Natural Magnetite: an efficient catalyst for the degradation of organic contaminant

Iron (hydr)oxides are ubiquitous earth materials that have high adsorption capacities for toxic elements and degradation ability towards organic contaminants. Many studies have investigated the reactivity of synthetic magnetite, while little is known about natural magnetite. Here, we first report the reactivity of natural magnetites with a variety of elemental impurities for catalyzing the decomposition of H2O2 to produce hydroxyl free radicals (•OH) and the consequent degradation of p-nitrophenol (p-NP). We observed that these natural magnetites show higher catalytic performance than that of the synthetic pure magnetite. The catalytic ability of natural magnetite with high phase purity depends on the surface site density while that for the magnetites with exsolutions relies on the mineralogical nature of the exsolved phases. The pleonaste exsolution can promote the generation of •OH and the consequent degradation of p-NP; the ilmenite exsolution has little effect on the decomposition of H2O2, but can increase the adsorption of p-NP on magnetite. Our results imply that natural magnetite is an efficient catalyst for the degradation of organic contaminants in nature.

Text S2 The digestion procedure of magnetite samples before ICP-AES analysis.
Text S3 The procedure of surface site density analysis.

Figure S7
UV-Vis spectra of p-NP during its degradation by TS magnetite.

Figure S8
The pH-C(H s ) curves (a) and Cran-V curves (b) for natural magnetites obtained from the acid-base potentiometric titration.
Text S1 The discussion on the Mö ssbauer characterization results of natural magnetite samples.
The Mö ssbauer spectral parameters of the TS and ZK magnetite samples indicate the sole phase of magnetite with hyperfine field (B hf ) of ~49.1±0.1 T (tetrahedral site A) and 46.0±0.3 T (octahedral site B) and isomer shifts (IS) of 0.29±0.1 and 0.66±0.1 mm s -1 , respectively ( Figure S5 and Table S2). The relative ratios of the A to B site area is almost 0.5. The Mö ssbauer spectral parameters of TS and ZK magnetites are similar to those of pure magnetite 1 . This suggests that most of the trace metal cations do not enter the structure of magnetite, which is consistent with the EDS analysis results. For HN sample, its Mö ssbauer parameter is identical to that of maghemite (γ-Fe 2 O 3 ), with B hf of ~51.9 T and IS of 0.329 mm s -1 , respectively (Table S2). From ICP-AES measurements (Table 3) and Electron probe microanalysis ( and Fe 3+ on the octahedral sites of pleonaste. The relative area of ilmenite in DM is 19.9%, obviously higher than that in PZH (8.2%), while the relative area of pleonaste in DM is 3.7%, lower than that in PZH (5.4%), indicating the higher ilmenite content and lower pleonaste content in DM than that in PZH, which is consistent to the XRD analysis results. 6 Text S2 The digestion procedure of magnetite samples before ICP-AES analysis.
1.0 g of sample was mixed with by 0.6 mL of HNO 3 (50% v/v) and 0.6 mL of HF in a digestion vessel. The vessel was tightly sealed and heated at 100 o C for 3 days to evaporate the solvent. The obtained residue was digested by repeating the above procedure. Then the residue was completely dissolved by 2.4 mL HCl, and then diluted 2000 times with Milli-Q water for ICP-AES analysis. 7 Text S3 The procedure of surface site density analysis.
The surface site density (D s ) of magnetite particles, i.e., the number of proton-active sites (per nm 2 ), was determined by acid-base potentiometric titration 5 , by applying Eq. (1) : where N A is Avogadro's constant (6.02×10 23 ), S is the BET surface area (m 2 g −1 ), C s is the concentration of magnetite particles (1.0 g L −1 ), and H s is H + adsorption capacity (mol L −1 ). H s can be derived from Eq. (2): where V e is the titration volume of alkali at the equivalence point, given by the intersection of the Gran titration curve and the X-axis 6 , and C alkali is the concentration of alkali used in titration. In addition, the intrinsic acidity constants of the surface (pK a1 and pK a2 ), and the zero point of charge (pH zpc ), were obtained from the Gran titration curves ( Figure S8).
In the titration, the sample was mixed with NaCl solutions of different concentrations (c)   The rate of • OH generation increased gradually as the magnetite dosage was increased from 0 to 1.5 g L −1 . The • OH generation rate followed zero-order kinetics (linear regression). The rate of • OH generation also increased with a rise in H 2 O 2 concentration.
When the H 2 O 2 concentration increased from 5 to 60 mmol L -1 , the kinetics changed from zero-order to a power function.

Figure S8
The pH-C(H s ) curves (a) and Cran-V curves (b) for natural magnetites obtained from the acid-base potentiometric titration. From the acid-base titration curves, the intrinsic acidity constants of magnetite surface (pK a1 and pK a2 ), and the zero point of charge (pH zpc ) were from Figure S8.a. The surface site density (D s ) was obtained from Eqs. (1)-(2) and Figure S8.b.