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                                                                       DOI: 10.3303/CET2189092 
 

 
 

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

 

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Paper Received: 6 May 2021; Revised: 11 August 2021; Accepted: 11 November 2021 
Please cite this article as: Thi N.D., 2021, Evaluating the Removal of Hexavalent Chromium (Cr -VI) in Wastewater by Low-Cost Adsorbent 
Modified from Waste Fly Ash, Chemical Engineering Transactions, 89, 547-552  DOI:10.3303/CET2189092 

CHEMICAL ENGINEERING TRANSACTIONS 

VOL. 89, 2021 

A publication of 

The Italian Association 
of Chemical Engineering 
Online at www.cetjournal.it 

Guest Editors: Jeng Shiun Lim, Nor Alafiza Yunus, Jiří Jaromír Klemeš 
Copyright © 2021, AIDIC Servizi S.r.l.
ISBN 978-88-95608-87-7; ISSN 2283-9216 

Evaluating the Removal of Hexavalent Chromium (Cr -VI) in 
Wastewater by Low-Cost Adsorbent Modified from 

Waste Fly Ash 
Nga Dinh Thi 
Hochiminh City University of Natural Resources and Environment, 236B, Le Van Sy St., Ward 1, Dist. Tan Binh, Hochiminh 
City, Vietnam 
dtnga@hcmunre.edu.vn 

The Cr (VI) is a toxic element to human and other organisms. In this paper, the adsorption of Cr (VI) to fly ash
zeolite (FAZ) was investigated in batch modes. The effect of pH (2 – 9), initial Cr (VI) concentration (0.2 – 1.4 
mg/L), adsorbent amount (0.01 – 3.00 g), and contact time (0 – 5 h) was investigated. These experiments were 
carried out in an Erlenmeyer flask containing 50 mL solution of Cr (VI) at room temperature. The Langmuir and 
Freundlich models were applied for determining the adsorption characteristic and isotherms. The FAZ was 
tested for removing Cr (VI) containing in actual metal plating industry effluent. The result showed that in the 
investigated values of pH the adsorption ability decreased when pH increased. The adsorbent dose of FAZ 
provided the best capacity at 0.05g FAZ in 50 mL solution; the suitable adsorption time was observed at 3 h
duration; the 0.8 mg/L of Cr (VI) concentration was appropriated in this study. The Langmuir model was matched 
to the experimental records (R2 = 0.9922) which demonstrated that the Cr (VI) adsorption on FAZ belonged to
monolayer type. In order reach the standard regulation at the column B, QCVN 40:2011/BTNMT for effluent
industrial wastewater (Cr (VI) concentration is under 0.1 mg/L), it took 6 h duration for the adsorption to drop
the initial Cr (VI) concentration of 0.803 mg/L to 0.091 mg/L. It can be suggested that FAZ is a good material for 
treating of Cr (VI) in aqueous solution and industrial wastewater. 

1. Introduction
People have been facing the dark side of industrialization all around the world which is environmental pollution.
The main concerns about surface and ground water resources are the contamination of organic matters, 
nutrients, microorganisms and heavy metal (Malise et al., 2020). The representative members of heavy metals 
include cadmium, arsenic, chromium, copper, lead, and mercury (Ru et al., 2021). Heavy metals are contained
with high concentration in the effluent of many industries such as metal processing, landfill leachate, mining,
pulp and paper; and pesticides (Guo et al., 2020). It has been well-known that heavy metal ions can destroy the 
enzyme system in living organs, thus cause many diseases related to nervous, digestive, respiratory, 
reproductive, and circulatory systems (Abbas et al., 2016). Chromium is reported to be the seventh most 
abundant elements on earth. It can enter the plants cells by penetrating through the ion channels on the 
membrane, accumulate in the food chain and become poisons to animal and human. It has been recorded that
Cr can destroy many organs such as kidney, lung, brain and immune system. Therefore, the study of heavy 
metals, especially Cr removal in water by appropriate agents is very essential for the suitable development of 
the world. The environmental engineers have been developing new material that can achieve high treatment 
efficiency with low cost (Duan et al., 2020). Among them, the application of adsorption from waste is considered
an expected trend as it could achieve both economic and environmental benefits at once (Kang et al., 2020)
The electric generation from coal is main energy sector in the world. It was reported that about 750,000 t of ash 
are generated annually worldwide in which the coal fly ash proportion is estimated as 40 % (Xing et al., 2019). 
People have been tried to reuse the waste coal fly ash in several fields such as construction materials
(Muthusamy et al., 2020); catalytic synthesis (Sikarwar et al., 2018) ,and adsorbents (Kobayashi et al., 2020). 
A large amount of this waste has been dumped and this process could cause pollution for the soil, the ground 

547



water, and the air and then further made negative effect to the human health or other organisms (Asl et al., 
2019). The advantages of developing FA-based adsorbent are including: low cost; recycle waste fly ash; 
environmental protection; and high efficiency (Pham and Le, 2021). It is necessary to find a suitable approach 
to convert this coal fly ash to useful product and apply in the environmental remediation. This study aims to 
investigate the hexavalent chromium removal by using zeolite modified from waste fly ash of power plant. 

