Wetting properties of poultry litter and derived hydrochar

Abstract

Detailed assessment of hydrochar wetting properties, which could provide an essential understanding of underlying mechanisms during its application to soils, is lacking. We characterized hydrochar produced from hydrothermal carbonization (HTC) performed on poultry litter at various temperatures and for different times in terms of hydrophobicity and surface free energy properties. Hydrochar was more hydrophobic than untreated poultry litter, and its hydrophobicity increased with increasing HTC temperature (contact angle > 130°). These changes were correlated with degradation of hemicellulose and cellulose. Hydrochar produced at 250°C contained mostly lignin and displayed high hydrophobicity over both prolonged wetting periods and repeated wetting cycles. Surface free energy was calculated using the Owens–Wendt–Rabel–Kaelble and Wu models, with the latter resulting in lower standard errors. The surface free energy decreased as HTC treatment severity increased from 26 mJ/m² in the poultry litter to 8 mJ/m² after treatment at 250°C for 60 min. The dispersive component fraction of the surface free energy increased with increasing treatment severity. This study demonstrated that changes in the physical composition of hydrochar due to increased treatment severity increase its hydrophobicity and decrease its surface free energy. Moreover, due to non-persistent hydrophobicity, hydrochar produced at temperatures lower than 250°C will likely not show adverse effects on soils.

Full text

RESEARCH ARTICLE
Wetting properties of poultry litter and
derived hydrochar
Vivian Mau
1
, Gilboa AryeID
2
*, Amit Gross
1
1Department of Environmental Hydrology and Microbiology, Zuckerberg Institute for Water Research, Ben
Gurion University of the Negev, Campus Sde Boqer, Midreshet Ben Gurion, Israel, 2French Associates
Institute for Agriculture and Biotechnology of Drylands, Jacob Blaustein Institutes for Desert Research, Ben
Gurion University of the Negev, Campus Sde Boqer, Midreshet Ben Gurion, Israel
*aryeg@bgu.ac.il
Abstract
Detailed assessment of hydrochar wetting properties, which could provide an essential
understanding of underlying mechanisms during its application to soils, is lacking. We char-
acterized hydrochar produced from hydrothermal carbonization (HTC) performed on poultry
litter at various temperatures and for different times in terms of hydrophobicity and surface
free energy properties. Hydrochar was more hydrophobic than untreated poultry litter, and
its hydrophobicity increased with increasing HTC temperature (contact angle >130˚). These
changes were correlated with degradation of hemicellulose and cellulose. Hydrochar pro-
duced at 250˚C contained mostly lignin and displayed high hydrophobicity over both pro-
longed wetting periods and repeated wetting cycles. Surface free energy was calculated
using the Owens–Wendt–Rabel–Kaelble and Wu models, with the latter resulting in lower
standard errors. The surface free energy decreased as HTC treatment severity increased
from 26 mJ/m
2
in the poultry litter to 8 mJ/m
2
after treatment at 250˚C for 60 min. The disper-
sive component fraction of the surface free energy increased with increasing treatment
severity. This study demonstrated that changes in the physical composition of hydrochar
due to increased treatment severity increase its hydrophobicity and decrease its surface
free energy. Moreover, due to non-persistent hydrophobicity, hydrochar produced at tem-
peratures lower than 250˚C will likely not show adverse effects on soils.
Introduction
Maintenance of soil fertility has been an issue in agriculture since its inception [1]. Continuous
land cultivation leading to nutrient depletion, and soil erosion resulting in loss of soil organic
matter are extant problems to this day [1,2]. To solve them, soil amendments such as animal
manure and compost are commonly used. However, these can lead to pollution of surface and
groundwater, and spread of pathogens, heavy metals, and pharmaceuticals [3]. Recently, these
materials have begun to be dried and treated by pyrolysis to improve their properties and
reduce the presence of pathogens [4]. Biochar, the product of pyrolysis, has been widely inves-
tigated in the last 20 years as a potential soil amendment, and has been found to improve soil
PLOS ONE | https://doi.org/10.1371/journal.pone.0206299 October 26, 2018 1 / 15
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OPEN ACCESS
Citation: Mau V, Arye G, Gross A (2018) Wetting
properties of poultry litter and derived hydrochar.
PLoS ONE 13(10): e0206299. https://doi.org/
10.1371/journal.pone.0206299
Editor: Jorge Paz-Ferreiro, RMIT University,
AUSTRALIA
Received: June 20, 2018
Accepted: October 10, 2018
Published: October 26, 2018
Copyright: ©2018 Mau et al. This is an open
access article distributed under the terms of the
Creative Commons Attribution License, which
permits unrestricted use, distribution, and
reproduction in any medium, provided the original
author and source are credited.
