Melanin-embedded materials effectively remove hexavalent chromium (CrVI) from aqueous solution
Cuong et al. Environmental Health and Preventive Medicine
Melanin-embedded materials effectively VI remove hexavalent chromium (Cr ) from aqueous solution
An Manh Cuong 0
Nguyen Thi Le Na 0
Pham Nhat Thang 3
Trinh Ngoc Diep 3
Ly Bich Thuy 2
Nguyen Lai Thanh 0
Nguyen Dinh Thang 0 1
0 Department of Biochemistry and Molecular Biology, Faculty of Biology, VNU University of Science, Vietnam National University , 334 Nguyen Trai St., Thanh Xuan Dist, Hanoi , Vietnam
1 Key Laboratory of Enzyme and Protein Technology, VNU University of Science , Hanoi , Vietnam
2 Institute for Environmental Science and Technology, Hanoi University of Science and Technology , Hanoi , Vietnam
3 High school for Gifted Students, VNU University of Science , Hanoi , Vietnam
Background: Currently, it is recognized that water polluted with toxic heavy metal ions may cause serious effects on human health. Therefore, the development of new materials for effective removal of heavy metal ions from water is still a widely important area. Melanin is being considered as a potential material for removal of heavy metal from water. Methods: In this study, we synthesized two melanin-embedded beads from two different melanin powder sources and named IMB (Isolated Melanin Bead originated from squid ink sac) and CMB (Commercial Melanin Bead originated from sesame seeds). These beads were of globular shape and 2-3 mm in diameter. We investigated and compared the sorption abilities of these two bead materials toward hexavalent-chromium (CrVI) in water. The isotherm sorption curves were established using Langmuir and Freundlich models in the optimized conditions of pH, sorption time, solid/liquid ratio, and initial concentration of CrVI. The FITR analysis was also carried out to show the differences in surface properties of these two beads. Results: The optimized conditions for isotherm sorption of CrVI on IMB/CMB were set at pH values of 2/2, sorption times of 90/300 min, and solid-liquid ratios of 10/20 mg/mL. The maximum sorption capacities calculated based on the Langmuir model were 19.60 and 6.24 for IMB and CMB, respectively. However, the adsorption kinetic of CrVI on the beads fitted the Freundlich model with R2 values of 0.992 for IMB and 0.989 for CMB. The deduced Freundlich constant, 1/n, in the range of 0.2-0.8 indicated that these beads are good adsorption materials. In addition, structure analysis data revealed great differences in physical and chemical properties between IMB and CMB. Interestingly, FTIR analysis results showed strong signals of -OH (3295.35 cm− 1) and -C=O (1608.63 cm− 1) groups harboring on the IMB but not CMB. Moreover, loading of CrVI on the IMB caused a shift of broad peaks from 3295.35 cm− 1 and 1608.63 cm− 1 to 3354.21 cm− 1 and 1597.06 cm− 1, respectively, due to -OH and -C=O stretching. Conclusions: Taken together, our study suggests that IMB has great potential as a bead material for the elimination of CrVI from aqueous solutions and may be highly useful for water treatment applications.
