Discuss the physical, chemical, and physicochemical industrial and hazardous waste treatment technologies.

R E S E A R C H R E V I E W

Polycyclic aromatic hydrocarbons and volatile organic compounds in biochar and biochar-amended soil: a review T A N U S H R E E D U T T A 1 , E I L H A N N K W O N 2 , S A T Y A S U N D A R B H A T T A C H A R Y A 3 ,

B Y O N G H U N J E O N 4 , A K A S H D E E P 5 , 6 , M I N O R I U C H I M I Y A 7 and KI-HYUN KIM1

1Department of Civil and Environmental Engineering, Hanyang University, 222 Wangsimni-Ro, Seoul 04763, Korea, 2Department of Environment and Energy, Sejong University, Seoul 05006, Korea, 3Department of Environmental Science,

Tezpur University, Assam 784028, India, 4Department of Natural Resources and Environmental Engineering, Hanyang

University, Seoul 04763, Korea, 5Central Scientific Instruments Organisation (CSIR-CSIO), Sector 30 C, Chandigarh 160030,

India, 6Academy of Scientific and Innovative Research, CSIR-CSIO, Sector 30 C, Chandigarh 160030, India, 7USDA-ARS

Southern Regional Research Center, 1100 Robert E. Lee Blvd., New Orleans LA 70124, USA

Abstract

Residual pollutants including polycyclic aromatic hydrocarbons (PAHs), volatile organic compounds (VOCs),

and carbon (aceous) nanoparticles are inevitably generated during the pyrolysis of waste biomass and remain

on the solid coproduct called biochar. Such pollutants could have adverse effects on the plant growth as well as

microbial community in soil. Although biochar has been proposed as a ‘carbon negative strategy’ to mitigate the

greenhouse gas emissions, the impacts of its application with respect to long-term persistence and bioavailability

of hazardous components are not clear. Moreover, the co-occurrence of low molecular weight VOCs with PAHs in biochar may exert further phytotoxic effects. This review describes the basic need to unravel key mechanisms

driving the storage vs. emission of these organics and the dynamics between the sorbent (biochar) and soil

microbes. Moreover, there is an urgent need for standardized methods for quantitative analysis of PAHs and

VOCs in biochar under environmentally relevant conditions. This review is also extended to cover current

research gaps including the influence of biochar application on the short- and long-term fate of PAHs and VOCs

and the proper control tactics for biochar quality and associated risk.

Keywords: biochar, environmental pollutants, PAHs, soil, sorption capacity, VOCs

Received 26 January 2016 and accepted 22 March 2016

Introduction

Environmental pollutants such as polycyclic aromatic

hydrocarbons (PAHs) and volatile organic compounds

(VOCs) have a detrimental effect on soil quality and

plant growth. PAHs are carcinogenic and persistent

pollutants that are ubiquitously distributed in the envi-

ronment and are some of the most difficult organic

contaminants to treat (Edwards, 1983; Cerniglia, 1992).

The VOCs, especially ethylene, may trigger various

plant and microbial responses known as ‘soil volatilo-

mics’ by mimicking plant hormones and through

additional mechanisms (Insam & Seewald, 2010). The

ppm-level VOCs can regulate the seed germination

rates, herbivore resistance, weed response, and nutrient

uptake (Simms & Rausher, 1987; Ryu et al., 2003; Klinke

et al., 2004; Kloepper et al., 2004; Baldwin et al., 2006;

Zhang et al., 2007; Almeida et al., 2009; Graber et al.,

2010). The co-occurrence of low molecular weight

(LMW) VOCs with PAHs in biochar may further

amplify the phytotoxic effects relative to the PAHs

alone (Gell et al., 2011).

The primary source of PAHs, VOCs, and black carbon

pollutants in the environment is the incomplete com-

bustion of fossil fuels (Khalili et al., 1995; Simoneit,

2002; Lemieux et al., 2004). Unburned hydrocarbons

(UHCs that include PAHs and VOCs) are generated

during the combustion of solid-phase fossil fuels such

as coal; mass transport limitation between the oxidant

and the solid fossil fuels provides favorable conditions

for the volatilization and the gas-phase addition

reactions (Kwon & Castaldi, 2008). These reactions are

controlled by the particle size of the solid-phase fossil

Correspondence: Ki-Hyun Kim, tel. +82 2 2220 2325;

fax +82 2 2220 1945, e-mail: [email protected]; Eilhann Kwon,

tel. +82 2 3408 4166, fax +82 2 3408 4320, e-mail: ekwon74@sejong.

ac.kr

© 2016 The Authors. Global Change Biology Bioenergy Published by John Wiley & Sons Ltd. This is an open access article under the terms of the Creative Commons Attribution License,

which permits use, distribution and reproduction in any medium, provided the original work is properly cited.990

GCB Bioenergy (2017) 9, 990–1004, doi: 10.1111/gcbb.12363

fuels (Kwon & Castaldi, 2009). Compared to the solid-

phase fossil fuels, the generation of UHCs in lignocellu-

losic biomass is expedited because of the high volatile

matter contents (Kwon et al., 2013).

The pyrolysis of biomass (i.e., thermal degradation

under oxygen-limiting conditions) gives rise to three

different product fractions: pyrolytic oil, syngas, and

biochar. The solid residue (biochar) produced from

pyrolysis of biomass is a porous carbon-rich material

intended for environmental, agricultural, and industrial

applications (Masiello, 2004). The gaseous fraction is a

mixture of noncondensable gases (including H2 and

CO), and the liquid (bio-oil) fraction is a complex mix-

ture of organic compounds. In particular, PAHs and

VOCs formed by thermal cracking of the biomass

(Demirbas, 2000; Zeng et al., 2011) tend to concentrate

in the bio-oil fraction of the pyrolysis products (Yu

et al., 2007) as a result of dehydrogenation and gas-

phase addition reactions (Kwon et al., 2013). These com-

pounds become trapped inside the pores of biochar or

deposited onto biochar during the cooling process (Buss

et al., 2015).

Biochar can act as a carbon sink by remaining in the

soil for more than 100 years (Lehmann, 2007). Conse-

quently, biochar has attracted attention as a possible

strategy to withdraw CO2 from the atmosphere (Sohi

et al., 2010; Many�a, 2012). Furthermore, its application

to soil can also have the beneficial effects of improving

crop yield and reducing the loss of soil nutrients

through leaching (Jeffery et al., 2010; Singh et al., 2010).

However, VOCs released from biochar are known to

possess stimulatory or inhibitory effects on plant pro-

ductivity (Deenik et al., 2010) and microbial processes

(Graber et al., 2010; Khodadad et al., 2011). For example,

Spokas (2010) reported the production of ethylene at

varying rates from different biochars that might be

involved in plant and soil microbial responses to bio-

char additions and may also act as a nitrification inhibi-

tor (McCarty & Bremner, 1991). Furthermore, humans

can be exposed to biochar-associated PAHs either

directly through inhalation of particles or indirectly

through the ingestion of fruits/vegetables grown in bio-

char-amended soil. Such exposure can pose a significant

threat to human health due to the toxic, mutagenic, and

carcinogenic effects of PAHs (Chen & Liao, 2006). Addi-

tionally, both PAHs and VOCs can have bactericidal

properties that would adversely affect the structure of

soil microbial community (Ward et al., 1997). This could

also partly explain the lack of any substantial changes

in the soil microbial biomass carbon and activity after

biochar amendment (Van Zwieten et al., 2009; Yoo &

Kang, 2011).

