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Morphological Properties and Nutrient Status of Different Waste Derived Slow Pyrolyzed Biochars
Authored by Md Faruque Hossain
Abstract
Slow
pyrolyzed (500±50 °C) ten different waste derived biochar viz. animal bone,
corn stover, wood chips, sewage sludge, sugarcane bagasse, green coconut palms,
nutshells, potato peels, water hyacinth and organic waste were analyzed to know
their physicochemical properties and nutrient contents. Results provided the
fact that water hyacinth biochar had the best nutrient status along with
excellent physical properties like water holding capacity (509%) and CEC (300
cmolc kg-1) whilst potato peel biochar was the second best among all
categories. The average particle size of wood chips biochar 0.82 μm2 was the
largest along with the maximum pore depth. However, the region of this biochar
occupied by remarkably small particles, which was 47.42%. The corn stover
biochar, on the other hand, had the smallest average particle size (0.18 μm2)
and the lowest particle area (9.19%). Biochar wood chips (51.3%) and biochar
potato peels (49.4%) had the highest organic C value, while biochar nutshell
had the lowest (15.31%), respectively. Nutrient content varies depending on the
variation in the feedstock mostly N, P, K, and S in total content. Animal bone
biochar (3.89%) and biochar nutshells (3.32%) exhibited the highest total N
content. Total N biochar content derived from potato peel, water hyacinth, and
organic matter had around 3 ppm, which was much higher than the remaining
biochar content. In the analysis, high phosphorus concentrations resulted in
biochar derived from animal bone feedstock (8.44%), whereas other biochars such
as potato peel, water hyacinth, and organic waste were less than 1%. The
biochar potato peel and the biochar water hyacinth had higher total K content
than other biochars. All the biochars exhibited equal total S concentration. Biochar
derived from animal bone (2.34%) and potato peel (2.72%) had a higher
percentage of total K compared with other biochar. Biochar related wastes
showed a very low concentration of heavy metals such as Cr, Pb, Cd, and Ni. The
highest chromium content resulted in biochar sewage sludge (0.746 ppm). The
concentration of total chromium was similar to that of both sugarcane bagasse
biochar and nutshell biochar. The overall amount of lead and cadmium in all of
the biochar was below the detection mark. In comparison, the biochar sewage
sludge contained a high amount of nickel (1.06 ppm) relative to other biochars.
This is perhaps due to the high amount of pollutants present in the sewage
sludge feedstock.
Keywords:
Characteristics; Nutrient content; Waste; Slow pyrolyzed; Biochars
Introduction
The
challenge of satisfying the increasing need for food in this twenty-first
century has put enormous pressure on soil health. People are forcing soil to
yield beyond its capacity with the help of inorganic fertilizers and other
agrochemicals; which, in turn deteriorating her quality to sustain the
production for the future. Extensive industrialization and natural resource
exploitation have resulted in environmental degradation and pollution with the
rising economy. Large quantities of waste were dispersed in thousands of
contaminated sites spread all over the globe. The use of these wastes, as well
as other by-products as feedstock for bioenergy pyrolysis represents a
significant environmental management and economic achievement. Waste can be
reused as biochar, a solid product rich in carbon as 65 to 95 percent [1-4].
Biochar
is a solid carbonaceous material that is produced from heating biomass at or
above 250 °C in the absence of limited air. It was originally intended for use
in sequestration of carbon (C) and soil health improvement, but has now
expanded toward environmental management [4-6]. In soil application, biochar
has a long history, is associated with the discovery of “Terra Preta di Indio”
(also called Amazonian Dark Earths) in Amazonia, where the dark earth soils
(accounting for * 10 percent of Amazonia) were created using the slash-and-char
technique by the deliberate addition of biochar [7]. For hundreds to thousands
of years the dark earth soils maintained high fertilities [8]. This discovery
motivated scientists to intentionally create biochar using modern artificial
techniques to recreate the ancient agricultural miracle in Amazonia. Biochar
feedstock includes diverse biomasses, especially waste and lowvalue biomass
such as agricultural straw, livestock manure, wood chips, and sawdust [2-4,9].
