Removal of hexavalent chromium upon interaction

Environ Sci Pollut Res (2017) 24:16786–16797 DOI 10.1007/s11356-017-9322-9

RESEARCH ARTICLE

Removal of hexavalent chromium upon interaction with biochar under acidic conditions: mechanistic insights and application Bharat Choudhary 1 & Debajyoti Paul 1,2 & Abhas Singh 1,3 & Tarun Gupta 1,3

Received: 18 January 2017 / Accepted: 19 May 2017 / Published online: 31 May 2017 # Springer-Verlag Berlin Heidelberg 2017

Abstract Chromium pollution of soil and water is a serious environmental concern due to potential carcinogenicity of hexavalent chromium [Cr(VI)] when ingested. Eucalyptus bark biochar (EBB), a carbonaceous black porous material obtained by pyrolysis of biomass at 500 °C under oxygenfree atmosphere, was used to investigate the removal of aqueous Cr(VI) upon interaction with the EBB, the dominant Cr(VI) removal mechanism(s), and the applicability to treat Cr(VI)-contaminated wastewater. Batch experiments showed complete removal of aqueous Cr(VI) at pH 1–2; sorption was negligible at pH 1, but ~55% of total Cr was sorbed onto the EBB surface at pH 2. Detailed investigations on unreacted and reacted EBB through Fourier transform infrared spectroscopy and X-ray photoelectron spectrometry (XPS) indicate that the carboxylic groups in biochar played a dominant role in Cr(VI) sorption, whereas the phenolic groups were responsible for Cr(VI) reduction. The predominance of sorption–reduction mechanism was confirmed by XPS studies that indicated ~82% as Cr(III) and ~18% as Cr(VI) sorbed on the EBB surface. Significantly, Cr(VI) reduction was also facilitated

by dissolved organic matter (DOM) extracted from biochar. This reduction was enhanced by the presence of biochar. Overall, the removal of Cr(VI) in the presence of biochar was affected by sorption due to electrostatic attraction, sorption–reduction mediated by surface organic complexes, and aqueous reduction by DOM. Relative dominance of the aqueous reduction mechanism depended on a critical biochar dosage for a given electrolyte pH and initial Cr(VI) concentration. The low-cost EBB developed here successfully removed all Cr(VI) in chrome tanning acidic wastewater and Cr(VI)-contaminated groundwater after pH adjustment, highlighting its potential applicability in effective Cr(VI) remediation.

Responsible editor: Guilherme L. Dotto

Acidic industrial wastewater from leather tanning, electroplating, and metallurgical applications is a major contributor of Cr release into the environment, leading to widespread contamination of soil and water (Kotaś and Stasicka 2000; ATSDR 2008). In the wastewater, Cr occurs predominantly in both Cr(III) and Cr(VI) oxidation states. Although trace amounts of Cr(III) are essential for metabolism, Cr(VI) is known to be mutagenic and carcinogenic (Bagchi et al. 2002). Under natural environmental conditions, Cr(III) precipitates as Cr(OH)3(s) that remains insoluble but Cr(VI) exists as highly soluble and mobile oxyanionic species (HCrO4−, CrO42−, and Cr2O72−), which pose a serious threat to water bodies (Kimbrough et al. 1999). Hence, Cr(VI) removal from acidic industrial wastewater is required prior to its safe disposal.

Electronic supplementary material The online version of this article (doi:10.1007/s11356-017-9322-9) contains supplementary material, which is available to authorized users. * Debajyoti Paul [email protected]

1

Centre for Environmental Science and Engineering, Indian Institute of Technology Kanpur, Kanpur, Uttar Pradesh 208016, India

2

Department of Earth Sciences, Indian Institute of Technology Kanpur, Kanpur, Uttar Pradesh 208016, India

3

Department of Civil Engineering, Indian Institute of Technology Kanpur, Kanpur, Uttar Pradesh 208016, India

Keywords Biochar . Eucalyptus globulus bark . Hexavalent chromium . Dissolved organic matter . X-ray photoelectron spectroscopy . Sorption–reduction . Chromium wastewater

