Critical Review pubs.acs.org/est
Activity and Reactivity of Pyrogenic Carbonaceous Matter toward Organic Compounds J. J. Pignatello,*,† William A. Mitch,‡ and Wenqing Xu§ †
Department of Environmental Sciences, The Connecticut Agricultural Experiment Station, New Haven, Connecticut 06504-1106, United States ‡ Department of Civil and Environmental Engineering, Stanford University, 473 Via Ortega, Stanford, California 94305, United States § Department of Civil and Environmental Engineering, Villanova University, Villanova, Pennsylvania 19085, United States ABSTRACT: Pyrogenic carbonaceous matter (PCM) includes environmental black carbon (fossil fuel soot, biomass char), engineered carbons (biochar, activated carbon), and related materials like graphene and nanotubes. These materials contact organic pollutants due to their widespread presence in the environment or through their use in various engineering applications. This review covers recent advances in our understanding of adsorption and chemical reactions mediated by PCM and the links between these processes. It also covers adsorptive processes previously receiving little attention and ignored in models such as steric constraints, physicochemical effects of confinement in nanopores, π interactions of aromatic compounds with polyaromatic surfaces, and very strong hydrogen bonding of ionizable compounds with surface functional groups. Although previous research has regarded carbons merely as passive sorbents, recent studies show that PCM can promote chemical reactions of sorbed contaminants at ordinary temperature, including long-range electron conduction between molecules and between microbes and molecules, local redox reactions between molecules, and hydrolysis. PCM may itself contain redox-active functional groups that are capable of oxidizing or reducing organic compounds and of generating reactive oxygen species (ROS) from oxygen, peroxides, or ozone. Amorphous carbons contain persistent free radicals that may play a role in observed redox reactions and ROS generation. Reactions mediated by PCM can impact the biogeochemical fate of pollutants and lead to useful strategies for remediation.
1. INTRODUCTION Pyrogenic carbonaceous matter (PCM) refers to the solid pyrolysis products of fresh or fossilized biomass. It includes combustion soot, chars formed in fires, and carbons such as charcoal, biochar, and activated carbon produced for advantageous uses, such as heating and cooking, soil conditioning, water and soil purification, catalyst support, and others. PCM formed by natural processes or discharged to the environment is commonly referred to as black carbon. Charcoal and biochar are usually made from plant biomass in a reactor under anoxic or low-oxygen conditions and at temperatures typically below 800 °C. Chars produced in wildfires typically had experienced conditions of limited air below 800 °C.1 Activated carbon (AC) is made from plant biomass, polymers, or coal under anoxic conditions at temperatures usually well above 900 °C, and then is “activated” with gases or reagents to modify pores and surfaces.2 This review addresses the activity and reactivity of PCM toward organic compounds, incorporating insight as appropriate from studies of reference materials such as carbon nanotubes (CNTs), graphene, and graphite that share similar physicochemical features. It is broadly interested in the role PCM plays in contaminant fate in the soil and sediment environment, the use of PCM for stabilizing or remediating contaminated soil or sediment, and the use of PCM in water © XXXX American Chemical Society
purification. Pyrogenic carbon can make up as much as half the total organic carbon in soil3−6 and even more in some impacted areas. It is well-known that PCM sorbs most compounds more effectively and less linearly than non-pyrogenic natural organic matter, and thus can potentially contribute significantly to overall sorption in geomedia, especially at low contaminant concentrations. Its contribution has been difficult to assess, however, due to the difficulties in characterizing PCM in geosamples, an incomplete understanding of sorption mechanisms and the influence of pyrolysis conditions, and a poor understanding of the effects of environmental weathering on PCM. Sorption by activated carbon,7,8 environmental black carbon,4 soot,9 and biochars10−12 has been discussed in several good reviews and comprehensive studies. Those investigations focused mainly on molecular structure−activity relationships for neutral compounds, emphasizing apolar compounds. It is generally agreed that sorption of such compounds is dominated by van der Waals forces and solvophobic effects. This review will not reiterate that discussion, but will instead focus on fundamental aspects of sorption at the molecular level that are Received: March 1, 2017 Revised: May 31, 2017 Accepted: July 7, 2017
