Phosphate removal in constructed wetland with rapid cooled basic oxygen furnace slag

Accepted Manuscript Phosphate removal in constructed wetland with rapid cooled basic oxygen furnace slag Jong-Hwan Park, Jim J. Wang, Seong-Heon Kim, Ju-Sik Cho, Se-Won Kang, Ronald D. Delaune, Dong-Cheol Seo PII: DOI: Reference:

S1385-8947(17)31107-5 http://dx.doi.org/10.1016/j.cej.2017.06.155 CEJ 17236

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

16 May 2017 24 June 2017 26 June 2017

Please cite this article as: J-H. Park, J.J. Wang, S-H. Kim, J-S. Cho, S-W. Kang, R.D. Delaune, D-C. Seo, Phosphate removal in constructed wetland with rapid cooled basic oxygen furnace slag, Chemical Engineering Journal (2017), doi: http://dx.doi.org/10.1016/j.cej.2017.06.155

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Phosphate removal in constructed wetland with rapid cooled basic oxygen furnace slag

Jong-Hwan Parka, Jim J. Wanga,**, Seong-Heon Kimb, Ju-Sik Cho c, Se-Won Kangc, Ronald D. Delauned, Dong-Cheol Seob,*

a

School of Plant, Environmental and Soil Sciences, Louisiana State University Agricultural Center, Baton Rouge, LA 70803, USA b

Divison of Applied Life Science (BK21 Program) & Institute of Agriculture and Life Science, Gyeongsang National University, Jinju, 52828, South Korea

c

Department of Bio-Environmental Sciences, Sunchon National University, Sunchon, 57922, South Korea d

Department of Oceanography and Costal Sciences, College of the Coast and Environment, Louisiana State University, Baton Rouge, LA 70803, USA

*Corresponding Author: Corresponding Author: Divison of Applied Life Science (BK21 Program) & Institute of Agriculture and Life Science, Gyeongsang National University, Jinju 52828, South Korea. Phone: +82-55-772-1963; fax: +82-55-772-1969; e-mail address: [email protected] **Co-corresponding Author: School of Plant, Environmental, and Soil Sciences, Louisiana State University AgCenter, Baton Rouge, LA 70803, USA. Phone: +1 (225) 578 1360; Fax: +1 (225) 578 1403; e-mail: [email protected]

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Abstract The objective of this study was to evaluate adsorption characteristics of phosphate by rapid cooled basic oxygen furnace slag (RC-BOFS) through various conditions and removal rate of phosphate in small-scale constructed wetland with RC-BOFS as filter material. The phosphate adsorption by RC-BOFS was rapid in the first 0.5 h and the pseudo-second-order kinetic model fit the data better than the pseudo-first-order kinetic model. The maximum phosphate adsorption capacities of RC-BOFS under different pH were in the following order: 3.57 mg P g-1 (pH 5) > 2.47 mg P g-1 (pH 7) > 1.46 mg p g-1 (pH 9). Small-size RC-BOFS (0.8-2.3 mm) was more efficient with 23% higher phosphate adsorption than big-size RCBOFS (2.3-4.6 mm). Characterization of RC-BOFS before and after phosphate adsorption by XRD, FTIR and SEM-EDS indicated that phosphate adsorption by RC-BOFS was dominated by metal oxide and precipitation by calcium and was closely related to the slag chemical properties. The phosphate saturation time in constructed wetland with coarse sand was predicted about 292 days, whereas the longevity of constructed wetland with adding about 25% RC-BOFS to the coarse sand can significantly increase up to 1,349 days. It was concluded that the horizontal flow constructed wetland with sand 75%:RC-BOFS 25% ratio could achieve high phosphate removal rate and near-neutral pH for meeting the acceptable water quality discharge standard from water treatment plant.

KeyWords: Constructed wetlands; Horizontal flow; Langmuir isotherm; Phosphate; Rapid cooled basic oxygen furnace slag; Saturation time.

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1. Introduction Constructed wetland (CW) technology was developed in 1970s as an alternative ecological technology for wastewater treatment [1]. This technology has been widely applied to domestic wastewater treatment in rural areas and small village in South Korea [2,3]. It possess several advantages compared with conventional wastewater treatment plants, such as low investment, maintenance and operation cost, utilization of renewable energy sources (wind and solar energy), and tolerance over variation of wastewater volume and level [4]. Constructed wetland design is based on natural components including soil, water plant and microorganism for wastewater treatment [5]. Particularly, the main treatment of nitrogen and organic matter contained in wastewater is by biological reaction. For example, nitrogen is removed through a combination of ammonification, nitrification and denitrification processes by associated bacteria with these in CWs [6,7]. Organic carbon can be removed by both aerobic microbial mineralization and anaerobic microbial methane formation [8,9]. For this reason, the removal of nitrogen and organic matter can be maintained stably even for long-term operation. However, the phosphate treatment mechanisms in CWs are adsorption/precipitation by filter media, uptake by aquatic plant, and immobilization by microbes [10-12]. Overall, phosphate removal occurs mainly as an effect of adsorption and/or precipitation by filter media with minerals based soil such as calcium, iron, magnesium and aluminum [13,14]. In particular, adsorption and precipitation can easily saturate the adsorption sites during pollutant treatment, thereby decreasing the treatment efficiency. Therefore, the selection of filter media with high adsorption capacity is essential to the design of CWs due to the consideration for the longevity of CWs. For this reason, study on new filter media to enhance the phosphate adsorption capacity has become a priority for