Data Availability Statement: All relevant data are
within the paper.
Funding: This study was funded by the Israeli
Ministry of Environmental Protection (132-2-1,
http://www.sviva.gov.il), and the Rosenzweig–
Coopersmith Foundation. Vivian Mau received
financial support from the Israeli Ministry of
National Infrastructures, Energy and Water
Resources (216-01-044, http://archive.energy.gov.
il), the Israeli Ministry of Science and Technology
(3-14568, https://www.gov.il/en/Departments/
ministry_of_science_and_technology), the Rieger
Foundation, and the Zuckerberg Scholarship Fund

fertility in some cases [5,6]. A meta-analysis on the effect of biochar application to soils on
crop productivity determined that positive effects are most noticeable in soils with acidic to
neutral pH and coarse to medium texture, at application rates of 100 t/ha [7]. Crop yield
increase is mainly due to improved water-holding capacity and nutrient availability. Moreover,
of the studied feedstocks, poultry litter demonstrated the greatest improvement in crop pro-
ductivity [7].
Today, another possible treatment is being considered to produce soil amendments. Hydro-
thermal carbonization (HTC) is a fairly new technology that converts wet organic matter into
a char-like compound termed hydrochar. Common feedstock for this technology are animal
manures, sewage sludge and lignocellulosic agricultural waste [8–11]. Typically, HTC takes
place in a temperature range of 180–250˚C, under autogenous pressure, and reaction times
varying from minutes to several hours [12]. Anoxic or low oxygen concentrations are required
to retain the carbon in its solid form. Treating wet organic matter by HTC is intrinsically more
energy efficient than other thermal processes, such as pyrolysis, because a specific phase
change is avoided by heating at pressures greater than saturated pressure [13,14]. Therefore, it
has been suggested for the treatment of animal manure, which has a naturally high moisture
content. Traditionally, hydrochar is considered an energy source [14,15]; however, hydrochar
has also been proposed as a potential soil amendment due to some similarities with the proper-
ties of biochar [8], but only a handful of studies have explored this possibility [10,16,17]. More-
over, a detailed comparison reveals many physico-chemical differences between the two
products [15]. For example, hydrochar possesses an acidic pH, high N concentration, and ali-
phatic structure, whereas biochar has a basic pH, low N concentration, contains mostly aro-
matic structures, and has high thermal stability [9,18,19]. Consequently, the effect of
hydrochar on soil properties needs to be thoroughly investigated.
A characterization of hydrochar wetting properties is essential to understanding the under-
lying mechanisms during its application to soil. Hydrochar has been found to be more hydro-
phobic than the raw material from which it is produced, based on analysis of functional
groups on its surface, and the ease of separation of hydrochar from the liquid phase [11,12,20].
Some studies have indirectly assessed hydrochar hydrophobicity by measuring equilibrium
moisture contents [21–24]. However, to the best of our knowledge, only two studies have
directly investigated hydrochar hydrophobicity [16,20]. Results from those studies suggested
that temperature and time of digestion, or in other words, the severity of the HTC, affect the
hydrophobic properties of the hydrochar, although the effect may be substrate-dependent. For
example, an increase in temperature from 200 to 250˚C resulted in increased hydrophobicity
for hydrochar produced from corn digestate but not for hydrochar produced from woodchips
[16]. Knowledge of hydrochar wetting properties is still limited and warrants special
consideration.
This study provides a detailed assessment of the wetting properties of hydrochar produced
from poultry litter. Poultry litter, a mix of poultry manure, bedding materials, feathers, and
spilled feed, was chosen because its direct application to fields can result in spread of patho-
gens, and contamination of water bodies [25], and stringent regulations such as the European
Union nitrates directive result in the need to treat it [26]. Moreover, produced quantities are
on the rise as poultry production is growing at a global annual rate of 1.6% [27], generating
625–938 million tons of litter [28,29]. The natural moisture content of poultry litter indicates
that treatment by HTC would be more energetically efficient than pyrolysis [14]. Finally, posi-
tive experiences with application of poultry litter-derived biochar to soils [7,25] indicates that
this feedstock has potential as a soil amendment. By characterizing hydrochar wetting proper-
ties through hydrophobicity and surface free energy components, it is possible to elucidate its
behavior as a potential soil amendment. Specifically, this study had three main goals: (i) to
Hydrophobicity of poultry litter hydrochar
PLOS ONE | https://doi.org/10.1371/journal.pone.0206299 October 26, 2018 2 / 15
for Students at the Zuckerberg Institute for Water
Research (http://in.bgu.ac.il/en/bidr/ziwr/Pages/
default.aspx). The funders had no role in study
design, data collection and analysis, decision to
publish, or preparation of the manuscript.
