Figure 1 showed the XRD patterns of the plane trees' bark biochar, ZnAl-LDH, ZnAl -LDH modified plane trees' bark biochar and ZnAl-LDH modified plane trees' bark biochar after phosphate adsorption. As shown in Fig. 1a, it could be revealed that there are two typical characteristic peaks of activated carbon at 2θ = 24.5 and 43, without any impurities. Figure 1b showed the XRD patterns of ZnAl-LDH. The samples showed the obvious diffraction peaks at 2θ = 10.2, 20.3, 34.7, 62.1, which conformed to (003), (006), (009) and (110) crystal faces of hydrotalcites. Compared to activated carbon, the ZnAl-LDH/biochar in Fig. 1c exhibited characteristic peaks at 2θ = 11.21, 21.87, 35.20, 62.10, corresponding to (003), (006), (009) and (110) crystal plane, respectively. The above results showed that the Zn-Al LDH was successfully modified onto plane trees' biochar. The diffraction peak shape was relatively sharp, good symmetry, indicating the formed ZnAl-LDH onto the biochar possessed good crystal structure and high crystallinity. After phosphate adsorption, the position of the diffraction peaks remained unchanged in Fig. 1d, indicating that the adsorbed phosphate was mainly intercalated in the layers of the hydrotalcite.

As shown in Fig. 2, in the FTIR spectrum of biochar, the broad band at 3422 cm originated from stretching vibration of O-H. The peaks at 1566 cm and 1470 cm attributed to the stretching vibration peaks of C = O and C = C, respectively. In the FTIR spectrum of ZnAl-LDH, the broad band at 3400 cm attributed to the stretching vibration of laminated (M)-OH, 1600 cm was the bending vibration of interlayer H-O-H, and 1100 cm was absorption peak of interlayer sulfate ions. Moreover, the peaks at 427 cm and 612 cm belonged to lattice vibrations of Al-O and Mg-O, respectively. In the BC/ZnAl-LDH composites, the characteristic absorption peaks of hydrotalcite was observed. Therefore, hydrotalcite loaded onto biochar hold promise for effective absorption of phosphate.

As shown in Fig. 3a and b, the biochar showed honeycomb-shaped pores with smaller sizes. Moreover, the as-prepared ZnAl-LDH was shown in the layered stacking structure in Fig. 3c. The biochar/ZnAl-LDH exhibited both the porous structure and layered structure in Fig. 3d, indicating the formed composites. Combined with the XRD results, the ZnAl-LDH was successfully in-situ precipitated onto the biochar. The ZnAl-LDH modified biochar might have good phosphorus removal capacity, owing to the component of the hydrotalcites.

According to the experimental research, the adsorption effect of plane trees' bark biochar/ZnAl-LDH for phosphate on different initial concentrations was shown in Fig. 4a. Plane trees' bark biochar/ZnAl-LDH composites had a good adsorption effect for phosphate. As the sample was as little as 10 mg, the adsorption ratio was about 93% for the 25 mL of 20 mg/L PO. Moreover, as the initial concentration of phosphate ions increased, the overall adsorption ratio of the sample decreases. This was mainly because the sample dosage was only 10 mg, the number of adsorption sites provided was limited. As the initial concentration of phosphate increased, adsorption sites was occupied by phosphate molecular until the active site was completely occupied. The remaining phosphate ions were free in the solution, and the adsorption ratio was low. As shown in Fig. 4b, the single biochar showed little adsorption effect for phosphate, compared to the blue adsorption mother liquor in the first volumetric flask. The third solution in the volumetric flask was colorless after adsorption (10 mg of biochar/ZnAl-LDH, 25 mL, 20 mg/L). The experimental results indicated that the phosphorus removal was successfully achieved by loading Zinc Aluminum hydrotalcite onto activated carbon, which expanded the application of activated carbon in water treatment.

According to the experimental research, the adsorption effects of plane trees' bark biochar/ZnAl-LDH at 20 mg/L phosphate at different adsorption time was shown in Fig. 4c. The adsorption ratio had reached 60% in the first 5 min, indicating that the adsorption process was very efficient. As the adsorption time increased, the adsorption ratio of phosphate ions continuously increased. And then, the adsorption process gradually became very slow, and finally reached equilibrium in the two hours. At the initial stage, the plane trees' bark biochar/ZnAl-LDH composites could provide more adsorption sites. The concentration of phosphate in the aqueous solution was relatively high, and the adsorption driving force between the material and the adsorbate P was so large that the adsorption rate at the initial stage was very fast. Therefore, the adsorption ratio was as high as 72% in the first 5 min. As the adsorption time increased, the adsorption reached an equilibrium state. Therefore, based on the above analysis, 2 h was the adsorption equilibrium time. The temperature experiment shows that the high temperature is beneficial for the adsorption process (Fig. 4d). However, the adsorption ratio does not increase significantly, so the room temperature adsorption is more economical.

The saturated adsorption capacity of PO on plane trees' bark biochar/ZnAl-LDH was calculated by initial concentration adsorption data, which was fitted by both the classic Langmuir and Freundlich isotherm models. The adsorption data at 25 ℃ were linearly fitted in Fig. 5, and the relevant parameters were shown in Table 1. Moreover, the correlation coefficient (R) indicated that the adsorption process of Planed-Trees' bark/Zn-Al LDH for phosphate could be better explained by Langmuir equation, revealing the adsorption was mainly mono-molecular layer adsorption. As seen from Table 1, the saturated adsorption capacity was 103.1 mg/g, which indicates that the plane trees' bark biochar/ZnAl-LDH prepared by co-precipitation method has good adsorption capacity for phosphate ions in aqueous solution.

