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20CHEMICAL SCIENCES
EXTRACTION OF HEAVY METAL IONS FROM AQUEOUS SOLUTIONS
Obushenko T.,
Senior Lecturer National Technical University of Ukraine «Igor Sikorsky Kyiv Polytechnic Institute» Tolstopalova N., PhD, Associate Professor National Technical University of Ukraine «Igor Sikorsky Kyiv Polytechnic Institute»
Chyrieva M.
Student National Technical University of Ukraine «Igor Sikorsky Kyiv Polytechnic Institute»
ABSTRACT
In this work, model cobalt aqueous solutions were investigated using anionic surfactants and isoamyl alcohol as the organic phase. The regularities of solvent sublation of Co2 + ions from aqueous solutions are investigated and the following rational conditions of the purification process are revealed: surfactant - sodium dodecyl sulfate; pH 10-10.5; the molar ratio of Co2+: surfactant = 1: 2; process duration - 20 min, organic phase - isoamyl alcohol; volume of organic phase - 10 cm3; the volume of the aqueous phase is 200 cm3. For these conditions, the degree of withdrawal is 95.6%.
Keywords: sublation, heavy metals, surfactant, extractant, substrate, degree of extraction.
Introduction. Increasing the amount of polluted water and lack of clean water is already one of the urgent problems today. One of the most important environmental tasks is the treatment of wastewater from enterprises in various industries. Heavy metals (Me) are among the most biologically dangerous pollutants. The main sources of heavy metal pollution are metallurgical, metalworking and chemical plants. The toxicity of the metal is due to its effect on the metabolism of living organisms and human health. Most heavy metal ions have carcinogenic, mutagenic properties and have a cumulative effect [1].
Given the modern methods of removing heavy metal ions from wastewater, it is advisable to develop innovative technologies for wastewater treatment from heavy metal ions. The quality of treated wastewater must meet the needs until its return for industrial and technical needs of the enterprise, or before discharge into the city sewer system. Solvent sublation is such a perspective method. This method is based on a combination of flotation and extraction methods, based on the passage of gas bubbles through the aqueous phase and the removal of the pollutant substance (sublate) in the organic phase. The organic phase should be lighter than the aqueous, and not dissolve in it. In the process of solvent sublation, surfactants are used, which play the role of collectors, binding to heavy metal ions in water-insoluble hydrophobic sublates, which due to their hy-drophobic properties, adhesion forces bind to the bubbles and are removed from the aqueous phase in organic.
The purpose of this work is to develop a technology of solvent sublation of wastewater from heavy metals on the example of cobalt (II) ions.
To achieve this goal it is necessary to: theoretically justify the choice of method of effective wastewater treatment from heavy metal ions; to study
the physico-chemical laws of the process of solvent sublation as a method of removal of cobalt ions, to propose a method of recovery of the extractant and the basic scheme of purification.
Solvent sublation technology was first proposed as a type of ion flotation in cases where the formation of foam having the required properties for ion flotation is impossible or it is necessary to quantitatively separate the foam for further analysis [2]. A characteristic feature of this flotation process is the method of separating the floated substance (sublate) by concentrating it in a layer of organic liquid on the surface of the aqueous phase.
Advantages and disadvantages of solvent subla-
tion:
- no foam; - the ability to work with large volumes of water bodies, the concentration of which may exceed a ratio of 100: 1, this gives the solvent sublation a great potential in the analysis of elements that are in the water in micro- and nano-quantities;
- the active substance is carried out by gas bubbles and enters the upper layer of the hydrophobic liquid without mixing the phases; - the degree of removal in the process of solvent sublation does not depend on the ratio of the volumes of aqueous and organic phases; -in many cases, the substance to be recovered is concentrated in the organic phase, which greatly facilitates its further processing.
Traditionally, the main disadvantages include low, compared to flotation, productivity due to low gas consumption, which does not destroy the upper layer of organic liquid in the distribution apparatus [3-4].
All solvent sublation systems consist of the following elements [5-6]:
- gas source (it can be both a conventional compressor and a gas cylinder, the most common gases are air, argon and nitrogen);
- gas flow rate and pressure control systems; - rotameter (used to measure gas flow);
- column (usually used glass or plastic columns, at the bottom of which is a partition with the required porosity).
Experimental. The solvent sublation was performed in a glass column 3 made in the form of a cylinder (volume 250 cm3, column diameter 34 mm, H = 375 mm). Nitrogen from cylinder 1 was fed to the column. The sample for analysis was taken from the upper
part of the column. The experimental installation is shown in Fig. 1.
A model solution of Co2+ was prepared with CoCl2-6H2O. Standard solution: 1 g / dm3. Working concentration: 10 mg / dm3. The following collectors were used: sulfanol (CisH29SO3Na) and sodium lauryl sulfate (sodium dodecyl sulfate) (Ci2H2sSO4Na). The pH was adjusted with solutions of alkali NaOH with a concentration of 0.1 M and HCl with a concentration of 0.1 mol / dm3.
