In this paper, spinel ferrite with high crystallinity and high saturation magnetization was successfully prepared from steel pickling sludge by adding iron source and precipitator in the hydrothermal condition. The obtained spinel ferrite was characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), vibrating sample magnetometer (VSM), and Zeta potential methods and investigated as an adsorbent for removal of Pb2+ from aqueous solution. Batch experiments were performed by varying the pH values, contact time, temperature and initial metal concentration. The result of pH impact showed that the adsorption of Pb2+ was a pH dependent process, and the pH 5.8 ± 0.2 was found to be the optimum condition. The achieved experimental data were analyzed with various kinetic and isotherm models. The kinetic studies revealed that Pb2+ adsorption onto spinel ferrite followed a pseudo-second order model, and the Langmuir isotherm model provided the perfect fit to the equilibrium experimental data. At different temperatures, the maximum Pb2+ adsorption capacities calculated from the Langmuir equation were in the range of 126.5–175.4 mg/g, which can be in competition with other adsorbents. The thermodynamic results showed that the spinel ferrite could spontaneously and endothermically adsorb Pb2+ from aqueous solution. The regeneration studies showed that spinel ferrite could be used five times (removal efficiency (%) >90%) by desorption with HNO3 reagent.

INTRODUCTION

With the rapid development of the lead-acid rechargeable battery industry, water pollution caused by lead ion has become a major environmental problem. As a highly toxic heavy metal ion, lead cannot be biodegraded and easily accumulates in the human body to induce lead poisoning. Removal of lead ions in the water is an important and highly significant need. During the past decades, various techniques have been developed to reduce the pollution of lead ions, including leaching-solvent extraction (Silva et al. 2005), electrolysis (Vegliò et al. 2003), ion exchange (De Villiers et al. 1995; Parkpian et al. 2002), membrane separation (Chaudry et al. 1998) and microbiological methods (Chen & Lin 2001; Ryu et al. 2003; Shanableh & Omar 2003). Many of them, however, may be essentially limited by economic feasibility, technical difficulty and relatively low recovery. Adsorption, compared with the above approaches, is considered as a most promising technique for removal of heavy metals from wastewater because of its high removal efficiency, easy operation and less residue production.

Spinel ferrites have the general formula MeFe2O4 (AB2O4), where Me2+ (A) can be any divalent ion such as Pb2+, Cd2+, Fe2+, Mn2+, Ni2+, Zn2+, Mg2+, etc. or their combination, and Fe3+ (B) can be substituted by trivalent Al3+, Cr3+, etc. (Goldma 2006). If multi-ions simultaneously occupy the position Me2+ (A) or the position Fe3+ (B), complex ferrite will be formed; on the other hand, the ions will be removed from solution. During the past decades, the use of spinel ferrites to immobilize heavy metal ions from wastewater has attracted particular attention on the basis of the above theory (Tamaura et al. 1991; Demirel et al. 1999; Perales-Perez & Umetsu 2002). As we know, the practical application of spinel ferrites is a cost decision, so finding an interesting potential market to get the low-cost spinel ferrites is the core problem. Steel pickling sludge, a common waste from the steel process of pickling and polishing, is mainly composed of iron compounds. At present, the main treatment for such waste is landfill, but unidentified components of sludge may pose a potential risk to the soil and groundwater (Flyhammar 1997). Therefore, exploring a viable and environment friendly technology to efficiently reuse and recycle steel pickling sludge is becoming an increasing topic.

As a potentially valuable resource, the steel pickling sludge can be mineralized into ferrite with various methods, including sonochemical process (Yang et al. 2007), citrate precursor techniques (Shafi et al. 1997), coprecipitation (Prasad & Gajbhiye 1998), mechanical alloying (Yang et al. 1999), sol-gel (George et al. 2006), shock wave (Dhara & Bhargava 2003), reverse micelle (Liu et al. 2001), hydrothermal (Kale et al. 2004) and ultrasonically assisted hydrothermal processes (Zhou et al. 2005). Because the ferrite preparation process belongs to the solid–liquid reactions with the uneven complicated multi-phase system, the traditional methods cannot achieve high conversion efficiency. The hydrothermal reaction is able to produce highly crystallized and weakly agglomerated powder. Moreover, its main process factors are easy to control (Meskin et al. 2006).