The overall observed effects of biochar amendment in

soil suggest a mixed role of PAHs and VOCs in the

plant–soil system. Yet there is a paucity of quantitative data on the sorbed organics due to methodological chal-

lenges resulting from overlapping chromatographic

peaks and interference from other compounds with sim-

ilar elution times. The detection of volatile organics is

particularly challenging by the strong sorption capacity

of biochar, which often has a negative effect on the

extraction methods. The extraction efficiency of VOCs

from biochar is a function of the compound to be

detected and the biochar itself (Raguso & Pellmyr,

1998), leading to unequal extraction efficiencies for vari-

ous biochars with a range of sorbed organic com-

pounds. Moreover, even biochar produced from the

same feedstock under the same pyrolysis conditions

yields diverse levels of VOCs (Spokas et al., 2012).

The goal of this review was to summarize studies on

the chemical nature of the organic compounds sorbed

on biochar surfaces with a particular focus on the meth-

ods of detection of the organic compounds, the effects

of these compounds on plant growth, microbial biomass

and their abundance, microbial responses to biochar

application, and the ability of biochar to influence the

sorption characteristics of contaminated soil. This

review critically analyzes the findings of previous stud-

ies characterizing the PAHs and VOCs in biochar sam-

ples while delineating the research gaps requiring

further investigation. For example, although many stud-

ies have shown that VOCs remain sorbed on the surface

of biochar (Spokas et al., 2011), there is still a lack of

understanding regarding the fate of the sorbed organic

compounds after application of the biochar to soil under

environmentally relevant conditions. As such, the scope

of future research is wide open with respect to the use

of biochar and its associated effects.

Methods of detection of PAHs and VOCs from

biochar and biochar-treated soil

Despite above-described impacts of VOCs on soil biota,

only a limited number of studies have examined the

diversity of VOCs in biochar. Spokas et al. (2011) stud-

ied VOCs in biochar using gas chromatography–mass spectrometry (GC-MS) with headspace desorption at

150 °C for 10 min. They detected 140 compounds (with molecular weights below 100 Da) in over 70 biochars

produced from different feedstocks and pyrolytic condi-

tions. Clough et al. (2010) also confirmed the presence

of VOCs in biochar using solid-phase micro-extraction

(SPME) and GC-MS methods. More recently, Buss et al.

(2015) used a semiquantitative comparison against the

spectra library for the detection of VOCs in biochar.

Although headspace instrumentation has been reported

as an ideal tool for the determination of sorbed VOCs

(Bernardo et al., 2010; Spokas et al., 2011), current

© 2016 The Authors. Global Change Biology Bioenergy Published by John Wiley & Sons Ltd., 9, 990–1004

PAH AND VOC IN BIOCHAR AND BIOCHAR-AMENDED SYSTEMS 991

methodologies are not reliable enough for quantitative

estimation of VOCs in biochar. Ultrahigh-resolution

mass spectrometric techniques, such as Fourier trans-

form ion cyclotron mass spectrometry (FT-ICR MS),

have also been adapted for direct molecular characteri-

zation of biochar materials (Podgorski et al., 2012). Cole

et al. (2012) applied laser desorption ionization (LDI)

and negative electrospray ionization [(�) ESI] for char- acterization of VOCs in biochar. In the case of PAHs in

biochar, simple approaches for quantitative extraction,

separation, and detection have been presented by Hilber

et al. (2012) and Fabbri et al. (2013). However, both

groups stressed the necessity for a more standardized

protocol for quantitative analysis of PAHs in biochar,

which should be a prerequisite for biochar quality con-

trol and risk assessment.

Sample preparation, extraction, and clean-up

Biochar samples usually go through several processing

steps before the characterization of PAHs. Methods of

detection generally commence with drying of the sam-

ples at 40 °C overnight, followed by grinding and siev- ing at 0.25 mm (Hilber et al., 2012). The sieved samples

are generally stored in a dry place at room tempera-

ture. Prior to extraction, the sample is thoroughly

mixed and an aliquot is used to determine the dry

weight of the biochar. Extraction of PAHs is carried

out using either a Soxhlet extraction method or an

accelerated solvent extraction method. The method of

extraction and the choice of solvent are crucial parame-

ters for quantification of carbonized materials because

hydrophobic contaminants are tightly bound to the aro-

matic matrix (Jonker & Koelmans, 2002). Both toluene

and dichloromethane are common solvents used for

extraction of PAHs in biochar (Freddo et al., 2012; Hil-

ber et al., 2012), but the best extraction results have

been reported using the Soxhlet extraction method for

36 h with 100% toluene as a solvent (Hilber et al.,

2012). However, this finding was later contradicted by

Cole et al. (2012) and Fabbri et al. (2013), who reported

that toluene offered a decent level of efficiency only for

the extraction of carbonaceous materials (sp2-hybri-

dized 2D carbons materials), whereas in the case of

LMW PAHs (particularly naphthalene), a far better

extraction efficiency was obtained using acetone/cyclo-

hexane than with 100% toluene (Fabbri et al., 2013).

Cole et al. (2012) used a mixture of water and methanol

to extract hydrophilic polar compounds, particularly

those originating from pyrolyzed cellulose or hemicel-

lulose. After extraction, the extracts are concentrated

and cleaned by liquid–liquid partitioning over silica gel, for example using dimethylformamide/Milli-Q

water (9 : 1, v/v).

For the analysis of VOCs, solvent extraction is less

common with the exception of the study by Bernardo

et al. (2010) in which extraction with dichloromethane

was used for the removal of organics with high-to-

medium volatility. In most cases, VOCs are desorbed

thermally from the biochar (Spokas et al., 2011) or

extracted by microextraction methods (solvent-free

methodology) (Clough et al., 2010). However, optimiza-

tion of the solid-phase microextraction (SPME) method

for analysis of VOCs in a range of matrices by Higashi-

kawa et al. (2013) showed good performance for soils

but poor efficiency for biochar. The strong sorption

capacity of biochar markedly reduced the efficiency of

SPME. As mentioned above, the extraction efficiency of

VOCs from biochar is known to be a function of the

target compound and the biochar (Raguso & Pellmyr,

1998) and even biochar produced using the same feed-

stock and pyrolysis procedures possesses diverse level

of VOCs (Spokas et al., 2012). Therefore, further refine-

ment of the analytical techniques is essential before

accurate quantification can be achieved by headspace

technology. In conclusion, purpose-driven VOCs/PAHs

detection methods are on demand. For regulatory pur-

pose (for the biochar to meet sub-ppm requirement),

rigorous extraction methods are necessary to determine

the total PAHs/VOCs content of biochar. Different ana-

lytical methods should then be applied to assess the

bioavailability and environmental behavior of persis-

tent contaminants under environmentally relevant con-

ditions.

