Article pubs.acs.org/est
Degradation of p‑Nitrophenol on Biochars: Role of Persistent Free Radicals Jing Yang,†,‡ Bo Pan,*,† Hao Li,† Shaohua Liao,† Di Zhang,† Min Wu,† and Baoshan Xing*,‡ †
Faculty of Environmental Science & Engineering, Kunming University of Science & Technology, Kunming 650500, P. R. China Stockbridge School of Agriculture, University of Massachusetts, Amherst, Massachusetts 01003, United States
‡
S Supporting Information *
ABSTRACT: Generation of environmentally persistent free radicals (EPFRs) on solid particles has recently attracted increasing research interest. EPFRs potentially have high reactivity and toxicity. However, the impact of EPFRs on organic contaminant behavior is unclear. We hypothesized that EPFRs in biochars can degrade organic contaminants and play an important role in organic contaminant behavior. We observed obvious degradation of p-nitrophenol (PNP) in the presence of biochars, through the detection of NO3− as well as organic byproducts. The extent of PNP degradation was correlated to the intensity of EPR signals of biochar particles. tert-Butanol (a •OH scavenger) did not completely inhibit PNP degradation, indicating that •OH could not fully explain PNP degradation. The decreased PNP degradation after tert-butanol addition was better correlated with reduced PNP sorption on biochars. PNP degradation through the direct contact with EPFRs in biochar particles could be an important contribution to the PNP concentration reduction in the aqueous phase. The coating of natural organic matter analogue (tannic acid) on biochars did not considerably inhibit PNP degradation, suggesting the ability of biochars to degrade PNP in soil and natural water. Similar EPFR-promoted degradation was observed for five different types of biochars and one activated carbon, as well as one additional chemical (p-aminophenol). Therefore, organic chemical degradation by EPFRs in biochars can be a common process in the environment and should be incorporated in organic chemical fate and risk studies.
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INTRODUCTION Free radicals in solid particles have recently attracted a great deal of research attention. Several investigators reported that in thermal treatment systems, organic components may be partially pyrolyzed, and degradation byprodcuts (such as organic free radicals) could complex with transitional metals and thus stabilized, forming environmentally persistent free radicals (EPFRs).1−3 Similarly, macromolecular organic constituents could also be pyrolyzed and broken bonds were altered into macromolecular free radicals.4 In comparison to common free radicals, these solid-phase free radicals generally have very long lifetimes, varying from days3,5 to months.4 Importantly, EPFRs were showed to maintain their reactivity, and also have significant toxicity. For example, Dellinger and his co-workers reported that EPFRs may result in pulmonary and cardiac dysfunction.6−8 We have previously reported that during the pyrolysis of biomass, a large amount of free radicals were generated in the produced biochars. Negative impacts of biochar on plant were also observed because of the oxidative stress from these free radicals, such as decreased germination rate and inhibited plant growth.4 Therefore, these free radicals keep their reactivity although they are stabilized. Zhou and his co-workers investigated the degradation of organic contaminants in biochars systems.9,10 They mostly focused on the engineering © XXXX American Chemical Society
application of the free radicals in biochars. They emphasized that the free radicals in biochars may activate other smallmolecular free radicals, such as hydrogen peroxide9 or even oxygen,11 then catalyzing organic contaminant degradation. The role of EPFRs themselves in organic contaminant degradation, fate and risks has not been investigated, which could be one of the important missing links in organic contaminant environmental fate and behavior. Hence, research is urgently needed to understand how EPFRs break down organic contaminants. Another line of studies have reported high sorption of various contaminants by biochars. However, comparing to activated carbon or nanoparticles, the surface areas and pore volumes of biochars are actually very low. The surface hydrophobicity was also not as strong as activated carbons (ACs) and carbon nanotubes (CNTs). Therefore, the observed high sorption of organic contaminants on biochars could not be fully explained by their surface properties. Considering the strong electron paramagnetic resonance (EPR) signals in biochars and the possible contact between organic contamiReceived: August 20, 2015 Revised: November 20, 2015 Accepted: December 21, 2015
