*Corresponding author: Department of Water Resources Engineering, Faculty of Engineering, Universitas Brawijaya, 65156, Indonesia E-mail address: [email protected] (Moch Sholichin)
doi: https://doi.org/10.21776/ub.pengairan.2023.014.02.6 Received: 03-03-2023; Revised: 08-09-2023; Accepted: 29-11-2023
P-ISSN: 2086-1761 | E-ISSN: 2477-6068 © 2023 [email protected]. All rights reserved. 153
Vol. 14 No. 02 (2023)
Jurnal Teknik Pengairan: Journal of Water Resources Engineering
Journal homepage: https://jurnalpengairan.ub.ac.id/index.php/jtp
Original research article
Effectiveness of Acacia Wood Charcoal and Coconut Shell Analysis in Reducing Concentrations of Cr (VI) Pollution from Industrial Waste Using Adsorption Method at Laboratory Scale
Moh. Sholichin
a*, Bambang Ismuyanto
b, AS. Dwi Saptati N.H
baDepartment of Water Resources Engineering, Faculty of Engineering, Universitas Brawijaya, 65156, Indonesia
bDepartment of Chemical Engineering, Faculty of Engineering, Universitas Brawijaya, 65156, Indonesia
A R T I C L E I N F O A B S T R A C T Keywords:
Adsorption of pollution;
Acacia wood;
Concentration of Cr (VI);
Coconut shell
Controlling the concentration of Cr(VI) in industrial wastewater is imperative to adhere to environmental quality standards and mitigate the risk of polluting river waters, ensuring the safety of living organisms and public health. Acacia wood charcoal and coconut shell charcoal have emerged as effective adsorbents capable of reducing Cr(VI) levels. This research seeks to identify the optimal adsorption pH and evaluate the impact of initial adsorbate concentration on Cr(VI) removal, comparing the efficacy of acacia wood charcoal and coconut shell charcoal.
The study systematically manipulated process variables, encompassing 2, 4, and 6 pH levels and adsorbate concentrations of 30, 40, and 50 ppm. Charcoal characterization techniques, such as XRF, FTIR, and SNI, were employed alongside analyzing Cr(VI) levels utilizing UV-visible spectrophotometry. Results elucidated that coconut shell charcoal exhibited a higher percentage removal of Cr(VI) at concentrations of 30% (41%), 40% (38%), and 50 ppm (31%), outperforming acacia wood charcoal, which displayed removal percentages of 16%, 14%, and 7%, respectively. Consequently, the research underscores the superior efficacy of coconut shell charcoal in Cr(VI) removal compared to acacia wood charcoal under the specified conditions.
The findings contribute valuable insights to optimizing adsorption processes for industrial wastewater treatment, promoting sustainable practices and environmental stewardship.
1. Introduction
Along with the high activity of mining, industrialization processes, laboratories, and daily activities, heavy metal pollution in soil and waters has become a global problem, which is important to handle and prevent it. One of the harmful liquid waste pollution from industrial activities is Cr(VI) pollution. Cr(VI) is found in many industrial wastes such as the textile industry, metal plating (chrome), paint factories, ink factories, leather and tanning factories, and oil refineries [1]. Based on quality standards in waters, the maximum allowable total Cr content is 0.1 mg/L, while for Cr(VI) content, it is 0.05 mg/L, which is regulated in Kep- 03/MENKLH/11/1991. At low concentrations, Cr(VI) can cause a high level of toxicity in living things. Cr(VI) poisoning can cause skin irritation, accumulation in the liver, and systemic poisoning effects. In addition, it can cause inflammation of the respiratory tract and lung cancer if chromate vapor is inhaled, as well as chromate salts, which cause skin damage (chrome ulcers) [2]. Judging from the
effects that can be caused by Cr(VI), it is very important to treat chromium-containing waste. Various methods have been developed to reduce and remove heavy metal content, especially in liquid waste. The method that is generally easier to apply is adsorption. This method is environmentally friendly, simpler, highly effective, and economical, so it is efficient for water or wastewater treatment [3].
Adsorption consists of a dynamic system (column) and a static system (batch). Adsorption in a batch system describes the performance of the adsorbent, which is contacted with several solutions, and then changes in the quality of the adsorbent are observed after a certain period. Batch systems are easy to use in laboratory studies but are less suitable for industrial applications [4]. In contrast, the adsorption column system has a better approach to industrial applications than the batch system because of the adsorbent’s ability to adsorb until it is optimally saturated by contacting the adsorbent with a fresh solution. Fixed-bed columns are widely used in various chemical industries [4].
