Adsorption removal of Mn²⁺ and NH₄⁺–N from electrolytic manganese metal wastewater by modified phosphate ore flotation tailings

Abstract

Electrolytic metal manganese wastewater (EMMW) is industrial wastewater produced in the process of producing electrolytic metal manganese, which contains Mn²⁺ and NH₄⁺–N pollutants. How to deal with low concentrations of Mn²⁺ and NH₄⁺–N from EMMW is an urgent problem for electrolytic manganese enterprises. In this study, Mn²⁺ and NH₄⁺–N were removed via adsorption by modified phosphate ore flotation tailings (MPOFTs). The results indicated that the concentration of Mn²⁺ and NH₄⁺–N in the EMMW decreased from 165 mg L⁻¹ to 0.001 mg L⁻¹ and from 70 mg L⁻¹ to 1.73 mg L⁻¹, respectively, when the solid–liquid ratio of MPOFTs to EMMW was 1 : 200, the reaction temperature was 323 K, the reaction time was 2.5 h and the initial solution pH was 9.0. The adsorption process was more consistent with the pseudo-first-order adsorption kinetic model and Weber's intraparticle diffusion model, and the results showed that this adsorption reaction was an endothermic reaction. This study provided a new research idea for EMMW and POFT treatment and disposal.

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Water impact

Electrolytic manganese metal wastewater (EMMW) contains a low concentration of Mn²⁺ and NH₄⁺–N, which easily pollute the surrounding ecological environment. Phosphate ore flotation tailings (POFTs) are solid wastes that are produced by phosphate ore after flotation. In this study, Mn²⁺ and NH₄⁺–N were removed via adsorption by modified phosphate ore flotation tailings (MPOFTs) with H₂C₂O₄. The results indicated that the concentration of Mn²⁺ in the EMMW decreased from 165 mg L⁻¹ to 0.001 mg L⁻¹, and NH₄⁺–N decreased from 70 mg L⁻¹ to 1.73 mg L⁻¹, when the solid–liquid ratio of MPOFTs to EMMW was 1 : 200, the reaction temperature was 323 K, the reaction time was 2.5 h and the initial solution pH was 9.0. The adsorption process was more consistent with the pseudo-first-order adsorption kinetic model and Weber's intraparticle diffusion model, and the results showed that this adsorption reaction was an endothermic reaction. NH₄⁺–N was removed mainly by struvite (MgNH₄PO₄·6H₂O), and Mn²⁺ was removed mainly by Mn(OH)₂, Mn₃O₄, Mn₃(PO₄)₂·7H₂O and Mn(OH)₂. This study provided a new research idea for EMMW and POFT treatment and disposal.

1 Introduction

Electrolytic metal manganese is a single metal which is obtained from manganese ore by acid leaching and then sent to the electrolytic cell for electrolytic precipitation. In 2021, China produced more than 1.31 million tons of metal manganese annually, accounting for about 96.5% of the world's total production.^1,2 Nowadays, electrolytic manganese residue (EMR) in China has exceeded 160 million tons.^3–5 Owing to historical and technical reasons, these piles of EMR will produce a large amount of electrolytic metal manganese wastewater (EMMW). EMMW containing low concentrations of Mn²⁺ and NH₄⁺–N will cause harm to human production, life and physical health.^6–8 How to deal with low concentrations of Mn²⁺ and NH₄⁺–N from EMMW is an urgent problem for electrolytic manganese enterprises.

Mn²⁺ and NH₄⁺–N can be removed by catalytic oxidation, chemical oxidation, physicochemical adsorption, breaking point chlorination, membrane treatment, biophysical methods, electrochemical, chemical precipitation and stripping methods.^9–19 In industrial production, Mn²⁺ and NH₄⁺–N in EMMW are often removed by neutralization precipitation and flocculation precipitation methods, and Mn²⁺ and NH₄⁺–N were main removed in the form of solidified precipitates and converted to ammonia gas, respectively. The above methods can remove high concentrations of Mn²⁺ and NH₄⁺–N, but low concentrations of Mn²⁺ and NH₄⁺–N also remain in the EMMW.²⁰ Thus, the development of low concentration Mn²⁺ and NH₄⁺–N removal technology is extremely important for electrolytic manganese enterprises.