1 - cylinder, 2 - rotameter, 3 - solvent sublation column, 4 - Schott filter. Fig. 1 - Experimental installation.
The model solution in the required amount was transferred to a volumetric flask, made the calculated amount of surfactant to achieve the desired ratio of Me: surfactant. The solution was quantitatively transferred to a solvent sublation column and 10 cm3 of organic phase (isoamyl alcohol) were added. Turned on the gas supply, which was controlled by a flow meter and bubbled the gas through the solution to a constant residual concentration of cobalt, which. determined by standard photometric methods [7].
The main characteristic of the system is the degree of removal X - the ratio of the difference between the initial and residual concentrations to its initial concentration, expressed as a percentage:
(cm ,0 _ CM ,res)
XM - ■
c
•100%,
M ,0
where Xm - the degree of removal of the test ion, Cm, res - the residual concentration of the ion in the test solution, Cm, o - the initial concentration of the ion.
Research results and discussion. The efficiency of the solvent sublation is influenced by number of fac-
tors. Process parameters such as the acidity of the medium, the duration of the process, the choice of extract-ant, the number of reagents are determined by the properties of the complexing agent and the metal and should be studied on a case-by-case basis. The determining factor in choosing the pH of the solution is the stability of the complex compound. Collectors are added to the system in order to increase the hydrophobicity of the formed complexes.
Selection of extractant. Of great importance in solvent sublation is the selection of the extractant. The removal process with isoamyl alcohol, heptane, isooctane, octanol and undecyl alcohol was investigated. The working volume of the aqueous solution containing Co2 + ions is 200 cm3, the concentration is 0.01 mg / cm3. Surfactant - sodium dodecyl sulfate (SDS). It has been investigated that the best extractant is isoamyl alcohol, which has a branched structure and, as a consequence, a better ability to retain sublates. Comparative characteristics of the extractants by the degree of removal are shown in Fig. 2. The volume of the organic phase is 10 cm3. According to the data obtained, the highest degree of seizure is 95.6%.
95,6
eö
<+H
O (L> (U
<-i ¿
W O
n s
O p
i-h
100 90 80 70 60 50 40 30 20 10 0
isoamyl alcohol
heptane isooctan octanol
undecyl alcohol
Fig. 2 - Comparative characteristics of extractants by the degree of removal of Co2 + ions.
Dependence of the degree of removal of Co2 + ions on the ratio of Co2 +: surfactant. The graph (Fig. 3) shows the results of experiments with the following ratios of Co2 +: SDS = 1: 0.5; 1: 1.5; 1: 2; 1: 2.5. With a Co2 +: SDS ratio in the range of 1: 0.5 to 1: 1.5, the degree of removal is not maximal, due to the insufficient
amount of surfactant for complete binding of cobalt to the sublate. It was investigated that the ratio of Co2+: SDS=1: 2 is the most optimal and the degree of removal is 95.6%.
cd
>
O
a;
i—
<+-
o
100 90 80 70 60 50 40 30
a; a;
tg 20
Ci
10 0
0,5
1
1,5
2,5
Ratio Co:surfactant
Fig. 3 - Dependence of the degree of removal of cobalt (II) ions on the molar ratio of Co: SDS (process duration
- 20 minutes; pH 10.5; organic phase - isoamyl alcohol).
The decrease in the degree with a further increase in the ratio can be explained by the appearance of excess surfactants in the aqueous phase. As a result, it is difficult to transfer the substance from the aqueous to the organic phase and its accumulation on the interface of the water-organic layer.
Dependence of the degree of removal of Co2 + on the pH of the solution. The pH of the test solution was
100
varied in the range of 2-11. The effect of pH on the degree of removal, the duration of the process - 20 min, the molar ratio of Co2 +: SDS = 1: 2, the volume of the aqueous and organic phases (isoamyl alcohol) - 200 cm3 and 10 cm3 respectively. According to the obtained data (Fig. 4), the best removal of cobalt (II) takes place at pH 10-10.5, when the cobalt sublate is in the form of a neutral hydroxide molecule.
i 80
CÜ
>
2 60
40
i—
<+H
O
u 20 OJ
M 0
pH
11
Fig. 4 - Dependence of the degree of removal of cobalt (II) ions on pH.
0
2
3
1
3
5
7
9
The dependence of the degree of removal of Co2+ on the duration of the solvent sublation. The solvent sublation was performed for 40 minutes. The sample was taken for analysis every 5 minutes. As can be seen from Fig. 5, the optimal duration of the process was 20
minutes with 95.6% removal of cobalt at a ratio of Co: SDS = 1: 2. When the process lasts longer than 20 minutes, there is a decrease in the degree of removal, which can be explained by the partial transition of the substrate from the organic to the aqueous phase.