Up to now, no report has ever been published regarding conversion of steel pickling sludge to spinel ferrite and then application for adsorbing lead ion. Therefore, the objectives of this work were: (1) to assess the feasibility of preparing spinel ferrite using steel pickling sludge by adding iron source and precipitator in the hydrothermal condition, (2) to thoroughly characterize the spinel ferrite by X-ray diffraction (XRD), vibrating sample magnetometer (VSM), field emission scanning electron microscopy (FE-SEM), and Zeta meter methods, and (3) to evaluate the spinel ferrite's adsorption properties (pH effects, adsorption kinetics, isotherms, thermodynamics) and reusability toward Pb2+.

MATERIALS AND METHODS

Materials

The steel pickling sludge used in this experiment was provided by Nanjing Hazardous Waste Management Centre, China. Before use, the sample was subjected to conventional pretreatment processes. Briefly, it was washed thoroughly with deionized water to remove any solute salt and dirt, dried at 80 °C for a week to a constant weight and ground with a mechanical grinder to obtain the fine powder. Simulated wastewater (1,000 mg Pb2+/L) was prepared by dissolving appropriate amounts of Pb(NO3)2 in deionized water. The required concentrations in experiments were obtained by appropriately diluting the stock solution. All chemical reagents used in this work are analytical grade, purchased from Sinopharm Chemical Reagent Co., Ltd, China.

Spinel ferrite synthesis and characterization

The spinel ferrite powder (SFP) was synthesized by the following procedures: (1) 1.0 g steel pickling sludge powder was mixed with a certain amount of FeCl3·6H2O in 20 mL deionized water under ultrasound for 5 min; (2) the mixture was maintained at a pH value of 9.0–10.0 by adding NH3·H2O and then the mixture was transferred to 50 mL Teflon-lined stainless steel autoclave and kept in an oven at 200 °C for 8 h; and (3) the black product obtained from the cooled autoclave was rinsed several times with ethanol and dried at 60 °C for 12 h.

The crystalline phase of SFP was analyzed by XRD (D8 Advance, Bruker, Germany) with Cu Kα radiation. Its morphology and micro-structure were investigated by FE-SEM (Quanta 250FEG, FEI, USA). The magnetic property of material was measured at room temperature by VSM (HH-15, NanDa Instrument, China). The zeta potential was measured by a Zeta meter (ZetaPALS, Brookhaven Instruments, USA), using 0.01 M KCl solution as a background electrolyte.

Adsorption tests

Batch experiments were performed by mixing a certain amount of SFP with wastewater in 250 mL conical flasks, and the mixtures were agitated in an air bath oscillator at a speed of 200 rpm. After adsorption, samples were withdrawn and separated by a strong magnet, and the residual Pb2+ concentration was determined by inductively coupled plasma spectrometer (ICP-AES, Optima 7000DV, PerkinElmer, USA). The removal efficiency (R, %) and the adsorption capacity (qe, mg/g) were calculated from the following equations (Boudrahem et al. 2011):
formula
1
formula
2
where C0 and Ce (mg/L) are the initial and final concentrations of Pb2+ in conical flasks, respectively, V (L) is the volume of the wastewater, and m (g) is dosage of SPF.

To determine the pH effect on adsorption, 0.1 g SFP was left in contact with 100 mL wastewater (50 mg Pb2+/L) at 25 °C for 3 h with continuous shaking. 0.1M HCl or NaOH was used to adjust the initial solution to pH values of 2.0, 3.0, 4.0, 5.0, 6.0 and 7.0.

For kinetic studies, 0.04, 0.08 and 0.1 g SFP was respectively loaded in 250 mL conical flasks containing 100 mL wastewater (50 mg Pb2+/L). Then the flasks were capped and agitated on a thermostatic shaker at 25 °C. Samples were withdrawn at predetermined time intervals (0, 5, 10, 20, 30, 40, 50, 60, 90, 120, 150, 180, 210, 240, 270, 300 and 330 min) for the analysis of residual Pb2+ concentration in solution.

For adsorption isotherm studies, 0.1 g SFP was separately added to 100 mL wastewater with various concentrations of 10, 20, 40, 50, 60, 80 and 100 mg Pb2+/L. The mixtures were shaken at 25 °C for 3 h.

For adsorption thermodynamics studies, 0.1 g SFP was equilibrated with 100 mL wastewater containing 50 mg Pb2+/L at 298, 308 and 318 K for 3 h.

For investigating the effect of competing ions, Ca2+, Mg2+, K+ and Na+ in varying concentrations were added to 100 mL Pb2+ solution (50 mg/L) suspended with 0.1 g SFP. The mixture was shaken at 25 °C for 3 h.

