Carbon Composites for Cr(VI) Removal Prepared from Harmful

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Iron/Carbon Composites for Cr(VI) Removal Prepared from Harmful Algal Bloom Biomass via Metal Bioaccumulation or Biosorption Yanbin Cui, Haoxin He, and John D. Atkinson ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04921 • Publication Date (Web): 06 Dec 2018 Downloaded from http://pubs.acs.org on December 11, 2018

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ACS Sustainable Chemistry & Engineering

Iron/Carbon Composites for Cr(VI) Removal Prepared from Harmful Algal Bloom Biomass via Metal Bioaccumulation or Biosorption

Yanbin Cui1, Haoxin He1, John D. Atkinson1* 1 Department

of Civil, Structural, and Environmental Engineering, State University of New York

at Buffalo, Buffalo, NY 14260, USA

* Corresponding

author. Tel: 1 716 645-4001. Email: [email protected] (John D. Atkinson)

Address: 233 Jarvis Hall, North Campus, State University of New York at Buffalo, Buffalo, NY 14260, USA

1 ACS Paragon Plus Environment

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ABSTRACT Iron/carbon (Fe/C) composites efficiently remove Cr(VI) because of synergistic adsorption and reduction effects. This study uses harmful algal bloom (HAB) biomass and ferric ammonium citrate (FAC) or ferric nitrate as precursors for preparing Fe/Cs with a one-pot synthesis. The investigation uniquely differentiates material and performance impacts associated with two iron loading approaches, bioaccumulation (metal uptake by living algae) and biosorption (metal deposition onto dry algae). As-prepared Fe/Cs are up to 70% mesoporous with iron loading reaching 8.3 wt%. Uniformly dispersed nanoparticles (20-50 nm) are observed in all Fe/Cs, and microscale particles are present on the surface of biosorption samples due to sintering. Fe3O4 is the dominant iron species in Fe(NO3)3 added samples, while Fe0 dominates samples prepared with FAC, attributed to the reducing atmosphere generated during FAC pyrolysis. Up to 4.0 wt% nitrogen doping is achieved, from nitrogen in HAB biomass and iron precursors. Fe/Cs remove up to 165 mg/g Cr(VI) at pH = 2 and 73 mg/g Cr(VI) at pH = 6, with rapid kinetics. Magnetic properties (>16 emu/g) from reduced iron nanoparticles facilitate Fe/C separation and reuse, and samples maintain 73-82% of their removal capacity after five removal/recovery cycles. This work is important because it converts HAB biomass waste into functional materials with value in environmental applications.

Key words: Harmful algal bloom, Fe/C composite, Porous carbon, Cr(VI) removal, Material reuse


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INTRODUCTION Chromium is widely used in, and released from, applications including leather tanning, steel production, electroplating, mining, and wood preservation.1 Aqueous chromium exists as Cr(III) or Cr(VI), with carcinogenic and mutagenic Cr(VI) being 100 times more toxic than Cr(III).2 Cr(VI) is also more water-soluble and mobile in surface water,3 increasing human exposure. Of the many reported Cr(VI) control methods,4-8 adsorption and reduction are effective and scalable.9-11 Specifically, iron/carbon (Fe/C) composites synergistically remove aqueous Cr(VI),12, 13 with carbon providing adsorption sites and iron mediating reduction. For example, an Fe/C, prepared using SBA-15 templating, achieved 97% Cr(VI) removal while maintaining 80% of its capacity after seven cycles.14 Despite demonstrated value for Cr(VI) removal, Fe/C preparation remains non-continuous and energy intense, requiring several discrete heating steps.15-17 Porous carbon, prepared in a separate process, is typically impregnated with an iron salt via incipient wetness or excess solution impregnation, and then heated to generate nanoparticles. Post-preparation modifications tailor the chemistry of the support and/or metal.18 The inefficient process makes Fe/C production unsuitable for scale-up. Given the value of these materials, there is an opportunity to improve cost-effectiveness and scalability by streamlining preparation and applying waste-based precursors. Blue-green algae is a component of many harmful algal blooms (HABs),19 reaching up to 105 cells/ml in eutrophicated waters to create economic and ecological concerns.20 This biomass has high carbon and nitrogen content, making it an effective carbon precursor. For example, Bai et al. converted HAB biomass into negative carbon electrodes.21 Another HAB-derived carbon achieved 156 mg/g Cr(VI) removal at pH = 1.22 To improve Cr(VI) removal specificity and 3 ACS Paragon Plus Environment