2. Materials and methods
2.1 Chemical reagents and materials

The chemicals including NaOH, HCl, and K2Cr2O7 were in analytical level (Merck, Germany). Distilled water that 
used for diluting and dissolving reagents during experiment was prepared by Aquatron water still equipment 
(Stuart, England). The waste fly ash was taken from Duyen hai thermal power plant, pretreatment and further 
alkali modification to be FAZ. Before doing the experiment, characteristic of FAZ sample was determined by 
BET machine (SURFER, Thermo Fisher Scientific Co., US). As a result, this FAZ had surface space of 66.4 
m2/g, the pore volume was 15.3 cm3/g. 
The specific Cr (VI) concentration was synthesized by dissolving K2Cr2O7 reagent in distilled water with desired 
weight. The actual electroplating industry wastewater was collected from Mocal Company in Protrade Industrial 
Park in An Tay, Ben Cat Town, Binh Duong Province, Vietnam. In the treatment system of the factory, 
wastewater was separated into different lines according to its characteristic. In the line which contained chromic 
acid, the treatment went through steps of: pre-treatment, oxidation-reaction; and then adsorption by activated 
carbon to achieve the required standard before discharging. In this study, the the Cr (VI) removal test was 
carried out using wastewater collected from the treatment systems of the factory before the adsorption step. A 
volume of 50 mL wastewater containing Cr (VI) (C = 0.803 mg/L) was used for the FAZ adsorption test.  

2.2 Experimental procedure 

Batch experiments of Cr (VI) removal by FAZ were performed an Erlenmeyer containing 50 mL of Cr (VI) 
solutions with designed concentrations. Tested solution was shaken at 120 rpm in ambient condition. The effects 
of initial pH (2– 9), contact time (0 – 5 h), adsorbent dose (0.01 – 0.30 g), and Cr (VI) concentration (0.2 – 1.4 
mg/L) were investigated. The NaOH 0.1 M or HCl 0.1 M solutions was used for pH adjustment. In order to 
determine the potential of adsorbent in Cr (VI), the adsorption capacity (Eq(1)) and removal rate (Eq(2))  are 
shown as bellow: 

𝑞𝑞𝑒𝑒 =
(𝐶𝐶0 − 𝐶𝐶𝑒𝑒) ∗ 𝑉𝑉

𝑀𝑀
(1) 

𝑅𝑅 =  
(𝐶𝐶0 − 𝐶𝐶𝑒𝑒)

𝐶𝐶0
∗ 100% (2) 

where: Co is the original concentration of adsorbate (mg/L); Ce and qe are the concentration of adsorbate (mg/L) 
and the adsorption ability (mg/g) at the balance stage; V is the experimental volume (L); M is the adsorbent 
weight (g).  

2.3 Analysis 

During the experimental batches, liquid samples from the reactor were withdrawn from the reactor after a 
designed time. After that, samples were filtered by 0.45 µm pore filter paper, the obtained filtrate was used to 
analyze the Cr (VI) concentration. The quantitative of chromium in liquid samples was processed by following 
the instruction in TCVN 6658:2000. By following the instruction, chromium (VI) calibration curve was established; 
before analyzing, the samples those contained Cr (VI) were reacted with 1,5-diphenylcarbazide reagent to turn 
up a specific color and then further measured by spectrophotometer at 540 nm wavelength by UV – VIS (TCVN, 
6658:2000). 