Theories

Kinetic models

To investigate the controlling mechanism of the adsorption process such as mass transfer, diffusion control and chemical reaction, the well-known kinetic models pseudo-first order model (Lagergren 1898) and pseudo-second order model (Ho 2006) were used to test experimental data. Their linear forms are expressed as:

Pseudo-first order equation:
formula
3
Pseudo-second order equation:
formula
4
where qe and qt (mg/g) represent the amount of metals adsorbed at equilibrium and at time t (min), and k1 (min−1) and k2 (g/(mg min)) are the rate constants for the pseudo-first order and pseudo-second order, respectively.

Isotherm models

The Langmuir isotherm model (Langmuir 1918) assumes that a single molecular layer is adsorbed on the adsorbent uniform coverage of the surface, and each molecule on the surface has equal activation energy. Its linear form can be written by:
formula
5
where Ce (mg/L) is the equilibrium concentration of Pb2+, qe (mg/g) is the amount of Pb2+ adsorbed under equilibrium, qmax (mg/g) is the maximum adsorption capacity, and b (L/g) is a Langmuir constant related to the affinity of the binding sites and energy of adsorption.
The Freundlich isotherm model (Freundlich 1906) is usually used to describe the adsorption characteristics of a heterogeneous surface. The linear form of the isotherm equation is given by:
formula
6
where KF and n are Freundlich constants, with n giving an indication of how favorable the adsorption process is and KF (mg/g) related to the bonding energy.
The Dubinin-Radushkevich (D-R) model (Dubinin 1960) can evaluate in depth the mechanism of Pb2+ adsorption on SFP, as well as distinguish between physisorption and chemisorption. Its linear equations can be represented by:
formula
7
formula
8
formula
9
where qm (mg/g) is the theoretical saturation adsorption capacity, β (mol2 J−2) is a constant correlated with the mean free energy of adsorption, ɛ is the Polanyi potential, R (8.3145 J/(mol K)) is the universal gas constant, T (K) is the absolute temperature in Kelvin, and E (mol2/kJ2) is the mean free energy of adsorption per molecule of the adsorbate when transferred to the surface of the solid from infinity in the solution.

Thermodynamics

Thermodynamic parameters, including the Gibbs free energy change (ΔG°, kJ/mol), the enthalpy change (ΔH°, kJ/mol) and the entropy change (ΔS°, kJ/(mol K)) are critical for determining if the process is endothermic or exothermic, and the spontaneity of the adsorption process, which can be calculated by (Aydın & Aksoy 2009):
formula
10
formula
11
formula
12
where K is the adsorption equilibrium constant, Cs (mg/g) is the amount of Pb2+ adsorbed per weight unit of SFP after equilibrium, Ce (mg/L) is the phosphate concentration in solution at equilibrium, R (8.3145 J/(mol K)) is the universal gas constant, and T (K) is the temperature. ΔH° and ΔS° were obtained from the slope and intercept of the plot of ln K versus 1/T.

RESULTS AND DISCUSSION

Characterizations

The structure of SFP prepared from steel pickling sludge was analyzed by XRD and the result is shown in Figure 1(a). The wide-angle XRD pattern exhibited seven characteristic peaks that appeared at 2θ = 30 °, 36 °, 43 °, 54 °, 57 ° and 63 °. It has been assigned to the typical crystal planes of (Ni, Mg)Fe2O4, demonstrating the feasibility of producing spinel ferrite from steel pickling sludge. Figure 1(b) shows the FE-SEM image of SFP. The SFP particles presented a relatively uniform spherical shape 50–110 nm in diameter. From Figure 1(c), the saturation magnetization of SFP was measured as 116.4 emu/g, which was greater than the ferrite material derived from electroplating sludge (Chen et al. 2010). The complete separation between SFP and solution only needed 2 min (insert image). Figure 1(d) exhibits the change of surface charge of SFP. With pH increasing from 2.0 to 7.0, zeta potential of SFP decreased from 5.40 to −13.01 mV. The point of zero charge (pHpzc) was obtained at pH 4.35.
Figure 1

Characterizations of SFP prepared from steel pickling sludge: (a) XRD, (b) VSM, (c) FE-SEM, (d) Zeta potential.

Figure 1

Characterizations of SFP prepared from steel pickling sludge: (a) XRD, (b) VSM, (c) FE-SEM, (d) Zeta potential.

