The aim of this work is to study the performances of isomeric α-, β-, and γ-FeOOH (goethite, akaganéite and lepidocrocite, including five samples named as Gth1 and Gth2, Aka1 and Aka2, and Lep, respectively) for removing hexavalent chromium (Cr(VI)) from aqueous solutions. The adsorption mechanisms were explored by kinetic and isothermal experiments. Adsorption efficiencies under the different pH values, anions, and the levels of adsorbate and adsorbent were also measured. Results showed that the Cr(VI) adsorption by isomeric FeOOH could be best described by pseudo-second-order kinetic model. The processes of Cr(VI) isothermal adsorption could be greatly fitted by the Langmuir and Freundlich equations with the high correlation coefficients of R2 (>0.92). Also, there were the optimum pH values of 3.0–8.0 for FeOOH to adsorb Cr(VI), and their adsorption capacities were tightly related with the active sites of adsorbents. Cr(VI) adsorptions by these adsorbents were easily influenced by H2PO4–, and then SO42–, while there were little effects by Cl–, CO32– and NO3–. These obtained results could provide a potentially theoretical evidence for isomeric FeOOH materials applied in the engineering treatment of the polluted chromate-rich waters.
Chromium as an important metal element is widely used in the manufacturing industry (Cheng et al. 2015; Xie et al. 2015). Chromium pollutions in natural water and drinking water have received considerable attention, due to their adverse effects on the environment and human health. The maximum level of chromium contaminant in domestic water supplies has been set to be 50 μg/L (Attar et al. 2014). Therefore, it is essential to remove heavy metal ions such as Cr(VI) before the wastewater was discharged into the natural water bodies. Among the highly efficient and simple water treatment technologies, adsorption for removing heavy metals has been widely used (Lu et al. 2017). In recent years, the metal oxide materials such as iron oxyhydroxides have been developed and used as promising adsorbents, because their numerous oxygen vacancies can intimately contact and interact with heavy metal ions in wastewater (Ren et al. 2013; Wu et al. 2016; Ni et al. 2017).
Iron oxyhydroxides including FeOOH have stable chemical properties, high specific surface area and fine particle structures, so they have taken an important role in purifying contaminants in the natural environments (Wang et al. 2016; Samanta et al. 2018). For instance, FeOOH materials not only can act as the Fenton-like catalysts for the oxidation of organics (Lin et al. 2014), but also as the excellent adsorbents for toxic heavy metals such as Cr(VI) (Wang & Lo 2009; Hu et al. 2014).
There is a high removal efficiency for FeOOH adsorbing Cr(VI) and the other contaminants on their surfaces by precipitation, ion exchange, adsorption, etc. (Jia et al. 2013; Zhang et al. 2013; Wang et al. 2015; Zeng et al. 2015). The external factors such as pH values, competitive anions, and reaction times will influence on the adsorption capacity of FeOOH (Ren et al. 2012; Hu et al. 2014; Ni et al. 2017). For example, Wu et al. (2016) documented that α-FeOOH could achieve the Cr(VI) removal efficiency of 94.5% at pH 9.0. Also, Xu et al. (2012) found that the adsorption of Cr(VI) onto β-FeOOH-coated sand was related with pH, temperature and adsorbent dosage. It was reported that in reaction solutions initially containing 80 g/L adsorbent and 5 mg/L Cr(VI), the Cr(VI) maximum removal efficiency was 58% at pH 7.0, but it would decrease rapidly at the higher pH values. This case could result from hydrolysis of the oxygen-containing groups on the surface of FeOOH (Namasivayam & Sureshkumar 2008). As the pH values of system change, the surface hydroxyl groups of FeOOH also change and form –OH2+ or –OH (Wang & Giammar 2015; Xie et al. 2015). Moreover, Cr(VI) anion can also be attracted by the positively charged functional group in the FeOOH by the electrostatic interaction to achieve their adsorption roles (Wu et al. 2016; Xie et al. 2015). On the other hand, it has been documented in many studies that the adsorbed Cr(VI) by adsorbents was easily released, due to the adsorbent's active sites for Cr(VI) competed by the inorganic anions and organic groups existing in the environmental media (Gagrai et al. 2013; Yu et al. 2014; Zhang et al. 2016).