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DOI: 10.1021/acs.est.5b04042 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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Environmental Science & Technology nants and macromolecular free radicals, organic contaminants could react directly with free radicals in biochars. We thus hypothesize that in biochar sorption systems (without any addition of free radical-active chemicals such as hydrogen peroxide or persulfate), organic chemicals can be directly degraded on biochar surface by macromolecular free radicals. This process along with sorption contributes to the decrease of the aqueous-phase concentration. Ignoring the degradation in biochar sorption system will greatly overestimate the sorption and provide wrong information to understand organic contaminant fate, and insufficient assessment of biochar functions and applications. p-Nitrophenol (PNP) is a priority pollutant according to U.S. EPA classification. This chemical is mutagenic and carcinogenic. It is widely used in organic synthesis industry and in the production of pesticides.12 The degradation pathways of PNP have been widely investigated in the literature. We thus used PNP as a model adsorbate, and its degradation could be well compared to the literature results. In addition, the thermal treatment of carbon-rich solid particles is very common, and the generated solid particles (such as biochar, activated carbon, carbon-based nanoparticles) were widely used as effective adsorbents for various contaminants, including organic pollutants. The result of this study will provide fundamental information regarding EPFRs generation mechanism in the thermal treatment and their roles in organic contaminant fate and behavior.
The specific surface areas of biochars were analyzed by Brunauer Emmett and Teller (BET) nitrogen adsorption technique at 77 K. Elemental C, N, O, and H contents of biochar were determined using a vario MICRO elemental analyzer (Elementar, Germany). Fourier Transform Infrared (FTIR) spectra of powdered biochar mixed with KBr (ratio 1:80) were obtained using a Varian 640-IR FTIR. The surface elemental compositions of the biochars were analyzed by X-ray photoelectron spectroscopy (XPS, PHI5000 VersaProbe II, ULVACPHI, Inc.). Al Ka (15 kV, 50 W) was used as the X-ray source. The obtained spectra were standardized using a C 1s peak at 284.8 eV. Electron Paramagnetic Resonance (EPR) Measurements. Biochar particles within 2 days of their production were loaded into a micropipette (from Germany, 1.0 mm in i.d., 1.5 mm in o.d., and 125 mm in length) and sealed with vacuum grease (Germany) at one tip. These loaded biochars were continuously monitored for free radical signals on an EPR spectrometer (Bruker, A300−6/1, Xband) with a single cavity, a modulation of 100 kHz and microwave frequencies of 9.2−9.9 GHz. The typical parameters for EPR measurement were as follows: the sweep width was 100 G, the modulation amplitude was 1.00 G, and the resolution in the X axes was 1024 points. For solid particles, the EPR microwave power was set specifically to 31 dB (or 0.131 mW) and the sweep time was 81.92 ms. Batch Sorption Experiments. All the solid particles were subject to batch sorption experiments for PNP. To avoid any possible impact from different amount of solid particles, the aqueous:solid ratio was fixed at 100:3 (w:w). PNP was dissolved in deionized water (Milli-Q water, which was purged by N2 for 10 min right before use) as a stock solution (800 mg/ L). The stock solution was diluted with deionized water to a series of concentrations (50, 100, 150, 200, 250, 300, 350, and 400 mg/L). The same concentration sequence of PNP solution without solid particles was used as the initial concentration references. The batch sorption experiments were carried out in 15 mL vials with Teflon-lined screw caps. All the vials were continuously shaken on an air-bath shaker in dark at 25 °C for 7 days. All the vials were then centrifuged at 2500g for 10 min. The supernatants were sampled for PNP and NO 3 − quantification. PNP was analyzed by high performance liquid chromatography (HPLC, Agilent Technologies 1260) equipped with a reversed-phase C18 column (5 mm, 4.6 × 150 mm) and an UV detector at 318 nm. The mobile phase for PNP was 35:65 (v:v) of methanol and deionized water at a flow rate of 1.0 mL/min and a column temperature of 25 °C. The supernatants were also tested by an ion chromatographic technique and a UV spectrophotometer (UV-2600, Shimazu) at 202 nm for NO3− quantification (Supporting Information (SI) Figures S1).13 The Impact of TBA on PNP Sorption and Degradation. Experiments for probing the effect of •OH on PNP degradation were performed under the same conditions as in the sorption experiments, except that TBA was added. TBA was used as a typical •OH scavenger. Five to 45 μL TBA were added in PNP-biochar systems and PNP concentrations were monitored as above. Analysis of Intermediates and Final Products. Determination of intermediates was carried out on gas chromatography coupled with mass spectrometry (GC/MS, Agilent 7890A-5975C). The GC column was operated at 80 °C for 3 min and then increased to 250 °C at the rate of 20 °C min−1.