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researchers in the last two decades. Numerous scientists have evaluated the potential of various filter media such as zeolite, aluminum oxide, limestone, peat, and mesoporous materials for treating phosphate in CWs [15-18]. These filter media however have several disadvantages including insufficient removal rate, high cost requirement, resource depletion, and low applicability for wastewater. Therefore, seeking alternative material for phosphate adsorption in CWs has become the focus for scientists in the past few years. The slag discharged from the steel industry is one of the industrial wastes, the amount of which is about 27 million tons per year, and it is expected that the amount will be increased every years. Although slag has been used as raw material for cement and fertilizer and for construction and road materials [19,20], its wide use applicability has not been adequately tested. Over the past decades, many studies has reported that slag can be an effective material for the adsorption of phosphate from wastewater [3,21-23]. However, because of the high pH, the most slags studied are limited applicability to the wastewater treatment. Moreover, high pH in water adversely affects aquatic ecosystem such as plants and microorganisms [24,25]. Recently, the rapid cooled basic oxygen furnace slag (RC-BOFS) derived from economical and environmentally friendly new slag process (Baosteel Slag Short Flow) has been developed, which is derived from the molten steel slag and is treated in the roller body with compressed air and cooling water through the tilting device of the slag ladle and cinder scraper [26]. In a very recent study, Park et al. [3] reported that the free CaO content of the slag derived from BSSF is reduced due to the rapid cooling method, which would allow it maintain a lower pH and higher phosphate adsorption capacity than that of conventional slags. However, in the previous study, only phosphate adsorption capacity of

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RC-BOFS was compared with those of conventional slags, but no investigation under various conditions for application the actual CW was performed. Effects of several parameters such as initial phosphate concentration and pH, particle size and slag dosage, which are important for treating phosphate in CWs were not studied. Therefore, in this work, we evaluated adsorption characteristics of phosphate by RCBOFS in detail under various conditions. Then, based on the adsorption experiments, we estimated the saturation time for phosphate removal as RC-BOFS is applied in CW. In addition, adsorption mechanisms of phosphate on RC-BOFS were investigated using X-ray diffractometer (XRD), Scanning Electron Microscope (SEM) equipped with Energy Dispersive Spectrometer (EDS), and Fourier transform infrared spectroscopy (FTIR). Finally, the effectiveness of horizontal flow CWs with sand and RC-BOFS was evaluated for treating hydroponic wastewater containing high phosphate.

2. Materials and methods 2.1 Materials The RC-BOFS was obtained from Gwangyang Iron and Steel Works, POSCO, South Korea. The physicochemical characteristics of the RC-BOFS were analyzed, and the results are shown in Table 1. The bulk density and porosity of RC-BOFS were 1.74 g cm-3 and 34%, respectively. The composition of RC-BOFS was mainly CaO (36.7%), Fe2O3 (24.2%), SiO2 (13.4%), MgO (6.4%) and Al2O3 (3.3%). The RC-BOFS used in this study contain 1.12% free CaO which is an advantage for maintaining low pH as compared to BOFS (Free CaO = 4-6%) obtained from conventional method [27]. Generally the hydration reaction of free-CaO results in a high pH value in the aqueous phase [28].

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Insert Table 1 here.

2.2 Methods 2.2.1 Batch experiment To establish the time dependence of phosphate adsorption on RC-BOFS, a series of suspensions in 100 mL glass Erlenmeyer flasks was prepared, each containing 2 g of RCBOFS and 50 mL of phosphate solution having different concentration (10-200 mg L-1). The initial pH value was adjusted to 7 by drop-wise addition of 0.1 M HCl or NaOH solutions with stirring. After equilibration under shaking for different time internals up to 24 h at 25oC, the mixture samples were separated by centrifugation at 4000 rpm and the solution was filtered through a Whatman GF/C filter. The adsorption behavior of the system for the process design and operation control is very important factors. To understand adsorption mechanism well, the pseudo-first-order model and pseudo-second-order model were used to investigate adsorption kinetics of phosphate onto RC-BOFS. These models as described by Lagergren [29] and Ho and McKay [30] respectively were fit to the experimental data to elucidate the phosphate adsorption mechanism onto the RC-BOFS. They are in the form:



log( −  ) = log − .

 

=

6

  

+



(Eq. 1) (Eq. 2)

where qt and qe (mg/g) are adsorbed phosphate amount at time t (h) and equilibrium, k1 (1/h) and k2 (g/(mg‧h)) are the rate constants for the pseudo-first-order and pseudo-second-order adsorption kinetics, respectively. The plot of log (q e-qt) vs. t should give a linear relationship from which k1 and q e can be determined from the slope and the intercept of the plot, respectively. The linear plots of t/qt against time t can give the pseudo-second-order adsorption rate constant k2 from the slope and q e can be calculated from the intercept. To determine the adsorption capacity of the RC-BOFS and to evaluation the adsorption isotherm, the adsorption experiments were carried out at different pH (5, 7 and 9), particle size (0.8-2.3 and 2.3-4.6 mm) and dosage (1-5 g). All pH adjusted using 0.1M HCl and NaOH. All experiments were carried out at pH 7 except for pH-dependent experiments. Furthermore, the results were simulated using Freundlich and Langmuir models. These models were used to fit phosphate adsorption date for the RC-BOFS. The Freundlich (Eq. 3) and Langmuir (Eq. 4) equations are expressed, respectively, by

q = KCe1 / n

(Eq. 3)

abCe 1 + bCe

(Eq. 4)

q=

where q (mg g-1) is the amount of the phosphate adsorbed per unit weight of RC-BOFS, Ce (mg L-1) is the equilibrium phosphate concentration of solution. K and n are the Freundlich constant representing phosphate adsorption capacity and intensity, respectively. a (mg g-1) is Langmuir constant indicating maximum adsorption capacity of phosphate and b is the binding strength [31].