Method validation using soil and biochar samples

Validation is an important step to ensure the reliability

of the chosen method for analysis of biochar and soil

samples. In fact, to this end, many studies have used a

certified soil material with known concentrations of

PAHs together with a reference biochar sample. For

example, Fabbri et al. (2013) tested the accuracy of a

method developed for quantification of PAHs in biochar

on a soil matrix. A natural soil containing 15 PAHs with

concentrations ranging from 1.14 to 12.9 mg kg�1 was used together with an internal reference biochar sample

(industrial-scale slow pyrolysis orchard prunings bio-

char). The reference biochar was homogenized and

mixed with an agricultural soil (dried and sieved

2 mm) at a 1.16% (w/w) amendment level. This concen-

tration corresponded to an application of 36 t biochar

ha�1 (assuming a soil with 1.2 g cm�3 density and 0.3 m depth), which is comparable to the range cur-

rently used for agricultural application of biochar (20– 60 t ha�1) (Baronti et al., 2010). For headspace methods, commonly employed for the identification of VOCs, vial

temperature and equilibration time are the most crucial

© 2016 The Authors. Global Change Biology Bioenergy Published by John Wiley & Sons Ltd., 9, 990–1004

992 T. DUTTA et al.

parameters for method development (Friant & Suffet,

1979; Penton, 1992). Spokas et al. (2011) conducted pre-

liminary experiments using several biochar samples for

optimization of these factors. Based on the results of the

preliminary experiments, the biochars were thermally

heated to 150 °C for 10 min in a sealed headspace vial prior to injection.

Separation of PAHs and VOCs and detection by GC-MS

Fabbri et al. (2013) and Rombola et al. (2015) employed

GC-MS analyses using a 6850 Agilent HP gas chro-

matograph connected to a 5975 Agilent HP quadrupole

mass spectrometer. Separation of PAHs was achieved

by on-column injection of ~1 lL of the extract in a fused silica capillary column (stationary-phase poly[5% diphe-

nyl/95% dimethyl]siloxane, 30 m 9 0.25 mm i.d.,

0.25 mm film thickness), using helium as a carrier gas

at a constant flow of 1 mL min�1 (Hilber et al., 2012). Solvent mixtures containing different amounts of ana-

lytes and constant amounts of internal standards were

used for calibration.

Headspace analysis of thermally desorbed com-

pounds is the most commonly applied method for

detection of VOCs in biochar samples (Bernardo et al.,

2010; Clough et al., 2010). Spokas et al. (2012) presented

a modified method wherein the headspace sampler was

adapted to direct the flow of the effluent from one of

the columns to a mass spectrometer (MS) detector and

the effluent from the other column was connected

directly to a thermal conductivity detector (TCD), which

was in series with a flame ionization detector (FID),

thus enabling the determination of LM VOCs that

would otherwise be lost in the air/water peak of MS.

However, the study remained qualitative due to the co-

occurrence of a large number of peaks leading to over-

lapping peaks and co-eluting interferences. On the other

hand, Buss et al. (2015) identified a series of organic

compounds in the toluene extract of biochar by employ-

ing a semiquantitative scan method using Varian 2011

ion trap MS in full-scan mode.

Factors controlling the formation and retention of

PAHs and VOCs in soil: sorption vs. degradation

Soils and sediments are the ultimate sink for PAHs and

VOCs. In particular, at low contaminant concentrations

(<10–6 solubility), carbonaceous materials, including char, charcoal, and coal, are the primary geo-sorbents (Macloed

& Semple, 2002). Microbial degradation of organic com-

pounds in soil is strongly influenced by many factors such

as pH, soil type, soil aeration, soil nutrient status, water

availability, bioavailability of PAHs, and the presence of a

microbial community capable of degrading PAHs

(Cerniglia, 1992; Kastner et al., 1998; Breedveld & Spar-

revik, 2000; Volkering & Breure, 2003; Zhang et al., 2006;

Ding et al., 2010). Furthermore, apart from the pyrolytic

gases, biochar has the ability to absorb volatile com-

pounds present in the environment that could influence

the sorption behavior of diverse organic contaminants

with structurally similar functional groups (Cheng et al.,

2008). Sorption–desorption and degradation are the two mechanisms that dictate the final concentrations of toxic

organic compounds in soil.

Role of biochar in sorption and retention of PAHs

and VOCs in soil

Effect of feedstock, type of pyrolysis, and pyrolysis temperature

Among various pyrolytic conditions, the pyrolytic tem-

perature plays a critical role in determining the quantity

and type of compounds released from biochar (Cole

et al., 2012; Keiluweit et al., 2012). Biochars produced by

fast pyrolysis and steam gasification contain greater

quantities of PAH than those produced by slow pyroly-

sis (Cole et al., 2012). This could be because slow pyrol-

ysis provides more chance of PAH loss in gaseous

forms to the atmosphere, whereas in fast pyrolysis the

PAHs tend to become sorbed on biochar surfaces (Hale

et al., 2012). The dominant fractions of PAHs in slow

pyrolysis biochars are produced between 350 and 550°C (Hale et al., 2012). The temperature (pyrolysis) depen-

dence of solvent-extractable PAH content in biochar

was experimentally evidenced by Keiluweit et al. (2012),

who reported that the amount of PAHs in biochars

produced between 400 and 600 °C greatly exceeds the quantities in biochars produced from the same feed-

stocks at higher or lower temperatures. Quilliam et al.

(2012) quantified the concentration of 16 priority PAHs

in biochar-amended soil (treated for 3 years with two

different concentrations of biochar made from wood or

rice husk) and found that the quantity of PAH is

strongly influenced by the chemical composition of the

feedstock (Table 1) and the pyrolytic conditions (Figs 1

and 2).

Schimmelpfennig & Glaser (2012) suggested that bio-

chars generated under different pyrolytic conditions

should show distinctive diversity of PAH composition.

In particular, the combination of naphthalene/phenan-

threne ratio and total PAH content can be used to assess

the production procedure used. Among the various

methods employed for producing biochars, those pro-

duced using a hydrothermal method (i.e., steam gasifi-

cation) or a rotary kiln (pyrolysis) procedure were

found to contain lower amounts of PAHs than the

threshold numbers set for environmental application

© 2016 The Authors. Global Change Biology Bioenergy Published by John Wiley & Sons Ltd., 9, 990–1004

PAH AND VOC IN BIOCHAR AND BIOCHAR-AMENDED SYSTEMS 993

(Schimmelpfennig & Glaser, 2012). Naphthalene was

reported to be the most abundant PAH in biochar

(Freddo et al., 2012; Hale et al., 2012; Hilber et al., 2012;

Fabbri et al., 2013; Rombola et al., 2015), followed by

phenanthrene and fluorene (Rombola et al., 2015). Typi-

cally, solvent-extractable PAHs occur within a range of

0.7–1.7 mg kg�1 in biochar from different origins (Hale et al., 2012; Hilber et al., 2012; Rombola et al., 2015). In

addition, the duration of pyrolysis and surface area of

the biochar have additional impacts on the amount of

PAHs in biochar. How all these factors are related to

the extent of PAH bioaccumulation as a function of time

has not been meaningfully evaluated.