In fact, adsorption methods have the characteristics of simple operation, high efficiency, reusable adsorbent, and less possibility of secondary pollution.²¹ They are suitable for the removal of Mn²⁺ and NH₄⁺–N with low concentrations. However, the available adsorbents are difficult to recycle due to their high cost and poor stability, which limit the development of adsorption technology.^22–24 Therefore, finding a low-cost adsorbent is essential to achieving the removal of Mn²⁺ and NH₄⁺–N with low concentrations.

Phosphate ore flotation tailings (POFTs) are solid wastes that are produced by the enrichment of P₂O₅ from low-grade phosphate ore.²⁵ Approximately 10 million tons of POFTs are released in China annually, and the massive accumulation of POFTs has brought about environmental pollution problems.^26,27 POFTs are rich in available elements such as P, Mg, Ca and Si, which are high-quality resources with potential for development and utilization.^28–30 Therefore, effective utilization of Ca, Mg, P and other resources in POFTs is essential for its resource utilization.

In this study, modified phosphate ore flotation tailings (MPOFTs) were used to remove Mn²⁺ and NH₄⁺–N. The influence of the concentration of MPOFTs, different pH, different reaction time, and different aeration intensity on the removal efficiency of Mn²⁺ and NH₄⁺–N was investigated. This study provided a new method for Mn²⁺ and NH₄⁺–N removal, and POFT resource utilization.

2 Materials and methods

2.1 Materials

The EMMW used in this study was collected from an electrolytic manganese enterprise in Guizhou Province in China. The POFTs were sampled from a phosphate mining company in Chongqing and stored at room temperature. Sodium hydroxide (NaOH), sulfuric acid (H₂SO₄), oxalic acid dihydrate (C₂H₂O₄·2H₂O), and sodium oxalate (Na₂C₂O₄) used in the experiment were produced by Chengdu Kelong Chemical Co., Ltd., and all chemical reagents were all of analytical grade.

2.2 Experimental processes

H₂C₂O₄-modified POFTs (MPOFTs #1): first, 0.5 mol L⁻¹ H₂C₂O₄ solution was prepared (solution #1). Following a solid–liquid ratio of 1 : 5, ground POFTs were added to solution #1. Second, the solid–liquid mixture was stirred adequately at a constant temperature of 353 K for 1.5 hours to obtain a slurry (slurry #1). The pH of slurry #1 was maintained at 8 with NaOH, and slurry #3 was obtained after 6 hours of reaction. Finally, H₂C₂O₄-modified POFTs were obtained.

Na₂C₂O₄-modified POFTs (MPOFTs #2): first, 0.5 mol L⁻¹ Na₂C₂O₄ solution was prepared (solution #2). Following a solid–liquid ratio of 1 : 5, ground POFTs were added to solution #1. Second, the solid–liquid mixture was stirred adequately at a constant temperature of 353 K for 1.5 hours to obtain a slurry (slurry #2). The pH of slurry #2 was maintained at 8 by NaOH, and slurry #4 was obtained after 6 hours of reaction. Finally, Na₂C₂O₄-modified POFTs were obtained.

Adsorption process of MPOFTs: the effects of the concentration of MPOFTs, initial solution pH, reaction time, and reaction temperature on the removal efficiencies of Mn²⁺ and NH₄⁺–N were explored. Based on the adsorption behavior of the MOPTs on Mn²⁺ and NH₄⁺–N in the EMMW, the adsorption kinetics and adsorption thermodynamics of NH₄⁺–N were studied.

