Removal of phosphate and hexavalent chromium from aqueous solutions by engineered waste eggshell

Abstract

In this work, a novel adsorbent derived from waste eggshell (ES) was used for phosphate and hexavalent chromium adsorption. EDS, XRD, and FTIR analyses demonstrated that α-FeOOH was successfully loaded onto the eggshell, thus increasing the available sorption sites and facilitating the adsorption abilities of phosphate and hexavalent chromium. The maximum phosphate and hexavalent chromium adsorption capacities on α-FeOOH modified eggshell (F-ES) were 248.73 and 41.57 mg g⁻¹, which were much greater than that of the original eggshell 89.74 and 11.81 mg g⁻¹, indicating that the modification process significantly improved the phosphate and hexavalent chromium removal abilities. Besides, the kinetics data were both well fitted with pseudo-first-order, pseudo-second-order, and Richie kinetics models, which proved that the adsorption process onto the eggshell was controlled by multiple mechanisms. In addition, the removal rates of phosphate and hexavalent chromium increased with increasing adsorbent dosage. However, the removal rates showed downward trends with pH increased from 3 to 9 and ionic strength increased from 0 to 0.1 M.

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1. Introduction

Although phosphate is an essential nutrient for biomass growth in most aquatic environments, an excessive presence of it will bring about some undesirable effects such as low water quality, outbreak of algalbloom, decrease of dissolved oxygen etc.^1,2 Especially, the generated cyanotoxins may pose a risk to human health. Therefore, it is of vital importance to explore effective technologies for phosphate removal before discharging it to aquatic environments.^3,4

Cr(VI) is one of the most dangerous pollutants in aquatic environments due to its strong oxidizing capacity and dissolving power in water. This pollutant can exist as Cr₂O₇²⁻, CrO₄²⁻, and HCrO₄⁻ in water and always causes gastric damage, lung cancer, kidney damage, and etc. to human bodies.^5,6 Although WHO and USEPA recommend that the maximum concentration of total chromium is 100 μg L⁻¹ in drinking water environment,⁷ the real concentrations in natural waters are much higher than the recommended value. Hence, Cr(VI) remediation becomes an essential issue waiting to be solved by effective and economical approaches.⁸

Because adsorption is becoming an efficient and economical technology for pollutant removal in recent years, more and more attentions have been paid to the explorations of effective and low cost adsorbents. Yan and Chen⁹ proposed that the waste seafood shells exhibited potential environmental remediation capacity. Trazzi et al.¹⁰ used biochar and sugar cane bagasse to remove phosphate and obtained 12.80 and 15.50 mg g⁻¹ phosphate sorption capacities. Jung et al.¹¹ used waste marine macroalgae for phosphate removal and demonstrated that this material could be utilized for fertilizer after sorption process. Arshadi and coworkers¹² prepared Spondias purpurea seed waste loaded by iron nanoparticles for phosphate removal and achieved satisfied removal rates. Pan et al.¹³ utilized modified peanut straw biochar for Cr(VI) removal and obtained excellent adsorption capacity. Chen and coworkers¹⁴ prepared biochar derived from municipal sludge to remove Cr(VI) and proved that surface precipitation and ion exchange were the main mechanisms. Agrafioti et al.¹⁵ used biochars derived from rice husk and organic solid wastes for Cr(VI) removal and achieved satisfied effects. In our previous study, we prepared pectin-stabilized nanoscale zero-valent iron for Cr(VI) removal and obtained good results.¹⁶

In the present study, a novel engineered waste eggshell was prepared to remove phosphate and hexavalent chromium from aqueous solutions. The mechanism of functionalization of the eggshell was that α-FeOOH particles loaded onto the multihole eggshell surface through covalent bond interaction and electrostatic attraction. A series of experiments were carried out to evaluate the adsorption capacities and discuss the adsorption mechanisms.

2. Materials and methods

2.1. Chemicals and reagents

All solutions used in this experiment were prepared using deionized water. All chemicals and reagents used in this experiment were guarantee reagents. Besides, sodium hydroxide (NaOH), hydrochloric acid (HCl), ferrous sulfate (Fe₂(SO₄)₃), potassium dihydrogen phosphate (KH₂PO₄), and potassium dichromate (K₂Cr₂O₇) were purchased from Aladdin and Qiangsong Fine Chemicals.

