Th e widespread use of chromium (Cr) has a deleterious impact
on the environment. A number of pathways, both biotic and
abiotic in character, determine the fate and speciation of Cr
in soils. Chromium exists in two predominant species in the
environment: trivalent [(Cr(III)] and hexavalent [Cr(VI)]. Of
these two forms, Cr(III) is nontoxic and is strongly bound to soil
particles, whereas Cr(VI) is more toxic and soluble and readily
leaches into groundwater. Th e toxicity of Cr(VI) can be mitigated
by reducing it to Cr(III) species. Th e objective of this study was
to examine the eff ect of organic carbon sources on the reduction,
microbial respiration, and phytoavailability of Cr(VI) in soils.
Organic carbon sources, such as black carbon (BC) and biochar,
were tested for their potential in reducing Cr(VI) in acidic and
alkaline contaminated soils. An alkaline soil was selected to
monitor the phytotoxicity of Cr(VI) in sunfl ower plant. Our
results showed that using BC resulted in greater reduction of
Cr(VI) in soils compared with biochar. Th is is attributed to the
diff erences in dissolved organic carbon and functional groups that
provide electrons for the reduction of Cr(VI). When increasing
levels of Cr were added to soils, both microbial respiration
and plant growth decreased. Th e application of BC was more
eff ective than biochar in increasing the microbial population and
in mitigating the phytotoxicity of Cr(VI). Th e net benefi t of BC
emerged as an increase in plant biomass and a decrease in Cr
concentration in plant tissue. Consequently, it was concluded
that BC is a potential reducing amendment in mitigating Cr(VI)
toxicity in soil and plants.
The Infl uence of Biochar and Black Carbon on Reduction
and Bioavailability of Chromate in Soils
G. K. Choppala, N. S. Bolan,* M. Megharaj, Z. Chen, and R. Naidu
Excessive concentrations of heavy metals released from indus-
trial activities adversely aff ect biological functions in soil (Giller
et al., 1998). Toxicity of heavy metals on the microbial com-
munity in soils has been reported (McGrath et al., 1995; Wang
et al., 2007). Of these heavy metals, chromium (Cr) is one of
the most severe and high-risk pollutants that threaten fl ora and
fauna with eff ects that last for a lifetime. Several industries,
such as dye production, leather tanning, anodizing of alumi-
num, cooling towers, electroplating, and steel production, use
Cr at diff erent stages of their manufacturing cycle (Darrie,
2001; Banks et al., 2006). Th e toxicity, mobility, and bioavail-
ability of Cr mainly depend on its speciation.
Generally, Cr occurs in diff erent oxidation states from −2 to
+6; however, the most common species are Cr(III) and Cr(VI).
Th ese two species of Cr prevail in the environment with dif-
ferent physiochemical properties and biochemical reactivities
(Kotaś and Stasicka, 2000). Chromium(VI) is highly mobile,
mutagenic, teratogenic, and thermodynamically metastable in
soil and exists as chromate (CrO4
dizing environments (Faybishenko et al., 2008). In contrast,
Cr(III) is an essential dietary nutrient (>200 μg d−1) (Lukaski,
1999) that mediates normal glucose metabolism, cholesterol,
and fat in humans (Kimbrough et al., 1999) and forms stable
inorganic and trivalent hexaqua ions [(Cr(H2O)6)3+] and hexa-
coordinate stable complexes (Bartlett, 1991).
Chromium(VI) in soils can be eff ectively reduced to Cr(III)
by using organic carbon (OC) sources, such as biosolids com-
post, and farmyard and poultry manures, through diff erent
mechanisms (Park et al., 2008; Tokunaga et al., 2003). Organic
carbon stimulates microbial communities and increases dis-
solved organic carbon (DOC), which acts as an electron source
for the reduction of Cr(VI) (Bolan et al., 2003). Th e poten-
tial of OC in reducing Cr(VI) depends on the contaminant
he continuous influx of wastewater streams pol-
luted with heavy metals into fresh water streams cul-
minates in severe contamination of water and soil.
−) complexes in strongly oxi-
2−) and bichromate
−, H2CrO4, and HCr2O7
Abbreviations: ADH, Adelaide Hills; BC, black carbon; CEC, cation exchange
capacity; CWB, chicken waste biochar; DOC, dissolved organic carbon; FTIR, Fourier
transform infrared spectroscopy; KPB, Kulpara Bay.
Centre for Environmental Risk Assessment and Remediation, University of South
Australia; Cooperative Research Centre for Contamination Assessment and
Remediation of the Environment, University Parade, Mawson Lakes, SA 5095,
Cooperative Research Centre for Contamination Assessment and Remediation
of Environment, PO Box 486 Salisbury South, SA 5106, Australia. Assigned to
Associate Editor Andrew Tye.
Copyright © 2012 by the American Society of Agronomy, Crop Science Society
of America, and Soil Science Society of America. All rights reserved. No part of
this periodical may be reproduced or transmitted in any form or by any means,
electronic or mechanical, including photocopying, recording, or any information
storage and retrieval system, without permission in writing from the publisher.
J. Environ. Qual. 41
Received 15 Apr. 2011.
