Development of hollow δ-FeOOH structures for mercury removal from water

Mercury (Hg) is one of the most toxic metals that can affect human health, due to its volatility, persistence, and bioaccumulation (Yu et al. 2016). Severe damage to the nervous system, kidneys, and other organs has been reported after Hg exposure (Shen et al. 2014). Thus, there is great concern about the release of mercury into the environment; for example, through mining, and/or urban and industrial waste disposal (Ahamad et al. 2017). Among the main chemical forms of Hg, inorganic Hg(II) species in water pose a serious threat because of their mobility and complexity, showing features of wide distribution, low concentration and difficulty in control (Santana et al. 2016).

The US Environmental Protection Agency (USEPA), and WHO have set/recommended maxima of 1 μg-Hg L⁻¹ for water and 5 μg-Hg L⁻¹ for wastes discharged to the environment (Homayoon et al. 2017). In recent years, various techniques have been used to remove Hg(II) from water, including solvent extraction, precipitation, coagulation, ion-exchange, reverse osmosis, membrane separation, coagulation, and photoreduction (AlOmar et al. 2017). Adsorption is considered an efficient and low cost technique for Hg (II) removal from wastewater effluents (Burakov et al. 2018).

Iron oxyhydroxide (FeOOH) nanoparticles are cheap, harmless and eco-friendly (Patra & Kim 2017), and used widely for efficient removal of various metals from aqueous media (Kokkinos et al. 2015). Iron hydroxides have been used widely as decontaminants to remove oxyanions like arsenite, arsenate, chromate, vanadate, phosphate, etc (Davantès et al. 2016). Recently, δ-FeOOH, magnetic iron oxyhydroxide, has been reported as an efficient adsorbent for Hg(II) from contaminated water (Maia et al. 2019). The maximum Hg(II) adsorption capacity of δ-FeOOH at pH 7 was determined as 35 mg g⁻¹. δ-FeOOH has a significant number of -OH groups on its surface, providing an attractive platform for coordination with Hg(II) metal species. Thus, to pursue high adsorptive performance, it was interesting to increase the number of active surface hydroxyl groups on it.

In this work, δ-FeOOH hollow structures were synthesized on a mesoporous silica surface, which was then treated with NaOH to dissolve the silica. The δ-FeOOH hollow structures were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), BET specific surface and zeta potential. The effect of variables on Hg(II) adsorption was investigated and the material was used to treat mercury-contaminated water from the Doce River, Governador Valadares, Minas Gerais, Brazil.

(NH₄)₂Fe(SO₄)₂.6H₂O (CAS: 7783-85-9), NaOH (CAS: 1310-73-2), H₂O₂ 30% (w/w) (CAS:7722-84-1), and cetyltrimethylammonium bromide (CTAB) (CAS: 57-09-0) were purchased from Vetec (Brazil). Sigma-Aldrich supplied HgCl₂ (CAS 7487-94-7) and tetraethyl orthosilicate (TEOS) (CAS: 78-10-4). All chemicals were of analytical grade and used as supplied. The samples and standards were prepared using ultrapure water with a resistivity of 18.2 MΩ cm, which was obtained from a Thermo Scientific Barnstead™ Nanopure™ water purification system.

The mesoporous silica was synthesized as by Coelho et al. (2013) reported by dissolving 1.63 g of CTAB in a mixture of 30 mL of ultrapure water and 10 mL of 1 mol L⁻¹ NaOH, adding 3.7 ml of TEOS and stirring the mixture for 24 hours at pH 11. After the contact time, the white precipitate was collected by centrifugation and washed several times until the pH of the final wash was 7. The material obtained was dried in a vacuum desiccator at ambient temperature for 48 hours. To remove the CTAB, 1,0 g of the dried solid was calcined in a muffle furnace at 550 °C for 5 hours.

MCM (1.0 g) was dispersed into 50 mL of 0.3 mol L⁻¹ of Fe²⁺ solution and sonicated for 5 minutes. Fe²⁺ was precipitated as a green rust in the presence of MCM with 50 mL of 2 mol L⁻¹ NaOH, after which 10 mL of 30% H₂O₂ was added immediately, with stirring. Stirring continued for 30 minutes. The reddish brown precipitate FeOOH/MCM formed was washed several times with deionized water and dried in a vacuum desiccator at room temperature.

δ-FeOOH hollow structures were prepared by dissolving 1.0 g of δ-FeOOH/MCM in 50 mL of 5 mol L⁻¹ NaOH and stirring at room temperature for 2 hours. Finally, the yellowish-brown material obtained was washed three times with deionized water and dried in a vacuum desiccator.

Powder X-ray diffraction (XRD) data were collected from 5 to 80° 2θ at a step width of 0.02°, 0.6 s per step, using Cu-Kα (λ = 1.540560 Å) radiation in a Rigaku Geigerflex diffractometer equipped with a graphite diffracted-beam monochromator. Silicon was used as the external standard. Morphology analyses were conducted with a JEOL JEM 2000EXII transmission electron microscope. The specific surface of the nanoparticles was determined by the BET method, using N₂ adsorption/desorption in an Autosorb 1 Quantachrome instrument and the Barrett-Joyner-Halenda (BJH) method to determine the pore diameter distribution (Barrett et al. 1951). The zeta potential was measured by dynamic light scattering (Zetasizer Nano ZS, Malvern, UK). OriginPro (OriginPro, Student Version, 2021) software was used for data processing.