Introduction

Environ Sci Pollut Res (2017) 24:16786–16797

Several chemical and biological methods have been suggested for Cr(VI) remediation, which have certain limitations (Barrera-Díaz et al. 2012). The chemical methods for removal of Cr(VI) from industrial wastewater use ferrous sulfate, sodium sulfite, and sodium metabisulphite, etc. for reduction of Cr(VI) to Cr(III) in acidic media followed by its removal as metal hydroxide precipitates after pH adjustment of media to 7.0–8.5 (Hackbarth et al. 2016). However, the existing chemical methods are ineffective for treatment of wastewater containing trace amounts of Cr(VI) (Barrera-Díaz et al. 2012). Further, these methods also generate Cr-containing secondary sludge and toxic sulfur dioxide gas. The biological method employs microbial reduction of Cr(VI) (Cheung and Gu 2007), which may not be effective because Cr(VI) acts as a bactericidal toxicant and limits the growth and effectiveness of microorganisms (Yurkow et al. 2002). Biological treatment can also be expensive when special aeration facility for wastewater treatment is required (Mamais et al. 2016). Compared to the existing chemical and biological methods, the use of biochar in pollutant removal has tremendous potential because of its eco-friendly and economic production (Ahmad et al. 2014; Tan et al. 2015). Biochar is a carbonaceous black porous material obtained by pyrolysis of biomass under oxygen-free atmosphere. The oxygen-containing functional groups on the biochar surface likely play a critical role in environmental applications (Chen et al. 2014). While activated carbon has been shown to effectively remove aqueous Cr(VI) (Mohan and Pittman 2006), its production compared to biochar is significantly more energy intensive and environmentally less beneficial in terms of the carbon footprint of a wastewater treatment facility (Thompson et al. 2016). Past studies show successful removal of aqueous Cr(VI) under highly acidic media (pH 1–2) using biochar derived from sugar beet tailing (Dong et al. 2011), oak wood, and oak bark (Mohan et al. 2011), coconut coir (Shen et al. 2012), oily seeds of Pistacia terebinthus L. (Deveci and Kar 2013), municipal wastewater sludge (Zhang et al. 2013), rice husk, organic solid wastes, and sewage sludge (Agrafioti et al. 2014). However, the exact mechanisms of biochar-mediated Cr removal and its resultant speciation are not clearly understood. The current hypothesis on Cr(VI) removal mechanisms is based on a series of investigations of Cr(VI) uptake on biomass (Park et al. 2004; Park et al. 2005; Park et al. 2007), but these studies were not directly conducted on biochar. Park et al. (2005) proposed two possible mechanisms: (1) Cr(VI) adsorption on the biomass surface followed by its reduction to Cr(III) and (2) direct reduction to Cr(III) facilitated by biomass-derived dissolved organic matter (DOM). However, the identity of sorbed and reduced Cr byproducts after reaction with biochar is not known. Further, the role of predominant biochar functional groups on Cr(VI) sorption– reduction is poorly understood. Moreover, it is unclear under what conditions does the surface-mediated Cr(VI) reduction

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mechanism predominate over the DOM-mediated direct Cr(VI) reduction mechanism. A few studies, using XPS analysis, have qualitatively identified the presence of both Cr(III) and Cr(VI) on the biochar surface (Dong et al. 2011; Zhang et al. 2013). Similarly, Cr(III) and Cr(VI) were detected as sorbed forms of Cr on coconut coir biochar using Cr K-edge X-ray absorption near edge structure (XANES) spectroscopy (Shen et al. 2012). However, XANES, being a bulk technique, could not distinguish whether the reduced product is formed on the biochar surface or in the bulk phase. Therefore, detailed investigations on the speciation of Cr upon interaction with biochar would identify these gaps. Furthermore, past investigations on biochar application have largely been limited to simulated Cr-containing water. Few studies, if any, have tested the efficacy of biochar in reducing Cr(VI) in field-derived contaminated groundwater or wastewater. In this study, we have used the biochar produced from pyrolysis (at 500 °C) of Eucalyptus globulus bark (EBB) to investigate the removal of Cr(VI) upon interaction with the biochar under acidic media. The main objectives of this study were (i) to identify the governing mechanism of Cr(VI) removal by systematic dissolved phase and solid phase investigations, specifically to understand (a) the role of oxygencontaining functional groups (carboxylic and phenolic) present on the EBB surface and (b) the role of DOM in the removal of aqueous Cr(VI) and (ii) to evaluate the efficacy of the proposed method in removal of Cr(VI) from Cr-contaminated acidic industrial wastewater and groundwater.

