Removal of inorganic mercury from aquatic environments by multi-walled carbon nanotubes
© Yaghmaeian et al. 2015
Received: 21 May 2014
Accepted: 11 July 2015
Published: 28 July 2015
Mercury is considered as a toxic heavy metal in aquatic environments due to accumulation in bodies of living organisms. Exposure to mercury may lead to different toxic effects in humans including damages to kidneys and nervous system.
Materials and methods
Multi-walled carbon nanotubes (MWCNTs) were selected as sorbent to remove mercury from aqueous solution using batch technique. ICP instrument was used to determine the amount of mercury in solution. Moreover, pH, contact time and initial concentration of mercury were studied to determine the influence of these parameters on the adsorption conditions.
Results indicate that the adsorption strongly depended on pH and the best pH for adsorption is about 7. The rate of adsorption process initially was rapid but it was gradually reduced with increasing of contact time and reached the equilibrium after 120 min. In addition, more than 85 % of initial concentration of 0.1 mg/l was removed at 0.5 g/l concentration of sorbent and contact time of 120 min. Meanwhile, the adsorption process followed the pseudo second-order model and the adsorption isotherms could be described by both the Freundlich and the Langmuir models.
This study showed that MWCNTs can effectively remove inorganic mercury from aqueous solutions as adsorbent.
KeywordsAdsorption Aqueous solutions Heavy metals Nano material
Water resource pollution with industrial effluents is known as a serious environmental problem, nowadays .Of these, scientists have focused mostly on the presence of mercury due to its bioaccumulation in organisms, toxic effects and persistence in environment [2, 3]. In addition, it should be noted that mercury has been widely used in various industrial fields as an element including chlor-alkali, pharmaceutical, producing barometer and thermometer, mining, dental practices. It is proved that high concentrations of mercury are released constantly from the mentioned industries to the environment [4, 5].
From the toxicological point of view, the level of toxicity of mercury is highly related to its chemical form . To put it another way, mercury transforms biologically, physically, and chemically through its cycle in nature, which results in the formation of various forms of mercury. Organic mercury is the most toxic among these forms . Mercury is mostly in its inorganic (Hg+2) or methylmercury forms in aquatic environments . However, at the presence of specific kind of bacteria the inorganic form of mercury transforms to methylmercury which is highly toxic for human and other organisms at food chain . Exposing to mercury results in neurological disorders, damage to central nervous systems, and also negatively affects the kidney and liver . Considering these facts, there should be a proper way to handle this element and remove it from the environment.
Various methods have been used to remove mercury from water and wastewater, including chemical precipitation, ion exchange and membrane methods . Nevertheless, these methods have their own weaknesses including high level of either energy or chemical compound is needed, and most importantly these methods are not able to remove low concentration of mercury from the environment. However, adsorption due mostly to its high performance, recoverability, and reactive ability of adsorbent can be considered as a suitable method in terms of economy [11, 12].
In adsorption process, it is needed to have an adsorbent with wide specific surface. The surface results from the existence of tiny pores at it which the chemical property, area, size and distribution of these pores influence the level of an adsorbent’s specific surface. Different materials, such as fruit shell , chitosan , marine macroalga , bagasse pith , furfural , and rubber  have been applied as adsorbents to remove mercury from aqueous environments.
After the discovery of carbon nanotubes, scientists paid specific attention to them because of their particular efficiency in construction, electricity, chemistry, and physics. Also, these materials have widely been used to produce nano-structured materials, nanocomposites, sensors, and gas adsorption. In 2004, when EPA proposed a research into the environmental application of these materials , wide ranges of studies were conducted on these nanotubes which have tiny pores with uniform size and also wide specific surface . Usage of carbon nanotubes was studied to remove pollutants, such as fluoride , dichlorobenzene , trihalomethanes , zinc , chromium , nickel [26, 27], and cadmium  from water and waste water. In this paper, Multi-walled carbon nanotubes) MWCNTS (were used to remove inorganic mercury from aqueous solutions.
Materials and methods
Batch reactors with the same volumes of 250 ml were used. The reactors were filled with 100 ml of mercury solution with the concentrations of 0.1, 1 and 10 mg/l. The pH of the solutions were adjusted by nitric acid 0.1 N and NaOH 0.1 N (Merk). After adjusting the pH, the solutions were agitated under the temperature of 25 °C and 150 rpm on an Incubator Shaker (Innova 4340, USA). Thereafter, the solution was passed through 0.2 μm Millipore filter in order to separate the adsorbents from the aqueous solutions. Then, the pH was adjusted and lowered to <2 by using nitric acid. It should be noted that the prepared solutions were kept in glass containers at 4 °C. Besides, Cold vapour inductively coupled plasma optical emission spectrometry (Spectro, Germany) was applied to measure the concentration of mercury. This method has high sensitivity, excellent detection limits, rapid analysis and Easy to use. Also chemical and spectral interference is less than other methods . For the reliable determination of mercury all variables were measured at least twice.
considered variables in previous studies the removal of metal ions from aquatic environments by carbon nanotubes
pH, contact time, initial metal ion concentration. Isotherm models
adsorption kinetic, Isotherm models
pH, contact time, agitation speed
pH, ionic concentration, Isotherm models
pH, contact time, initial metal ion concentration, adsorbent’s concentration, Isotherm models
pH, contact time, agitation speed, adsorbent’s concentration, adsorption kinetic, Isotherm models
pH, contact time, initial metal ion concentration, temperature, adsorption kinetic, Isotherm models
contact time, pH, ionic strength, foreign ions
where q denotes the amount of Hg2+ adsorbed on adsorbent at any time (mg/g), C0 denotes the initial Hg2+ concentration (mg/l), Ct denotes the concentration of Hg2+at any time (mg/l), V denotes the volume of the solution (l) and m denotes the adsorbent mass (g).
