Evaluation of the Mechanism of Phosphate Removal Using Oyster Shell Powder in Aqueous Environments

Abstract

Eutrophication is caused by the inflow of nutrients, such as phosphorus and nitrogen, into closed waterbodies from wastewater. Calcination of oyster shells greatly increases their capacity for phosphate removal; however, information available on this mechanism and the capacity for phosphate removal under different initial pH values and temperatures is less. Herein, we investigated the utilization of oyster shells for phosphate removal under different pH and temperature conditions. Oyster shell powder (OSP) was calcined in a muffle furnace at temperature ranges of 200–1000 °C. Each OSP sample was added to a phosphate solution and the suspension was shaken under different pH and temperature conditions. The main component of OSP changed from CaCO₃ to CaO after calcination at approx. 800 °C. The amount of phosphate removal by the calcined OSPs at 800 and 1000 °C was higher than that removal by the other OSPs. Further, the amount of calcium elution from the OSPs calcined at 800 and 1000 °C was higher than that elution from the other OSPs. This was because the solubility of CaO was higher than that of CaCO₃. The amount of phosphate removal by the OSP and calcined OSPs at 200–600 °C was the highest at pH 5–7, and increased with increasing reaction temperature. These findings suggested that the mechanism of phosphate removal may involve adsorption in the OSP and OSPs calcined at 200–600 °C, whereas it is associated with coagulation settling and adsorption in the OSPs calcined at 800 and 1000 °C.

Introduction

With the rapid development of the mariculture industry, global mariculture production of oysters has been more than approx. 5.3 million tons.¹⁾ However, only some oyster shells are effectively utilized while most are discarded after incineration.¹⁾ The accumulation sites of oyster shells may release odor because gases (such as H₂S, amines, and NH₃) are produced from the flesh or microbes attached to the shells.²⁾ In South Korea, illegal dumping of oyster shells is an environmental and social problem.³⁾ Thus, developing new methods to reduce the number of oyster shells and utilize them is necessary.

Phosphorus is one of the three main macronutrients of fertilizers and is indispensable for crop production.⁴⁾ In Japan, phosphorus rock supplies are almost entirely dependent on imports and will be depleted worldwide in the coming decade.⁵⁾ Eutrophication, which is an aquatic environmental problem in Japan, is caused by the inflow of nutrients, such as phosphorus and nitrogen, into closed waterbodies from domestic and industrial wastewater.⁶⁾ The progression of eutrophication consequently leads to red tides and blue-green algae, which can be harmful to mariculture and tourism industries. Therefore, recovering and recycling phosphorus from environmental waters is necessary.

Approximately 96% of the oyster shells are composed of CaCO₃, which is used as a building material and calcium supplement,⁷⁾ and for eliminating phosphorus from water.^3,8–16) Calcination of oyster shells at 800 °C greatly increases their capacity for phosphorus removal,¹⁷⁾ and optimal phosphate removal occurs at pH 11.⁸⁾ Additionally, the amount of phosphate removed by non-calcined oyster shells increases with increasing solution temperature from 20 to 30 °C.⁹⁾ Although previous studies have investigated the phosphate removal capacity of untreated or calcined oyster shells (at 800 °C), knowledge on the phosphate removal capacity at different calcination temperatures is unclear. The phosphate removal mechanism may be associated with the formation of hydroxyapatite,¹⁷⁾ and calcium elution may be associated with this mechanism. However, little is known about the comprehensive mechanism and capacity for phosphorus removal under various conditions, such as initial pH and temperature, and its relationship with calcium elution.

Accordingly, to bridge these gaps, the present study aimed to assess the utilization of oyster shells for phosphorus removal and investigate the capacity and mechanism of oyster shells to remove phosphorus under different pH and temperature conditions.

Experimental

Materials and Chemicals

Potassium dihydrogen phosphate, HCl, and NaOH were purchased from FUJIFILM Wako Pure Chemical Corporation (Osaka, Japan). Oyster shell powder (OSP) was obtained by washing the oyster shells acquired from Miyagi, Japan, with purified water, drying them at 50 °C for 24 h, and then pulverizing them until the particle size was less than 500 µm. The OSP was thermally treated in a muffle furnace at 200, 400, 600, 800, and 1000 °C for 2 h, and the corresponding samples were denoted as OSP200, OSP400, OSP600, OSP800, and OSP1000, respectively. Pure water was used as the experimental system.