Materials and methods Biochar preparation The Eucalyptus globulus plant bark was collected from the Indian Institute of Technology Kanpur campus. About 200 g of the collected biomass was thoroughly cleaned with ultrapure water (>18.2 MΩ cm at 25 °C), air-dried overnight, and grounded to a fine powder. The powdered biomass was collected in a large silica crucible and pyrolyzed at 500 °C in a muffle furnace for 2 h in an oxygen-free atmosphere (N2 gas at 10 psi). The pyrolyzed EBB was cooled down to room temperature in a desiccator and passed through an ASTM 60mesh sieve to obtain 5. Considering that only ~20% of Cr was sorbed onto the EBB in the Cr(III)-only control experiment at pH 2, under otherwise identical conditions, the increased TOT-Cr sorption in the main experiment indicated significant uptake of chromium as Cr(VI) on EBB at pH 2.

low and high molecular weight humic substances that form soluble complexes with Cr(III) (Kotaś and Stasicka 2000; Lin et al. 2012; Graber et al. 2014). The EBB-extracted DOM concentrations showed a near-linear increase from 11 to 36 mg/L with increasing EBB dosage from 1 to 5 g/L (Fig. 3b). The DOM concentration, upon reaction with 40 ppm of aqueous Cr(VI), decreased by 27 ± 4% of the unreacted concentrations; for example, the DOM extracted from 1 g/L EBB decreased from 11 to 8 ppm after reaction

Effect of sorbent dosage and DOM on Cr(VI) removal For an initial Cr(VI) concentration of 40 ppm at pH 2, a systematic increase in EBB dosage resulted in a non-linear decrease in the equilibrated dissolved Cr(VI) concentrations from 60% at 1 g/L to ~0% at 4 g/L (Fig. 3a). The corresponding dissolved Cr(III) concentrations, estimated from the difference of measured TOT-Cr and dissolved Cr(VI), increases from 22 to 100% of the initial Cr. These results suggest that an increase in sorbent dosage leads to increased reduction of dissolved Cr(VI) to Cr(III). Interestingly, the amount of Cr sorbed on EBB was minimal (~20%) at 1 g/L and negligible at higher dosages. The decrease in Cr sorption and a simultaneous increase in dissolved Cr(III) at higher EBB dosage is indicative of the role of EBB-derived DOM on Cr(VI) removal. DOM extracted from eucalyptus-derived biochar, produced under similar conditions as ours, were reported to reductively solubilize Mn and Fe from sandy soils at acidic pH (Graber et al. 2014). Additionally, biochar-extracted DOM consists of

Fig. 2 Effect of pH on aqueous Cr(VI) in presence of EBB. The inset shows the effect of pH on Cr(III) under same experimental conditions


