Reusable magnetite nanoparticles–biochar composites for the efficient removal of chromate from water

Biochar (BC) and magnetite (Fe3O4) nanoparticles (MNP) have both received considerable recent attention in part due to their potential use in water treatment. While both are effective independently in the removal of a range of anionic metals from aqueous solution, the efficacy of these materials is reduced considerably at neutral pH due to decreased metal adsorption and MNP aggregation. In addition to synthetic metal oxide–biochar composites for use in treatment and remediation technologies, aggregates may also occur in nature when pyrolytic carbon is deposited in soils. In this study, we tested whether magnetite synthesized in the presence of biochar leads to increased removal efficiency of hexavalent chromium, Cr(VI), at the mildly acidic to neutral pH values characteristic of most natural and contaminated aqueous environments. To do so, magnetite nanoparticles and biochar produced from ground willow were synthesized to form composites (MNP–BC). Batch studies showed that MNP–BC markedly enhanced both adsorption and reduction of Cr(VI) from aqueous solution at acidic to neutral pH as compared to MNP and BC separately, suggesting a strong synergetic effect of hybridizing Fe3O4 with BC. Mechanistically, the Cr(VI) removal processes occurred through both adsorption and intraparticle diffusion followed by reduction to Cr(III). Synchrotron-based X-ray absorption spectroscopy analyses confirmed that Cr(VI) was reduced at the surface of MNP–BC, with electrons derived directly from both biochar and magnetite at low pH, while at near-neutral pH, biochar increased Cr(VI) reduction by inhibiting MNP aggregation. Extended X-ray absorption fine structure fitting results confirmed that the Cr(III) precipitates consist of Cr(OH)3 and chromite (Cr2FeO4) nanoparticles. Our results demonstrate that MNP–BC composites have great potential as a material for the treatment of chromate-containing aqueous solutions across a wide range of pH values, and provide information valuable broadly relevant to soils and sediments that contain biochar.


Results and discussion
Characterization of adsorbents. The elemental analysis of BC showed that it is primarily composed of C (76 wt%) and O (10 wt%). The specific surface area of BC as measured by Brunauer-Emmet-Teller (BET) is 319 m 2 /g (Table S2). BC is conductive; the sheet resistance of BC decreased as BC concentration increased (Fig. S1). TEM and HRTEM-SAED images ( Fig. 1) of MNP-BC show crystalline magnetite nanoparticles in the composites at various MNP to BC weight ratios. The MNP particles exhibited generally irregular and rounded morphologies, with a size range between approximately 10-20 nm, and in agreement with Fe 3 O 4 , ferrozine 43 results showed Fe(II)/Fe(III) ratios of approximately 0.5. The particle sizes were estimated using the TEM scale bars by measuring a number of particles (n = [15][16][17][18][19][20]. DLS analysis showed that size ranges of MNPs increased with increasing pH (Fig. S2); this is likely attributed to their agglomeration at higher pH, as shown in previous studies [5][6][7] . SEM images of MNP-BC composites (Fig. S3) showed MNPs are widely distributed on the surface of BC, evidencing that BC prevents MNP agglomeration.
FT-IR spectra (Fig. S4A) of MNPs and MNP-BC show a distinct band at 594 cm −1 that can be ascribed to Fe-O groups, consistent with the presence of magnetite 44 . The spectra also showed stretching vibrations of water molecules (free O-H) of MNPs and MNP-BC at 3210 cm −1 and 1410 cm −1 , respectively. BC has strong bands at 3425 cm −1 and 1590 cm −1 , corresponding to -OH and -C=O groups 22,45 , respectively. The same bands are observed in the MNP-BC composite. The vibrational band at 1110 cm −1 is assigned to stretching of the C-O-C moiety.