CrVI; CMB; IMB
Currently, environmental pollution caused by rapid
industrialization and technological advances is a
worldwide problem. It is recognized that water polluted with
toxic heavy metals can have serious effects on human
]. There are many types of materials which
have been being used to remove heavy metals from
aqueous effluents; these include activated carbon,
plantleaf materials, chitosan gel, and hydrotalcite [
However, these materials are not fully effective nor cost
Chromium is considered to be one of the key
contaminants in the wastewaters of many industries, such as
plating-electroplating, dying-pigmenting, film-photography,
leathering and mining. Although both hexavalent
chromium (CrVI) and trivalent chromium (CrIII) are
predominant species in industrial effluents, the CrVI is more toxic
than CrIII. More seriously, the CrVI is considered as a
mutagenic agent, which may cause adverse public health
Melanin is synthesized in humans, animals, invertebrate
animals, bacteria, and fungi by oxidation of phenol or
indole compounds [
]. Besides its role in pigmentation,
melanin has many other important biological functions; it
serves as an electron transporter, ion balancer, free radical
acceptor as well as antioxidant, antibacterial, antivirus,
and anticancer agent [
]. Thus, melanin has been widely
considered to be a potential material for use in various
industries including agriculture, pharmacy, medicine, and
Recently, melanin powder (but not melanin bead) has
also been examined for its ability to eliminate heavy
metal ions (e.g., lead, cadmium, copper, and ferrous) in
aqueous solutions [
]. Generally, for removing of
heavy metal ions, a material in powder form should have
very high sorption capacity. However, there is no
guarantee that a high sorption capacity for the material exists
in bead form [
]. For practical conditions, such as in
drinking water treatment, the bead form (rather than in
powder form) of a material is the most popular and
suitable form to avoid the possibility of being stuck when
water flow passes through the material column. To date,
there has been no study evaluating the use of melanin
originated from squid ink sacs for removal of chromium
ions, although a previous report showed that melanin
secreted from Aureobacidium pullulans could also
adsorb CrVI from waste water . However, the
different source of melanin may have big different capacity in
removing of CrVI ion. In this study, we used two different
melanin sources: one was isolated from squid ink sacs,
which are considered as waste material of seafood
processing companies and named as IMB (Isolated Melanin
Bead), and the other was derived from sesame seeds
(purchased from Xi’an Green Spring Technology Co., LTD,
China) and named as CMB (Commercial Melanin Bead).
These two melanin powders were used to make
melaninembedded beads for investigating their abilities to remove
hexavalent chromium ions (CrVI). This study also aimed
to compare the capacity of CrVI uptake by the two
melanin-embedded beads. Comparisons were made by
examining differences in their physical and chemical
properties due to their different source of origin.
Melanin isolation from squid ink sacs
The method used for isolating melanin has been
described previously [
]. Briefly, squid ink sacs
collected from the seafood company were broken down
to collect ink liquid. This liquid (50 g) was dissolved
into 200 mL of 0.5 M HCl. The mixture was then
sonicated for 15 min in a sonicator followed by
stirring for 30 min. The mixture was then incubated at
4 °C for 48 h before centrifuging at 10,000 rpm at 5 °
C for 15 min to collect the pellet. The pellet was
washed with acetone for three times then washed
with distilled water for three times. The melanin
pellet was dried at 60 °C, grinded, sieved through
150 μm, and then stored at room temperature.
Method for making spherical melanin-embedded beads
Melanin beads were made according to a previously
published protocol [
]. Briefly, melanin powder was
embedded using sodium alginate as a cohesion agent.
Sodium alginate was dissolved in 20 mL of distilled
water and incubated in a water bath incubator at 70 °C
to completely dissolve it before adding 5 g of melanin
powder with continuous stirring. The mixture solution
was drawn into a syringe and then eluted drop by drop
into CaCl2 solution (5%) to create beads with spherical
form and with a diameter of 2–3 mm. Next, the melanin
beads were dried out and dipped into 5% CaCl2 solution
for 24 h before washing with distilled water for three
times and drying to unchanged weight.
Fourier-transform infrared analysis
Infrared spectra of the material beads were obtained
using a Fourier-transform infrared spectrometer (FTIR
Affinity - 1S, SHIMADZU, Kyoto, Japan) [
Morphology and purification of melanin powder isolated
from squid ink sac was investigated by scanning electron
microscope, model NANOSEM450 (Netherlands), and
surface property of melanin bead was examined under
Carlzeiss stereo-microscope, model stemi SV2000 (Germany).
Sorption experiments and CrVI analytical methods
Experiments were conducted at room temperature.
Batch equilibrium sorption experiments were carried
out in 250 mL Erlenmeyer flasks containing
potassium dichromate (K2Cr2O7) solutions (100 mL) of
known concentrations (varying from 5 to 200 mg/L).
