Magnetic Steel Slag Biochar for Ammonium Nitrogen Removal from Aqueous Solution

: In this study, magnetic steel slag biochar (MSSB) was synthesized from low-cost steel slag waste to investigate the effectiveness of steel slag biochar composite for NH 4 -N removal and magnetic properties in aqueous solution. The maximum adsorption capacity of NH 4 -N by MSSB was 4.366 mg/g according to the Langmuir model. The magnetic properties of MSSB indicated paramagnetic behavior and a saturation magnetic moment of 2.30 emu/g at 2 Tesla. The NH 4 -N adsorption process was well characterized by the pseudo-second order kinetic model and Temkin isotherm model. This study demonstrated the potential of magnetic biochar synthesized from steel slag waste for NH 4 -N removal in aqueous solution.


Introduction
Due to the development of steel industries, the annual domestic generation of crude steel in South Korea reached about 71 million tons in 2019 and it has shown an increasing trend over the years. [1] However, steel industries also produce a considerable amount of waste materials. Particularly, steel slag is produced as a byproduct from steel-making and refining processes and occupies about 40% of annual steel output in South Korea [2]. For decades, the proper treatment or reuse of steel slag has become an issue and steel industries are seeking efficient techniques to reclaim it [3,4].
Basic oxygen furnace (BOF) slag mainly consists of SiO 2 , CaO, Fe 2 O 3 , Al 2 O 3 , and some alkaline substances. It has been used as lime, silicate, and iron fertilizer [5][6][7][8]. However, there is a limit to the broad application of raw BOF slag as fertilizers because of nitrogen deficiency. In general, nitrogen is an essential component for plant growth and nitrogen fertilizer is expensive [9].
Biochar is solid biomass such as wood, crop residues, or agricultural waste that is pyrolyzed under oxygen-free conditions [10,11]. It has been used as a soil amendment [12], carbon sequestration agent [13], and adsorbent for the removal of organic and inorganic compounds (e.g., NH 4 -N, Cu, Zn, and Pb) in aquatic environments [14][15][16][17][18][19]. However, biochar in the form of fine powder is difficult to separate from aqueous solution after adsorbing pollutants [10,20]. Filtration and centrifugation steps are commonly used for this separation, but the pollutants on the biochars may desorb during the processes [21,22]. To overcome the problem of powdered biochar recovery, some studies have introduced Energies 2021, 14, 2682 2 of 12 magnetic properties into biochar by pure chemical (e.g., iron oxide, ferromanganese binary oxide, and iron oxy-hydroxides) immobilization [10,11,23]. Magnetic biochars prepared by combining iron oxide and orange peel powder indicated efficient sorption capabilities to remove organic pollutants and phosphate from an aqueous solution [10]. The adsorption capacity of salicylic acid, naproxen, and ketoprofen from aqueous solution was identified by synthesized magnetic biochars using walnut shell and ferric chloride [21]. Wang and Zhang demonstrated the adsorption capacities of Bisphenol A from wastewater by magnetic biochars synthesized from grapefruit peel and maghemite; the main adsorption mechanisms were H-bond and π-π electron-donor-acceptor interaction [23]. However, the application of pure iron oxide chemicals increases the cost and thereby reduces the economic benefits of biochar derived from waste biomass.
In this study, we developed inexpensive magnetic biochar combining raw BOF slag and biochar to explore whether the magnetic slag biochar composite can maximize the benefits of the two components. Differences in biomass types and compositions are principal factors influencing the physicochemical characteristics of biochar. The introduction of magnetic BOF slag may affect the physicochemical characteristics and performances of biochar. Research on the effect of magnetic field on physicochemical properties of aqueous solutions was also reported previously by Szatylowicz and Skoczko [24]. Therefore, this study was designed to investigate the effectiveness of magnetic slag biochar composite for NH 4 -N removal from aqueous solution, and to determine the physicochemical characteristics of magnetic slag biochar such as adsorption capacities, volume of surface area, surface structure, and magnetic properties.