Zeta potential measurements, an indicator of surface charge, versus pH ( Fig. S4B) of MNPs, MNP-BC and BC showed that the isoelectric points-the pH values at which the particles carry no net electrical charge-for MNP and MNP-BC are near pH 6 and pH 5, respectively, whereas BC has an isoelectric point near pH 2. This demonstrates that the presence of organic functional groups slightly lowers the isoelectric point of the magnetite. . XPS spectra (Fig. S4E,F) are also consistent with the presence -C=O groups in both MNP-BC and BC. Organic functional groups (e.g., carboxyl), such as those present on humic substances, confer a negative charge which has the effect of lowering the overall surface charge of magnetite and decreases its isoelectric point (IEP) 39 . XRD analyses confirmed that MNPs and MNP-BC contain Fe 3 O 4 at various time intervals after synthesis and also after exposure to atmospheric conditions, i.e., exposure to O 2 (Fig. S5). Our XRD patterns show diffraction peaks at 2θ = 35.42° and 43.05°, which correspond to the (220) and (311) planes and represent the face-centered cubic inverse spinal structure of crystalline Fe 3 O 4 12 . The deviation in plane 400 at 43.05° over time can be attributed to a minor transformation of magnetite to maghemite, but we did not observe diffraction peaks consistent with maghemite. In addition to magnetite, the reflection of quartz is also observed in MNP-BC.
The potentiometric titration data were modeled using a non-electrostatic, discrete site surface complexation model to determine the proton binding constants and corresponding site concentrations for reactive surface sites of BC, MNPs and MNP-BC. A model that uses three sites for BC and MNP-BC and one amphoteric site for MNPs yielded the best fits to the experimental data for each sorbent, where variance, V(Y), values were in the range of 0.1 < V(Y) < 20 47 (Fig. S6). The pK a values for site 1 in BC and MNP are approximately 2.4 to 3.0, within the range of pK a values of carboxyl groups (pK a = 1.7-4.7) [48][49][50] . The pK a value for site 1 in MNPs of 5.0 corresponds to Fe-O (pK a = ~ 5). The pK a values calculated for sites 2 and 3 of BC and MNP-BC, ranging between 6.3 to 9.0, correspond to lactones (pK a = ~ 6) and phenolic hydroxyl (pK a = ~ 9.5) groups (Table S3) 44,[48][49][50][51][52] . The proton-active functional groups, including -C=O, -OH and Fe-O, that were used in modeling the potentiometric titration data were also detected in both FT-IR and XPS analyses, as is discussed above.
Effect of pH on adsorption and reduction of Cr(VI). The pH-dependent adsorption kinetics of Cr(VI) for BC, MNPs and MNP-BC were studied in the pH range of 2-7 ( Fig. 2), levels relevant to those found from acid mine drainage (AMD) and natural waters associated with rivers and groundwater. pH significantly influences the adsorption capacities of sorbents, as both the sorbent net surface charge and the aqueous speciation of Cr(VI) change with pH ( Fig. S7) 35 . At the lowest pH tested (pH 2), complete removal of Cr from solution was achieved for all of sorbents ( Fig. 2A). However, the experimental results showed that magnetite dissolved at pH 2 and the resulting dissolved Fe ions subsequently adsorbed to BC when the pH was adjusted to 2 in the composites. To further test the mechanism of Cr(VI) removal at pH 2, Fe 2+ and Fe 3+ were dissolved into a solution (without forming MNP) in the presence of BC and Cr (VI). We found that Cr(VI) was also removed from the solution completely. Hence, the results demonstrate that instead of magnetite, dissolved Fe 2+ played the dominant role in Cr(VI) removal at pH 2. At pH 3, BC removed only 60% of the total Cr(VI) from solution, which was further reduced to 10% at pH 5, and to no measureable removal at pH 7 ( Fig. 2B-D). However, MNPs and the MNP-BC composites showed higher Cr(VI) removal efficiencies of between 90 and 100% at pH 3. The adsorption capacity , electrostatic repulsion between the adsorbent and adsorbate inhibits Cr(VI) adsorption to BC above pH 2 and MNPs above pH 6. However, our results showed that the repulsive electrostatic interaction between the MNP-BC and Cr(VI) species does not inhibit the removal of Cr(VI) from solution to MNP-BC even at pH 7, above the isoelectric point at pH 5. The lack of pH effects on Cr(VI) removal by MNP-BC at neutral pH suggests that adsorption is governed by surface complexation rather than electrostatic interactions [34][35] . Our results showed that MNPs are effective to adsorb Cr(VI) up to pH 5; however, the removal efficiency decreases by pH 7, likely due to aggregation of MNPs as demonstrated by the DLS measurements (Fig. S2). The results show that the Cr(VI) removal efficiency of MNP-BC, even at a lower BC loading of 1 g L −1 BC, was much higher at neutral pH as compared to MNP or BC alone, suggesting a positive effect of BC on the reactivity of MNPs in the composite materials.