100
95
=n 90 c3
> 85
o S
(L)
!-h <+H
O
80 75
(U (U
&70
65
Me: SDS
1
0,5 1
1,5 2
2,5
0
10 20 30
Duration of prosess, min
40
50
Fig. 5
Dependence of the degree of removal of cobalt (II) ions on the duration of the process at different molar ratios.
The effect of the molar ratio of Me: surfactant for pH 9-11 was studied in more detail. The obtained data are shown in Fig. 6.
pH 9 pH 10 pH 11 pH 12
0,00 0,50 1,00 1,50 2,00 2,50
Ratio Me:surfactant
3,00
Fig. 6 - Dependence of the degree of removal of cobalt (II) ions at different ratios (process duration - 20 min).
Therefore, the obtained data indicate that the molar ratio is an important factor in the process of solvent sublation. However, only the right pH will achieve the maximum degree of removal.
Dependence of the degree of removal of cobalt ions on the initial concentration of the working solution. Initial concentrations of the working solution studied: 0.001 mg / cm3; 0.01 mg / cm3; 0.1 mg / cm3. The process time is 40 minutes, the degree of removal was examined every 5 minutes. Surfactant - sodium dodecyl sulfate, extractant - isoamyl alcohol. The volume of the aqueous phase is 200 cm3, the volume of the organic phase is 10 cm3.
It was obtained that the highest degree of removal - 97.48% is achieved at an initial metal concentration of 0.1 mg / cm3. That is, with increasing concentration of the initial working solution, the degree of removal increases (Figure 3.6). This can be explained by the fact that the coagulation process takes place in the system, even the intensive formation of aggregates was visually noticeable. The decrease in the degree of removal with decreasing concentration can be explained by a decrease in the probability of convergence of particles and, accordingly, the ability to coagulate.
> O
S
(U
!-h <+H
O
(L> (U
!-h
w
100 95 90 85 80 75 70 65 60 55 50
C=0,001 Mr/Mfl C=0,01 Mr/Mfl C=0,1 Mr/M^
50
0 10 20 30 40
Duration of process, min
Fig. 7 - Dependence of the degree of removal on the duration of the process at different molar concentrations
Extragent regeneration study. It is known from previous experimental studies that during the solvent sublation the organic phase is saturated with the sub-late. It is economically and ecologically expedient to regenerate the spent extractant by separating the organic solvent and the substrate by distillation. Distillation - the process of separating solids or liquids (or mixtures thereof) into constituent parts (components) by evaporation followed by condensation without access of air. The distillation process is based on the different ability of substances to turn into a vapor state depending on temperature and pressure.
Regeneration of the extractant was carried out in a laboratory installation, which is shown in Fig. 8. In the
distillation process, the mixture was heated in a flask (distilling cube) using a flask heater. Since isoamyl alcohol forms an azeotropic mixture with water (boiling temperature = 95 °C), which contains 41% alcohol, the first fractions of the distillate were obtained at a temperature of 95 °C.
To remove water, a small amount of Na2SO4 (anhydrous) was added to a container with an azeotropic mixture. Due to its high hygroscopicity and insolubility in organic solvents, sodium sulfate is used for their dehydration. Next, at a temperature of 131 °C, the second fraction of the distillate was obtained - isoamyl alcohol proper. At the end of the distillation in the distilling cube contained the remnants of the sublat.
Installation for distillation
Fig. 8 - Laboratory installation for distillation.
The quality of the obtained isoamyl alcohol was evaluated using refractometry - an optical method of analysis, which is based on the measurement of the refractive index (n) of the test substance using a refrac-tometer.
The value of the refractive index of isoamyl alcohol: for the original alcohol (before solvent sublation) -1.4065; after distillation - 1.4057; reference - 1.4058. Based on the obtained data, it can be concluded that the obtained distillate has a quality not worse than the original isoamyl alcohol, which was used for solvent subla-tion.
Conclusions. An urgent problem today is the protection of the environment from pollution by toxic industrial waste. Paying attention to the ecological situation on the planet, technologies that use closed cycles
with minimal waste generation are becoming competitive. The imperfection of the existing technologies of wastewater treatment contaminated with heavy metal ions necessitates the search for such methods that allow to treat wastewater to the maximum allowable concentration and to carry out the regeneration of valuable components. The solvent sublation method satisfies the following requirements. One of the characteristic features of this method is the possibility of repeated concentration of metal ions in small volumes of organic solvent. It is established that the process of sublaton is influenced by a number of factors: pH of the solution, molar ratio of Me: surfactant, initial concentration of extracted metal ions, duration of the process. The regularities of solvent sublation of Co2 + ions from aqueous solutions are investigated and the following rational
conditions of the purification process are revealed: surfactant - sodium dodecyl sulfate; pH 10-10.5; the molar ratio of Co2 +: surfactant = 1: 2; process duration - 20 min, organic phase - isoamyl alcohol; volume of organic phase - 10 cm3; the volume of the aqueous phase is 200 cm3. For these conditions, the degree of withdrawal is 95.6%. Recovery of spent flotation extractant (isoamyl alcohol) by distillation is proposed.
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