2.1 Process for the preparation of BC/CS
With the increase in the portion of cattail, N and P adsorption first increased and then decreased (see Fig. 2a). Compared with 0% cattail addition, 20% cattail addition significantly increased N and P adsorption on biochar, and 60% cattail addition showed the most N and P adsorption, with N adsorption 0.4407 mg·g-1 more than P adsorption. Zhang et al.19 egreed rice rust sludge biochar through copyrolysis; thus, it is supposed that biochar with 60% cattail addition possesses a rather optimized structure. Considering its N and P adsorption capacity, 60% cattail addition was selected as the process parameter for cattail addition.
The great difference in charring temperature in N and P adsorption is shown in Fig. 2b. Compared with N adsorption, P adsorption increased by 1.6707 mg·g-1 at 500°C. N adsorption was greatly affected by temperature. At 400°C, N adsorption on the charring biochar was 2.1577 mg·g-1 higher than that at 300°C. At 500℃, N and P adsorption reached the peak. This result was similar to the research of Wang et al.20, who found that the best charring temperature of sludge straw biochar through copyrolysis was 503.19°C. Shi et al.21 also reported that sludge biochar possessed the best P adsorption capacity at 700°C. Thus, it is estimated that the biomass of plants shows good modification to sludge biochar and reduces the temperature requirement for the preparation of sludge biochar.
The effects of charring time on N and P adsorption also showed great differences (Fig. 2c). With increasing charring time, N adsorption first decreased and then increased afterward, in contrast to a decrease in P adsorption. P adsorption on biochar after 8 h of charring decreased significantly. Different charring times showed great differences in N and P adsorption. After 0.5 h of charring, the difference in N and P adsorption equaled 1.9232 mg·g-1. Wang et al.22 prepared biochar from cattail and phragmites with a 20 min charring time, in addition to a sludge biochar preparation with a 15 min charring time by Mao et al.23. Given N and P adsorption, 0.5 h was selected as the process parameter for charring time.
The effect of the activator on N was similar to that on P, but a large difference in adsorption occurred, as shown in Fig. 2d. With potassium hydroxide as an activator, P adsorption on the biochar was 3.0535 mg·g-1 higher than N adsorption. N adsorption was greatly affected by the types of activators. N adsorption with potassium hydroxide as an activator was 1 mg·g-1 higher than that with zinc chloride and phosphoric acid. Studies have shown that biochar with potassium hydroxide as an activator possesses excellent structures, such as a large BET surface area and rich pores, and biochar with potassium hydroxide as an activator also shows good N and P adsorption24. Thus, potassium hydroxide was selected as the process parameter for the activator type.
As shown in Fig. 2e, the effects of the immersion ratio on N adsorption were similar to those on P adsorption; both tended to rise first and then decrease. The immersion ratio showed a greater effect on N adsorption. The difference between N and P adsorption on biochar with an immersion ratio of 4 mL·g-1 was 2.2580 mg·g-1. Potassium hydroxide was used as an activator to obtain biochar from mushroom residue25and water chestnut shells26at a mass ratio of 4:1, and the results of N and P adsorption on biochar were similar to the largest N and P adsorption in this study at an immersion ratio of 4 mL·g-1. Accordingly, 4 mL·g-1 was selected as the process parameter for the immersion ratio.
As shown in Fig. 2f, the effects of activator concentrations on N and P adsorption were similar. If the concentration was less than 300 mg·L-1, N and P adsorption rose rapidly and then decreased slowly. A great difference occurred between the effects of activator concentrations on N and P adsorption. Their difference reached 0.7571 mg·g-1 at a concentration of 300 mg·L-1. The rise in the activator concentration significantly inhibited N adsorption. At activator concentrations of 400 and 900 mg·L-1, N adsorption on biochar reached 8.6089 and 5.9588 mg·g-1, respectively. In light of the effects of different activator concentrations on N and P adsorption on biochar, 300 mg·L-1 was selected as the process parameter for activator concentration.