Materials
Pine sawdust powder collected from Pohang National Forestry Cooperative Federation in South Korea was stored in a dried phase in an incubator (30 • C). BOF slag powder (Table 1) was obtained from a steel-making company (POSCO, Pohang, Korea) and used as a substrate for slag biochar complexation.

Preparation of Biochar
BOF slag was ball-milled to fine powder (Air jet mill JM-LB, Dongsung Science Co., Ltd., Seoul, Korea) for effective slag biochar synthesis. Two different types of biochar were synthesized by a fixed bed pyrolysis system (Mirae SI Co., Ltd., Gwangju, Korea): pine sawdust biochar (PB) from only pine sawdust, and magnetic steel slag biochar (MSSB) from pine sawdust mixed with BOF slag (5:2 w/w) (Figure 1). The chemical composition and crystal structure of the two types of biochar were compared. The fixed bed pyrolysis system containing a quartz tube (25 mm inside diameter and 240 mm height) was operated as follows: each sample was located in the quartz tube; its temperature was increased from room temperature (RT) to 500 • C at 10 • C/min in nitrogen atmosphere (N 2 flow rate: 100 mL/min); maintained at that temperature (500 • C) for 1 h; and then cooled to RT.

Preparation of Biochar
BOF slag was ball-milled to fine powder (Air jet mill JM-LB, Dongsung Science Co., Ltd., Seoul, Korea) for effective slag biochar synthesis. Two different types of biochar were synthesized by a fixed bed pyrolysis system (Mirae SI Co., Ltd., Gwangju, Korea): pine sawdust biochar (PB) from only pine sawdust, and magnetic steel slag biochar (MSSB) from pine sawdust mixed with BOF slag (5:2 w/w) ( Figure 1). The chemical composition and crystal structure of the two types of biochar were compared. The fixed bed pyrolysis system containing a quartz tube (25 mm inside diameter and 240 mm height) was operated as follows: each sample was located in the quartz tube; its temperature was increased from room temperature (RT) to 500 °C at 10 °C/min in nitrogen atmosphere (N2 flow rate: 100 mL/min); maintained at that temperature (500 °C) for 1 h; and then cooled to RT.

Physicochemical Property Analysis of Slag Biochar Complex
The chemical compositions of biochar were analyzed by an elemental analyzer (Vario Micro cube, Elementar Analysensysteme GmbH, Langenselbold, Germany). A Brunauer-Emmett-Teller (BET) surface analysis device (Nanoporosity-XQ analyzer, Mirae SI Co., Ltd., Gwangju, Korea) was used to measure physical adsorption capacities and surface area of gas molecules on the surface of MSSB. BET surface area and pore size distribution were analyzed by drawing an isothermal adsorption line using the Barrett-Joyner-Halenda (BJH) equation [25,26]. The aggregated form of MSSB was analyzed using a scanning electron microscope (SEM: JSM-6510, JEOL Co., Ltd., Tokyo, Japan) and energy-dispersive X-ray spectroscopy (EDS: Emax, Horiba, UK). To measure the magnetic properties of synthesized slag biochar, the residual magnetism of synthesized biochar was measured using a physical property measurement system (PPMS-9, Quantum Design Inc., San Diego, CA, USA). The chemical composition of MSSB was analyzed using an X-ray diffractometer (XRD: X'pert PRO MPD, Malvern PANalytical B.V., Almelo, The Netherlands).

Batch Adsorption Experiments
To evaluate the efficiency of MSSB as an NH 4 -N adsorbent, 0.1 g of MSSB was added to each of five Pyrex bottles containing 100 mL of NH 4 -N solution that had a concentration of 1 ≤ [NH 4 -N] ≤ 5 mg/L and each bottle was sealed then stirred in a shaking incubator (VS-8480SF, Vision Science Co., Ltd., Daegu, Korea) at 200 rpm and a temperature of 25 • C. At 10 min, 30 min, 1 h, 3 h, 6 h, 9 h, and 24 h, a sample was collected from each bottle, then NH 4 -N concentrations were determined according to standard methods [27]. An adsorption isotherm was measured using MSSB at 1 ≤ [NH 4 -N] ≤ 5 mg/L. The reaction time determined by adsorption kinetics was 24 h.

