Removing nitrogen and phosphorus simultaneously in stormwater runoff using permeable asphalt pavement system with a zeolite-regulated reservoir

Stormwater runoff is identified as a major source of pollution in water bodies, and to limit the impact of these pollutants on groundwater quality, permeable asphalt pavement systems (PAPS) have been built worldwide. But so far, few have considered zeolite or regulated zeolite as a post-treatment in reservoirs in PAPS. This study aimed at investigating the efficiencies of modified zeolites in removing NH4 þ-N and TP from stormwater runoff and providing a novel insight into the research on the reuse of stormwater runoff by PAPS. The effect of PAPS with a zeolite-regulated reservoir on removing nutrient in stormwater was explored by artificial rainfall experiments and temporary storage experiments. Results showed that the removal rate of NH4 þ-N and TP in PAPS with a regulated-zeolite reservoir increased by 23.7% and 37.2%, respectively, during simulated rainfall events when compared to those without a regulated-zeolite reservoir. TP was mainly removed by the form of phosphorus precipitation such as Zr(H2PO4)2. Ion exchange and adsorption during the temporary storage period were considered as the main approaches for removal of NH4 þ-N and TP. This study can help develop an optimal strategy for the operation of PAPS in treating stormwater runoff from urban roads. This is an Open Access article distributed under the terms of the Creative Commons Attribution Licence (CC BY 4.0), which permits copying, adaptation and redistribution, provided the original work is properly cited (http://creativecommons.org/licenses/by/4.0/). doi: 10.2166/wrd.2020.057 om http://iwaponline.com/jwrd/article-pdf/10/2/106/701491/jwrd0100106.pdf er 2021 Hui Luo Zhaoqian Jing (corresponding author) Zeyu Zhang Mengni Tao Yin Wang Chen Chen College of Civil Engineering, Nanjing Forestry University, 159 Longpan Road, Nanjing, 210037, China E-mail: zqjing@njfu.edu.cn Lin Guan Nanjing Municipal Design and Research Institute Co., Ltd, 31 Tongren Street, Nanjing, 210008, China


INTRODUCTION
In recent years, a series of problems like deterioration of urban water environment, frequent occurrence of flood disasters, and severe runoff pollution has become increasingly prominent. Notably, urban non-point source pollution has been gradually identified as one of the most important reasons for degradation of urban water quality (Line et al. ; Page et al. ). The road, a critical element of urban catchments, is closely linked to drainage facilities and also a key source of urban non-point source pollution. The source-based, decentralized, and small-scale low impact development (LID) rainwater control system is a novel management system featuring excellent performance in solving drainage problems induced by urbanization (Zhao et al. ). The permeable asphalt pavement system (PAPS) is a typical infiltration facility with strong adsorption capacity and purification capacity for rainwater runoff, which can further maximize the efficacy of LID technology in rainwater control. The permeable pavement system has been applied as an essential element of water control system in many countries for years, such as the sustainable urban drainage system of the UK (Construction Industry Research and Information Association ) and the water-sensitive urban design of Australia (Wong ).
PAPS is a special type of permeable pavement with hotmix asphalt mixture as the top wear layer. The most distinct feature of the permeable pavement system from a typical hot-mix asphalt mixture is that the ratio of fine aggregate content is significantly decreased in the mix material (Dietz ; Brodie ). The base layer is made of permeable concrete, large particle-sized permeable asphalt mixture (LSPM), and graded gravel. The reservoir is mainly made of large porous materials like graded gravel, which makes it a good temporary reservoir for rainwater to penetrate, pass through the top permeable surface layer, and slowly infiltrate into subgrade soil (Zhao & Zhao ). The reservoir has multiple functions, including reducing stormwater, replenishing groundwater, regulating atmospheric humidity, alleviating urban heat island effect, etc. ( Jiang et al. ; Sounthararajah et al. ).