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capacity, Fe/C composites can be made from HAB biomass impregnated with iron via bioaccumulation or biosorption.23 Bioaccumualtion is an active, metabolically driven process where algae cells uptake iron internally, storing iron supplied in excess of cell demand.24, 25 This process has been exploited for aqueous metal remediation,26 but no efforts to date take advantage of the corresponding waste products (metal-impregnated HAB biomass) to produce useful materials. Biosorption is a passive process where metals are complexed, adsorbed, ion exchanged, or precipitated onto dry algal powder.27,

28

Both methods are well-understood,

however, they have never been compared in the context of materials engineering for Fe/C preparation. Specifically, mechanistic differences associated with the iron addition method (e.g., iron in the living cell vs. on the dead cell) will impact Fe/C physiochemical properties, and corresponding Cr(VI) removal. If HAB biomass is to be considered a practical Fe/C precursor, such investigations are necessary. In this work, therefore, Fe/Cs are prepared from HAB biomass using iron bioaccumulation or biosorption with living algae or dry algal powder, respectively. Produced materials are characterized to relate iron precursor and loading method to Fe/C physiochemical properties and Cr(VI) removal. Regeneration and reuse experiments highlight the long-term value of the materials. Methods reported in this study convert HAB waste into a product for pollution prevention, providing direct and indirect environmental benefits while reducing Fe/C production costs. This interdisciplinary, proof-of-concept study has implications for sustainable waste management, environmental engineering, and materials science/engineering.


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METHODOLOGY Fe/C Preparation Blue-green algae was collected from Chautauqua Lake (42.1579N, 79.3965W), in Western New York. After washing with water to clear residual debris associated with the natural material and then filtering, the algal suspension was divided into three parts. The first part was used to prepare Fe/C through iron bioaccumulation using living algae cells, and the second part was used to prepare Fe/C through iron biosorption using dry algal biomass. For both methods, ferric nitrate (Fe(NO3)3, Fisher, 97%) and ferric ammonium citrate (FAC, Amresco, reagent grade) were considered as iron precursors. An activated carbon (AC, no Fe) control was prepared using the third part of the algal suspension. For iron bioaccumulation, algae (5.0 g/L) was cultivated using a growth media reported by Ichimura.29 An Fe(NO3)3 or FAC stock solution was added to achieve an initial iron concentration of 10 mM, and the pH was adjusted to neutral using NaOH or HCl. Algae were then cultivated in 500 ml flasks on a shaking bed (100 rpm). After 7 d, the biomass was filtered, washed, and dried at 80 oC for 24 h. Dry biomass was heated in N2 (0.5 SLPM) at 10 oC/min to 600 oC in a tube furnace (Lindberg) for 1 h. Temperature was then ramped (10 oC/min) to 900 oC,

the carrier gas was switched to CO2 (0.5 SLPM), and the conditions were held for 30 min.

After washing with water and ethanol, products were dried under vacuum at 60 °C overnight. Samples produced using iron bioaccumulation are labeled Alga-X, where X is Fe(NO3)3 or FAC. For iron biosorption, algal biomass was immediately filtered, dried at 80 oC for 24 h, and then manually ground into a powder. 2 g algal powder was covered with 40 ml of 0.2 M FAC or Fe(NO3)3 on a shaking bed (100 rpm) for 24 h, followed by washing and drying at 80 oC for 24