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A saturation time for long-term phosphate treatment is essential in constructed wetlands is used. Therefore, the amount of phosphate adsorption of RC-BOFS was evaluated by mixing RC-BOFS with coarse sand generally used as filter media in a series of ratios. Two grams of sample (total mass) of Coarse sand (C) + RC-BOFS (S) in different C:S ratios (C100S0, C75S25, C67S33, C50S50, C33S67, C25S75 and C0S100), were placed in glass Erlenmeyer flask contained 50 mL of phosphate solution in the concentration of 10-200 mg L-1. After 24h equilibration on a rotary shaker at constant room temperature (25±0.2 oC), the samples were separated by centrifugation at 4000 rpm, and the supernatant was filtered through a 0.45 µm GF/C filter and analyzed for phosphate. The concentrations of phosphate in residual solutions were analyzed by the molybdenum blue-ascorbic acid method [32]. The amount of phosphate adsorbed per mass unit of RC-BOFS was calculated by difference between the initial and equilibrium concentrations in solution. To establish adsorption mechanism of RC-BOFS for phosphate, SEM-EDS, FTIR, and XRD techniques were employed to analyze the RC-BOFS characteristics before or after phosphate adsorption. The mineral compositions of RC-BOFS before and after phosphate adsorption were examined using XRD (Bruker D8 Advance, Germany) operating at 40 kV and Cu-Kα radiation as X-ray source (λ=0.154 nm). The scanned 2θ range was 5°–90° with a scanning step of 0.01. SEM-EDS (Philips XL 30S FEG, Netherlands) were used to investigate the morphology and elemental composition of RC-BOFS before and after phosphate adsorption. The surface chemical properties of the RC-BOFS were examined by FTIR (Bruker VERTEX 70, BRUKER OPTICS, Germany). The spectra were obtained in a wavelength range of 400 to 4,000 cm-1 with 128 successive scans at a resolution 2 cm-1.

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2.2.2 Small-scale constructed wetland experiment The small-scale CW in this experiment consisted of HF beds with coarse sand, RCBOFS, and 75% coarse sand+25% RC-BOFS as a filter media. The beds consisting of HF are shown in Fig. 1.

Insert Fig. 1 here.

The horizontal flow (HF) beds constructed with 5 mm acrylic plate (Fig. 1) were 0.20 m (width) x 0.20 m (length) x 0.20 m (height) representing 0.008 m3 total volumes. The acrylic plate was used for its easiness to describe the actual site CW. With acrylic plate, the monitoring of CW can be carried out for a long time without structural changes. For horizontal bed, each filter media was filled up to 0.18 m from the bottom of the beds. The inflow of wastewater was controlled by zigzag type structure, which would help increase the hydraulic retention time in the HF bed [2]. Iris pseudoacorus plant was transplanted in the HF beds. The hydroponic wastewater was collected from a strawberry hydroponic cultivation located at Gyeongsangnam-do Agricultural Research & Extension Services, South Korea. The wastewater discharged from hydroponic cultivation had a pH of 6.8±0.4 and phosphate of 12.4±2.2 mg L-1. The amount of hydroponic wastewater injected into a HF bed was 0.8 L day-1 and the hydraulic retention time (HRT) of the hydroponic wastewater in HF bed was 3.2 days. The samples of the influent and effluent were taken every week and phosphate concentration and pH were measured for 7 months (from Mar. 2015 to Oct. 2015).

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3. Results and Discussion 3.1 Adsorption characteristics 3.1.1 Effect of time The contact time between the polluted water and the adsorbent is limited by the space available activate site. Kinetic experiment can provide information on the contact time required to achieve optimal conditions for phosphate adsorption. In this study, the phosphate adsorption by RC-BOFS was rapid in the first 0.5 h and then slowly reached equilibrium after 24 h (Fig. 2). Two distinct stages of phosphate adsorption by RC-BOFS were identified. The fact that the quick initial rate of adsorption during the first few minutes of contact was followed by a slower one until equilibrium state was likely due to the existence of abundant vacant active RC-BOFS sites of different accessibility [33]. The adsorption process continued for a progressive saturation of these active sites with time. Han et al. [34] reported that the rapid initial removal is largely attributed to the phosphate precipitation with exchangeable and dissolved Ca2+ rather than the adsorption. The liberated Ca2+ from the exchange site or from the dissolution of CaCO3, CaO and Ca(OH)2 are preferably precipitated by phosphate in high pH solution, which gives a high initial rate of adsorption [35]. However, in the case of RC-BOFS, pH after kinetic experiment under different initial phosphate concentration at 24 h was range of 8.2-8.8. Therefore, phosphate adsorption of RC-BOFS may have occurred with ion exchange mechanisms of phosphate hydrolysis products (H2PO4-, HPO42-) and the precipitation of the metallic salts of phosphate better than effect of precipitation by Ca2+. The effect of contact time on the amount of phosphate adsorbed by RC-BOFS was observed at the initial concentration in the range from 10 to 200 mg L-1. The values k1 and k2, calculated qe values determined from the pseudo-first-order and pseudo-second-order model 10

along with the corresponding correlation coefficients (R2) are presented in Table 2. Our results indicated that the pseudo-second-order kinetic model (R2 = 0.9302-0.9996) fit the data better than the pseudo-first-order kinetic model (R2 = 0.8670–0.9515). It was also confirmed that chemisorption was the rate-limiting for the adsorption of phosphate onto RC-BOFS. These results indicate that the reaction rate is proportional to the number of the active sites present on the RC-BOFS surface.

Insert Fig. 2 here.

Insert Table 2 here.