*Corresponding author (Nanthi.Bolan@unisa.edu.au).
© ASA, CSSA, SSSA
5585 Guilford Rd., Madison, WI 53711 USA
Journal of Environmental Quality
ENVIRONMENTAL BENEFITS OF BIOCHAR
1176 Journal of Environmental Quality
concentration, microbial activity, and reactivity of OC (Chen
and Hao, 1998). Other natural materials for the reduction of
Cr(VI) in soil include organic and humic acids (Deng and
Stone, 1996; Nakayasu et al., 1999).
Application of carbonaceous materials such as black carbon
(BC) and biochar to soils can act as sink and eff ectively reduce
anthropogenic CO2 emissions into the environment (Lehmann
et al., 2006). Black carbon is an important stable carbon pool
in soils. Conversion of plant biomass to biochar can lead to
sequestering nearly 50% of initial carbon, whereas burning and
biological decomposition can only retain up to 3% and <10
to 20% after 5 and 10 yr, respectively (Schmidt and Noack,
2000; Lehmann et al., 2006). Biochar obtained from pyrolysis
of various biomasses and normal plant biomasses is a potential
inhibitor of nitrous oxide emissions originating from grazing
pastures (Sohi et al., 2009; Taghizadeh-Toosi et al., 2011).
Adding organic amendments immobilizes and decreases the
availability of toxic heavy metals (Gadepalle et al., 2007; Park
et al., 2011a). However, when organic matter decomposes,
adsorbed metals may be released into the soils (Namgay et
al., 2010). Black carbon derived from plant biomass signifi -
cantly increases the retention of cations in soils and enhances
soil fertility while decreasing off site pollution in agricultural
landscapes (Cheng et al., 2006; Van Zwieten et al., 2010).
Incorporation of biochar into the soil increases water-holding
capacity and nutrient availability to plants and reduces acidity
in soils, thereby decreasing aluminum toxicity to plants and
soil microorganisms (Glaser et al., 2002).
It has been reported that several OC sources, includ-
ing brown seaweed (Park et al., 2004), phenols (Elovitz and
Fish, 1995), powdered leaves (Suseela et al., 1987), biosolids
composts, farm yard manure, chicken manure (Bolan et al.,
2003), and organic acids (Deng and Stone, 1996), are able
to reduce Cr(VI) to Cr(III) with subsequent immobilization
of Cr(III). Organic amendments constitute a higher percent-
age of carbon and release DOC on incubation in soils. Th e
addition of organic amendments increases DOC and micro-
bial respiration, which in turn enhances biotransformation of
metal(loid)s, including Cr reduction (Tokunaga et al., 2001;
Park et al., 2011b). Several studies have commented on the iso-
lation and characterization of Cr(VI)-resistant bacteria in con-
taminated soils (Megharaj et al., 2003; Patra et al., 2010; He et
al., 2010). However, the response of microbes in the presence
of added BC and biochar to enhance Cr reduction in soils is
not well documented. Although several reports indicated that
the application of organic amendments in reducing Cr toxicity
in soils has potential value, there are limited data (Table 1) on
the benefi cial eff ect of organic amendments, especially biochar
on the mitigation of metal phytotoxicity in soils. No study has
reported on the role of pH of organic amendments on Cr(VI)
reduction in soils.
One of the goals of this experiment was to investigate the
impact of BC and biochar addition on the response of microbes
in Cr(VI)-contaminated soils. Th is study examines the eff ect of
two organic amendments that vary in pH and carbon fractions
(BC and chicken waste biochar [CWB]) on the mitigation of
Cr(VI) phytotoxicity in soils.
Materials and Methods
Soil Collection and Characterization
Bulk samples of two diff erent soils were collected from uncon-
taminated surface soils (0–10 cm depth) in South Australia.
Th e collected soils varied widely in their origin, mineralogy, and
properties. Adelaide Hills (ADH) soil was classifi ed as kurosol,
and Kulpara Bay (KPB) soil as chromosol. Th e air-dried soil
samples were disaggregated with agate mortar, homogenized,
and passed through a 2-mm sieve to ensure for uniformity of
soil particles. Th e samples were stored in polythene bags at
room temperature until further analysis.
Th e soil samples were analyzed for pH, cation exchange
capacity (CEC), and organic matter. Each soil sample (5 g) was
weighed into a 50-mL centrifuge tube, mixed with 12.5 mL of
0.01 mol L−1 CaCl2, and shaken in an end-over-end shaker for
30 min, and pH was measured with a pH electrode (Smartchem-
lab; TPS, Springwood, Australia) after calibration.
Th e CEC of soils was determined using a method described
by Gillman and Sumpter (1986), which involves the displace-
ment of cations on exchange sites with 0.2 mol L−1 NH4Cl. Th e
soil (2 g) soil was taken in a 50-mL polythene centrifuge tube,
mixed with 20 mL 0.2 mol L−1 NH4Cl, and shaken for 2 h. It
was then centrifuged at 1789 g for 20 min and fi ltered through
0.45-μm syringe fi lters. Displaced cations (Ca, Mg, K, and Na)
were determined by ICP–MS (7500ce; Agilent, Santa Clara,
CA). Total organic matter was estimated using the oxidizable
dichromate method (Walkley and Black, 1934).