For VOCs, biochars produced by hydrothermal car-

bonization (HTC) or fast pyrolysis were found to con-

tain the largest number of sorbed volatiles (Spokas

et al., 2011). In contrast, gasification, thermal or chemi-

cally processed biochars, that is, chemically/steam-acti-

vated carbons, soil kiln mound, and open pit biochars

had low-to-nondetectable levels of VOCs. The list of

identified compounds and their retention times are pro-

vided in Table 2.

The slow pyrolysis biochars generally exhibited

variable contents of sorbed VOCs. There were no

clear dependencies of the composition of sorbed

VOCs on the feedstock, except for a study by Rom-

bola et al. (2015) that reported significantly higher con-

centrations of volatile fatty acids (VFAs) in biochars

derived from poultry litter (4–9 mg g�1) than in those produced from corn stock (1–4 mg g�1). A wide array of VOCs derived from the thermal degradation of

polysaccharides, proteins, and lipids (e.g., VFAs) were

detected in poultry litter biochar. Biochars produced

at low pyrolysis temperatures (<350 °C) generally con- sisted of short carbon chain aldehydes, furans, and

ketones. In contrast, sorbed aromatic compounds and

long carbon chain hydrocarbons were dominant in

biochars produced at elevated temperatures (>350 °C) (Fig. 1). Moreover, the presence of oxygen during

pyrolysis also led to a reduction in sorbed VOCs. As

stated earlier, even biochars created under similar

pyrolysis conditions and from equivalent feedstock

may exhibit chemically diverse types of VOCs (Spokas

et al., 2011).

Table 1 Effect of feedstock on sorbed polycyclic aromatic hydrocarbons (PAHs) of biochar

Order PAH (lg g�1) BC1 BC2 BC3 BC4 BC5 BC6 BC7 BC8 BC9 BC10

1 Naphthalene 1.57 2.39 0.44 0.47 0.93 2.58 0.78 0.49 27.1 6.68

2 Acenaphtylene 0.50 0.04 0.01 0.02 0.12 0.71 0.10 0.05 5.27 0.77

3 Acenaphthene 0.62 0.05 0.01 0.07 0.08 0.28 0.24 0.22 2.14 0.21

4 Fluorene 0.25 0.10 0.05 0.08 0.04 0.92 0.59 0.26 6.42 0.24

5 Phenenthrene 0.25 0.56 0.31 0.27 0.36 3.88 0.49 0.33 9.92 0.79

6 Anthracene 0.03 0.07 0.03 0.03 0.04 0.65 0.19 0.12 3.24 0.14

7 Fluoranthene 0.14 0.11 0.08 0.11 0.05 2.46 0.10 0.09 3.15 0.27

8 Pyrene 0.07 0.08 0.08 0.12 0.04 2.58 0.16 0.07 3.72 0.31

9 Cyclopenta[c,d]pyrene# 0.01 0.01 0.01 0.01 0.01 0.05 0.04 n.d. n.d n.d.

10 Chrysene# 0.05 0.02 0.02 0.03 0.02 0.92 0.42 0.17 0.97 0.03

11 Benz[a]anthracene# 0.04 0.05 0.04 0.04 0.02 0.83 0.46 0.08 1.00 0.04

12 5-methylchrysene# 0.11 0.02 0.02 0.02 0.09 0.27 0.21 n.d. n.d. n.d.

13 Benzo[b]fluoranthene# 0.02 0.04 0.04 0.05 0.02 0.70 0.29 0.05 0.62 0.04

14 Benzo[k]fluoranthene# 0.02 0.04 0.02 0.02 0.01 0.43 0.39 0.07 0.12 BD

15 Benzo[j]fluoranthene# n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.

16 Benzo[a]pyrene# 0.02 0.10 0.01 0.05 0.02 0.67 0.32 0.06 0.56 0.02

17 Indenol[1,2,3-cd]pyrene# 0.01 0.13 n.d. 0.02 0.01 0.50 0.27 n.d. 0.16 BD

18 Dibenzo[a,h]anthracene# 0.02 0.01 0.01 0.01 0.01 0.08 0.21 0.19 0.06 BD

19 Benzo[ghi]perylene# 0.01 0.01 0.01 0.02 0.01 0.53 n.d. n.d. 0.19 BD

20 Dibenzo[a,e]pyrene# n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.

21 Dibenzo[a,h]pyrene# n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.

22 Dibenzo[a,i]pyrene# n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.

23 Dibenzo[a,l]pyrene# n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.

16 EPA PAHs 3.6 3.8 1.2 1.4 1.8 19 5.0 2.2 64 9.5

# 15 EU PAHs 0.32 0.43 0.18 0.27 0.22 5.0 2.6 0.62

Values represent the mean of four replicates � SE. BD, below detection limits. BC1, hardwood sawdust; BC2, wood waste; BC3, wood waste; BC4, hardwood; BC5, wood chips/manure; BC6, macadamia nut shells;

BC7, distillers grain; BC8, distillers grain (Fabbri et al., 2013); BC9, rice husk; BC10, wood (Quilliam et al., 2012).

© 2016 The Authors. Global Change Biology Bioenergy Published by John Wiley & Sons Ltd., 9, 990–1004

994 T. DUTTA et al.

Fate of biochar-associated PAHs and VOCs

The environmental fate of biochar-associated PAHs that

are added to soil is still poorly understood. Soil type

can have a significant effect on PAH degradation, for

example soils with enhanced sorptive capacity (due to

high levels of SOM) may show a reduced mass transfer

rate of PAHs in soil solution (Macloed & Semple, 2002).

However, in contaminated soils with high pollutant

levels, sorbent saturation would result in more freely

available PAHs, which could lead to increased levels of

degradation (Cornelissen et al., 2005; Rhodes et al.,

2010). Additionally, as biochar is capable of stimulating

soil microbial activity, its addition to soil could strongly

influence the degradation of naturally occurring PAH in

soil.

Alternatively, as biochar can effectively absorb organic

contaminants, it could decrease microbial mineralization

by limiting the bioavailability of the contaminants

(Rhodes et al., 2008, 2010; Xia et al., 2010; Quilliam et al.,

2012). The sorbing abilities of biochar might prove to be

beneficial from the perspective of groundwater quality,

as sorption of PAHs onto biochar surfaces would pre-

sumably limit the rate of PAH leaching from soils (Hale

et al., 2012). However, the latter assumption was contra-

dicted by Quilliam et al. (2012) in a study determining the

effect of biochar addition on mineralization and leaching

of PAH after rainfall. Their data showed that PAH con-

centrations measured 3 years after the addition of bio-

char were significantly higher than those of unamended

soils. These results suggested that biochar might act as a

source of PAH in soil. In a stark contrast to their original

hypothesis, biochar was found to increase the leaching of

PAH through soils. This was ascribed to the release of

significant amounts of dissolved organic carbon (DOC)

following the application of fresh biochar to soil. Dis-

solved organic carbon, together with dust particles from

biochar, binds PAHs and facilitates their leaching

through soil. Nonetheless, the levels of PAHs in soils

were well within the maximum acceptable limits of 5– 50 lg g�1, as reported previously by Carlon (2007). It was, however, difficult to assess whether the PAHs found

in biochar-treated soil were indigenous to the soil or

sorbed on the biochar surface.

