# Enhanced Phosphorus Removal from Wastewater Using RSPRC and a Novel Reactor
## 3 Results and Discussion
### Phosphorus Removal Characteristics of RSPRC  
As can be seen from Figure 6, there has been a gradual rise in the phosphorus removal rate with the increasing adsorbent dosage. The phosphorus removal rate of RSPRC reached a peak when the adsorbent dosage was 6 g. However, the adsorption capacity (Qe, mg/g) or amount of phosphorus adsorbed by RSPRC had a downward trend when the dosage of adsorbent increased. Similarly, [PERSON] et al. (2014) discovered that the phosphate sorption capacity was decreased with increasing dose of layered double hydroxides, because with the increasing dose, the adsorbent mass increased in the same volume of adsorbate solution, while the mass of the adsorbate to be sorbed remained the same [22]. As the dosage increased, although the available active sites were increasing, the increase in the density of the adsorbent caused the adsorption active sites to be stacked, so the adsorption efficiency was lowered, resulting in a slow increase in the removal rate of phosphorus and the decline in the amount of phosphorus adsorbed by RSPRC [23; 24]. Figure 7 provides the results obtained from the experiment of studying the effect of contact time on the phosphorus removal of RSPRC. As the contact time increases, the effluent P concentration decreases and the phosphorus removal rate increases. With the increase in contact time, different dosages of adsorbent have similar changes in phosphorus removal rate. In the interval of 0-8 h, the effluent P concentration decreased slowly. In the case of the 8-24 h interval, there was a sharp drop in the effluent P concentration. However, in the 24-72 h interval, the effluent P concentration declined gradually, and after 72 h, the effluent P concentration tended to be stable and basically reached the adsorption equilibrium. The observed correlation between contact time and phosphorus removal effect might be explained as follows: there are more active sites on the surface and pores of the RSPRC, where the initial adsorption exists for a short time, so the phosphate ions rapidly occupy the active site, and the adsorption rate is faster. With the extension of time, RSPRC adsorbs more and more phosphorus from the wastewater and the active sites are fewer, which can result in a decline in the removal rate and the effluent P concentration [25]. [PERSON] et al. (2009) also found that an almost 70% removal of  
Figure 5: SEM images of RSPRC. The magnifications from (**a**–**d**) are 5080, 2230, 4070, 26260.  
phosphate by fly ash was reached in a short time and progressively increased with the contact time. However, there were no further increases in the percentage of phosphate removal after 20 h [26]. The experimental results on the effect of initial phosphorus concentration on the phosphorus removal of RSPRC are presented in Figure 8. The adsorption capacity (Qe, mg/g) or amount of phosphorus adsorbed by RSPRC significantly increased when the initial P concentration increased. As the contact time is increased, the higher the initial P concentration is, the faster the amount of phosphorus adsorbed by RSPRC is increased. This result indicates that, under the reaction conditions with a high P concentration, the amount of phosphorus adsorbed by RSPRC increased and the efficiency of phosphorus removal was significantly improved [27]. These relationships may partly be explained by the fact that there are sufficient adsorption sites on the surface of the adsorbent. At the same  
Figure 6: Effect of RSPRC dosage on phosphorus removal efficiency. Figure 7: Effect of contact time on phosphorus removal efficiency. time, the initial phosphate concentration in the solution is high, so a sufficient driving force for mass transport promotes the adsorption of phosphate by the RSPRC [28; 29]. [PERSON] et al. obtained the result that the adsorption capacity of electric arc furnace steel slags ranged from 0.09 to 0.28 mg/g and did not increase according to initial P concentrations above 10 mg /L, suggesting that a limit in P removal was reached [30].