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Comparison of biochars derived from different types of feedstock and their potential for heavy metal removal in multiple-metal solutions

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Three different types of feedstocks and their biochars were used to remove Cr(III), Cd(II), Cu(II) and Pb(II) ions from a mixture of multiple heavy metals. The effect of the initial concentration of heavy metals in solution has been analysed, and kinetics modelling and a comparison of the adsorption capacity of such materials have been performed to elucidate the possible adsorption mechanisms. The results show that the adsorption capacity is dependent on the type of feedstock and on the pyrolysis conditions. The adsorption capacity of the biomass types is ranked as follows: FO (from sewage sludge)>> LO > ZO (both from agriculture biomass waste)>> CO (from wood biomass waste). Biochars, which are the product of the pyrolysis of feedstocks, clearly improve the adsorption efficiency in the case of those derived from wood and agricultural biomasses. Complexation and cation exchange have been found to be the two main adsorption mechanisms in systems containing multiple heavy metals, with cation exchange being the most significant. The pore structure of biomass/biochar cannot be neglected when investigating the adsorption mechanism of each material. All the disposal biomasses presented here are good alternatives for heavy metal removal from wastewaters.
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Comparison of biochars derived
from dierent types of feedstock
and their potential for heavy metal
removal in multiple-metal solutions
JingJing Zhao1,2, Xin-Jie Shen1, Xavier Domene3,4, Josep-Maria Alcañiz3,4, Xing Liao1 &
Cristina Palet
2
Three dierent types of feedstocks and their biochars were used to remove Cr(III), Cd(II), Cu(II) and Pb(II)
ions from a mixture of multiple heavy metals. The eect of the initial concentration of heavy metals
in solution has been analysed, and kinetics modelling and a comparison of the adsorption capacity of
such materials have been performed to elucidate the possible adsorption mechanisms. The results show
that the adsorption capacity is dependent on the type of feedstock and on the pyrolysis conditions. The
adsorption capacity of the biomass types is ranked as follows: FO (from sewage sludge)>> LO > ZO
(both from agriculture biomass waste)>> CO (from wood biomass waste). Biochars, which are the
product of the pyrolysis of feedstocks, clearly improve the adsorption eciency in the case of those
derived from wood and agricultural biomasses. Complexation and cation exchange have been found
to be the two main adsorption mechanisms in systems containing multiple heavy metals, with cation
exchange being the most signicant. The pore structure of biomass/biochar cannot be neglected when
investigating the adsorption mechanism of each material. All the disposal biomasses presented here are
good alternatives for heavy metal removal from wastewaters.
Chromium, copper, cadmium and lead are the main heavy metal species in the wastewater industry1,2. Relatively
modest concentrations of Cr(III), Cd(II) and Pb(II) have toxic eects on the environment and humans. Cu(II) is
also a potential toxicant at high doses3. According to the World Health Organization (WHO), maximum concen-
tration limits of Cr(III), Cu(II), Cd(II) and Pb(II) (<0.55 mg/L for Cr(III), <0.017 mg/L for Cu(II), <0.01 mg/L
for Cd(II), and <0.065 mg/L for Pb) have been established for irrigation water47. To address heavy metal con-
tamination, biosorption is a promising technique for the removal of contaminants from wastewaters due to its
low cost and eco-friendly nature compared with other methods8,9. Biosorption processes are based on the use
of feedstocks or biomasses, which are usually wastes from agriculture, wood from forests, and sewage industrial
sludge1012.
On the other hand, biochar is a porous carbonaceous material obtained during the oxygen-limited pyrolysis
of biomass derived from a variety of feedstocks13. Biochar has proven to be eective in the removal of heavy metal
contaminants from wastewaters due to its specic properties, such as a large surface area, a porous structure,
surface-enriched functional groups and the presence of some mineral components14,15. e heavy metal adsorp-
tion eciency of biochars can vary widely depending on the types of feedstocks and the pyrolysis temperature16,17.
e most commonly used feedstock to produce biochar is agricultural waste, such as corn, rice, fruit peels, and
wood from forests. In addition, biochar derived from original materials, such as daily manure, wastewater sludges
and micro algae, has also been studied in the last decade1820. erefore, a large body of literature focuses on the
1Oil Crops Research Institute of Chinese Academy of Agricultural Sciences/Key Laboratory of Biology and Genetic
Improvement of Oil Crops of the Ministry of Agriculture, Wuhan, 430062, China. 2GTS-UAB Research Group,
Department of Chemistry, Facultat de Ciències, Universitat Autònoma de Barcelona, 08193, Cerdanyola del Vallès,
Catalunya, Spain. 3Centre for Research on Ecology and Forestry Applications (CREAF), 08193, Cerdanyola del Vallès,
Spain. 4Universitat Autònoma Barcelona, 08193, Cerdanyola del Vallès, Spain. JingJing Zhao and Xin-Jie Shen
contributed equally. Correspondence and requests for materials should be addressed to X.L. (email: liaox@oilcrops.
cn) or C.P. (email: cristina.palet@uab.cat)
Received: 15 February 2019
Accepted: 24 June 2019
Published: xx xx xxxx
OPEN
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use of biochar to remove heavy metals, such as Pb(II), Cu(II), Cr(III), Cd(II), Ni(II) and Zn(II), which are the
most studied metals from wastewaters21,22.