Hg(II) adsorption was investigated by batch method studies to examine the effect of various parameters – pH, contact time, initial Hg(II) concentration, and interferents – and determine the conditions leading to maximum Hg(II) sorption capacity. All experiments were carried in duplicate at 25 ± 0.1 °C in 50 ml, metal free tubes metal stirred at 200 rpm, using 10 mg of adsorbent. The effect of pH on Hg(II) adsorption was investigated at pH = 3, 5, 7, 9 and 11.

The Hg(II) adsorption rate was determined by batch experiments with varying reaction times. Solutions were in contact with hollow δ-FeOOH nanoparticles for a period of time between 0 and 360 minutes. Adsorption isotherms were obtained by varying the initial Hg(II) concentration in the range 0–40 mg/L. The effect of interferent anions on Hg(II) removal from solution was investigated by performing adsorption experiments in the presence of solutions containing 0 and 50 mg L⁻¹ of Cl⁻, NO₃⁻, SO₄²⁻, PO₄³⁻. Binary mixtures of Hg²⁺ ions with different Cu²⁺, Co²⁺, Cd²⁺, Zn²⁺, Ni²⁺, Na⁺ or Mg²⁺ metals in 0.1 mg L⁻¹ concentration were investigated to determine the adsorption capacity of hollow δ-FeOOH in the presence of competitive cations. The initial and residual concentrations of mercury were determined with an inductively coupled plasma mass spectrometer (ICP-MS) (Perkin Elmer NexIon 300D). The analytical conditions are summarized in Table 1.

The water contaminated with mercury was collected in Governador Valadares, Minas Gerais, Brazil, in November 2015, using the procedure adopted by Environmental Company of the State of São Paulo (CETESB or Environmental Company of the State of São Paulo) (CETESB 2011). The samples were collected at two sites on the Doce River: P1 is within the area serviced by the water and sewage treatment system at 18°52′55.31 (S) 41°57′.86″ (W) and P2, on the urban perimeter, is at 18°51′22.54 (S) 41°56′10.55″ (W).

A nano-adsorbent filter was developed using the procedure reported by Maia et al. (2019) to remove mercury from water. 5 mL of a standard solution containing 50 μg-Hg(II) L⁻¹ was applied to each filter, which then drained by gravity. The filtrate was collected after 20 minutes of continuous flow.

The TEM image for the δ-FeOOH/MCM hybrid in Figure 2(a) shows a nanoparticle agglomerate, with a clear contrast between the dark δ-FeOOH and pale amorphous SiO₂ that can be seen. It is impossible to define a characteristic form. The δ-FeOOH hollow structures were obtained by treating δ-FeOOH/MCM with 5 mol L⁻¹ NaOH to remove the silica. It is possible from the STEM image (Figure 2(b)) to infer the formation of cavities on the nanoparticle surfaces, confirming that much of the nanomaterial has a hollow interior.

The N₂ adsorption–desorption isotherms for the δ-FeOOH/MCM and hollow δ-FeOOH are shown in Figure 3. The isotherms exhibit high N₂ adsorption, reaching 170 and 156 cm³ g⁻¹ at p/p₀ = 0.95 for δ-FeOOH/MCM and hollow δ-FeOOH, respectively. A type IV isotherm was exhibited, characteristic of mesoporous materials, and a type H3 hysteresis loop, which is related to the filling of the mesopores due to capillary condensation. The BET area decreased from 94 m² g⁻¹ in δ-FeOOH/MCM to 63 in hollow δ-FeOOH. This occurred because part of the silica was removed in the treatment with NaOH. The pore size distribution (Figure 3, inset) calculated from the desorption branch of the N₂-isotherms by the BJH method, showed an average pore size of 20 nm, thus confirming the presence of mesopores.

The effect of pH on Hg(II) adsorption onto hollow δ-FeOOH is shown in Figure 4(a). As the pH increased up to 9, mercury adsorption also increased. However, at pH 11, there was a slight decrease. The Hg(II) adsorption capacity at pH 9 was 8.01 mg g⁻¹ corresponding to a removal efficiency of 93.3%. Subsequent studies at pH 7 showed Hg(II) removal efficiency of 92.4%.

The change in solution pH results in the formation of different aqueous Hg(II) species, including Hg²⁺, HgOH⁺, and Hg(OH)₂ (Lu et al. 2014), and different hollow δ-FeOOH surface charges. When the mercury solution pH is below the point of zero charge (PZC) of hollow δ-FeOOH (PZC = 4.72, Figure 4(b)), the surface of hollow δ-FeOOH is positively charged and interacts very weakly with mercury cations, due to protonation of the surface functional groups, and competition between the mercuric and H⁺ ions present in the solution (Husnain et al. 2016). On the other hand, when pH exceeded the PZC, the hollow δ-FeOOH surface was negatively charged and more functional groups were available for metal cation binding due to deprotonation, resulting in electrostatic attraction between Hg(II) and hollow δ-FeOOH. At pH above 9, insoluble metal hydroxide species (Hg(OH)₂) are formed, lowering adsorption efficiency (Shen et al. 2014).