Toxicity evaluation

Evaluation of the hazardous nature of adsorbent is an essential step for its use. In this work, the toxicity characteristic leaching procedure (TCLP) method was selected for assessing the leachability of toxic metals from steel pickling sludge and SFP. Figure 2(a) shows that the Pb, Cd and Cr were leached from the raw sludge, and the concentrations of metals were all below the USEPA regulatory levels, indicating that raw sludge had little environmental impact. From Figure 2(b), it is obvious that only Cr was detected, and its TCLP concentration in leachate (pH 4.93 ± 0.05) was considerably lower than the level of raw sludge, deducing that the SFP could immobilize the Pb, Cd and Cr properly.
Figure 2

Toxicity characteristics leaching procedure (TCLP) extracted various metals in (a) raw steel pickling sludge and (b) spinel ferrite powder (SFP).

Figure 2

Toxicity characteristics leaching procedure (TCLP) extracted various metals in (a) raw steel pickling sludge and (b) spinel ferrite powder (SFP).

Effect of pH

The solution pH is one of the important factors for sorption of heavy metal ions, which not only significantly influences the hydrolytic species of metal ions but also affects the surface charge of adsorbents (Azouaou et al. 2010; Boudrahem et al. 2011). To study the effect of this parameter on the Pb2+ adsorption on SFP, the solution initial pH was controlled within the range of 2.0 to 7.0 to avoid lead hydroxide precipitation. Figure 3 shows the effect of wastewater initial pH on the uptake of Pb2+. At low pH values, there was excessive protonation on the SFP's surface, resulting in a decrease in the sorption of Pb2+, which was consistent with the literature (Kobya et al. 2005). On increasing the pH of Pb2+ solution from 2.0 to 6.0, the removal percentage increased from 12.07 to 75.53%. This can be attributed to the fact that the negative charge on the SFP surface (Figure 1(d)) obviously increases and thus attracts the positively charged Pb2+ more strongly. When pH > 6.5, the lead hydroxide precipitation was partly formed, leading to the higher removal efficiency of Pb2+. Subsequent studies were conducted at pH 5.8 ± 0.2.
Figure 3

Effect of pH on the removal of Pb2+ by SFP from aqueous solution.

Figure 3

Effect of pH on the removal of Pb2+ by SFP from aqueous solution.

Kinetics studies

The effects of dosage and contact time on the removal of Pb2+ by SFP are shown in Figure 4(a). From the figure, it can be seen that Pb2+ sorption occurred rapidly. The removal efficiency of Pb2+ increased gradually with increasing contact times and reached a plateau afterward. On increasing the SFP dose from 0.04 to 0.1 g, the removal efficiency of Pb2+ increased from 52.68 to 96.20%. This may be ascribed to an increased contact surface and available active sites resulting from the increased dosage of the adsorbent. At an amount of adsorbent higher than 0.1 g, the incremental Pb2+ removal became very low (only 3.0%); therefore, dosage of 0.1 was chosen for the following experiments.
Figure 4

The effect of dosage and contact time on the uptake of Pb2+ onto SFP.

Figure 4

The effect of dosage and contact time on the uptake of Pb2+ onto SFP.

From Table 1, it can be found that the R2 values (0.9773, 0.9493 and 0.9623 for 0.04, 0.08 and 0.1, respectively) of the pseudo-first order model were lower than those of the pseudo-second order model (0.9939, 0.9984 and 0.9979). The theoretical qe cals calculated from the pseudo-first order equation were 48.5, 30.8 and 59.3 mg/g for 0.04 g, 0.08 g and 0.1 g, respectively, which were markedly different from experimental data (67.4, 50.3 and 48.1 mg/g). However, for the pseudo-second order model, the values of qe cal were in good agreement with experimental data. These results indicated the good applicability of the pseudo-second order model for Pb2+ removal by SFP under the experimental condition. On the other hand, the overall rate of Pb2+ adsorption on SFP was controlled by a chemical process, which was in accordance with the results obtained by an other study (Yan et al. 2014).

Table 1

Pseudo-first order and pseudo-second order kinetic parameters for the adsorption of Pb2+ onto SFP

Dosage (g)qe exp (mg/g)Pseudo-first order
Pseudo-second order
qe cal (mg/g)R2k2 × 10−3 (g/(mg min))qe cal (mg/g)R2
0.04 67.4 48.5 0.9773 0.19 72.4 0.9939 
0.08 50.7 30.8 0.9493 0.33 55.3 0.9984 
0.1 48.1 59.3 0.9623 0.38 51.2 0.9979 
Dosage (g)qe exp (mg/g)Pseudo-first order
Pseudo-second order
qe cal (mg/g)R2k2 × 10−3 (g/(mg min))qe cal (mg/g)R2
0.04 67.4 48.5 0.9773 0.19 72.4 0.9939 
0.08 50.7 30.8 0.9493 0.33 55.3 0.9984 
0.1 48.1 59.3 0.9623 0.38 51.2 0.9979 