The Cr(VI) reduction capacities of BC, MNPs and MNP-BC were also tested between pH 2-7 ( Fig. 3). BC alone was effective in reducing Cr(VI) to Cr(III) at pH 2 ( Fig. 3A), and after 10 days, 1 g L −1 BC was able to completely reduce a solution of 170 µM Cr(VI). At pH 3, only 20% of the total Cr(VI) in solution was reduced by BC in 10 days (Fig. 3B), and BC did not reduce a measureable concentration of Cr(VI) at pH 5 or 7, even at the lowest tested Cr(VI) concentration of 85 µM (Fig. 3C,D; Fig. S9). The reduction capacity of Cr(VI) to Cr(III) by BC is almost certainly controlled by the adsorption step. At a lower pH, where the BC surface is more positively charged, adsorption is more favorable and electron transfer from BC to Cr(VI) can occur, as shown in previous biochar studies 29,451 . The transformation products generated by pyrolysis of lignin and cellulose of the parent biomass are thought be the source of electron-donating moieties in biochar 51 . These moieties include phenolic and carboxylic functional groups to which Cr(VI) species can bind, while quinones and polycondensed aromatic structures (graphitic sheets) are a source of π-electrons and capable of transporting π-electrons to phenolic and carboxylic functional groups [21][22]51 . Our results demonstrate that Cr(VI) reduction by BC is influenced by protonation of the functional groups and Cr(VI) speciation. www.nature.com/scientificreports/ The kinetics of the Cr(VI) reduction to Cr(III) by MNP-BC is much higher compared than those of MNPs at all pH values tested (pH 2-7) (Fig. 3). Cr(VI) reduction was measured concurrently with Cr(VI) sorption over time for each pH experiment, and we found that the Cr(VI) reduction kinetics closely matched Cr(VI) adsorption kinetics (Figs. 2 and 3). The results support our assertion that adsorption was the rate limiting step for Cr(VI) reduction by BC, MNPs and MNP-BC. The results from the reusability test showed that MNP-BC can be recycled multiple times to remove Cr(VI) from solution, as 10 mM Fe+ 1 gL −1 BC still can remove 80% of 170 µM of Cr(VI) from solution at its fifth reuse (Fig. S10). Since BC is conductive and enhances electron transfer along with preventing aggregation of MNPs, the cooperation between MNPs and BC leads to a higher efficiency in Cr(VI) adsorption and reduction by the composite across a wide range of pH. However, surface absorption of Cr(VI) should not impact the hydrodynamic size of the particles of magnetite in the composites. The strong cooperative behavior of MNP-BC for both adsorption and reductive immobilization make it a promising material for removal of Cr(VI) from aqueous solution.

Effect of oxidants on reduction of Cr(VI).
In natural waters and wastewaters, oxidants can coexist with heavy metals [52][53][54] . Thus, it is essential to evaluate how oxidants could influence the reduction capacity of MNP-BC composites. The effects of dissolved oxygen (O 2 ) and the most prevalent oxidants in drinking and wastewater systems [i.e., potassium permanganate (KMnO 4 ), sodium chlorite (NaClO 2 ) and sodium hypochlorite (NaClO)] 53 on the reduction of Cr(VI) by MNP-BC were tested (Fig. S11). O 2 did not have a measurable influence on the rate or extent of Cr(VI) reduction. However, the presence of KMnO 4, NaClO 2 and NaClO slowed the kinetics of reduction. The removal efficiency of Cr(VI) was reduced from approximately 100-70% in the presence of KMnO 4 and NaClO 2 , and reduced further to 40% by NaClO over the 10 day course of the experiments . The results clearly show that these oxidants impede Cr(VI) reduction by consuming electrons from MNP-BC. Nonetheless, the composites still removed Cr(VI) from solution, even in the presence of oxidants..

Cr(VI) adsorption and reduction kinetics.