Thus, the development of biochar will achieve the resource utilization goal of
this biomass waste, a valueadded process. Besides, during biochar processing,
by-products such as bio-oil and syngas can be collected and used as bioenergy
[10]. Biochar, similar to activated C (AC), has a large specific surface area,
a porous structure with an abundance of usable surface groups and can be used
as a sorbent or passivator to extract or immobilize inorganic and organic
contaminants from water or soil [2,11-14]. Because of these advantages, such as
improving soil health, reducing climate change, mitigating greenhouse gas (GHG)
emissions (e.g. CO2, CH4, and N2O), remediating water and soil pollution,
managing and using waste biomass, and generating bioenergy (e.g. bio-oil,
syngas), biochar technology has attracted public interest in the past decade
[15,16]. Recently, increasing studies have recommended biochar beyond soil
health improvement and sequestration as a management tool to connect
environmental protection and bioenergy development [4,12,13,17]. However, the
new concept of biochar application in the field is still new, with only a few
research activities being conducted here in Bangladesh, and is focused on the
biochar made from various waste materials such as bovine bone, maze things,
wood chips, sewage bagasse, green cocoon palm, nutshell, potato peel and water
hyacinths, as well as organic waste. This research work, therefore, carried out
to use waste as a feedstock for the production of biochars and compare its
characteristics with each other.
Materials
and Methods
Feedstock
selection
Ten
different types of wastes were collected to produce different types of biochar
from the capital of Bangladesh. All the feedstock selected are potentially
waste materials and collected from different sources. Animal bones (Fish,
Chicken, Cow, and Goat) collected from the meat market in Malibagh Bazar, home
leftover, and leftover after the Eid festival (Eid-ul-Azhar). Corn Stovers were
collected from Sher-e-Bangla Agriculture University. Wood chips were collected
from sawmill (Rahim Timber and Sawmill) from Abul Hotel, Chowdhury Para, Dhaka.
Sewage sludge was collected from Pagla Sewage Treatment Plant. Sugarcane
bagasse was collected from the roadside sugarcane juice shop. Green coconut
palm collected from the coconut market near Malibagh Bazar. Nutshells were
collected from the peanut shop near Wari Bazar. Potato peels were arranged from
the small agro-food processing industry in Puran Dhaka. Water hyacinths were
collected from a pond, located in Keraniganj, Dhaka. Organic wastes were
collected from kitchen leftover vegetables and other municipal wastes. After
collection, the materials were packed in polythene bags, and the bags were tied
with strings to prevent any air-exchange between the atmosphere and the sample
itself.
Before production of biochar, all the waste derived
feedstocks were well dried for few days under the sunlight. After properly
drying all the feedstocks, one by one were processed and pyrolysis was done in
a specially designed kiln. A wasted pressure cooker was collected from a
recycle shop in Old Dhaka then, it was fixed, and a stainless-steel pipe is
attached in the upper part of the cooker. The whole pressure cooker was air
tightened by using heat resistance rubber in the head of the cooker. The pipe
was used to remove syngas that produce in the cooker (Figures 1&2).
Individual feedstock was placed in the bally of the
cooker and then the head of the cooker is locked that no oxygen can enter
inside the cooker. The cooker was then placed on the gas Stover for burning.
Approximate temperature 450 to 550 °C was maintained after one hour. The
feedstock was burnt for 3 hours maintain the above-mentioned temperature. After
the completion of the process, the cooker was removed from the gas stover and
it was kept on the floor to cool down. The head of the cooker was not opened
because it can readily oxidize in contact with atmospheric air. After the
biochar cooled down, the lid of pot opened and screened through a 0.25mm
stainless sieve and then kept in plastic jars with paper tags indicating
source, manufacturing date etc., (Table 1).
Table 1: Types of waste used to produce biochar and
symbols.
Laboratory analysis and analytical procedure
To determine the water holding capacity by mass ASTM
(2010) method was followed. The morphological properties of biochars were
analyzed by Scanning Electron Microscopic (SEM) imaging. A range of SEM images
(Magnification: 2000× to 10,000×) were captured with a JEOL JSM-6490 operating
at 20KV at the Center for Advanced Research in Sciences (CARS), University of
Dhaka. Image analysis was done with ImageJ version 2.0 with appropriate
threshold and size range values.
The pH, electrical conductivity (1:10 ratio) and cation
exchange capacity (CEC) of biochar samples were measured as described in
Rayment and Higginson (1992). Organic carbon of the feedstock and biochar were
determined by wet oxidation method of Walkley and Black [18]. Total N of the
samples was determined by Kjeldahl steam distillation method (Jackson 1962).
The concentration of P, K and S in feedstocks and biochars were analyzed after
digestion with nitric-perchloric acid (Jackson 1962). Total P was measured
calorimetrically using a spectrophotometer by developing yellow color with
vanadomolybdate, total K by flame photometer and total S by turbidimetric
method using spectrophotometer (Jackson, 1962). Statistical analyses were done
by using Microsoft Excel 2016 and Minitab 2019.