Surface runoff typically contains various pollutants that
can reduce the qualities of urban water resources, including total suspended solids (TSS), nutrients such as phosphorus (TP) and nitrogen (TN), hydrocarbons, and heavy metals (Kayhanian et al. ). Generally, stormwater runoff contains substantial pollutants that cannot be completely absorbed by PAPS, such as nutrients like nitrogen and phosphorus (Bentarzi et al. ). Such nutrients will flow into groundwater and cause severe pollution, which impedes water restoration and destroys the regional ecological environment (Chen et al. ). Thus, it is of great importance for environmental protection to improve the quality of water that infiltrates into soil, especially water on urban roads with a certain traffic flow (Zhao et al. ). Biological, chemical, and physical methods are common methods for removing nitrogen and phosphorus in polluted water, among which, the physical method is the most common one, especially the physical adsorption method (Lin et al. ). The adsorption method features many advantages, including low cost, high accessibility to adsorption materials, high efficiency, simple process, stable effects, and easy operation (Ganrot et al. ). Zeolite, as a microporous material, displays unique physicochemical properties, such as the presence of strong acidic centers, large specific surface area, and a precisely defined system of micropores and channels, and can be an ideal material for the reservoir layer in roads. Natural zeolites are characterized by good performance in cation exchange and ammonium removal, whereas it is almost incapable of removing phosphate because of the electro-negativity (Kuroki et al. ; Guaya et al. ). Metal oxide could provide large numbers of coordination sites and exhibits good sensitivity toward phosphate even at trace levels. Thus, it is necessary to modify the properties of natural zeolites in order to equip it with phosphate removal capacity in addition to its ammonium removal capacity.
Numerous studies have shown that compared to natural zeolites, the adsorption capacity of inorganic salt-regulated and acid activated zeolites was greatly improved, which enabled regulated zeolites to increase pore structure and remove nitrogen and phosphorus from water bodies simul- There are no good methods to remove nitrogen and phosphorus simultaneously with PAPS in the existing studies. The aim was to investigate and characterize the effect of a PAPS with a zeolite-regulated reservoir in removing pollutants. In this study, sodium chloride ion exchange and zirconium oxychloride deposition baking and a two-step method were applied to modify zeolites for generating denitrified and dephosphorized zirconiumsodium-regulated zeolites (Zr/Na/Z). The process of surface modification of zeolites was analyzed using a scanning electron microscope (SEM)-energy dispersive spectrometer (EDS) and X-ray diffraction (XRD). The capacity of regulated zeolites to adsorb NH 4 þ -N and TP at different pH and initial concentrations was also investi-

Preparation of synthetic rainwater
Synthetic water was prepared based on the research findings  Table 2.

Experimental device
An artificially simulated rainfall device was designed for simulating natural rainfall indoors in this study. The system included four main parts: rainfall nozzle, water

Static adsorption
Simulated water samples for adsorption tests were prepared by a mixture of NH 4 Cl, KH 2 PO 4 and deionized water.
Adsorption experiments were performed using a water bath in a 200 mL conical flask with a cover. NH 4 þ -N or TP solution (100 mL) of different concentrations and a certain amount of zeolites were added into the conical flask.
Then, the conical flask was submerged in a water bath shaker with a constant temperature and shaken at 25 C for 24 h (150 r/min).

Artificial rainfall schedule
In order to carry out simulated rainfall events with a rainfall intensity similar to natural rainfall, the rainfall intensity in the Chicago rainfall model (Bentarzi et al. ) was  adjusted to an interval of 5 min. The whole process of the artificial rainfall experiment was divided into 24 periods.
The time-span rainfall of the rainfall generator was calculated based on the rainfall intensity and typical rainfall scenario with a rainfall duration of 120 min and the parameters of test devices. The synthetic scenario is shown in Figure 2.

Experimental process
Before the artificially simulated rainfall tests, synthetic rainwater was taken out from the water tank using a polyethylene plastic bottle and set as raw water. Then, the conductivity and values of the raw water were measured and recorded. When the outlet of the discharging pipe was found to be suspended with water droplets for the first time, the initial filtered runoff water sample was collected for water quality analysis. By comparison with the original water sample, the removal effects of permeable asphalt roads before and after modification on pollutants in the initial pavement runoff were determined. The filtered runoff water samples were collected at 0 min, 30 min, 45 min, 60 min, 90 min, and 120 min, respectively, after outflow. A total of 13 water samples (including the raw water sample) were collected.

Adsorption kinetics for modified zeolite
The adsorption data were applied to three different kinetic models: pseudo-first order, pseudo-second order, and intraparticle diffusion models. In both the pseudo-first order and pseudo-second order models, the adsorption steps, including external diffusion, internal diffusion, and adsorption, are lumped together. Nonlinear forms of the pseudo-first order and pseudo-second order equations (Chi & Ramarao ) are given in Equations (1) and (2), as well as intraparticle diffusion in Equation (3): where q e and q t are the amount of dye adsorbed (mg/g) on the adsorbents at equilibrium and at time t, respectively, k 1 is the rate constant of adsorption (1/h), k 2 is the rate constant of pseudo-second order adsorption (g/mg h), k p is the rate constant for intraparticle diffusion (1/h), and c is the intercept.

Chemical compositions and surface characterization
The chemical compositions of all materials were analyzed with X-ray fluorescence (XRF) by the Advanced Analysis and Testing Center. The materials before and after the leaching and adsorption tests were characterized by SEM and XRD to evaluate the changes in morphology, crystal structure, and the chemical compositions of the surface. For all analytical tests, the materials were dried in an oven at 105 C for 24 h.

Water quality assessment
The measured values of water samples collected at each period were averaged and recorded as the final measured values. The interception rates and removal rates of different pollutants in the stormwater by permeable pavement system before and after modification were calculated according to Equation (4): where η i is the removal rate for nitrogen and phosphorus with different forms in rainfall runoff by permeable pavement  (5) and (6): where P i is the pollution index of the assessment factor I, C i is the measured value of the assessment factor i (mg/L), and C si is the standard value of the assessment factor i (mg/L): where PI is the comprehensive pollution assessment index; P i  (Table 3), which provides a basis for water quality assessment (Zhu et al. ).