2.4 Adsorption isotherm models 

Langmuir model was widely applied to define the monolayer adsorption. The Freundlich model has been used 
to describe both monolayer and multilayer processes. In Langmuir model, it is assumed that: the adsorption 
happens on the surface of the material; the adsorption is homogeneous at all points at the surface; there is no 
reaction between adsorbent and the target adsorbate (Ru et al., 2021). The Langmuir model (Eq(3)) and 
Freundlich model (Eq(4)) are indicated below: 

548



𝐶𝐶𝑒𝑒
𝑞𝑞𝑒𝑒

=
1

𝑏𝑏𝑞𝑞𝑚𝑚
+
𝐶𝐶𝑒𝑒
𝑞𝑞𝑚𝑚

(3) 

ln(𝑞𝑞𝑒𝑒) =  ln(𝐾𝐾𝐹𝐹) + �
1
𝑛𝑛
� ln 𝐶𝐶𝑒𝑒 (4) 

Where: Ce, concentration of pollutant at the equilibrium stage (mg/l); qe, adsorption ability at the balance stage 
(mg/L); qm and b, KF the adsorption constants (L/mg); n, the adsorbent intensity. 

3. Results and discussion
3.1 Effect of pH on Cr (VI) adsorption

Figure 1 illustrates the effect of pH on Cr (VI) removal of the FAZ. The result indicated that the removal 
adsorption capacity decreased when pH was increased. The adsorption capacity gradually dropped from 0.79 
to 0.46 mg/g together with the change of pH from 2 to 9. In the previous study, Mor et al. (2007) demonstrated 
that the removal of Cr (VI) ions from aqueous solution by activated charcoal was strongly affected by pH, the 
removal efficiency decreased from 99 % at pH 2 to 60 % at pH 10. The same trend of Cr (VI) removal in different 
pH value was also elucidated by other authors (Jang et al., 2020). This phenomenon can be explained that in 
the solution happens the chemical equilibrium: 2CrO4- + 2H+ ↔ Cr2O72- + H2O. In the acidic condition the Cr2O72- 
is the dominant and CrO4- is dominant alkaline condition. The present of Cr2O72- and H+ ions can provide the 
favor environment for the adsorption process (Asl et al., 2013). When pH increases, the present of OH- in the 
solution could complete with CrO42- in attaching to the surface of adsorbent resulting in the reduction of 
adsorption capacity (Mor et al., 2007).  

Figure 1: The effect of pH on Cr (VI) removal of FAZ adsorbent 

3.2 Effect of FAZ dosage and contact time on Cr (VI) adsorption 

The effect of FAZ dosage on removal efficiency and adsorption capacity is shown in Figure 2a. The results 
demonstrated that when the FAZ dosage increased from 0.01 to 0.30 g, the removal rate was improved but the 
adsorption capacity was reduced. Remarkably, the highest adsorption capacity was achieved at the dosage of 
0.01 g however the removal efficiency at this dosage was only 50.8 %. At the adsorbent dosage of 0.05 g the 
removal efficiency reached 94.6 % and it was not increased significantly when the adsorption dosage increased 
in later level up to 0.30 g. This dose was chosen for the other experiments in this study.  Figure 2b describes 
the removal rate during operational time. It is noticeable that the removal rate sharply increased in the first two 
hours of the operational time and to reach 91.2 % at the contact time of 3 h and the adsorption ability at this 
point was as 0.74 mg/g. The removal efficiency was eligible improve in later phase at contact time of 4 h (94.2 
%) and 5 h (94.6 %). This could be explained by the fact that the active positions were stepwise saturated, so 
the chromium ions had no ability to compete with the functional groups in the material. Previous studies have 
described that an initial Cr (VI) concentration of 10 mg/L in the dose of adsorbent 0.5 g/100 mL solution could 
reach the equilibrium after 120 min (Mor et al., 2007). Dakiky et al. (2002) also found out that the necessary 
contact time for achieving equilibrium condition of Cr (VI) is 2 h (Dakiky et al., 2002). It can be assumed that the 
treatment of Cr (VI) by fly ash based adsorbent could obtain the equilibrium condition after 3 h contact time. 

549



Figure 2: The effect adsorbent dosage (a) and contact time (b) on Cr (VI) adsorption 

3.3 Effect of adsorbate concentration on adsorption capacity 

The experiment was carried out in various concentrations in the range of 0.2 to 1.4 mg/L, the amount of 
adsorbent was fixed at 0.05 g of FAZ in 50 mL solution for each batch test; pH value was adjusted at 3. The 
result indicated that the adsorption ability of Cr (VI) by FAZ was enhanced with the enlargement of initial 
concentration from 0.2 to 0.8 mg/L corresponding to improved adsorption ability from 0.18 to 0.62 mg/g. When 
the chromium (VI) concentration was changed from 0.8 to 1; 1.2; and 1.4 mg/l, the adsorption capacity was 
insignificantly improved (Figure 3). In the earlier study, Khan et al investigated the removal of chromium (VI) by 
using rice husk carbon adsorbent and revealed that the removal rate of Cr (VI) decreased from 62.7 to 43.2 % 
when the concentration of Cr (VI) risen from 60 to 120 mg/L (Khan et al., 2016). It was elucidated that when the 
amount of adsorbent was constant, the higher adsorbate concentration the less active sites and capillaries of 
adsorbent were available. In other words, the adsorbent became saturated resulting the decrease of adsorption 
capacity in the later phase of the experiment (Mor et al., 2007).  