Physicochemical Properties of OSP

The physical properties before phosphate removal were evaluated. Scanning electron microscope (SEM; SU1510, Hitachi High-Technologies Corporation, Japan) was used to evaluate the surface morphology of the OSP (acceleration voltage: 15.0 keV; beam diameter: 5 µm). Further, thermogravimetric-differential thermal analysis (TG-DTA) was performed using a TG8210 (Rigaku, Japan) instrument with the following measurement conditions (target temperature: 1000 °C; atmosphere: 150–200 mL/min; heating time: 10 °C/min; sampling time: 1 s).

The physical properties before and after phosphate removal were also evaluated. X-ray diffraction (XRD; MiniFlex II, Rigaku, Japan) was used to analyze the difference in the crystal phase of the OSP (radiation source: CuKα; tube voltage: 30 kV; tube current: 15 mA). Electron probe microanalysis (JXA-8530F, JEOL, Tokyo, Japan) was performed to determine phosphorus distribution on the OSP surface (acceleration voltage: 15.0 keV; beam diameter: 5 µm). An X-ray photoelectron spectroscopy system (AXIS-NOVA, Shimadzu Corporation, Japan) with Al radiation was used to determine the binding energy of phosphorus at the OSP. The above experiments of phosphorus removal were performed under the following conditions: initial concentration: 1000 mg/L, solvent volume: 50 mL, weight of each OSP: 0.5 g, temperature: 25 °C, contact time: 24 h, and agitation speed: 100 rpm.

Phosphate Removal Capacity and the Amount of Calcium Elution

Each OSP sample (0.05 g) was added to 50 mL phosphate solution (1500 mg/L). The suspension was shaken at a reaction temperature of 25 °C and agitation speed of 100 rpm for 24 h using a water bath shaker, and then filtered through a 0.45-µm membrane filter. The phosphate concentration in the filtrate was measured using inductively coupled plasma–optical emission spectroscopy (ICP–OES; iCAP-7600, Thermo Fisher Scientific Inc., Japan). The amount of phosphate removed was calculated as follows:

  

(1)

where, q is the amount removed (mg/g), C₀ is the initial concentration (mg/L), C_e is the equilibrium concentration (mg/L), V is the solvent volume (L), and W is the weight of each OSP (g). Additionally, the change in the phosphate removal capacity of each OSP was evaluated under different experimental conditions. To examine the effect of pH, the pH of the phosphate solution (100 mg/L) was adjusted to 3, 5, 7, 9, and 11 using HCl or NaOH. The pH of the solution was measured using a digital pH meter (F-73; Horiba Ltd., Japan). To examine the effect of temperature, the reaction temperature of the phosphate solution (100 mg/L) was set to 5, 25, and 45 °C.

To determine the amount of eluted calcium, each OSP sample (0.05 g) was added to 50 mL purified water, the suspension was shaken, and the calcium concentration in the filtrate was measured using ICP–OES. The pH and temperature conditions were identical to those described above. These experiments were performed in duplicate to obtain the mean amount of phosphate removal and calcium elution.

Results and Discussion

Physicochemical Properties of Each OSP before Phosphate Removal

Figure 1 shows the SEM images of each OSP before phosphate removal. OSP, OSP200, and OSP400 consisted of a thin layer and a dense structure. OSP600, OSP800, and OSP1000 comprised spherical particles with smooth surfaces, suggesting that calcination above 600 °C affected the OSP particle structure. OSP before calcination consists of a dense particle distribution, whereas this structure changes to peanut-shaped grains after calcination above 800 °C.¹⁰⁾

Fig. 1. SEM Images of Each OSP before Removed

Figure 2 shows the TG-DTA profiles of each OSP before phosphate removal. Thermogravimetric measurements significantly decreased at approx. 700–830 °C, and the DTA curve showed an endothermic peak at approx. 800 °C. This combination of TG-DTA curves suggested the occurrence of dehydration and degradation reactions; namely, CaCO₃ decarboxylation occurred at approx. 700–830 °C, followed by CaO formation. Furthermore, the TG-DTA results were consistent with those of a previous study.¹⁸⁾