Melanin was added into the K2Cr2O7 solution with
various ratios of solid/liquid and placed on a shaker
at 200 rpm for various time settings. The solution
was then centrifuged at 10,000 rpm for 10 min. In
the acidified medium, CrVI reacted with diphenyl
carbazide to form a purple-violet colored complex. The
concentration of CrVI in the supernatant was
determined colorimetrically using a spectrophotometer
(Shimadzu). Absorbance was measured at wavelength
(λ) of 540 nm [
]. Standard curves were generated
and depicted in Fig. 1. Adsorption efficiencies were
calculated using following formula:
H: Adsorption efficiency (%)
Co: Initial concentration (mg/L)
Ce: Equilibirium concentration (mg/L)
Method for determining the isotherm adsorption equations
Freundlich adsorption model: The Freundlich model is
used to describe the adsorption model from liquids and
can be expressed as the following equation [
ln qe ¼ ln K F þ n
qe ¼ qmax
Ce þ qmax
Ce: concentration at equilibrium stage (mg)
qe: adsorption capacity at equilibrium stage (mg/g)
qmax: maximum adsorption capacity (mg/g)
KL: adsorption constant for Langmuir (L/mg)
KF, 1/n: adsorption constants for Freundlich (L/mg)
Langmuir adsorption model: The Langmuir model,
which is mainly used to determine the maximum
adsorption capacity, is expressed as the following
In this study, all experiments were repeated three times,
and the collected data were analyzed with the appropriate
statistical tests. To compare the two groups, the
MannWhitney U test (for non-parametric comparisons) or
Student’s t test (for parametric comparisons) were used.
Significance was set at three levels with P < 0.05 [
Synthesis of spherical melanin beads
After purification, isolated melanin pellet was lyophilized
to obtain the intact natural squid melanin. Then the
melanin sample was examined by the scanning electron
microscope (SEM), which showed high purity without
contamination by any cellular components (Fig. 2). This
purified melanin was more than enough for treatment of
heavy metal ions in adsorption experiments [
To produce the spherical melanin-embedded beads,
melanin powder was added into the binding agent
solution containing alginate at different percentages, which
varied from 3 to 15%, to form a mixture before dropping
into the CaCl2 solution to form beads (Fig. 3). The
results showed that at low percentages of alginate (3 and
4%), the formed melanin beads were not stable and were
easily broken since the concentration of the binding
agent was insufficient. At the high percentages of
alginate (12 and 15%), the formed melanin beads did not have
spherical shape because the viscosity of the mixture was
too high. Percentages of alginate in the range of 5–10%
were optimal to form stable melanin beads with
spherical shape (Fig. 3).
In addition, neither alginate content (in the range of
3–15%) nor the drying method (un-drying,
lowtemperature drying, high-temperature drying, or freezing
drying) had any significant effect on the sorption
capacities of the beads (Fig. 4). However, alginate at 5% was
chosen because it yielded the highest productivity and
uniformity of the melanin beads.
Effect of pH on CrVI sorption by melanin-embedded
The effect of pH on the efficiency of CrVI removal by
melanin beads was evaluated for the following set
conditions: shaking rate of 200 rpm at 30 °C, solid/liquid ratio
of 10 g/L, shaking time of 1 h, and CrVI initial
concentration of 200 mg/L. The results are shown in Fig. 5.
The removal efficiencies of CrVI by IMB or CMB were
better at lower pH values and reached the maximum at
pH 1–2 (Fig. 5a). However, IMB had a much higher
sorption capacity compared to that of CMB at any pH
value. In particular, at the optimized pH (1–2), the
sorption capacity of IMB was almost threefold higher
than that of CMB.
Effect of sorption time on CrVI removal efficiency
The effect of sorption time on the efficiency of CrVI
removal by melanin beads was also evaluated for the
following set conditions: pH of 2, shaking rate of
200 rpm at 30 °C, initial CrVI concentration of
200 mg/L, and the solid/liquid ratios of 20 g/L. The
results indicated that the longer the sorption time,
the higher the removal efficiency. However, the
removal efficiency quickly increased during the first
hour then slowly increased and reached the highest
values around 96% at 2 h for IMB and 67% at 6 h
for CMB (Fig. 5b). In general, at any sorption time,
IMB was more effective than CMB at removing CrVI.