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h. Iron-loaded algal powder was then subjected to the same pyrolysis, activation, washing, and drying procedures described above. Samples produced using iron biosorption are labeled Pow-X, where X is Fe(NO3)3 or FAC. The AC control was prepared according to this same procedure, without addition of FAC or Fe(NO3)3. Fe/C Characterization Physical properties of prepared materials were determined with N2 adsorption-desorption isotherms measured at -196 oC in a sorption analyzer (TriStar II 3020). For determining surface area, the BET equation was applied to adsorption data from 0.01 < P/P0 < 0.2. Total pore volume was measured at P/P0 = 0.98. Pore size distributions were found using Density Functional Theory (DFT), assuming slit-shaped pores. Thermo-gravimetric analysis (TGA) was applied to determine thermal stability and decomposition mechanisms. 10-15 mg dry algal biomass (after iron loading; before pyrolysis/activation) was heated from 25 to 700 °C at 10 °C/min in air. Surface texture and morphology were qualitatively assessed using a field emission scanning electron microscope with energy-dispersive X-ray spectrometer (FESEM-EDS, Hitachi S4000). Surface elemental composition was quantified with EDS. Transmission electron microscopy (TEM, JEOL JEM 2010) was completed using 200 kV accelerating voltage. The average size of iron nanoparticles deposited in the carbon matrix is reported using average Martin diameters. Carbon, hydrogen, and nitrogen content were measured with a CHN bulk elemental analyzer (PerkinElmer 2400). Iron content was determined by inductively coupled plasma-optical emission spectroscopy (ICP-OES, Thermo Scientific iCAP 6000). Carbon content reported from EDS was standardized based on bulk CHN data to determine concentrations of inorganic elements (including Na, K, Mg, Al, Ca, Si, P, and S). With this information, oxygen content was determined by difference from 100%. 6 ACS Paragon Plus Environment

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X-ray diffraction (XRD, Rigaku Ultima IV diffractometer) patterns were recorded using CuK (λ = 1.5406 Å) radiation with a step size of 0.05◦ from 10◦ to 80◦, at 4◦/min. Surface functional groups were identified using Fourier transform infrared (FTIR) spectroscopy (Perkin-Elmer Spectrum Two), equipped with an attenuated total reflectance (ATR) accessory. Spectra were collected from 400-4000 cm−1 with 4 cm−1 resolution against an air background. Magnetic properties were quantified in air at room temperature with a vibrating sample magnetometer (Quantum Design, Evercool II). The magnetization-magnetic field (M-H) curve was obtained using a magnetic field from −7 to +7 T. The pH of the point of zero charge (pHpzc) was determined using a pH meter (S220 SevenCompact, Mettler Toledo). 0.01 M NaCl was adjusted to pH between 2 and 12 using 0.1 M HCl or NaOH. 50 mg of prepared materials were mixed with 10 mL of each solution, followed by agitation at 150 rpm for 48 h. The pH of the supernatant was then measured, and pHpzc is the point where pHfinal vs. pHinitial crosses the line pHfinal = pHinitial. Fe/C Application for Cr(VI) Removal Batch Cr(VI) removal tests were conducted in 15 ml solutions (20-200 mg/L) containing 15 mg of material, agitated at 150 rpm (room temperature) and with initial pH adjusted to between 2 and 12 using HCl or NaOH. Sealed flasks were shaken for 7 d to ensure equilibrium. After centrifugation, the concentration of Cr(VI) in the supernatant was determined using a spectrophotometer (SpectraMax i3) and the 1,5-diphenylcarbazide colorimetric test.30 100 μl supernatant (diluted, if necessary), 3 μl H2SO4, and 6 μl 1,5-diphenylcarbazide were added to a 96-well plate. Absorbance was recorded at 540 nm, and concentration was determined based on a standard curve. Equilibrium removal capacity, qe (mg/g), was determined using Eq. 1. 7 ACS Paragon Plus Environment


qe = V(C0-Ce)/W

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(1)

where C0 and Ce (mg/L) are initial and equilibrium Cr(VI) concentrations, respectively. V is solution volume, and W is the mass of dry adsorbent (g). Langmuir and Freundlich models were applied to equilibrium removal data. For kinetic studies, Cr(VI) solutions (C0 = 100 mg/L) were sampled at different times, and removal capacity at time t, qt (mg/g), was calculated by: qt = V(C0 - Ct)/W

(2)

where C0 and Ct (mg/L) are the initial Cr(VI) concentration and the concentration at any time t, respectively. Adsorption kinetics were modeled using pseudo-first-order and pseudo-secondorder models. Fe/C Regeneration and Reuse After Cr(VI) (C0 = 100 mg/L) equilibrium removal tests, Fe/Cs were recovered by magnetic separation and washed with deionized water and 0.01 M NaOH (3 times each), followed by a rinse with ethanol.14 After drying under vacuum at 60 °C overnight, regenerated materials were reused for Cr(VI) (C0 = 100 mg/L) removal. 5 cycles were completed. To determine the stability of impregnated iron species in as-prepared Fe/Cs, materials when shaken in an acidic solution (pH = 1) for 7 d, and the concentration of released iron was then determined with ICP-OES.