The kinetics of adsorption and reduction are key factors in designing an adsorptive material for Cr(VI) removal from solution 34,44 . Adsorption proceeded rapidly for the first few hours of each experiment, and then gradually reached equilibrium (Fig. 3). Both pseudo-firstorder and pseudo-second-order models yielded good fits of Cr(VI) adsorption to BC kinetics data (R 2 ca. 0.9; www.nature.com/scientificreports/ Figs. S12A and S13A). In contrast, higher correlation coefficients were achieved using a pseudo-second-order model (R 2 > 0.92) for Cr(VI) adsorption to MNP-BC than using a pseudo-first order model (R 2 < 0.6), indicating that chemical adsorption could be the rate-limiting step in the reduction process for Cr(VI) by both BC and MNP-BC (Figs. S12B and S13B; Table S4). The adsorption processes may also be diffusion or intraparticlediffusion controlled. Intraparticle diffusion reactions occur as soon as the reactants (i.e., sorbate) enter into solution 29,[31][32]55 . BC is mesoporous as observed from the porosity analysis, and also hydrophobic. Thus, the process of wetting of biochar mesopores likely influences the Cr adsorption/reactions process in these mesopores. Therefore, the kinetics data were also analyzed using an intraparticle diffusion model to determine if the adsorption/reactions mechanisms by BC and MNP-BC were controlled by varying diffusion rates. The results indicate that the binding of Cr(VI) onto BC and MNP-BC is governed by both surface adsorption and intraparticle diffusion, followed by redox reactions that reduce Cr(VI) to Cr(III) (Figs. S12 and S13; Table S5). The kinetics of Cr(VI) reduction by BC, MNPs and MNP-BC were additionally modeled to determine the order and rates of Cr(VI) reduction at varying pH. The reaction kinetics of a pseudo-first-order kinetic model are typically rate proportional to dissolved Cr(VI) concentration ([Cr(VI) soln ]) 56-60 , according to: where, K int (h −1 ) is the intrinsic first-order rate coefficient.
Previous studies determined that Cr(VI) reduction by biosorbents follows a pseudo-first-order model 29,59 . Our results show that a pseudo-first-order fits the experimental data of Cr(VI) reduction by BC at pH 2 fairly well (liner correlation coefficient R 2 = ~ 0.8), but provides a poor fit to reduction experiments conducted at pH 3 (R 2 = ~ 0.5) (Fig. S14A,B). Since Cr(VI) reduction by BC was rapid during the first two hours, modeling the kinetics data using two pseudo-first-order kinetics relationships, one for the fast step (< 2 h) and a second for the slow step (> 2 h), provides the best fit to our experimental data (Fig. S14C,D). The reduction rate constant (K int ) decreased with increasing pH and Cr(VI) concentration; a similar observation was also shown in other Cr(VI) reduction studies 57,58 (Table S6). The higher Cr(VI) concentration could add more complexity to the system by changing chromate aqueous speciation and polymerization, thereby influencing the reduction rate 61 .
The reduction experiments by MNP-BC were modeled using second-order kinetics, according to the following rate law [57][58][59][60][61] The second-order kinetics model provides better fit than does the pseudo-first-order model for MNPs and MNP-BC (Fig. S15). The Fe(II) concentration in the composite material also influenced the rate of Cr(VI) reduction by MNPs loading on MNP-BC. With decreasing Fe(II) concentration in the composite, the actual ate of chromate reduction (Kobs) is decreased (Table S7). Overall, the kinetics models suggest that Cr(VI) reduction by BC followed first-order kinetics, and that the rate and order of the reduction depended on both pH and Cr concentration. Cr(VI) reduction could also be influenced by BC concentrations at acidic pH levels (e.g., pH 2 and 3). Since BC could not reduce measurable Cr(VI) at pH 5 and 7, it is expected that pH is the dominant factor for Cr(VI) reduction by BC. The 2nd order kinetics model best fits the results for Cr(VI) reduction by MNPs and MNP-BC, as Cr(VI) reduction was controlled by pH and Cr concentration as well as the Fe concentration in the MNPs and in MNP-BC.

Cr(VI) adsorption and reduction mechanisms.