3.1.2 Effect of pH In order to explore the maximum adsorption capacities of phosphate by RC-BOFS, adsorption experiments were conducted with different initial phosphate concentration and pH and the results are presented in Fig. 3A. The values of the Freundlich and Langmuir constants and of the correlation coefficients for RC-BOFS studied at different pH are presented in Table 3. Using the Freundlich isotherm, the phosphate adsorption capacity (K) of RC-BOFS under different pH decreased in the order of pH 5 (0.8283) > pH 7 (0.4998) > pH 9 (0.2536). In this study, the values of 1/n in the adsorption isotherms for all tested slags were less than unity, suggesting a favorable adsorption process. Based on the Langmuir adsorption isotherm, the maximum phosphate adsorption capacities (a; mg P g-1) of RC-BOFS under different pH were in the following order: pH 5 (3.57) > pH 7 (2.47) > pH 9 (1.46). It can be found that the phosphate adsorption tends to decrease with the increase of pH. Similar result was also observed for 11

phosphate adsorption from other researches [36]. In general, the surface of filter medium in aqueous solution of high pH would carry more negative charges which would repulse the negatively charged pollutants. Consequently, the lower phosphate adsorption at higher pH values may be a result of increased repulsion between the more negatively charged PO43species and negatively charged surface sites. On the other hand, we did notice a generally much lower final equilibrium solution pH after phosphate adsorption in this study due to the low pH of RC-BOFS (Table 1). For example, in the case of initial pH 7, pH of the supernatant after the batch experiment ranged from 8.2 to 8.8, which was much lower than those reported for conventional slags after phosphate adsorption (pH range of 10-12) in the effluents [35,37]. Highly alkaline leachates from steel slags can pose a threat to the aquatic environment [38] as well as the dissemination of alkaline materials from the filter media can especially cause plant toxicity and be a source of highly alkaline effluent water in CWs [39]. This highlights the superiority of RC-BOFS over conventional slags for potential use of treating wastewater in actual wetland sites. Park et al. [3] reported that the equilibrium pH of the solution was dependent on the volumes and properties of slags as well as the concentration of phosphate. Therefore, it is important to consider these conditions when RCBOFS is used for the treatment of actual wastewater.

3.1.3 Effect of particle size The adsorption isotherms for phosphate by RC-BOFS under different particle size in the adsorption experiment are shown in Fig. 3B. These fitted parameter using Freundlich and Langmuir models are given in Table 3. Freundlich isotherm calculated adsorption coefficient (K) of RC-BOFS (0.8-2.3 mm) and RC-BOFS (2.3-4.6 mm) for phosphate were 0.57 and

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0.17, respectively. On the other hand, the maximum adsorption capacities (a) of of RC-BOFS (0.8-2.3 mm) and RC-BOFS (2.3-4.6 mm) for phosphate determined based on the Langmuir model were 2.81 and 2.28 mg P g-1 RC-BOFS, respectively. The maximum phosphate adsorption capacity (a) of RC-BOFS (0.8-2.3 mm) was higher than those in RC-BOFS (2.34.6 mm), corresponding to the greater specific surface area of finer RC-BOFS size than the larger size (0.38 m2 g-1 vs 0.18 m2 g-1). More metal oxides/hydroxides and silicon oxides are exposed with decreasing RC-BOFS size during the grinding process. This can provide more active sites for phosphate adsorption and result in the increase of phosphate adsorption capacity with decreasing RC-BOFS size. In addition, smaller particles may be effective to filter organic matter and increase the retention time of pollutants in wastewater in constructed wetlands. On the other hands, it also known that extremely smaller particle of filter media may lead to clogging and insufficient contact between pollutants and filter media in constructed wetlands [40]. The effective size and the uniformity coefficient of filter media are closely related to the saturation time and pore clogging situation of constructed wetlands. According to the literature, effective size and uniformity of filter media for pollutants removal in constructed wetlands are 0.2–1.0 mm and less than 3, respectively [41,42]. The effective size and the uniformity coefficient of the RC-BOFS used in this study were 0.8 mm and 2.25, respectively (Table 1), and satisfied the aforementioned criteria.

Insert Fig. 3 here.

Insert Table 3 here.

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3.1.4 Effect of RC-BOFS dosage The effect of RC-BOFS dosage on the phosphate adsorption in solutions was shown in Fig. 4. The RC-BOFS dosage was varied from 1 to 5 g and the initial phosphate concentration was ranged from 10-200 mg L-1. The removal efficiency of phosphate by RCBOFS increased as adsorbent dosage increase. As the RC-BOFS dosage increase, the number of adsorption sites in the RC-BOFS will increased, as a result increase the removal efficiency of phosphate from the solution. The removal efficiency of phosphate in initial concentration of 200 mg L-1 was found to be 28.7, 48.6, 57.9, 63.7 and 73.7% at adsorbent dose of 1, 2, 3, 4 and 5 g, respectively (Fig. 4B). On the other hand, the amount of phosphate adsorbed per unit mass of RC-BOFS decreased with the increasing RC-BOFS dosage (Fig. 4A). It was likely simply a function of the increase in total amount of active sites [43], the data indicate that not all of the added adsorption sites were available for binding. Additionally, the pH in solution increased with the increasing RC-BOFS dosage. The pH in initial phosphate solution of 200 mg L-1 was increased from 7 to 7.9, 8.7, 9.3, 9.9 and 10.6 at adsorbent dose of 1, 2, 3, 4 and 5 g, respectively. The increased solution pH at high dose, could also contribute to the lower removal efficiency. Based on the above results, the optimum dosage of RC-BOFS was 2 g (40 g L-1), considering the concentration of phosphate in wastewater discharged from agriculture and domestic in South Korea is 3.0-15.8 mg L-1 [2,44].