Biochar has good removal eciencies in single-metal systems but lower capacities in multiple-metal systems
due to the competition between the heavy metals present in wastewaters. Based on the literature, ve sorption
mechanisms have been proposed to explain biochar adsorption systems, which vary considerably with biochar
properties and the target metals. ese mechanisms include electrostatic interactions, cation exchange, compl-
exation with functional groups, metal precipitation and reduction of metal species9,14. However, few studies have
compared the sorption capacities of biochar derived from dierent types of feedstocks via dierent sorption
mechanisms in multiple-heavy-metal systems. erefore, it is necessary to study the sorption mechanisms of
heavy metals on biochar to improve the metal removal eciency and guide the application of biochar in the
future. Most importantly, biochar application can help solve the large worldwide problem of biomass disposal.
In this study, three types of feedstock from wood, agriculture and industrial sewage sludge wastes were used
to remove Cr(III), Cd(II), Cu(II) and Pb(II) ions from multiple-metal systems. Additionally, three biochars
were produced from poplar, corn and sewage sludge to determine the inuence of the pyrolysis process on the
adsorption systems. e objectives of this study are (1) to compare the adsorption capacities of the three dierent
types of feedstocks and derived biochars and (2) to evaluate the possible adsorption mechanisms of biochar in
multiple-heavy-metal systems.
Materials and Methods
Biomass and biochars. Biomass obtained from dierent sources, namely, poplar biomass (CO, from wood),
sewage sludge (FO, from industry sewage sludge wastes), corn (ZO) and Brassica napus (LO) biomasses (both
from agriculture wastes), were chosen to evaluate their adsorption capacities, and were all used to remove Cr(III),
Cd(II), Cu(II) and Pb(II) ions in multiple-metal systems. Additionally, three biochars (CL, ZL and FL) were
produced from poplar, corn and sewage sludge, respectively. ese biochars were thermally dried and pyro-
lysed at the Prat del Llobregat wastewater treatment plant (WWTP) (Barcelona, Spain), and all were produced
by slow pyrolysis processes. e temperature conditions and duration of the pyrolysis processes, together with
a description of the original biomasses, are listed in Table1. While poplar, corn, sewage sludge and their bio-
chars were kindly provided by the Centre for Research on Ecology and Forestry Applications (CREAF, Barcelona,
Spain), Brassica napus is produced in China and was kindly provided by the Oil Crops Research Institute, Chinese
Academy of Agricultural Sciences, Wuhan.
Chemical and reagents. All the chemicals were analytical grade. A 1,000 mg/L stock solution of a
multiple-element system was prepared by dissolving the required amounts of Cr(NO3)3.9H2O, Cu(NO3)2.3H2O,
Cd(NO3)2.4H2O and Pb(NO3)2 (all 99% from Panreac, Barcelona, Spain).
Characterization of adsorbents. Some physical and chemical properties of biochar including pH of mate-
rials, surface area, porosity, surface charge, functional groups, and mineral contents, play an important role in
explaining the process of sorption of metals. For this purpose, the morphologies of biomasses and their biochars
were analysed by scanning electron microscopy (SEM) at the Electron Microscopy Facilities of the Universitat
Autònoma de Barcelona (UAB, Catalunya, Spain). Attenuated total reectance Fourier transform infrared spec-
troscopy (ATR-FTIR, Tensor 27, Bruker, USA) was performed to identify the chemical functional groups present
on the adsorbents. FTIR data were obtained in the wavenumber range of 600 to 4000 cm1 with an average of
16 or 64 scans at 4.0 cm1 resolution at Servei d’Anàlisi de Química (UAB, Catalunya, Spain). A Flash 2000 C.E.
Elemental Analyzer (ermo Fisher Scientic, USA) was used to analyse the C and H components of the bio-
chars. A Flash EA 1112 Elemental Analyzer (ermo Fisher Scientic, USA) was used to analyse N. e O/C,
H/C and N/C ratios were calculated from the molar concentrations of the elements of interest, and each ratio was
calculated by dividing the total weight of the element by its molecular weight23. Brunauere Emmette-Teller tech-
nique (BET, TriStar II 3020, Micromeritics, USA) is performed to calculate the surface area and pore structure
of materials. e sample is heated at 200 °C for 4 hours under nitrogen vacuum condition for mesoporous meas-
urement. Zeta potential (Zen 3600, Malvern, USA) was performed to indentify the surface charge of materials.