2.2. Preparation of engineered waste eggshell

The waste eggshells were collected from the student restaurant in Wuhan University. Firstly, the waste eggshells were washed with deionized water and vacuum-dried. Then, the dried eggshells were ground and screened in the range of 30–124 μm. After that, 2 mol L⁻¹ NaOH solution was added to 1 mol L⁻¹ Fe₂(SO₄)₃ solution until the mixture pH reached 11.0, then the suspension was reacted for 72 h under 30 °C condition. Next, eggshell powder was added to the suspension solution and reacted for 48 h under stirring condition. Finally, the precipitation was washed by deionized water and vacuum-dried under 60 °C condition, obtaining α-FeOOH modified eggshell.

2.3. Characterization of engineered waste eggshell

Phosphate was measured by ammonium molybdate spectrophotometric method using spectrophotometer at 700 nm. Cr(VI) was measured using spectrophotometer at 540 nm after complexation with 1,5-diphenylcarbazide. Surface morphology of the samples was determined using scanning electron microscopy (SEM) (ZEISS, Germany) equipped with an energy dispersive X-ray fluorescence spectroscopy EDS for analyzing surface elements. FTIR spectroscopy spectra of the samples were taken with a Nicolet 5700 spectrometer using KBr pellets in the range of 4000–400 cm⁻¹. XRD spectra were obtained with an X-ray Diffractometer X'Pert Pro, PANalytical.

2.4. Adsorption kinetics and isotherms

In order to control quality, all the kinetics and isotherm experiments were conducted in duplicate and the differences of two measurements were lower than 3%.

Adsorption kinetics of phosphate and hexavalent chromium on engineered eggshell were carried out by adding 0.05 g adsorbent to 200 mL Erlenmeyer flasks containing 100 mL 100 mg L⁻¹ phosphate and 20 mg L⁻¹ hexavalent chromium solution at 25 °C and pH 3.0 in a mechanical shaker. Samples were taken from 0 to 48 h to measure the phosphate and hexavalent chromium concentrations.

Adsorption isotherms of phosphate and hexavalent chromium on engineered eggshell were conducted by adding 0.05 g adsorbent to 200 mL Erlenmeyer flasks containing 100 mL phosphate and hexavalent chromium solution at 25 °C and pH 3.0 in a mechanical shaker. The concentrations of phosphate and hexavalent chromium increased from 5 to 500 mg L⁻¹. After 24 h reaction, the concentrations of phosphate and hexavalent chromium were measured.

2.5. Effects of dosage, pH, and ionic strength

The effect of adsorbent dosage was discussed by changing adsorbent dosage from 0.1 to 1.5 g L⁻¹ at pH 3.0 for phosphate and hexavalent chromium adsorption. For pH effect, the pH of the solution was varied from 1 to 9 at dosage 0.5 g L⁻¹. For ionic strength, NaCl concentration in the solution was varied from 0 to 0.1 M at pH 3.0 and dosage 0.5 g L⁻¹. During the experiment, the initial concentrations of phosphate and hexavalent chromium were maintained at 100 and 20 mg L⁻¹.

3. Results and discussion

3.1. Characterization of engineered eggshell

The SEM image and corresponding EDS spectrum of α-FeOOH modified eggshell were shown in Fig. 1. Comparing with raw eggshell displayed in some studies,^17–19 large amounts of small particles on the engineered eggshell surface might be assigned to α-FeOOH particles. In addition, the EDS spectrum showed that C, O, Ca, and Fe were the main elements of engineered eggshell, suggesting that iron oxides hydrate were successfully loaded onto eggshell. The mechanism of functionalization of the eggshell might be that α-FeOOH particles loaded onto the multihole eggshell surface through covalent bond interaction and electrostatic attraction.

Fig. 1 SEM image of engineered eggshell and corresponding EDS spectrum.