The kinetics of Hg(II) adsorption are shown in Figure 5(a). Preliminary studies on the rate of mercury removal by hollow δ-FeOOH at pH 7 indicated that some 89.7% of the total mercury was removed after 60 minutes’ adsorption, suggesting that Hg(II) adsorption on hollow δ-FeOOH is rapid initially but decreases gradually with time. Subsequently, the amount adsorbed increased slightly, and a maximum removal efficiency of 91.7% (8.13 mg g⁻¹) was determined. This could arise from the high specific surface of δ-FeOOH due to its small particle size.

The linear plot of t/q_t versus t is shown in Figure 5(a). The correlation coefficient for Hg(II) was 0.999. Moreover, the values of q_e calculated show good agreement with the experimental data (Table 2). Hg(II) adsorption in hollow δ-FeOOH can thus be approximated favorably by the pseudo-second-order adsorption model. The value of k₂ was calculated as 0.1151 g mg⁻¹ min⁻¹ and q_e (calculated) was 8.141 mg g⁻¹.

The effect of the initial Hg(II) concentration on adsorption capacity was also studied (Figure 6). The value of q_e when the Hg(II) concentration was 1.5 mg L⁻¹ was 2.56 mg g⁻¹, showing that the adsorbent is effective in adsorbing Hg(II) even at lower concentrations.

The maximum Hg(II) adsorption capacity calculated using the Langmuir model was 89.1 mg g⁻¹, which is higher than that found for δ-FeOOH (Maia et al. 2019), possibly due to the increased number of hydroxyl groups (main binding site) in the hollow interior of the nanoparticles. The value of R_L (=1/(1 + k_LC₀)) was below 1.0, indicating that the adsorption process was favorable, which was further supported by the 1/n values below 1 in the Freundlich model.

The maximum Hg(II) adsorption capacity of hollow δ-FeOOH was compared with that of other iron oxides and hydroxides reported previously in the literature. The 89.1 mg g⁻¹ found for hollow δ-FeOOH exceeded those for materials like FeOOH and FeMnOOH (6.4 and 11.9 mg g⁻¹) (Kokkinos et al. 2015); Fe₃O₄ and Fe₃O₄@SiO₂ (24.80 and 29.52 mg g⁻¹) (Afraz et al. 2017), and Fe-Mn/graphene oxide composite (32.9 mg g⁻¹) (Tang et al. 2016), but was lower than the 200 mg g⁻¹ found for mesoporous α-FeOOH adsorbent (Patra & Kim 2017), with its high specific surface (152.4 m² g⁻¹).

Coexisting ions in solution affect many aspects of the adsorption process (Zhou et al. 2017). For example, different species can compete for the limited number of active sites, influence electrostatic interactions and impede the ion transfer from solution to the adsorbent surface (Tan et al. 2015).

Figure 7(a) shows that Hg(II) adsorption efficiency was affected little by the presence of NO₃⁻ or SO₄²⁻. The decrease of about 5% adsorbed mercury in the presence of Cl⁻ was due to the formation of aqueous Hg(II)-Cl complexes; that is, HgCl₃⁻ and HgCl₄²⁻ (Shan et al. 2015), leading to electrostatic repulsion from the negative surface of hollow δ-FeOOH. With phosphate, the reduction was slightly greater at about 7%, because species formed that have little or no affinity with the variable-charge surfaces (Sarkar et al. 2000).

Figure 7(b) shows the competitive adsorption of Hg(II) in the presence of various metal ions in water. There was no significant interference in adsorption capacity at the concentration studied.

The filter containing 0.5 g of hollow δ-FeOOH was tested initially for the treatment of water contaminated with a mercury standard at 50 μg-Hg L⁻¹. After filtration, the concentration was 1.150 μg-Hg L⁻¹, a removal efficiency of 97.7%. Samples of water collected from the Doce River and filtered in the same way contained Hg concentrations below the limit of detection – see Table 4.

Hollow, mesoporous δ-FeOOH structures were fabricated successfully using a simple, eco-friendly method. They are an effective tool for Hg(II) adsorption from aqueous solutions. Studies showed that their efficiency in Hg(II) adsorption is about 92.4% at pH 7.

The results of kinetic tests showed that Hg(II) adsorption was fast, occurring in the first 60 minutes with approximately 90% removal. The material offered high adsorption capacity (89.1 mg g⁻¹) with low interference for Hg(II) removal. The nanomaterial filter produced was efficient in mercury removal from water from the Doce River, lowering the mercury concentration below the maxima recommended by WHO and USEPA.

The authors are grateful to CNPq, FAPEMIG, Rede Mineira de Química (RQ-MG), and CAPES for the financial support and fellowships.

There are no conflicts to declare.

All relevant data are included in the paper or its Supplementary Information.