The formulas of Mn²⁺ and NH₄⁺–N adsorption removal efficiencies are as follows:

where C_Mn²⁺ (mg L⁻¹⁻¹) and C_NH4⁺ (mg L⁻¹⁻¹) are the concentrations of Mn²⁺ and NH₄⁺–N. Subscripts 0 and t denote the initial and sampling time, respectively.

2.3 Adsorption kinetics and adsorption thermodynamics experiments

Since the Mn²⁺ in the EMMW was almost completely removed when the initial solution pH was 9.0, in this study, the adsorption of Mn²⁺ by the MPOFTs was no longer considered. The concentration of NH₄⁺–N, adsorption times and adsorption temperatures was measured. The initial solution pH of the system was adjusted with NaOH solution or H₂SO₄, and the solution was quickly separated from the adsorbent through a 0.22 μm aqueous filter membrane after the reaction.^31–33 The calculation formula and analysis methods are shown in the ESI.†

3 Results and discussion

3.1 Effect of POFT modification

As shown in Fig. 1, the concentration of Mn²⁺ in the EMMW decreased from 165.00 mg L⁻¹ to 85.07 mg L⁻¹, 89.53 mg L⁻¹ and 92.45 mg L⁻¹, respectively, when 5 g mL⁻¹ of MPOFTs #1, MPOFTs #2, and POFTs were added to the EMMW, respectively. The concentration of NH₄⁺–N decreased from 70.00 mg L⁻¹ to 35.29 mg L⁻¹, 41.77 mg L⁻¹ and 44.42 mg L⁻¹, respectively. The pH and concentrations of Mn²⁺ and NH₄⁺–N in the EMMW that was treated with MPOFTs were lower than those in the EMMW that was treated with POFTs. MPOFTs #1 outperformed MPOFTs #2 in processing Mn²⁺ and NH₄⁺–N in the EMMW. According to the results after modification, the POFT treatment effect on the EMMW improved, and the modification of H₂C₂O₄ had a better effect on the POFT adsorption capacity than the modification of Na₂C₂O₄. At the same time, the use of Na₂C₂O₄ introduced as Na⁺ into the system produced by-products and affected the treatment results. In summary, modification can improve the adsorption capacity of POFTs, and H₂C₂O₄-modified POFTs are more suitable for the treatment of EMMW.

Fig. 1 Treatment effect of POFT modification (1#: MPOFTs #1; 2#: MPOFTs #2 and 3#: POFTs).

3.2 Effects of the concentration of MPOFTs and the initial solution pH

In Fig. 2a, the concentrations of Mn²⁺ and NH₄⁺–N decreased from 102.05 mg L⁻¹ and 45.58 mg L⁻¹ to 48.49 mg L⁻¹ and 31.01 mg L⁻¹, respectively, and the final solution pH of the system increased from 7.57 to 8.34, when the concentration of MPOFTs was increased from 1 g L⁻¹ to 30 g L⁻¹. The results indicated that the concentration of MPOFTs was proportional to the removal effect of Mn²⁺ and NH₄⁺–N. The surface area and the number of adsorption sites were increased by the increase of the concentration of MPOFTs. The MPOFTs were weakly alkaline, so when the concentration of MPOFTs was increased, the pH of the system increased accordingly. In Fig. 2b, when the initial solution pH increased from 8.0 to 9.5, the concentration of Mn²⁺ that remained in the EMMW decreased from 85.07 mg L⁻¹ to 0.001 mg L⁻¹, and the concentration of NH₄⁺–N decreased from 35.29 mg L⁻¹ to 18.96 mg L⁻¹. The results showed that weakly alkaline reaction condition were more suitable for the adsorption system. In summary, when the concentration of MPOFTs was 5 g L⁻¹, the reaction time was 1 h, the reaction temperature was 293 K, and the initial solution pH was 9.0, the concentrations of Mn²⁺ and NH₄⁺–N in the EMMW were 0.001 mg L⁻¹ and 33.60 mg L⁻¹, respectively, and the final solution pH of the system was 8.41.