Competing interests: The authors have declared
that no competing interests exist.

characterize the effect of HTC treatment temperature and time on poultry litter-derived
hydrochar hydrophobicity; (ii) to investigate the persistence of its hydrophobicity during wet-
ting cycles; and (iii) to determine the surface free energy components of the hydrochar that
govern the hydrophobic effects.
Materials and methods
Hydrochar production
Hydrochar was produced from poultry litter originating from a chicken farm in the Negev
region of Israel following the procedure outlined by Mau et al. [30]. Briefly, the poultry litter
consisted of chicken feces, wood chips, spilled feed, and feathers. Clean wood chips were
placed in the house when a new flock of chicks was introduced. When the chickens achieved
maturity at 22 weeks, the farm staff removed the poultry litter from the house. With permis-
sion from the farm owner, the researchers obtained the poultry litter for experiments in the
laboratory. It was then dried at 105˚C for 24 h, and aggregates were crushed using a mortar
and pestle and then sieved through a no. 8 mesh. The dried and homogenized feedstock was
stored in a desiccator prior to HTC experiments. The dried poultry litter was mixed with dou-
bled-distilled water at a solid-to-water ratio of 1:3. The poultry litter sludge was placed in HTC
reactors, one of which had a temperature probe to provide a representative measure of the
temperature inside the reactors. Reactors were heated by immersion in Paratherm HR heat-
transfer fluid (Conshohocken, PA) that was preheated to temperatures of 180˚C, 220˚C and
250˚C. Once the reactors reached the set temperatures, carbonization was run for 5, 30 and 60
min. When the desired reaction time had elapsed, the reaction was quenched by placing the
reactors in an ice bath. All combinations of temperature and reaction time were conducted in
triplicate using the same poultry litter to ensure repeatability. All subsequent analyses were
performed separately for each sample or replicate. The generated hydrochar was separated
from the liquid phase and oven-dried at 105˚C for 24 h. The dried hydrochar was ground
using a mortar and pestle and then sieved. The <150 μm fraction was used in the study.
Fiber analysis was conducted to determine the poultry litter and hydrochar contents of
hemicellulose, cellulose and lignin by the van Soest method of neutral detergent fiber, acid
detergent fiber and acid detergent liquid (aNDF–ADF–ADL) [31]. This was performed with a
fiber analyzer (model A200, Ankom Technology, Macedon, NY) following the manufacturer’s
procedures [32–34] at the Forage and Feed laboratory (Gedera, Israel). Due to the need for a
large amount of sample for these analyses, they were performed in duplicates from a composite
sample formed by mixing the triplicate samples of each treatment.
Contact-angle measurements
Contact angle (CA), a measure of hydrophobicity, was estimated by the sessile drop method
and the Wilhelmy plate method. The first method measured the stability of hydrochar wetta-
bility over time [35], and the initial CA was used for the calculation of surface free energy com-
ponents. The Wilhelmy plate method [36] was used to estimate the advancing and receding
dynamic CA under multiple wetting cycles, thus assessing the persistence of hydrophobicity.
Applying both methods provides a more complete evaluation of the hydrophobic properties of
the material and its behavior over time.
Sessile drop method. Hydrochar hydrophobicity was assessed using the sessile drop
method according to Bachmann et al. [37]. Specifically, the hydrochar particles were sprinkled
on double-sided adhesive tape placed on a glass slide. The excess particles on the slide were
removed by gentle tapping until a one-grain-thick hydrochar layer was obtained. The CA
formed between the hydrochar and a water droplet was monitored as a function of time. A 10-
Hydrophobicity of poultry litter hydrochar
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μL droplet of double-distilled water was placed on the hydrochar-coated slide and the CA was
measured over a period of 3 min at 30 frames per second using an optical goniometer
(OCA20, Dataphysics, Filderstadt, Germany). The horizontal view of the water drop on the
hydrochar slide was used to calculate the CA with the software SCA 20 (Dataphysics). The CA
was measured for hydrochar produced at all temperatures and reaction times, as well as for the
untreated poultry litter. In a similar manner, the initial advancing CAs of four additional wet-
ting liquids (ethylene glycol, formamide, glycerol and diiodomethane) were measured. The
initial advancing CAs for all wetting liquids were used to calculate the surface free energy com-
ponents as detailed in the section on surface free energy calculations.