Insert Fig. 4 here.

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3.1.5 Advantage of RC-BOFS over other adsorbents as filter material for phosphate removal Table 4 presents the comparison of RC-BOFS with adsorbents derived from industrial by-products, and natural and synthetic materials for phosphate adsorption reported in literature. In general, RC-BOFS showed higher phosphate adsorption capacity than many of natural materials but were comparable with or lower than those of synthetic materials and industrial by products. However, direct comparison of phosphate adsorption capacities of these materials was impossible and misleading because the experimental conditions used for evaluating the adsorption capacities of these materials especially slags were different, and the same materials showed different physico-chemical properties due to varying conditions. On the other hand, although conventional slags have higher phosphate adsorption capacities, they have limited use as filter material for constructed wetlands. The main mechanism of phosphate removal from aqueous solution by conventional slags such as BSFS, BOFS, and EAFS were reported to be precipitation in the form of hydroxyapatite through the reaction of calcium eluted from the slag surface and phosphate in solution with high pH [58]. It can be explained by CaO-slag dissolution and HAP precipitation process as follow:

Free CaO + H2O
Ca2+ + 2OH5Ca2+ + 3PO43- + OH-
Ca(PO4)3(OH)

(Eq. 5) (Eq. 6)

The free CaO released from conventional slag and associated high pH are main controlling parameters in phosphate removal. Ranieri et al. [59] reported that high pH in CW could adversely influence plant growth such as intracellular metabolic activity inhibition,

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biomass reduction, and cell growth decline. Additionally, microbial activities were sensitive to pH in CW [60]. Therefore, conventional slags were limited use as a filter material due to its cause of high pH of effluent in CW (Table 4). On the other hand, the free CaO content of RC-BOFS is very low (Table 1), due to that free CaO from surface of RC-BOFS was removed during the rapid cooling process of molten slag in BSSF. For this reason, when using RC-BOFS as a filter material, the pH can be kept neutral as observed in this study. It is also true that many synthetic materials such as magnetic iron oxide nanoparticle (MION), zirconium oxide nanoparticle (ZON), and iron oxide coated granular activated carbon (Fe-GAC) with high phosphate adsorption have been developed in recent years. However, these materials are difficult to apply to full-scale CWs because manufacturing processes are complicated and costly. In addition, the application of these synthetic materials could cause the release of secondary pollutants. For example, Fe concentration released from Fe-GAC was found to be more than 2 mg L-1 and it was difficult to satisfy the effluent water quality standards of Europe [61]. Natural materials such as sand, dolomite and limestone have been used as filter medium in constructed wetland, but they have relatively low adsorption capacity compared to RC-BOFS. Peat has a high phosphate adsorption capacity, but due to the small particle size, its application as a filter material in CW is also limited [40]. Based on the above comparison, RC-BOFS is considered to be an excellent filter material for CWs because it can meet the high adsorption capacity and stable effluent water quality standards as compared with conventional slags, and other industrial by-product and synthetic materials.

Insert Table 4 here. 16

3.2 Predict of saturation time The effect of the mixing ratio of coarse sand(C): RC-BOFS(S) on the amount of phosphate adsorbed is shown in Fig. 5A. The Langmuir’s maximum adsorption capacity (LM) of phosphate in C100S0, C75S25, C67S33, C50S50, C33S67, C25S75 and C0S100 treatment were 0.10, 0.43., 0.89, 1.45, 1.83, 2.20, and 2.47 mg g-1, respectively. Ciupa [62] reported that the treatment efficiency of phosphate by sand in the CWs was stable at the beginning, but it decreased due to saturation of the adsorption site over time. Additional, the removal rate of phosphate by sand would be used up after only a few months in full-scale CWs, whereas that of others would persist up to several years [63]. The most important characteristic of the sands that determined their phosphate removal capacity was their calcium content. Pant et al. [64] reported that some local sands having high Fe and Al content in Canada have high adsorption capacity for phosphate. However, content of Fe, Ca, Mg and Al of coarse sand used in this study was 2,428, 2,012, 489, and 3 mg kg-1, respectively, which was greatly lower compared with that of RC-BOFS. Therefore, the amount of phosphate adsorbed increased as the mixing ratio of RC-BOFS increased. The pH values at coarse sand:RCBOFS ratios of C100S0, C75S25, C67S33, C50S50, C33S67, C25S75 and C0S100 were 7.2, 7.4, 7.7, 7.9, 8.3, 8.4 and 8.7, respectively, showing that the all coarse and RC-BOFS ratios except C0S100 were suitable for meeting the acceptable water quality discharge standard from water treatment plant (pH is between 5.8 and 8.5 in South Korea). Prochaska and Zouboulis [40] reported that the use of mixed 10:1 (w/w) ratio of sand to dolomite as a filter media in CWs, which meets the aforementioned discharge standard. The selection of filter media was such to increase phosphate removal rate, without inducing high pH values, which should not affect water bodies including plant and microbial by effluent discharged from the

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CWs. Based on this result, it is considered that the longevity of the CW by the phosphate saturation can be extended by adding the RC-BOFS (Fig. 5B). Prediction of saturation time for phosphate in simulated CW was evaluated only by adsorption capacity of the RC-BOFS without considering the microbial degradation and plant uptake. Consequently, if the coarse sand is used for the currently operating CW (inflow: 10 m3 day-1 including 6 mg phosphate L1

; outflow: 10 m3 day-1 including 1 mg phosphate L-1; area: wide 10 m × length 10 m × depth

1 m), the phosphate saturation time is predicted about 292 days, but it is considered that the longevity of the wastewater including phosphate can be extended up to 1,349 days by adding 25% RC-BOFS to the coarse sand. Especially, phosphate saturation time in CW with 25% RC-BOFS was 4.6 times higher than that with 100% coarse sand. Therefore, it is determined that the amount of phosphate adsorbed can be increased by adding the RC-BOFS, and that the longevity of the CW can be extended due to the phosphate saturation by adding the RCBOFS to the coarse sand in the CWs.