To better understand the processes of adsorption and reduction, we studied the local molecular coordination environment of Cr on BC, MNPs and MNP-BC using XPS, FT-IR, XANES and extended X-ray adsorption fine structure (EXAFS) analyses. The XPS results indicate that after Cr(VI) adsorption, the intensity of the O 1s peak at the binding energy of 530.3 eV increased. This suggests that the Cr(VI) adsorption involved Fe-OH and C-OH functional groups, resulting in changes to the O1s intensity (Fig. S16). The FT-IR spectra show that the relative intensity of Fe-O peaks in MNPs, Fe-O and -OH peaks in MNP-BC, and C=O and -OH peaks in BC, shifted downward in the Cr-loaded samples (Fig. S17), suggesting that Cr(VI) complexed with these functional groups. Synchrotron based-XRF mapping of Cr(VI)-laden MNP-BC shows heterogeneous distribution of Cr on the composite surface (Fig. S18), while Fe had a varied correlation with Cr over the sample matrix (R 2 = 0.66-1.00) suggesting that both Cr sorption and reduction might occur simultaneously for both MNPs and BC (Fig. S19).
The XANES spectra of the Cr-laden samples clearly indicate the reduction of Cr(VI) to Cr(III) (Fig. 4). The Cr(VI) standard (K 2 Cr 2 O 7 ) has a distinct and well-characterized pre-edge feature at 5990 eV, which is absent in the spectrum of trivalent Cr (Cr(OH) 3 ). The linear combination fitting (LCF) of the XANES spectra show that more than 80% of the Cr(VI) reduced to Cr(III) by BC, MNPs and MNP-BC (Table S8).
The local coordination environment of Cr associated with BC, MNPs and MNP-BC was analyzed using EXAFS fitting (Fig. 5). Converted to R space, the first shell at 1.98-2.02 Å in the samples are consistent with literature values Cr(III)-O bonds having lengths of 1.98 Å (Table S9). The coordination number of Cr(III)-O is 6 in the samples, likely representing an octahedral geometry. Since BC contains a variety of ligands, including carboxylate, quinone and phenolic groups, Cr(III) can complex with any of them 34 . The second shell present in all samples is consistent with Cr-Cr bonding at a distance ~ 3.1 Å, and indicates the presence of amorphous Cr(OH) 3 precipitates, formed as a result of Cr(VI) reduction to Cr(III). The Cr 2p at binding energy of 577.20 eV from XPS analysis also supports the presence of a Cr(OH) 3 precipitate (Fig. S20) 62  www.nature.com/scientificreports/  www.nature.com/scientificreports/ Cr 2 FeO 4 nanoparticles exists in the sample system (Fig. S21). The size of the Cr 2 FeO 4 particle in the sample system is between approximately 0.5 µm to less than 1.0 nm in diameter.

Crystal transformation of magnetite nanoparticles. The binding of MNPs on BC was analyzed by
FT-IR (Fig. S17). The free stretching C=O band in BC is usually assigned at 1590 cm −1 . In the composite upon complexation with MNPs, the intensity of the C=O BC band in MNP-BC is decreased. However, the interaction between BC and MNPs does not appear to be affected by Cr(VI) adsorption, as shown by the unchanging C=O band, indicative of strong binding and complexation with magnetite [34][35] . The DOC leaching data shows that less than 2% of the DOC leached from BC and MNP-BC composites, indicating that BC is stable at low pH.
To better understand the transformation of magnetite in the composites after Cr(VI) adsorption and reduction, Cr(VI)-loaded MNPs and MNP-BC samples were analyzed with XRD, XPS, Fe K-edge XANES and EXAFS, and 57 Fe Mössbauer spectroscopy. The XRD and XPS spectra indicated no detectable difference between magnetite in the original MNP-BC at higher Fe concentration (10 mM Fe+ 1 g L −1 BC) and Cr(VI)-loaded MNP-BC implying no transformation from magnetite to maghemite (Figs. S22 and S23). However, XRD analysis of Cr(VI)loaded MNP-BC at the lower Fe concentration (2 mM Fe+ 1 g L −1 BC) showed that the diffraction peaks at 2θ = 35.42° and 43.05°, which correspond to the (311) and (400) planes, respectively, are not well indexed to the face-centered cubic inverse spinal structure of crystalline Fe 3 O 4 , indicating some transformation in magnetite (S22C and S22D).