Wilhelmy plate method. The dynamic CA was evaluated by the Wilhelmy plate
method as described by Bachmann et al. [36] with slight modifications [38]. Specifically,
glass slides (76 mm long, 26 mm wide and 1 mm thick) were covered with a double-sided
adhesive tape. Each slide was sprinkled with hydrochar particles until the tape on both
sides and edges was completely concealed [36]. Excess particles were removed until a one-
grain-thick hydrochar layer was obtained. Slides were secured (one at a time) to an elec-
tronic micro balance (DCAT 11, Dataphysics) with surfaces perpendicular to a glass vessel
containing double-distilled water. Slides were immersed in the glass vessel to a depth of 5
mm at a rate of 0.2 mm/s and subsequently emersed at the same rate. Four immersion/
emersion cycles were performed with 1-s pause between cycles. The immersion/emersion
process and the corresponding CA calculations were performed with SCAT-12 software
(Dataphysics).
The advancing and receding CAs were calculated from the total force (Ft, kg) measure-
ments during immersion and emersion, respectively, according to Eq 1.
cos y¼ðFtþVrgÞ
lwglv ð1Þ
where θ(˚) is the CA, V(m
3
) is the volume of the immersed section of the slide, ρ(kg/m
3
) is
the density of the wetting liquid, g(m/s
2
) is the acceleration due to gravity, l
w
(m) is the wetted
perimeter and γ
lv
(N/m) is the surface tension of the wetting liquid at the liquid/vapor inter-
face. For each cycle, the CA hysteresis (i.e., the difference between the advancing and receding
CAs) was calculated, serving as an additional measure for hydrophobicity persistence.
Three measurements were performed for each hydrochar replicate produced at 180˚C,
220˚C and 250˚C at a reaction time of 60 min, and the poultry litter. Thus, in total, 9 measure-
ments were performed for each treatment temperature and the untreated material.
Surface free energy calculations
Additional quantitative insights into the surface characteristics of the poultry litter and
derived hydrochars can be gained from the surface free energy (mJ/m
2
) at the solid/air
interface, which can also be viewed as the interfacial tension (mN/m). Following the con-
cept that interfacial tensions (solid/air or liquid/air) are comprised of additive polar and
dispersive (i.e., apolar) components, one can calculate the interfacial tension at the solid/
air interface from knowledge of interfacial tension of the wetting liquids and their corre-
sponding CA. As mentioned, the sessile drop method was used to measure the CA formed
by ethylene glycol, formamide, glycerol and diiodomethane on the surface of the poultry
litter and derived hydrochar. The surface tension of the wetting liquids and their polar
and dispersive components are shown in Table 1.
The consistency between two models that quantify the solid surface free energy and its
components were examined: (i) the Owens–Wendt–Rabel–Kaelble (OWRK) method [39,40]
Hydrophobicity of poultry litter hydrochar
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(Eq 2) and (ii) the Wu method [41] (Eq 3).
glð1þcos yÞ ¼ 2ffiffiffiffiffiffiffiffi
gd
sgd
l
qþ2ffiffiffiffiffiffiffiffi
gp
sgp
l
qð2Þ
gl1þcos yð Þ ¼ 4gd
sgd
l
gd
sþgd
lþgp
sgp
l
gp
sþgp
l
  ð3Þ
where γ(mJ/m
2
) is the interfacial tension; θ(˚) is the measured CA, the subscripts land s
stand for the liquid and solid phases, respectively, and the superscripts dand pstand for the
dispersive and polar components, respectively. The surface free energy calculations were car-
ried out using the SCA 21 software (Dataphysics).
Statistical analysis
Analysis of variance was conducted to determine statistical differences between treat-
ments (p<0.05). Treatment temperature and time were set as the fixed factors, and the
above-described measured or calculated parameters were the dependent variables. When
a significant difference was detected, Tukey’s post hoc test was performed. For surface
free energy experiments, an exponential decay regression analysis was performed. The
severity factor was set as the independent variable, and the surface free energy was set as
the dependent variable. Analysis of variance analyses were performed with Statistica ver-
sion 10 (StatSoft, Tulsa, OK), and regression analyses were performed with SigmaPlot
12.5 (Systat Software Inc., Chicago, IL).