Fig. 2 Effects of the concentration of MPOFTs and the initial solution pH (a: the concentration of MPOFTs; b: the initial solution pH).

3.3 Effects of the reaction time and reaction temperature

When the reaction time increased from 0.5 h to 3.0 h, the concentration of Mn²⁺ was decreased, and NH₄⁺–N decreased from 41.20 mg L⁻¹ to 21.78 mg L⁻¹, as well as the final solution pH decreasing from 8.48 to 8.16 (Fig. 3a). The final solution pH of the reaction was 8.25, the concentration of Mn²⁺ was 0.001 mg L⁻¹ and the concentration of NH₄⁺–N was 22.20 mg L⁻¹, when the reaction time was 2.5 h. The concentration of NH₄⁺–N did not meet the Integrated Wastewater Discharge Standard (GB8978-1996); hence, the effect of the reaction temperature on the adsorption results was studied and next explored. In Fig. 3b, as the reaction temperature increased, the concentration of Mn²⁺ remained at 0.001 mg L⁻¹. When the reaction temperature increased from 293 K to 343 K, the concentration of NH₄⁺–N decreasing from 22.20 mg L⁻¹ to 0.001 mg L⁻¹. It could be shown that an increase in temperature could enhance the decreases in the concentrations of Mn²⁺ and NH₄⁺–N in the EMMW. Because of the energy consumption, 323 K was selected as the reaction temperature.

Fig. 3 Effects of the reaction time and reaction temperature (a: reaction time; b: reaction temperature).

3.4 Adsorption isotherms, kinetics and thermodynamics

The adsorption isotherm curve is shown in Fig. S1,† and the fitting result of the adsorption isotherm model is shown in Table 1. The “q_m” and “K_L” values of the adsorption isotherm obtained by the Langmuir model fitting were 2.0698 (mg g⁻¹) and 61.0809 (L mg⁻¹), respectively. The “K_F” and “n” values of the adsorption isotherm obtained by the Freundlich model fitting were 0.3051 (mg g⁻¹) and 1.1467 (g L⁻¹), respectively. And the “n” value was greater than 1, indicating that NH₄⁺–N was suitable for adsorption by MPOFTs. The “R²” values obtained by the Langmuir model and Freundlich model fitting were 0.9155 and 0.9966, respectively. Therefore, the adsorption behavior of MPOFTs on NH₄⁺–N was more in line with the description of the Freundlich model. This adsorption process is dominated by uneven surface adsorption.

Table 1 The data of adsorption isotherm, adsorption kinetics, Weber's intraparticle diffusion model and adsorption enthalpy model

Isotherms	Parameters	R ^2
Langmuir model	q m (mg g^−1) = 2.0698	0.9155
	K L (L mg^−1) = 61.0809
Freundlich model	K F (mg g^−1) = 0.3051	0.9966
	n (g L^−1) = 1.1467
The quasi-first-order adsorption model	q e1 (mg g^−1) = 7.3342	0.9914
	K 1 (min^−1) = 0.0311
The quasi-second-order adsorption model	q e2 (mg g^−1) = 10.5574	0.9440
	K 2 (min^−1) = 0.1193
Weber's intraparticle diffusion model	C 1 (mg g^−1) = 5.2507	0.9842
	K i1 (mg g^−1 min^−1/2) = 0.6420
	C 2 (mg g^−1) = 10.3179	0.9034
	K i2 (mg g^−1 min^−1/2) = 0.1970
Adsorption enthalpy model	ΔH (kJ mol^−1) = 67.754	0.9186
	C = −24.1555