According to the above two kinetic equations, the data of phosphate adsorption on the plane trees' bark biochar/ZnAl- LDH were fitted in Fig. 6, and the relevant parameters was shown in Table 2. From Table 2, it could be seen that R was obviously better than R, and the adsorption capacity calculated by the quasi-second order kinetic equation approached to actual adsorption capacity, with a significant difference from that of the quasi-first order kinetic model. Therefore, the quasi-second order kinetic model could better describe the adsorption process of phosphate onto biochar/ZnAl-LDH, indicating that the adsorption of phosphate was mainly chemical adsorption.

As shown in Fig. 7, the new peaks appeared at 1038 cm and 1365 cm in the sample after adsorption, which belonged to the P-OH and P = O stretching vibration of phosphate. The above results proved that phosphate had been effectively adsorbed on the surface of the sample. Meanwhile, the infrared absorption peak at 1110 cm conformed to stretching vibration peaks of SO, and the intensity of infrared peak decreases after adsorption of phosphate, indicating that the sulfate between the layers was replaced by phosphate. Associated with XRD analysis, it could be concluded that the adsorbed phosphate was intercalated in the layers of the hydrotalcite, which was consistent with the structural characteristics of hydrotalcite-like materials. In the range of 400 cm to 700 cm, the peaks of BC/ZnAl-LDH were characteristic lattice vibrations of M-OH (Zn-OH and Al-OH). However, the intensity of these peaks decreased after P-adsorption, which indicated that M-OH was replaced by PO. The new M-O-P composites was formed in the ways of Complexation or ligand exchange.

To find out the mechanism, XPS survey was also performed, and the results was shown in Fig. 8. There were the elements of C, O, Zn, and Al in the both biochar/ZnAl-LDH and biochar/ZnAl-LDH(P). Moreover, a notable P 2p peak appeared at 143.9 eV in the biochar/ZnAl-LDH(P), revealing the adsorbed phosphate onto biochar/ZnAl-LDH. The atomic ratio of the main elements such as C, O, Zn, Al in BC/ZnAl-LDH sample was 50.97%, 36.77%, 7.4% and 4.86%. After P adsorption, the atomic ratio of C, O, Zn, Al, P in BC/ZnAl-LDH(P) sample was 53.35%, 33.63%, 3.99% 3.22%, and 5.62%. Meanwhile, the S2p peak noticeably weakened because sulfate ions between layers were replaced by phosphate ions. The O1s spectrum of the biochar/ZnAl-LDH were composed of M-O (3.95%), M-OH (54.26%), C = O (12.31%), O-C = O (29.48%). However, after phosphate adsorption, the content of C = O and O-C = O decreased to 0.86% and 27.25% because part of CO in the ZnAl-LDH was exchanged by PO. The percentage of M-O increased fron 3.95-30.18%, while that of M-OH decreased from 54.26 to 41.69% due to replacement of OH by P-O bond. Similarly, in the both Zn2p and Al2p spectrum, the content of Zn-O and Al-O increased, and that of Zn-OH and Al-OH decreased. That may be due to the formation of P-O-Al and P-O-Mg bonds by ligand exchange or complexation. Above all, the adsorption mechanism of phosphate contained mainly interlayer anion exchange, surface complexion and ligand exchange.

10 mg of biochar/ZnAl-LDH composites were added to 7 mL of 50 mg/L PO stirring for 2 h. Then, the sample of adsorbed phosphate were centrifuged to obtain the upper clear solution. The equilibrium concentration of phosphate (Ce) was determined by Phosphorus molybdenum blue spectrophotometric method. The phosphate removal ratio was 93% by 10 mg biochar/ZnAl-LDH for 7 mL of 50 mg/L PO43-. Then, the absorbed sample were soaked in 20% NaCO solution for phosphate desorption, stirring for 2 h. The desorption efficiency (%) were calculated.

After analysis, the desorption ratio was 61%, compared to the absorption ratio of 93%. The recovered phosphorus could be reused. In addition, the effect of NaCO concentration on desorption efficiency were investigated. As shown in Fig. 9, it can be seen that in sodium carbonate solutions with mass ratios of 0.28%, 1.4%, 5.6%, 10%, and 20%, the desorption rates of phosphate ions are 13.8%, 26.1%, 57.4%, 56.3%, and 61.0%, respectively. Therefore, a suitable sodium carbonate desorption solution is around 5.5%. The desorption mechanism might be that adsorbed phosphate molecular could be replaced by the high concentration of CO in the solution and phosphate molecular released.

The adsorption-desorption Cycle experiments have also been carried out in Fig. 10. The first round of adsorption was 93%, and that of desorption was 57.4%. The second round of adsorption was 38.64%, and that of desorption was 28.64%. This might be due to only a portion of the adsorbed phosphate in the first round were desorbed, and about 35.6% of the active sites still occupied. Therefore, the second round of adsorption was about 30%. The desorption ratio was 28.46%, and 73.65% of adsorbed PO was released in the second round. Besides, the results revealed that single NaCl solution had no desorption effect for PO. Since Zn(OH) and Al(OH) are amphoteric hydroxides, they are easily dissolved in both acidic and basic solutions. Above all, NaCO solution was a suitable desorption solution. The phosphate adsorbed by ZnAl-LDH released, and it is expected to be reused.