Triplicate measurements were performed and each sample was measured 3 times to determine the zeta potential
values. Inductively coupled plasma optical emission spectrometry (ICP-OES) using a Varian 725-ES Radial ICP
Optical Emission Spectrometer (Varian Inc., USA) was used to analyse K, Ca, Mg and P. e pH of the materials
was measured as follows (as reported elsewhere): biochars were prepared in triplicate by adding water at a ratio of
Materials Description Pyrolysis (temperature) Time(min)
CO Populus nigra (Poplar) wood
CL Biochar of populus Slow pyrolysis 500–550 °C 15
ZO Carozo Zea mays
ZL Biochar of Carozo slow pyrolysis 400–500 °C 120
FO Sewage sludge
FL Biochar of Sewage sludge slow pyrolysis 500–550 °C 15
LO Brassica napus
Table 1. Feedstock biomasses and biochar preparation procedure description.
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1:10 (biochar/g:deionized water/mL) and vertically agitating for 24 h at a speed of 60 rpm. en, the suspensions
were vacuum ltered with Whatman 42 lter paper, and the pH was measured immediately23. e components of
the biochars and the pH were analysed at CREAF, and the equipments were all from Servei d’Anàlisi de Química
(UAB, Catalunya, Spain).
Batch adsorption experiments. Adsorption experiments were carried out at room temperature
(25 ± 1 °C). Multiple-metal solutions (containing Cr(III), Cu(II), Cd(II) and Pb(II)) were prepared from 1,000
ppm initial stock solutions of each metal, and the initial heavy metal concentration ranged from 5 to 100 ppm.
Batch experiments were performed by adding 25.00 mg of adsorbent in 5.00 mL tubes and then adding 2.50 mL
of heavy metal aqueous solutions, adjusted to pH 4.0. e tubes were then placed on a rotary mixer (CE 2000
ABT-4, SBS Instruments SA, Barcelona, Spain) and shaken at 25 rpm for 24 h. e two phases were separated by
decantation and ltered through 0.22 μm Millipore lters (Millex-GS, Millipore). e concentrations of heavy
metals in the supernatant phase were analysed by ICP-mass spectrometry (MS) (XSERIES 2 ICP-MS, ermo
Scientic, USA). e adsorption of the selected heavy metals by the adsorbents was expressed as the adsorption
percentage calculated by using Eq. (1). Furthermore, the capacity of the adsorbent was calculated by using Eq. (2):
=
×Adsorption
CC
C
%
()
100
(1)
e0
0
q
CC V
m
() (2)
ee0
=
−×
where qe (mg/g) is the capacity of the adsorbent, expressed as the amount of heavy metal per adsorbent mass unit
at equilibrium; V (L) is the volume of the heavy metal solution; C0 and Ce are the initial and equilibrium heavy
metal concentrations in solution (both in mg/L), respectively; and m (g) is the dry weight of the adsorbent. To
study the adsorption mechanism, it is more convenient to convert qe into mmol/g. All the results are expressed as
the mean value of duplicate measurements.
Results and Discussion
SEM characterization analysis. SEM was used to study the morphological structure of the biomass and
biochar. As shown in Fig.1, biochars CL and ZL show more pore structures than CO and ZO, respectively.
Furthermore, FL did not have much change in porosity compared with the original biomass FO (see Fig.1).
Additionally, LO has been shown to have a higher heavy metal removal eciency than CO and ZO (see Fig.2),
which can be explained by the high porosity and the large pore size of LO. is behaviour clearly shows that LO
has a pore structure similar to that of CL and ZL, even before pyrolysis (see Fig.1). us, the results presented
here demonstrate that pore structure is a key factor that can inuence the sorption of heavy metals onto biomass,
and Bagreev et al. reported similar results2426.
ATR-FTIR characterization analysis. ATR-FTIR analysis was carried out to identify the functional groups
present in the dierent adsorbents that might be involved in the sorption process. FTIR spectra of biomass and
their biochars are shown in Fig.3(a–d). e wavenumbers and approximate assignments of the vibrational modes
for the FTIR spectra are listed in Table2. e peaks at 3200–3270 cm1 and 1780–1710 cm1 correspond to the
O-H and C=O stretching vibrations, respectively, which conrms the presence of carboxyl groups on the adsor-
bents27. Carboxyl acid groups are very useful for the adsorption of heavy metal ions and can be found in most of
the adsorbents studied here (ZO, ZL, CO, CL, LO), except for FO (sewage sludge) and the corresponding biochar
FL (see Fig.3c). ese dierences can be explained by the compositions of ZO, CO, and LO, which are cellulose
and lignin-based biomasses that contain carboxyl groups. However, FO and FL are from industry sewage sludge,
and their main components are carbon, hydrogen, oxygen and nitrogen, which are suitable for the production of
activated carbon28.