Equilibrium studies

The analysis of equilibrium data is essential to understand the adsorption process and to be able to compare different adsorbents under different operational conditions. The relationship between various initial Pb2+ concentrations and equilibrium adsorption capacities was investigated. It is obvious from Figure 5 that the equilibrium adsorption capacities of SFP ranged from 33.71 mg/g to 120.7 mg/g, 35.78 mg/g to 157.1 mg/g, and 36.84 mg/g to 174.1 mg/g at 298 K, 308 K, and 318 K, respectively. Especially, the equilibrium adsorption capacities of SFP were in the order of qe 318 > qe 308 > qe 298 in the whole adsorption process, suggesting an endothermic process for Pb2+ adsorption onto SFP.
Figure 5

Effect of initial concentration on the uptake of Pb2+ onto SFP.

Figure 5

Effect of initial concentration on the uptake of Pb2+ onto SFP.

The calculated isotherm constants are summarized in Table 2. It can be discovered that the adsorption processes in all conditions were accurately described by the Langmuir model due to the higher values of R2, indicating that the Pb2+ removal by SFP corresponded to a homogeneous system. The maximum adsorption capacities (qmax) predicted from the Langmuir equation were in the range of 126.5–175.4 mg/g, which were similar to or even greater than those of other adsorbents (Table 3), implying that SFP possessed a significant potential for Pb2+ removal from practical wastewater. The Langmuir constant b is commonly used to calculate the dimensionless separation factor, RL (RL = (1 + bC0)−1), which can evaluate the favorability of adsorption. The adsorption process is favorable if the RL values lie between 0 and 1, while the process is unfavorable if the values of RL > 1 (Meitei & Prasad 2014). In this case, the calculated RL values at different temperatures were all within the range of 0–1 (0.049–0.340 for 298 K, 0.031–0.243 for 308 K and 0.007–0.072 for 318 K), indicating the favorable nature for adsorption of Pb2+ from wastewater by SFP. Additionally, the 1/n values obtained from the Freundlich equation were 0.2879, 0.2674 and 0.2461 for 298 K, 308 K and 318 K, respectively, which all lie between 0 and 1, reflecting the favorable adsorption and high affinity between the optimum hydroxyapatite prepared from alkaline residue (O-HAP) and Pb2+ (Zhao et al. 2010), demonstrating the results from RL studies.

Table 2

Langmuir, Freundlich and D-R parameters for the adsorption of Pb2+ onto SFP

T (K)Langmuir
Freundlich
D-R
qmax (mg/g)b (L/mg)R2KF1/nR2E (kJ/mol)R2
298 126.5 0.1941 0.9917 38.01 0.2879 0.9583 19.61 0.9385 
308 161.3 0.3100 0.9915 56.57 0.2674 0.9142 22.36 0.8943 
318 175.4 1.295 0.9999 80.32 0.2461 0.8309 23.57 0.8588 
T (K)Langmuir
Freundlich
D-R
qmax (mg/g)b (L/mg)R2KF1/nR2E (kJ/mol)R2
298 126.5 0.1941 0.9917 38.01 0.2879 0.9583 19.61 0.9385 
308 161.3 0.3100 0.9915 56.57 0.2674 0.9142 22.36 0.8943 
318 175.4 1.295 0.9999 80.32 0.2461 0.8309 23.57 0.8588 
Table 3

Comparison of adsorption capacity (qmax) of various adsorbents for Pb2+ adsorption

Adsorbentsqmax (mg/g)References
SFP 126.5–175.4 This study 
Carbonate hydroxyapatite 101 Liao et al. (2010)  
Non-activated charcoal of oak wood origin 31.08 Machida et al. (2005)  
FGD gypsum 161.3 Yan et al. (2015)  
Chitosan-based granular adsorbent 178.57–465.12 Zhu et al. (2015)  
Functionalized chrysotile nanotubes 83.96 Sun et al. (2015)  
Water hyacinth based activated carbon 118.8 Huang et al. (2014)  
Mercapto groups functionalized CeO2 nanofiber 90.9 Yari et al. (2015)  
Modified hide waste 14.60–32.36 Kong et al. (2014)  
Adsorbentsqmax (mg/g)References
SFP 126.5–175.4 This study 
Carbonate hydroxyapatite 101 Liao et al. (2010)  
Non-activated charcoal of oak wood origin 31.08 Machida et al. (2005)  
FGD gypsum 161.3 Yan et al. (2015)  
Chitosan-based granular adsorbent 178.57–465.12 Zhu et al. (2015)  
Functionalized chrysotile nanotubes 83.96 Sun et al. (2015)  
Water hyacinth based activated carbon 118.8 Huang et al. (2014)  
Mercapto groups functionalized CeO2 nanofiber 90.9 Yari et al. (2015)  
Modified hide waste 14.60–32.36 Kong et al. (2014)  