Results and discussion
The surface functional groups of MWCNTs
Figure 1(d) depicts the FTIR spectra of multi-walled carbon nanotubes. This spectrum displays major peaks at 3755, 3443, 2923, 1633, 1459 and 1051 cm−1. The peak at 3755 cm−1 is associated with free hydroxyl groups. The peak at 3443 cm−1 is attributed to the O–H stretch from carboxyl groups (O = C − OH and C − OH). The peak at 2923 cm−1 is associated with (CH2). The peak at 1633 cm−1 is related to carbonyl groups. The peak at 1459 cm−1 is associated with carboxylic acids and phenolic groups (O–H). The peak at 1051 cm−1 is attributed to the (C–O). These functional groups can prepare numerous chemical sorption sites on surface of the MWCNTs [24, 28, 30, 32, 33].
Effect of contact time
Effect of primary concentration
Effect of solution’s pH
After the test, the pH was measured again and it was cleared that in the primary pH of upper than 5, the final pH decreases. The release of H2+ from the surface of carbon nanotubes into the solution could be the reason of this decrease. In the same contact time, the decrease of final pH was higher for the high concentration of mercury then the low concentration of mercury. When the primary concentration of mercury increases, the amount of adsorption of Hg+2 also gets higher by which the rate of H2+ release from the adsorbent surface increases and causes to the drop of the pH level of the solution . This process can be the indicator of chemical adsorption of inorganic mercury on the surface of the carbon nanotubes in pH of over 5.
Effect of adsorbent’s concentration
Effect of ionic concentration
Adsorption isotherms describe the distribution of metal ions between the liquid and solid phase at equilibrium. Adsorption balance of metallic ions is usually studied by the Freundlich and Langmuir adsorption isotherm. Langmuir isotherm is the indicator of active surface adsorption on the homogenous surface, while the Freundlich isotherm is used for heterogeneous surfaces . The linear form of Langmuir and Freundlich equations are as below.
Where Ce denotes the equilibrium concentration of Hg+2 (mg/l), qe denotes the amount adsorbed (mg/g), qm denotes the theoretical saturated adsorption capacity (mg/g), K denotes the Langmuir constant (l/mg). The values of K and qm were calculated by plotting Ce/qe versus Ce. KF and n are the Freundlich constants related to adsorption capacity and adsorption intensity, The Freundlich constants n and KF were calculated by plotting log qe versus log Ce.
The parameters of Langmuir and Freundlich isotherm models for the removal of Hg+2 by MWCNTS
KF (l/ g)
The adsorption kinetic model can provide suitable information for designing the removal of pollutants from water and wastewater. In order to assess the adsorption kinetic of inorganic mercury on the surface of carbon nanotubes, the pseudo-first and pseudo-second orders of kinetic equations were applied.
Where qt is the amount of Hg+2 adsorbed at any time (mg/g), qe is the amount of Hg+2 adsorbed at equilibrium (mg/g), K1 is the adsorption rate constant (1/min).
A straight line log (qe -qt) versus t indicates the applicability of the pseudo-first order kinetic model.
Kinetic parameters for the removal of Hg+2 by MWCNTS at different mercury concentrations
primary Hg+2 concentration (mg/l)
qe, exp (mg/g)
qe, cal (mg/g)
qe, cal (mg/g)
k2 (g/mg min)
Qu et al.  reported that nanomaterials have been widely used to remove heavy metals from water due to their large surface area, high reactivity, short intra particle diffusion distance and low temperature modification. In spite of that Tang et al.  suggested the reuse and management of the used nanomaterials is an important issue and has not been considered seriously. Only a few relevant studies are available in literature. It would be worthwhile to investigate the reusability of the used nanomaterials.
Multi-walled carbon nanotubes were assessed as adsorbent to remove inorganic mercury from aqueous solutions. The adsorption rate of Hg2+ on the surface of adsorbent is highly affected by pH, and the increase of pH from 3 to 7 increases the percentage of removal. The best contact time is 120 min. Also, the increases of primary concentration of inorganic mercury and ionic concentration solution have negative effect on adsorption process. Finally, the process of adsorption follows both Freundlich and Langmuir isotherms, and the pseudo-second order adsorption kinetic can well describe adsorption process. The present study indicated that Carbon nanotubes have high efficiency in adsorbing of mercury.
Inductively coupled plasma optical emission spectrometry (End-on-Plasma)
Multi-walled carbon nanotubes
This research was supported by Tehran University of Medical Sciences & health Services (Project No. 17314).
Our aim of this research is Removal of mercury from water and waste water that it plays an important role to maintain environmental and human health. Usage of nano materials as a new technology has been increased in different research. Hence, efficiency of carbon nanotubes to remove mercury from water and waste water was studied in this paper.
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