RESULTS AND DISCUSSION
Performance of regulated zeolites

SEM-EDS characterization of natural zeolites and regulated zeolites
The natural zeolites were featured by a rough surface and a relatively compact structure with the presence of particulate debris. After modification by sodium hydroxide, the particle fragments disappeared, making the surface of the regulated zeolites become relatively porous, coarse, and heterogeneous. Plentiful porous channels were Zr and Na 2þ during the regulation (Figure 3).

XPS characterization of natural zeolites and regulated zeolites
To gain further insights into the composition to be modified, the XPS spectra of zeolite before and after were analyzed. Compared with the wide-scan XPS of natural zeolite (Figure 4), new peaks appeared on the surface of Z/Na/Zr at 1,071.8 eV, 180.8 eV, and 182.8 eV, which corresponded to Na 1s, Zr 3d2/5 and Zr 3d3/2, proving the successful loading of Na and Zr elements. The sodium Level of water quality Clean Relatively clean Slight pollution Medium pollution Heavy pollution Serious pollution species on the surface of Z/Na and Z/Na/Zr were further investigated and Na 1s was decomposed by XPS PEAK, and the results indicated that the area of Na of Z/Na and Z/Na/Zr samples were stable in the modification. The large portion of Zr existence in Z/Na/Zr might be explained by the presence of partially small particles over the surface of zeolite, which were located deeper (3-5 nm) than the XPS sampling depth.

XRD characterization of natural zeolites and regulated zeolites
The XRD diffraction patterns of Z/Na and Z/Na/Gr are shown in Figure 5.

Adsorption kinetics
The kinetic curves of adsorption for ammonium and phosphorus by Z/Na/Zr are shown in Figure 6. Z/Na/Zr With the reaction progressing, the active sites for adsorption  were occupied and the exchangeable cations were replaced, leading to the decrease in the concentrations of ammonium.
The results of nonlinearly fitting to the quasi-first order and the quasi-second order kinetic models are shown in Figure 6.
The quasi-first order kinetics of adsorption for ammonium and phosphate by Z/Na/Zr, the equilibrium adsorption capacity was obtained by fitting quasi-secondary kinetics, theoretical equilibrium adsorption capacity, adsorption rate constant, and fitting correlation coefficients were obtained by model fitting, which are listed in Table 4 In the simulated rainfall experiment, the concentration of NH 4 þ -N in the outflow water showed an increasing trend with the duration of the rainfall (Figure 7(a)). The zeolite   Notably, the removal rate of NH 4 þ -N reached up to 80.1%, which was significantly greater than that in the pavement with natural zeolite reservoir and in accordance with the Grade IV standard for surface water quality. These results demonstrated that the zirconium-sodium-regulated zeolite reservoir improved the quality of runoff in the pavement.
Dynamic reducing capability for TP in stormwater runoff Assessment of effluent quality from regulated pervious asphalt pavement In this study, the fifth level standard for surface water in China is defined as the basic category. The degrees of effluent pollution are determined by Equations (2) and (3) based on the comprehensive pollution index method. Results showed that during the simulated rainfall event, the quality of water collected from the permeable pavement system containing a natural zeolite reservoir during the six sampling periods was inferior, and the water storage was graded as the fifth level standard (

CONCLUSIONS
The capacities of PAPS using a regulated and unregulated reservoir to remove NH 4 þ -N and TP in stormwater runoff were compared in this study by designing and synthesizing zirconium-sodium-regulated zeolites. The removal efficiencies on pollutants during a rainfall event and a temporary storage were measured, and water qualities were assessed.
The mechanisms for the regulated zeolite reservoir to purify stormwater runoff were investigated. Results showed that the removal efficiency for pollutants was significantly increased during the water storage period regardless of the regulation status. The efficiencies of the PAPS with regulated zeolites in removing NH 4 þ -N and TP were above 90% with extension of the storage duration. The modification for zeolites increased porous sizes, specific surface areas, and gross pore volumes, facilitating the entrance of zirconium and sodium elements into zeolite pores and amplifying exchangeable cations in zeolites. The diffraction pattern by XRD suggested the crystal structure of zeolites remained unchanged during the process of modification.
Ion exchange was the major method for the regulated zeolite reservoir to remove ammonium, and phosphate was mainly removed via complexation action. The effluent achieved the Grade V standard for PAPS with regulated zeolite as the reservoir in an artificial rainfall event. Therefore, it is recommended to further investigate various enhancement and combination technologies (e.g., ecological ditch, ecological slope, bioretention, constructed wetland, and other series and combined technologies) to practically minimize the impacts on pollutant removal involving nitrogenous and phosphorus compounds and other heavy metals.