As shown in Fig. 2g, immersion time showed no significant impact on P adsorption, in contrast to a rather large impact on N adsorption. The 6-8 h immersion time significantly increased N adsorption, in contrast to decreased P adsorption. Studies have shown the large impacts of activator concentrations on immersion time. If there was a high activator concentration, then the immersion time was short, and vice versa. Song et al.27 obtained biochar with potassium hydroxide as an activator at a concentration of 1 g·mL-1 for 12 h, in contrast to a 15% concentration of potassium hydroxide for 4 h by Guo et al.28. Considering the effects of immersion time on N and P adsorption, 6 h was determined as the process parameter for immersion time.
2.2 Process for the preparation of BC/SC
2.2.1 Impacts of sizing amounts on BC/SC
The impacts of sizing amounts on N and P adsorption on BC/SC were similar (Fig. 3); both decreased first and then increased afterward. The increase in the content of polyethylene inhibited N adsorption on BC/SC, especially when sizing amounts reached 40% and 50%, and the difference between N and P adsorption tended to be large. Meanwhile, with the increase in sizing amounts, the mechanical strength of controllable biochar rose, with little difference between N and P adsorption, which suggested a similar impact of the N and P solutions on BC/SC. It was found that only when the mechanical strength exceeded 100 N·(cm2)-1 could long-term water scouring and air shock occur29. Although the mechanical strength exceeded 100 N·(cm2)-1 with a sizing amount of 50%, in light of the complexity of the water environment and N and P adsorption, a 60% sizing amount was determined for the preparation of BC/SC.
2.2.2 Impacts of molding pressures on BC/SC
Different molding pressures showed different impacts on N and P adsorption on BC/SC (Fig. 4). With increasing molding pressure, P adsorption on BC/SC decreased gradually, whereas N adsorption rose first and then decreased afterward. This result indicated that polyethylene played a role in the modification of BC/CS, leading to some changes in the physical and chemical properties of BC/CS and strengthening N adsorption, which was proven in the performance of BC/CS. The mechanical strength of BC/SC increased with molding pressure and reached more than 140 N·(cm2)-1 under a molding pressure of 5 N·(cm2)-1. In light of the complexity of the water environment and N and P adsorption, a molding pressure of 5 N·(cm2)-1 was determined for BC/SC preparation.
2.2.3 Impacts of molding temperature on BC/SC
A similar tendency occurred between the impacts of molding temperatures on N and P adsorption on BC/SC (Fig. 5). Both N and P adsorption rose first and decreased later. N and P adsorption reached their peaks at temperatures of 155°C and 160°C, respectively. When it rose from 155℃ to 160℃, P adsorption rose significantly. This result suggested that increasing the temperature to a certain point promoted the modification of BC/CS by polyethylene and strengthened P adsorption. Below 160℃, increasing the temperature enhanced the mechanical strength of BC/SC. When the temperature exceeded 160℃, its mechanical strength decreased significantly. This result indicated that temperatures beyond a certain point showed a strong negative effect on the structure. In light of N and P adsorption and mechanical strength, a temperature of 160℃was determined for the preparation of BC/SC.
2.2.4 Impacts of forming time on BC/SC
The impacts of different forming times on the N and P adsorption of BC/SC were similar (Fig. 6). Both N and P adsorption rose first and later decreased. Generally, a significant difference occurred between the N and P adsorption of BC/SC. This result indicated different roles of biochar composition in BC/SC N and P adsorption capacity, and a certain prolonged forming time would strengthen its N and P adsorption capacity. Too long of a forming time led to a decrease in N and P adsorption on BC/SC, as well as in mechanical strength. This illustrated the destruction of polyethylene on its structure, leading to a decrease in its mechanical strength. Considering N and P adsorption and mechanical strength, a forming time of 95 minwas determined for the preparation of BC/SC.