Results and Discussion
Physical and morphological characteristics of biochars
The sugarcane bagasse biochar possessed the highest
water retention of 574% that is nearly ten times more than that of sewage
sludge biochar, which may be due to increased porosity of sugarcane bagasse
(Table 2). Both corn stover and water hyacinth biochar also have high
percentage of water holding capacity. Corn stover biochar demonstrated the
second highest water retention of 525% followed by the water hyacinth biochar
(509%). On the other hand, animal bone biochar (67%) and sewage sludge biochar
(55%) have very low water holding capacity than other biochars. Biochar could
be a competent amendment to light soils especially for newly developed lands
with high sand deposited, as provides high water holding capacity. Improving
biochar performance, feedstock selection and manufacturing conditions demand a
comprehensive understanding of structure and particle distribution. Biochars
are typically comprised of abundant minerals and organic structures. The
surface morphology of all the biochar materials was highly diverse in
structural composition.
After analyzing the images with ImageJ software (Table 2
and Figure 3) wood chips biochar’s average particle size 0.82 μm2 was the
biggest along with its highest pore volume (Figure 3). However, this biochar’s
area occupied by particles in surprisingly high which is 47.42%. In contrast,
corn stover biochar possessed the smallest average particle size is 0.18 μm2 and
lowest area occupied by particles (9.19 %). The highly spongy and honeycomb
like porosity of these biochars may grant high surface area which are likely to
increase soil aeration, water holding capacity, and nutrient retention when
incorporated in soil.
Chemical characteristics of biochars
Most of the biochar found to be alkaline in nature (pH
6.4 to 10.02) may be high dissolution of base cations (Table 3). Due to the
production methods and high temperature increases the pH value of biochars
probably in consequence of the relative concentration of non-pyrolyzed
inorganic elements that are already present in the original feedstocks [19].
Biochar’s alkaline property can be described to four
broad categories: surface organic functional groups, carbonates, soluble organic
compounds and other inorganic alkalis including oxide, hydroxides, sulfates,
sulfides and orthophosphates [20]. Increased pH of biochar amended acid soils
may help to reduce Al-toxicity and increase P availability. Electrical
conductivity was very high for biochars produced from potato peels and organic
waste and in animal bone, corn stover and young coconut palm EC value is
moderate (Table 3). Other biochars had relatively lower EC; however, high EC
results may be due to high soluble salt concentrations. Biochars produced from
potato peels and domestic organic waste demonstrated higher EC (9.77 and 9.9
mS/cm, respectively) that may be due to their high K content (Table 5).
Cation exchange capacity indicates the ability of
biochar to hold cationic nutrients. Soils with high CEC values are able to
retain cationic fertilizers (K+ and NH4+) in the root zone and prevent nutrient
leaching. The WH biochar showed highest CEC (300.16 Cmolc/kg) (Figure 4) which
is almost four times of the most mineral soils (≤15 Cmolc/kg) indicated this
biochar could be an interesting soil amendment for sandy soils [21]. Animal
bone biochar had the lowest CEC that is 25.65 Cmolc/kg followed by sewage
sludge biochar (27.6 Cmolc/kg) (Figure 4) and predicted that K, Ca, Mg, Na and
P in the biomass promote the formation of O-containing groups on biochar surface
during pyrolysis, resulting in higher CEC [22]. Biochars with higher CEC.
Biochars with high CEC can also be an environmental management option for
remediating soil or water contaminated with heavy metals [22].
Carbon content and nutrient status of biochar
Results indicated that wood chips biochar (51.3%) and
potato peels biochar (49.4%) possessed the highest organic C content,
respectively whereas nutshell biochar holds the lowest (15.31%) (Table 4). This
stable form of organic C would extensively affect physicochemical properties of
soil. High-temperature biochar exhibits a high degree of aromatic C structures
that are resistant to degradation, as they do not provide labile fraction of C
to soil microbes [19]. Biochar is generally regarded as relatively inert when
compared to their feedstocks. The carbon of biochars tends to be present in the
soils for hundreds to thousands of years, depending on the feedstock and type
of pyrolysis [23].
The increase in soil C and nutrient status is due to
thermal humiliation which means loss of volatile compounds (H and O mainly) of
the original material and comparatively small losses of alkali nutrients by
volatilization [24]. Pyrolysis alters the nutrient content in the resulting
biochar, which therefore affects nutrient availability to plants. Nutrient
content mostly N, P, K and S in total content can vary according to the
variation in feedstock. Animal bone biochar (3.89%) and nutshells biochar
(3.32%) had the highest content of total N (Table 5). Total N contents of
biochar, produced from potato peel, water hyacinth and organic matter had
around 3 ppm that was much higher than remaining biochar. Mainly the influence
of feedstock is particularly evident in the case of total P. In the study, high
concentration of phosphorus resulted in biochar produced from feedstock of
animal bone (8.44%) whereas other biochars like potato peel, water hyacinth
and, organic waste showed lower than 1%. Potato peel biochar and water hyacinth
biochar had higher total K content than other biochars. All the biochars showed
similar concentration of total S. Biochar produced from animal bone (2.34%) and
potato peel (2.72%) had the higher percentage of total K than other biochar.