Fig. 2. Thermal Analysis of the OSP under an Air Atmosphere

Physicochemical Properties of Each OSP before and after Phosphate Removal

Figure 3 shows the XRD patterns of each OSP before and after phosphate removal. Before phosphate removal, the peak pattern of CaCO₃ was observed in OSP, OSP200, OSP400, and OSP600, those of CaCO₃ and CaO were observed in OSP800, and only that of CaO was observed in OSP1000. These results were similar to those reported previously.¹⁰⁾ Peak patterns of CaCO₃ and CaO were observed in OSP800 because of the transformation of CaCO₃ into CaO at a calcination temperature of 800 °C. After phosphate removal, the peak pattern of CaCO₃ was observed in OSP, OSP200, OSP400, and OSP600 as before phosphate removal. After phosphate removal in OSP800, peak patterns of CaCO₃ and Ca(OH)₂ were detected, and a light peak pattern of Ca₅(PO₄)₃(OH), a type of hydroxyapatite, was observed. After phosphate removal in OSP1000, Ca(OH)₂ and Ca₅(PO₄)₃(OH) peak patterns were observed. Calcium phosphate exists in various forms under different pH conditions, such as calcium monohydrogen phosphate at pH 4–6, a mixture of calcium monohydrogen phosphate and hydroxyapatite at pH 6–7, and hydroxyapatite at pH >7. These findings suggested that the peak pattern of Ca₅(PO₄)₃(OH) was detected because the pH value after phosphate removal by OSP800 and OSP1000 was approx. 7.

Fig. 3. XRD Patterns of Each OSP (a) before and (b) after Phosphate Removal

Figure 4 shows the binding energies of phosphorus before and after phosphate removal by each OSP. After phosphate removal, P(2p) and P(2s) peaks were detected in all the OSPs, and this result was consistent with that of a previous study.¹⁷⁾ Figure 5 shows the qualitative analysis of each OSP surface before and after phosphate removal. After phosphate removal, the phosphorus intensity increased in all the OSPs. These results indicated that phosphate was adsorbed onto the OSP surface.

Fig. 4. Binding Energies of Phosphorus before and after Phosphate Removal by Each OSP

Fig. 5. Qualitative Analysis of Each OSP Surface before and after Phosphate Removal

Comparison of Saturated Amount of Phosphate Removal by Each OSP

Figure 6 shows the saturated amount of phosphate removal by each OSP. The saturated amounts of phosphate removal by OSP, OSP200, OSP400, OSP600, OSP800, and OSP1000 were 154.0, 259.2, 213.7, 250.9, 320.6, 379.5 mg/g, respectively. The amount of phosphate removal by OSP1000 was the highest among all OSPs. The pH after phosphate removal was approx. 6.5, regardless of the OSP type. The phosphate removal capacity of OSP was previously reported to increase when the calcination temperature exceeded 800 °C,¹⁷⁾ and the same trend was observed in our study. This may be due to a change in the main component of the OSP from CaCO₃ (with low solubility) to CaO (with high solubility).

Fig. 6. Saturated Amount of Phosphate Removal by Each OSP

Initial concentration: 1500 mg/L, sample volume: 50 mL, adsorbent: 0.05 g, temperature: 25 °C, contact time: 24 h, and agitation speed: 100 rpm.

Effect of pH and Temperature on the Amount of Phosphate Removal by Each OSP

Figures 7 and 8 show the effects of pH and temperature on the amounts of phosphate removal by each OSP and calcium elution from each OSP. In OSP, OSP200, OSP400, and OSP600, the amount of phosphate removal was higher at pH 5–7 than at pH 9–11. This may be because H₂PO₄⁻ and HPO₄²⁻ are more abundant form among the ionic forms of phosphoric acid at pH 5–7, which tend to form hydroxyapatite. Meanwhile, a previous study reported that the optimal phosphate removal by OSP800 occurred at pH 11,⁸⁾ which was consistent with our results for OSP800. These results suggested that the optimum pH for phosphate removal varies depending on the calcination temperature of OSP. Additionally, the amount of phosphate removal by OSP–OSP600 increased with increasing adsorption temperature. This was similar to that of a previous study, which reported that the amount of phosphate removal by non-calcined oyster shells increased with increasing solution temperature from 20 to 30 °C.⁹⁾ This may be because the reaction of OSP decarboxylation (5CaCO₃ + 3H₂PO₄→Ca₅(PO₄)₃(OH) + 5CO₂ + 4H₂O) occurred with increasing reaction temperature in phosphoric acid solution. In OSP800 and OSP1000, the amount of phosphate removal was higher than that removal by the other OSPs under all the pH and temperature conditions. A previous study reported that the calcination of oyster shells at 800 °C greatly increased their capacity for phosphorus removal, and this mechanism is associated with another mechanism besides adsorption.¹⁷⁾ The results of our study showed the same tendency.