Herein in the present study, to systemically understanding the adsorption mechanism of Cr(VI) by isomeric α-, β-, and γ-FeOOH, the Cr(VI) kinetic and isothermal adsorptions were studied in detail, and we further investigated and explored the effect actions of pH, adsorbate concentration, adsorbent content and anions.
MATERIALS AND METHODS
Synthetic and analytic methods for isomeric FeOOH adsorbents
The used five adsorbent samples of isomeric α-, β-, and γ-FeOOH were goethite of Gth1 and Gth2, akaganéite of Aka1 and Aka2, and lepidocrocite of Lep. These samples were prepared according to the synthetic methods reported by previous researchers (Xiong & Zhou 2008; Snow et al. 2011). The size distributions of the sample particles were measured by using a laser diffraction instrument (Nano ZS90 Zetasizer, Malvern, UK) that allowed the measurement of particle sizes ranging from 0.3 nm to 5 μm. The specific surface area was estimated by using specific surface analysis and pore size analyzer (Autosorb IQ3, Quantachrome Instruments, USA).
Kinetic and isothermal adsorption experiments
Firstly, under the different reaction times, batch experiments on kinetic adsorption for each of five FeOOH adsorbents were developed in triplicate of a series of 50-mL centrifuge tubes containing 20 mL reaction solutions initially with 1 g/L adsorbent and 60 mg/L adsorbate. In another series of 20 mL reaction solutions initially with the different concentrations (10–160 mg/L) of adsorbate and 1 g/L adsorbent, the isothermal adsorption experiments were also developed for the corresponding adsorbent.
The above reaction solutions (and those in the below experiments) initially with pH 7.0 (except for the reactions under different pH values) included 0.01 mol/L NaNO3 electrolyte (except for the reactions under anion actions), and were shaken in a horizontal shaker for 24 h (except for kinetic experiments) at 25 ± 0.2 °C and 180 rpm.
Batch experiments under various levels of adsorbate and adsorbent
Batch experiments on the adsorbate concentration actions on Cr(VI) adsorption were carried out in 20 mL reaction solutions with 10–120 mg/L of Cr(VI) and 1 g/L FeOOH. Another series of batch experiments on the adsorbent content actions on Cr(VI) adsorption were also performed in 20 mL reaction solutions with 0.25–2.5 g/L of isomeric FeOOH and 60 mg/L Cr(VI).
Batch experiments under different pH values and anions
Batch experiment was operated in 20 mL reaction solutions (initially with 60 mg/L adsorbate and 1 g/L FeOOH adsorbent) adjusted their pH to the desired values of 3‒12. Additionally, the other batch experiment was also carried out in 20 mL reaction solutions in which NaNO3, NaCl, Na2SO4, Na2CO3, or NaH2PO4 orderly acted as electrolyte solutions with their strengths of 0.001, 0.01, and 0.1 mol/L, respectively.
RESULTS AND DISCUSSION
Adsorption kinetics of Cr(VI) on isomeric FeOOH
The data of Cr(VI) adsorption kinetics for isomeric FeOOH (Gth1, Gth2, Aka1, Aka2 and Lep) as shown in Figure 1(a), the uptake of Cr(VI) could be divided into two stages: fast and slow stages. In fast stage, about 70%–90% of the Cr(VI) was removed within the first 120 min. This was due to the significant difference of Cr(VI) concentration between in the solution and the adsorbent (i.e. a high concentrations gradient). In the following slow stages, the low concentration gradients and depletion of active sites induced that the adsorption process slowed down and gradually reached their equilibrium states (Wang et al. 2014).