A
DOI: 10.1021/acs.est.7b01088 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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Figure 1. (A) The minimum average cluster size of chars versus heat treatment temperature.20 (B) Schematic representation of PCM’s polyaromatic sheets with their edge functional groups. Counter ions are not shown. Sheets may be substituted with aliphatic chains (not shown) and may be covalently linked to, or stacked by weak forces with other sheets. (C) Electron donating capacity (EDC) and electron accepting capacity (EAC) associated with redox-active surface functional groups.23
decomposition reactions proceeding through polyaromatic intermediates and leading to conglomeration of primary particles into aggregates of various sizes.19 Features of PCM shared by most of its forms and central to its behavior are high polyaromaticity and high nanoporosity. With heating under anoxic conditions the structure of woody or cellulosic char evolves from a transition phase consisting mainly of biopolymers with cellulosic crystallinity mostly preserved, to an amorphous phase of thermally altered biomolecules, to a composite phase in which emerging clusters of graphene sheets are randomly mixed with the amorphous phase, and finally to a turbostratic state comprised of disordered graphitic microcrystallites.18 Quantitative structural changes in a series of wood chars heated for 2 h at different temperatures are revealed by advanced 13C NMR methods.20 At 300 °C the char largely retains its lignin and cellulose character. By 350 °C the carbohydrate moieties completely disappear. By 400 °C the lignocellulosic features are lost, leaving predominantly aromatic resonances. From 500 to 700 °C, there is a progressive decrease in aromatic C−H and C−O groups, an increase in nonprotonated C, and an increase in aromatic cluster size. The mean minimum ring cluster size increases roughly linearly with temperature from about 8 carbons at 300 °C to about 76 carbons at 700 °C (Figure 1).20 The polyaromatic rings are rimmed with heteroatom groups including keto, quinone, lactone, pyrone, ether, hydroxyl, carboxyl, and various types of amines (Figure 1). Fuel soots and carbons prepared at very high temperatures tend to have fewer of these groups. These groups can affect rings electronically. Carbon surfaces are amphoteric. The proton acidity of PCMs is mainly supplied by carboxyl and hydroxyl groups. The proton basicity can be supplied by O and N functional groups and possibly by C−π electrons.21 The intrinsic pKa of carboxyl (3−
seldom taken into account in structure−property free energy relationships. Such investigations are useful, not only for improving fate models, but for designing carbonaceous materials with superior properties for contaminant removal by adsorption or potentially to assist their removal by chemical reaction. Among environmental scientists and engineers, PCM has historically been considered merely as a passive sorbent that is capable of capturing, concentrating, and sequestering contaminants in sediments, soil, or water streams.13,14 Indeed, researchers have even added AC to sediments to reduce the mobility and bioavailability of hydrophobic contaminants.15,14,16,17 If PCM were merely passively removing contaminants from solution, adsorption within the pore system would protect the contaminant from microorganisms and shield it from light and potential reactants in the aqueous phase. Over the past decade, however, researchers have begun to recognize features of PCM that can promote degradation of sorbed contaminants. PCM can mediate certain reactions, facilitate electron transfer between microbial species and molecules, and produce reactive oxygen species (ROS) from some bulk oxidants. PCM may also have intrinsic reactivity toward some compounds. The reactions in which PCM can participate have not been fully explored, and the underlying mechanisms are still open to scrutiny, yet reactions mediated or catalyzed by PCM can impact the environmental fate of contaminants and lead to useful strategies for remediation. The review discusses their potential implications and suggests avenues for further research.