Fig. 7. Effect of pH and Temperature on the Amount of Phosphate Removal by Each OSP

(A) Initial concentration: 100 mg/L, initial pH: 3, 5, 7, 9, and 11, sample volume: 50 mL, adsorbent: 0.05 g, temperature: 25 °C, contact time: 24 h, and agitation speed: 100 rpm. (B) Initial concentration: 100 mg/L, sample volume: 50 mL, adsorbent: 0.05 g, temperature: 5, 25, and 45 °C, contact time: 24 h, and agitation speed: 100 rpm.

Fig. 8. Effect of pH and Temperature on the Amount of Calcium Elution from Each OSP

(A) Initial pH: 3, 5, 7, 9, and 11, sample volume: 50 mL, adsorbent: 0.05 g, temperature: 25 °C, contact time: 24 h, and agitation speed: 100 rpm. (B) Sample volume: 50 mL, adsorbent: 0.05 g, temperature: 5, 25, and 45 °C, contact time: 24 h, and agitation speed: 100 rpm.

Under all pH and temperature conditions, the amount of calcium elution from OSP800 and OSP1000 was higher than that elution from OSP–OSP600. Previous studies have reported that the solubility of CaO is higher than of CaCO₃.^19,20) Thus, the amount of calcium elution from OSP800 and OSP1000 might be higher than that elution from the other OSPs because OSP800 and OSP1000 contain CaO. Additionally, the amount of calcium elution from each OSP was higher at pH 3–5 than at pH 7–11, which was consistent with a previous study that reported an increase in the solubility of CaCO₃ under acidic conditions.²⁰⁾

These findings suggested that the mechanism of phosphate removal may involve surface adsorption in OSP–OSP600 with low-calcium elution, whereas it is associated with other mechanisms, such as coagulation settling and adsorption in OSP800 and OSP1000 with high-calcium elution. Particularly, the phosphate removal mechanism may be associated with coagulation settling in the 5 °C temperature range. In OSP–OSP600, the phosphate removal efficiency may increase at high temperatures. The amount of phosphate removal by OSP decrease in the presence of coexisting anions,¹⁷⁾ and those of our study will be lower in real environmental water. Further research is needed to confirm the capacity of phosphate removal by OSP in coexisting anions. However, a previous study has suggested that phosphate can be recovered using OSP for conversion to fertilizer and can contribute to reducing the consumption of phosphorus for fertilizers.¹⁷⁾ The results of this study reveal that the optimum conditions of the phosphate removal by each OSP, and will be fundamental data to solving the shortage of phosphorus rock.

Conclusion

In this study, we evaluated the utilization of oyster shells for phosphate removal under different pH and temperature conditions. The TG-DTA and XRD results showed that the main component of OSP changed from CaCO₃ to CaO after calcination at approx. 800 °C. The amount of phosphate removal by OSP and calcium elution from OSP increased when the calcination temperature of OSP exceeded 800 °C. This was because the solubility of CaO was higher than that of CaCO₃. The amount of phosphate removal by OSP800 and OSP1000 was higher than that removal by the other OSPs under all pH conditions. Additionally, the amount of phosphorus removal by OSP–OSP600 increased with increasing temperature, and was higher at pH 5–7 than at pH 9–11. These findings suggested that the phosphate removal mechanism may involve surface adsorption on OSP–OSP600, whereas it may be associated with coagulation settling and adsorption on OSP800 and OSP1000.

Conflict of Interest

The authors declare no conflict of interest.

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