Despite many complexities, sorption is a key charac-

teristic factor determining the fate of organic com-

pounds associated with biochar (Quilliam et al., 2012).

Fig. 1 Trends in relative peak areas of toluene,

ethylene + acetylene, ethanol, 2-pentylfuran, and benzene in

biochar as a function of pyrolysis temperature [data source:

Spokas et al. (2011)].

Fig. 2 Extractable polycyclic aromatic compounds (PAHs, mg kg�1), volatile matter by Py-GC-MS (toluene/naphthalene ratio), and volatile fatty acids (VFAs, mg g�1) of biochar from corn stalk and poultry litter as a function of pyrolysis temperature [numbers rep- resent mean values from two replicates; data source: Rombola et al. (2015)].

© 2016 The Authors. Global Change Biology Bioenergy Published by John Wiley & Sons Ltd., 9, 990–1004

PAH AND VOC IN BIOCHAR AND BIOCHAR-AMENDED SYSTEMS 995

The sorption capacities of soils themselves are found to

be altered after the application of biochar (Table 3). This

can help reduce short-term risks as desorption of PAHs

can be suppressed. However, one line of evidence indi-

cates that this may not always be remedially beneficial;

rather, the benefit depends on the sorption strength and

factors inherent to the contaminants, char, and soil that

can interactively influence the retention of PAHs over

time. Future studies should thus be directed to the

assessment of various factors controlling the extent of

PAH sorption on biochar surface, the rate of its satura-

tion, the role of microbes in accumulation of PAH in the

soil solution (increasing bioavailability), and their per-

sistence in the soil environment (due to decreased

bioavailability). All these factors should be evaluated

prior to large-scale application of biochar in agricultural

and contaminated soils.

Effects of biochar-released PAHs and VOCs on soil microbial processes

Apart from the human health and environmental impli-

cations, the release of PAHs and VOCs into the soil via

biochar application has the potential to affect microbial

Table 2 List of identified volatile organic compounds (VOCs) in biochar and their retention times (RT) by the headspace thermal

desorption–GC/MS method (Spokas et al., 2011)

Order Organic compound RT # Organic compound RT

1 Ethanol 8.3 40 Toluene 21.1

2 Furan 8.8 41 1,3 Dichloropropene (e) 21.9

3 2-Propenal 9.2 42 1,1,2 Trichloroethane 22.4

4 1,1,2 Trichloro 1,2,2 trifluoro ethane 9.3 43 Tetrachloroethene 22.5

5 1,1 Dichloroethane 9.4 44 Hexanal 23.3

6 Propanal 9.4 45 Dibromochloromethane 23.4

7 Acetone 9.7 46 Hexene 23.5

8 Carbon disulfide 10 47 1,2 Dibromoethane 23.8

9 Pentane 2-methyl 10.7 48 Cyclopentanone 23.8

10 Methyl acetate 10.8 49 Chlorobenzene 25

11 Dichloromethane 10.9 50 Ethylbenzene 25.2

12 2 Methoxy 2 methyl propane 11.4 51 1,2 Dimethylbenzene 26

13 1,2 Dichloroethane (z) 11.5 52 1,4 Dimethyl benzene 26.1

14 Hexane 12 53 1,3 Dimethyl benzene 26.1

15 Methyl propanal 12.1 54 2-Hexanone 5-methyl 26.1

16 Ethyl acetate 12.5 55 Furfural 26.3

17 1,1 Dichloroethane 12.7 56 2-Hexanal 26.5

18 Trimethyl ester boric acid 13.2 57 Styrene 26.8

19 2 Methyl furan 13.2 58 2-Heptanone 27.4

20 Butanol 13.8 59 Tribromomethane 27.4

21 2,3 Butanedione 13.9 60 1 Methylethyl benzene 27.7

22 Trans 1,2 dichloroethane 14.2 61 Heptanal 27.7

23 Methyl ethyl ketone 14.2 62 Hexamethyl trisiloxane 28.3

24 Trichloromethane 14.5 63 1,1,2,2-Tetrachloroethane 28.8

25 2-Butanol 14.8 64 2-methyl-2-cyclopentene-1-one 28.9

26 Cyclohexane 15.3 65 Benzaldehyde 31.2

27 1,1,1 Trichloroethane 15.3 66 1,3-dichlorobenzene 31.5

28 Carbon tetrachloride 15.6 67 Octanol 31.6

29 Benzene 16.3 68 1,4-Dichlorobenzene 31.7

30 1,2 Dichloroethane 16.5 69 1,2,3-Trimethylbenzene 32.8

31 3-Methyl-butanol 17 70 1,2-Dichlorobenzene 32.9

32 2-Pentanone 17.1 71 1,2-Dibromo 3-chloropropane 35.3

33 Trichloroethene 17.9 72 1-Dodecane 36.1

34 Methylcyclohexane 18.2 73 1,2,4-Trichlorobenzene 37.7

35 1,2 Dichloropropane 18.6 74 Hexachlorobutadiene 38.2

36 3 Pentanone 18.8 75 Naphthalene 39

37 Pentanal 19 76 1,2,3 Trichlorobenzene 39.6

38 Bromodichloromethane 19.2

39 1,3 Dichloropropene (z) 20.4

© 2016 The Authors. Global Change Biology Bioenergy Published by John Wiley & Sons Ltd., 9, 990–1004

996 T. DUTTA et al.

processes in soil. For instance, the cycling of nitrogen (N)

in soil can be altered as a result of modification of soil

nitrification (Sverdrup et al., 2002; Maliszewska-Kordy-

bach et al., 2007) and changes in the abundance and

structure of denitrifying communities (Guo et al., 2013).

A number of studies reported an initial noxious effect on

plants (Gell et al., 2011) or microbial communities when

certain biochars were applied to soil (Dempster et al.,

2012). Moreover, the transient release of inhibitory or

toxic compounds from biochar has been postulated as a

mechanism contributing to the reduction in greenhouse

gas (GHG) emissions (e.g., nitrous oxide) from soil. In

this regard, Spokas and Reicosky (2009) were the first to

report that the release of organic compounds from bio-

char inhibits soil microbial processes (generation of

GHG). They detected the production of ethylene in soils

amended with biochar that appeared to suppress nitrifi-

cation and methanotrophic activities.

As the VOCs have the potential to inhibit or stimulate

microbial and plant processes, they influence the plant

and microbial response to biochar amendments. Because

of great variabilities in biochar-associated VOCs, their

chemical dissimilarity could play an important role in

the observed plant and soil microbial responses to bio-

char amendment of soil (Spokas et al., 2011). As reported

by Sun et al. (2015), the structure of the soil microbial

community structure was greatly altered by application

of fresh biochar. However, the authors suggested that

such effects induced by fresh biochar might be short-

lived, although this was not shown experimentally. It is

evident that further research is required to fully under-

stand the mechanisms underlying the effects of biochar

on the structure of soil microbial communities.