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RESULTS AND DISCUSSION Morphology and Physical Properties of Fe/Cs The AC control consists of layered graphene sheets and has a heterogeneous surface (Supplementary Figure 1), consistent with other reports for HAB-based carbons.22,

31

Fe/Cs

prepared from living algae cultivation (i.e., Fe bioaccumulation) have similar surface morphology as AC, while samples prepared from algal powder (i.e., Fe biosorption) exhibit micro-scale surface imperfections that add roughness (Figure 1). For a given preparation method, no visible differences are observed for the two considered iron precursors. Backscatter electron micrographs (Figure 1e-h) distinguish two material phases for biosorption Fe/Cs, with EDS identifying the white surface microparticles as iron-based. Similar metal aggregation was observed by Qiu et al. when carbonizing iron-loaded cellulose.32 It is notable that these surface iron-containing particles are not present on bioaccumulation Fe/Cs. TEM, however, shows metal nanoparticles (20-50 nm) homogeneously dispersed throughout carbon matrices in all Fe/C composites (Figure 1i-l). Iron particles in Alga-X samples are wrapped by multiple intact graphene layers, contrasting the iron particles in Pow-X samples that are on the surface or embedded into fewer graphene layers. This suggests that iron nanoparticles are formed in the interior of carbonized algal biomass when using bioaccumulation, compared to added iron density on the carbon surface when using biosorption. Iron bioaccumulated in living algae cells is metabolically transported through the cell wall, which contains most of the lignin,33 and stored internally.34 On the other hand, biosorbed iron attaches exclusively to exterior surfaces of dead cells and is more likely to sinter into microscale particles when heated.



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Figure 1. SEM (first row, a-d), backscattered electron SEM (second row, e-h) and TEM (third row, i-l) of Alga-FAC, Pow-FAC, Alga-Fe(NO3)3, and Pow-Fe(NO3)3

The AC control has a Type I N2 adsorption isotherm (Supplementary Figure 2a), including rapid pore filling at P/P0 < 0.05 followed by a plateau until saturation. The sample shows no desorption hysteresis loop, representative of a microporous adsorbent (Table 1). All Fe/Cs have more total pore volume than AC and exhibit combined Type I and IV N2 adsorption isotherms that include gradual adsorption increases with increasing relative pressure and well-developed H4 hysteresis loops during desorption.35 Fe/Cs also have broader pore size distributions than AC that include micro- and mesopores (up to 30 nm, Supplementary Figure 2b and 2c). These results conflict with several Fe/C studies that report decreasing pore volume after iron impregnation, 10 ACS Paragon Plus Environment

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attributed to pore blockage.14,

36, 37

In this work, independent of metal loading method, iron

nanoparticles are generated in situ – at the same time as the porous carbon support. This is different than reports that impregnate the iron precursor onto a prefabricated, porous carbon and should be considered a unique advantage of this approach. While total pore volume increases, BET surface area decreases and pore widths widen for all Fe/Cs prepared here, compared to AC. Two justifications explain these observations. First, iron catalyzed carbon gasification may broaden micropores to generate mesoporosity.38 Second, extensive sample washing procedures may remove weakly attached iron nanoparticles from the support structure,39 leaving internal vacancies (i.e., pores) that are 10-30 nm in size. As TEM shows (Figure 1i-l), iron nanoparticles in Pow-X samples are less stably anchored into the carbon matrix, making them more likely to be washed away and resulting in consistently larger pore volumes than Alga-X samples. Samples prepared using Fe(NO3)3 exhibit larger mesopore volume and pore widths than FAC added samples. It is possible that added oxidizing capacity associated with thermally decomposing nitrate groups removes more reactive carbon sites during pyrolysis, expanding internal porous structures. Table 1. Physical properties of HAB-derived materials

a

Sample

SABETa (m2/g)

VTotalb (cm3/g)

VMicrob (cm3/g)

VMesob (cm3/g)

dporec (nm)

dparticled (nm)

AC

340

0.12

0.11

0.01

1.6

-

Alga-FAC

220

0.13

0.06

0.07

5.5

26

Pow-FAC

250

0.15

0.07

0.08

4.6

28

Alga-Fe(NO3)3

200

0.16

0.05

0.11

6.7

32

Pow-Fe(NO3)3

240

0.18

0.06

0.12

7.2

36

SABET = BET surface area, b V = volume, c d = average pore width, d d = average particle size (Martin diameter)