Results and discussion
The poultry litter and derived hydrochar were characterized in terms of fiber composition,
specifically hemicellulose, cellulose and lignin fractions. The hemicellulose fraction decreased
significantly (p<0.05) from 28% in the poultry litter to less than 7% in hydrochar produced at
all temperatures after 60 min (Fig 1). The HTC process degraded hemicellulose, a hydrophilic
fiber, resulting in a hydrochar composed of more hydrophobic fibers—cellulose and lignin
[15]. At the treatment temperature of 250˚C, significant cellulose degradation was observed
(p<0.05). The resulting hydrochar was mainly composed of lignin fibers, differentiating it
from those produced at 180 and 220˚C. In total, the fibers accounted for 38–67% of the hydro-
char composition. The remaining 33–62% consisted of ash, sugars, protein, fat and starch. For
example, at 250˚C, the cellulose fraction underwent degradation, probably forming sugars—
which were not measured, thus reducing the fraction of the hydrochar composition that was
accounted for. A general chemical characterization of the poultry litter and hydrochars focus-
ing on elemental composition and FTIR spectra can be found in Mau et al. [30].
Table 1. Surface free energy components of the wetting liquids.
Liquid Surface free energy (mJ/m
2
)
γ
l
γ
ld
γ
lp
Water 72.8 22.1 50.7
Ethylene glycol 47.7 26.3 21.4
Formamide 58.2 39.0 19.2
Glycerol 63.4 29.0 34.4
Diiodomethane 50.8 50.8 0
https://doi.org/10.1371/journal.pone.0206299.t001
Hydrophobicity of poultry litter hydrochar
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Time-dependent contact angle
The time-dependent CA of a water drop placed on the surface of poultry litter before and after
the HTC treatments (Fig 2) served as a quantitative measure for the degree of hydrophobicity
and its persistence. For each surface, the initial advancing CA was calculated from the first
frame (corresponding to about 33 ms). The average value obtained from all hydrochar samples
was 145˚ ±3˚ with no significant difference between HTC production temperature and time
(p>0.15). The initial CA obtained for the poultry litter was 138˚ and found to be significantly
different (p<0.05) from all hydrochar samples.
For all samples, a rapid decrease in CA was observed in the first few seconds of measure-
ment (Fig 2). This rapid decrease continued for the untreated poultry litter, and at a slower
rate for samples generated at 180˚C. After 67 s, the CA for poultry litter decreased to 22˚ and
could no longer be determined with precision. After 3 min of measurement, the CA decreased
to 33˚, 40˚, 63˚ for the 180˚C hydrochar produced after 5, 30 and 60 min, respectively (Fig 2).
With respect to the hydrochars produced at temperatures of 220 and 250˚C, after the initial
rapid decrease, the CAs remained elevated, relatively constant and statistically similar to each
other. In total, after 3 min, the reduction in CA was of 13˚ or less, remaining above 130˚ (Fig
2). These results are similar to sessile drop method experiments performed on woodchip-
derived hydrochar that was carbonized at 200 and 250˚C for 6 h [16]. The woodchip hydrochar
Fig 1. Fiber composition of poultry manure and hydrochar. Data for hydrochar produced at 180˚C, 220˚C and
250˚C after 60 min of treatment is shown. Lowercase letters indicate significant differences between samples for each
fiber.
https://doi.org/10.1371/journal.pone.0206299.g001
Hydrophobicity of poultry litter hydrochar
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was also highly hydrophobic (initial CA >130˚), and over 5 s, the CA decreased by about 5
and 15˚ for hydrochar produced at 250 and 200˚C, respectively.
The marked difference between the degree of hydrophobicity of untreated poultry litter and
hydrochar can be explained in part by the fiber composition of the materials. The poultry litter
consisted of 28% hemicellulose, a hydrophilic fiber, which was degraded to less than 7% during
HTC.
It should be noted that in the sessile drop method, the sample’s roughness can influence CA
measurements, leading to a higher CA than would be measured on a smooth surface [37]. This
Fig 2. Contact angle of water drop formed on the surface of poultry litter and derived hydrochar. Data for hydrochar generated at
180˚C, 220˚C, and 250˚C at 5, 30 and 60 min is shown. Standard errors were less than 2% for all curves, except for poultry litter and
hydrochar produced at 180˚C after 5 and 30 min, where it was up to 13%. Photos demonstrate drops created on samples after 60 min of
carbonization.
https://doi.org/10.1371/journal.pone.0206299.g002
Hydrophobicity of poultry litter hydrochar
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might explain in part why the CAs measured in this study were so much higher than those
measured by He et al. [20] on compressed disks of sewage-sludge-derived hydrochar.
Surface free energy components
As already noted, the initial advancing CAs from the sessile drop method for poultry litter and
hydrochars were similar. Since water is a highly hydrophilic wetting liquid (72 mJ/m
2
) and the
hydrochars were found to be highly hydrophobic, it was difficult to distinguish between the
surface characteristics of the different hydrochars from the initial advancing CA, when water
was the wetting liquid. Therefore, further CA measurements were conducted with the sessile
drop method using other wetting liquids with lower total surface tension and different polar
and dispersive components (Table 1).