As illustrated in Fig. 2, the XRD spectrum of F-ES indicated that numerous peaks were appeared at 2θ = 21.2°, 27.1°, 29.4°, 36.4°, 39.5°, 43.2°, 46.9°, 48.6°, 52.9°, 60.7°, and 64.8°. Among these peaks, 29.4°, 39.5°, 43.2°, 48.6°, 60.7°, and 64.8° corresponded to calcium carbonate,²⁰ which confirmed that calcium carbonate was the main component of waste eggshell. In addition, the obvious peaks of F-ES at 2θ = 21.2°, 36.4°, and 52.9° were assigned to α-FeOOH²¹ compared with ES, which also demonstrated that the eggshell surface was covered with α-FeOOH.

Fig. 2 XRD pattern of engineered eggshell.

FTIR spectra of raw eggshell and engineered eggshell were given in Fig. 3. Overall, the abundant and broad band at 3000–3500 cm⁻¹ was ascribed to OH-stretching.²² The prominent peak appeared at 1639 cm⁻¹ was assigned to N–H. In addition, the Fe–O–H bending vibration of α-FeOOH was attributing to the peak at 891 and 785 cm⁻¹ of F-ES spectrum, indicating the presence of α-FeOOH onto eggshell surface.²³ Moreover, the obvious peak at 1024 cm⁻¹ corresponded to C–O stretching vibration.²⁴ Especially at 510 cm⁻¹ of F-ES spectrum, the new peak was ascribed to Fe–O stretching vibration,²³ suggesting that iron compounds were loaded onto eggshell. Based on the FTIR analysis, it could be concluded that the oxygen containing functional groups and α-FeOOH supplied the main sorption sites for phosphate and hexavalent chromium ions.

Fig. 3 FTIR spectrum of engineered eggshell.

3.2. Adsorption kinetics and isotherms

The specific relationship between the sorption capacity and equilibrium concentration could be discussed by adsorption isotherms. The isotherm results for phosphate and hexavalent chromium on eggshell and engineered eggshell were shown in Table 1 and Fig. 4. Overall, for phosphate adsorption, Langmuir model (R² ≥ 0.95) better fitted the sorption data than Freundlich model (R² ≥ 0.85), besides, the sorption data were both well fitted with Freundlich–Langmuir (R² ≥ 0.99) and Redlich–Peterson (R² ≥ 0.99) models, which indicated that multiple adsorption mechanisms were presented onto waste eggshell. It can be seen from Langmuir model that the maximum P adsorption capacities of eggshell and engineered eggshell were 89.74 and 248.73 mg g⁻¹, meaning that α-FeOOH modified eggshell possessed much higher P sorption ability than original eggshell. In addition, for Redlich–Peterson model, the parameter S_max of F-ES was six times higher than that of ES, which also demonstrated that the adsorption ability of waste eggshell was significantly increased after modification. While for Cr(VI) removal, the sorption data were both well fitted with Langmuir (R² ≥ 0.99), Freundlich–Langmuir (R² ≥ 0.99) and Redlich–Peterson (R² ≥ 0.99) models. For Langmuir model, the maximum Cr(VI) sorption capacity on F-ES was nearly four times greater than that of ES, suggesting that the modification method certainly improved the removal capacity of Cr(VI) on waste eggshell. Similarly, the parameters S_max in Freundlich–Langmuir and Redlich–Peterson models also showed the same trends, proving that α-FeOOH modified eggshell possessed excellent Cr(VI) removal ability. Based on the adsorption isotherms analyses, it could be concluded that the engineered eggshell exhibited much greater phosphate and hexavalent chromium removal capacities than original eggshell.

Table 1 Best-fit parameters for kinetics and isotherm models of P and Cr(VI) sorption onto ES and F-ES