Organic Carbon Sources and Characterization
Black carbon was prepared from a South Australian common
weed (Solanum elaeagnifolium). Th e weeds were collected,
washed with water, dried in air under shade for 1 wk, and oven
dried at 80°C for 2 d. Th e plant residue (100 g) was placed in
a ceramic cup and pyrolyzed aerobically in a muffl e furnace for
3 h at 300°C. Nearly 25 to 30% of the weight of dried weed
Table 1. Selected references on the remediation of chromium(VI) phytotoxicity by organic amendments.
Amendment Plant used ObservationReference
reduced Cr in roots and no change in Cr
concentration in shoots
reduced Cr concentration in plant tissue
dry matter of shoot and roots increased
growth of seedlings increased
reduced availability of Cr for plant uptake
reduced Cr uptake by plant
decreased Cr in plant extract
increased root and shoot growth
Banks et al. (2006)
Cattle compost and straw
Hog dung and cattle dung compost Triticum vulgare
Bark of Pinus radiata
Farm yard manure
Bolan et al. (2003)
Rendina et al. (2006)
Lee et al. (2006)
Bolan and Thiagarajan (2001)
Yadav et al. (2009)
Branzini and Zubillaga (2010)
Singh et al. (2007)
Sesbania punicea and Sesbania virgata
www.agronomy.org • www.crops.org • www.soils.org 1177
plant material was converted to BC after pyrolysis. Th e charred
residues were ground to <250 μm size and treated three times
with 1 mol L−1 HCl for 4 h and then four times with a 1:1
mixture of 1 mol L−1 HCl and 1 mol L−1 hydrofl uoric acid to
remove silica (Qiu et al., 2008) and other inorganic materials
without damaging the surface property of BC (Chun et al.,
2004). Th e protonated BC was washed several times with dis-
tilled water until no further change in pH occurred (Qiu et al.,
2008). Black carbon was dried in an oven at 60°C for 3 d and
stored in a glass container. Chicken waste biochar was obtained
from Pacifi c Pyrolysis (NSW, Australia). Th e CWB was pre-
pared from poultry litter that was pyrolyzed at 550°C. Th e
CWB samples were air dried, ground, sieved through 250-μm
mesh, and stored in glass container.
Th e BC and CWB samples before and after treatment
with Cr(VI) were analyzed by Fourier transform infrared
(FTIR) spectroscopy to monitor and confi rm the role of sur-
face functional groups in Cr(VI) reduction. Th e infrared spec-
tra of untreated and Cr(VI)-loaded BC and CWB (100 mg
Cr(VI) kg−1) samples were recorded using a spectrum 400
FTIR (PerkinElmer, Waltham, MA) with DRIFT at Flinders
University, South Australia. Th e amendment powders were
blended with spectroscopic grade KBr at the ratio of 1:10 in an
agate mortar and pressed to prepare translucent pellets. Spectra
of the samples were scanned between 500 and 4000 cm−1 at a
resolution of 4.0 cm−1. Th e samples of BC and CWB were ana-
lyzed for nitrogen, carbon, hydrogen, and sulfur composition
using an elemental analyzer (EA1110; CE Instruments, Wigan,
UK). Surface area was analyzed using a surface area and pore
size analyzer (Gemini V; Micromeritics, Norcross, GA). Th e
BC and CWB samples were out-gassed in a vacuum at 105°C
for 8 h before measuring the surface area. Th e samples’ surface
area was estimated using the BET (Brunauer–Emmett–Teller)
model. Th e structure and morphology were examined via scan-
ning electron microscopy (JSM-2701F; JEOL, Tokyo, Japan).
Reduction of Chromium(VI) in Soils
To examine the eff ect of BC and CWB on the reduction of
Cr(VI), acidic (ADH) and alkaline soils (KPB) were selected
because these soils have contrasting pH levels. Th e air-dried
soil samples were mixed with two levels of organic amend-
ments (0 and 50 mg kg−1), mixed with two levels of Cr(VI)
(K2Cr2O7) (0 and 500 mg Cr(VI) kg−1 soil) to monitor Cr(VI)
reduction, and incubated at fi eld capacity.
Spiked soils are likely to have a homogenous distribution
of Cr(VI) species, whereas naturally contaminated soils con-
tain diff erent levels of Cr(VI) in various zones with high spatial
variability. In the laboratory, the experiments were conducted
using spiked soils under controlled conditions such as moisture,
temperature, and mixing time, whereas naturally contaminated
soils experience environmental variations that may change soil
pH and Eh, the two primary parameters that determine the
speciation and distribution of Cr(VI) in soil. Moreover, spiking
the soils with Cr(VI) salt (K2Cr2O7) directly results in a “salt
eff ect,” which is likely to aff ect Cr(VI) availability in soils. Care
was taken to reduce this artifact by dissolving Cr(VI) salt in
water and repeating the wetting–drying cycle two times before
starting the experiments.