NH 4 -N Adsorption Kinetics and Isotherms
NH 4 -N adsorption by MSSB was examined for 24 h to measure the time taken to reach equilibrium, and to find the adsorbed amount of NH 4 -N in MSSB at equilibrium. The adsorption equilibrium capacity q e [mg/g] was calculated as where C 0 and C e [mg/L] denote, respectively, the initial and equilibrium concentrations of adsorbate in the solution, V [L] is the volume of solution, and W [g] is the mass of adsorbent in the solution.
The kinetics of experimental data were fitted using a pseudo-first-order equation and pseudo-second-order equation where q t [mg/g] represents adsorption capacity of adsorbate at time t [min], q e [mg/g] is the adsorption equilibrium capacity of adsorbate, and k 1 [min −1 ] and k 2 [g·(mg·min) −1 ] is the adsorption-rate constants [28,29].
To calculate NH 4 -N adsorption isotherms of MSSB, the data were fitted with the Langmuir equation the Freundlich equation where q e [mg/g] is the amount of adsorbate adsorbed at equilibrium,  [30][31][32]. Originpro software (version 9.0) was used to draw figures and fit isotherm curves by both linear and nonlinear regression methods.

Physicochemical Properties of MSSB
To observe the crystal structure of MSSB, the XRD peak patterns of PB, BOF slag powder, and MSSB were compared ( Figure 2). The XRD pattern of PB showed that it is amorphous; this trait influences the structure of MSSB. XRD patterns of BOF slag powder and MSSB had five predominant peaks (Figure 2, triangles) that match the 220, 311, 400, 511, and 440 planes of magnetite (JCPDS 19-0629); this result illustrates that BOF slag powder included Fe 3 O 4 and that pyrolysis successfully combined pine sawdust with BOF slag powder components.
The chemical composition and structural characterization of MSSB were identified by SEM images and EDS analysis ( Figure 3). These measurements confirmed the presence of BOF slag powder on the surface of MSSB (Figure 3a). The base morphology of MSSB showed porous linked-fibrous shape, which is similar to the morphology of pine sawdust [33]. The EDS spectra showed that MSSB includes Fe, Ca, Al, Mg, Mn, C, and O ( Figure 3c); most of the ions (Fe, Ca, Al, Mg, and Mn) in MSSB seemed to derive from BOF slag powder components, and most C ions seemed to derive from pine sawdust.
MSSB and PB had distinct elemental compositions (CHN) and pH ( Table 2). MSSB included BOF slag powder and therefore had less C than PB did and higher pH. MSSB had smaller specific surface area, pore size, and adsorption/desorption area than PB did. Similar results were also observed under synthesized magnetic biochar condition compared to non-magnetic biochar condition although the magnetic biochar was synthesized using commercial chemicals [10]. The chemical composition and structural characterization of MSSB were identified by SEM images and EDS analysis ( Figure 3). These measurements confirmed the presence of BOF slag powder on the surface of MSSB (Figure 3a). The base morphology of MSSB showed porous linked-fibrous shape, which is similar to the morphology of pine sawdust [33]. The EDS spectra showed that MSSB includes Fe, Ca, Al, Mg, Mn, C, and O ( Figure  3c); most of the ions (Fe, Ca, Al, Mg, and Mn) in MSSB seemed to derive from BOF slag powder components, and most C ions seemed to derive from pine sawdust.
MSSB and PB had distinct elemental compositions (CHN) and pH ( Table 2). MSSB included BOF slag powder and therefore had less C than PB did and higher pH. MSSB had smaller specific surface area, pore size, and adsorption/desorption area than PB did. Similar results were also observed under synthesized magnetic biochar condition compared to non-magnetic biochar condition although the magnetic biochar was synthesized using commercial chemicals [10].   The chemical composition and structural characterization of MSSB were identified by SEM images and EDS analysis (Figure 3). These measurements confirmed the presence of BOF slag powder on the surface of MSSB (Figure 3a). The base morphology of MSSB showed porous linked-fibrous shape, which is similar to the morphology of pine sawdust [33]. The EDS spectra showed that MSSB includes Fe, Ca, Al, Mg, Mn, C, and O ( Figure  3c); most of the ions (Fe, Ca, Al, Mg, and Mn) in MSSB seemed to derive from BOF slag powder components, and most C ions seemed to derive from pine sawdust.
MSSB and PB had distinct elemental compositions (CHN) and pH ( Table 2). MSSB included BOF slag powder and therefore had less C than PB did and higher pH. MSSB had smaller specific surface area, pore size, and adsorption/desorption area than PB did. Similar results were also observed under synthesized magnetic biochar condition compared to non-magnetic biochar condition although the magnetic biochar was synthesized using commercial chemicals [10].   previously [20,[34][35][36], possibly because we did not use commercial chemicals to synthesize magnetic biochars (Figure 4, PPMS indicated that MSSB was paramagnetic with no external magnetism. Saturation magnetic moment of MSSB was 2.30 emu/g at 2 Tesla, which is lower than reported previously [20,[34][35][36], possibly because we did not use commercial chemicals to synthesize magnetic biochars ( Figure 4, Table 3).