Fe 2p 1/2 and Fe K-edge XANES of Cr(VI)-loaded MNP-BC (Figs. S23 and S24) showed some transformation of magnetite in MNP-BC after Cr(VI) reduction to Cr(III). The coordination number and the inter atomic distance calculated from Fe K-edge EXAFS spectra (k 3 ·χ(k)) fitting shows that the samples exhibited better fits to the magnetite crystallography ( Fig. S25 and Table S10) as compared to oxidized Fe mineral phases, suggesting that magnetite did not convert completely to an oxidized Fe(III) mineral. The linear combination fitting (LCF) (Table S11) of the Fe XANES spectra showed that magnetite had partially transformed to oxidized Fe phases (likely ferrihydrite and goethite) after complete reduction of 350 µM Cr(VI) to Cr(III) in 4 days, implying that the reduction of Cr(VI) to Cr(III) consumed a fraction of the Fe(II) (Fig. S24).
Cr-loaded MNP-BC was further analyzed by Mössbauer spectroscopy. The spectra of Cr-loaded MNP-BC (10 mM Fe+ 1 g L −1 BC) are dominated by two magnetically ordered hyperfine sextets (HFD Site 1 and HFD Site 3; Table S12) which constitute approximately 65% of the complete spectral area. The hyperfine parameters revealed these sextets to correspond to octahedral and tetrahedral lattice sites, respectively (Fig. 6). However, accurate fitting of the spectra also required the inclusion of a collapsed sextet in addition to a doublet which are indicative of a superparamagnetic, or poorly ordered phase. This collapsed sextet was present in all samples, but the doublet was only present for samples collected at t = 0 and t = 2 days. It was not possible to identify either of these components, however they likely correspond to superparamagnetic magnetite, or potentially a maghemite like phase meaning that it is not possible to explicitly exclude the presence of maghemite in these samples. The prominent doublet at lower Fe concentration (2 mM Fe+ 1 g L −1 BC) in MNP-BC, located in the middle of both spectra, accounts for about 10% of the spectral area and likely corresponds to a paramagnetic Fe(III) phase, such as lepidocrocite or nano-goethite (Fig. S26).

Conclusions
The reductive immobilization of Cr(VI) to nontoxic Cr(III) via a coupled reduction-sorption process is amongst the best remediation strategies to treat Cr(VI)-contaminated water. Magnetite and biochar can effectively remove Cr(VI) from aqueous solutions at low pH, but their individual efficiencies decrease at neutral pH 13 . Here we demonstrate that magnetite nanoparticle-biochar composites provide a robust, reusable adsorbent that can transform Cr(VI) to Cr(III). The adsorption-reduction processes suggest that the strong cooperation between MNP and BC underpins the improved adsorption and reduction capacity. Moreover, the kinetics of Cr(VI) adsorption by MNP-BC is rapid, reaching equilibrium within an hour. The chemical process of Cr(VI) removal by MNP-BC occurs through adsorption and intraparticle diffusion, followed by redox reactions that reduce chromium to its trivalent state. Spectroscopic analyses showed the complete reduction Cr(VI) to Cr(III) by MNP-BC, leading to an immobilized Cr(III)-complex including Cr(OH) 3 and chromite (Cr 2 FeO 4 ). The ability of the MNP-BC composite to both absorb and reduce redox-sensitive metals suggests it as a cost-effective, promising and reliable sorbent in the treatment of contaminated water.

Preparation of Fe 3 O 4 nanoparticles (MNPs) and Fe 3 O 4 -biochar composites (MNP-BC). The
MNPs and MNP-BC composites were prepared according to Liu et al. 39 and Yuan et al. 40 . Briefly, 2 g FeCl 3 ·6H 2 O and 1 g FeCl 2 ·4H 2 O were dissolved in 200 mL water that had been bubbled with N 2 gas for 30 min. While continuing to bubble with N 2 , the solution was vigorously stirred for 5 min at 60 °C. A solution of 1.5 M NH 4 OH was added dropwise into solution until the pH value of the mixture reached 8.0 and a black precipitate formed. The mixture was then aged for 30 min at 60 °C. The solid precipitate was centrifuged and separated from water, washed four times with water and then dried at 40 °C in a vacuum oven. To prepare MNP-BC, BC was added to the initial ferric/ferrous chloride solution described above, at several MNP to BC ratios. The addition of NH 4 OH and subsequent preparation steps were identical to those conducted for the synthesis of MNPs.