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with Cr(VI) (Fig. 3b). This decrease in DOM was accompanied by a reduction of dissolved Cr(VI) to Cr(III) by 22–38% of the initial Cr(VI), with increasing amounts of extracted DOM (Fig. 3c). Studies have shown that reduction of aqueous Cr(VI) results in simultaneous oxidation of DOM to CO2 (Testa et al. 2004; Dong et al. 2014). Cr(VI) reduction was significantly enhanced when equilibrations were performed with EBB-containing suspensions instead of only the extracted DOM. With an increase in the EBB dosage from 1 to 5 g/L, Cr(VI) removal increased from 41 to 99% (Fig. 3c). Since EBB contained ~15% by mass of oxidizable carbon (Table S1), an increase in dosage provides more potential electron donor sites for the reduction of Cr(VI) (Bolan et al. 2003). The equilibrated DOM in EBB-containing suspensions, however, did not exhibit a clear trend. Whereas for 1–2 g/L EBB, the DOM decreased by ~65% of the levels recorded in Cr-free conditions, an increase by ~16% was observed for 3–5 g/L EBB (Fig. 3b). The enhanced DOM relative to Cr-free conditions and increasing Cr(III) concentration for ≥3 g/L EBB dosage could be attributed to oxidative dissolution of biochar in the presence of Cr(VI) and solubilization of Cr(III) sorbed on EBB (Hsu et al. 2010). At low sorbent dosage (1–2 g/L), the simultaneous decrease in equilibrated DOM and dissolved Cr(VI) and increase in the dissolved Cr(III) indicates that even though Cr(VI) was converted to Cr(III), the oxidizable organic carbon did not solubilize and remained bound to the EBB. It is likely that the speciation of dissolved Cr(III) at lower dosages was different from that at higher dosages where it seems correlated with DOM. However, this study did not probe these speciation changes. Nevertheless, these results point to a critical dosage of EBB required to completely reduce Cr(VI) and solubilize into Cr(III) at an acidic pH. Cr presence on biochar surface

Fig. 3 a Dissolved and sorbed Cr after reaction with EBB at different dosages (1–5 g/L) at pH 2. b Variation in the amount of EBB-extracted DOM, before and after reaction with aqueous Cr(VI) only, and after reaction of Cr(VI) with EBB. c Comparison of Cr(VI) removal by EBB and by the EBB-extracted DOM only. Error bars indicate one standard deviation of the mean of triplicate experiments, which is similar to the size of the symbols

The dominant oxidation states and the coordination environments of Cr associated with the EBB surface were probed by XPS. XPS survey scans indicate ~2.7% decrease in atomic carbon and a similar increase in oxygen in the washed-EBB compared with the raw-EBB (Table 1; Fig. S4), which could be due to oxidation of surface functional groups by increased free proton concentration (Chen and Yang 2006; Datsyuk et al. 2008). After equilibration with Cr(VI), however, significant changes in carbon (decrease from 83.6 to 49.0%) and oxygen (an increase from 16.4 to 43.0%) were observed in the Cr-EBB (Table 1). These changes coincide with the detection of 8% Cr on the EBB surface, indicating enhanced oxidation of surface functional groups and their role in chromium sorption. Variations in the abundance of EBB surface functional groups prior and after equilibration with Cr(VI) were analyzed by deconvoluting respective high resolution XPS spectra

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Table 1 Atomic contents of C, O, and Cr in survey scans and chemical composition of C1s and O1s from deconvoluted peaks of high resolution XPS spectra of biochar samples Components

Survey scan

C1s

O1s

Relative atomic %

Binding energy (eV)