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Elemental Analysis and Surface Chemistry Mass yields of Fe/Cs are higher than that of AC because of added mass from iron species (Table 2). Iron content in Fe/Cs reaches 8.3 wt%, with loading via bioaccumulation resulting in 16-43% more iron than loading via biosorption. Alga-FAC has 23% more iron than Alga-Fe(NO3)3, indicating that FAC is more easily assimilated by living algae.40 No iron loading difference was observed for biosorption Fe/Cs prepared with FAC versus Fe(NO3)3, consistent with the entirely physical nature of that method. Oxygen content in Fe(NO3)3 added samples is about 20% higher than in FAC added samples. Thermal decomposition of Fe(NO3)3 generates O2 and nitrogen oxides,41 facilitating partial oxidation of reactive carbon sites and iron species during heating.42 This result, therefore, is consistent with the expanded pore size distributions described in Table 1 and earlier text. The AC control contains notable N content (2.8 wt%), attributed to cyanobacteria’s ability to fix nitrogen in natural environments.43 This observation coincides with past studies that describe HAB biomass as a precursor for nitrogen-doped carbon.44,

45

The

nitrogen content of Fe/Cs, however, is 18-42% higher than AC. For these samples, added nitrogen not only comes from the HAB carbon precursor, but also from ammonium and nitrate groups in FAC and Fe(NO3)3, respectively. This suggests an opportunity to simultaneously impregnate carbon materials with Fe and N. Nitrogen functional groups add basicity to Fe/C products, increasing their pHpzc compared to AC (Table 2).46


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Table 2. Mass yield, pHpzc and elemental composition of HAB-derived AC and Fe/C composites

Sample

Production Yield

Elemental Composition (wt%) pHpzc

(%)

Ca

Ha

Na

Ob

Non-Fe Inorganic Elements c

Fe d

AC

32.5

6.44

54.6

0.7

2.8

18.8

23.2

ND

Alga-FAC

41.6

8.35

51.2

0.7

4.0

16.7

19.1

8.3

Pow-FAC

39.1

8.29

55.4

0.6

3.9

15.4

18.9

5.8

Alga-Fe(NO3)3

40.3

7.13

49.8

0.6

3.4

18.2

21.7

6.4

Pow-Fe(NO3)3

38.4

6.82

52.3

0.5

3.5

19.1

19.1

5.5

ND = Not detected, a by CHN bulk elemental analysis, b by difference, c by SEM-EDS, d by ICP-OES

Thermogravimetric analysis was used to identify decomposition mechanisms and ash contents of considered precursor combinations (Supplementary Figure 3). All samples share three weight loss regions, attributed to removal of physically adsorbed moisture (25-120 oC), release of volatile components from both carbon and iron precursors (200-350 oC), and lignin combustion (420-500 oC).47 A white gray ash (4.8 wt%) was recovered from the AC control precursor (i.e., dry algal powder), which is believed to consist of inorganic oxides.48 All iron-loaded samples, however, generate more ash that has a brown/red color. Additional ash (6.6-11.4 wt%) in ironloaded samples compared to AC is assigned to Fe2O3, in approximate agreement with iron loading detected by elemental analysis (Table 2). Crystalline structures of AC and Fe/Cs were characterized with XRD (Figure 2a). For all Fe/Cs, peaks at 30.3°, 35.5°, 43.1°, 57.1°, and 62.5° are observed, corresponding to magnetite (Fe3O4, PDF#04-012-7038).49 Others have reported that the primary pyrolysis product of Fe(NO3)3 is Fe2O3,50 which is inconsistent with results observed here. For the one-pot pyrolysis method applied in this work, partially reduced iron oxides may be caused by the Boudouard reaction,



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which generates carbon monoxide during biomass pyrolysis that may reduce iron oxides.51 Regardless of iron loading method, FAC samples are dominated by zero valent iron (ZVI, PDF#04-013-5554), with smaller magnetite contributions. FAC decomposition during algae carbonization generates reducing gases, including CH4, CO, and NH3,52 that are capable of reducing metal oxides to their zero valent form.53 Wang et al. similarly reported graphene oxide functionalization with nitrogen and simultaneous iron reduction via FAC decomposition.54 In established algae cultivation procedures, FAC is often added to culture media because it provides both N and Fe nutrients.24 Small diffraction peaks in all XRD patterns, including the AC control, are attributed to mineral impurities associated with, for example, SiO2, CaSO4, MgO, and Al2O3.55