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The adsorption capacity of MPOFTs on the NH₄⁺–N in the EMMW increased first and then tends to be flat (Fig. S2a†). When the reaction time was longer than 150 minutes, the adsorption capacity reaches equilibrium. This is because in the initial stage of adsorption, the adsorption sites in the MPOFTs were not saturated, and at the same time the concentration of the adsorbent and the NH₄⁺–N in the solution was very different, the mass transfer resistance was small, and the adsorbent can quickly adsorb in the EMMW. The adsorption capacity of NH₄⁺–N increased rapidly. With the extension of the adsorption time, the available NH₄⁺–N adsorption sites are occupied, at the same time the concentration gradient difference was reduced, and the mass transfer resistance increased, resulting in the slowdown of the adsorption rate and the adsorption capacity tending to a stable value. As shown in Fig. S2b† and Table 1, the “q_e” values of the quasi-first-order adsorption model and the quasi-second-order adsorption model were 7.3342 (mg g⁻¹) and 10.5574 (mg g⁻¹), respectively, and the “R²” values were 0.9914 and 0.9440, respectively. The fitting degree of the quasi-first-order adsorption model is better than that of the quasi-second-order adsorption model. Therefore, the quasi-first-order adsorption model is more suitable for describing the adsorption behavior of MPOFTs on NH₄⁺–N, indicating that the adsorption behavior is the same as physical adsorption and chemical adsorption. As shown in Fig. S2d† and Table 1, the internal diffusion behavior can be divided into two stages. The first stage was the diffusion process of NH₄⁺–N to the surface of the MPOFTs, and the second stage was the internal diffusion process of NH₄⁺–N. The “K_i1” is greater than “K_i2”, indicating that the first stage is a fast process, while the second stage is a gradual process. The “R²” values of the two stages were 0.9842 and 0.9034, respectively, indicating that the Weber's intraparticle diffusion model fitting can describe the adsorption behavior.³⁴

During the adsorption process, the free energy changes are shown in Table S1.† As shown in Fig. S3† and Table 1, ΔG was a negative number in the reaction temperature range of 293 K to 343 K, indicating that this behavior was favorable and spontaneous from the perspective of adsorption thermodynamics. ΔH was 67.754 kJ mol⁻¹, and the adsorption enthalpy became a positive number, indicating that the process was an endothermic reaction. The saturated adsorption capacity increased with the increase of temperature. Increasing the temperature was beneficial to the effect of MPOFTs on the adsorption behavior of NH₄⁺–N in the EMMW.

3.5 Removal mechanism

In Table S2,† the contents of CaO, MgO, P₂O₅, SO₃, SiO₂, F and Fe₂O₃ were 57.26%, 16.89%, 9.78%, 8.16%, 3.53%, 1.70% and 1.06%, respectively, with a total content of 98.38%. The MPOFTs were composed mainly of CaO, MgO, P₂O₅, SO₃, SiO₂ and Fe₂O₃, the contents of these elements were 64.13%, 22.14%, 6.52%, 0.73%, 3.15% and 1.67%, respectively, with a total content of 98.34%. The contents of P, S and F were decreased after modification. The results show that H₂C₂O₄ can reduce the contents of P, S and F in POFTs. Fig. 4 shows that the relative pressure P/P₀ was in the range of 0.6 to 0.8. The POFTs before and after modification exhibit obvious hysteresis loops. Therefore, the curve is a type IV curve, which proves that the POFTs before and after modification had a mesoporous structure.³⁵ When the relative pressure P/P₀ is in the range of 0.8 to 1.0, the hysteresis loop has an upward trend, indicating that the structure was mesoporous with a larger pore size. The POFTs had an obvious pore size distribution from 2.41 nm to 18.61 nm, indicating that the POFTs had a mesoporous structure, but the pore size was too small, which was not conducive to adsorption (Fig. 4). The MPOFTs had an obvious pore size distribution from 9.21 nm to 33.06 nm, which was beneficial for adsorption. In Table S3,† the BET surface area and BJH desorption pore volume of the MPOFTs were 6.5239 m² g⁻¹ and 0.038057 cm³ g⁻¹, respectively, which were larger than those of the POFTs before modification. The above results show that the MPOFTs are more suitable for use as an adsorbent.