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EXPERIMENTAL SECTION Materials. Five types of common biomass, namely, pine wood (PI), corn stalks (CN), peanut shells (PT), rice (RC), and wheat (WT) straws were collected from farm fields, dried and chopped into pieces below 2 mm. Two major biopolymers, lignin (LG, dealkaline, TCI) and cellulose (CL, Sigma-Aldrich), were used as model biomass. p-Nitrophenol (PNP, Analytical reagent) was purchased from Aladdin Reagent (Shanghai) Co. Tannic acid (TA, analytical reagent) was purchased from Tianjin Chemical Reagent Company. Other chemicals including 5,5-dimethyl-1-pyrroline-N-oxide (DMPO, ≥ 99.0%, GLC), tert-butanol (TBA, ≥ 99.5%) and N,O-bis(trimethylsilyl)trifluoroacetamide (BSTFA, Derivatization grade for GC analysis) were obtained from Sigma-Aldrich and used without further purification. Biochar Production and Characterization. All the biomass materials were processed using a temperature programming method (kept at 100 °C for 30 min, and then pyrolyzed for 4 h at temperatures of 200, 350, 500, and 700 °C, respectively). The oven was purged with continuous flow of N2 during pyrolysis. The obtained biochars were ground to below 0.25 mm and stored in amber vials until further use. Biochars produced at temperatures of 200, 350, 500, and 700 °C were noted as the suffixes of 2, 3, 5, and 7, respectively. For example, PI2, PI3, PI5, and PI7 represent biochars generated using pine wood at 200, 350, 500, and 700 °C, respectively. The surface coating of biochars by TA was performed in sorption experiments. Briefly, 7.5 g of biochars were mixed with 500 mL of 5 g/L TA solution at 25 °C for 2 days. The mixture was then centrifuged and the particles were washed repeatedly with deionized water under negligible detection of TOC in the supernatant. The particles were then freeze-dried. The coated TA was in the range of 51−140 mg/g depending on biochar types. B
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Environmental Science & Technology The other experimental conditions were: EI impact ionization 70 eV; helium as the carrier gas; injection temperature 280 °C; source temperature 100 °C. Kinetics Study. The apparent change of aqueous-phase PNP concentrations was monitored in biochar sorption systems using three initial PNP concentrations (50, 200, 400 mg/L). The solid:aqueous ratio was also fixed at 100:3 (w:w). The sorption experiments were conducted in 100 mL amber glass vials with Teflon-lined caps. All the vials were kept in dark and shaken vertically in an air-bath shaker in dark at 25 °C. Samples were taken at 2, 4, 6, 8, 12, 24 h and then every day until the concentration of PNP in the supernatants leveled off or approached zero. At the selected time points, the vials were centrifuged at 2500g for 15 min, 0.5 mL of the supernatants were taken out for PNP determination. OH Capture. The •OH generation in biochar system was investigated through DMPO (0.3 M in 0.01 M PBS solution) spin trapping.14 Briefly, 10 mg biochar particles were mixed with 100 μL water and 100 μL DMPO. The mixture was shaken by hands for 2 min to disperse the mixture, and then loaded into a micropipette and sealed with vacuum grease at one tip. This micropipette was inserted into the standard resonator for high sensitivity continuous-wave EPR measurements. The concentration of DMPO−OH adducts was determined by the intensity of the second line (at low magnetic field) for its representative 4-line spectra and compared with the intensity of standard TEMPOL (4-hydroxyl2,2,6,6-tetramethylpiperidine-1-oxyl, a stable radical). For liquid samples, the EPR microwave power was 9 dB (or 21 mW) and the sweep time was 20.48 ms.