In particular, at the same sorption time of 2 h, the
sorption capacity of IMB was 2.8-fold higher than
that of CMB.
Effect of solid/liquid ratios on sorption efficiency
To investigate the effect of solid/liquid ratios on the
efficiency of CrVI adsorption, we tested solid/liquid ratios in
the range of 1–30 g/L with the following set conditions:
pH of 2, shaking rate of 200 rpm at 30 °C, shaking time
of 1 h, and CrVI initial concentration of 200 mg/L. The
results showed that the removal efficiency increased
rapidly as the solid/liquid ratio increased from 1 to 20 g/L
and increased only slightly from 20 to 30 g/L. The
maximum removal efficiencies reached 95 and 35% for IMB
and CMB, respectively, at the solid/liquid ratio of 30 g/L
(Fig. 5c). At the same solid/liquid ratio, IMB was much
more effective than CMB at eliminating CrVI.
Effect of initial concentration of CrVI on sorption
The effect of the initial concentration of CrVI on the
efficiency of CrVI removal by melanin beads was evaluated
for the following set conditions: pH of 2, shaking rate of
200 rpm at 30 °C, solid/liquid ratio of 20 g/L, and
sorption time of 2 h for IMB or 4 h for CMB. The initial
concentrations of CrVI were in the range of 5–200 mg/L.
The CrVI removal efficiencies and the sorption capacities
of CMB and IMB are shown in Fig. 5d and presented in
Tables 1 and 2. The maximum capacities for IMB and
CMB were 19.6 and 6.24, respectively. These results
indicate that while CMB is not that efficient at
eliminating CrVI, IMB is efficient and serves as a promising
material for CrVI removal due to its high sorption
capacity, especially as bead form. Previous studies have
tested numerous materials (e.g., activated carbon, sludge,
plant-leaf materials, and chitosan gel) for CrVI removal
from aqueous solution and have shown that these
materials as powder form had sorption capacities of wide
range from 6 mg/g to 50 mg/g [
]. Our study shows
that IMB (in bead form) is a highly effective material for
removing CrVI in water.
CrVI sorption kinetics
The results of this study indicate that an increase of
initial concentration can lead to a decrease of CrVI removal
efficiency and increase of the sorption capacity. From
the isotherm adsorption results of CrVI at different
concentrations on IMB and CMB at optimized conditions,
we then examined the suitable isotherm adsorption
model for adsorption of CrVI on IMB and CMB using
the two common models of Langmuir and Freundlich.
The results are shown in Fig. 6. The isotherm equations
deduced from Langmuir and Freundlich models were
presented as follows:
Equation of the Langmuir model for CMB (Fig. 6a): Cqe
= 0.051 Ce + 0.492; R2 = 0.885
Equation of the Freundlich model for CMB (Fig. 6c):
ln qe = 0.491lnCe − 0.237; R2 = 0.989.
Equation of the Freundlich model for IMB (Fig. 6d): ln
qe = 0.601lnCe + 0.655; R2 = 0.992
Parameters for isotherm adsorption of CMB and IMB
are summarized in Table 3. The results suggest that the
Freundlich model is more suitable than the Langmuir
model to describe the sorption mechanism of CrVI on
melanin bead since the R2—coefficient value of the
Freundlich model—was higher than that of the Langmuir
model. The data also indicate that the surfaces of IMB or
CMB are not uniform, and therefore, the distributions of
reaction centers on the surface of the materials probably
follow an exponential equation rather than a linear one. In
the Freundlich model, the mechanism and the rate of
adsorption are functions of the constants 1/n and KF. For
a good absorbance, the 1/n value should be 0.2 < 1/n < 0.8,
and a smaller value of 1/n indicates better adsorption and
formation of strong bonds between the adsorbate and
]. In this study, the 1/n values of 0.49
and 0.6 for CMB and IMB, respectively, demonstrate that
both IMB and CMB are good materials for adsorption of
CrVI; however, IMB has a much better adsorption capacity
compared to CMB.