154 Coconut shells and wood, which are agricultural wastes, contain lignocellulosic, which can be converted to form materials with a high carbon content through the carbonization process to be used as adsorbents. Charcoal is a solid material with pores and is the result of the carbonization or heating process of a material with carbon (C) [5]. Several studies have proven that coconut shell charcoal can be used as an adsorbent, including for the adsorption of Cr(VI) [6];
adsorption of Pb metal [7], and adsorption of phenol in water [8]. In addition, wood charcoal can also be used as an adsorbent in heavy metal adsorption. Several studies have proven that wood charcoal can be used as an adsorbent, including for the adsorption of Cr metal [9], adsorption of Cr(VI) [10], and adsorption of Fe and Mn [11]. Adsorption of Cr(VI) by (continuous) column system using various adsorbents has also been extensively studied. Some of these adsorbents include sargassum ceramics [12], coconut shell charcoal [13], iron oxide-modified bentonite [14], and zeolite ceramics [15].
In the adsorption process, several factors can affect the adsorbent surface area, physical and chemical properties of the adsorbate, pH, temperature, porosity of the adsorbent, and chemical characteristics of the adsorbent surface [16].
Knowledge of adsorption pH is very important because pH affects the surface charge of the adsorbent, the degree of ionization, and the adsorbate species [10]. Based on the explanation above, research can be carried out to determine and compare the percentage of Cr(VI) removal through the column adsorption method with coconut shell charcoal and wood charcoal as adsorbents using various concentrations of Cr(VI).
Based on the explanation above, research can be carried out to determine and compare the percentage of Cr(VI) removal through the column adsorption method with coconut shell charcoal and acacia wood charcoal as adsorbents using various concentrations of Cr (VI).
2. Method
2.1. Research Location
The research was conducted at the Chemical Engineering Laboratory, Faculty of Engineering, University of Brawijaya for testing using UV-Visible Spectrophotometry Test equipment, while testing using XRF and FTIR Test equipment was carried out at the Minerals and Materials Laboratory, Faculty of Mathematics and Natural Sciences, State University of Malang. Liquid waste samples were taken from sources of industrial waste pollution in the Malang Regency area as a basic for research, and modifications were made by making synthetic samples to adapt to existing equipment in the laboratory with similar characteristics. The detail equipment
used is a UV-VIS spectrophotometer, orbital shaker, furnace, oven, stopwatch, analytical balance, pH-meter, fixed bed column, separating funnel, beaker glass, Erlenmeyer, watch glass, volume pipette, dropping pipette, measuring pipette, measuring cup, filtering flask, mortar and pestle, porcelain cup, spray bottle, rubber bulb, stand and clamps, spatula. The materials used are coconut shell charcoal, acacia wood charcoal, aquademin, diphenylcarbazide, glass wool, H2SO4, H3PO4, NaOH, K2Cr2O7, and acetone. The major equipment used in the research can be seen in Figure 1.
2.2. Research Variables and Research Design
The research variables consist of fixed variables and independent variables. Fixed variables in this study are as follows :
• The batch adsorption process is carried out for 60 minutes.
• The column adsorption process was carried out for 30 minutes.
• The particle size of the adsorbent is -20+30 mesh.
• Cr(VI) concentration in batch adsorption was 30 ppm, with a rotating speed of 150 rpm and a mass of 2 grams of adsorbent.
• The adsorption flow rate of the column is 5 mL/minute with an adsorbent height of 14. cm and glass wool 0.5 cm.
• Dimensions of the adsorption column: H = 40 cm, ID = 2 cm.
The independent variables in this study are as follows:
• Adsorbent for acacia wood charcoal and coconut shell charcoal
• pH concentration with a value of 2, 4, 6
• concentration of Cr(VI) with values of 30, 40, and 50 ppm
2.3. Research Procedure
Acacia wood charcoal and coconut shell charcoal are crushed and sifted to a particle size of -20+30 mesh, then dried in the oven at 105°C for 1 hour to remove the water content contained in the charcoal. Charcoal characterization with the XRF (X-ray Fluorescence Spectrometer) test aims to determine the composition of elements and/or metal oxide compounds and their concentrations using a spectrometric method. The FT-IR (Fourier Transform Infra Red) test aims to determine the functional groups contained in charcoal.