The E calculated from D-R equations gives important information about the adsorption type. The adsorption type is physisorption if E < 8 kJ/mol, while the process is chemical in nature if the values of E > 16 kJ/mol, and 8 < E< 16 kJ/mol indicates ion exchange (Hamayun et al. 2014). In present work, the E values were all greater than 16, suggesting that the removal of Pb2+ by SFP was a stronger chemical process, which was consistent with the results of kinetic studies.

Thermodynamics

The thermodynamic parameters are listed in Table 4. As the temperature rose from 298 to 318 K, the absolute values of ΔG° increased, reflecting more efficient adsorption at higher temperature. The positive values of ΔS° revealed the increase of randomness at the interface of the solid-solution during the adsorption of Pb2+ on the active sites of SFP. Furthermore, the positive values of ΔH° confirmed that this adsorption process was exothermic in nature. In addition, according to the ΔH° value, the procedure can be classified as a physical (2.1 < ΔH° < 20.9 kJ/mol) or a chemical (20.9 < ΔH° < 418.4 kJ/mol) process (Saǧ & Kutsal 2000). In this case, the ΔH° was 58.60 kJ/mol, indicating that the nature of Pb2+ adsorption is chemisorption, which is consistent with the results of kinetics study and D-R model.

Table 4

Thermodynamic parameters for the adsorption of Pb2+ on SFP

Temperature (K)ΔG° (kJ/mol)ΔH° (kJ/mol)ΔS° (J/mol)
298 −3.57 58.60 0.21 
308 −5.13 
318 −7.74 
Temperature (K)ΔG° (kJ/mol)ΔH° (kJ/mol)ΔS° (J/mol)
298 −3.57 58.60 0.21 
308 −5.13 
318 −7.74 

Effect of competing ions

Alkali and alkali-earth metal ions such as K+, Na+, Ca2+ and Mg2+, which exist widely in natural water sources, may compete with Pb2+ for the available adsorptive sites (Dong et al. 2010). Hence, it is important to investigate the interference of competing ions for Pb2+ adsorption onto SFP. Figure 6 shows the effect of various ions of different concentrations on Pb2+ adsorption by SFP. Under the given experimental conditions, K+, Na+, Ca2+ and Mg2+ have shown negligible effect on the removal of Pb2+. These results confirm that SFP has the capability of selectively and efficiently removing Pb2+ from aqueous solution when the above competing ions are in the range of 0–200 mg/L.
Figure 6

Effect of competing ions on the removal of Pb2+ by SFP from aqueous solution.

Figure 6

Effect of competing ions on the removal of Pb2+ by SFP from aqueous solution.

Regeneration

At present, the significance of removing heavy metal ions relates to the process of desorption and reuse in industry processes rather than simple adsorption and disposal (Kumar et al. 2007). In order to assess the reusability of SFP, continuous adsorption/desorption cycles were conducted by mixing 0.1 g SFP and 100 mL wastewater (50 mg Pb2+/L) at a speed of 200 rpm. At every 3 h sampling time interval, the entire solution was withdrawn from the conical flask to analyze Pb2+ concentration, followed by the addition of 50 mL HNO3 (1.0 M) to desorb the saturated SFP. Such adsorption/desorption cycles were carried out using the same SFP batch eight consecutive times. As seen from Figure 7, Pb2+ removal was higher than 90% during the initial five cycles but sharply declined to 72.13% at the eighth cycle. In general, the relatively good regenerability was more conducive to its practical application.
Figure 7

Sustained removal of Pb2+ by SFP.

Figure 7

Sustained removal of Pb2+ by SFP.