Fourier transform infrared analysis
In general, on the surface of the melanin material, there
are many chemical groups including hydroxyl, carboxyl,
and ether, which have been proposed to be responsible
for sorption of metal ions by formation of chemical
bonding. The chemical-sorption ability of the material
depends on factors such as quantity of active centers, its
accessibility, and affinity between active centers and
metal ions [
]. The surfaces of IMB and CMB were
observed under stereo-microscope and presented in
Fig. 7. The differences in surface structure of IMB and
CMB are clearly distinguishable. It showed that the
intensities of peaks of the hydroxyl, carboxyl, and ether
groups in IMB were very clear and sharp, while the
intensities of these corresponding peaks in CMB were
not so clear especially for hydroxyl group. This result
indicated that the distribution of chemical groups on the
surface of IMB may be denser on the surface of CMB
and might lead to difference in numbers of chemical
linkages formed between melanin and CrVI ion.
Conversely, FTIR analysis was used to analyze the
functional groups on the surfaces of the native and
CrVI-bound IMB and CMB; results are shown in
Figs. 8 and 9. IMB and CMB showed completely
different FTIR spectra. While IMB had the broad
absorption peaks at 3296 cm− 1 and 1608 cm− 1 due
to the presence of the –OH and –C=O groups,
], there were almost no peaks at
these sites on the surface of CMB (Fig. 8a and
Fig. 9a). Although many other sorption peaks were
observed, it is difficult to interpret all. After loading CrVI,
the FTIR spectra of CrVI-bound IMB and CrVI-bound
CMB were presented in Fig. 8b and Fig. 9b. The results
indicated that the adsorption of CrVI on the surface of
IMB may have caused a shift of the broad peaks at
3296 cm− 1 and 1608 cm− 1 to 3354 cm− 1 and 1597 cm− 1,
respectively, due to –OH and –C=O stretching (Fig. 8b).
Chromium pollution originated from plating and
electroplating industries, iron and steel industries, and
inorganic-chemical production represents a huge
problem for environmental health [
]. Exposure to
chromium ions, especially CrVI, may cause diseases related to
the digestive system and lung; such complications can
include epigastric pain, nausea, diarrhea, hemorrhage,
and cancer [
]. Thus, it is essential to eliminate CrVI
from wastewater before disposal. There are many
methods which can be applied to remove CrVI from
aqueous solutions; these methods include ion exchange
], chemical precipitation [
], reduction [
], solvent extraction [
], membrane separation [
], and reverse
osmosis and biosorption [
]. However, these different
methods have different disadvantages, such as low
removal efficiency, expensive equipment, high operating
cost, and high energy requirement [
In this study, we investigated the ability of melanin (as
a material in bead form) to remove CrVI from aqueous
solution. Two different natural melanin sources, one
originating from plant (commercial one) and the other
extracted from ink sacs of squid (isolated one), were
used for making melanin-embedded beads; the beads
were called CMB and IMB, respectively. In many Asian
countries, the seafood industry is one of the most
important industries which provide great economic
benefit for the country. Squid and octopuses are
processed in many seafood processing companies for export.
Nevertheless, ink sacs of squid and octopuses are wastes
in these seafood companies. More importantly, melanin
accounts for about 16–18% in total weight of the sac
]. Thus, utilization of these wastes for melanin
production will have great impact since melanin has not
only been considered as a potential material for heavy
metal removal but also for many other applications, such
as medicine and cosmetics [
To examine the effect of IMB and CMB on removing
CrVI, the effect of various parameters such as pH,
sorption time, and solid-liquid ratio on CrVI sorption were
conducted, and isotherm models including Freundlich
and Langmuir were applied to fit experimental data. In
accordance with previous studies [
], the data
showed that IMB and CMB both had the highest
sorption capacities as pH 1–2 and that the Freundlich model
was the best model to represent the sorption model of
CrVI on IMB and/or CMB.