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Figure 1. PNP aqueous-phase concentration change with contact time in biochar sorption systems. Biochars produced from pine wood (PI), lignin (LG) and cellulose (CL) were separately presented in panels A, B, and C, respectively. PNP was stable in its original solution, but approached zero in biochars obtained at pyrolysis temperature higher than 200 °C. This figure presents the PNP degradation kinetics at initially applied PNP concentrations (C0PNP) of 400 mg/L. The data for C0PNP of 50 mg/L and 200 mg/L are presented in SI Figures S2 and S3, respectively.
RESULTS AND DISCUSSION Evidence of PNP degradation in biochar sorption system. PNP was stable in their background solutions as demonstrated by the constant concentrations of the stock solution and absence of degradation products. However, in biochar sorption systems, it was not the case. The first observation was that in most of the biochar-involved systems, PNP concentrations approached zero in contrast to equilibrium aqueous-phase concentrations in common sorption experiments. For example, in PI (pine wood) sorption systems, PNP aqueous-phase concentrations reached constant values with extended time (Figure 1A). The equilibrium time was in the range of 72−96 h depending on the initially applied PNP concentrations. This is a very typical case of sorption equilibrium. However, for PI3, PI5 as well as PI7, PNP aqueous-phase concentrations kept decreasing to below detection limit. Similar PNP concentration decreasing kinetics was also observed for LG and CL biochars (Figure 1B and C). The gradual disappearance of PNP in aqueous phase may suggest chemical alteration of PNP molecules. According to our GC-MS analysis, various byproducts of PNP were detected, clearly indicating PNP degradation, which will be discussed with more details later. Nitrate is the primary chemical degradation product of PNP and thus monitoring of nitrate is a useful approach to evaluate the degree of PNP degradation, as used in other studies.15,16 Thus, we also measured NO3− concentrations in this work. As presented in Figure 2, no NO3− was detected in PNP reference solutions (without any solid particles). However, in systems with biochars, significant amount of NO3− was detected. We first noted that NO3− was detected in the presence of biochar when PNP was absent, because of the release of NO3− from biochars.
However, the concentration was around the detection limit of 0.01 mg/L, which was 2 orders of magnitude lower than the lowest NO3− concentration detected in PNP-biochar systems. Because of the sorption of NO3− on solid particles, the actual generation of NO3− was calculated as the sum of aqueous-phase and solid-phase concentrations (calculated through aqueousphase concentrations based on NO3− sorption experiments). NO3− sorption was described well by Langmuir model (SI Table S2). These fitting results were used to calculate solidphase concentration based on the measured aqueous-phase NO3− concentrations. Although the potential competition with PNP byproducts may alter NO3− sorption, this method provides a practical calculation to estimate PNP degradation from NO3− generation. In PNP-biochar system, NO3− concentration increased with PNP concentration after a 7 day reaction in PI5 sorption system (Figure 2A, up to 40 mg/L where the initially applied PNP concentration was 400 mg/L). The increased NO3− generation with fixed biochar amount but increased PNP concentration suggested that NO3− was originated from PNP. NO3− generation decreased when biochar amount was reduced (Figure 2B), and NO3− concentration kept increasing with extended reaction time (Figure 2C). Hence, PNP degradation was accelerated by biochar particles. More importantly, when C
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Figure 2. NO3− generation in PNP-biochar system after a 7 day equilibration. NO3− concentrations were calculated as the sum of aqueous and solidphase (adsorbed) concentrations. NO3− generation was measured in sorption systems of biochars produced from pine wood at different temperatures (panel A). Biochar produced from pine wood at 500 °C (PI5) was selectively used in sorption experiment to compare different solid:aqueous ratios (B). NO3− was progressively generated in PNP-biochar systems (C).