The parameters tested to determine the character and quality of the charcoal are based on SNI 01-1682-1996 for coconut shell charcoal and SNI 01-1683-1989 for acacia wood charcoal, which include :
Water content (%) = 𝑊1
𝑊2 x 100 (1)
Figure 1. The major equipment used in the research are (a) X-Ray Fluorescence (XRF), (b) Fourier Transform Infrared (FTIR), and (c) Spectrophotometer UV-VIS
(b) (c)
(a)
155 where W1 is sample weight loss (grams), W2 is sample weight (grams).
The amount of parts lost on heating 950°C : (%) = 𝑊1−𝑊2
𝑊1 x 100 (2) where W1 is the sample weight before heating (gram), W2 is the sample weight after heating (gram)
Ash content (%) = 𝑊1−𝑤2𝑊 x 100 (3) Where W is sample weight before incubation (gram), W1 is sample weight plus cup after drying (grams), and W2 is empty cup weight (gram).
Preparation of 500 ppm Cr (VI) mother liquor was carried out by dissolving 1.4145 g K2Cr2O7 using aquademin up to a volume of 1000 mL, then diluting the Cr (VI) mother liquor with a concentration of 30 ppm using aquademin up to a volume of 100 mL to determine the adsorption pH and concentration of 30, 40, and 50 ppm using aquademin until the volume reaches 350 mL for column adsorption.
Preliminary research to determine the adsorption Ph was carried out using acacia wood charcoal and coconut shell charcoal to obtain an adsorption pH to remove Cr (VI). The pH variations used were 2, 4, and 6 with a 30 ppm Cr (VI) concentration of 100 Ml, 2 grams of adsorbent mass, and 150 rpm rotation speed for 60 minutes.
The adsorption process of the column system was carried out using acacia wood charcoal and coconut shell charcoal as adsorbents, which aimed to obtain adsorption adsorbate concentrations to remove Cr(VI). The concentration variations used were 30, 40, and 50 ppm with and adsorbent height of 14.5 cm and 0.5 cm of glass wool, a flow rate of Cr (VI) solution of 5 mL/minute for 30 minutes, and an adsorption pH of pH 2.
Analysis of the Cr(VI) content in the samples was carried out according to a standard method using a UV-visible spectrophotometer, namely by preparing a standard solution and a calibration curve with a concentration of 0.2, 0.4, 0.6, 0.8, and 1 ppm. Absorbance measurements were carried out on a UV-Vis Spectrophotometer with a maximum wavelength of 530-540 nm. The concentration of the Cr(VI) sample was then measured with the maximum wavelength obtained on the UV-Vis Spectrophotometer. The following equation can calculate batch system adsorption results:
Percent allowance Cr(VI) (%) = (C0 – Cf)/C0 x 100 (4) q = adsorption capacity (mg/g) = C0 – Cf) x V/M (5) Where C0 is concentration before adsorption (ppm), Cf is concentration after adsorption (ppm), V is the volume of adsorbate used (mL), and M is mass adsorbent (g).
The following equation can calculate The following equation can calculate the operation conditions and adsorption results of the fixed bed column
Volum of efluen (Veff) = F x ttotal (6) Total amount Cr(VI) (mtotal) = ( C0 x F x ttotal) /1000 (7) Maximum adsorption column capacity
qtotal (mg) = 1000𝐹 ∫0𝑡𝑡𝑜𝑡𝑎𝑙(𝐶0 − 𝐶𝑡 )𝑑𝑡 (8) Adsorption Equilibrium (qe-exp) = qtotal/m (9) Total Cr(VI) removal efficiency
(E%) = qtotal/mtotal x 100% (10)
where, F is the flow rate (mL/min), ttotal is total residence time (min), and m is the total dry weight of the adsorbent in column (g).
3. Result and Discussion 3.1. Adsorbent Characterization
Based on the results of the FTIR analysis, the functional groups owned by each adsorbent can be identified, namely acacia wood charcoal and coconut shell charcoal. A comparison of the functional groups of acacia wood charcoal and coconut shell charcoal can be seen in Table 1.
Based on the FTIR analysis of coconut shell charcoal, the absorption areas between 3010-3095 and 675-995 cm-1 indicate the presence of the C-H alkene functional group. In the absorption area, 3010-3100 and 690-900 cm-1 indicate the presence of functional groups C-H aromatic rings. The absorption area of 1050-1300 cm-1 indicates the presence of the C-O alcohol/ether/carboxylic acid/ester functional group.