Figure 2. (a) XRD patterns and (b) FTIR spectra for HAB-derived AC and Fe/C composites

FTIR analysis of AC and Fe/Cs is presented in Figure 2b. For all samples, sharp bands around 2925 cm-1 and 2853 cm-1 are assigned to asymmetric and symmetric C–H stretching, respectively.56 Similarly, absorption peaks at 1470 and 1380 cm-1 are attributed to C–H asymmetric and symmetric bending, respectively.57 A smaller band at 983 cm-1 is associated with 14 ACS Paragon Plus Environment

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CH=CH2 stretching.58 Absorbance peaks at 1580 and 1260 cm-1 are also found in all samples and caused by N–H bending and C–N stretching, respectively,59 most likely attributed to nitrogen doping on the carbon surface that results from inherent nitrogen content in HAB biomass. A strong peak at 1036 cm−1 is caused by C–O stretching60 and is only observed for Fe/Cs prepared with Fe(NO3)3, consistent with proposed oxidation caused by surface reactions with NO2 or O2 released from the iron precursor during heating. A peak at 1120 cm-1 is assigned to the stretching vibration of the C–N bond59 and is not found for AC, demonstrating added nitrogen doping associated with the use of both iron precursors. An absorption peak at 584 cm−1 is only present in Fe/C samples prepared using Fe(NO3)3, potentially corresponding to the stretching vibration of Fe–O–Fe in Fe3O4.61 Physiochemical properties of Fe/Cs vary based on iron precursor and metal loading approach. Bioaccumulated iron is stored in algae cells during cultivation, generating internal iron nanoparticles that are surrounded by multiple graphene layers after the sample is pyrolyzed and activated. In contrast, iron nanoparticles on the surface of samples prepared via biosorption are prone to sintering into microscale particles. Fe3O4 is the main iron species in Fe(NO3)3 added samples, while ZVI is present in FAC added samples due to the reducing atmosphere associated with FAC decomposition. Nitrogen doping is achieved in all Fe/Cs due to elevated N in the HAB biomass precursor and the iron precursor, especially for FAC. To investigate the environmental applications of these materials, and to relate performance to these material properties, Cr(VI) removal and material reuse studies were conducted. Cr(VI) Removal: Effect of pH pH controls adsorbate speciation and solubility and impacts availability of adsorbent surface functional groups. Figure 3 describes equilibrium Cr(VI) removal for pH values from 2 to 12, 15 ACS Paragon Plus Environment


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and with C0 = 200 mg/L. Removal decreases by 56-74% as pH increases from 2 to 6 for all samples, followed by more gradual decreases as solutions become more basic. Cr(VI) is dominantly present in oxyanion forms (HCrO4-, Cr2O72-, and CrO42-)1 that are electrostatically attracted to positive surface functional groups at low pH. As the concentration of H+ increases (i.e., pH decreases), carbon surfaces are increasingly protonated, providing additional Cr removal sites. Also in acidic conditions, Cr(VI) is more readily reduced to Cr(III) in the presence of Fe0 and Fe3O4.62 Therefore, peak removals are noted at the lowest pH; Alga-FAC removes 165 mg/g due to increased adsorption and reduction. Cr(VI) removal capacity of the FAC added samples decreases more gradually with increasing pH than the other samples. This observation is attributed to (1) their higher pHpzc values that are associated with additional nitrogen doping, and (2) Fe0-mediated reduction being less impacted by pH than adsorption.

Figure 3. Effect of pH on Cr(VI) removal (C0 = 200 mg/L) using HAB-derived materials


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Cr(VI) Removal: Isotherms and Kinetics Batch Cr(VI) removal studies evaluate the performance of HAB-derived AC and Fe/Cs in nearneutral, environmentally-relevant conditions (pH = 6.0). While AC has the largest microporosity and surface area of the considered samples, its Cr(VI) removal capacity is surpassed by mesoporous Fe(NO3)3 added samples for Ce > 70 mg/L (Figure 4a). The mesoporous structure in Fe/Cs provides penetrating channels and accommodating space, allowing the material to fully utilize its elevated total pore volume when the concentration gradient is large.11, 63 At C0 = 20 mg/L, Alga-FAC and Pow-FAC achieve 96.3 and 95.5% removal, respectively – an important finding given the likelihood of low Cr(VI) concentrations in environmental applications. Over the entire tested concentration range, these samples consistently outperform the others.