Effect of presence of soil and aging on the sorption capacity of biochar

Although biochar application has been found to

improve the sorption behavior of contaminated soils

(Table 4), the presence of soil has a negative impact on

the sorption capacity of the biochar; for example, a

decrease in sorption capacity for hydroquinone and

diuron of 28% and 60%, respectively, was observed

(Yang & Sheng, 2003; Cheng et al., 2008). Likewise, a

reduction in phenanthrene logarithmic Freundlich parti-

tioning coefficient (from 4.3 to 2.2) was also observed

when biochar produced from pine needles at 400 °C was mixed with soil at a concentration of 0.1% w/w

(Chen & Yuan, 2011). In agricultural soils, application of

biochar was found to result in significantly higher levels

of PAHs (Fabbri et al., 2013) (Table 3). In particular, the

concentration of naphthalene increased from

0.0098 lg g�1 in untreated soil to 0.0263 lg g�1 in bio- char-amended soil; this large difference in naphthalene

content postbiochar amendment was attributed to the

initial high concentration of naphthalene in the biochar

(1.75 lg g�1). However, given the low values of total PAHs reported in the literature for the slow pyrolysis

biochars (Freddo et al., 2012; Hale et al., 2012) and the

recommended rates of biochar application for agricul-

ture practices, the elevated levels of PAHs in biochar-

amended soil (especially for slow pyrolysis biochars)

are not of universal concern. Nonetheless, some bio-

chars do have levels of sorbed PAHs (Keiluweit et al.,

2012; Kloss et al., 2012; Fabbri et al., 2013) that exceed

existing guidelines for land application of commercial

biochar (Hilber et al., 2012). Therefore, the development

Table 3 Polycyclic aromatic hydrocarbon (PAH) concentrations of an agricultural soil and corresponding biochar-amended soil

(1.16% w/w) [adopted from Fabbri et al. (2013)]

Order PAH (lg g�1) Soil Soil + biochar

1 Naphthalene 0.0098 � 0.0002 0.0263 � 0.0046 2 Acenaphthylene n.d. n.d.

3 Acenaphthene n.d. n.d.

4 Fluorene 0.0023 � 0.0008 0.0033 � 0.0006 5 Phenanthrene 0.0118 � 0.0036 0.0212 � 0.0063 6 Anthracene 0.0003 � 0.0002 0.0014 � 0.0014 7 Fluoranthene 0.0035 � 0.0010 0.0075 � 0.0030 8 Pyrene 0.0031 � 0.0007 0.0069 � 0.0020 10 Chrysene 0.0007 � 0.0003 0.0014 � 0.0010 11 Benzo[a]anthracene 0.0039 � 0.0007 0.0057 � 0.0009 13 Benzo[b]fluoranthene 0.0067 � 0.0014 0.0091 � 0.0029 14 Benzo[k]fluoranthene 0.0005 � 0.0001 0.0014 � 0.0003 16 Benzo[a]pyrene 0.0001 � 0.0002 0.0019 � 0.0009 17 Indeno[1,2,3-cd]pyrene 0.0023 � 0.0008 0.0040 � 0.0022 18 Dibenzo[a,h]anthracene# 0.0009 � 0.0002 0.0014 � 0.0004 19 Benzo[ghi]perylene 0.0046 � 0.0011 0.0070 � 0.0013

Values in the table indicate the mean values � SD of four replicates.

© 2016 The Authors. Global Change Biology Bioenergy Published by John Wiley & Sons Ltd., 9, 990–1004

PAH AND VOC IN BIOCHAR AND BIOCHAR-AMENDED SYSTEMS 997

of analytical procedures for the quantitative determina-

tion of PAHs in biochar and biochar-amended soils is

critical.

With respect to aging, direct evidence for a change in

biochar sorption capacity is scarce. Several reports indi-

cate changes in the physicochemical characteristics of

biochar due to aging (Nguyen et al., 2008; Cheng & Leh-

mann, 2009). For instance, aging at 110 °C increased the pH of biochar by three units and the cation exchange

capacity by 50% (Hale et al., 2011). However, despite

changes in physicochemical characteristics, the sorption

capacity of biochars (especially those produced at high

temperature) was not greatly affected by the presence of

soil or by aging of the biochar (Hale et al., 2011)

(Table 5).

These results need to be tested using biochars pro-

duced from a wide range of feedstock materials and

with diverse manufacturing processes. Also, further

investigation is needed to test whether these conclu-

sions could be extended to biochars produced at low

temperature and for soils of different origins with

diverse characteristics.

Biochar and soil microbes

Effects of biochar on microbial biomass and activity

Both surface area and pore size distribution vary greatly

depending on feedstock properties and pyrolytic condi-

tions (Downie et al., 2009). Interestingly, when biochar

comes in contact with soil, its surface area and pore

volume is significantly altered as a result of clogging of

the pores by sorbed organic and mineral substances

(Pignatello et al., 2006; Joseph et al., 2010). In addition,

mineralization of VOCs may influence the porosity of

biochar (Lehmann et al., 2011).

Soil microorganisms live in various types of micro-

habitats, which supply the resources for their specific

metabolic activities (Fig. 3). For example, oxidized sur-

faces of soil aggregates are favorable habitats for aerobic

microbes, whereas semi-aquatic species and denitrifiers

live inside the moist core of soil peds (Lehmann et al.,

2011). This creates a highly differential redox condition,

mainly because of sorption of organic matter on the bio-

char particles and oxidation of the biochar carbon

(Liang et al., 2006; Lehmann 2007). This in turn greatly

influences the decomposition of organic matter, metal

mobility, and microbial activity in the soil (Fig. 3).

Moreover, the internal porosity of biochar may allow

soil microbes to avoid consumption by grazers and

might conserve mineral nutrients along with the car-

bonaceous substrates (Pietik€ainen et al., 2000; Saito &

Marumoto, 2002; Warnock et al., 2007).