2. FUNDAMENTAL STRUCTURE OF PCM The charring of solid biomass takes place via solid-state reactions,18 whereas carbonaceous soot is formed in the gas phase by a series of free radical condensation and B
DOI: 10.1021/acs.est.7b01088 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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partition coefficient, Kow), most likely due to anthracene’s smaller critical diameter affording it a larger sorption domain. Steric effects can also affect rates. Gas diffusivity in zeolites decreases exponentially as the minimum critical molecular diameter approaches the pore diameter, showing significant effects even at ∼10% of pore diameter.35 Showing a consistent trend, the uptake rate of aromatic amines by a wood char decreased with increasing molecular size in the order pyridine > quinoline > prometon (a triazine herbicide), and was slower for the cationic than the neutral form of the molecule.36 The slower rate of the cation may be due to its larger hydration shell or the need for co-diffusion of a counterion (here, Cl−) to maintain electrical balance. Conformational/configurational factors plausibly leading to restricted approach to the surface include chain inflexibility (normal alkanes better approach the surface than cyclic alkanes),37 ortho-substitution limiting ring coplanarity of biphenyls (e.g., PCBs),38−42 and other shape factors.43,44 To date, steric effects have not been explicitly incorporated in ppLFER models for PCM8,9,12,30,31 and related materials,32 with the exception of one study that introduced a molecular “planarity toggle parameter” (1 or 0).30 Improved efforts to capture steric contributions to sorption free energy are clearly needed. This will be a major challenge because models will have to include terms for both solute molecular size/shape and pore size distribution. Kinetic models for ionic and ionizable solutes may have to take into account hydration sphere size and counterion diffusivity. Pore size distribution of PCM in environmental samples is difficult to determine because particles are not easy to isolate. A further complication is the presence of dissolved organic matter, which can compete for sites and block pores in a manner that may also depend on contaminant molecular size and PCM pore size distribution.45−47 When adsorbed to nanoporous PCM, a nonionic organic compound will coat surfaces and fill up pores with a liquid- or solid-like phase of the compound, beginning with the smallest pore sizes. 48 Water is also abundant in pores. The physicochemical environment within a carbon slit-nanopore differs, often sharply, from that on an open surface or within the bulk phase. The local gas pressure inside a pore at ambient external pressure depends on diameter, distance from the center, and whether calculated normal or tangential to the wall, but in micropores can reach thousands or tens of thousands of bars.49 Confining a pure substance to nanoscale dimensions can greatly affect its liquid viscosity, melting point (Tm), crystal structure, adsorptivity, mobility, and chemical reactivity.49,50 The effects of nanoconfinement are not usually taken into account in adsorption or reaction models. Figure 2 depicts some of the expected effects of nanoconfinement for neutral molecules. The difference in melting temperature in nanopores compared to the bulk state, ΔTm, is related to the difference between the fluid−fluid and fluid−wall attractive energies, and inversely related to pore width.49,50 For organic molecules, which generally have stronger fluid−wall than fluid−fluid interactions in carbon nanopores,49 Tm tends to be much higher inside carbon nanopores than in the bulk for example, CCl4 by 57 K, benzene by 60 K, aniline by 31 K, and methanol by 42.5 K in AC fibers.50 Hence, compounds that are liquids at ordinary temperature may freeze inside a carbon nanoporethis includes all but methanol of the examples given. The contact layer near the wall may freeze first.50 In fact, even on the flat open surface of graphite, the molecular layer of
6), pyridine (4−6), pyrone (∼5−8), pyridone (8−11), and phenolic (9−10) groups21,22 can explain the broad titration curves that are observed. Carboxyl, hydroxyl, and heterocyclic amine groups are active sites for interaction with metal ions and ionizable organic compounds. Oxygen functionalities, especially quinone, are believed to participate in the redox behavior of PCM.23 Polyaromaticity contributes to the electron conductivity of PCM, which, together with ring size, increases as the heating temperature increases.20 As we shall see, conductivity influences the reactivity of PCM. Despite a popular conception that the polyaromatic surface is water repelling, it is actually mildly attractive to water: the binding energy of the first water molecule is ∼10 kJ/mol.24 And water molecules cluster around polar groups.25,26 Chars, ACs, and soots are disordered porous solids with quasi slit-shaped nanopores and larger pores. Here, nanopores refer to micropores ( microporous AC.138 However, the microporous AC was the most reactive PCM toward nitroaniline decay, suggesting that both the pore size and the geometry of contaminants are important.138 Different studies have pointed to the importance of quinone