2.3 Performance of BC/SC
2.3.1 Element content, BET surface area and pore structure
The contents of C and H in BC/SC were higher than those in BC/CS, but the O content was lower (Table 3). The reason can be ascribed to the addition of polyethylene (with a simplified structure -[-CH2-CH2-]n-), which led to a significant increase in its content of C and H and a decrease in its content of O. The lower the content of functional groups containing O was, the stronger the hydrophobicity of the biochar was29. The O/C value of BC/SC was far lower than that of BC/CS. This result indicated that the hydrophobicity of BC/SC was stronger than that of BC/CS. This might be attributed to the hydrophobicity resulting from the addition of polyethylene. The values of H/C and O/C could reflect biochar stability, and low values show an aromatic ring structure and strong stability of biochar31. It is generally believed that the most stable biocha should have an O/C value less than 0.232. The O/C value of BC/SC, 0.17, was lower than that of BC/CS. Thus, it suggested a rather strong stability of BC/SC, its ability to exist in the environment for a long time, and great potential for eutrophicated water pollution treatment. The lower the (O+H)/C value was, the lower the biochar polarity was8. Accordingly, the polarity of BC/SC was lower than that of BC/CS. In summary, the addition of polyethylene changed the elemental composition of BC/CS and impacted the physical and chemical properties of BC/CS, making BC/SC different from BC/CS in terms of surface functional groups, hydrophilia or hydrophobicity, and stability.
The BET surface area (SBET) of BC/SC was significantly lower than that of BC/CS (Table 4). This result indicated the full combination of polyethylene and BC/CS. There was little difference in the surface area of micropores (Smic) between BC/SC and BC/CS. Considering the important role of micropores in biochar adsorption of pollutants, BC/SC still possessed the capacity of pollution adsorption. A large difference occurred between BC/SC and BC/CS in pore volume (Vpore) and micropore volume (Vmic), also indicating the full combination of polyethylene and BC/CS. The average pore diameter (Dpore) of BC/SC was similar to that of BC/CS, indicating no significant impacts of polyethylene addition on the average pore diameter. In sum, the addition of polyethylene greatly impacted the pore structure of BC/CS.
BC/SC was characterized by a coarse surface, abundant particles and loose dispersal, indicating a significant change in surface morphology. The addition of polyethylene enabled a close bond with BC/CS particles. Its particles were obvious, with increased coarseness, and pore diameters of different sizes were clearly visible with obvious filmy materials (Fig. 7). Considering the large amount of C crystals on the surface of BC/CS, it was suggested that the film materials were mainly formed by C crystals.
2.3.3 Surface functional groups and surface crystals
The adsorption of BC/SC reached a peak near BC/SC, illustrating the existence of —OH (Fig. 8). In contrast, the peak strength of BC/CS was just slightly weaker, suggesting no significant impact of polyethylene addition on the surface OH functional groups of BC/CS. CH3 appeared at the absorption peak near 1455.06 cm-1, in contrast to a C—O—C oscillation near 1016.51 cm-1. In comparison to BC/CS, the peak strength of BC/SC was weaker, and a significant decrease in C—O—C functional groups was observed. Near 694.39 cm-1, an adsorption peak was observed on BC/CS, illustrating the existence of a benzene ring, while no adsorption peak was observed on this site on BC/SC. This result indicated that polyethylene addition inhibited functional groups on the surface of BC/CS. Consequently, polyethylene addition played a negative role in the surface functional groups of BC/CS.
The surface of BC/CS was rich in SiO2 crystals, in contrast to the richness in C23H48 and reduction in SiO2 and Na3CrO4 in BC/SC, which indicated the full coverage of polyethylene on the surface of BC/CS (Fig. 9). No material containing K was observed on the surface of BC/CS and BC/SC after the addition of KOH as an activator, which suggested that the spillover effects of metallic ions can be avoided during environmental treatment through BC/SC. BC/SC contained 38.5% of C crystals. After a comparison with the BC/CS results through SEM, it is suggested that the film materials on BC/SC were mainly C crystals. Considering that film materials were mainly observed on the edge of pores, it was inferred that the formation of film materials occurred during the flow of polyethylene with C crystals wrapped at the molding temperature.