organic matter of pine needle tissue was extracted to make its structure fluffy and form porous structure. The exposure of the fibers contributes to a greater specific surface area, making the attachment to lanthanum ions easier and providing a greater surface area and surface active functionality.
The presence of black particles can also be observed from TEM Figure 1c of the LH pine needles, and the disordered mosaic of particles of varying sizes in the pores resulting from the uneven distribution of wrinkles. It was further proved that La modified pine needles.
(2) X-ray diffraction analysis As shown in Figure 2 are X-ray diffraction (XRD) patterns of LH pine needles, the diffraction peak shape of XRD has a great relation to whether the sample is crystalline or amorphous: if the peak shape is a steamed bread peak, it is a standard amorphous; if the peak shape is a sharp peak, it is a crystal. The sharper the peak, the smaller the half height width, the smaller the grain size, the higher the crystallinity [28]. It can be seen that the diffraction peaks of La match well with the cubie structure of La (JCPDS NO.83-1345) in the fixed figure. The results showed that LH pine needle were successfully prepared. Moreover, the characteristics of the amorphous solids of the pine needles were not changed after the pine needles were loaded with the metals La, indicating the structural stability of the modified pine needles [29].

Adsorption of Phosphorus in Reclaimed Water with LH Pine Needles as Recycling Adsorbent
In the actual wastewater adsorption experiment, the original water from the secondary effluent of the treated sewage plant was extracted by adding standard, and the recovery rate was above 80%. Reclaimed water was selected from typical WWTPs in Changchun. The different treatment processes (Anaerobic-Anoxic-Oxic (A2/O) and Anoxic Oxic (A/O)) of three WWTPs were selected, providing 81.2% of the entire city in daily processing capacity. Therefore, the study of reclaimed water has practical research value (Tables 1 and 2). The adsorption of phosphorus for reclaimed water with standard addition by LH pine needles followed. The content of the actual water body composition is very complex, and there were a lot of

Adsorption of Phosphorus in Reclaimed Water with LH Pine Needles as Recycling Adsorbent
In the actual wastewater adsorption experiment, the original water from the secondary effluent of the treated sewage plant was extracted by adding standard, and the recovery rate was above 80%. Reclaimed water was selected from typical WWTPs in Changchun. The different treatment processes (Anaerobic-Anoxic-Oxic (A 2 /O) and Anoxic Oxic (A/O)) of three WWTPs were selected, providing 81.2% of the entire city in daily processing capacity. Therefore, the study of reclaimed water has practical research value (Tables 1 and 2). The adsorption of phosphorus for reclaimed water with standard addition by LH pine needles followed. The content of the actual water body composition is very complex, and there were a lot of coexisting pollutants that were both organic and inorganic; the ionic strength and pH would have a great influence on the adsorption process of LH pine needles. Therefore, in the experiment process, the water of the typical wastewater treatment plant in Changchun was selected as the base water to determine the phosphorus adsorption of LH pine needles. The sample was collected three times, and 100 mL of each water sample was taken. The basic test results showed that, when counting number of samples that meets the standard in Table 2, 80% of outflows met the emission standard of GB18918-2002 after adsorption. The adsorption efficiency of phosphorus is approximately 62% in five kinds of water, of which the quality of the second effluent from the southern wastewater treatment plant was the best. The primary test results are shown in Table 3.  Figure 3 shows the impact of LH pine needles adsorbing P in actual wastewater. The efficiency of adsorbing P could be 41-43% at 25 • C. In a broad pH, LH pine needles adsorption to P had a high efficiency, and in the low pH, the adsorption became stronger.
determine the phosphorus adsorption of LH pine needles. The sample was collected three times, and 100 mL of each water sample was taken.
The basic test results showed that, when counting number of samples that meets the standard in Table 2, 80% of outflows met the emission standard of GB18918-2002 after adsorption. The adsorption efficiency of phosphorus is approximately 62% in five kinds of water, of which the quality of the second effluent from the southern wastewater treatment plant was the best. The primary test results are shown in Table 3.  Figure 3 shows the impact of LH pine needles adsorbing P in actual wastewater. The efficiency of adsorbing P could be 41-43% at 25 °C. In a broad pH, LH pine needles adsorption to P had a high efficiency, and in the low pH, the adsorption became stronger.
The whole adsorption efficiency was 88-93%, this is consistent with that found in synthetic wastewater. The effects of standard addition were better than that without standard addition. Further, the components in the raw water were complicated-there are many kinds of interfering ions. When PO4 3− was at a low concentration, it was disadvantageous to occupy the active position. The concentration of phosphate increases after the addition of the standard, and with the free diffusion effect, the efficiency of LH pine needles clearly increased. The whole adsorption efficiency was 88-93%, this is consistent with that found in synthetic wastewater. The effects of standard addition were better than that without standard addition. Further, the components in the raw water were complicated-there are many kinds of interfering ions. When PO 4 3− was at a low concentration, it was disadvantageous to occupy the active position. The concentration of phosphate increases after the addition of the standard, and with the free diffusion effect, the efficiency of LH pine needles clearly increased.