groups, polycondensed aromatic structures, and porosity in surface-mediated electron transfer reactions by PCM. One study suggests that pyrone groups are also theoretically capable of reversibly accepting electrons.144 Sun et al.113 examined the dual functions of PCM as “geobattery” (electron storage) and “geoconductor” (direct electron transfer to a solute at the surface) in electrochemical cells. As the pyrolysis temperature used to make the black walnut chars increased from 400 °C to above the percolation temperature (about 600 °C), the geoconductor function increased exponentially in concert with an increase in bulk conductivity. Concurrently, the geobattery function decreased in concert with a decline in the quinone content. More research is needed to better understand types of PCM and which properties are most important for redox mediation in biotic and abiotic reactions. Several interesting developments in PCM-mediated nucleophilic reactions have appeared. The examples of RDX, DDT, and DDE reacting with a surface species derived from oxidation of hydrogen sulfide on graphite were mentioned above. A number of papers report that AC and functionalized CNT can promote base-catalyzed dehydrochlorination of compounds such as lindane145,146 and 1,1,2,2-tetrachloroethane22,145−147 in neutral-to-slightly alkaline solution. The attacking nucleophile has been assigned to a base-deprotonated surface group, such as −CO2−, −O−, or −NH2 (eq 5a), although there is some uncertainty about this proposed pathway.147 Activated carbon also can mediate the base hydrolysis of bromomethane and other simple alkyl bromides without direct involvement of surface groups.148 Hydrolysis obeys a two-term rate law for reactions occurring in both aqueous and adsorbed states. The kinetics and products (primarily methanol) of the adsorbed-state reaction are consistent with a bimolecular SN2 reaction between adsorbed bromomethane and adsorbed OH− (eq 5b). A mechanism involving anion exchange of hydroxide at surface sites is implicated because hydrolysis is accelerated by pre-adsorption of quaternary ammonium surfactants, which H
DOI: 10.1021/acs.est.7b01088 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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Environmental Science & Technology Table 1. Reactions of Dangling Bonds in PCM reactions reaction with molecular oxygen H2O2 production
equations
Cm ·, Cm : + O2 → Cm(O2 ), Cm(O), CmOO·, CmO· H2O
Cm(O2 ) ⎯⎯⎯⎯→ H 2O2 + Cm(− 2H+) H2O
+
CmOO· ⎯⎯⎯⎯→ HO2 · + Cm(− H ) H2O
CmOO· ⎯⎯⎯⎯→ (Cm)+ + O−2 ·
(7) (8)
(9)
HO2 · + O2− · + H 2O → H 2O2 + O2 + OH− H2O2 decomposition and ROS formation
H2O
H 2O2 + PCM ⎯⎯⎯⎯→ O2
(6)
(10)
(11)
(a) H 2O2 + CmO·, Cm · → OH · + OH− + (CmO)+ , (Cm)+ +
+
+
(b) H 2O2 + (CmO) , (Cm) → HO2 · + H + CmO·, Cm·
(c) H 2O2 + Cm ,nrad → OH · + OH− + (Cm ,nrad )+ Me
n+
(n − 1) +
+ H 2O2 → Me
(n − 1) +
Me reaction with organic compounds
+ H 2O2 → Me
n+
OH · + RH → R· + H 2O
+ HO2 · + H
+ OH · + OH
(b) RH + Cm,nrad → products
OH · + PCM ⎯⎯⎯⎯→ products
PCM → OH · + H 2O2 a
(14)a (15)a
(18)
(19) (20)
O3 + PCM → OH · + products hν
(13)
(17)
(a) RH + CmO·, Cm · → products
other ROS formation pathways
−
(12b)
(16)
PCM + RH → products
H2O
+
(12a)
(21)
(22)
Me is a redox-switchable metal from the mineral component of PCM.
in biomass chars is typically ∼1018 to 1019 spins g−1.154,155,153 Similar levels of PFR are also found in AC,156 atmospheric particles (PM2.5),157 diesel exhaust,158 and hexane soot.159 Increasing the final pyrolysis temperature increases the PFR intensity and decreases the O/C radical ratio as indicated by the EPR g value.154,155 Addition of phenolic compounds or transition metal salts to the biomass precursor increases the PFR concentration and raises and lowers, respectively, the O/C radical ratio of the char.160 Pure lignin affords char with higher PFR concentration and greater O character than pure cellulose char, suggesting that aromaticity in the precursor is important.154,153 H2O2 Production. When the cooled char product is submerged in water, a portion of the nondissociatively incorporated oxygen is liberated as H2O2 (eqs 7−10).153 For pure lignin and cellulose chars,153 the H2O2 concentration reached a maximum at ∼1 h and then declined to a low steadystate value, indicating that H2O2 is both produced and destroyed by the PCM. Aging experiments were performed under high vacuum or in moist air.153 Samples stored under a vacuum for a month behaved as though fresh. For samples stored in air, net H2O2 production after a 1-h mixing period with water declined rapidly during the first 10−30 h of storage, but then remained at low levels for at least a month of storage. These results indicate that dioxygen destroys H2O2-producing sites. Possible reactions leading to H2O2 include hydrolytic elimination of H2O2 from Cm(O2) (eq 7), hydrolytic elimination of HO2· from Cm-OO· (eq 8), and direct electron transfer from radicals to dioxygen giving O2−· (eq 9).155 The HO2·/O2−· species (pKa, 4.88) may disproportionate to H2O2 via eq 10 and analogous reactions.
increase the density of anion exchange sites, and is inhibited by the addition of inert competitive anions in the order of their binding affinity for ion exchange resins (i.e., Br− < NO3− < ClO4−).