NH 4 -N Adsorption by MSSB in Aqueous Solution
Time-dependent NH 4 -N adsorption by MSSB and PB was investigated using solutions that had initial 1 ≤ [NH 4 -N] ≤ 5 mg/L. All NH 4 -N adsorption capacities of MSSB increased as time increased to 24 h. The adsorption capacities of NH 4 -N at equilibrium by MSSB increased from 1.0 to 2.72 mg/g as the initial NH 4 -N concentration increased from 1 to 5 mg/L (Figure 5a). Similarly, the adsorption capacities of NH 4 -N at equilibrium by PB increased from 1.0 to 2.91 mg/g as the initial NH 4 -N concentration increased from 1 to 5 mg/L (Figure 5b). However, the maximum NH 4 -N adsorption capacity of MSSB was lower than that of PB. This might be attributed to smaller surface area and pore size at MSSB than at PB (Table 2) or to the considerable amount of metal oxides derived from BOF slag at MSSB (Table 1). Biochars can carry a lot of negative charges on the surface [15] and some metal oxides of BOF slag such as MgO and CaO can be dissolved in aqueous solution, which could form cations around MSSB [37,38]. Consequently, the metal cations seemed to compete with NH 4 -N for the adsorption sites of MSSB, resulting in a decrease in the capacity of adsorbent material for NH 4 -N [39].