According to Gomez et al. (2014), chemically recalci-

trant biochar serves as a substrate for microbial activity,

as confirmed by isotopic analyses of phospholipid fatty

acids (PLFAs), which clearly depicted the incorporation

of biochar carbon into the bacterial biomass. However,

during a 12-month incubation study, a decrease in

microbial biomass was observed following biochar

addition, which was consistent with the earlier findings

of Dempster et al. (2012). This decrease was attributed

Table 4 Summary of selected studies showing the effects of biochar on the sorption capacities of soils

Order Amendment Contaminant Effect Reference

1 Sawdust-derived biochar (700 °C) Terbuthylazine Sorption increased in amended

soils by a factor of 63

Wang et al. (2010)

2 Sawdust-derived biochar Atrazine and acetochlor Kcd increased by a factor of 1.5

for acetochlor. Sorption of atrazine

also increased (not quantified)

Spokas et al. (2009)

3 Hardwood-derived biochar (450 °C)

mixed at 30% v:v

Polycyclic aromatic

hydrocarbons (PAHs)

>40% reduction in PAH Beesley et al. (2010)

4 Pinus radiata-derived biochar (350 °C)

0.1 a,d 0.5% application rate

Phenanthrene Kcd increased by a factor of 2–51 Zhang et al. (2010)

5 Pinus radiata-derived biochar (700 °C)

0.1 a,d 0.5% application rate

Phenanthrene Kcd increased by a factor of 6–700 Zhang et al. (2010)

6 Wheat-derived biochar (0.05%,

0.5% and 1%)

Diuron Sorption increased by 7–80%,

1% amendment

Yang et al. (2006)

7 Eucalyptus spp derived biochar (450 °C)

0.1%, 0.5%, 1.0%, 2.0%, and

5.0% application rates

Diuron Sorption capacity of the amended

soil increased by 7–80%

Yu et al. (2006)

8 Eucalyptus spp derived biochar (450 °C)

0.1%, 0.2%, 0.5%, 0.8%, and

1.0% application rates

Diuron Sorption capacity of the amended

soil increased by 5–125%

Yu et al. (2006)

Kcd is sorption coefficient.

© 2016 The Authors. Global Change Biology Bioenergy Published by John Wiley & Sons Ltd., 9, 990–1004

998 T. DUTTA et al.

to reduced availability of substrate although another

possible reason for the decrease might be suppression

of microbial growth due to the release of growth-inhi-

biting chemical compounds such as PAHs and VOCs.

However, the decrease was found to be partially buf-

fered by large biochar addition rates suggesting that

biochar, when abundant, may become a substrate for

microbial activity.

Yoo & Kang (2011) reported an increase in microbial

biomass-N following application of biochar (produced

from swine manure) to soil, which was indicative of

microbial nitrogen immobilization. However, a lack of

any substantial change in the microbial biomass carbon

from biochar additions to the soil was reported (Van

Zwieten et al., 2009; Yoo & Kang, 2011). Only recently, a

significant increase in soil microbial biomass after bio-

char application as measured by total PLFA abundance

was reported over a 30-month incubation study (Jiang

et al., 2016). Nonetheless, microbial biomass did not

show a linear relationship with biochar addition rates.

Influence of biochar amendment on microbial abundance in soil

Microbial abundance in biochar-amended soil has been

investigated by various methods including total geno-

mic DNA (Grossman et al., 2010; Jin, 2010), culture and

plate counting method (Jackson, 1958; O’Neill et al.,

2009), substrate-induced respiration (Zackrisson et al.,

1996; Steiner et al., 2004, 2009; Wardle et al., 2008; Kolb

et al., 2009), fumigation–extraction (Jin, 2010; Liang et al., 2010), PLFA (Gomez et al., 2014; Jiang et al., 2016),

staining, and direct observation of individual biochar

particles (Pietik€ainen et al., 2000; Warnock et al., 2007;

Jin, 2010). Using a PLFA study, Gomez et al. (2014)

demonstrated that the addition of a wood-derived fast

pyrolysis biochar could positively influence the

microbial abundance of temperate soils. Nearly

12 months after biochar application, the microbial com-

munity composition was found to have shifted toward

a Gram-negative bacteria-dominated community (rela-

tive to fungi and Gram-positive bacteria). Another incu-

bation study by Jiang et al. (2016) confirmed these

results by showing an increase in the grouped abun-

dance of signature PLFAs of Gram-negative bacteria

and actinobacteria after application of biochar. Relative

to the bacteria, fungi were negatively affected by

increasing the dose of biochar. This could have impor-

tant implications for SOM decomposition, methane

emissions, and cycling of nitrogen and sulfur. In con-

trast, a few other studies have reported positive effects

of biochar additions on the abundance of mycorrhizal

plant and fungi (e.g., Warnock et al., 2007; Steinbeiss

et al., 2009; Yoo & Kang, 2011). Based on quantitative

real-time polymerase chain reaction (PCR), such effects

were also estimated with respect to the bacterial, fungal,

and archaeal biomass; the results demonstrated no sig-

nificant treatment effects on bacterial and archaeal gene

copy number, whereas a significant effect on fungal

gene copy number was observed (Yoo & Kang, 2011).

Microbial abundance is largely governed by nutrient

and carbon availability, pH, and bacterial adhesion to

biochar surfaces. The addition of fertilizer to soil

reduces the enhancing effect of biochars on microbial

reproduction (Steiner et al., 2009). Blackwell et al. (2010)

also observed a noticeable increase in arbuscular mycor-

rhizal colonization in the root zone of a wheat crop cul-

tivated in biochar-treated soil with no or low fertilizer

application. However, nodule formation by rhizobia is

promoted by addition of P-containing fertilizer, but not

by N-containing fertilizer (Ogawa & Okimori, 2010); in

Table 5 Carbon-normalized Freundlich coefficient [Log KFr (ng kg�1) (ng L�1)�nF] of unaged, biologically aged, chemically aged (at 60 and 110 °C), and physically aged biochar with and

without soil [data source: Hale et al. (2011)]

Aging regime

Log KFr (ng kg �1) (ng L�1)�nF

Biochar Biochar + soil Soil

Unaged 6.17 � 0.18 7.14 � 0.11 3.53 � 0.12 Biologically

aged

5.54 � 0.35 6.78 � 0.35

Chemically

aged (at 60 °C)

5.22 � 0.38 5.39 � 0.09

Chemically

aged (at 110 °C)

5.95 � 0.12 5.89 � 0.02

Physically aged 6.21 � 0.20 5.77 � 0.33

O bs

ti na

te

fr ac

ti on

Biochar M

ineralized nutrient elem

ents

Soil microorganisms

Pore size distribution

Surface area

Energy

Labile organic matter

Sorption behavior

Fig. 3 An overview of interactions between soil processes,

biochar, and microbial diversity [adapted from Lehmann et al.

(2011)].

© 2016 The Authors. Global Change Biology Bioenergy Published by John Wiley & Sons Ltd., 9, 990–1004

PAH AND VOC IN BIOCHAR AND BIOCHAR-AMENDED SYSTEMS 999

fact, P fertilization strengthens the symbiotic relation-

ship between leguminous plants and microorganisms.

In contrast, the abundance of nonsymbiotic microbes in

soil is augmented by higher nutrient availability (Tay-

lor, 1951). This may be due to biochar-mediated reten-

tion and slow release of nutrients in soil (Lehmann

et al., 2011).

Generally, an increase in pH value within a gradient

from 3.7 to 8.3 enhances the microbial biomass in soil

(Aciego Pietri & Brookes, 2008). However, whereas an

increased pH up to 7 may increase bacterial abundance,

fungal growth in soil is sometimes inhibited at high pH

levels (>7) (Rousk et al., 2009). Interestingly, the pH of soils may fluctuate significantly depending on the pH

and liming value of biochars (Lehmann et al., 2011). Bio-

chars possess a wide range of pH values (<4 to >12) (Lehmann, 2007) and thus create highly different chemi-

cal ambience for the microorganisms. Moreover, adhe-

sion of bacterial populations to biochar surfaces also

protects against leaching in soil (Pietik€ainen et al., 2000).