Figure 3. EPR measurement of biochars and DMPO trapping. EPR signals of pine wood (PI) and biochars produced at different temperatures (PI2, PI3, PI5 and PI7 for 200 °C, 300 °C, 500 °C, and 700 °C, respectively) were illustrated in panels A and B. Generation of OH free radicals was determined based on DMPO trapping (C).
Figure 4. Overestimation of PNP sorption because of its degradation in biochar sorption systems. Because of the generation of NO3− and thus PNP degradation in biochar sorption system (A), PNP sorption was generally overestimated. Sorption isotherms before (open symbols) and after (gray symbols) the correction of PNP degradation were presented together for comparison (B). Ignoring PNP degradation could result in 1.1 to 1.9 times overestimation of the sorption (C).
(SI Figures S4 and S5). The pyrolysis at 700 °C generally decreased free radical signals. The elevated temperature may result in the breakdown and reorganization of organic structures, thus leading to the disappearing of free radicals.19 In order to examine the reactivity of the free radicals, the potential of these radicals promoting the generation of •OH in aqueous phase was determined through DMPO trapping. According to the measurements as presented in Figure 3, the generation of •OH was consistent with the intensity of EPR signals of biochar particles. Although EPR signals were correlated with PNP degradation, the above results did not necessarily suggest that PNP degradation was resulted from •OH in aqueous phase, as discussed extensively in the following sections. Similar PNP degradation kinetics (SI Figures S2 and S3) and EPR signals (SI Figures S4 and S5) were observed for the biochars from model biomass components (lignin and cellulose). Potential Contribution of PNP Degradation to Apparent PNP Sorption by Biochars. Although sorption
biochars produced from different temperatures were compared, NO3− generation increased with biochar pyrolysis temperature except for PI7. PNP dissipation/degradation rate was reported to increase with increasing pH.17 If pH played a significant role in our experimental system, high PNP degradation should be observed for PI7 (with the highest pH, see SI Table S1). However, PI7 had relatively low content of NO3− (Figure 2A) in the sorption system, correlating with low PNP degradation. Previous studies stated that free radicals were generated in thermally treated solid particles,18 and the free radicals will react with organic molecules. We detected strong free radicals signals in our biochar samples, and the signal intensity of free radicals correlated well with the extent of PNP degradation. For example, no free radicals were detected in the original solid particles. Free radical signals increased dramatically when PI was pyrolyzed up to 500 °C (Figure 3A). But, when the temperature increased to 700 °C, the free radical signals decreased greatly (Figure 3B). Similarly for lignin and cellulose, we also observed the increased free radical signals up to 500 °C D
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Figure 5. PNP sorption and degradation by biochars as affected by tannic acid coating. The EPR signals of biochars were hardly changed after TA coating (A), but the generation of OH free radicals were greatly inhibited (B). The apparent PNP sorption was decreased after TA coating (Figure 4B and SI Figure S16A), but NO3− generation was not affected (C).
Figure 6. PNP degradation in PI5 as affected by TBA. The generation of NO3− decreased with increased TBA application (A). PNP degradation ratios were comparable at different C0PNP (B). The addition of TBA decreased PNP solid-phase concentrations, indicating obvious competitive sorption between TBA and PNP (C). The solid line in panel C is a virtual line as a guide for the eye.