Kinetics and Isotherms of MSSB
To describe NH4-N adsorption equilibrium data, pseudo-first order and pseudo-second order kinetic models were fitted using linear regression analysis ( Figure 6, Table 4). Compared to the pseudo-first kinetic model, the correlation coefficients from the pseudosecond kinetic model were higher for 1, 2, and 5 mg/L of initial NH4-N concentration and similar for 3 and 4 mg/L of initial NH4-N concentration; this result indicates that the pseudo-second kinetic model showed mostly higher correlation with the experimental data than the pseudo-first kinetic model. A similar result was reported previously [47]; the pseudo-second kinetic model was better fit for experimental data than the pseudofirst kinetic model when pine sawdust and wheat straw biochars were used for NH4-N adsorption. This trend may suggest a mechanism in which a fast initial step that is limited by general diffusion is followed by a slow second step that is limited by diffusion in small pores or by slow adsorption [48]. Previous results have shown that NH 4 -N adsorption capacities by biochars can be influenced by types of materials and initial NH 4 -N concentrations. Biochars derived from wheat straw, corn straw, or peanut shells obtained a maximum NH 4 -N adsorption capacity of 2.4 mg/g at initial [NH 4 -N] = 50 mg/L [11]. Rice straw biochars achieved a maximum adsorption capacity of 2.67 mg/g at initial [NH 4 -N] = 50 mg/L [16]. Pinechip biochars achieved a maximum NH 4 -N adsorption capacity of 0.52 mg/g at initial [NH 4 -N] = 79 mg/L [40]. Biochars prepared from sorghum distillers grain attained a NH 4 -N adsorption capacity of 2.06 mg/g at initial [NH 4 -N] = 100 mg/L [41]. Wood chip biochars obtained a maximum NH 4 -N adsorption capacity of 0.96 mg/g at initial [NH 4 -N] = 80 mg/L [42]. Various biochars indicated the potential to remove NH 4 -N from an aqueous solution. The NH 4 -N adsorption capacities by MSSB in this study were comparable to previous studies.
NH 4 -N can be adsorbed by three mechanisms: (1) surface complexation between NH 4 -N and surface hydroxyl species, (2) surface co-precipitation as Mg(NH 4 )PO 4 with magnesium from biochars and phosphate from biochars or solutions, and (3) formation of NH 4 -C bonds between NH 4 -N and delocalized lone pair electrons in biochars [43]. In this study, MSSB includes BOF slag powder that contains 5.1% MgO and 2.1% P 2 O 5 (Table 1), so surface precipitation in the form of Mg(NH 4 )PO 4 seems likely to be a possible mechanism of NH 4 -N adsorption. MgO hydration process can be described as follows [37]: Water diffuses inside porous MgO particles, adsorbs, and reacts on the surface, then Mg 2+ and OH − ions are released. The dissolution process causes high supersaturation, which induces rapid Mg(NH 4 )PO 4 precipitation according to the following reaction and P 2 O 5 is easily reacted with water, releasing H 3 PO 4 [44]: In addition, NH 4 -N adsorption to biochars can be explained as an electrostatic reaction and NH 4 + in aqueous solutions can displace Ca 2+ present on biochar surfaces [45]. The BOF slag powder from which MSSB was synthesized included 35.0% CaO, which easily reacts with water sequentially: Ca(OH) 2 is produced, then the Ca(OH) 2 is dissociated into Ca 2+ and OH − [38,46]. Therefore, Ca 2+ displacement may also contribute to NH 4 -N adsorption (Table 1).

Kinetics and Isotherms of MSSB
To describe NH 4 -N adsorption equilibrium data, pseudo-first order and pseudosecond order kinetic models were fitted using linear regression analysis ( Figure 6, Table 4). Compared to the pseudo-first kinetic model, the correlation coefficients from the pseudo-Energies 2021, 14, 2682 8 of 12 second kinetic model were higher for 1, 2, and 5 mg/L of initial NH 4 -N concentration and similar for 3 and 4 mg/L of initial NH 4 -N concentration; this result indicates that the pseudo-second kinetic model showed mostly higher correlation with the experimental data than the pseudo-first kinetic model. A similar result was reported previously [47]; the pseudo-second kinetic model was better fit for experimental data than the pseudofirst kinetic model when pine sawdust and wheat straw biochars were used for NH 4 -N adsorption. This trend may suggest a mechanism in which a fast initial step that is limited by general diffusion is followed by a slow second step that is limited by diffusion in small pores or by slow adsorption [48].