To combine the effects of varying HTC temperatures and reaction times, we used the sever-
ity factor (SF), developed by modeling HTC kinetics according to hydrochar oxygen loss [42].
The model was calibrated with data from HTC of feedstocks ranging from cellulose to sub-
bituminous coal treated at temperatures of 120–390˚C and reaction times ranging from 1 min
to 6 months.
SF ¼50 t0:2expð 3500=TÞ ð4Þ
where t is the reaction time in seconds, and T is the temperature in K. According to the SF, dif-
ferent combinations of time and temperature that lead to the same SF may result in similar
effects on reactions and hydrochar properties. The SFs of the studied poultry litter and derived
hydrochars are presented in Table 2.
In Fig 3, the CA as a function of SF is presented for the wetting liquids employed (Table 1),
excluding diiodomethane. The latter exhibited instantaneous and complete wetting (i.e.,
CA = 0) when placed on any one of the surfaces. It should be noted that the total surface ten-
sion of diiodomethane is moderate (50.8 mJ/m
2
), but its polar fraction is zero. Namely, wetting
interaction took place between the dispersive fraction of the liquid and solid phases, implying
a highly hydrophobic nature for the hydrochar.
In contrast to the similar CAs obtained for water, the CAs obtained for ethylene glycol,
formamide and glycerol were sensitive to the HTC treatments. Specifically, for these wetting
liquids, the CA as a function of SF (Fig 3) exhibited a sigmodal-like pattern of increasing CA
with increasing SF. In general, the CA values corresponded to the total surface tension, where
the CA obtained for glycerol (63.4 mJ/m
2
) was highest and that obtained for ethylene glycol
(47.7 mJ/m
2
) was lowest. At the highest SF (0.32), however, no significant differences could be
observed among the three wetting liquids.
The measured CA values (Fig 3) were further used to calculate the surface free energy and
its components using the Wu and OWRK models. The resultant total surface free energies as a
function of SF are presented in Fig 4, together with regression curves and 95% confidence
bands. Both models produced total surface free energy values of similar magnitude, which fol-
lowed a similar trend. The Wu model resulted in smaller standard errors than the OWRK
model, especially for samples with SF >0.13. As the SF increased, the surface free energy
decreased following an exponential decay. The Wu model demonstrated a better correlation
with the SF (Fig 4). The total surface free energy of the untreated poultry litter (i.e., SF = 0) was
about 26–30 mJ/m
2
, whereas the surface free energy for hydrochar with SF 0.11 was about
10 mJ/m
2
. These values are relatively low, implying that the hydrochar surface is extremely
hydrophobic and may indirectly impact crop yields by inducing water repellency when added
to mineral soils. In comparison, agricultural cultivated loess soils have a surface free energy of
39–68 mJ/m
2
[43,44]. These high surface free energy values demonstrate the hydrophilic
Hydrophobicity of poultry litter hydrochar
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nature of the mineral soils in comparison to the hydrochar. Therefore, the addition of hydrochar
to mineral soils could significantly change its hydrophobicity. Nevertheless, more research is nec-
essary to assess the impact of hydrochar on soil’s hydraulic properties in conjunction with crop
yields. It should be noted, however, that the hydrophobicity of hydrochar produced at tempera-
tures lower than 250˚C was not persistent, as will be shown in the following section.
Table 2. Polar (γ
p
) and dispersive (γ
d
) surface free energy components calculated using OWRK and Wu models for hydrochar generated at various reaction temper-
atures and times. The effect of temperature and time is combined in the severity factor (SF; [42]). Values are presented as mean ±standard error. In parentheses is the
fraction of total surface free energy.