Figure 3: The effect initial Cr (VI)  concentration on adsorption capacity 

3.4 Adsorption isotherms 

After conducting experiments, the adsorption kinetics of FAZ was determined. Based on the Langmuir model, 
the good linear relationship between Ce/qe on the Ce was obtained which indicated at high the correlation 
coefficient (R2 = 0.9922) (Figure 4a). The maximum adsorption level was calculated as qm = 0.797 mg/g. From 
the observed data, the adsorption trend (RL) that equals to 1/(1+ bC0) was calculated, RL value of 0.806. 
Previous researches introduced the status of isotherm in the value range of RL as following: RL > 1: unfavorable; 
RL = 1: linear; 0 < RL < 1: favorable; and RL = 0: irreversible (Ma et al., 2019). For that reason, the value of RL 
in this study is in the expected range. Therefore, it can be assumed that the adsorption route was fixed with 
Langmuir isotherm and it can be considered that the adsorption of Cr (VI) on FAZ is monolayer type. Figure 4b 
shows the relationship between lnqe and lnCe by applying Freundlich model. The obtained straight line showed 
R2 as 0.6084. It meant that Freundlich model is not appropriate for the observed data from this experiment and 
this model is not suitable to describe the adsorption process of Cr (VI) on FAZ in this study. The better match of 
the Langmuir equation than that Freundlich model in study of removing Cr (VI) in the solution by different 
adsorbents was also found out in the former researches (Mor et al., 2007).  

550



Figure 4: The isotherm schemes of Cr (VI) adsorption by FAZ: a) Langmuir model; b) Freundlich model 

3.5 The Cr (VI) removal in actual metal plating industry wastewater 

The change of chromium concentration in the solution during the experimental period is illustrated in the Figure 
5. It is clear to observe that the adsorption capacity was gradually increased from 0 to 5 h with the increase of
removal efficiency from 48.00 % to 87.96 % and it reached 88.63 % after 6 h operation corresponding to the Cr
(VI) concentration as 0.091 mg/L. This effluent value of Cr (VI) could satisfy the required standard regulated at
the column B, QCVN 40: 2011/BTNMT for effluent industrial wastewater (QCVN, 2011). It was demonstrated
that the actual metal plating industry wastewater which contained competitive ions from many other metals could 
affect the adsorption of chromium to the adsorbent (Wen et al., 2011). From the experimental result, it can be
demonstrated that the FAZ could provide a feasible approach in the treatment of Cr (VI) in the electroplating
industry wastewater.

Figure 5: The removal Cr (VI) in metal plating industry wastewater by FAZ 

4. Conclusions
This study found out that the FAZ is effective to remove Cr (VI) containing in synthesis and actual electroplating 
industry wastewater. The results suggests that he suitable operational conditions of the tests as at acidic 
condition, pH = 2. At the adsorption dosage of 0.05 g, FAZ could perform well for the treatment of 50 mL Cr (VI) 
(C = 0.8 mg/L) at contact time of 3 h. The study of adsorption isotherms revealed the best suitable of the 
Langmuir model in this case study with the correlation coefficient of 0.9922. The adsorbent also demonstrated 
the high potential of treatment Cr (VI) electroplating industry wastewater. The FAZ adsorbent could remove Cr 
(VI) from 0.803 g/L to 0.091 mg/L after 6 h to achieve the requirement of Cr (VI) level containing in industry
effluent (under 0.1 mg/L) indicated in QCVN 40:2011/BTNMT. In order to have fully understanding in this
process, it is necessary to conduct further experiment to investigate the effect of other operational factors such
as temperature. The adsorption of concomitant heavy metals in actual wastewater is essential to investigate.

Acknowledgements 

The author would like to thanks to Hochiminh City University of Natural resources and Environment for 
supporting the facilities and to express the appreciation to MSc. Vo N. H. Linh who assisted for experiment. 

551



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	Evaluating the Removal of Hexavalent Chromium (Cr -VI) in Wastewater by Low-Cost Adsorbent Modified from Waste Fly Ash