It was found that the experimental data agreed well with the fitted equation (R2 = 0.999), which indicated that the adsorption process may be chemisorption involving valence forces through sharing or exchanging electrons between adsorbent and adsorbate (Lu et al. 2017). The calculated adsorption amount of Cr(VI) by Aka2 (β-FeOOH) was 21.5 mg/g, while those by the other four adsorbents were about 7–10 mg/g (as presented in Table 1). Apparently, among the five isomeric FeOOH adsorbents, Aka2 had the highest Cr(VI) adsorption capacity. The reason could be a larger specific surface area of 161.5 m2/g for Aka2 (while 35.0–82.1 m2/g of Brunauer–Emmett–Teller (BET) for the other adsorbents, see Figure 2(a)), and more active sites on its surface are beneficial to remove contaminants. Moreover, Aka2 had a larger mean particle size (a declined trend for five adsorbents being Aka2 > Gth1 ≈ Gth2 ≈ Lep > Aka1, see Figure 2(b)) in aqueous solutions. This could also be because of the smaller Aka2 adsorbent particles easily assembling into the large agglomeration.
Adsorption isotherm of Cr(VI) on isomeric FeOOH
Meanwhile, Ce (mg/L) is the concentration of adsorbate remaining in solution at equilibrium state; b (L/mg) is the Langmuir bonding energy coefficient; Qm (mg/g) is the adsorption maximum, and Qe (mg/g) is the adsorption amount of adsorbate by per unit mass of adsorbent. Also in Freundlich equation, constant K relates to adsorption capacity, and exponent n indicates the adsorption intensity. A higher n (>1) is an indicator for the favorable adsorption process.
Figure 3 shows the Cr(VI) isotherm adsorptions for isomeric FeOOH in solutions initially with pH 7.0 and the corresponding parameters are listed in Table 2. It was found that Langmuir and Freundlich models were suitable for describing the adsorption behavior of Cr(VI) on FeOOH. The correlation coefficient R2 for Langmuir and Freundlich isotherm models were larger than 0.92. It suggested that the Cr(VI) might be adsorbed on the surfaces of adsorbents in the mixed form of monolayer and multilayer. The theoretical adsorption capacity for Aka2 calculated from Langmuir model was 30.3 mg/g. It was larger than those (12.7–15.1 mg/g) for Gth1, Gth2, Aka1 and Lep. Also, it was calculated that the Langmuir constant b had a positive value and the Freundlich constant n was up 1. It implied there were the favorable conditions for Cr(VI) adsorptions by FeOOH adsorbents. Further, according to the fitting results in Table 2 for the physical meanings of the K and n parameters (Wu et al. 2016; Yang et al. 2017), it could be inferred a decreasing trend (i.e. Aka2 > Aka1 ≈ Gth1 ≈ Lep ≈ Gth2) for Cr(VI) adsorption capacities of isomeric FeOOH.
|Name||Langmuir constants||Freundlich constants|
|R2||Qm (mg/g)||b (L/mg)||R2||K||n|
|Name||Langmuir constants||Freundlich constants|
|R2||Qm (mg/g)||b (L/mg)||R2||K||n|
Actions of pH on Cr(VI) adsorption by isomeric FeOOH
Cr(VI) adsorption by isomeric FeOOH at various pH values of 3 to 12 as shown in Figure 4, the capacities of Cr(VI) adsorption by FeOOH adsorbents decreased with increasing pH values. As the solution pH values increased, the adsorption capacities for Aka2 decreased quickly in the pH range of 3–6. While in the pH range of 4–7 (or 6–9), it retained the constants for the other four adsorbents (or for Aka2), and finally at the values than pH 9, it decreased sharply for all adsorbents. Comparatively, Aka2 had a higher Cr(VI) adsorption capacity in a wider pH range.
Apparently, the adsorbents achieved the higher Cr(VI) removal rate at the lower solution pH values, while they had a sharp drop at the higher pH values. This is due to a correlation between solution pH and the surface charge of adsorbents and the zeta potential values of five FeOOH adsorbents being just at the lower pH values of 4.0–4.5 (Figure 5). Also, it has been recognized that the surface of FeOOH has a negative/positive charge and mainly exhibits strong electrostatic repulsion to the anion when the solution pH values are above/below the zeta potential values (Wang et al. 2017; Li et al. 2018).