Insert Fig. 5 here.

3.3 Mechanisms Effluent pH is an important factor of consideration when choosing filter material in CWs for treating wastewater. In this study, phosphate removal by RC-BOFS was evaluated in three initial solution pH of 5, 7, and 9 which gave the final equilibrium solution pH ranges of 6.9-7.2, 8.2-8.8, and 9.7-10.2, respectively, depending on the concentration of phosphate (Fig. 3). The result of final pH 8.2-8.8 (with initial pH 7) represents most acceptable pH condition 18

(< pH 9) for potential usage of RC-BOFS as filter material in CWs. Therefore, phosphate adsorption mechanisms of RC-BOFS were investigated using XRD, SEM-EDS, and FTIR techniques on RC-BOFS sample adsorbed in phosphate solution adjusted pH 7 of 200 mg L-1, which gave potential effluent pH of 8.2-8.8. X-ray diffraction spectroscopy scans of the RC-BOFS sample before and after phosphate adsorption are shown in Fig. 6. According to the XRD results, RC-BOFS before phosphate adsorption primarily consisted of Ca, Mg, Al and Fe-bearing mineral phases such as dicalcium silicate, and divalent metal oxides, and so on. The presence of these minerals indicates that the RC-BOFS has the potential for phosphate removal by precipitation and adsorption. The intensities of several peaks of RC-BOFS sample after phosphate adsorption decreased compared to the pattern of RC-BOFS before phosphate adsorption, which can be ascribed to the hydrolysis of Ca2Fe2O5, Ca2SiO4, Ca14Mg2(SiO4)8, Ca3Mg(SiO4)2, CaFe2O4, CaO, and FeO [65]. Especially, the RC-BOFS sample before phosphate adsorption showed the broad and high peaks with near 43θ and 62θ, which were attributed to divalent metal oxides and iron oxide [66], whereas they decreased in phosphate adsorbed RC-BOFS. This suggests the involvement of these oxides in sorption of phosphate. However the XRD results did not show obvious formation of any calcium phosphate minerals. As the formation of calcium phosphate minerals would take place at pH levels above 8 [67,68] it is likely that there was insufficient mass of formation or the precipitation was in amorphous form [65].

Insert Fig. 6 here.

The FEG-SEM images and EDS scans of RC-BOFS samples before and after 19

phosphate adsorption are shown in Fig. 7. Changes in the surface structure of RC-BOFS were observed after phosphate adsorption. The RC-BOFS sample after phosphate adsorption was covered with a finely distributed crystalline substance along with amorphous particle, whereas the surface of RC-BOFS sample before phosphate adsorption was primarily amorphous particle (Fig. 7a). In a study of phosphate adsorption by BOFS without being subjected to rapid cooling process, Pratt et al. [66] reported that phosphate adsorbed surface of BOFS sample was consisted of (1) P-rich amorphous Fe oxides and (2) Fe/P/O and Ca/P/O precipitates. Additionally, Baker et al. [69] used SEM/EDS to evaluate the phosphate distribution in an exhausted slag material treating synthetic wastewater and showed that phosphate was closely associated with a thin Fe oxide on slag surface. In our study, the EDS spectra confirmed that the surface of RC-BOFS before phosphate adsorption was mainly composed of Fe, Ca, Si, Mn, and Mg. After phosphate adsorption, the RC-BOFS sample predominantly consisted of Ca, Fe, Si, K and P (Fig.7b). The presence of strong EDS peak of P along with the presence of newly formed distributed crystalline substance indicates the interaction of phosphate with RC-BOFS, likely in Ca/P/O precipitates [67].

Insert Fig. 7 here.

The FTIR spectra of RC-BOFS samples before and after phosphate adsorption is shown in Fig. 8. The adsorption bands at 450-600 cm-1 indicated Al2O3, Fe2O3, MgO, ferrites and most metallic oxides, which are a common feature of different slags [70]. According to Viswanathan et al. [71], the band at 1085, 1030, 950, and 865 cm-1 in slag can be attributed to P-O of phosphate group. In our study, it could observe that spectra of RC-BOFS before and 20

after phosphate adsorption were slightly different (Fig. 8). There was broad shoulder peak at 1030 cm-1 of P-O stretching in RC-BOFS after phosphate adsorption, which was not evident before phosphate adsorption. Correspondingly, there was slight decrease in the intensity of Fe-O stretching at 585 cm-1 in the phosphate-loaded RC-BOFS sample, suggesting the interaction of Fe oxide with phosphate as a result of adsorption. On the other hand, the broad beak around 1030 cm-1 could be due to the influence of Si-O stretching, which typically occurs at similar region [70]. From the above results, it can be concluded that the main mechanism of phosphate adsorption by RC-BOFS is dominated by metal oxide while precipitation by calcium is also plausible given the final solution pH being in the range of 8.2-8.8 which would begin to favor the Ca-P precipitation [67,68].

Insert Fig. 8 here.