CONCLUSIONS

The present study indicated that the spinel ferrite synthesized using the steel pickling sludge as raw material showed excellent adsorption performance for Pb2+ from aqueous solution. Increasing the solution's initial pH increased the negative surface charge of spinel ferrite, and the negative surface enhanced the removal efficiency of Pb2+. When the dosages of adsorbent and initial pH were, respectively, fixed at 0.1 g and 5.8 ± 0.2, the process of adsorption was relatively rapid and approached equilibrium within 180 min. The kinetic data correlated well with the pseudo-second order model, indicating that the chemical reaction was the rate-limiting step. Equilibrium study showed an excellent fit between experimental data and the Langmuir isotherm model, and the maximum adsorption capacities predicted from the Langmuir equation were relatively greater than for other adsorbents. The negative ΔG° values and positive ΔH° and ΔS° values signified that the adsorption reaction was spontaneously endothermic and increased randomness at the solid/liquid interface. Thus, the spinel ferrite from steel pickling sludge can be considered as a promising candidate for Pb2+ removal from wastewater.

ACKNOWLEDGEMENTS

The authors would like to acknowledge National Natural Science Foundation of China (No. 51278248), Environmental Protection Department of Jiangsu Province, China (No. s201118) and Jiangsu Provincial Education Ministry of China (No. KYZZ_0129) for their financial support.