Furthermore, a decrease in the intensity of the peaks corresponding to carboxyl (–COOH) and hydroxyl
(–OH) groups is observed in the FTIR spectra aer pyrolysis, probably due to the loss of functional groups in
the lignocellulosic materials with increasing temperature. e decrease in the H/C and O/C atomic ratios for
biochars (Table3) conrms this hypothesis. On the other hand, a reduction in the amounts of negative surface
charges (related to functional groups such as –COOH, –COH and –OH) will increase the pH of the biochar and,
thus, the metal adsorption eciency of such materials29. In this sense, ZL material has higher basicity than CL
(measured as explained in the Characterization section, which pH values are shown in Table3) that can explain
also its higher heavy metals adsorption. e adsorption results for these biochars were as follows: LO > ZL > CL
(see Fig.2). e nding that LO has a higher adsorption eciency than ZL and CL can be explained by the higher
amount of carboxyl functional groups on the surface of LO that are available to react with heavy metals (see
Fig.3d).
Furthermore, BET analysis show that for all biochar systems they have higher surface area values than the
corresponding biomass. ese results are collected in Table3. As expected, large surface increase adsorption.
e zeta potential values at pH 4.0 are showed in Table3. It can be seen that the zeta potential values of bio-
mass (CO, ZO) were more close to zero value than biochars (CL, ZL), which means less negatively charged than
biochar. It means that more negative charge on the surface of biochar leading to more chance for electrostatic
interactions with heavy metal. e negative charge values at pH 4 ranked as: ZL > FO > CL > FL > CO > LO >
ZO. erefore, the surface charge increased aer the pyrolysis process except the case of FL. is behavior can
explain the increase on the heavy metal adsorption when using biochar systems.
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Mineral composition analysis. Based on the literature, mineral composition, including potassium (K), cal-
cium (Ca), magnesium (Mg) and phosphorus (P) in biomass and biochar, is also responsible for metal adsorption
from aqueous solutions14,30. As seen from the results of the mineral composition analysis of all adsorbents under
study (collected in Table3), the mineral concentrations of FO and FL (from sewage sludge) are much higher
than those of agriculture waste (ZO, LO) and wood biomass (CO). Furthermore, the concentrations of mineral
components (K, Ca, Mg, P) increased aer pyrolysis (see Table3). e pre-concentration of minerals on biochar
is mainly due to the formation of biomass ash during pyrolysis. FO and FL have higher mineral concentrations
that can provide more opportunities to adsorb heavy metals from water, which can explain the adsorption results
(FO > LO > ZO > CO), as shown in Fig.2. erefore, this behaviour illustrates the importance of mineral com-
position in the adsorption process.
Figure 1. Images of biomasses and biochars. (a) Images of raw CO, CL, ZO, ZL, FO, FL and LO. (b) SEM
images of CO, CL, ZO, ZL, FO, FL and LO.
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e LO biomass yielded promising adsorption results without any modication or pyrolysis process, probably
as a result of both its higher porosity level and the important amount of functional groups on its surface, such as
carboxyl and hydroxyl groups. erefore, the authors believe that this material should be studied in more depth
in the future.
Next, the adsorption properties of such materials, namely, sewage sludge (FO), wood waste (CO), and agri-
cultural waste (ZO), together with their biochar materials (FL, ZL and CL) will be presented. Followed with the
kinetic modelling and the inuence of the initial heavy metal concentrations in solution.
Comparison of biosorbent adsorption properties. As indicated in the previous section, four feed-
stocks and three biochars were used to remove Cr(III), Cd(II), Cu(II) and Pb(II) ions in multiple-metal systems:
biomass CO (poplar from wood), FO (sewage sludge from solid industrial waste), ZO (corn from agriculture
Figure 2. Adsorption of Cr(III), Cd(II), Cu(II), and Pb(II) ions by CO, CL, ZO, ZL, FO, FL and LO in the
multiple-metal aqueous system. Initial metal concentration of 0.18 mmol of each metal, contact time of 24 h,
initial pH of 4.0, and 25 mg of adsorbent in 2.5 ml of initial solution.
Figure 3. ATR-FTIR spectra of CO and CL (a), ZO and ZL (b), FO and FL (c), and LO (d).