Fig. 4 BET analysis of the POFTs and MPOFTs.

In Fig. 5, XRD results indicated that the main components of the POFTs were magnesite ((Mg,Fe)CO₃), dolomite (CaMg_0.77Fe_0.23(CO₃)₂, CaMg(CO₃)₂), gypsum (CaSO₄·2H₂O), fluorapatite (Ca₅F(PO₄)₃, Ca_5.164F_0.959P_2.892O_11.523) and quartz (SiO₂). The main components of the MPOFTs were whewellite (CaC₂O₄·H₂O, Ca(COO)₂·H₂O), calcium phosphate (Ca₂P₂O₇), newberyite (MgHPO₄·3H₂O), collinsite (MgCa₂(PO₄)₂·2H₂O) and sodium magnesium phosphate (MgNaPO₄·6H₂O). The peaks of CaMg(CO₃)₂, Ca₅(PO₄)₃F, (Mg, Fe)CO₃, Ca_5.164F_0.959P_2.892O_11.523 and CaMg_0.77Fe_0.23(CO₃)₂ decreased in number and weakened, and the characteristic peaks of CaSO₄·2H₂O and Ca₅F(PO₄)₃ disappeared. The results show that CaSO₄·2H₂O, Ca₅F(PO₄)₃, (Mg, Fe)CO₃, CaMg(CO₃)₂, CaMg_0.77Fe_0.23(CO₃)₂ and Ca₅(PO₄)₃F in the POFTs were decomposed during the modification process, and then Ca²⁺, PO₄³⁻ and Mg²⁺ were transformed into their free states and reacted with C₂O₄²⁻, and eventually formed CaC₂O₄·H₂O, Ca₂P₂O₇, MgHPO₄·3H₂O, MgCa₂(PO₄)₂·2H₂O and MgNaPO₄·6H₂O. The main reaction equation is shown in eqn (3)–(11):

CaMg(CO₃)₂ + 4H⁺ → Ca²⁺ + Mg²⁺ + 2H₂O + 2CO₂ (3)

Mg²⁺ + NaOH + PO₄³⁻ + Ca²⁺ + 6H₂O → MgNaPO₄·6H₂O + Ca(OH)₂ (4)

Mg²⁺ + 2PO₄³⁻ + 2Ca²⁺ + 2H₂O → MgCa₂(PO₄)₂·2H₂O (5)

Ca(OH)₂ + H₂C₂O₄ → Ca(COO)₂·H₂O + H₂O (6)

Mg²⁺ + PO₄³⁻ + 4H₂O → MgHPO₄·3H₂O + OH⁻ (7)

Ca(OH)₂ + H₂C₂O₄ → CaC₂O₄ + 2H₂O (8)

H₂C₂O₄ + CaSO₄·2H₂O + 2NaOH → CaC₂O₄ + Na₂SO₄ + 2H₂O (9)

2Ca⁺ + 2PO₄³⁻ + H₂O → Ca₂P₂O₇ + 2OH⁻ (10)

Ca₅F(PO₄)₃ + 5H₂C₂O₄ + 10NaOH → NaF + 3Na₃PO₄ + 5CaC₂O₄ + 10H₂O (11)

Fig. 5 XRD analysis of different samples (1#: POFTs; 2#: MPOFTs; 3#: MPOFTs after adsorption).