Raw-EBB

Washed-EBB

Cr-EBB

Raw-EBB

Washed-EBB

Cr-EBB

Avg. ± SD

C

86.3

83.6

49.0

O

13.7

16.4

43.0

Cr C═C

0.0 63.6

0.0 49.2

8.0 47.1

284.6

284.4

284.5

284.5 ± 0.1

C–C C–O

– 26.1

17.6 25.7

37.6 –

– 285.5

285.0 285.6

284.9 –

285.0 ± 0.1 285.6 ± 0.0

C–O–C C═O Carbonates

–

–

7.1

–

–

286.4

286.4

2.8 6.6

3.2 4.3

8.2 –

288.1 290.1

288.6 290.6

288.4 –

288.4 ± 0.2 290.4 ± 0.2

π–π* O–Cr

0.9 –

– –

– 9.1

293.5 –

– –

– 529.9

293.5 –

O═C O–C═O O–C

33.6 28.0 38.4

23.5 38.9 37.6

63.5 21.1 6.3

531.5 532.1 533.4

531.6 532.1 533.5

531.4 532.6 533.6

531.5 ± 0.1 532.3 ± 0.3 533.5 ± 0.1

SD standard deviation based on the binding energy of respective biochar samples

(Fig. 4). For the C1s spectra, a major peak at 284.6 eV (C═C coordination), and four minor peaks at 285.5 eV (C–O), 288.1 eV (C═O), 290.2 eV (carbonate) and 293.5 eV (shake up satellite due to π–π* transitions in aromatic ring) were identified in the raw-EBB spectrum (Fig. 4a) (Puziy et al. 2008; Ganguly et al. 2011; Tien et al. 2011). Whereas the washed-EBB spectrum shows a new peak at 285 eV corresponding to the C–C coordination (Fig. 4b), equilibration with Cr(VI) resulted in major changes on the EBB surface. These changes include complete loss of C–O coordination and formation of a new peak at 286.4 eV corresponding to (7%) C– O–C coordination. Whereas the C═O coordination increased to 5%, no carbonate was observed in the Cr-EBB spectrum (Fig. 4c; Table 1; also see XRD patterns in the Supplementary Material Fig. S3). The deconvoluted O1s spectrum for the raw-EBB shows three peaks at 531.5 eV (O═C coordination), 532.1 eV (O–C═O), and 533.4 eV (O–C) (Fig. 4d; Table 1) (Cheng et al. 2006; Chen and Yang 2006). In comparison, the washed-EBB spectrum shows 10.9% increase in the carboxylic functional group (O–C═O) (Fig. 4e; Table 1), suggesting oxidation of EBB surface carbon at pH 2. Equilibration with Cr(VI) decreased the O–C═O and O–C coordination (17.8 and 31.3%, respectively), with a corresponding increase (40%) in the relative O═C coordination (Fig. 4f; Table 1). Enhancement in the C═O peak intensity was also observed in the FT-IR spectra of Cr-EBB at 1384 cm−1 (Fig. 1). The increased C═O carbonyl formation suggests oxidation of phenolic hydroxyl (O–C) groups (Mohan et al. 2011; Dong et al. 2014). Note that the concentration of phenolic group (1.2 mmol/g) in the EBB is much higher than the carboxylic (0.63 mmol/g) and lactonic (0.34 mmol/g) oxygen-containing

groups (Table S1). Furthermore, a decrease in the O–C═O coordination and a corresponding increase (~9%) in the O– Cr coordination indicates Cr complexation with oxygencontaining groups on the EBB surface, possibly with the carboxylic groups, in which the oxygen atom donates an electron and reduces the Cr atom (Chen and Yang 2006). The new peak at 529.9 eV in the O1s deconvoluted Cr-EBB spectrum (Fig. 4f), also detected as a major peak in the Cr2O3 standard (Fig. S5), is attributed to the oxygen coordinated to Cr in Cr2O3 (Ma et al. 2014). These results indicate that oxygencontaining functional groups on biochar surface are involved in the sorption and reduction of Cr(VI) on the EBB surface. The high resolution XPS Cr2p spectrum of Cr-EBB shows peaks at 577.2 eV (for Cr2p3) and 586.9 eV (for Cr2p1), which are similar to the peaks in the Cr2O3 standard spectrum (576.4 and 586.2 eV, respectively). This suggests that sorbed Cr on the EBB was predominantly present as Cr(III) species (Fig. 5a). To identify the nature of sorbed-Cr species, the Cr2p3 regions in the Cr-EBB is compared with the deconvoluted spectra for Cr(III) and Cr(VI) standards. A relative abundance of 18.5% of the deconvoluted peak at 579.2 eV in Cr-EBB indicates the minor presence of Cr(VI) (Fig. 5b); the binding energy is similar to the Cr2p3 peak of K2Cr2O7 (Cr(VI)) standard (Fig. 5d). Although presence of minor Cr(VI) and major Cr(III) on the sorbent surface on sugar beet tailing biochar was inferred previously (Dong et al. 2011), the sorbed-Cr species were not identified and quantified. In our study, the deconvoluted peaks of Cr-EBB spectrum at 576.5 and 577.6 eV (Fig. 5b) corresponded well with the deconvoluted Cr2O3 spectrum (Fig. 5c) showing similar (within 5%) relative peak abundances suggesting the


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Fig. 4 Peak deconvolution of high resolution XPS C1s (a–c) and O1s (d–f) spectra of raw-EBB, washed-EBB, and Cr-EBB. Mean and standard deviation of binding energy for respective coordinations are shown by vertical dashed lines and shaded areas, respectively

presence of Cr2O3-like species. However, the presence of Cr(OH)3-like species at 577.2 eV could not be ruled out (Biesinger et al. 1997). Additional evidence for the presence of Cr2O3-like or Cr(OH)3-like species on the EBB surface comes from the presence of 529.9 eV peak in both the deconvoluted O1s spectra of Cr-EBB (Fig. 4f) and the Cr2O3 standard (Fig. S5).