Figure 4. (a) Cr(VI) removal isotherm at pH = 6 using HAB-derived materials (curves are Langmuir modeling) and (b) Cr(VI) removal kinetics with C0 = 100 mg/L (curves are pseudo-second-order modeling)

Comparable performances for Alga- and Pow- samples, when the iron precursor is constant, suggests that there is no functional preference for internal vs. surface iron particles in this application. This indicates that the largely mesoporous structure of these samples facilitates 17 ACS Paragon Plus Environment


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Cr(VI) diffusion to internal iron species in Fe/Cs prepared via bioaccumulation, allowing similar removal performance as biosorption samples. Additionally, given that electron transfer can occur through graphene layers in aqueous systems,18 the reducing capacity of impregnated iron species may not vary for internal versus external particles. nZVI functions as a reactive electron donor that directly reduces Cr(VI), a process that is more efficient than multi-step Cr(VI) reduction mediated by Fe3O4.64 As such, it is observed that samples prepared using FAC have improved performance compared to those prepared with Fe(NO3)3. Additionally, added nitrogen content in FAC samples supplies more positively charged functionalities (such as amine, nitro, and pyridinic) on the carbon surface to interact with Cr(VI).65 Langmuir and Freundlich equilibrium adsorption models were applied to removal data to expand analyses. Calculated fitting parameters are listed in Supplementary Table 1, and for all samples, the correlation coefficient for the Langmuir model is larger than that of the Freundlich model, suggesting that active sites are homogeneously distributed. This fitting result is consistent with other Cr(VI) removal reports using Fe/Cs.14,

32

Materials prepared in this study exhibit competitive Cr(VI) removal

performance compared to other biosorbents and carbon-based materials, including Fe/Cs, reported in the literature (Table 3).


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Table 3. Comparison of removal capacities of various materials used for Cr(VI) removal Material

C0 (mg L−1)

qmax (mg g−1)

pH

Ref

Cyanobacterium biosorbent

100

23

3.0

66

Oedogonium algal biosorbent

100

35

2.0

28

HAB-derived AC

200

156

1.0

22

Marine red algae-derived AC

100

66

1.0

67

Corn cob-derived magnetic AC

125

57

2.0

68

Iron doped ordered mesoporous carbon

1000

257

5.0

14

Magnetic carbon nanoadsorbents

120

29

7.0

69

HAB-derived AC

200

132

2.0

This study

200

165

2.0

This study

200

73

6.0

This study

HAB-derived Fe/C

All Fe/Cs exhibit fast Cr(VI) removal upon initial exposure, and 90% of equilibrium Cr(VI) removal is achieved in about 5 h. Cr(VI) removal for the AC control is notably slower, likely caused by diffusion limitations associated with the sample’s narrow micropores.70 Pseudo-firstorder and pseudo-second order models are used to fit kinetics data.71 Regression coefficients and calculated rate constants are included in Supplementary Table 2. The R2 value of the pseudosecond-order model (all > 0.99) suggests that this model describes observed Cr(VI) removal better than the pseudo-first order model. Alga-FAC not only shows the highest removal capacity, but also the fastest removal kinetics. These findings are attributed to the sample’s well-dispersed nZVI in accessible mesopores.32 Fe/C Regeneration and Reuse Magnetic powdered materials are easily recovered for regeneration and reuse. Saturated magnetization (Ms) values of Alga-FAC, Pow-FAC, Alga-Fe(NO3)3, and Pow-Fe(NO3)3 are 21.9, 19 ACS Paragon Plus Environment


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19.1, 16.3 and 18.4 emu/g, respectively (Figure 5a-b), in contrast with weak paramagnetic characteristics of AC. Alga-FAC has the highest Ms value, which is consistent with its maximum bulk iron content that includes notable ZVI. While Pow-Fe(NO3)3 has less iron than AlgaFe(NO3)3 (Table 2), its Ms value is larger. This might be associated with its surface microscale iron particles that are caused by high temperature sintering. Gangopadhyay et al. reported that larger iron particles have higher Ms values at room temperature.72 For all Fe/Cs, no hysteresis loops were observed in magnetization curves, and the near-zero coercivity and remanence values indicate that as-prepared samples are superparamagnetic, attributed to the presence of nanosized iron particles. 69