3.4 Variations of phosphate concentration and pH in influent and effluent in smallscale constructed wetland The variations of phosphate concentration and pH in influent and effluent in HF CW under different filter media (sand, RC-BOFS, sand+RC-BOFS) are shown in Fig. 9. In the HF bed with coarse sand, phosphate concentrations in inflow and effluent were 11.8±1.6 and 6.2±1.1 mg L-1, respectively, showing that a phosphate removal rate of 46.4±12.3%. In the HF bed with RC-BOFS, concentration and removal rate of phosphate in effluent were 0.7±0.3 mg L-1 and 94.2±2.8%, respectively. In the HF bed with sand+RC-BOFS, the phosphate concentration was 1.2±0.5 mg L-1 in effluent. The removal efficiency of phosphate in the HF 21

bed with sand+RC-BOFS was 89.3±4.4%. The removal rates of phosphate in HF bed were higher in the order of RC-BOFS > sand+RC-BOFS >> sand. The pH in effluent discharged from in HF beds with sand, RC-BOFS, sand+RC-BOFS were 7.3±0.2, 8.9±0.6 and 7.7±0.2, respectively. In this study, phosphate removal efficiency in HF bed with coarse sand, which is the major filter media commonly used in CWs, was lower than those reported for other HF bed. On the other hand, removal rate of phosphate in the HF bed with 75% coarse sand and 25% RC-BOFS was higher than that in HF bed with sand only, clearly indicating the greater phosphate adsorption capacity of RC-BOFS. Although, the removal rate of phosphate in the HF bed containing RC-BOFS only was highest among all three beds, its use as filter media for CWs is considered to be limited due to the effluent pH discharge limit of 8.5 (Fig. 9). Based on the above results, even though phosphate removal rate in HF bed with sand+RC-BOFS was lower than that in HF bed with RC-BOFS, HF bed with Sand+RCBOFS was a suitable pH for meeting acceptable water quality discharge standard from water treatment plant (pH is between 5.8 and 8.5 in South Korea). The amount of phosphate removed in HF bed with sand+RC-BOFS during 7 months was 0.15 mg g-1. Considering the phosphate adsorption amount of sand+RC-BOFS (0.43 mg g-1), the saturation of phosphate was predicted to be after 16 months. This value can only be used as theoretical estimate as it did not consider to other mechanism such as plant uptake, microbial decomposition, and long-term sedimentation [40]. Therefore, the saturation time on phosphate in constructed wetland with sand+RC-BOFS may actually be even longer.

Insert Fig. 9 here.

22

Conclusions In this work, the adsorption characteristics of phosphate by RC-BOFS were evaluated in detail through various conditions. The results demonstrate that the parameters including initial phosphate concentration, initial pH and RC-BOFS size and dosage have important influences of phosphate adsorption by RC-BOFS. Maximum adsorption capacity of phosphate by RC-BOFS increased with lower solution pH and smaller sample particles. At initial pH 5 and 7, RC-BOFS exhibited adsorption capacities of 3.62 and 2.47 mg P g-1, respectively. The results of XRD, FTIR and SEM-EDS revealed the phosphate adsorption by RC-BOFS was dominated by metal oxides along with precipitation by calcium at neutral initial solution pH. The small–scale CW experiment showed that RC-BOFS as filter medium significantly improved the phosphate removal rate of effluent from 46.4% using sand to 94.2% with amount of phosphate adsorbed as 0.083 and 0.146 mg P g-1, respectively, during 7 month experiment. Even with partial inclusion of RC-BOFS in sand (75% sand and 25% RC-BOFS), the phosphate removal rate was significantly enhanced to 89.3% with phosphate adsorption of 0.151 mg P g-1. The phosphate saturation time was predicted to extend from 292 days for sand to 1,349 days for 75% sand and 25% RC-BOFS mixture, greatly improving the filter longevity of constructed wetland. Overall, horizontal flow constructed wetland with sand 75%:RC-BOFS 25% ratio could achieve high phosphate removal rate and near-neutral effluent pH for meeting the acceptable water quality discharge standard from water treatment plant.

23

Acknowledgments This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea Government (MSIP), [NRF-2017R1A2B4004635].

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32

Table 1 Physico-chemical characteristics of RC-BOFS. Physico-chemical characteristics Porosity (%)

Content 34

Bulk density (g/cm3)

1.74

d10 (mm)

0.8

d60 (mm)

1.8

Specific surface (m2/g)

0.38

Uniformity coefficient (d60/d10)

2.25

pH (1:5H2O)

9.1

SiO2 (%)

13.4

Fe2O3 (%)

24.2

Al2O3 (%)

3.3

MgO (%)

6.4

CaO (%)

36.7

Free CaO (%)

1.12

33

Table 2 The parameter estimates and coefficients of determination (R2) for fit of the kinetic equation to experimental data of phosphate adsorption at RC-BOFS Initial

Pseudo-first order model

concentration

Pseudo-second order model

(mg L-1)

qe

k1

R2

qe

k2

R2

10

0.084

0.104

0.8670

0.129

2.508

0.9302

25

0.214

0.104

0.9200

0.423

1.415

0.9801

50

0.325

0.108

0.9515

0.810

1.145

0.9925

100

0.228

0.070

0.9050

1.308

1.957

0.9982

200

0.310

0.098

0.8929

2.424

1.671

0.9996

34

Table 3 The parameter estimates and coefficients of determination (R2) for fit of the isotherm equation to experimental data of phosphate adsorption at RC-BOFS Freundlich adsorption isotherm

Langmuir adsorption isotherm

Condition

pH

Particle size (mm)

K†

1/n‡

R2

a§

b¶

R2

5

0.8283

0.361

0.9630

3.57

0.258

0.9632

7

0.4998

0.339

0.9580

2.47

0.121

0.9532

9

0.2536

0.347

0.9226

1.46

0.068

0.9340

0.8-2.3

0.5667

0.372

0.9388

2.81

0.165

0.9678

2.3-4.6

0.1741

0.542

0.9909

2.28

0.036

0.9573

†

K: adsorption capacity of phosphate. 1/n: an empirical parameter related to the intensity of sorption. § a: maximum adsorption capacities of phosphate (mg g-1). ¶ b: binding strength constant of phosphate. ‡