4.2. PCM-Mediated Generation of Reactive Oxygen Species and the Intrinsic Redox Reactivity of PCM. Because biomass carbonization takes place in the absence of a liquid phase, the matrix of the solid on cooling is left with unsatisfied valencies, or “dangling bonds” that exist as Ccentered or O-centered unshared electrons (known as free radicals, Cm· and CmO·, respectively) or C-centered unshared electron pairs (known as singlet- or triplet-state carbenes, Cm:),149−151 which are often delocalized into π-systems. Illustrations of matrix Cm· and Cm: appear in Figure 1B. The dangling bonds can subsequently undergo a variety of reactions, which appear in eqs 6−21 of Table 1. Persistent Free Radicals Formation. When a fresh char is cooled and exposed to air, many of the C-centered dangling bonds combine with dioxygen, dissociatively or non-dissociatively, to give both valence-saturated and radical products (eq 6). Chemisorption of dioxygen in this way may continue for days. The carbenes are rapidly annihilated,152 and many of the radicals are annihilated over a few hours.153 However, some radicals can persist for long times due to extensive πdelocalization or their inaccessibility in the matrix.150 The concentration of these so-called persistent free radicals (PFR) I
DOI: 10.1021/acs.est.7b01088 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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Environmental Science & Technology H2O2 Decomposition and ROS Formation. It is well-known that AC can decompose H2O2 in water to O2 (eq 11).110,161 Decomposition is not strictly catalytic as the rate declines with repeated addition.153 Upon treatment of lignin char with a large excess of H2O2, the EPR signal intensity declines by about ∼20%140,143 but the O/C radical ratio is unchanged.153 Knowing the loss of H2O2, it was possible to calculate that about one spin was annihilated per 2 molecules H2O2 decomposed,153 suggesting a low degree of catalysis by the PFR (eq 12a and 12b) or that H2O2 decomposition takes place also at non-PFR matrix sites (Cm,nrad) (eq 13), or both. The decomposition of indigenous or added H2O2 generates ROS including HO2·/O2−· and hydroxyl (OH·), which are detected by spin-trapping (DMPO) or radical-trapping (salicylic acid) agents.154,162 Salicylate-trapped OH· formed by addition of H2O2 to ACs correlated with the initial PFR concentration suggesting that some PFR produce OH· by reduction of H2O2 (eq 12a). Transition metal constituents such as oxides of Fe or Cu may also be a source of ROS via Fentontype reactions (eqs 14−15). In studies of atmospheric fine particles, Dellinger and co-workers157,163 proposed that semiquinone radicals coordinated to metal oxide surfaces (e.g., CuO) reduce O2 to H2O2, which then generates OH· by Fenton-type back-reactions with the particles. The involvement of metals in ROS production is clearly an important issue that requires elucidation. However, metals clearly cannot be the sole source of ROS by PCM given the reactivity of chars from purified precursors.164,153 Reactions with Organic Compounds. Hydroxyl generated by the combination of PCM and H2O2 may attack organic compounds (eq 16); examples include p-nitrophenol,164 2chlorobiphenyl,162 diethyl phthalate,155 methyl t-butyl ether,161 trichloroethene,161 4-chlorophenol,165 and 1,3-dichloropropene.166 In one case,162 both the trapped OH· and the observed rate constant for organic compound loss correlated with initial PFR concentration. It was also shown that PCM can transfer an electron to peroxymonosulfate ion (HOOSO3−), forming sulfate radical, SO4−·, which is a useful oxidant.160 PCM can react directly with some compounds, as well. Sorption of chloropicrin to various ACs or activated charcoals resulted in its spontaneous reduction to about three equivalents of chloride.167 For the lignin char-mediated degradation of pnitrophenol, ROS from decomposition of indigenous H2O2 was found to play only a minor role, since the HO· scavenger tbutanol164 and the H2O2 scavenger catalase153 had little effect. It was concluded that the predominant loss pathway of pnitrophenol was its direct reaction with the char (eq 17).153 Moreover, reaction of a large excess of p-nitrophenol barely affected the EPR signal intensity and g value, meaning that only a small fraction (