Subsamples were taken at various intervals and extracted with
1 mol L−1 KH2PO4 at a soil/solution ratio of 1:10 for 1 h, and
the solution was fi ltered through a 0.45-μm fi lter. Th e complex
formed by reacting Cr(VI) with 1,5-diphenyl carbazide provides
a pink color, and the intensity of color is proportional to the
amount of Cr(VI). Th e absorbance of the color complex was
measured spectrophotometrically at 540 nm (USEPA, 1995).
Th e amount of Cr(VI) reduction was estimated from the
decrease in the concentration of Cr(VI) in soil solution using
where Cr is the amount reduced (mg kg−1), Ci is the initial con-
centration (mg kg−1), and Cf is the concentration of KH2PO4–
extractable Cr(VI) (mg L−1), V is the solution volume (L), and
W is the weight of the air-dried soil sample (kg). Th e decline
in Cr(VI) concentrations may be due to adsorption and reduc-
tion mechanisms. However, there was no signifi cant diff er-
ence in the concentrations of Cr(VI) extracted by water and
KH2PO4, indicating less possibility of adsorption of Cr(VI) by
soils (Bolan et al., 2003).
Th e fi rst-order decay rate equation as described by Lan et
al. (2007) was used to calculate the Cr(VI) reduction rate. Th e
net rate of change in residual Cr(VI) equals the reduction rate
(Vdec), which is expressed by the following fi rst-order diff eren-
where k is the fi rst-order reduction rate constant (per time),
C is the concentration of Cr(VI) in the KH2PO4–extractable
Cr(VI) (mg L−1), and t is the incubation time of Cr(VI) and
amendments in soil (days). Th e equation has the following
C = C0e−kt 
where C0 is the initial amount of Cr(VI) recovered at Day 0 of
the experiment in the soil.
Th e rate constant k was estimated by using the nonlinear
least square method. Th e Cr(VI) half-life (time taken to reduce
Cr(VI) concentration to half of the initial value; i.e., C = C0/2)
was calculated as:
Microbial Activity in Soils
Th e infl uence of BC and CWB on the microbial activity and
Cr(VI) reduction was determined by monitoring basal respira-
tion as the amount of CO2 released from the microbial com-
munity after 7 d of incubation in acidic and alkaline soils that
had been treated with Cr(VI) and organic amendments. Soil
microbial respiration experiments were conducted in three
replicates. Each sample of 18 g of soils treated with diff erent
levels of chromate (0, 50, 100, 250, and 500 mg Cr(VI) kg−1)
and amendments (0 and 50 mg kg−1) was placed in 50-mL
1178 Journal of Environmental Quality
centrifuge tubes fi tted in a 250-mL bottle containing 20 mL
0.05 mol L−1 NaOH solution to capture released CO2 from
Th e amount of CO2 captured was measured by titrating
the NaOH solution with 0.5 mol L−1 HCl after the precipita-
tion of barium carbonate formed by adding BaCl2 to NaOH.
Phenolphthalein diluted in 100 mL ethanol (60%, v/v) was
used as an indicator (Fernandes et al., 2005). Th e amount of
CO2 produced by soil respiration was calculated based on the
consumed HCl solution for titration (Bloem et al., 2006):
CO2 produced = [Mol. wt. of CO2 × vol. of HCl consumed
(blank − sample) × conc. of HCl×1000]/(Dry wt. of soil×time×2) 
Plant Growth Experiment
Alkaline soil (KPB) was selected to observe the phytoavailabil-
ity of Cr(VI) because the eff ect of organic amendments on Cr
reduction was more pronounced in this soil. Soil samples were
treated with diff erent levels of Cr(VI) (0, 50, 100, and 250 mg
kg−1) using K2Cr2O7 as a Cr(VI) source and incubated at fi eld
capacity for 1 d. Th ese soil samples were subsequently mixed
with 5% BC and CWB and transferred to pots.
Th e method developed by Stanford and DeMent (1957)
was used for plant growth and conducted under glasshouse
conditions to examine the eff ect of three amendments on the
plant uptake of Cr(VI). Plastic pots (600 mL capacity; 10 cm
diameter) were fi lled with 500 g sand. Sunfl ower (Helianthus
annuus L) seeds, 10 in each pot, were sown, and Hoagland solu-
tion was supplied after germination. Th e number of seedlings
was reduced to four per pot 14 d after germination. Seedlings
were further reduced to two per pot before transferring them
to contaminated soils. Th irty days after germination, the seed-
lings and the sand medium were transferred to another set of
pots that contained 200 g of treated soils with diff erent levels
of Cr(VI) and amendments.
Rhizon samplers (one per pot) (Rhizosphere Research
Products, Wageningen, Th e Netherlands) were placed hori-
zontally at 1 cm from the bottom of the pot. Each treatment
was performed in triplicate. Pore water samples were collected
using the Rhizon sampler at 1, 2, 3, and 4 wk after the estab-
lishment of the pot experiment and analyzed for Cr(VI) using
the colorimetric method.