The Analysis of the Recycling Efficiency of LH Pine Needles
In order to enhance the using efficiency of the adsorbent and decrease the production cost, reclaimed saturation LH pine needles have been studied.
Taking into consideration the characteristics of raw needle adsorbent, modified LH pine needles, melting point, thermostability, vapour pressure, as well as physical and chemical characteristics, chemical solvent reclamation was chosen to reclaim the LH pine needles.
In our previous studies, LH pine needles had a better performance in acidic conditions than that in neutral or alkaline conditions. Lewis plays a main role in the adsorbing process under the alkaline condition in which the ligand changing function is inactive. The alkaline eluent was used to avoid decreasing the loading La when reclaiming the saturated LH pine needles. The elution rate of phosphate was calculated according to the ratio of phosphate in the eluent as Equation (3): where C e (mg/L) is the concentration of phosphate in the eluent after equilibrium, q e (mg/g) is the adsorbing capacity after equilibrium, V (mL) is the volume of the eluent and m (g) is the amount of the LH pine needles. The elution rate of phosphate in LH pine needles under different eluents is displayed in Table 4. Table 4. Elution rate of phosphate in regeneration solvent.

Regeneration Solvent NH 3 ·H 2 O NaCl NaOH
Elution rate (%) 63 59 91 The regeneration results show that NH 3 ·H 2 O, NaCl, and NaOH all had an acceptable performance (≥59%) in eluting phosphate on the LH pine needles, signifying it is good for reclaiming LH pine needles under the alkaline condition. The elution rate of NaCl was lower than that of NaOH, which potentially results from the accelerating adsorption process by Cl − .
It is suitable for bidentate phosphate to transfer into monodentate under high pH. Accordingly, NaOH elution was chosen as the regeneration solvent. After some cycling, the reclaimed LH adsorbent also had a good performance for phosphate adsorbing.
Regenerating LH pine needle is the reverse reaction process of adsorbing by decreasing the adhesive force. The regeneration formula is presented in Equation (4): La(HPO 4 ) n + 2nOH − →La(OH) 2n + nHPO 4 2− (4) Figure 4 shows the influence of cycling on the adsorption capacity of the reclaimed LH pine needles. Using a 0.1 mol/L NaOH (pH = 10.5) solution, the saturated LH pine needles were eluted 10 times. During the first time, the adsorption capacity of reclaimed LH pine needles could be recovered as much as 90% as the raw LH pine needles. After 2-5 cycles, the adsorption capacity of reclaimed LH pine needles could be recovered higher than 90%.