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Figure 5. (a, b) Magnetic properties of HAB-derived materials; (c) magnetic isolation of Fe/C after Cr(VI) removal; (d) Cr(VI) capacity recovery of Fe/Cs applied in five consecutive cycles

High magnetization values allow Fe/Cs to respond to an external magnetic field. A magnet was put next to the vial after equilibrium Cr(VI) removal (Figure 5c) and suspended particles were attracted to the wall in < 10 min, facilitating Fe/C recovery for reuse. Collected Fe/Cs were regenerated using NaOH as the stripping solution, allowing HCrO4- to be exchanged with OHand removed from the material.73 Supplementary Table 3 compares released iron concentrations after shaking Fe/Cs at pH = 1 for 7 d. Released iron from Alga-X samples is 6-15 times lower than that of Pow-X samples. This demonstrates added iron stability for the bioaccumulation



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approach, which was expected based on the lack of surface iron and embedding of impregnated nanoparticles. To evaluate Cr(VI) capacity recovery efficiency, regenerated samples were subjected to five consecutive use cycles. All Fe/Cs recover > 90% of their initial capacity during the first regeneration. With added cycles, capacity recovery of FAC samples drops more rapidly than Fe(NO3)3 samples, potentially due to consumption or oxidation of ZVI, which is not recovered during regeneration.14 An added reduction step may improve regeneration of FAC samples, but would add cost and time to the process.18,

74

Iron particles in Alga-FAC are more stably

impregnated in the carbon matrix, adding physical resilience to rigorous washing procedures and translating to a better capacity recovery than Pow-FAC (Supplementary Table 3). Cr(VI) capacity decreased gradually with cycling, but all samples had at least 73% of their initial performance after five cycles. This reuse study highlights the regenerability of as-prepared, HAB-based Fe/Cs, adding economic feasibility to the improved materials production process.

CONCLUSION AC is prepared from many waste-based precursors (e.g., wood, nut shells) to reduce costs. While HAB biomass has been considered for AC preparation, its use for producing Fe/C composites is not thoroughly described. Specifically, no Fe/C or other materials preparation studies have exploited algae’s ability to naturally uptake and fix iron from the environment. Therefore, this study compares properties and Cr(VI) removal performance of HAB-based Fe/Cs produced using this method with those produced using established procedures. Results suggest that Fe/Cs produced from HAB biomass using Fe bioaccumulation or biosorption can be competitive


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materials for environmental applications, specifically Cr(VI) removal. Because HAB biomass proliferates without arable land or fertilizer, these reported methods should represent an alternative option to landfilling natural HAB biomass, including biomass that was used for metal removal from natural water bodies. The conversion of waste into useful materials that can be applied for aqueous remediation is an impactful outcome of this analysis.

SUPPORTING INFORMATION Additional SEM and TEM images, N2 adsorption-desorption isotherms, TG analyses, constants corresponding to equilibrium adsorption and kinetics models, and acid-leaching data

ACKNOWLEDGEMENTS The authors acknowledge the State University of New York at Buffalo's Furnas Hall Materials Characterization Laboratories for providing access to the N2 adsorption analyzer, XRD, TGA, and SEM-EDS. The authors also thank Fan Sun and Dr. Hao Zeng (University at Buffalo, Physics) for assistance with the magnetization testing.

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GRAPHICAL ABSTRACT

Synopsis: Fe/C materials prepared from harmful algal bloom biomass remove Cr(VI) effectively, with favorable properties for material recovery, regeneration, and reuse.


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


6 0 4 0 2 0 0 2

4

6

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p H ACS Paragon Plus Environment

1 0

1 2


a

b


Page 38 of 42

Page 39 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


a

b

c

d




Page 40 of 42

Page 41 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41


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1 0 0 P r P r P r P r

9 0 8 0

(% )

7 0 6 0

T G

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Page 42 of 42

5 0

e c e c e c e c

u r u r u r u r

s o s o s o s o

r f r f r f r f

o r o r o r o r

A C A lg a -F A C P o w d e r-F A C A lg a -F e ( N O 3) 3

P re c u rs o r fo r P o w d e r-F e (N O 3

) 3

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T e ACS m Paragon p e r a Plust u Environment re (oC )

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Carbon Composites for Cr(VI) Removal Prepared from Harmful

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