We thus call for attention that carbonaceous adsorbents, especially biochars, can contain free radicals and promote the degradation of organic contaminants. If the degradation was not appropriately taken into consideration, the sorption capacities of these adsorbents will be overestimated, which may lead to erroneous modeling and prediction of organic chemical fate and toxicity. PNP Degradation As Affected by Tannic Acid Coating. The coating of organic matter on biochars is a natural process in the environment, which can affect biochar sorption/ degradation characteristics. For this purpose, tannic acid (TA) was used as an organic matter surrogate. As presented in Figure 5B, TA coating has greatly inhibited the generation of •OH, as suggested by low DMPO−OH signals. However, the free radical signals of the solid particles were hardly changed after this coating (Figure 5A). In addition, NO3− generation was comparable at the same C0PNP (Figure 5C), and thus PNP degradation was not altered notably. If the primary degradation would have occurred in aqueous phase, significant •OH generation should be detected using DMPO trapping after TA coating. This was definitely not the case in this research. With these results, we propose that PNP degradation on biochar solid particles is an important process. It should be noted that the apparent sorption of PNP on biochar decreased after TA coating (Figure 4B and SI Figure S16A). The decreased direct contact of PNP to biochar particles did not inhibit PNP degradation. Previous studies concluded that organic matter may behave as an electron shuttle, which may enhance the electron-transfer rate and thus accelerate PNP degradation.21 This process may have occurred in this study when TA was coated on biochars. However, the exact contribution of electron shuttle to PNP degradation could not be obtained in this experiment. Future experiments are required to specifically address this point. PNP Degradation As Affected by TBA. Tert-butanol (TBA) is reported to effectively inhibit the activities of •OH
curves could be obtained for biochars in common batch sorption experiments, the obvious degradation of PNP may result in wrong estimation of sorption characteristics. In order to establish a reliable sorption isotherm, the actually adsorbed PNP concentrations should be determined. However, extraction of PNP from biochar particles had very low recovery with high variations, probably because of the strong retention of PNP in biochar porous structure. Thus, PNP solid-phase concentrations could not be directly measured and a mass balance calculation could not be applied for degradation calculation. Previous studies have reported that NO3− was the primary degradation byproducts, and PNP degradation generally started from denitration.20 We used NO3− as an indicator of PNP degradation. Considering the possibility of PNP degradation without or before denitration, NO3− would be a conservative parameter to index PNP degradation. PNP degradation was thus calculated from NO3− through 1:1 stoichiometic relationship. The commonly used mass balance calculation of PNP sorption will result in clear overestimation of PNP sorption. The most extraordinary case was observed for lignin-derived biochars. For example, in the sorption system of LG5, PNP sorption was overestimated by 200% if PNP degradation was not properly considered (Figure 4C). Biochars produced at 200, 350, 500, and 700 °C all showed different degrees of sorption overestimation (SI Figures S6−S8). This overestimate may have occurred commonly in the literature. Many carbonaceous adsorbents were involved with thermal treatment. We observed strong free radicals signals in biochars from peanut shells, wheat straw, corn stalk, rice straw, even activated carbon (SI Figures S9−S11). These free radicals resulted in PNP degradation at different extents (SI Figures S12−S15). For example, the free radicals in activated carbon produced up to 20% degradation in 7 day sorption experiments (SI Figure S15). E
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Figure 7. Schematic diagram of PNP degradation in biochar sorption systems. These byproducts were detected using GC-MS as presented in SI Figure S19.
