Two biochars derived from wattle bark (BA) and coffee husks (BC) used to remove
methylene blue (MB) which achieved the yield values of 33.3 % to 35.08% at 500 oC of pyrolysis.
Adsorption increased with the rising of the dose until 2 g/L for BC, and 5 g/L for BA, respectively.
When rising pH, the increasing trend was observed for BA, and BC, from 67 % at pH of 2 to 93% at
pH of 12. The equilibrium time depended on the initial MB concentration and the MB removal
accounted for over 83% at the first 30 minutes. The experiment data fits better with Avrami, and
Elovich than Pseudo-first-order equation (PSO), and Pseudo-second-order model (PFO).
Adsorption process comprised more than one reaction pathway including integer-kinetic order, and
multiple kinetic orders or fractionary kinetic order.
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DOI: 10.15625/vap.2019.000215
602
STUDY OF BIOCHAR FROM GRO-WASTE FOR DYE ADSORPTION:
CAPACITY AND KINETICS
Nguyen Xuan Cuong
1*
, Tran Thi Cuc Phuong
2
, Vo Thi Yen Binh
2
1
Center for Advanced Chemistry, Institute of Research and Development, Duy Tan University
2
Faculty of Environmental Engineering Technology, Hue University, Quang Tri Campus, Viet Nam
Email: nguyencuongqt2008@gmail.com
ABSTRACT
Two biochars derived from wattle bark (BA) and coffee husks (BC) used to remove
methylene blue (MB) which achieved the yield values of 33.3 % to 35.08% at 500
o
C of pyrolysis.
Adsorption increased with the rising of the dose until 2 g/L for BC, and 5 g/L for BA, respectively.
When rising pH, the increasing trend was observed for BA, and BC, from 67 % at pH of 2 to 93% at
pH of 12. The equilibrium time depended on the initial MB concentration and the MB removal
accounted for over 83% at the first 30 minutes. The experiment data fits better with Avrami, and
Elovich than Pseudo-first-order equation (PSO), and Pseudo-second-order model (PFO).
Adsorption process comprised more than one reaction pathway including integer-kinetic order, and
multiple kinetic orders or fractionary kinetic order.
1. INTRODUCTION
Dyes cause primarily to colored compounds in water (Hao et al., 2000; Santos & Boaventura,
2015), in which azo dyes like MB is being used extensively (Hao et al., 2000; Tan et al., 2016). To
treat these dyes, many methods have been introducing such as coagulation, advanced oxygen
process, membrane, and adsorption (Hao et al., 2000; Park et al., 2019). However, many of them are
expensive, high energy consumption and toxic secondary generation. Biochar - a low cost
carbonaceous material to eliminate contaminants, has attracted much attention. In general, the
studies of biochar for dye removal and decolorization have achieved significant results. Moreover,
biochars produced from agro-waste bring other benefits in terms of waste minimization, recovery
and reuse, and environmental conservation.
The focus of this work is to compare the adsorption capacity of biochars derived from wattle
bark, and coffee husks for MB removal and to clarify the adsorption kinetics. These materials using
to produce biochars in this study are not yet investigated for dye removal.
2. MATERIALS AND METHODS
2.1. Raw materials
The wattle bark and the coffee husks collected from local processing factories. MB is an
organic cation dye which chosen to test adsorption potential of biochars.
2.2. Biochar production
Fig.1. Raw materials, biochars and SEM images at 10 µm.
Hồ Chí Minh, tháng 11 năm 2019
603
The prepared materials heated in the furnace which programmed with the heating rate of
10
o
C/min until 500
o
C and kept there for 2 hours of retention time. Then, biochars crushed and
passed through the sieve with a size of 0.25 mm (60-mesh).
2.3. Characteristics of biochar
The chemical properties of biochars were tested by, pH, moisture, volatile fraction, ash, and
elemental analysis. Moisture, volatile fraction and ash determined by ASTM D-1762-84 standard.
pH value was measured by Multi-parameter Water Quality Meter (HQ40d, Hach, USA). The pH of
point of zero charge (pHpzc) of biochars also was determined according to pH drift method. Surface
of biochars were studied using a scanning electron microscope (SEM).
2.4. Batch adsorption studies
Batch experiments were implemented to determine the influence of operational factors such
as dose, time, temperature, pH, etc. on the adsorption process. The adsorption experiments used 50
mL of MB solution. After adsorption finished, suspensions were centrifuged at 4.000 rpm for 10
minutes. The solution was then measured by UV-spectrophotometer at 665 nm.
2.5. Adsorption kinetics
The kinetic models consist of PFO, PSO, Avrami fractional-order equation, and Elovich
model (chemisorption).
3. RESULTS AND DISCUSSION
3.1. Biochar characterization
pHpzc of BC and BA were found to be 8.82, and 5.48, respectively. BC moisture obtains only
0.2% and BA was 2.04. The values of ash were 7.16, and 7.48 % for BA, and BC, respectively.
3.2. Adsorption capacity and influent factors
3.2.1. Effect of doge and pH
pH changed did not affect significantly on MB mass removal by BA while this rate rose
gradually for BC (Fig. 2a). With pH > 5.48 (pHpzc) for BA and > 8.82 (pHpzc) for BC, meaning that
the biochar surface becomes negative charge which absorbed positive MB molecule, and
consequently enhanced the adsorption capacity. The adsorption of BA increased with the rising of
dose while that trend is the case of BC until 2 g/L. When the dose was bigger than 2 g/L, the
adsorption of BC leveled off and then dropped at 81.07% at 10 mg/L. This might be due to the high
lignin content (21 - 28%) in coffee husks which cannot be converted to biogas (Oosterkamp, 2014).
3.2.2. Effect of contact time and reaction temperature
For the initial MB strengths of 10 to 20 mg/L, the equilibrium time just was about 30 minute
or lower. Generally, at the first 30 minutes, the MB removal ability of three biochars accounted for
over 83%, except for BC with initial MB of 60 mg/L (Fig. 3).
Fig.2. Effect on MB adsorption of: a) dose; and b) pH.
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604
Fig. 3. Effect of time on MB adsorption of: a) BM; b) BC; and c) BM.
3.3. Adsorption kinetics
RSS (residual sum of squares) values are lowest in the following order: Avrami, Elovich,
PSO, and PFO. This indicates that data fit better with Avrami and Elovich than remaining models
(Fig. 4). The better fitting of PSO, and Elovich compared to PFO indicating that besides physical
adsorption mechanism the test also is controlled by chemisorption as the assumption of PSO
(Shawabkeh & Tutunji, 2003; Sun et al., 2013) and Elovich (Vaghetti et al., 2009).
Fig. 4. Non-linear kinetic model for MB adsorption: a) BA; b) BC.
4. CONCLUSIONS
Adsorption efficiency increased with the rising of the dose until 2 g/L for BC, and 5 g/L for
BA, and BC, respectively and those values are also the optimal points of dose. The range of pH is
appropriate for using biochars without any pH adjustment. The increasing of temperature enhanced
the MB removal ability, but this trend is uneven at different concentrations. The experiment data
fits better with Avrami, and Elovich than PSO, and PFO.
Acknowledgment
This research is funded by Vietnam National Foundation for Science and Technology
Development (NAFOSTED) under grant number 105.99-2019.25.
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