In the absorption area, 1180-1360 cm-1 indicates the presence of C-N amine/amide functional groups. In the absorption area, 1340-1470 cm-1 indicates the presence of the C-H alkane functional group. The absorption area of 1500-1600 cm-1 indicates the presence of functional groups C=C in aromatic rings. The 3500-3650 cm-1 absorption area indicates the presence of functional groups O-H carboxylic acid monomers.
In the absorption area, 3590-3650 cm-1 indicates the presence of monomer alcohol/phenol O-H functional groups.
Chemically, Cr(VI) can be bonded with oxygen-containing groups such as -CO, -C-OH, -COOH, etc., on the carbon surface [17]. In coconut shell charcoal, the possible functional groups that can adsorb Cr(VI) are the hydroxyl and carboxyl groups [18]. In the results of the FTIR analysis, it was identified that there was a C-O (carbonyl) group, which was shown at vibrations of 1050-1300 cm-1 and was strengthened by the presence of O-H (hydroxyl) groups at 3500-3650 cm-1.
Based on the FTIR analysis of acacia wood charcoal, the absorption area ranging from 675-995 cm-1 indicates the presence of the C-H alkene functional group. The 690-900 cm- 1 absorption area indicates the presence of C-H functional groups in aromatic rings. In the absorption area, 2850-2970 and 1340-1470 cm-1 indicate the presence of the C-H alkane functional group. The absorption area of 1500-1600 cm-1 indicates the presence of functional groups C=C in aromatic rings.
Functional groups can come from the raw material of the charcoal itself. The presence of lignocellulose in raw materials is indicated by bonds such as hydroxyl, aromatic, and carbonyl compounds [19]. The –OH functional group can come from cellulose, and the ether and phenol functional groups can come from lignin [20], [21]. The C=C functional group is a group in the form of carbon with a high purity number, in which the element O or H is released, which initially combines with element C [22]. In general, changes in functional groups can be caused by the influence of carbonization temperature and activation time. Carbonization with higher temperatures will result in changes in functional groups, namely shifts, loss of absorption wave numbers or reduced absorption levels, and the formation of unstable radical compounds that then react to new compounds [23].
156
Table 1. Comparison of the functional groups of acacia wood charcoal and coconut shell charcoal
Functional groups Acacia Wood
Charcoal
Coconut Shell Charcoal
C-H, alkene ✓ ✓
C-H, aromatic ring ✓ ✓
C-O, alcohol/ether/carboxylic acid/ester - ✓
C-N, amines/amides - ✓
C-H, alkanes ✓ ✓
C=C, aromatic ring ✓ ✓
O-H, carboxylic acid monomers - ✓
O-H, alcohol monomer/phenol - ✓
Table 2. Characteristics of XRF on acacia wood charcoal and coconut shell charcoal Component Acacia Wood Charcoal
(%)
Coconut Shell Charcoal (%)
P2O5 1.7 5.4
K2O 8.99 66.5
CaO 80.4 14.9
Cr2O3 0.66
Fe2O3 4.04 8.73
NiO 0.2 0.5
CuO 0.99 1.8
Yb2O3 1.6 2
TiO2 0.2 -
MnO 0.41 -
BaO 1 -
Table 3. Results of characterization of acacia wood charcoal adsorbents
SNI 01-1683-1989 Content Acacia Wood Charcoal
Water content Maximal 6% 4%
The missing piece at 950°C heating
Maximal 30% 31%
Ash Content Maximal 4% 53%
Table 4. Characterization results of coconut shell charcoal adsorbents
SNI 01-1682-1996 Content Coconut Shell
Charcoal
Water content Maximal 6% 5%
The missing piece at 950°C heating
Maximal 15% 34%
Ash Content Maximal 3% 54%
XRF analysis aims to determine the presence of elements and metal oxide compounds in their composition in the adsorbents of acacia wood charcoal and coconut shell charcoal. The content of these elements is an impurity compound contained in the adsorbent.
These impurities are spread on the adsorbent grid and cover the surface of the adsorbent so that they can clog the adsorbent’s pores, and the adsorbent’s ability to absorb gas or solution is not optimal [24]. Impurities in the adsorbent are related to the ash content, affecting the charcoal’s quality as an adsorbent. The resulting ash is a metal oxide containing minerals that cannot evaporate during the ashing process [25].
Based on the data in Table 2, there are more metal oxide compounds in acacia wood charcoal than in coconut shell charcoal. So it can be said that the differences in metal oxide impurities and their proportions in the adsorbent are
determined by the type of adsorbent material and its respective chemical content.