Mechanism of Cr(VI) removal by EBB The mechanism of Cr(VI) removal by EBB can be best explained by a combination of processes: sorption due to electrostatic attraction, sorption and reduction mediated by EBB surface organic complexes, and aqueous reduction by DOM (Fig. 6) whose relative dominance depends on the specific pH,

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Fig. 5 a Comparison of high resolution XPS Cr2p spectra of K2Cr2O7 [Cr(VI)-Std], Cr2O3 [Cr(III)-Std], and Cr-EBB. Peak deconvolution of XPS Cr2p spectra of b Cr-EBB, c Cr(III)-Std, and d Cr(VI)-Std. Relative abundances of deconvoluted peaks are given within brackets

EBB dosage, and Cr(VI) concentrations. The interaction of Cr(VI) with EBB is facilitated by electrostatic attraction of Cr(VI) anionic species by the protonated biochar surface at pH ≤2 (Fig. 2). Cr sorption on EBB occurs as Cr(VI), evident from the macroscopic uptake of Cr(VI) relative to Cr(III) under otherwise identical conditions (Fig. 2). XPS investigations reveal that this Cr(VI) uptake is likely facilitated by the presence of carboxylic (O–C═O) groups on the EBB surface (Table 1; Fig. 4). A majority of this sorbed Cr(VI) is reduced to Cr(III) by coupled oxidation of phenolic hydroxyl (O–C) groups abundantly present in the EBB (Figs. 1 and 5; Table 1). The surface-bound Cr(III) is likely present in a coordination environment similar to Cr(III) oxides and/or hydroxides. While macroscopic equilibration data combined with XPS and FT-IR investigations reveal the relative chemical forms of Cr and organic matter in EBB-mediated Cr(VI) sorption and reduction to Cr(III) on EBB, the study is insufficient to resolve the relative time-scales of these sorption and reduction processes. Furthermore, direct Cr(VI) reduction by DOM derived from EBB is also found to be a significant mechanism that

likely competes for Cr(VI) sorption, particularly when sorbent dosage exceeds a critical level (2 g/L at pH 2 for 40 ppm initial Cr(VI); Fig. 3). While Cr sorption–reduction is significant

Fig. 6 Schematic showing the detailed mechanism supported by experimental data for Cr(VI) removal using EBB

Environ Sci Pollut Res (2017) 24:16786–16797 Table 2 Application of EBB for Cr(VI) removal from chrome tanning liquor (CTL) and contaminated groundwater

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Sample ID

Initial [Cr(VI)] (ppm)

Avg. [Cr(VI)] in filtrate (ppm)