Plants were harvested 4 wk after being transferred to
Cr(VI)-contaminated soils. After harvesting plants, the shoots
and roots were separated, washed with deionized water, and
oven dried, and their dry weights recorded. Th e plant materi-
als were analyzed for metal content using nitric acid digestion
(Zarcinas et al., 1987). Th e ground plant material (0.1–0.5 g)
was weighed directly into a 75-mL digestion tube with 5 mL
of concentrated nitric acid and left to cold digest in a fume
cupboard overnight. After keeping the samples overnight,
the tubes were heated using a temperature-controlled diges-
tion block (AIM 500 Block Digestion System; AI Scientifi c,
Australia) programmed to slowly increase the temperature to
140°C until 1 mL of digest remained in the tube. Th e tubes
were brought to room temperature before dilution with deion-
All measurements, including pH, CEC, OM (%), Cr(VI)
concentration in reduction, and soil respiration experiments,
were calculated from triplicates of each treatment. All calcula-
tions and standard deviations of the replicates were determined
using Microsoft Excel.
Th e relationship between Cr(VI) reduction in soils (acidic
and alkaline) and days of incubation in the presence of BC and
CWB and the relationship between DOC and half-life were
derived using regression analysis in grapher software (Version
7; Golden Software, Golden, CO). Statistical signifi cance of
the experimental data was analyzed using a statistical software
tool (PASW Statistics, Version 18.0.0; SPSS, Inc., Chicago, IL)
at a signifi cance level of p < 0.05. Amendment effi ciency on
decreasing Cr(VI) phytotoxicity was compared using Duncan’s
multiple range test using the SPSS software
Results and Discussion
Soil and Organic Matter Characteristics
Th e two soils collected from South Australia were used for
Cr(VI) reduction, and these varied considerably in their physi-
cal and chemical properties. Th e pH varied from acidic (4.60)
in ADH soil to alkaline (8.17) in KPB soil. Organic matter was
in the range of 1.06 to 6.73%, and CEC ranged from 3.84 to
6.36 cmol kg−1. Th e pH of BC was 3.52, and the pH of CWB
was 8.85. Concentrations of nitrogen, carbon, and hydrogen
are listed in Table 2. Th e surface areas (BET) of BC and CWB
were 3.35 and 7.27 m2 g−1, respectively. Scanning electron
microscope images demonstrated that BC and CWB had a fi ne
pore structure, which refl ected the large surface area (Fig. 1).
Th e scanning electron microscopy images were similar between
BC and CWB in that they both showed fi brous channels.
Eff ect of Organic Amendments on Reduction
Th e amount of Cr(VI) reduced is calculated from the decrease
in the concentration of Cr(VI) in soil solution. Reduction was
higher in acidic than in alkaline soils. Th e rate of Cr(VI) reduc-
tion increased with the addition of amendments in both soils,
and there was a signifi cant diff erence in the rate of reduction
between the amendments. Th e addition of BC at 5% applica-
tion rate to contaminated acidic and alkaline soils spiked with
500 mg of Cr(VI) kg−1 resulted in complete reduction within
Table 2. Values of pH, elemental composition, cation exchange capacity, and surface area of black carbon and chicken waste biochar.
————————————— wt% —————————————
Surface area (BET)‡
Chicken waste biochar
† Cation exchange capacity.
‡ Brunauer Emmett Teller surface area.
www.agronomy.org • www.crops.org • www.soils.org 1179
6 and 10 d, respectively. In contrast,
CWB (5% addition) resulted in reduc-
tions of 197.6 and 219.4 mg Cr(VI) kg−1
in 14 d for acidic and alkaline soils (500
mg of Cr(VI) kg−1), respectively (Fig. 2).
Half-life values for BC, CWB, and con-
trol soil were 0.49, 10.66, and 19.25 d
in acidic soil and 0.91, 11.41, and 21.73
d in alkaline soil, respectively.
Reduction of Cr(VI) to Cr(III) in soils
has often been found to be rapid, reach-
ing the maximum within days (Ross et al.,
1981). Th e rate of reduction of Cr(VI)
was as follows: BC > CWB > control
soil. Th e reduction results indicated that
adding organic amendments, such BC and CWB, enhanced
the rate of Cr(VI) reduction in the soil. Th is is consistent with
the fi ndings of Losi et al. (1994) and Bolan et al. (2003), who
reported substantial increases in the rate of reduction of Cr(VI)
in the presence of organic amendments.
Th e results indicated that BC is more eff ective than CWB
in enhancing Cr(VI) reduction. Th e eff ect of BC and CWB in
enhancing Cr(VI) reduction may be attributed to the presence
of several oxygen-containing acidic (carbonyl, lactonic, carbox-
ylic, hydroxyl, and phenol) and basic (chromene, ketone, and
pyrone) functional groups (Goldberg, 1985; Boehm, 1994).
Moreover, BC and CWB comprise disordered polycyclic aro-
matic hydrocarbon sheets that are highly porous with high
surface area. Th ese sheets donate π-electrons for the Cr(VI) to
be reduced (Wang et al., 2010). Th e resultant Cr(III) either
adsorbs or participates in surface complexation with organic
amendments (Hsu et al., 2009a).
A lesser degree of Cr(VI) reduction was mediated by CWB
in comparison to BC despite the presence of several functional
groups. Th e acidic functional groups dissociate its protons and
act as Brønsted acids in soil with a pH range of 5 to 7. Th e
high pH (8.8) of the CWB may prevent the dissociation and
oxidation of phenolic and hydroxyl groups, thereby limiting
the supply of protons for Cr(VI) reduction.