The Analysis for LH Pine Needles Adsorbing Phosphate Mechanism
Electrostatic force and ligand exchange are most common during adsorption [30]. Some anionssuch as PO4 3− , NO3 − and ASO2 − are adsorbed and affected by several factors. However, it is difficult to locate a reasonable adsorption mechanism, as well as the complex system of variable anion concentrations and pH in natural waters [31]. pH plays an important role in the adsorbing process because PO4 3− is exchanged with the OH ligand of the adsorbance. At the end of the adsorption, the solution pH decrease may result from the deprotonation of the La active cite (La-OH2 + ↔ La-OH + H + ). This result indicates that the main process for PO4 3− adsorption was electrostatic attraction.
FT-IR spectra study of the adsorption process often appears the presence of several kinds of structural OH groups. In the FT-IR spectra, the main characteristic peak at 3445 cm −1 in Figure 5 could be attributed to -OH from the adsorbed H2O [32]. The peak at 814 cm −1 could be attributed to -CH in aromatic ring structure [33]. The peak at 2920 cm −1 is attributed to -CH2− from the biopolymer, the peak at 1617 cm −1 was attributed to C=O and C=C. The peak at 1262 cm −1 was attributed to C-O from aromatics and -OH from phenols, while the peaks at 672 cm −1 and 524 cm −1 are attributed to La-OH in La(OH)3 [34]. The peak at 1417 cm −1 is attributed to NO3 − from the compound La(NO3)3·6H2O. After adsorbing, there was a new peak present at 1060 cm −1 , which could be attributed to PO4 3− [35,36]. The peak attributed to La-O moved from 524 cm −1 to 571 cm −1 . There was a new peak at 484 cm −1 , which could be attributed to O-P-O [36]. The essence of these changes is that the metal activity points of lanthanum are occupied by phosphate.  After 10 times, the adsorption capacity of reclaimed LH pine needles could be recovered as much as 80% of the raw. This result indicates that LH pine needles have a good ability to regenerate.