% removal avg. ± SD

CTL + no EBB

20

20.12

–

CTL + 2 g/L EBB

20

nd

100 ± 1.5

CTL + 4 g/L EBB CTL + No EBB

20 40

nd 40.1

98.5 ± 1.3 –

CTL + 2 g/L EBB

40

8.48

78.8 ± 0.8

CTL + 4 g/L EBB Groundwater + 2 g/L

40 25

1.58 nd

96.9 ± 1.3 99.5 ± 0.5

nd not detected, SD standard deviation based on triplicates

below this critical level, above this threshold the reduced Cr(III) prefers to stay solubilized due to increased availability of EBB-derived DOM. Although direct reduction by solubilized DOM in aqueous media contributed up to ~40% of Cr(VI) conversion to Cr(III), the presence of EBB clearly enhances Cr(VI) conversion to Cr(III) by providing surface sites as well as DOM for simultaneous sorption–reduction and direct reduction mechanisms, respectively (Fig. 3c). Overall, this study has improved upon the mechanistic understanding of biochar-mediated Cr(VI) removal by identifying and quantifying dominant Cr and organic matter species, and reaction pathways that were not fully explored in previous studies (Park et al. 2005; Park et al. 2007; Dong et al. 2011). Application of biochar for Cr(VI) removal Our results show that EBB-mediated removal of Cr(VI) is effective in acidic media. Therefore, EBB can be readily used at the sources that generate acidic Cr(VI)-contaminated wastewater, e.g., electroplating wastewater having pH 2.2 and chrome tanning wastewater having pH 3.0–3.2 (Ahmed et al. 2016; Hackbarth et al. 2016). About 40% of chromium used for leather tanning is discharged in the effluent, in which it occur both as Cr(VI) and Cr(III) (Saha and Orvig 2010). Batch experimental results (Table 2) show that both 2 and 4 g/L of EBB lead to complete Cr(VI) removal from an initial 20 ppm Cr(VI)-spiked CTL solution. However, an increased EBB dosage (4 g/L) was needed for almost complete removal of Cr(VI) from the 40 ppm Cr(VI)-spiked CTL, highlighting the importance of a critical mass of sorbent required for Cr(VI) removal. Experimental results for the contaminated groundwater also show complete removal of 25 ppm of initial Cr(VI) in water by EBB (Table 2). As per the results shown in Table 2, 50 mL of 20 ppm Cr(VI)-spiked CTL solution was completely removed by EBB at 2 g/L dosage (i.e., 100 mg in 50 mL). This gives a minimum Cr(VI) removal capacity of EBB to be 10 mg/g. This is comparable with the Cr(VI) sorption capacity of other recently reported biomaterials: 7.44 mg/g for pineapple-peelderived biochar (Wang et al. 2016), 9.55 mg/g for mercaptoacetic acid modified Litchi peel (Yi et al. 2017), 17.8 mg/g for

nanoscale zero-valent iron-modified biochar (Dong et al. 2017), 31.1 mg/g for rice husk pretreated with hydrothermal carbonization technology (Ding et al. 2016), etc.

Conclusions Cr(VI) removal by eucalyptus bark biochar (EBB), developed in this study, occurs due to a combination of mechanisms such as sorption by electrostatic attraction, chemisorption, and reduction mediated by oxygen-containing surface organic complexes and direct aqueous reduction by EBB-derived dissolved organic matter (DOM). The relative contributions of these mechanisms depend on the electrolyte pH, biochar dosage, and Cr(VI) levels. EBB is effective in removing Cr(VI) at acidic pH (≤2) only. Cr uptake on EBB occurs as Cr(VI), facilitated by the carboxylic and phenolic groups contained in biochar. On the other hand, phenolic groups present on EBB surface facilitate Cr(VI) reduction. A majority of surface Cr (~82%) on biochar is likely present as Cr(III) species while a smaller fraction (~18%) is present as Cr(VI) species. The sorbed Cr(III) occurs in a Cr2O3 and/or Cr(OH)3-like coordination environment. For a given pH and initial Cr(VI), this sorption–reduction mechanism of Cr(VI) removal is significant below a critical dosage of biochar. When the EBB dosage exceeds this level, direct Cr(VI) reduction by EBB-derived DOM competes with Cr(VI) sorption causing Cr(III) to stay completely solubilized due to increased availability of DOM at very high dosages. Finally, the EBB completely removed Cr(VI) present in contaminated water from a leather processing tannery and groundwater under acidic conditions. The eco-friendly and low-cost biochar holds promise as an effective remediation agent for removal of highly mobile and toxic Cr(VI) ions contained in acidic wastewater. Acknowledgements This work was supported by Ministry of Environment and Forest, India (Grant#19/45-2010-RE) awarded to DP and TG to carry out this research. AS acknowledges support by the Indian Institute of Technology Kanpur Faculty Initiation Grant (INI-IITK-CE20130151). We sincerely thank the two anonymous reviewers and Guilherme L. Dotto (Editor) for the thoughtful and thorough reviews, which have significantly improved the clarity of the manuscript.

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