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waste), and LO (Brassica napus from agriculture waste). Additionally, biochars obtained by the pyrolysis of
CO, FO and ZO (CL, FL and ZL, respectively) were evaluated as heavy metal adsorbents. ese seven sorbents
show dierent biosorption capacities for the dierent metal ions, as shown in Fig.2. In general, for all metals,
biochars have better sorption capacity than the original biomass, which can be explained by surface changes
during the pyrolysis process, such as changes in porosity, functional groups and mineral content. Based on the
literature, high pyrolysis temperatures lead to increased porosity and surface area compared with the original
biomaterial (as shown in Fig.1 and Table3). High porosity and large surface areas can increase the adsorption
of metals31. High temperature also increases the concentration of minerals (K, Ca, Mg and P) on the surface
of sorbents that can be used for ion exchange with heavy metals3133. Minerals from biomass are not burned,
so the pyrolysis process acts as a mineral pre-concentration step. e adsorption percentages according to the
type of feedstock were ranked as follows: sewage sludge (FO)>> agriculture waste biomass (LO) > (ZO)>>
wood biomass (CO). is ranking can be explained by the dierent mineral compositions and functional
groups present, which is conrmed by the measured adsorption capacity (see Table3, Fig.3 and part of the
eect of the initial concentration).
Eect of contact time. e eect of the contact time between sorbents (CO, CL, ZO, ZL, FO, FL) and
heavy metals in multiple-metal systems (Cr(III), Cu(II), Cd(II), Pb(II)) was studied. For that purpose, adsorption
experiments were performed (as indicated in the experimental section) for dierent times (5, 15, 30, 45, 60, 120,
240, 360, 540, 1440 and 2880 minutes) for each adsorbent (Fig.4). In general, biochars from ZO (from agricul-
ture) wastes were more eective than those from sewage sludge and wood biomass. Adsorption equilibrium was
reached at dierent times for each biomass or biochar. ZO and ZL were both eective at adsorption of all metal
ions and reached equilibrium in 5 minutes. In the case of CO, the equilibrium time diered as a function of the
heavy metal, so adsorption equilibrium was reached in 5 minutes for Pb(II) and Cd(II), in 1 h for Cu(II), and in
24 h for Cr(III). Additionally, CL reached equilibrium slowly compared with CO, requiring approximately 8 h for
Cr(III), Cu(II) and Pb(II) and 24 h for Cd(II). In contrast, biochar from sewage sludge (FL) was less eective than
FO (especially for Cd(II)). FL also needed a longer time than FO to reach adsorption equilibrium (approximately
6 h), while FO adsorption of all metals took only 5 minutes. us, 24 h was chosen as the optimal contact time for
further adsorption experiments.
Wave numbers (cm1) Assignments
Sorbents
CO CL ZO ZL FO FL LO
3200–3700 O-H stretching 3346 3337 3303 3331
2700–3000 C-Hn stretching 2884 2912 2912
1780–1710 Carboxylic Acid
C=O stretching 1737 1698 1723 1723 1737
1750–1630 Ketone, Ester, Amide
C=O Stretching 1688 1672 1606 1644 1640
1644 1686
1421 1599
1000–1200 C=O/C-O-C 1229 1236 1242 1236
750–870 C-N/R-O-C/R-O-CH3
stretching
aromatic C-H 1029 1029 1029 1008 1008 1022
868
Table 2. FTIR spectral band assignments for CO, CL, ZO, ZL, FO, FL and LO before use.
CL wood ZL corn FL sewage sludge
Components Cellulose (50%),
Hemicellulose (25–35%),
Lignin (15–25%)
Cellulose/glucan (37%),
Xylan (21%),
Lignin (18%)
Carbon (50–70%),
Hydrogen (6–7.3%),
Oxygen (21–24%),
Nitrogen (15–18%)
Temperature 500–550 °C 400–500 °C 500–550 °C
pH 8.2 10.3 8.7 (FO 8.0)
BET surface area m²/g 15.0 (CO 10.8) 22.3 (ZO 17.1) 31.4 (FO 18.5)
Zeta potential (pH 4)
mV 13.6 (CO 2.12) 32.1 (ZO 0.857) 5.24 (FO 17.1)
H/C 0.026 0.29 (ZO 1.7) 0.054 (FO 0.17)
O/C 0.15 0.090 (ZO 0.74) 0.22 (FO 2.5)
K g/kg 6.6 23 (ZO 9.4) 9.1 (FO 4.1)
Ca g/kg 9.6 2.6 (ZO 0.22) 89 (FO 41)
Mg g/kg 1.3 1.2 (ZO 0.15) 12 (FO 5.5)
P g/kg 2.0 1.8 (ZO 0.20) 51 (FO 24)
Table 3. Physicochemical properties of CL, ZL and FL. H/C and O/C values are the molar ratio.