Sample SF OWRK Wu
γ
p
γ
d
γ
p
γ
d
(mJ/m
2
) (mJ/m
2
)
Poultry litter 0.00 28.8 ±7.0 1.5 ±0.9 (0.0 ±0.0) 25.9 ±5.0 0.2 ±0.2 (0.0 ±0.0)
180˚C, 5 min 0.07 5.3 ±3.8 13.6 ±7.5 (0.6 ±0.2) 7.8 ±3.2 10.6 ±4.5 (0.5 ±0.2)
180˚C, 30 min 0.10 4.1 ±2.8 8.0 ±3.7 (0.6 ±0.3) 7.8 ±4.0 10.5 ±6.0 (0.5 ±0.2)
180˚C, 60 min 0.11 2.0 ±0.7 5.0 ±1.5 (0.7 ±0.1) 6.3 ±1.1 5.3 ±1.5 (0.4 ±0.1)
220˚C, 5 min 0.13 1.2 ±0.4 11.9 ±4.3 (0.8 ±0.1) 2.1 ±1.2 9.3 ±2.7 (0.8 ±0.1)
220˚C, 30 min 0.18 1.0 ±0.4 11.6 ±2.0 (0.9 ±0.0) 1.3 ±0.9 7.6 ±0.7 (0.9 ±0.1)
220˚C, 60 min 0.19 1.9 ±0.3 10.9 ±3.7 (0.8 ±0.1) 2.7 ±2.2 9.0 ±2.9 (0.7 ±0.2)
250˚C, 5 min 0.21 0.7 ±0.3 11.0 ±1.8 (1.0 ±0.0) 0.8 ±0.6 9.4 ±1.4 (0.9 ±0.1)
250˚C, 30 min 0.28 0.3 ±0.2 7.0 ±0.9 (1.0 ±0.0) 1.0 ±0.5 7.0 ±0.8 (0.9 ±0.1)
250˚C, 60 min 0.32 0.9 ±0.3 3.0 ±1.5 (0.6 ±0.2) 5.7 ±2.6 2.8 ±1.4 (0.4 ±0.2)
https://doi.org/10.1371/journal.pone.0206299.t002
Fig 3. Contact angles measured for poultry litter and hydrochar. Contact angles were measured using different test
liquids, poultry litter, and hydrochar generated at 180˚C, 220˚C, and 250˚C at 5, 30 and 60 min. The effect of
temperature and time is combined in the severity factor [42]. Bars indicate standard errors.
https://doi.org/10.1371/journal.pone.0206299.g003
Hydrophobicity of poultry litter hydrochar
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The OWRK and Wu models separate the surface free energy into dispersive and polar compo-
nents. The two components and the fraction of the total surface free energy associated with the
dispersive component are shown in Table 2. In general, the models agreed on the proportion of
surface free energy associated with the dispersive component. This represented a close to negligi-
ble fraction of the surface free energy for the poultry litter. As the SF increased, the dispersive
component increased and accounted for almost all of the total surface free energy, except for the
hydrochar produced at 250˚C after 60 min. In comparison, in agricultural cultivated loess soils,
the dispersive component represents one-third of the surface free energy [43].
Fig 4. Total surface free energy of hydrochar calculated using OWRK and Wu models. Calculation was performed with values from
poultry litter and hydrochar generated at 180˚C, 220˚C, and 250˚C at 5, 30 and 60 min. The effects of temperature and time are
combined in the severity factor [42]. Bars indicate standard errors. Solid lines indicate the regression line, and the dotted lines the 95%
confidence bands. Regression equation and coefficient of determination are shown for each model.
https://doi.org/10.1371/journal.pone.0206299.g004
Fig 5. Contact angles measured with the Wilhelmy plate method. (a) Advancing and (b) receding contact angles, and (c) hysteresis for poultry
litter and hydrochar generated at 180˚C, 220˚C, and 250˚C after 60 min of treatment. Bars indicate standard error.
https://doi.org/10.1371/journal.pone.0206299.g005
Hydrophobicity of poultry litter hydrochar
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Advancing and receding contact angles
The advancing CA measured during the first cycle was similar for all samples (Fig 5A). This is
in agreement with the similar initial advancing CAs obtained by the sessile drop method (Fig
2). However, the values obtained were smaller than the initial advancing CA measured with
the sessile drop method. This disparity has also been observed in other studies [37,45] where it
was found that in general, the Wilhelmy plate method underestimates the advancing CA
because the actual wetted perimeter is larger than the perimeter of the slide due to the rough-
ness created by the sample particles. Both methods, however, will yield higher advancing CAs
relative to an ideal smooth surface with the same chemical characteristics. In general, the CA
measured on rough hydrophobic surfaces (i.e., CA >90˚) is larger than it would be on an ideal
smooth surface [37].
With subsequent wetting cycles, the advancing CA decreased drastically, by more than 74˚,
for the poultry litter and hydrochar produced at 180 and 220˚C. The advancing CA of hydro-
char produced at 250˚C decreased only slightly with each wetting cycle; the advancing CA
measured in cycle 4 was only 13˚ lower than that from the first cycle. The behavior observed
for the 250˚C hydrochar was similar to the effect of the prolonged static contact time investi-
gated by the sessile drop method. In fact, the time elapsed to measurement of the advancing
CA in cycle 4 was similar to the prolonged static contact time (153 vs. 180 s). Therefore, hydro-
char produced at 250˚C is highly hydrophobic over a long wetting period as well as over
repeated wetting periods. On the other hand, hydrochar produced at 220˚C is hydrophobic
over long periods but not with subsequent wetting cycles.