Equations		Parameter 1	Parameter 2	Parameter 3	R^2
P
First-order	ES	k1 = 0.325	qe = 74.038		0.988
	F-ES	k1 = 0.407	qe = 188.765		0.991
Second-order	ES	k2 = 0.005	qe = 84.081		0.957
	F-ES	k2 = 0.002	qe = 211.264		0.964
Elovich	ES	α = 55.154	β = 0.057		0.870
	F-ES	α = 209.836	β = 0.024		0.864
Richie	ES	k = 0.00001	qe = 73.734	n = 1.006	0.990
	F-ES	k = 0.00001	qe = 188.338	n = 1.424	0.994
Langmuir	ES	K = 0.015	Smax = 89.741		0.966
	F-ES	K = 0.012	Smax = 248.734		0.957
Freundlich	ES	Kf = 7.974	n = 0.385		0.857
	F-ES	Kf = 18.555	n = 0.408		0.865
Freundlich–Langmuir	ES	Kr = 0.0006	Smax = 74.250	n = 1.907	0.993
	F-ES	Kr = 0.0002	Smax = 199.737	n = 2.062	0.992
Redlich–Peterson	ES	Kr = 0.0006	Smax = 1388.979	n = 1.452	0.993
	F-ES	Kr = 0.0002	Smax = 8730.992	n = 1.583	0.990
Cr(VI)
First-order	ES	k1 = 0.631	qe = 9.961		0.924
	F-ES	k1 = 0.340	qe = 39.135		0.987
Second-order	ES	k2 = 0.079	qe = 10.895		0.977
	F-ES	k2 = 0.009	qe = 44.236		0.958
Elovich	ES	α = 32.213	β = 0.567		0.950
	F-ES	α = 33.272	β = 0.116		0.876
Richie	ES	k = 0.027	qe = 12.279	n = 2.987	0.984
	F-ES	k = 0.6 × 10^−8	qe = 39.119	n = 1.000	0.988
Langmuir	ES	K = 0.033	Smax = 11.813		0.995
	F-ES	K = 0.040	Smax = 41.566		0.995
Freundlich	ES	Kf = 2.164	n = 0.284		0.919
	F-ES	Kf = 8.620	n = 0.266		0.869
Freundlich–Langmuir	ES	Kr = 0.0447	Smax = 12.344	n = 0.884	0.996
	F-ES	Kr = 0.027	Smax = 40.104	n = 1.153	0.997
Redlich–Peterson	ES	Kr = 0.036	Smax = 11.174	n = 0.991	0.995
	F-ES	Kr = 0.021	Smax = 66.432	n = 1.081	0.999

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Fig. 4 Adsorption isotherms of P and Cr(VI) sorption onto ES and F-ES.

The kinetics data of phosphate and hexavalent chromium adsorption onto eggshell and engineered eggshell were fitted with pseudo-first-order, pseudo-second-order, Elovich, and Richie kinetics models (Fig. 5), and the calculated parameters were listed in Table 1. For phosphate adsorption, pseudo-first-order and pseudo-second-order models both well fitted the removal data. Besides, the parameters q_e of F-ES were higher than that of ES, indicating that modified eggshell exhibited greater adsorption ability than pristine eggshell. For Elovich model, the parameter α of F-ES was four times higher than that of ES, which also demonstrated that engineered eggshell could be considered as promising adsorbent for highly concentrated phosphate remediation. In addition, Richie model data also showed the same tendency as Elovich. While for Cr(VI) removal, pseudo-first-order (R² ≥ 0.92), pseudo-second-order (R² ≥ 0.95), and Richie (R² ≥ 0.98) kinetics models both well fitted the experimental data, meaning that the reactions occurred onto eggshell surface showed diversity and multiplicity. The parameters q_e of F-ES in pseudo-first-order, pseudo-second-order, and Richie models were much greater than that of ES, suggesting that α-FeOOH modification process successfully changed the surface structure of eggshell and then improved the Cr(VI) removal ability.

Fig. 5 Adsorption kinetics of P and Cr(VI) sorption onto ES and F-ES.

3.3. Effects of dosage, pH, and ionic strength

The effects of adsorbent dosage on phosphate and hexavalent chromium adsorption were investigated with adsorbent dosage varied from 0.1 to 1.5 g L⁻¹. The removal efficiencies changed with dosage were given in Fig. 6. The phosphate removal rates increased from 10.51% to 98.41% and 25.92% to 99.85% on eggshell and engineered eggshell with dosage increased from 0.1 to 1.5 g L⁻¹. Similarly, Cr(VI) sorption rates raised from 11.38% to 74.07% and 55.90% to 98.46% on eggshell and engineered eggshell when adsorbent dosage increased from 0.1 to 1.5 g L⁻¹. This tendency was due to the reason that more surface areas and sorption sites were available for adsorption with increasing adsorbent dosage.^25,26

Fig. 6 Effect of adsorbent dosage.