Kinetics and Isotherms of MSSB
To describe NH4-N adsorption equilibrium data, pseudo-first order and pseudo-second order kinetic models were fitted using linear regression analysis ( Figure 6, Table 4). Compared to the pseudo-first kinetic model, the correlation coefficients from the pseudosecond kinetic model were higher for 1, 2, and 5 mg/L of initial NH4-N concentration and similar for 3 and 4 mg/L of initial NH4-N concentration; this result indicates that the pseudo-second kinetic model showed mostly higher correlation with the experimental data than the pseudo-first kinetic model. A similar result was reported previously [47]; the pseudo-second kinetic model was better fit for experimental data than the pseudofirst kinetic model when pine sawdust and wheat straw biochars were used for NH4-N adsorption. This trend may suggest a mechanism in which a fast initial step that is limited by general diffusion is followed by a slow second step that is limited by diffusion in small pores or by slow adsorption [48].  Langmuir, Freundlich, and Temkin adsorption isotherm models were applied to determine the mechanism of NH 4 -N adsorption onto MSSB (Figure 7). The isotherm parameters (Table 5) show that the equilibrium data were fit best by the Temkin isotherm; it had B (heat of adsorption) > 0; this result suggests that NH 4 -N adsorption is exothermic ( heat of adsorption): if B < 4.2 kJ/mol, the adsorption reaction is physical adsorption, whereas if B is 84-210 kJ/mol, the adsorption reaction is chemical adsorption [50]. In this study, B < 4.2 kJ/mol, so NH 4 -N adsorption on MSSB seemed to be physical adsorption; this adsorption can occur by weak electrostatic interactions such as Van der Waals interactions, dipole-dipole forces, and London forces [51]. Therefore, the possible mechanism of NH 4 -N adsorption on MSSB is electrostatic interaction between NH 4 + ion and negative charge on the surface of MSSB, which is due to the dissociation of oxygen-containing functional groups of biochar; this causes electrostatic attraction between biochar and positively charged molecules [52].
termine the mechanism of NH4-N adsorption onto MSSB (Figure 7). The isotherm parameters (Table 5) show that the equilibrium data were fit best by the Temkin isotherm; it had B (heat of adsorption) > 0; this result suggests that NH4-N adsorption is exothermic (Table  5) [49]. The type of adsorption can be classified according to the value of B (J/mol, heat of adsorption): if B < 4.2 kJ/mol, the adsorption reaction is physical adsorption, whereas if B is 84-210 kJ/mol, the adsorption reaction is chemical adsorption [50]. In this study, B < 4.2 kJ/mol, so NH4-N adsorption on MSSB seemed to be physical adsorption; this adsorption can occur by weak electrostatic interactions such as Van der Waals interactions, dipoledipole forces, and London forces [51]. Therefore, the possible mechanism of NH4-N adsorption on MSSB is electrostatic interaction between NH4 + ion and negative charge on the surface of MSSB, which is due to the dissociation of oxygen-containing functional groups of biochar; this causes electrostatic attraction between biochar and positively charged molecules [52]. The Langmuir isotherm model also showed a high correlation coefficient for NH4-N adsorption (Table 5) with a maximum adsorption capacity 4.366 mg/g. In general, this model is based on limited monolayer adsorption, so NH4-N adsorption on MSSB might be more monolayer adsorption than multilayer adsorption [53].   The Langmuir isotherm model also showed a high correlation coefficient for NH 4 -N adsorption (Table 5) with a maximum adsorption capacity 4.366 mg/g. In general, this model is based on limited monolayer adsorption, so NH 4 -N adsorption on MSSB might be more monolayer adsorption than multilayer adsorption [53].

Conclusions
This study demonstrated the NH 4 -N adsorption capacities and magnetic properties of biochars synthesized from low-cost steel slag waste and pine sawdust. MSSB had large specific surface area, micro-sized pores, and good magnetic properties. The maximum adsorption capacity of NH 4 -N by MSSB was 2.72 mg/g. Although the maximum NH 4 -N adsorption capacity of PB was higher than that of MSSB, the difference was not great. Adsorption of NH 4 -N onto MSSB illustrated that the pseudo-second order kinetic model and Temkin isotherm model were well fit for the experimental data. These results indicated the potential of magnetic biochars using low-cost steel slag waste for NH 4 -N removal from an aqueous solution, but their NH 4 -N adsorption capacities and magnetic properties can be influenced by functional parameters such as NH 4 -N concentration, the ratio of steel