Th e reduction mediated by CWB increased between 0 and
2 d after the incubation. Th is could be attributed to microbial-
mediated oxidation of biochar (Zimmerman, 2010), which
would increase the supply of protons for Cr(VI) reduction.
Chicken waste biochar contains 28.9 g kg−1 of nitrogen (N),
and oxidation of this ammoniacal N releases protons, thereby
contributing to the reduction of Cr(VI).
Th ere was a negative relationship between the half-life of
Cr(VI) reduction and concentration of DOC in soils amended
with organic carbon sources (Fig. 3), indicating that the rate of
reduction increased when an increase in the concentration of
DOC occurred. Easily oxidizable carbon and DOC have been
shown to correlate with Cr(VI) reduction, and only certain
components of DOC act as electron donors when this reduc-
tion occurs (Bolan et al., 2003) (Eq. ). Elovitz and Fish
(1995) identifi ed hydroquinones in natural organic matter as
potential electron donors for reducing Cr(VI) in soils.
Fig. 1. Scanning electron microscope image of black carbon (BC) and chicken waste biochar (CWB).
Fig. 2. Comparison between black carbon (BC) and chicken waste
biochar (CWB) eff ect on the Cr(VI) reduction trends in acidic and
Fig. 3. Relationship between dissolved organic carbon and half-life
of Cr(VI) reduction in acidic and alkaline soils in the presence and
absence of black carbon (BC) and chicken waste biochar (CWB) appli-
cation. Plus sign, BC in acidic soil; diamond, BC in alkaline soil; square,
CWB in acidic soil; circle, CWB in alkaline soil; multiplication symbol,
control acidic soil; triangle, control alkaline soil.
1180 Journal of Environmental Quality
2− + organic carbon + 14 H+ → 2Cr3+ + CO2 + H2O
Reduction of Cr(VI) is a proton-consuming reaction; hence,
the soils’ pH increased (data not shown). For the reduction
to proceed, protons (H+) and electrons (e−) are vital. Because
the chemically treated BC contains a higher number of pro-
tons than CWB, the enhanced reduction in the presence of
BC may be attributed to the availability of protons for the
reduction of Cr(VI). Th e resultant Cr(III) ions precipitated
as Cr(OH)3 or were adsorbed through pH-enhanced CEC in
soils (El-Shafey, 2005).
Confi rmation of the Activity of Functional Groups
Th e presence of surface functional groups and the modifi ca-
tions mediated by BC after Cr(VI) reduction were confi rmed
by FTIR spectra (Fig. 4). Th ere were several modifi cations in
the spectra of BC after reacting with Cr(VI) (Table 3). Th e
absorption peak at 3203 cm−1 indicates the presence of free
and intermolecular hydroxyl (-OH) groups, confi rming the
release of hydroxyl ions by Cr(VI) reduction (El-Shafey, 2005).
Th e broad peak observed at 2929.5 cm−1 was assigned to the
stretching vibrations of C-H groups and amine (NH2) groups.
Th e peak at 1704.4 cm−1 in the untreated BC represents native
free–COOH stretching, which was not present after treat-
ment with Cr(VI), indicating its participation in reduction
and confi rming the process of carboxylation of the BC surface
(Murphy et al., 2008).
Th e strong intensity of the band at 1442.9 cm−1 is attrib-
uted to the formation of carbonyl/carboxylic groups on the
surface of BC. Th ese carboxylic groups may serve as binding
sites for Cr(III) species resulting from the reduction of Cr(VI)
(Hsu et al., 2009a). Th e intensity of the peaks at 1442.9 and
1283.0 cm−1 in Cr(VI)-treated BC was reduced, which was
attributed to the C-O-CH3 (methoxyl) deformation and
bending of C-OH (hydroxyl) in phenolic structures, respec-
tively (Samal et al., 1995; Pandey, 1999). Th e decrease in peak
intensities indicated the oxidation of these functional groups in
BC on Cr(VI) reduction. Because Cr(VI) is a strong oxidizing
agent, primary and secondary alcohols reduced Cr(VI), and
alcohols were converted to carbonyl compounds (Elangovan
et al., 2008). Phenolic groups in the BC play an important
role in supplying electrons for the reduction of Cr(VI) (Hsu
et al., 2009b). Elovitz and Fish (1995) indicated that oxida-
tion of phenols forms the quinones containing carbonyl groups
on BC surfaces. Th e increased intensity at 1596.2 cm−1 in BC
after reacting with Cr(VI) indicates the consequent formation
of carbonyl and carboxylic groups. All the above-mentioned
changes in the functional groups confi rmed the participa-
tion of functional groups in reducing Cr(VI) and subsequent
adsorption of Cr(III) species.
Th e FTIR spectrum of untreated CWB showed peaks at
3061.8, 1584.2, 1439.5, 1115.1, 875.6, and 802.5 cm−1.