The main substances in MPOFTs after separate adsorption treatment were dolomite (CaMg(CO₃)₂, CaMg_0.77Fe_0.23(CO₃)₂), whewellite (CaC₂O₄·H₂O), calcium phosphate (Ca₂P₂O₇), manganese hydroxide (Mn(OH)₂), struvite (Mg(NH₄)PO₄·6H₂O) and quartz (SiO₂). By comparing 2# and 3#, the characteristic peaks of MgNaPO₄·6H₂O disappeared, and the characteristic peaks of Mn(OH)₂, switzerite (Mn₃(PO₄)₂·7H₂O), hausmannite (Mn₃O₄), manganite (MnOOH) and Mg(NH₄)PO₄·6H₂O appeared,⁷ indicating that Mn²⁺ was removed via precipitation in the form of Mn(OH)₂, Mn₃(PO₄)₂·7H₂O, Mn₃O₄ and MnOOH. Combining the results of adsorption kinetics and adsorption thermodynamics, it can be seen that NH₄⁺–N was removed mainly via adsorption and ion-exchange with MgNaPO₄·6H₂O and MgHPO₄·3H₂O; and some NH₄⁺–N was removed by struvite.

In Fig. 6a and b, the POFTs formed loose and united flocs, whereas the MPOFTs had a more compact structure, with obvious pore-like structures and flake and block-like substances on the surface, these substances could be CaC₂O₄·H₂O, Ca₂P₂O₇ and a small amount of MgNaPO₄·6H₂O combined with the XRD results. The EDS diagram shows that the POFT surface mainly contained elements Ca, Mg, O, and small amounts of P, Si, S and F. The types of the main elements did not change significantly, and the contents of P, Si, S and F were reduced in MPOFTs. As shown in the SEM image, compared with the MPOFTs, many spherical particle substances appeared on the surface, and these substances could be Mn(OH)₂ and Mg(NH₄)PO₄·6H₂O (Fig. 6c). As shown in the EDS diagram, the contents of Ca and O increased significantly, and the remaining elements did not differ substantially from those of the MPOFTs, but a small amount of Mn appeared, indicating that the Mn²⁺ in the EMMW was adsorbed into the MPOFTs.

Fig. 6 SEM and EDS images of analysis (a: POFTs; b: MPOFTs; c: MPOFTs after adsorption).

In Fig. 7, the absorption peaks at 2524.36 cm⁻¹ and 1048.11 cm⁻¹ were caused by H₂PO₄⁻ and PO₄³⁻ group stretching vibration, respectively, and the double peak in the range of 604.16 cm⁻¹ to 519.24 cm⁻¹ was produced by the PO₄³⁻ group. The broad peak at 881.41 cm⁻¹ and 1436.90 cm⁻¹ were caused by carbonate stretching vibration, and the absorption characteristic peak at 729.01 cm⁻¹ was produced by the vibration of the SO₄²⁻ group. Compared with the POFTs, the characteristic peak of carbonate absorption at 1436.90 cm⁻¹ declined after modification, indicating that the carbonate content decreased. The absorption peak of hydroxyl was identified at 1620.81 cm⁻¹, and the characteristic peak of oxalate was produced at 881.42 cm⁻¹, which was the absorption peak of the oxalate group, indicating that after modification by oxalic acid, the metal ions in the POFTs formed new substances. At 781.06 cm⁻¹, a distinct characteristic vibration peak of the H₂PO₄⁻ group appeared, which indicated that phosphate appeared after modification. The characteristic peak of SO₄²⁻ group absorption at 729.01 cm⁻¹ was significantly reduced, indicating that the sulfate content in the MPOFTs was reduced. The characteristic peaks of H₂PO₄⁻ groups in the MPOFTs were weakened, and the characteristic peaks of PO₄³⁻ groups were enhanced and increased, indicating that more H₂PO₄⁻ groups were converted to PO₄³⁻ groups. Sample 3# had a characteristic absorption peak of the hydroxyl group of bound water at 3444.08 cm⁻¹. The absorption peak at 2524.36 cm⁻¹ was caused by the vibration of the H₂PO₄⁻ group; this peak was roughly the same as that for the MPOFTs. At 1620.81 cm⁻¹, the hydroxyl characteristic peak of the sample shifted and became a broad peak, thereby indicating that the hydroxide content increased, and the characteristic absorption peak of SO₄²⁻ at 729.01 cm⁻¹ declined, indicating that the ion originally combined with sulfate radicals.