On the other hand, the positively charged surface of FeOOH is favorable for the adsorption of HCr2O7– and Cr2O72– in the acidic solution, while in the alkaline solution, their negatively charged surface is not conducive to adsorption of Cr2O72– or CrO42– (Ren et al. 2012; Lu et al. 2017). Thereby, in the present work, there was a fleetly decreased adsorption capacity for the isomeric FeOOH (except for Aka2) at the higher pH values of 8 to 12. A similar result obtained in the polluted waters with an original pH ranging in 6 to 8.5 was reported by Towler et al. (2009). It showed that in the basic surface waters polluted by Cr(VI), there was an advantage of not adjusting solution pH for FeOOH applied in removing the contaminant. Also, the pH-dependent Cr(VI) adsorption by FeOOH has great potential significance in engineering application.
Effect of adsorbate concentration and adsorbent content
As seen in Figure 6, with increase of Cr(VI) concentration in reaction solutions with initial pH 7.0, adsorption amount of Cr(VI) on FeOOH adsorbents increased. Meanwhile, in solutions with 40 mg/L Cr(VI), the adsorption amounts of Cr(VI) on FeOOH were about three times of those in solutions with 10 mg/L Cr(VI). This result on the Cr(VI) concentration impacts was consistent with that reported in the literature (Xie et al. 2015). However, according to the results of the above isothermal experiments, the adsorption amount would gradually slow down and eventually reach an adsorption saturation state, when the adsorbate concentration kept enhancing. It could be explained by that the presence of more Cr(VI) ions in per unit number of adsorbent sites leaded to a saturation of the coordination sites (Xie et al. 2015).
In addition, under the same reaction condition, Cr(VI) adsorption removal for isomeric FeOOH at different contents are shown in Figure 7. When the contents of five FeOOH adsorbents ranged in 0.25–2.5 g/L Cr(VI), their Cr(VI) adsorption capacities hardly had difference (Figure 7(a)). It indicated that the adsorbents with the contents of 0.25–2.5 g/L had the better constant adsorption capacities. In addition, the percent of Cr(VI) removal in Figure 7(b) increased with the FeOOH contents increasing. This could be explained by that the active sites increased as the FeOOH contents increased (Wang et al. 2013; Wang et al. 2015).
Effects of the coexisting anions
Under the consistent reaction condition, the results on impacts of coexisting anions (Cl–, NO3–, SO42–, CO32– and H2PO4–) at various concentrations (0.001, 0.01 and 0.1 mol/L) on the amounts of Cr(VI) adsorbed by isomeric FeOOH are shown in Figure 8. Three anions of Cl–, CO32– and NO3– had the negligible influence. It showed that the binding affinities of these anions towards the active sites of FeOOH were much weaker than chromate ions. However, both of SO42– and H2PO4– anions dramatically inhibited the Cr(VI) adsorption by FeOOH adsorbents, when their concentration increased from 0.001 to 0.1 mol/L, which mainly resulted from the surface complexation type and the electrostatic interaction. A consistent result on Cr(VI) adsorption by goethite was also reported by previous researchers (Yu et al. 2014; Ghosh et al. 2015). Moreover, these anions occurring in the aqueous phase of practical and natural water environments could also compete with chromate anions and affect the efficiencies of pollutant adsorption removal (Xie et al. 2015).
The batch experiments were carried out to study the effects of reaction conditions on the adsorption of Cr(VI) by isomeric FeOOH. Results showed that the optimum pH for Cr(VI) adsorption was in the range of 3 to 8. Under isothermal conditions, according to the Langmuir and Freundlich equations, it could be concluded that there was the order of Aka2 > Aka1 ≈ Gth1 ≈ Lep ≈ Gth2 for their Cr(VI) adsorption capacities at the equilibrium states. The optimum levels of adsorbate and adsorbent mainly depended on the number of active sites on surfaces of isomeric FeOOH. The anions of SO42– and H2PO4– dramatically inhibited Cr(VI) adsorption, while Cl–, CO32– and NO3– had negligible influence.
The authors acknowledge the National Natural Science Foundation of China (nos. 41472034, 40902018 and 31372133) and Jiangsu Provincial Key Laboratory of Environmental Materials and Engineering (nos. K14022 and K13058) for supporting for the present study.