The Analysis for LH Pine Needles Adsorbing Phosphate Mechanism
Electrostatic force and ligand exchange are most common during adsorption [30]. Some anions-such as PO 4 3− , NO 3 − and A S O 2 − are adsorbed and affected by several factors. However, it is difficult to locate a reasonable adsorption mechanism, as well as the complex system of variable anion concentrations and pH in natural waters [31]. pH plays an important role in the adsorbing process because PO 4 3− is exchanged with the OH ligand of the adsorbance. At the end of the adsorption, the solution pH decrease may result from the deprotonation of the La active cite (La-OH 2 + ↔ La-OH + H + ). This result indicates that the main process for PO 4 3− adsorption was electrostatic attraction.
FT-IR spectra study of the adsorption process often appears the presence of several kinds of structural OH groups. In the FT-IR spectra, the main characteristic peak at 3445 cm −1 in Figure 5 could be attributed to -OH from the adsorbed H 2 O [32]. The peak at 814 cm −1 could be attributed to -CH in aromatic ring structure [33]. The peak at 2920 cm −1 is attributed to -CH 2− from the biopolymer, the peak at 1617 cm −1 was attributed to C=O and C=C. The peak at 1262 cm −1 was attributed to C-O from aromatics and -OH from phenols, while the peaks at 672 cm −1 and 524 cm −1 are attributed to La-OH in La(OH) 3 [34]. The peak at 1417 cm −1 is attributed to NO 3 − from the compound La(NO 3 ) 3 ·6H 2 O. After adsorbing, there was a new peak present at 1060 cm −1 , which could be attributed to PO 4 3− [35,36].
The peak attributed to La-O moved from 524 cm −1 to 571 cm −1 . There was a new peak at 484 cm −1 , which could be attributed to O-P-O [36]. The essence of these changes is that the metal activity points of lanthanum are occupied by phosphate.
aromatics and -OH from phenols, while the peaks at 672 cm −1 and 524 cm −1 are attributed to La-OH in La(OH)3 [34]. The peak at 1417 cm −1 is attributed to NO3 − from the compound La(NO3)3·6H2O. After adsorbing, there was a new peak present at 1060 cm −1 , which could be attributed to PO4 3− [35,36]. The peak attributed to La-O moved from 524 cm −1 to 571 cm −1 . There was a new peak at 484 cm −1 , which could be attributed to O-P-O [36]. The essence of these changes is that the metal activity points of lanthanum are occupied by phosphate. Figure 5. FT-IR atlas of LH pine needles before and after adsorption. Figure 5. FT-IR atlas of LH pine needles before and after adsorption. Figure 6 shows the effect of initial-final pH value (3-11) on phosphate adsorption capacity. The point of zero charge (pHpzc) of pine needles is approximately 8. Owing to the fact that pH affected the electric charge in the solution and LH pine needles and therefore affected the ions interaction, the adsorbing capacity changed significantly with the changing of pH.
Water 2019, 11, x FOR PEER REVIEW 9 of 12 Figure 6 shows the effect of initial-final pH value (3-11) on phosphate adsorption capacity. The point of zero charge (pHpzc) of pine needles is approximately 8. Owing to the fact that pH affected the electric charge in the solution and LH pine needles and therefore affected the ions interaction, the adsorbing capacity changed significantly with the changing of pH.
When pH of solution was higher than pHpzc of pine needles, there was some negative charge on the surface of pine needles. While the pH of solution was lower than pHpzc of pine needles, there was some positive charge on the surface of pine needles. The adsorbing capacity to PO4 3− of LH pine needles changed a lot in the whole range of pH. LH pine needles had the highest adsorption of 5.49 mg/g at pH = 3. Other pH levels decreased the adsorption for PO4 3− . When the pH was about 8 (pHpzc = 8), the adsorption increased a little and then decreased significantly. In specific, the adsorption decreased by 19.2% at pH = 9.6. It could be divided into three stages: I, II and III. At stage I (pH from 3 to 8), a positive correlation of pH (ΔpH = pHinitial − pHend) showed that the adsorption of LH pine needles was a ligand exchange reaction. In fact, -OH2 + could occupy the metal binding position more easily than -OH. When the solution pH was less than pHpzc, the -OH on the surface of LH pine needles would be protonized: La-OH + H + ↔La-OH2 + . The positive charge on the surface attracted PO4 3− by electrostatic force. In the surface of metal oxide, the metal ion had less ligancy, thus, the surface of dried metal oxide showed Lewis acid position.
The metal ion on the surface was first coordinated with the water molecule. Figure 7 shows a La-O ligand bond that was formed by the active position of La and negative oxygen ion of PO4 3− under Lewis acid affection.
At stage II (8 < pH < 10), phosphonium ions existed in the form of HPO4 2− . Protonation of ≡La-OH became weaker when the pH was less than 10 [37]. In this case, the adsorption of phosphonium ions by the electromagnetic force in the solution is not the main way for it to decrease. Meanwhile, the negative LA-O − and HPO4 2− had a strong electrostatic repulsion, stopping HPO4 2− from approaching the surface of LH pine needles. The mechanism of ligand exchange is that the protonation of LH pine needles surface became weak, resulting in ΔpH decreasing. The increasing of Lewis acid function resulted in negative oxygen of HPO4 2− increasing.
At stage III (pH > 10), the adsorption dramatically decreased. The electrostatic repulsion of adsorbent lead ligand exchange reaction was disappearing. Lewis acid reaction controlled the adsorption process. When pH of solution was higher than pHpzc of pine needles, there was some negative charge on the surface of pine needles. While the pH of solution was lower than pHpzc of pine needles, there was some positive charge on the surface of pine needles. The adsorbing capacity to PO 4 3− of LH pine needles changed a lot in the whole range of pH. LH pine needles had the highest adsorption of 5.49 mg/g at pH = 3. Other pH levels decreased the adsorption for PO 4 3− . When the pH was about 8 (pHpzc = 8), the adsorption increased a little and then decreased significantly. In specific, the adsorption decreased by 19.2% at pH = 9.6. It could be divided into three stages: I, II and III. At stage I (pH from 3 to 8), a positive correlation of pH (∆pH = pH initial − pH end ) showed that the adsorption of LH pine needles was a ligand exchange reaction. In fact, -OH 2 + could occupy the metal binding position more easily than -OH. When the solution pH was less than pH pzc , the -OH on the surface of LH pine needles would be protonized: La-OH + H + ↔La-OH 2 + . The positive charge on the surface attracted PO 4 3− by electrostatic force. In the surface of metal oxide, the metal ion had less ligancy, thus, the surface of dried metal oxide showed Lewis acid position. The metal ion on the surface was first coordinated with the water molecule. Figure 7 shows a La-O ligand bond that was formed by the active position of La and negative oxygen ion of PO 4 3− under Lewis acid affection. were regular vertical and horizontal textures on the surface and honeycomb cavity within the texture. Alkali alcohol could change the hydrogen bond of woodiness, carbohydrate and cellulose molecule, leading to a fluffy structure [38]. A significant amount of fluffy structure could facilitate LH pine needles adsorbing a certain amount of contaminants, causing a lot of honeycomb cavities being occupied by contaminants. Figure 7. SEM chart and phosphorus adsorption mechanism of LH pine needles.