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To understand the dierent adsorption behaviours in multiple-heavy-metal systems, kinetic analysis was per-
formed to nd a model that explains the obtained results and to obtain information about the mechanisms of
heavy metal adsorption onto biomass and biochar systems.
Kinetic modelling. Two dierent kinetic models, the pseudo-rst-order (PFO) and pseudo-second-order
(PSO) models, have been widely used to describe adsorption. e PFO and PSO models assume that the rate of
metals adsorbed on the surface of sorbents is proportional to the number of unoccupied sites; PFO kinetics is
controlled by the physical process, and PSO kinetics is controlled by chemical processes, including valence forces
sharing or exchanging electrons between the adsorbent and adsorbate. e PFO and PSO mathematic model
expressions are given in Eq. (3) and Eq. (4):
Figure 4. Adsorption percentage of Cr(III), Cu(II), Cd(II), and Pb(II) by CO (a), CL (b), ZO (c), ZL (d), FO
(e) and FL (f) from the multiple-metal system at dierent contact times. Initial metal ions concentration of
0.18 mmol/L of metal, pH 4.0, and 25 mg of adsorbent.
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−=
.
qq q
k
tlog( )log()
2303 (3)
et e1
qkq tq
1111
(4)
tee
22
=
×+
where qt and qe are the capacity at time t and at equilibrium, respectively (and expressed as mmol/g), and k1 and
k2 are the rate constants. In most cases, the PFO equation is linear only over approximately the rst 30 minutes;
therefore, it is appropriate for the initial contact time but not for the whole range34. Kinetics modelling analysis
showed that the adsorption process did not t well with the PFO model but t well with the PSO model for all the
adsorbents. is result means that the adsorption of heavy metals on the surface of the adsorbents is a chemical
adsorption process, such as valence forces sharing or exchanging electrons between the adsorbent and adsorbate.
e relative constants found by applying the model are listed in Table4 only for Pb in the multiple-metal system.
Higher capacities of Pb(II) have been found except in the case of CO.
Eect of initial concentration. Five multiple-metal solutions (5 ppm, 25 ppm, 50 ppm, 75 ppm and 100
ppm) were prepared from 1,000 ppm stock solutions of each heavy metal. e initial concentration study provides
a signicant understanding of the competition between the four heavy metals during the adsorption process.
e adsorption capacity of all the adsorbents for the four metals is shown in Fig.5, and the adsorption capacity
for Pb(II) in multiple-metal systems is listed in Table5. As shown in Fig.5, the adsorption capacity of heavy
metals (Cr(III), Cu(II), Cd(II), Pb(II)) from dierent types of feedstocks is ranked as follows: FO (from sewage
sludge) > CO (from wood) and ZO (from agriculture). All adsorbents have the same priority for heavy metals,
such as Cr(III), Cu(II) and Pb(II) metal ions, and the adsorption of Cd(II) was much lower than that of the other
metals, probably due to the competition between the heavy metals.
e ranking of the adsorption of biochars from dierent types of feedstocks was ZL (from agriculture) > FL
(from sewage sludge) > CL (from wood), which could be explained by the dierent mineral compositions of the
adsorbents. High mineral amounts provide more possibilities for the exchange of heavy metals from the solution
(because the concentration of minerals is increased aer pyrolysis), which could increase the adsorption capacity.
As shown in Table3, the concentrations of some mineral components of FL (Ca 89.1 g/kg, P 51.2 g/kg) are much
higher than those of ZL (Ca 2.55 g/kg, P 1.83 g/kg) and CL (Ca 9.60 g/kg, P 2.00 g/kg), but potassium contents
are the exception (FL (K 9.10 g/kg), ZL (K 23.4 g/kg), and CL (K 6.60 g/kg)). is behaviour can be related to the
adsorption capacity values of these biochars (see Table5).
FL has a higher mineral content than FO; however, a slight decrease in the adsorption capacity for Cr(III) and
Pb(II) and a much higher decrease in the adsorption capacity for Cd(II) were observed for FL. e slight decrease
in FL capacity could be explained by the loss of functional groups on the surface of the biomass. As shown in
Fig.3, most of the functional groups of FL were lost during the pyrolysis process, which was conrmed by the
decrease in the H/C and C/H ratios (Table3).
Possible adsorption mechanism. In previous studies reported in the literature, ve mechanisms have
been proposed to govern metal sorption by biochar from aqueous solutions, namely, complexation, cation
exchange, precipitation, electrostatic interactions, and chemical reduction9,14. However, the role that each mecha-
nism plays for each metal varies considerably depending on the target metals and adsorbents. Fei et al. described
the molecular-level adsorption of Pb(II) and Cu(II) to peat biomass mainly through carboxyl groups (–COOH)35.
