Application of Novel Fulvic Acid-Coated Magnetite Nanoparticles for CO2−-Mediated Photoreduction of Cr(VI)

We here isolate fulvic acids from vermicompost to prepare and characterize novel fulvic acid-coated magnetite nanoparticles. UV-A irradiation of suspensions of the nanoparticles under different experimental conditions led to photo-reduction of Cr(VI). In anoxic conditions in the presence of formic acid, after 60 min of irradiation ca. 100% of Cr(VI) was reduced. Under these conditions, the carbon dioxide radical anions, CO2.-, mediated the photo-reduction of Cr(VI). However, the high reduction potential of Cr(VI) and the variety of reactive species generated upon UV-A irradiation make this nanomaterial also suitable for Cr(VI) photo-reduction also under aerobic conditions in the presence of formic acid or under anoxic conditions without the addition of formic acid. The possible photodegradation routes involved are discussed in detail.


Introduction
We have previously shown that anaerobic UV-A irradiation of 1,4-naphthoquinone (NQ) solutions containing formic acid results in the oxidation of formic acid by the triplet state of NQ to yield the carbon dioxide radical anion, CO 2 .-,which reduced HgCl 2 (aq) to Hg 2 Cl 2 (s) (Berkovic et al. 2012).
Humic acids (HA) and fulvic acids (FA) are operationally defined fractions of humic acids based on their solubility difference in acid and base.HA are soluble in solutions with pH > 2, whereas FA are soluble under all pH conditions (Mac Carthy 2001).The higher content of quinones in FA with respect to HA (Wu et al. 2013) motivated us toward the successful replacement of NQ with a commercial FA for the anaerobic UV-A photoreduction of HgCl 2 mediated by CO 2 .- (Berkovic et al. 2012).
Mixed-valence nanoscale iron oxides, such as magnetite, which present high surface areas and sorption capacities, are potential materials suitable for environmental remediation (Auffan et al. 2007).
These insoluble magnetic materials can be employed as nanoadsorbents of metal ions (Carlos et al. 2013) and organic dyes (Inbaraj and Chen, 2011) among other contaminants.
Since bare magnetite nanoparticles are susceptible to air oxidation (Maity and Agrawal 2007) and are easily aggregated in aqueous systems, their stabilization is desirable.HA-coatings enhance the stability of nanodispersions of the Fe 3 O 4 particles by preventing their aggregation (Shahid et al. 2017).
On the basis of the successful employment of FA for the reduction of HgCl 2 and because of the convenience of using magnetic nanoparticles as carriers of FA, the aim of the present work is to synthesize and characterize novel FA-coated magnetite nanoparticles (VFANP) employing FA extracted from vermicompost (VFA).It is apparent from the above discussion that irradiation of colloidal suspensions of these nanoparticles in the presence of formic acid should generate CO 2 − radicals.
Since the most prevailing forms of chromium in the environment are Cr(VI) and Cr(III), being Cr (VI) 1000 times more toxic than Cr (III) (Shahid et al. 2017), several water treatments involving the reduction of Cr(VI) to Cr(III) have been developed.They include bioreduction (Zhang et al. 2014;Karthik et al. 2017), electrochemical reduction (Rodriguez-Valadez et al. 2005;Barrera-Díaz et al. 2011), and photocatalysis (Wang et al. 2010;Wang et al. 2016), among others.For the anaerobic photo-reduction of Cr(VI) to Cr(III), we propose here the use of VFANP, which can be easily separated and removed by applying external magnetic fields.

Extraction of VFA
The fulvic acid sample (VFA) was extracted from vermicompost by a procedure proposed by Yang (Yang and Xing 2009).Briefly, a solution containing the 0.1 mol/L NaOH was mixed with vermicompost at 10:1 (v:w).The mixture was purged with N2 gas and shaken for 24 h at room temperature.The suspension was centrifuged and the supernatant was collected and acidified to pH 1, by addition of HCl, to precipitate and remove the humic acids fraction.The supernatant was adjusted to a pH of 6.0; this suspension was filtered with a membrane filter to obtain the VFA solution.The extract was dried in a rotary evaporator and then in an oven at 60 °C in vacuum.

Synthesis of Magnetic Nanoparticles
A modification of the procedure reported by Carlos (Carlos et al. 2012) for the synthesis of magnetite nanoparticles with a HA capping was used here to prepare VFANP.Briefly, 6.1706 g FeCl 3 .3H 2 O and 3.0185 g FeCl 2 .4H 2 O were dissolved in water and heated to 90 °C.Then, two aqueous solutions were added rapidly and sequentially: (1) 10 mL of 25% ammonium hydroxide and (2) 50 mL of 1.0% w/v VFA.The mixture was stirred at 90 °C for 30 min and then cooled to room temperature.The product was filtered through an HPLC filtration device with 20-nm-nylon filters, washed with water, then was dried at 60 °C for 5 h in vacuum.The black precipitate obtained was stored at room temperature as a dry black powder prior to use in the experiments.