(i.e., •OH scavenger) in the aqueous phase, which can react with •OH (K•OH = 5 × 108 M−1 s−1).22,23 When compared at molar concentrations, TBA (5−50 mg) was 4−40 times higher than PNP (400 mg/L in 10 mL) in the reaction systems. If PNP were mainly degraded by •OH in aqueous phase, the use of excessive TBA would result in substantial inhibition of PNP degradation. The degradation ratios were only progressively decreased with the increased TBA as illustrated in Figure 6A. The decreased Kd after TBA addition implied that TBA may compete with PNP for sorption sites on biochars (Figure 6C). The correspondingly decreased NO3− production (Figure 6A) or PNP degradation (Figure 6B) again confirmed that PNP degradation on biochar surface (instead of in the aqueous phase) was important. The decreased PNP degradation and Kd after TBA addition were also observed for CL5 and LG5 as presented in SI Figures S17 and S18. PNP Degradation by Free Radicals on Biochars. Most studies on organic contaminant degradation emphasized the important role of •OH in the aqueous phase.23 Our current study suggested that PNP degradation was hardly inhibited after the addition of •OH scavenger (n-butanol in aqueous phase or TA coating on solid particles). PNP degradation was correlated with the strength of free radical signals on solid particles after comparing biochars generated at different temperatures. Displacement of the adsorbed PNP resulted in decreased PNP degradation. These results strongly supported the surface PNP degradation on solid particle, most likely through direct contact with EPFRs. The electron-donor ability of the OH group in phenols favors electrophilic attack on ortho- and para-positions.24 It is well-known that nitro group is an electron withdrawing group, which also preferentially affect the ortho and para position of a benzenic ring.25,26 Thus, catechols and hydroquinone were identified as the primary degradation products (SI Figure S19). In the following degradation process, p-substituted catechols were transformed into catechol followed by the opening of aromatic ring,27 which were detected using GC-MS as presented in SI Figures S19. Investigators observed the catalytic degradation of organic contaminants by carbonaceous materials, such as carbon
nanotubes,28 graphene oxide29 as well as activated carbon,30 which was attributed to the facilitated electron transfer by graphitic surfaces, activation of chemical bonds by the zigzag carbon atoms at the edges or the defects of carbon nanomaterials,29 or the catalytic effect of carbon nanotube functional groups.28 As we detected EPR signals in carbon nanotubes and activated carbon (SI Figures S9C and S9D), the free radicals in these carbonaceous materials may have a positive contribution to organic contaminant degradation. Clearly, this process is not well addressed in the literature. Although further investigation is needed to differentiate PNP degradation on solid from aqueous phases in the presence of biochars, this study showed high degradation potential of biochar. Both sorption and degradation were also observed for p-aminophenol in biochar sorption systems (SI Figure S20) and the degradation accounted for up to 57% in the overall decrease of the aqueous phase concentration. Therefore, degradation of organic contaminants by EPFRs could be a common process, which has to be carefully considered when assessing the environmental roles of carbonaceous materials (especially for biochars) to organic contaminants. With the potential benefit in pollution control, soil remediation, and agricultural use, massive application of biochars in the environment is expected in the near future. While most of the investigators focused on biochar sorption properties when assessing the impacts of biochars on organic contaminants, this current study highlights that organic contaminant degradation is also an important process regulating their fate after biochar application, increasing organic contaminant removal. However, the exact role of EPFRs and EPFR-induced small-molecular free radicals in organic contaminant degradation, as well as EPFR fate in biochar aging need to be further investigated in the future.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.5b04042. Additional information as noted in the text(PDF) F
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AUTHOR INFORMATION
Corresponding Authors
*(B.P.) Phone: 86-871-65102829; fax: 86-871-65920530; email:
[email protected]. *(B.X.) Phone: 413-545-5212; fax: 413-577-0242; e-mail: bx@ umass.edu. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by the National Natural Scientific Foundation of China (41273138, 41222025), Recruitment Program of Highly-Qualified Scholars in Yunnan (2010CI109), and USDA McIntire-Stennis Program (MAS 00028). We are thankful for the financial support from the China Scholarship Council (CSC) for the Ph.D. fellowship to J.Y. in the U.S.
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DOI: 10.1021/acs.est.5b04042 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