Whereas both electrostatic interactions and complexation with biochar surfaces are responsible for Cr adsorption
and reduction36,37.
Until now, few studies have focused on the comparison of dierent types of feedstocks for the removal of heavy
metals from multiple-metal aqueous systems (more similar to real water situations). According to the results of
Lu et al., the adsorption of Pb(II) by sewage sludge-derived biochar mainly occurred through proton-active car-
boxyl (–COOH) and hydroxyl (–OH) functional groups on the biochar surface, as well as coprecipitation or com-
plexation on the mineral surfaces38. Aer comparison of the characterization and evaluation of the adsorption
capacity of dierent types of feedstocks, a new perspective has been found to explain the adsorption mechanisms
Pb(II)/Multi-metal
Pseudo-second-order model
k2(g/μmol/min) q2 (μmol/g) R2
CO 0.98 9.20 1.000
CL 0.26 14.7 0.998
ZO 12.0 12.7 1.000
ZL 0.26 16.7 0.999
FO 5.68 16.7 0.999
FL 0.66 16.5 0.999
Table 4. Adsorption kinetic constants for the adsorption of Pb(II) by CO, CL, ZO, ZL, FO and FL in the
multiple-metal system.
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Figure 5. Eect of the initial concentration of heavy metals on their adsorption by CO (a), CL (b), ZO (c), ZL
(d), FO (e) and FL (f) in multiple-metal systems. Experimental conditions were T = 25 ± 1 °C, pH 4.0, 25 mg of
adsorbent, 2.5 mL of metals solution and stirring for 24 hours.
qe(μmol/g) CO CL ZO ZL FO FL
Cr(III) 15.9 27.9 35.7 109 114 98.2
Cu(II) 9.10 31.7 13.4 128 88.3 99.7
Cd(II) 2.32 2.39 3.02 15.5 54.7 14.3
Pb(II) 6.75 17.5 8.39 35.9 35.1 33.7
Table 5. Adsorption capacity of biomass and biochar in multiple-heavy-metal systems.
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onto biomass and biochar adsorbents. We demonstrate that complexation and cation exchange are the two main
adsorption mechanisms, with the inuence of cation exchange being larger than that of complexation in the
present cases.
Furthermore, an increase in mineral concentrations was found in biochars aer the pyrolysis of the corre-
sponding biomasses, which could explain the increase in the adsorption capacity to heavy metals of the biochars
(CL and ZL, respectively)38. FO and FL (which have higher mineral concentrations than the other adsorbents
presented here and lack carboxyl groups on their surfaces) have higher adsorption capacities than CO and ZO
(which have lower mineral concentrations and more carboxyl groups). e adsorption capacities of FO and FL
are similar, which may be due to the similar porosity of both materials before and aer pyrolysis. Although there
is an increase in the mineral concentrations on the surface of FL (which will increase its adsorption capacity), the
loss of carboxyl groups as a function of temperature during pyrolysis can globally reduce the adsorption capacity
of FL for heavy metals14. erefore, FO and FL have similar adsorption capacities.
In summary, heavy metals are adsorbed on the surface of biomass/biochar via exchange mainly with Ca, K,
and Mg but also with protons from carboxyl and hydroxyl groups. In addition, if these latter functional groups are
present at high amounts on the bioadsorbent surface, they can also complex heavy metals from the aqueous solu-
tions (see Fig.6). Finally, it is important to note that the amount of either mineral or carboxyl groups can dier
depending on the composition of the original biomass and, in the case of biochars, as a function of the pyrolysis
conditions employed39,40.
Conclusions
Seven biosorbents have been used successfully for the removal of Cr(III), Cu(II), Cd(II) and Pb(II) from
multiple-metal aqueous systems. e biochars produced from wood and agriculture wastes have higher adsorp-
tion capacities than the initial biomasses do. is nding can be explained by an increase in porosity and a
pre-concentration of mineral components during the pyrolysis process. Complexation and cation exchange prob-
ablyare the two main adsorption mechanisms in multiple-heavy-metal systems, and these mechanisms are inu-
enced by the kind of feedstock and its mineral composition and by the pyrolysis treatment, being more eective
for agriculture waste than for wood biomass. e sludge can be used directly to remove heavy metals without
pyrolysis pretreatment.
In summary, these disposal biomasses can be used to remove heavy metals from multiple-metal aqueous
systems because of their low cost, eco-friendliness and availability. erefore, biomass could be an interesting
alternative to synthetic materials for heavy metal removal. Biochar can also be an alternative, even though it
requires biomass pretreatment.
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Acknowledgements
is research was supported by grants from the National Key Research and Development Program of China (No.