Conclusions
The role of removal P by adsorbent is known to be influenced by many factors. In this study, the influence of pH, concentration and Regenerable was studied. In that wide-area pH range, LH pine has an effective adsorption capacity for phosphorus, and has a strong adsorption effect in the low pH range. The adsorption of LH on phosphorus in the pH range of 3~11 is divided into three stages. The three main mechanisms of phosphorus adsorption are ligand exchange reaction, electrostatic interaction force and Lewis acid-base interaction.
The results show that at 25 °C, LH pine needle has an effective adsorption capacity for phosphorus of wide-area pH range in reclaimed water, and the adsorption effect is stronger in the low pH range. The highest adsorption efficiency can be achieved 92%, and the treatment capacity of low concentration raw water was also 41%. LH pine needles retain the characteristics of the original amorphous solid, which indicates the structural stability of the modified pine needle. The experimental study on recycling and reuse of LH pine needle showed that LH pine needle had good regeneration performance.

Conflicts of Interest:
There is no conflict of interest. At stage II (8 < pH < 10), phosphonium ions existed in the form of HPO 4 2− . Protonation of ≡La-OH became weaker when the pH was less than 10 [37]. In this case, the adsorption of phosphonium ions by the electromagnetic force in the solution is not the main way for it to decrease. Meanwhile, the negative LA-O − and HPO 4 2− had a strong electrostatic repulsion, stopping HPO 4 2− from approaching the surface of LH pine needles. The mechanism of ligand exchange is that the protonation of LH pine needles surface became weak, resulting in ∆pH decreasing. The increasing of Lewis acid function resulted in negative oxygen of HPO 4 2− increasing.
At stage III (pH > 10), the adsorption dramatically decreased. The electrostatic repulsion of adsorbent lead ligand exchange reaction was disappearing. Lewis acid reaction controlled the adsorption process. Figure 7 shows the adsorption to PO 4 3− mechanism of LH pine needles. Ligand exchange reaction, electrostatic reaction and Lewis acid reaction were PO 4 3− adsorption mechanism.
Within the entire adsorption process, Ligand exchange reaction and electrostatic reaction became weak with the increase in pH, decreasing adsorption. However, when Lewis acid increased, the adsorption also increased. The SEM analysis of LH pine needles after adsorption shows that there were regular vertical and horizontal textures on the surface and honeycomb cavity within the texture. Alkali alcohol could change the hydrogen bond of woodiness, carbohydrate and cellulose molecule, leading to a fluffy structure [38]. A significant amount of fluffy structure could facilitate LH pine needles adsorbing a certain amount of contaminants, causing a lot of honeycomb cavities being occupied by contaminants.

Conclusions
The role of removal P by adsorbent is known to be influenced by many factors. In this study, the influence of pH, concentration and Regenerable was studied. In that wide-area pH range, LH pine has an effective adsorption capacity for phosphorus, and has a strong adsorption effect in the low pH range.
The adsorption of LH on phosphorus in the pH range of 3~11 is divided into three stages. The three main mechanisms of phosphorus adsorption are ligand exchange reaction, electrostatic interaction force and Lewis acid-base interaction.
The results show that at 25 • C, LH pine needle has an effective adsorption capacity for phosphorus of wide-area pH range in reclaimed water, and the adsorption effect is stronger in the low pH range. The highest adsorption efficiency can be achieved 92%, and the treatment capacity of low concentration raw water was also 41%. LH pine needles retain the characteristics of the original amorphous solid, which indicates the structural stability of the modified pine needle. The experimental study on recycling and reuse of LH pine needle showed that LH pine needle had good regeneration performance.

Conflicts of Interest:
There is no conflict of interest.