REFERENCES

REFERENCES
Aydin
Y. A.
Aksoy
N. D.
2009
Adsorption of chromium on chitosan: optimization, kinetics and thermodynamics
.
Chemical Engineering Journal
151
,
188
194
.
Boudrahem
F.
Aissani-Benissad
F.
Soualah
A.
2011
Adsorption of lead(II) from aqueous solution by using leaves of date trees as an adsorbent
.
Journal of Chemical Engineering Data
56
,
1804
1812
.
Chen
D.
Hou
J.
Yao
L.
Jin
H.
Qian
G.
Xu
Z.
2010
Ferrite materials prepared from two industrial wastes: electroplating sludge and spent pickle liquor
.
Separation and Purification Technology
75
,
210
217
.
De Villiers
P.
Van Deventer
J.
Lorenzen
L.
1995
The extraction of species from slurries of insoluble solids with ion-exchange resins
.
Minerals Engineering
8
,
1309
1326
.
Demirel
B.
Yenigun
O.
Bekbolet
M.
1999
Removal of Cu, Ni and Zn from wastewaters by the ferrite process
.
Environmental Technology
20
,
963
970
.
Dhara
S.
Bhargava
P.
2003
A simple direct casting route to ceramic foams
.
Journal of the American Ceramic Society
86
,
1645
1650
.
Dong
L.
Zhu
Z.
Qiu
Y.
Zhao
J.
2010
Removal of lead from aqueous solution by hydroxyapatite/magnetite composite adsorbent
.
Chemical Engineering Journal
165
,
827
834
.
Flyhammar
P.
1997
Estimation of heavy metal transformations in municipal solid waste
.
Science of the Total Environment
198
,
123
133
.
Freundlich
H. M. F.
1906
Over the adsorption in solution
.
Journal of Physical Chemistry
57
,
385
470
.
George
M.
John
A. M.
Nair
S. S.
Joy
P. A.
Anantharaman
M. R.
2006
Finite size effects on the structural and magnetic properties of sol-gel synthesized NiFe2O4 powders
.
Journal of Magnetism and Magnetic Materials
302
,
190
195
.
Goldma
A.
2006
Modern Ferrite Technology
.
Springer-Verlag
,
New York
.
Hamayun
M.
Mahmood
T.
Naeem
A.
Muska
M.
Din
S. U.
Waseem
M.
2014
Equilibrium and kinetics studies of arsenate adsorption by FePO4
.
Chemosphere
99
,
207
215
.
Ho
Y. S.
2006
Review of second order models for adsorption systems
.
Journal of Hazardous Materials
136
,
681
689
.
Kale
A.
Gubbala
S.
Misra
R. D. K.
2004
Magnetic behavior of nanocrystalline nickel ferrite synthesized by the reverse micelle technique
.
Journal of Magnetism and Magnetic Materials
277
,
350
358
.
Kumar
G. P.
Kumar
P.
Chakraborty
S.
Ray
M.
2007
Uptake and desorption of copper ion using functionalized polymer coated silica gel in aqueous environment
.
Separation and Purification Technology
57
,
47
56
.
Lagergren
S.
1898
Zur theoric der sogenannten adsorption gelǒster stoffe
.
Handlingar
24
,
1
39
.
Langmuir
I.
1918
The adsorption of gases on plane surfaces of glass, mica and platinum
.
Journal of the American Chemical Society
40
,
1361
1368
.
Liao
D.
Zheng
W.
Li
X.
Yang
Q.
Yue
X.
Guo
L.
Zeng
G.
2010
Removal of lead(II) from aqueous solutions using carbonate hydroxyapatite extracted from eggshell waste
.
Journal of Hazardous Materials
177
,
126
130
.
Liu
J.
He
H.
Jin
X.
Hao
Z.
Hu
Z.
2001
Synthesis of nanosized nickel ferrites by shock waves and their magnetic properties
.
Materials Research Bulletin
36
,
2357
2363
.
Machida
M.
Yamazaki
R.
Aikawa
M.
Tatsumoto
H.
2005
Role of minerals in carbonaceous adsorbents for removal of Pb(II) ions from aqueous solution
.
Separation and Purification Technology
46
,
88
94
.
Meskin
P. E.
Ivanov
V. K.
Barantchikov
A. E.
Churagulov
B. R.
Tretyakov
Y. D.
2006
Ultrasonically assisted hydrothermal synthesis of nanocrystalline ZrO2, TiO2, NiFe2O4 and Ni0.5Zn0.5Fe2O4 powders
.
Ultrasonics Sonochemistry
13
,
47
53
.
Parkpian
P.
Leong
S. T.
Laortanakul
P.
Poonpolwatanaporn
P.
2002
Environmental applicability of chitosan and zeolite for amending sewage sludge
.
Journal of Environmental Science and Health A
37
,
1855
1870
.
Ryu
H. W.
Moon
H. S.
Lee
E. Y.
Cho
K. S.
Choi
H.
2003
Leaching characteristics of heavy metals from sewage sludge by Acidithiobacillus thiooxidans MET
.
Journal of Environmental Quality
32
,
751
759
.
Shafi
K. V. P. M.
Koltypin
Y.
Gedanken
A.
Prozorov
R.
Balogh
J.
Lendvai
J.
Felner
I.
1997
Sonochemical preparation of nanosized amorphous NiFe2O4 particles
.
Journal of Physical Chemistry B
101
,
6409
6414
.
Silva
J. E.
Paiva
A. P.
Soares
D.
Labrincha
A.
Castro
F.
2005
Solvent extraction applied to the recovery of heavy metals from galvanic sludge
.
Journal of Hazardous Materials
120
,
113
118
.
Tamaura
Y.
Katsura
T.
Rojarayanont
S.
Yoshida
T.
Abe
H.
1991
Ferrite process – heavy-metal ions treatment system
.
Water Science and Technology
23
,
1893
1900
.
Yan
Y.
Wang
Y.
Sun
X.
Li
J.
Shen
J.
Han
W.
Liu
X.
Wang
L.
2014
Optimizing production of hydroxyapatite from alkaline residue for removal of Pb2+ from wastewater
.
Applied Surface Science
317
,
946
954
.
Yan
Y.
Li
Q.
Sun
X. L.
Ren
Z. Y.
He
F.
Wang
Y. L.
Wang
L. J.
2015
Recycling flue gas desulphurization (FGD) gypsum for removal of Pb(II) and Cd(II) from wastewater
.
Journal of Colloid and Interface Science
457
,
86
95
.
Yang
J. M.
Tsuo
W. J.
Yen
F. S.
1999
Preparation of ultrafine nickel ferrite powders using mixed Ni and Fe tartrates
.
Journal of Solid State Chemistry
145
,
50
57
.
Yang
J.
Peng
J.
Liu
K.
Guo
R.
Xu
D.
Jia
J.
2007
Synthesis of ferrites obtained from heavy metal solutions using wet method
.
Journal of Hazardous Materials
143
,
379
385
.
Yari
S.
Abbasizadeh
S.
Mousavi
S. E.
Moghaddam
M. S.
Moghaddam
A. Z.
2015
Adsorption of Pb(II) and Cu(II) ions from aqueous solution by an electrospun CeO2 nanofiber adsorbent functionalized with mercapto groups
.
Process Safety and Environmental Protection
94
,
159
171
.
Zhao
X.
Wang
J.
Wu
F.
Wang
T.
Cai
Y.
Shi
Y.
Jiang
G.
2010
Removal of fluoride from aqueous media by Fe3O4@Al(OH)3 magnetic nanoparticles
.
Journal of Hazardous Materials
173
,
102
109
.
Zhou
J.
Ma
J.
Sun
C.
Xie
L.
Zhao
Z.
Tian
H.
Wang
Y.
Tao
J.
Zhu
X.
2005
Low-temperature synthesis of NiFe2O4 by a hydrothermal method
.
Journal of the American Ceramic Society
88
,
3535
3537
.