2018YFD0200904), the Agricultural Science and Technology Innovation Program of China (No. CAAS-ASTIP-
2013-OCRI), and Spanish research projects (Nos. CTM2015-65414-C2-1-R and AGL2015-70393-R). Also, China
Scholarship Council (No. 201509110114). All the authors are grateful to the UAB Microscopy Service (Servei de
Microscòpia Electrònica from UAB, Catalunya, Spain) for the SEM analysis; to M. Resina who helped perform the
analysis of heavy metals by ICP-MS.
Author Contributions
J.J. Zhao and X.J. Shen carried out most of the experiments and wrote the main manuscript text. X. Domene and
J.M. Alcañiz provided the biomass/biochar and participated in data collection. X. Liao and C. Palet contributed
valuable discussions and helped revise the manuscript. All authors reviewed and approved the nal manuscript
for publication.
Additional Information
Competing Interests: e authors declare no competing interests.
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... Differences in mineral content and composition were observed. Probably potassium, calcium, magnesium and phosphorus are involved in the adsorption of heavy metals through ion exchange mechanism, as reported previously [46]. Biomasses of plant origin are mainly composed of lignin and cellulose, as mentioned above, together with hemicellulose, low molecular weight compounds, lipids, proteins, starch, water, etc. [7]. ...
... However, the differences between the results of adsorption percentage (A%) are not only due to the characteristics of the biomass adsorbent but were also due to the concentration of contaminant, dosage of the adsorbent, pH of the solution, temperature, contact time, among others, that are factors that can affect the adsorption process [7]. Some of these factors were evaluated by the authors in previous work on the adsorption of heavy metals on adsorbent materials of plant origin [26,46,54,55]. ...
... However, the adsorption of heavy metals in pine sawdust was lower for the multi-metal aqueous solution than for each metal separately, being the nickel ion the most affected by the competition with the other two heavy metals for the adsorption sites of the adsorbent, as expected (as Ni was the less adsorbed, as seen in Figure 9). Zhao et al. [46], also reported a lower performance of sawdust as an adsorbent when comparing the results of A% obtained for the adsorption of a mixture consisting of Cr(III), Cd(II), Cu(II) and Pb(II), on poplar sawdust and two other agricultural residues. ...
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... This finding is aligned with the ratio of H/C and O/C from the elemental analysis illustrated in Table 7. These two ratios decrease by increasing the temperature [28], which confirms the disappearance of light components. Figure 9 illustrates that complex compounds such as aromatics and aliphatics can be seen even at high temperatures (900 • C). ...
... the ratio of H/C and O/C from the elemental analysis illustrated in Table 7. These two ratios decrease by increasing the temperature [28], which confirms the disappearance of light components. Figure 9 illustrates that complex compounds such as aromatics and aliphatics can be seen even at high temperatures (900 °C). ...
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... Various agricultural waste, woods, sewage sludge, and municipal solid wastes are commonly utilized as biochar feedstock materials (Oleszczuk et al. 2016). The possibility of biochar technology to reduce climate change, eliminate pollutants and promote plant growth, soil health, and fertility has piqued experts' curiosity (Zhao et al. 2019). Basically, BC is chemically enriched with carbon and can reduce the emission of greenhouse gases by sequestering carbon Rawat et al. 2019). ...
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Environmental Toxicology is the third volume of a three-volume set on molecular, clinical and environmental toxicology that offers a comprehensive and in-depth response to the increasing importance and abundance of chemicals of daily life. By providing intriguing insights far down to the molecular level, this three-volume work covers the entire range of modern toxicology with special emphasis on recent developments and achievements. It is written for students and professionals in medicine, science, public health or engineering who are demanding reliable information on toxic or potentially harmful agents and their adverse effects on the human body.
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In recent years, adsorption science and technology for water and wastewater treatment has attracted substantial attention from the scientific community. However, the number of publications containing inconsistent concepts is increasing. Many publications either reiterate previously discussed mistakes or create new mistakes. The inconsistencies are reflected by the increasing publication of certain types of article in this field, including “short communications”, “discussions”, “critical reviews”, “comments”, “letters to the editor”, and “correspondence (comment/rebuttal)”. This article aims to discuss (1) the inaccurate use of technical terms, (2) the problem associated with quantities for measuring adsorption performance, (3) the important roles of the adsorbate and adsorbent pKa, (4) mistakes related to the study of adsorption kinetics, isotherms, and thermodynamics, (5) several problems related to adsorption mechanisms, (6) inconsistent data points in experimental data and model fitting, (7) mistakes in measuring the specific surface area of an adsorbent, and (8) other mistakes found in the literature. Furthermore, correct expressions and original citations of the relevant models (i.e., adsorption kinetics and isotherms) are provided. The authors hope that this work will be helpful for readers, researchers, reviewers, and editors who are interested in the field of adsorption studies.