Th e peak at 3061.8 is attributed to the presence of hydroxyl
groups, and the peak at 1584.2 was assigned to the stretch-
ing vibrations of conjugated C=C bonds and carboxyl groups
(Hsu et al., 2009b). Th e vibrations of bonds in ring skel-
etal C-O and bond stretching in C-C contribute to bands at
1115.1 cm−1 (Shen et al., 2010). Th e band at 875.6 resulted
from the out-of-plane bending of aromatic C-H bonds (Yang
et al., 2007). Aromatic C-H in plane vibrations resulted
in the peak at 802.5 cm−1 (Moreno-Castilla et al., 2000).
Because no major changes in the intensities of bands in CWB
occurred before and after treatment with Cr(VI) (Fig. 4), the
illustration of peaks was confi ned to BC samples only.
Microbial Activity in Soils
Soil respiration, as measured by the amount of CO2 released,
signifi cantly decreased when Cr(VI) concentration in acidic and
alkaline soils increased, with the eff ect being more pronounced
in the alkaline soil. Because acidic soils reduce Cr(VI) faster than
alkaline soils (Fig. 2), there is less microbial toxicity of Cr(VI) in
the former soil, as indicated by higher respiration. Application
of BC and CWB increased microbial
activity in Cr(VI)-contaminated soils
(Fig. 5a), confi rming their contribution
to Cr(VI) reduction in soils and thereby
overcoming Cr toxicity. Th e addition of
BC and CWB at 5% application to acidic
contaminated soil (500 mg Cr(VI) kg−1)
increased microbial activity as measured
by increased respiration from 2.77 to 5.96
and 5.65 μg C-CO2 g−1 soil h−1, respec-
tively. In alkaline contaminated soils (500
mg Cr(VI) kg−1), the addition of 5% BC
and CWB increased respiration from 2.46
to 6.11 and 5.81 μg C-CO2 g−1 soil h−1,
respectively (Fig. 5b).
Th e application of BC to soils has
been shown to increase microbial respira-
tion when compared with nonamended
control soil (Major et al., 2010). Both BC
and CWB stimulate microorganisms in
their porous structures, thereby enhanc-
ing microbial respiration (Pietikäinen et
al., 2000; Steiner et al., 2008). Smith et al.
Fig. 4. Stretching frequencies observed for black carbon and biochar before and after treatment
www.agronomy.org • www.crops.org • www.soils.org 1181
(2010) used biochar that was prepared from switchgrass and
observed increased CO2 production in soils, which they attrib-
uted to the direct emission from biochar. Th ey further con-
fi rmed the production of CO2 from the biochar in the soils by
using the ;13C signatures of CO2 evolved during the incubation
and biochar, which were similar.
Th e addition of organic amendments such as manures has
been shown to induce Cr(VI) reduction in soils by microor-
ganisms in biotic and abiotic conditions (Losi et al., 1994).
Th e dominance of biotic reduction revealed the active role of
microorganisms. Moreover, Pérez-Piqueres et al. (2006) dem-
onstrated that adding amendments increases the microbial
activity as measured by respiration in soils. Th e addition of BC
and CWB increased the level of soil basal respiration in acidic
and alkaline contaminated soils by 2-fold. Greater microbial
activity in the presence of carbonaceous materials may increase
mineralization; hence, N and P levels increased (Kolb, 2007).
Biochar prepared from poultry litter contains higher N and P
contents than biochar prepared from wood or pine chips and
increases CEC in soils on its incorporation (Chan et al., 2008;
Biochar Farms, 2011).
Plant Growth Experiment
Th e results showed that the production of biomass decreased
with increasing concentrations of Cr(VI) in soils (Fig. 6).
However, applications of BC and CWB decreased Cr(VI)
toxicity, thereby increasing dry biomass. Plants grown in
BC-applied contaminated soil produced higher dry biomass
than plants grown in CWB-treated soil. Th e addition of 5%
organic amendments increased the plant dry matter yield by 7,
38, 59, and 72% for BC and 12, 35, 51, and 67% for CWB in
soils treated with 0, 50, 100, and 250 mg Cr(VI) kg−1, respec-
tively (Fig. 6).
Th e addition of BC and CWB resulted in a signifi cant
increase in biomass in the control soil. Th e addition of
Cr(VI) at 50 and 100 mg kg−1 reduced the biomass signifi -
cantly (P < 0.05) when compared with the control, but the
addition of BC and CWB increased the biomass. Th e addi-
tion of 250 mg Cr(VI) kg−1 soil resulted in the maximum
reduction in plant biomass.
More Cr was accumulated in root than in shoot. Applying
BC to Cr(VI)-contaminated soil substantially decreased the
concentration of Cr in plant tissue. Black carbon application
reduced Cr accumulation in shoots and roots by 95 and 81%,
Table 3. Identifi cation of functional groups in black carbon by Fourier transform infrared spectroscopy and the changes in these groups resulting
from chromium(VI) reduction.
——————— cm−1 ———————
Wavelength range AssignmentChanges in functional groups
hydroxyl (-OH) groups
C=O stretch in carboxylic acids
imines and oxines
C-H bending in alkanes
antisymmetrical stretch of–COO in carboxylic acids
C-O stretch in ethers
peak height decreased
carbonyl group formed
deformation of C-O-CH3
-COOH intensity decreased
bending of C-OH in phenolic groups
Fig. 5. Eff ect of Cr(VI) on basal respiration in acidic soil (a) and alkaline soil (b) in the presence and absence of black carbon (BC) and chicken waste
biochar application (CWB).