There are many materials which have been used to
remove chromium ions in effluents from various
industries. The removal capacities of these materials vary from
0.2 to 200 mg/g. In general, sorption capacities of
materials are different from their origins, for example:
plantoriginated materials (0.5–10 mg/g), activated carbon
materials (2–30 mg/g), coal (6.68 mg/g), hydrous
titanium oxide (5 mg/g), maghemite nanoparticles (1.5 mg/
g), and tannin gel (200 mg/g) [
]. In addition,
almost all materials used in previous studies were in
powder form, and therefore, their capacities in CrVI
removal would be significantly decreased after making
the bead form. Previous studies demonstrated that acidic
conditions at pH of 1 or 2 were good for the removal of
CrVI from water [
]. This study also introduced the
similar result. In practical conditions, CrVI pollutant
mostly comes from the mining and platting industries,
which normally have effluents with low pH values. That
means IMB should be a suitable material for treatment
of CrVI from industrial effluent. Besides that, in some
cases, CrVI pollutant may also come from natural water,
which has pH of 5–7. However, concentration of CrVI in
natural water is below 1 μg/L , while this study
showed that the adsorption capacity of IMB for CrVI was
about 7–8 mg/g at the pH of 6–7 (one third of that at
pH of 1–2). It means that IMB is also good enough for
removing of CrVI from natural water.
In this study, our results demonstrated that melanin
materials are potential for the removal of CrVI from water.
However, melanin from different sources have different
physical and chemical properties. Particularly, the
properties of IMB (melanin extracted from squid ink sacs) were
significantly different from those of CMB (melanin
extracted from plant). These results led to a difference in
the ability of these two melanin materials to eliminate
CrVI from aquaous solution. CMB had CrVI sorption
capacity of 6.24 mg/g while IMB had CrVI sorption capacity
of 19.8 mg/g. In summary, our study suggests that
melanin isolated from squid ink sacs (which are considered as
waste of seafood processing companies) can be used to
synthesis the melanin bead and applied in water treatment
to effectively remove CrVI ions.
CMB: Commercial Melanin Bead; CrVI: Hexavalent chromium;
FTIR: Fourier-transform infrared analysis; IMB: Isolated Melanin Bead
Availability of data and materials
Please contact author for data requests.
AMC, NTLN, PNT, and TND carried out the melanin bead synthesis,
adsorption experiments. NLT carried out the FITR analysis. LBT performed the
statistical analysis. NDT conceived and designed the study and drafted the
manuscript. All authors read and approved the final manuscript.
Ethics approval and consent to participate
Consent for publication
The authors declare that they have no competing interests.
Springer Nature remains neutral with regard to jurisdictional claims in
published maps and institutional affiliations.
1. Ohgami N , Yamanoshita O , Thang ND , Yajima I , Nakano C , Wenting W , Ohnuma S , Kato M. Carcinogenic risk of chromium, copper and arsenic in CCA-treated wood . Environ Pollut . 2015 ; 206 : 456 - 60 .
2. Thang ND , Yajima I , Kumasaka M , Kato M. Bidirectional functions of arsenic as a carcinogen and an anticancer agent in human squamous cell carcinoma . PLoS One . 2014 ; https://doi.org/10.1371/journal.pone. 0096945 .
3. Singha B , Naiya TK , Bhattacharya AK , Das SK . Cr(VI) ions removal from aqueous solutions using natural adsorbents-FTIR studies . J Environ Protection . 2011 ; 2 : 729 - 35 .
4. Mohan D and Pittman CY . Activated carbons and low cost adsorbents for remediation of tri- and hexavalent chromium from water . J Hazard Mater . 2006 ; 137 : 762 - 811 , 2006 .
5. EPA, Environmental Protection Agency, Environmental Pollution Control Alternatives, EPA/625/5-90/025, EPA/625/4-89/023, Cincinnati, US , 1990 .
6. Manivasagan P , Venkatesan J , Senthilkumar K , Sivakumar K , Kim SK . Isolation and characterization of biologically active melanin from Actinoalloteichus sp . MA-32. Int J Biol Macromol . 2013 ; 58 : 263 - 74 .