Based on the data in Table 3 and Table 4, the adsorbents of acacia wood charcoal and coconut shell charcoal were found to have a moisture content of 4% and 5%, respectively, where coconut shell charcoal was slightly more hygroscopic than acacia wood charcoal. This difference is due to the water content in acacia wood charcoal, which evaporates more than coconut shell charcoal. Low water content indicates that the water content in the charcoal has evaporated during the carbonization process [24].
Determination of the missing parts' content at 950°C heating to determine the content of compounds that have not evaporated during carbonization contained in charcoal at 950°C.
157
Figure 2. Graph of the relationship between pH and the percentage of Cr(VI) removal using acacia wood charcoal and coconut shell charcoal adsorbents
In this study, the adsorbents of acacia wood charcoal and coconut shell charcoal had a missing parts content at 950°C of 31% and 34%, respectively, in which the two charcoal levels were above the standard set. The missing part at 950°C still attached to the adsorbent and will affect the absorption of the adsorbent [26].
Ash content is inorganic substances like metal oxides and minerals that cannot evaporate during the ashing process. In this study, the adsorbents of acacia wood charcoal and coconut shell charcoal had an ash content of 53% and 54%, respectively, where the levels of the two charcoals were above the established standards. The presence of a high content of inorganic substances can cause high ash content. The ash content in the charcoal is expected to be as low as possible so that maximum adsorption occurs. This is due to the presence of minerals in the ash, such as sodium, calcium, potassium, and magnesium, which are scattered on the adsorbent lattice and cover the surface of the adsorbent. It can cause blockage of the adsorbent pores, and the ability of the adsorbent to absorb gas or solution is not optimal [24].
3.2. Adsorption Ph
Knowledge of the adsorption Ph is very important because pH affects the surface charge of the adsorbent, the degree of ionization, and the adsorbate species. So, the pH of the solution becomes an important controlling parameter in the heavy metal adsorption process.
Figure 2 shows the relationship between pH and the percentage of Cr(VI) removal by acacia wood charcoal and coconut shell charcoal. Experiments were carried out at pH 2, 4, and 6 to find the adsorption pH for maximum removal efficiency. For acacia wood charcoal and coconut shell charcoal, the percent adsorption removal was found to be 2.02 and 1.99, respectively. For both adsorbents, absorption decreases at higher pH values. Cr(VI) removal decreased from 40% to 15% as the pH changed from 2 to 4 for acacia wood charcoal and decreased from 57% to 20% for coconut shell charcoal. There was a sharp decrease in removal when the pH of the solution was increased from 2 to 4. The adsorption of Cr(VI) at low pH indicates that the binding of negatively charged chromium species (HCrO4−) occurs via electrostatic attraction to the surface functional groups, which are
positively charged (due to more ions). H+) on the surface of the adsorbent.
Meanwhile, at a pH that is too high, the amount of OH- increases, thus neutralizing some of the positive charges and increasing the electronegativity of the adsorbent.
Simultaneously, OH- can compete with HCrO4- for functional groups on the adsorbent [10]. Therefore, the percentage of Cr(VI) removal will decrease as the pH increases. Coconut shell charcoal has an adsorption percentage at pH 4 lower than pH 6, which may be due to the weakening of the electrostatic attraction between the adsorbate, which is oppositely charged, and the adsorbent due to the weak and undirected Van Der Waals attraction [10].
3.3. Effect of Initial Concentration on Cr (VI) Removal
The adsorption process is carried out using a column system where the adsorbent will always be in contact with the new Cr(VI) solution feed so that over time the adsorbent in the column will experience saturation [4]. By continuously feeding the Cr(VI) solution, the available sites on the surface of the adsorbent are increasingly limited. This causes the effluent concentration coming out of the column to increase over time. In a column system, the initial concentration of adsorbate is a factor that can affect adsorption [27].
Figure 3, such as (a) and (b), shows that at 5 minutes, there was a sharp decrease in concentration for each concentration variation with both acacia wood charcoal and coconut shell charcoal adsorbents and continued to increase at 10, 15, 20, 25 and 30 minutes At the beginning of the adsorption process, there was a very significant decrease in concentration because the adsorbent in the column was still fresh and had not been in contact with the Cr(VI) solution. In this initial stage the adsorbate is adsorbed most quickly and effectively because the number of adsorbent surface sites is greater than the amount of adsorbate entering. This causes the mass transfer zone to be reached at the end of the influent or the top of the column. When the adsorbate is continuously flowed into the column, the efficiency of the adsorbent will decrease progressively, which is caused by the top of the column experiencing gradual saturation [28].