Bacterial adsorption to biochar surfaces mostly depends

on the abundance of minerals, pore size and curvature,

and iso-electric points of the biochars (Cheng et al.,

2008).

Microbial response to biochar in soil

Soil type, vegetation, and other ecosystems largely gov-

ern the microbial responses to biochar (Noyce et al.,

2015). The microbial response to biochar addition to soil

is a function of the existing soil microbial community

and might also be linked to previous land-use patterns

(Gomez et al., 2014). Gul et al. (2015) comprehensively

covered most of the important aspects of microbial

response to biochar amendments in soils in their

recently published review. Therefore, in this section, we

focus on reports that have not been reviewed previ-

ously. Microbes exposed to labile biochar with fresh car-

bon sources respond by increasing CO2 respiration. On

the other hand, the microbial population has also been

shown to proliferate in the presence of biochar with

more refractory carbon sources (Spokas et al., 2010; Zim-

merman et al., 2011). Some researchers suggested that

the soil type also influences the response of the soil

microbes to biochar addition (Spokas & Reicosky, 2009).

Moreover, biochar application alters soil enzymatic

activity. Results of Yoo & Kang (2011) suggest that

increased enzyme activities are induced by microbial

proliferation during incubation of soil with biochar;

however, reduced activities might be partly explainable

by higher nutrient availability or chemical blocking of

substrates by biochar. Such observations need to be

examined by scaling up from experiments in jars to pots

to field studies.

Recently, several long-term (>12 months) field studies have examined the impact of biochars on growth, activ-

ity, and composition of microbial biomass (Jones et al.,

2012; Quilliam et al., 2012; Rousk et al., 2013; Domene

et al., 2014). Jones et al. (2012) and Domene et al. (2014)

reported that biochar application greatly increases

above ground biomass and foliar N. On the other hand,

in the 2nd year of the study, biochar addition improved

fungal and bacterial growth rate and turnover through

increased soil respiration. Moreover, Rousk et al. (2013)

showed that bacterial growth was promoted by biochar

amendments due to the release of large amounts of

labile C. In contrast, Quilliam et al. (2012) reported that

biochar amendment did not induce significant microbial

growth and mycorrhizal colonization in a temperate

agricultural soil even after 3 years of soil residency.

These studies illustrate that the impact of biochar on

soil microorganisms is a controversial issue that must

be addressed carefully through long-term field experi-

mentation. In this regard, use of either stable 14C or 13C

isotope labeling might be useful to navigate the role of

biochar carbon in soil biota (Watzinger et al., 2014).

Possible ways to mitigate the contamination of

biochar with PAHs and VOCs

It is well established that both biochar chemistry and

yield vary considerably according to production pro-

cess conditions (Novak et al., 2009; Keiluweit et al.,

2010; Lee et al., 2010) and surface oxidation (Boehm,

1994; Yao et al., 2012). For instance, the temperature

(pyrolysis) dependence of the solvent-extractable PAH

content of biochar was clearly demonstrated by Keilu-

weit et al. (2012); biochars produced at temperature

between 400 and 600 °C contained the maximum levels of PAH. Likewise, partially carbonized (hydrothermal

carbonization (HTC) and fast pyrolysis) biochars were

found to contain a large number of sorbed volatiles

(Spokas et al., 2011), whereas gasification, thermal or

chemically processed biochars, soil kiln mound, and

open pit biochars had low-to-nondetectable levels of

VOCs. The presence of oxygen during pyrolysis also

led to a reduction in sorbed VOCs. In addition, a

recently developed pyrolysis process utilizing CO2 as a

reaction medium provided a potential option to control

the PAH and VOC content in biochar. Several studies

experimentally showed that CO2 enhanced thermal

cracking and blocked the pathway to formation of

PAHs, which led to a significant reduction in condens-

able pyrolytic oil (Kwon et al., 2012, 2013, 2015; Cho

et al., 2015a,b). However, some of these observations

need to be validated using a wide array of feedstocks

and biochar manufacturing conditions. With more

knowledge and information regarding the impact of

© 2016 The Authors. Global Change Biology Bioenergy Published by John Wiley & Sons Ltd., 9, 990–1004

1000 T. DUTTA et al.

various feedstocks, production conditions, and different

pyrolysis technologies on the chemical characteristics of

sorbed VOCs associated with these biochars, it will be

possible to modify certain conditions to minimize the

contamination of biochar with volatiles.

Concerns and recommendations for future research

In the wake of worldwide concern over global warming,

studies focusing on the effect of soil management on

photochemically reactive gases and organic compounds

are of critical importance. In addition, efforts to unravel

the key mechanisms driving the sequestration vs. emis-

sion of these compounds are greatly desirable to formu-

late recommendations for future soil management

practices. The use of biochar has been suggested as a

possible strategy to combat the ever-increasing percent-

age of carbon in the earth’s atmosphere. However, rela-

tively little is known about the impacts of biochar

addition on long-term persistence of PAH and VOC or

its bioavailability. As sorption is a key process in deter-

mining the fate of PAHs or VOCs in soil, an improved

understanding of the sorption characteristics of biochar

in a broad range of soil environments (agricultural soil,

contaminated soil, reclaimed systems, etc.) will provide

valuable information on the regulation of PAHs after

treatment with biochar. In this context, it is desirable to

accurately assess the dynamic interactions between sor-

bent (biochar) and soil microbes and how these pro-

cesses ultimately affect the cycling of PAHs in the soil

layer and their persistence in the environment. In addi-

tion, the bioaccumulation of PAH grown in biochar-

amended soils requires further investigation. Likewise,

the variability of VOC composition accompanying the

application of biochar also calls for their characteriza-

tion before and after application to precisely assess their

potential effects of biochar application on the agro-eco-

system. Recent reports suggest that the co-occurrence of

low molecular weight (LMW) VOCs with PAHs in bio-

char is more phytotoxic than the presence of PAHs

alone. Consequently, they recommended that VOC con-

tent in biochar should be included in quality assessment

of biochar.

In conclusion, the composition of PAHs and VOCs in

biochar produced through various manufacturing meth-

ods and from a range of feedstock materials must be

thoroughly characterized, together with their interac-

tions with soil biota and the effect of the presence of soil

and aging of biochar, before recommending their appli-

cation for agricultural or reclamation purposes. Future

studies need to be directed toward the development of

precise quantitative assessment tools for PAHs and

VOCs associated with biochar and toward ascertaining

the mechanism of their retention or release, including

the role of soil biota, under multiple soil and climatic

conditions.

Acknowledgements

This study was supported by a grant from the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (MEST) (No. 2009-0093848). This work was also carried out with the support of the ‘Coop- erative Research Program for Agriculture Science & Technol- ogy Development’ (Project title: Study on model development to control odor from hogpens, Project No. PJ01052101) Rural Development Administration, Republic of Korea. The second author also acknowledges the support made by a National Research Foundation of Korea (NRF) Grant funded by the Kor- ean Government (MSIP) (No. 2014RA1A004893).

Conflict of interest

There is no conflict of interest.

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