35

Table 4 Filter materials used for phosphate adsorption Slags

Maximum adsorption capacity (mg g-1)

Initial pH

BOFS BFS BFS BOFS BFS EAFS BOFS EAFS Oyster shell Coal ash Fly ash

3.6 2.5 5.3 3.6 46.5 33.3 8.9 0.3 2.5 3.9 0.8 0.9 5.5-42.6

5.0 7.0 5.5 8.5 6.7 6.7 7.0

Synthetic materials MION Fe-Cu binary oxide ZON Fe-GAC

5.0 35.2 99.0 10.4

Industrial materials RC-BOFS

Final pH

6.9-7.2 8.2-8.8

References

In this study Xiong et al. [45] Oguz [46] Lu et al. [47]

12.0 11.3 12.3 Xu et al. [48] 9.9-11.4 Barca et al. [49] 11.6-12.4 Drizo et al. [50] Seo et al. [10] Drizo et al. [51] Chen et al. [52]

Yoon et al. [53] Li et al. [54] Su et al. [55] Kumar et al. [56]

Natural materials Zeolite 2.2 Sakadevan and Bavor [15] Peat 8.9 Xiong and Mahmood [57] Sand 0.1 Prochaska and Zouboulis [40] Dolomite 0.2 Limestone 0.7 Drizo et al. [51] BOFS; Basic oxygen furnace slag, BFS; Blast furnace slag, EAFS; Electric arc furnace slag, MION; Magnetic iron oxide nanoparticle, ZON; Zirconium oxide nanoparticle, Fe-GAC; Iron oxide coated granular activated carbon

36

Figure caption Fig. 1. Diagram of horizontal flow constructed wetland. Fig. 2. Effects of contact time on phosphate adsorption by RC-BOFS. Fig. 3. Phosphate adsorption isotherms by RC-BOFS under different pH (A) and particle size (B). Fig. 4. Effects of RC-BOFS dosage on phosphate adsorption. (A) Amount of phosphate adsorbed, (B) Removal rate of phosphate. Fig. 5. Effects of mixed ratio of coarse sand to RC-BOFS on phosphate adsorption (A) and predict of phosphate saturation time in constructed wetland (B). Fig. 6. X-ray diffractometer (XRD) pattern of RC-BOFS before and after phosphate adsorption. Fig. 7. Micrographs of RC-BOFS before and after phosphate adsorption using SEM-EDS. Fig. 8. Relative change in FTIR wave number identification of of RC-BOFS before and after phosphate adsorption. Fig. 9. Variations of phosphate concentration and pH in influent and effluent in small-scale constructed wetland.

37

Inlet

HF Constructed wetland

Outlet Sand Inlet

Outlet

Inducing plate

RC-BOFS

0.2 m Horizontal flow (HF)

Sand+RC-BOFS

Fig. 1.

38

Outlet

Adsorbed phosphate(mg g-1)

3

10ppm

25ppm

100ppm

200ppm

50ppm

2

1

0 0

4

8

12 16 Contact time (h)

Fig. 2.

39

20

24

Adsorbed phosphate (mg g-1)

A 4

pH 5 pH 7

3

pH 9

2

1

0 0

50 100 Equilibrium P concentration (mg L-1)

Adsorbed phosphate (mg g-1)

B 3

150

0.8-2.3 mm 2.3-4.6 mm

2

1

0 0

40 80 Equilibrium P concentration (mg L-1)

Fig. 3.

40

120

Adsorbed phosphate (mg g-1)

A

3 1g

2g

3g

4g

5g

2

1

0 10

25 50 100 Initial concentration (mg L-1)

200

Romoval rate of phosphate (%)

B 120 100 80 60 40 20 1g

2g

3g

4g

5g

0 10

25 50 100 Initial concentration (mg L-1)

Fig. 4.

41

200

A

3

LM (mg g-1)

y = 2.6304x + 0.0229 R² = 0.9674 2

1

0 0%

25% 50% 75% Added ratio of RC-BOFS to coarse sand

B

Fig. 5.

42

100%

before

Dicalcium silicate

after

Divalent metal oxides solid solution

Intensity

Iron oxide

10

20

30

40

50

2θ (degree)

Fig. 6.

43

60

70

80

Fig. 7.

44

Transmittance

Metallic oxides(Al2O3, Fe2O3, MgO) Al-O Si-O symmetrically stretching C-O vibration Si-OH bending

C-O stretching(CO32-)

Before

Si-O stretching

After

C-O vibration

1030

585 O-Si-O bending

400

600

800

Hydroxyl bending

1000 1200 1400 Wavenumber (cm-1)

Fig. 8.

45

1600

1800

Inflow

Sand

RC-BOFS

Sand+RC-BOFS

100

16 12 8 4

Removal rate of phosphate (%)

Phosphate (mg L-1)

20

80

60

40

20

0

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

0

Treatment (Day)

12

Inflow

Sand

RC-BOFS

Sand+RC-BOFS

Sand

RC-BOFS

S+R

Sand

RC-BOFS

S+R

10

8

pH

10 pH

6

4

8

2 6

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Treatment (Day)

Fig. 9.

46

0

Graphical abstract

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Highlights • RC-BOFS was an effective adsorbent for phosphate • Contact time, initial concentration and pH value affect the adsorption capacity. • Adsorption mechanisms were well described by SEM-EDS, XRD and FTIR • High phosphate adsorption capacity and near-neutral pH was observed in CW with sand and RCBOFS

48