Steady-State Irradiation Experiments
Ar-saturated colloidal suspensions of VFANP containing formic acid were irradiated in a Rayonet RPR-100 reactor equipped with eight RPR-3500 lamps.The samples were contained in 25-mL glass tubes.
After irradiation the nanoparticles were separated with the aid of a magnet.The assays were performed at pH = 2, where the main species of Cr(VI) is Cr 2 O 7 2− and its concentration was followed by UV-visible spectroscopy (ε 346 = 3112 M −1 cm −1 ) (Kumagai et al. 2011).All the experiments were performed by triplicate.

Characterization of VFA
Figure 1 shows the ATR transmittance spectrum of VFA.The highest absorption band at around 3400 cm −1 is assigned to the O-H stretching of carboxylic acids, phenols, and alcohols.The bands around 2920-2850 cm −1 are characteristics of the aliphatic C-H stretching (Andjelkovic et al. 2006;Martín et al. 2014).Bands at around 1350 and 1600 cm −1 are due to C=O stretches.The broad band at 1100-1000 cm −1 present only in the extract is characteristic of aromatic ethers and polysaccharides (Amir et al. 2003).The sharp band below 700 cm −1 corresponds to Fe-O stretching vibration, which is centered at ~630 cm −1 (Carlos et al. 2012).
From the C content of the VFA (9.80%) as determined by TOC analysis and the UV-visible absorption spectrum (Fig. 2a), the molar absorptivity at 280 nm per mole of organic carbon ε 280 = 119.5 L mol CO −1 .cm−1 was obtained.This is a useful parameter for the estimation of an operational molecular weight of humic substances MW.From the linear correlation between ε 280 and MW, a value of MW = 967 Da is obtained (Chin et al. 1994).The absorbance ratio at two different wavelengths in the visible region, 465 and 665 nm (namely E4/E6 ratio), has been long considered to inversely correlate with the molecular size and/or the formation of multiring aromatic systems in humic substances (Uyguner and Bekbolet 2005).The values of ε 280 = 119.5 L mol CO −1 .cm−1 , molecular weight (MW) = 967 Da, and E4/E6 = 3.30 obtained here for VFA are all in complete agreement with those reported for other fulvic acids (Chin et al. 1994).

Characterization of VFANP
The results of VFANP characterization are summarized in Table 1.SEM images of VFANP show that the average nanoparticle size is (13.2 ± 0.3) nm (Fig. 3).The specific surface area (S BET ) was (84 ± 4) m 2 g −1 .
The absorption at 650 cm −1 present in the ATR spectrum of VFANP (Fig. 1) confirms the presence of magnetite in the samples (Carlos et al. 2012).Significant differences in the bands assigned to C=O stretches at around 1400 and 1600 cm −1 are indicative of the interaction between carboxylate anions of VFA with the FeO surface, as observed for magnetite nanoparticles coated with dimercaptosuccinic and humic acids (Carlos et al. 2012;Yantasee et al. 2007;Liu et al. 2008).
Figure 4a shows the DTA-TGA curves of nanoparticles.The TGA curve exhibits a constant mass loss from ambient temperature to 800 °C.The endothermic peak width accompanied by a significant mass loss, below 200 °C, in DTA curve corresponding to desorption of physisorbed water in the sample.The exothermic peak at 430 °C is attributed to decomposition of fulvic acid present on the surface of the nanoparticle.This peak indicates the formation of Fe-FA complex, since the decomposition of fulvic acid cores without metallic bonds takes place at a higher temperature (about 450 °C).Finally, the exothermic peak at 630 °C assigned to a possible transition phase magnetite-hematite (Chen 2013) or a possible reduction of Fe 3 O 4 to FeO (Schnitzer and Kodama 1971).
The pH dependence of the zeta potential (ζ) of VFANP dispersions in 10 −3 M KCl aqueous solutions shows a pH of zero point charge (pH PZC ) of 5.7 (Fig. 4b).The shift of the pH PZC value with respect to the underivatized magnetite nanoparticles (pH PZC = 6.7) (Chang and Chen 2005) confirms the coverage with the organic material, since the VFA is negatively wavenumber (cm -1 ) charged in the whole pH range due to the deprotonation of carboxylic and phenolic groups (Fig. 4b).
The apparent diameter of the nanoparticles obtained from DLS measurements performed with 1 mg mL −1 of VFANP suspensions at pH = 4.5 ± 0.2 was 206.6 ± 1.9 and 1062.4 ± 70.6 nm in water and 2.10 M KCl, respectively.Thus, the aggregation is favored at high ionic strength, as expected from the reduction of the thickness of the electric double layer and repulsive forces under this condition (Hotze et al. 2010).
The magnetization curves of VFANP and Fe 3 O 4 at 300 K (Fig. 5) show superparamagnetic behavior, including zero coercivity (H c ) and remanence (M r ) (Luo et al. 2014).The saturation magnetization (M s ) of the underivatized magnetite nanoparticles and VFANP were 65.256 and 60.892 emu g −1 , respectively.The decrease of M s upon derivatization of magnetite with organic coatings was previously observed (Sun et al. 2009;Magnacca et al. 2014) and was assigned to quenching of surface moments, resulting in the reduction of magnetic moment in the coated particles (Kim et al. 2003).