7. Mbonyiryivuze A , Nuru ZY , Ngom BD , Mwakikunga B , Dhlamini SM , Park E , Maaza M. Morphological and chemical composition characterization of commercial sepia melanin . American Journal of Nanomaterials . 2015 ; 3 ( 1 ): 22 - 7 .
8. Nosanchuk JD , Casadevall A . Impact of melanin on microbial virulence and clinical resistance to antimicrobial compounds . Antimicrob Agents Chemother . 2006 ; 6 : 3519 - 28 .
9. Tarangini K , Mishra S . Production, characterization and analysis of melanin from isolated marine pseudomonas sp. using vegetable waste . Res J Engineering Sci . 2013 ; 2 ( 5 ): 40 - 6 .
10. Hong L , Simon JD . Current understanding of the binding sites, capacity, affinity, and biological significance of metals in melanin . J Phys Chem B . 2007 ; 111 ( 28 ): 7938 - 47 .
11. Hong L , Liu Y , Simon JD . Binding of metal ions to melanin and their effects on the aerobic reactivity . Photochem Photobiol . 2004 ; 80 : 477 - 81 .
12. Szpoganicz B , Gidanian S , Kong P , Farmer P . Metal binding by melanins: studies of colloidal dihydroxyindole-melanin, and its complexation by Cu(II) and Zn(II) ions . J Inorg Biochem. 2002 ; 89 : 45 - 53 .
13. Yu XH , Gu GX , Shao R , Chen RX , Wu XJ , Xu W. Study on adsorbing chromium (VI) ions in wastewater by Aureobacidium pullulans secretion of melanin . Adv Mater Res . 2011 ; 156 - 157 : 1378 - 84 .
14. Magarelli M , Passamonti P , Renieri C . Purification, characterization and analysis of sepia melanin from commercial sepia ink (Sepia Officinalis) . Rev CES Med Vet Zootec . 2010 ; 5 ( 2 ): 18 - 28 .
15. Kato M , Azimi MD , Fayaz SH , Shah MD , Hoque MZ , Hamajima N , et al. Uranium in well drinking water of Kabul, Afghanistan and its effective, low-cost depuration using Mg-Fe based hydrotalcite-like compounds . Chemosphere . 2016 ; 165 : 27 - 32 .
16. Bansal M , Singh D , Garg VK . A comparative study for the removal of hexavalent chromium from aqueous solution by agriculture wastes' carbons . J Hazard Mater . 2009 ; 171 ( 1-3 ): 83 - 92 .
17. Gupta S , Babu BV . Removal of toxic metal Cr(VI) from aqueous solutions using sawdust as adsorbent: equilibrium, kinetics and regeneration studies . Chem Eng J . 2009 ; 150 : 352 - 65 .
18. Aoyama M , Sugiyama T , Doi S , Cho NS , Kim HE . Removal of hexavalent chromium from dilute aqueous solution by coniferous leaves . Holzforschung . 1999 ; 53 : 365 - 8 .
19. Dakiky M , Khamis M , Manassra A , Mereb M. Selective adsorption of chromium(VI) in industrial wastewater using low-cost abundantly available adsorbents . Adv Environ Res . 2002 ; 6 ( 4 ): 533 - 40 .
20. Aksu Z , Acikel U , Kabasakal E , Tezer S. Equilibrium modelling of individual and simultaneous biosorption of chromium(VI) and nickel(II) onto dried activated sludge . Water Res . 2002 ; 36 : 3063 - 73 .
21. Garg UK , Kaur MP , Garg VK , Sud D. Removal of nickel (II) from aqueous solution by adsorption on agricultural waste biomass using a response surface methodological approach . Bioresour Technol . 2008 ; 99 ( 5 ): 1325 - 31 .
22. Liu Y , Simon JD . Metal-ion interactions and the structural organization of Sepia eumelanin . Pigment Cell Res . 2005 ; 18 ( 1 ): 42 - 8 .