0 20 40 60
0 2 4 6 8
% r em o val
pH
Acacia Wood Charcoal Coconut Shell Charcoal
158 (a)
(b)
Figure 3. Graph of Cr(VI) effluent concentration with column system at the time certain adsorbents using (a) acacia wood charcoal and (b) coconut shell charcoal
Figure 4. Graph of the relationship between the type of adsorbent and the final removal of Cr(VI) 30, 40, and 50 ppm
Figure 4 shows the relationship between the type of adsorbent and the final removal value of Cr(VI) at various initial concentrations of the adsorbate. Adsorption with coconut shell charcoal as an adsorbent had a higher final removal percentage than acacia wood charcoal at concentrations of 30 ppm as well as 40 and 50 ppm.
At an initial concentration of 30 ppm, a final elimination of 41% was obtained with coconut shell charcoal, while acacia wood charcoal was 16%. At an initial concentration of Cr (VI) 40 ppm, a final removal percentage of 38% was obtained with
coconut shell charcoal while acacia wood charcoal was 14%.
As for the initial concentration of Cr(VI) 50 ppm, the final removal percentage obtained with coconut shell charcoal and acacia wood is 31% and 7%, respectively. Based on these results, the adsorbent of coconut shell charcoal is more effective and gives a higher final result in Cr(VI) adsorption compared to acacia wood charcoal [29]. This can be influenced by the presence of functional groups that are owned by coconut shell charcoal. Based on the results of the FTIR analysis, it can be seen that coconut shell charcoal has 0
10 20 30 40 50
0 5 10 15 20 25 30
final concentration (ppm)
Time (minutes)
30 ppm 40 ppm 50 ppm
0 10 20 30 40 50
0 5 10 15 20 25 30
final concentration (ppm)
Time (minutes)
30 ppm 40 ppm 50 ppm
0 10 20 30 40 50
0 10 20 30 40 50
% final removal
First Consentration (ppm)
Acacia Wood Charcoal Coconut Shell Charcoal
159 carboxylate and hydroxyl functional groups, which can be used in the Cr(VI) adsorption process.
The Cr(VI) adsorption process mostly involves the electrostatic attraction between HCrO4- and the positively charged adsorbent. Some of the interactions that can occur between the adsorbent and HCrO4- are as follows [30]:
Adsorben─OH2++HCrO4- ↔ Adsorben─OH2CrO4-+ H+ (11) Adsorben─NH2++HCrO4-↔Adsorben─NH2CrO4- + H+ (12) Adsorben─COOH2++HCrO4-↔Adsorben─COOH2CrO4-+H
(13) In this adsorption mechanism, there is an interaction between the Cr(VI) molecule and the surface of the adsorbent in a solution with acidic conditions. The presence of hydroxyl and carboxyl groups identified through FTIR analysis in Figure 4.1 shows that coconut shell charcoal tends to be more polar [22]. The adsorption of Cr(VI) shows that the binding of negatively charged chromium species (HCrO4−) occurs through electrostatic attraction to the positively charged surface functional groups (because there are more H+ ions) on the surface of the adsorbent occurs according to equations (4) and (6) in on.
At each concentration variation with acacia wood charcoal and coconut shell charcoal adsorbents, a decrease in the final Cr(VI) removal value coincided with an increase in the concentration of the initial solution used. The adsorption process was carried out with the amount of adsorbent and the flow rate of influent entering the column fixed, namely 15 cm and 5 mL/minute. As the initial concentration of the solution increases, more Cr(VI) ions are available to interact with the surface of the adsorbent [31]. When the initial concentration of the adsorbate increases, the driving force for adsorption will also increase but the available sites on the adsorbent remain, causing a decrease in removal efficiency regardless of ion availability [32], [33].
4. Conclusion
The adsorption pH for coconut shell charcoal and acacia wood charcoal is pH 2. The lower the initial concentration of the adsorbate, the higher the percentage of removal shows higher yields on coconut shell charcoal and also on acacia wood charcoal, where the highest percentage of removal is obtained at a concentration of 30 ppm with a percentage of removal of 41% and 16% respectively. Coconut shell charcoal has a better Cr (VI) removal effectiveness than acacia wood charcoal.
Acknowledgments
The author is grateful for the assistance from Universitas Brawijaya, especially the Chemical Engineering Laboratory, and the State University of Malang for providing laboratory facilities to support this research.
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