Characterization of adsorbents. The BC, MNPs and MNP-BC were characterized by Co K-alpha X-ray powder diffraction (XRD, Rikagu Ultima IV), high resolution transmission electron microscopy equipped with selected area diffraction (HRTEM-SAED, JEM-ARM200cF S/TEM) and scanning electron microscopy (SEM, Zeiss EVO® MA 15). Fourier transform infrared spectroscopy (FTIR, Thermo Nicolet 8700) was employed to www.nature.com/scientificreports/ analyze the molecular structure. The particle size distributions and zeta potential of BC, MNPs and MNP-BC were measured using dynamic light scattering (DLS) and electrophoretic light scattering (ELS) analyses, respectively (ZetaSizer Nano, Malvern Instruments, UK). X-ray photoelectron spectroscopy (XPS) measurements were carried out using a Kratos Axis 165 instrument to determine the functional groups and Cr and Fe speciation. The detailed procedure of these methods is given in the Supporting Information (SI) section. Potentiometric titrations were carried out to determine the protonation constants (K a ) and corresponding site concentrations of proton-active surface functional groups, as described in Alam et al. 7,20 To determine the sheet resistance of BC, it was mixed with a polyethylene oxide (PO) polymer. The sheet resistance of the BC and PO composites at different weight ratios (20 mg:200 mg; 100 mg:100 mg; 200 mg:20 mg) was measured at room temperature using a linear four points probe with 1 mm spacing (Lucas Pro-4 4000, USA) and a Keithley 2601A SourceMeter. Dry combusted and water extracted total organic carbon (TOC) of BC were determined using a TOC analyzer (Shimadzu TOC-V/TN). To ascertain the ratios of Fe(II) and Fe(III) in MNPs, the ferrozine method 43 was used.

Cr(VI) adsorption and reduction experiments. Batch adsorption and reduction kinetics experiments
were conducted at various initial Cr(VI) concentrations (85 µM, 170 µM and 350 µM) and sorbent concentrations (1 g L −1 BC, 10 mM Fe, 4 mM Fe, 0.5 g L −1 BC + 10 mM Fe, 1 g L −1 BC + 4 mM Fe, and 1 g L −1 BC + 2 mM Fe) at pH 2, 3, 5 and 7. Initially, a 10 g L −1 MNPs and MNP-BC stock slurry was prepared. The reported Fe and BC + Fe iron concentrations were calculated from dilutions of the stock MNPs and addition of MNP to the MNP-BC slurry, respectively. The release of Fe from BC was negligible (less the few ppb) at the tested pH values. The adsorption and reduction kinetics experiments were run in duplicate and the standard deviations were less than 5%. Details of the Cr (VI) adsorption and reduction experiments and kinetics modeling are given in the SI section. www.nature.com/scientificreports/ Solid phase analyses after adsorption and reduction. Synchrotron X-ray absorption spectroscopy (XAS) analyses were carried out at the Hard X-ray Microanalysis beamline (HXMA 06ID-1) of the Canadian Light Source (CLS). Synchrotron-based X-ray fluorescence (XRF) mapping was carried out on thin sections of Cr-loaded BC, MNPs and MNP-BC at the Very Sensitive Elemental and Structural Probe Employing Radiation beamline (VESPERS 07B2-1) at CLS. 57 Fe Mössbauer spectroscopy was conducted to characterize changes to Fe bearing mineral phases in MNP-BC exposed to different concentrations of Cr(VI). The detailed XAS, synchrotron-XRF and 57 Fe Mössbauer spectroscopy method are described in the SI. Cr-loaded BC, MNPs and MNP-BC samples were further analyzed by XRD, FTIR and XPS to determine the crystallographic transformation of MNPs and the functional groups that interacted with Cr after the adsorption and reduction processes.