23. Ho YS , Chiang CC , Hsu YC . Sorption kinetics for dye removal from aqueous solution using activated clay . Sep Sci Technol . 2001 ; 36 ( 11 ): 2473 - 88 .
24. Wang YT , Xiao C . Factors affecting hexavalent chromium reduction in pure cultures of bacteria . Water Res . 1995 ; 29 : 2467 - 74 .
25. Mohanty K , Jha M , Meikap BC , Biswas MN . Removal of chromium(VI) from dilute aqueous solutions by activated carbon developed from Terminalia Arjuna nuts activated with zinc chloride . Chem Eng Sci . 2005 ; 60 : 3049 - 59 .
26. Tiravanti G , Petruzzelli D , Passiono R . Pretreatment of tannery wastewaters by an ion exchange process for Cr(III) removal and recovery . Water Sci Technol . 1997 ; 36 : 197 - 207 .
27. Zhou X , Korenaga T , Takahashi T , Moriwake T , Shinoda S. A process monitoring/controlling system for the treatment of wastewater containing chromium(VI) . Water Res . 1993 ; 27 : 1049 - 54 .
28. Kongsricharoern N , Polprasert C . Chromium removal by a bipolar electrochemical precipitation process . Water Sci Technol . 1996 ; 34 : 109 - 16 .
29. Seaman JC , Bertsch BM , Schwallie L . In situ Cr(VI) reduction within coarsetextured, oxide-coated soil and aquifer systems using Fe(II) solutions . Environ Sci Technol . 1999 ; 33 : 938 - 44 .
30. Calace N , Muro DA , Nardi E , Petronio BM , Pietroletti M. Adsorption isotherms for describing heavy metal retention in paper mill sludges . Ind Eng Chem Res . 2002 ; 41 : 5491 - 7 .
31. Pagilla K , Canter LW . Laboratory studies on remediation of chromium contaminated soils . J Environ Eng . 1999 ; 125 : 243 - 8 .
32. Chakravarti AK , Chowdhury SB , Chakrabarty S , Chakrabarty T , Mukherjee DC . Liquid membrane multiple emulsion process of chromium(VI) separation from wastewaters . Colloids Surf A Physicochem Eng Asp . 1995 ; 103 : 59 - 71 .
33. Aksu Z , Ozer D , Ekiz H , Kutsal T , Calar A . Investigation of biosorption of chromium(VI) on C. crispate in two staged batch reactor . Environ Technol . 1996 ; 17 : 215 - 20 .
34. Aksu Z , Gonen F , Demircan Z. Biosorption of chromium(VI) ions by Mowital B3OH resin immobilized activated sludge in a packed bed: comparison with granular activated carbon . Process Biochem . 2002 ; 38 : 175 - 86 .
35. Aoyama M. Removal of Cr(VI) from aqueous solution by London plane leaves . J Chem Technol Biotechnol . 2003 ; 78 : 601 - 4 .
36. Aoyama M , Kishino M , Jo TS . Biosorption of Cr(VI) on Japanese ceder bark . Sep Sci Technol . 2004 ; 39 ( 5 ): 1149 - 62 .
37. Aoyama M , Tsuda M , Seki K , Doi S , Kurimoto Y , Tamura Y. Adsorption of Cr(VI) from dichromate solutions onto black locust leaves . Holzforschung . 2000 ; 54 : 340 - 2 .
38. Mohan D , Singh KP , Singh VK . Removal of hexavalent chromium from aqueous solution using low-cost activated carbons derived from agricultural waste materials and activated carbon fabric cloth . Ind Eng Chem Res . 2005 ; 44 : 1027 - 42 .
39. Mohan D , Singh KP , Singh VK . Trivalent chromium removal from wastewater using low cost activated carbon derived from agricultural waste material and activated carbon fabric cloth . J Hazard Mater . 2006 ; 135 : 280 - 95 .
40. WHO. Guidelines for drinking-water quality . 2nd ed. vol. 2 . Geneva: World Health Organization; 1996 .