As presented in Fig. 7, the phosphate adsorption rates decreased from 37.48% to 10.03% and 95.39% to 41.16% with solution pH increased from 3 to 9. Besides, the Cr(VI) removal rates also decreased from 26.31% to 5.825 and 94.58% to 39.81% with changing pH condition. For phosphate, the main species of phosphate was H₂PO₄⁻ at pH of 3–7, while HPO₄²⁻ became the main ions at pH of 7–9.^10,27 Under low pH condition, the excellent phosphate removal rates might due to that the adsorbent surface was protonated and thus had affinity to negative ions H₂PO₄⁻.^28,29 At alkaline conditions, large amounts of OH⁻ would compete with HPO₄²⁻ for limited adsorption sites on eggshells, resulting in weak sorption abilities on eggshells. For Cr(VI) adsorption, under lower pH condition, the adsorbent surface was positively charged³⁰ so that the negative ions HCrO₄⁻ and Cr₂O₇²⁻ were effectively adsorbed onto eggshells. However, the surfaces of eggshells were negatively charged,³¹ thus decreasing the adsorbent' affinity to negative ions²⁵ so that the Cr(VI) removal rates dramatically reduced at higher pH environment. Additionally, phosphate and hexavalent chromium adsorption efficiencies were both decreased when pH was lower than 3 because eggshell was easily corroded under highly acidic environment.

Fig. 7 Effects of pH.

Fig. 8 showed that ionic strength slightly influenced phosphate and hexavalent chromium adsorption rates. The phosphate removal rates decreased from 37.34% to 26.03% and 94.96% to 85.01% on eggshell and engineered eggshell with ionic strength increased from 0 to 0.1 M. While hexavalent chromium removal rates reduced from 25.56% to 18.15% and 94.26% to 82.34% on ES and F-ES adsorbent with increasing ionic strength. The reason for this result might be that Cl⁻ ions would compete with phosphate and hexavalent chromium negative ions for available adsorption sites onto eggshells, so that the phosphate and hexavalent chromium removal abilities decreased.

Fig. 8 Effect of ionic strength.

3.4. Adsorption mechanisms

Table 2 showed that the modified eggshell material exhibited superior phosphate and hexavalent chromium adsorption capacities compared with similar materials. Because the numerous oxygen containing functional groups and α-FeOOH particles supplied surface areas and sorption sites, the engineered eggshell presented excellent phosphate and hexavalent chromium removal capacities. Comparing with original eggshell, the α-FeOOH modified eggshell exhibited three times higher than original phosphate adsorption capacity and four times higher than original hexavalent chromium adsorption capacity, which proved that the α-FeOOH modification process significantly improved the adsorption abilities of waste eggshell. As a result, the engineered waste eggshell could be considered as a low cost and promising adsorbent for phosphate and hexavalent chromium adsorption.

Table 2 Comparison of different materials for phosphate and Cr removal

	Adsorbent	Adsorption capacity (mg g^−1)	Reference
P	Lanthanum loaded biochar	46.37	32
	Zirconium(IV) loaded chitosan	71.68	33
	Kaolin clay	11.92	34
	Amorphous zirconium oxide	99.01	35
	Magnetic iron oxide nanoparticles	5.03	36
	ES	89.74	This study
	F-ES	248.73	This study
Cr(VI)	Termite nest	18.60	37
	Akadama clay	4.29	38
	Surfactant-modified kaolinite	27.80	39
	Tannic-acid immobilised powdered activated carbon	5.64	40
	Chitosan/mangiferin particles	0.14	41
	ES	11.81	This study
	F-ES	41.57	This study

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4. Conclusion

The α-FeOOH modified eggshell obviously increased the removal capacities of phosphate and hexavalent chromium due to the fact that the loaded α-FeOOH particles increased the surface areas and sorption sites. In addition, the numerous oxygen containing functional groups onto F-ES supplied abundant sorption sites for phosphate and hexavalent chromium ions. Based on the adsorption kinetics and isotherms results, the engineered waste eggshell could be considered as low cost and promising adsorbent for effective phosphate and hexavalent chromium removal. Moreover, this investigation not only proved that the waste eggshell could be used for potential pollutant remediation, but also suggested that the α-FeOOH modification approach was effective for improving the sorption ability of waste eggshell.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (NSFC) (51378400) and the National Science and Technology Pillar Program (2014BAL04B04, 2015BAL01B02).

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