Steady-State Irradiation Experiments
Ar-saturated aqueous colloidal suspensions of 2 g L −1 VFANP (pH = 2) containing 0.4 M formic acid, and 0.1 mM K 2 Cr 2 O 7 were irradiated at 350 nm (eight lamps).The decay of the Cr 2 O 7 2− concentration upon irradiation follows a first order kinetics with an apparent rate constant of 0.057 min −1 (Fig. 6a).
As can be seen in Fig. 6  radicals (Vaughan and Blough 1998) and these oxidative species also scavenged the yield of CO 2 − radicals (David Gara et al. 2007).These radicals are able to reduce Cr 2 O 7 2− to Cr 3+ according to reaction 3 (Ren et al. 2017) (see reduction potentials in Table 2).
Moreover, according to the strength as reducing agents of some of the triplet states of humic substances estimated by McNeill and Canonica (McNeill and Canonica 2016) (see reduction potential in Table 2), these excited states as well as charge-transfer states (Zhang and Blough, 2016) can also reduce Cr(VI).
Near-UV/visible photoionization of humic substances is known to generate solvated electrons and radical cations of phenolic groups, which rapidly deprotonate to yield phenoxyl radicals (Martin et al. 2017).The reduction of Cr 2 O 7 2− to Cr 3+ by solvated electrons and phenoxyl radicals is also thermodynamically feasible (Table 2), and thus these reaction could also contribute to the efficient photodegradation of Cr(VI).Even hydrogen peroxide (Zhang and Blough, 2016) could act as reducing agent (Table 2).
Experiments performed under Ar-saturation in the presence of VFANP but without formic acid showed a 48% degradation of Cr(VI) (Fig. 6).In the absence of HCOOH, CO 2 − is not formed but triplet states, phenoxyl radicals and solvated electrons could be responsible for the reduction of Cr(VI).
Blank irradiation experiments performed with Arsaturated K 2 Cr 2 O 7 solutions without nanoparticles and formic acid for 60 min showed only a 15% Cr(VI) consumption (Fig. 6b), indicating that the contribution of direct photolysis is a minor photodegradation route.Blank experiments in the dark during the same period revealed no Cr 2 O 7 2− consumption, in line with the reported slow reduction of Cr(VI) by FA (Wittbrodt and Palmer 1995).

Conclusion
We here clearly demonstrate that FA can be easily transformed into a magnetic additive for the photochemical treatment of water contaminated with Cr(VI).
The high reduction potential of Cr(VI) and the variety of reactive species generated upon UV-A irradiation make this nanomaterial suitable for Cr(VI) photoreduction under several conditions.The best performances were obtained under Ar-saturation and in the presence of formic acid.However, albeit with a lower efficiency, the nanomaterials also photosensitize the reduction of Cr(VI) either under aerobic conditions in the presence of formic acid or under anoxic conditions without the addition of formic acid.The possible photodegradation routes involved are discussed in deeply.
The photochemical treatment described here involves the use of UV-A light, which is in the region of the electromagnetic spectrum (320-400 nm) within the solar radiation reaching the Earth's surface.Thus, the procedure could be useful for Cr(VI) reduction in aqueous systems because solar radiation could be employed as light source and the nanomaterials could be easily removed by application of an external magnetic field.

Fig
Fig. 2 a UV-visible spectrum of a 0.82 g L −1 solution of VFA at pH = 7. b Excitation-emission matrix of VFA

Fig. 4 a
Fig. 3 SEM images and histogram from VFANP

Fig. 6 a
Fig. 5 Magnetization curves of Fe 3 O 4 and VFANP

Table 2
Literature redox potentials