Fig. 7 FTIR spectra of the samples (1#: POFTs; 2#: MPOFTs; 3#: MPOFTs after adsorption).

In Table S4,† the concentrations of Mg²⁺, Mn²⁺ and Ca²⁺ in the leachate of the POFTs were 22.38 mg L⁻¹, 0.03 mg L⁻¹ and 3.03 mg L⁻¹, respectively. The concentration of Mg²⁺ in the leachate of the MPOFTs sharply increased to 108.32 mg L⁻¹, Mn²⁺ was not detected, and the concentration of Ca²⁺ decreased to 0.01 mg L⁻¹. The result shows that Mg²⁺ was more easily washed out, and Mn²⁺ and Ca²⁺ formed a stable compound after modification. Compared with the sample 3# and sample 4#, the concentrations of NH₄⁺–N, Mn²⁺, and Ca²⁺ decreased to 1.73 mg L⁻¹, 0.001 mg L⁻¹ and 5.85 mg L⁻¹, respectively, and the concentrations of Zn²⁺, Sb²⁺ and Co²⁺ met the Integrated Wastewater Discharge Standard (GB8978-1996).

According to XRF, BET, XRD, SEM-EDS-mapping, FT-IR and ICP analysis, and combining with the other research results.^7,19,36–38 The reaction equations of the Mn²⁺ and NH₄⁺–N removal process can be obtained as follows:

Mn²⁺ + 2OH⁻ → Mn(OH)₂ (12)

4Mn(OH)₂ + 2O₂ + 2H⁺ → MnOOH + Mn₃O₄ + 5H₂O (13)

(Ca²⁺, Mg²⁺) + 2OH⁻ → (Ca, Mg)(OH)₂ (14)

3Mn²⁺ + 2PO₄³⁻ + 7H₂O → Mn₃(PO₄)₂·7H₂O (15)

Mg²⁺ + NH₄⁺ + PO₄³⁻ + 6H₂O → Mg(NH₄)PO₄·6H₂O (16)

MgNaPO₄·6H₂O + NH₄⁺ → Mg(NH₄)PO₄·6H₂O + Na⁺ (17)

MgHPO₄·3H₂O + NH₄⁺ + 3H₂O → Mg(NH₄)PO₄·6H₂O + H⁺ (18)

4 Conclusions

In this study, the POFTs were modified to treat low-concentration NH₄⁺–N and Mn²⁺ from EMMW. During the process of H₂C₂O₄ modification, gypsum, fluorapatite, magnesite and dolomite in the POFTs were transformed into whewellite, calcium phosphate, newberyite, collinsite and sodium magnesium phosphate. The results show that the concentration of Mn²⁺ and NH₄⁺–N in the EMMW decreased from 165 mg L⁻¹ to 0.001 mg L⁻¹ and from 70 mg L⁻¹ to 1.73 mg L⁻¹, respectively, when the solid–liquid ratio of MPOFTs to EMMW was 1 : 200, the reaction temperature was 323 K, the reaction time was 2.5 h and the initial solution pH was 9.0. NH₄⁺–N was mainly removed via adsorption and ion-exchange with MgNaPO₄·6H₂O and MgHPO₄·3H₂O; and some NH₄⁺–N was removed by struvite. Mn²⁺ was mainly removed by precipitation such as manganese phosphate and Mn(OH)₂. The adsorption behavior of NH₄⁺–N is a spontaneous endothermic reaction, and the increase of temperature is beneficial to the occurrence of the adsorption behavior. This study provides a new method and idea for EMMW treatment.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (52174386), the Science and Technology Plan Project of Sichuan Province (2021YFH0058), and the Opening Project of Key Laboratory of Solid Waste Treatment and Resource Recycle (SWUST), Ministry of Education, Southwest University of Science and Technology (21kfgk02).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2ew00507g

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