1182 Journal of Environmental Quality
respectively, in soil treated with 50 mg Cr(VI) kg−1. Th e CWB
decreased Cr concentration in shoots and roots by 57 and
29%, respectively, in soil treated with 50 mg Cr(VI) kg−1. At
higher Cr(VI) concentrations (250 mg kg−1), the decrease in
Cr accumulation was greater in the root zone (54%), whereas
for the shoot zone, the statistical signifi cant decrease in Cr
accumulation was only 43% compared with plants grown in
Cr-contaminated soils without BC application. Black carbon
decreased Cr deposition in roots and shoots greater than CWB,
which may be due to its higher reducing capability.
Reduction of Cr(VI) to Cr(III) is a known detoxifi cation
mechanism present in some plants (Zayed et al., 1998). Th e
greater degree of accumulation of Cr(VI) in roots may be due
to its immobilization or its reduction to labile Cr(III) by root
exudates, thereby decreasing toxicity to the plant (Vazquez et
al., 1987; Mishra et al., 1995). Zayed et al. (1998) detected
Cr(III) in plant tissues but detected no Cr(VI), Cr(V), or
Cr(IV). Howe et al. (2003) confi rmed the reduction of
Cr(VI) in or on roots by analyzing with X-ray absorption
Another possible reason for the decrease in Cr accumula-
tion and growth could be the inhibition of root cell division
(Barcelo et al., 1986). Th e resulting reduction in nutrient and
water uptake arising from the reduced root growth would
therefore be manifested by a corresponding decrease in shoot
growth (Shanker et al., 2005). Th ere was a negative relation-
ship between Cr concentration and production of biomass in
sunfl ower plant (Fig. 7). Th is is consistent with earlier research
performed on Cr and plant biomass (Tripathi et al., 2000;
Vajpayee et al., 2001).
In alkaline soils, application of BC and CWB decreased
Cr(VI) concentration in soil pore water with increasing incu-
bation time. In the absence of amendments, Cr(VI) concen-
tration was decreased to 1.47 mg Cr(VI) L−1 in 28 d, whereas
applying 5% BC and CWB decreased Cr(VI) concentrations
to below 0.5 and 1.35 mg L−1 in 14 d, respectively (Fig. 8).
Chromium(VI) is generally highly mobile in alkaline and
slightly acidic soils. However, application of BC and CWB
reduced the availability and mobility of Cr(VI) in alkaline soils
as measured by Cr(VI) in soil pore water.
Th e price of biochar is approximately US $500 per
metric ton (US Biochar Initiative, 2009). Based on the
results obtained from our study (5% w/w biochar applica-
tion = 50 kg m−3 soil), it can be estimated that the cost of
treating Cr(VI)-contaminated soil using biochar is around
US $25 m−3, whereas other technologies, such as the in situ
redox manipulation method, that reduce Cr(VI) to Cr(III)
cost around US $162 m−3 (Federal Remediation Technologies
Roundtable, 2011). However, preparation and practical appli-
cability of protonated BC in large-scale remediation needs to
Th is study demonstrated that BC can be used as an eff ective
reductant for the mitigation of Cr(VI) contamination in soils.
Th e reduction capacity of Cr(VI) was higher in acidic soil, and
the addition of BC and biochar signifi cantly increased reduc-
tion in alkaline soil. Dissolved organic matter enhanced the
Fig. 6. Eff ect of Cr(VI) on biomass in the presence of absence of black
carbon (BC) and chicken waste biochar (CWB) application.
Fig. 7. Relationship between tissue Cr(VI) concentration and dry biomass.
Fig. 8. Changes over time in Cr(VI) concentration in soil pore water.
BC, black carbon; CWB, chicken waste biochar; KPB, Kulpara Bay.
www.agronomy.org • www.crops.org • www.soils.org 1183
reduction of toxic Cr(VI) to less toxic and relatively immobile
Cr(III); the consequent increase in pH due to H+ consumption
in Cr(VI) reduction is likely to result in the immobilization of
Cr(III) through adsorption/precipitation reactions. Application
of BC and biochar increased basal respiration, with the eff ect
being more pronounced for BC than biochar. Chromium(VI)
inhibited the growth of sunfl ower plants, and application of BC
and biochar signifi cantly decreased phytotoxicity in plants and
decreased the bioavailability of Cr(VI) in soils by converting
Cr(VI) to nontoxic Cr(III). Application of BC also reduced
Cr(VI) in soil pore water. Black carbon and biochar appear to
strongly mitigate Cr contamination because they are highly
reactive with many functional groups and are able to donate
electrons to reduce Cr(VI) in soils. Th erefore, the reduction of
Cr(VI) by BC and its concurrent immobilization is an ideal
technology to mitigate Cr(VI)-contaminated soils.
Th e authors thank Pacifi c Pyrolysis, New South Wales, Australia
for supplying biochar and the Cooperative Research Centre for
Contamination Assessment and Remediation of the Environment
(CRC CARE), Australia for funding this research work in collaboration
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