Biochars are variable materials in terms of total nutrient content and nutrient
availability can vary in response to plant and soil. For plant growth,
available nutrient content is an important factor. The available N, P, K and S
in the biochars varied according to the feedstock. Available N content found
even in most of the biochars (Figure 5). In general, biochars are very low in
mineral forms of Nnamely, Nitrate-N and Ammonium-N. The effects of feedstock
type and its conversion processes on the speciation of availability of P and K
from biochars is not well understood yet. Pyrolysis process of biochar
production also plays an important role in this case. Due to pyrolysis
procedure, generally loss a smaller amount of P than C or N as it transforms to
less soluble minerals resulting in reduction of available P in biochars [25].
Available sulphur content in biochar vary depending on biochar production
processes like pyrolysis or gasification (>700 °C). Pyrolyzed waste showed
result of available sulfur content below 1% (Figure 5). Macro and micronutrient
concentrations of different biochars (Table 5).
The total sodium content of most of the biochar were low
in concentration (Table 6). Animal bone biochar had the maximum Na content
(0.62%), which was almost triple than that of corn stover biochar (0.04%).
Soluble Na attributed for the higher EC in soil. The
high EC of the analyzed biochar might be the reason of huge Na content. Same as
Na content, animal bone biochar had highest amount of Ca content (0.33%), as animal
bone feedstock contain mostly calcium. Water hyacinth biochar had the second
highest Ca content. In contrast, Mg contents found low in all the biochars.
High production temperature resulted biochar depleting decomposable substances,
volatile compounds and elements like O, H, N, S and as a consequence
concentration of their nutrients increase, including K, Ca, and Mg [26]. Total
Fe content of sewage sludge biochar was high (19.5%) while with 2.17% animal
bone biochar had the lowest amount of iron. Biochar demonstrated a slight
variation in total Zn content ranged from 0.43 to 13.5 mg kg-1. The highest
amount of Zn found in sewage sludge biochar (13.5 mg kg-1) but it was lower
(1350 to 2175 mg kg-1) than the previous studies reports [27]. Most of the biochar
resulted in low amount of copper content. Nutshell biochar and Water hyacinth
had below detection limit in terms of total copper content.
Heavy metals status of biochars
Total chloride content of the biochar was ranging from
30 to 4900 ppm (Table 7). Corn stover biochar (4900 ppm) and animal bone
biochar (3900 ppm) had high amount of chloride content; in contrast, nutshell
biochar had the lowest (30 ppm) chloride content. Biochars have heavy metals
inherent within their structure, derived from their source material, which
maybe accumulated and concentrated in ash fractions during pyrolysis [6]. Waste
derived biochar demonstrated very low concentration of heavy metals like Cr,
Pb, Cd and Ni (Table 7). Highest amount of chromium resulted in sewage sludge biochar
(0.746 ppm). Both sugarcane bagasse biochar and nutshell biochar had similar
concentration of total chromium. Total lead content and cadmium content in all
the biochar were below detection limit. In contrast, sewage sludge biochar was
containing high amount of nickel (1.06 ppm) than other biochars. This is maybe
due to high amount of pollutant present in the feedstock of sewage sludge
biochar [28].
Conclusion
Waste derived biochars displayed varying physicochemical
properties and nutrient content. The water hyacinth biochar exhibited high
surface area, water keeping, and cation exchangeability while the biochar of
domestic organic waste had increased the critical nutrient contents.
Consequently, feedstocks for biochar production must be carefully chosen to
meet the needs of a specific combination of soil crops. The biochars used in
this study are readily available and some have high potential in the agricultural
system to be adopted. Cost-benefit ratio, production cycle, production
temperature effect, and socioeconomic factors should, however, be considered
before implementation on the ground. The properties of biochar and the heavy
metal content themselves differ considerably depending on their material
sources and manufacturing conditions. They have different effects on heavy
metals and can give many benefits, either alone or in conjunction with other
amendments. There is no question that certain biochars are an efficient
solution-sorbent for heavy metals, but this is not an indication of their
efficacy in handling heavy metals in the environment as a wide range of
confusing ecological, biological, and physical ecosystem interactions must also
be considered.
Acknowledgement
None.
Conflict of Interest
No conflict of interest.
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