The receding CA was measured when the slide was emersed from the water. The receding
CA measured in cycle 1 was significantly smaller (p<0.05) than the advancing CA of that
same cycle for all samples excluding the 250˚C hydrochar (Fig 5B). In other words, in cycle 1,
hysteresis was high for poultry litter and hydrochar produced at 180 and 220˚C, and close to
zero for hydrochar produced at 250˚C (Fig 5C). With each wetting cycle, hysteresis decreased
for all samples, except for the 250˚C hydrochar where hysteresis remained relatively constant
and low throughout all cycles. The increase in receding CA observed from cycle 1 and 2 for
poultry litter and hydrochar produced at 180˚C was likely due to the methodology’s lower sen-
sitivity at low CA values. With each subsequent cycle, the receding CA converged to values
similar to the advancing CA and thus, hysteresis was negligible.
The significant decrease in receding CA compared to advancing CA in cycle 1 for poultry
litter and hydrochar produced at 180 and 220˚C may be the result of water adsorption to the
particles’ surface during measurement, whereas the surface was dry for the advancing CA mea-
surement. Thereafter, the advancing CA measured in cycle 2 was similar to the receding CA
measured in cycle 1 because it contained a similar amount of adsorbed water. It is well known
that ordination and/or configuration of hydrophilic and hydrophobic functional groups at the
surface is dependent upon their interaction with water. Specifically, when water is adsorbed,
polar functional groups interact with the water molecules. However, as the surface dries, polar
groups interact with each other, increasing the fraction of hydrophobic functional groups at
the surface [46–48]. This mechanism might explain the rapid decrease in advancing CA and
receding CA observed for poultry litter and hydrochar produced at 180 and 220˚C.
The measurements of advancing and receding CA showed that hydrochar hydrophobicity is
persistent for hydrochar produced at 250˚C, but not for hydrochar produced at lower tempera-
tures. This discernible difference can be explained by the hydrochars’ chemical composition. The
250˚C hydrochar has a lower amount of cellulose than the other hydrochars (Fig 1). Cellulose has
both hydrophobic and hydrophilic components, and can reorient itself at the surface, resulting in
more hydrophilic behavior [49]. Since the cellulose was degraded, the more hydrophobic lignin,
Hydrophobicity of poultry litter hydrochar
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which has less OH-binding sites for water [50], probably had a stronger influence on the hydro-
char’s hydrophobicity. A smaller presence of OH-bonds has been reported for this 250˚C hydro-
char [30]. Principal component analysis of FTIR spectra of these hydrochars also confirmed that
the 250˚C hydrochar is markedly different from the other hydrochars [30]. The persistent hydro-
phobicity of 250˚C hydrochar implies that it could have adverse effects on soil hydraulic proper-
ties; however, hydrochar produced at lower temperatures could be a good soil amendment. More
research is needed to determine hydrochar performance in supporting crop yield.
Conclusions
Wetting properties of poultry litter and derived hydrochar were investigated. A hydrophobic
material was generated by HTC, with treatment temperature having a greater influence than
time. Only hydrochar produced at 250˚C demonstrated hydrophobic behavior under repeated
wetting cycles, probably due to lower amounts of cellulose which resulted in less water adsorp-
tion and hydrophilic molecule reorientation at the surface. Due to this persistent hydrophobic-
ity, the hydrochar produced at 250˚C could negatively impact soil hydraulic properties;
however, based on its wetting behavior, hydrochar produced at lower temperatures would
likely not present a problem when added to soil. The total surface free energy decreased with
treatment severity. Poultry litter had a weak dispersive component, which increased with
increasing severity of HTC treatment. Further experiments should be conducted to correlate
hydrochar wetting characteristics with soil hydraulic properties, and subsequently with crop
yield. This study contributes basic knowledge toward utilizing hydrochar as a soil amendment,
thereby increasing its applications and value.
Acknowledgments
The authors would like to acknowledge Paratherm for donating the heat-transfer fluid used in
this research.
Author Contributions
Conceptualization: Vivian Mau, Gilboa Arye, Amit Gross.
Data curation: Vivian Mau.
Formal analysis: Vivian Mau.
Funding acquisition: Gilboa Arye, Amit Gross.
Investigation: Vivian Mau, Amit Gross.
Methodology: Vivian Mau, Gilboa Arye, Amit Gross.
Supervision: Gilboa Arye, Amit Gross.
Validation: Vivian Mau, Gilboa Arye.
Writing – original draft: Vivian Mau.
Writing – review & editing: Gilboa Arye, Amit Gross.
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