Dyes which are organic compounds could pollute water, and as a result, they are widely used in various industries , including petroleum, textiles, dyeing, photographic coloring, paper, rubber, leather, pharmaceuticals, plastics, and various other areas . These industries release dye-contaminated wastewater large amounts every year, most of these dyes irritate the skin, and the water-polluted dye reduces photosynthesis, and thus prevents light penetration . Each year, about 80,000 tons of manufactured and consumed dyes are used because they are inexpensive and come in various colors , and thus, their existence in wastewater concerns environmentalists . The dyes are commonly utilized but polluting and dangerous, and the Alizarin Red S dye (ARS) causes malignant disorders of the lungs and respiratory system . Because the Alizarin Yellow R dye (AYS) is used in large quantities in the production of textiles, paper printing, and cosmetics, and because this process leads to wastewater pollution, it poses serious health and environmental risks. It has been classified as an azo dye, the least biodegradable, toxic, carcinogenic, and mutagenic . A lot of research works have been conducted for the removal of these colors from water in various ways [8,9]. Over the past decade, a lot of work has been done to develop better techniques for removing pigments from water . In addition to the surface adsorption, methods such as silica  and silica gel  have been used. In this work, polyacrylic acid hydrogel (PAA) granules have been used, which are flexible polymer chains that make it easy for solute molecules with water to penetrate the hydrogel. They were colorless and  and were used to remove ARS and AYR dyes from aqueous solutions. Figure 1 shows the polymer before and after the adsorption process.
(BDH) provided the materials (ARS, AYR, NaCl, and PAA) and distilled water was used. Figure 2 illustrates the architecture of (ARS and AYR).
- The absorbance as a concentration function was decided by the UV-Vis technique. Absorption wavelengths for Alizarin Red S and Alizarin Yellow R were (425 and 360) nm, respectively.
- The determination of the required time of equilibrium between adsorbate and adsorbent, some of their concentrations were mixed with (0.1 g) of polyacrylic acid hydrogel beads and put in a water bath shaker at 20 °C. We took the samples from the solution at different times to determine the change in concentration over time.
- Adsorption isotherms: The measurement of the adsorption isotherms for dye solutions, we weighed (0.1 g) of the surface six round flasks, and then put to each (50 mL) dyes flask (ARS, AYR).We submerged these flasks in a water bath at (20 °C) for (ARS=210 minutes and AYR=60 minutes). When the mixture was separated, we determined the adsorption by the UV-Vis spectrophotometer.
The determination of the adsorption quantities is based on the following equation :
Qe= the adsorbate (mg/g) quantity.
C◦= initial concentration (mg/L).
Vs= solution volume (L).
m= the surfaces (g) mass
Ce= equilibrium concentration (mg/L).
We repeated the preceding procedure at various temperatures to track the adsorption of dyes on the surface as the temperatures varied.
Results and discussion
Contact time effects
The equilibrium time for (ARS and AYR) dyes is (210 min and 60 min), respectively of the adsorption rise as a function of time, as displayed in Figures 3 and 4.
For each equilibrium concentration, the adsorbed amount (Qe) was computed. Figure 5 illustrates Qe vs. Ce plotted to demonstrate the general adsorption isotherm scheme.
The overall scheme of the adsorption isotherm of ARS and AYR on the PAA surface highlights these wares as belonging to the (S3) class according to Giles classification, in which the surface adsorbate particles are bevel vertical .
Here, the Temkin's equation is based on:
Where, bT and kT are the Temkin constants.
Figure 6a, b demonstrates the graph of Qe vs. LnCe. The Temkin constants for (ARS and AYR) dyes were calculated, as represented in Table 1.
Depending on Freundlich Equation:
Where, KF and n refer to the Freundlich constants.
Figure 7a, b displays the plot of LnQe vs LnCe. These constants for (ARS and AYR) dyes were calculated, as listed in Table 2.
Depending on Langmuir Equation:
Where, K and a are the constants.
Figure 8a, b shows the plot of Ce/Qe vs. Ce. These constants for (ARS and AYR) dyes were calculated, as reported in Table 3.
The isotherms of adsorption of (ARS) were found obeys the Freundlich equation. The isotherms of (AYR) adsorption were found obeys the Langmuir equation.
Effect of temperature
With rising temperature (Endothermic process), the adsorption amount at (ARS and AYR) dyes rises.
By using the Vant Hoff-Arrhenius equation, the (ΔH) value is as follows:
Xm: stands for the maximum amount of adsorbed quantity.
R: stands for gas constant.
T: stands for temperature.
Figure 9 is the plotted LnXm vs. inverted temperature (1/T).
Table 4 represents how to get the (ΔG and ΔS) value by using the following equations:
The experimental results indicate that the adsorption (ARS and AYR) increases with the rise of temperature, and this is consistent with the thermodynamic properties. The positive values of (ΔH and ΔG) indicate an endothermic process. When the process is endothermic, this is evidence of an absorption process in addition to the adsorption process (sorption). With increasing temperature, surface-adsorbed molecules diffuse into the pores and the speed of their diffusion increases, which is in agreement with some studies [16,21,22]. The positive (ΔS) value indicates that the molecules arrangement in the solution was more regular than their arrangement on the surface .
Effect of ionic strength
The absorption of different concentration of NaCl (3.42x10-3M and 17.1x10-3 M) of (ARS and AYR) dyes was studied. It was found that when the salt concentration increases, the absorption decreases, as demonstrated in Figure 10.
The salt molecules crowded the dye molecules to be present on the surface, and this indicates that the salt took some sites on the PAA surface, so the adsorption decreased in the presence of salt.
The vibration rate is an important component that affects the dye adsorption process because it is related to the diffusion or movement of dye molecules towards the absorbent (PAA) surface . The shaking effect was studied, as demonstrated in Figure 11.
Effect of using wet PAA
The percentage of dyes removal increases when using wet PAA instead of dry, as shown in Figure 12.
Effect of initial dyes concentration
The proportion of dye removed was determined by altering the dye concentration at (T = 293K). A 50 mL solution of (ARS and AYR) was employed. After allowing the solution to reach equilibrium, the residual (ARS and AYR) concentration was calculated. The findings in Figure 13 indicate that the percentage of removed dyes is greater at the low concentrations than at the high concentrations. The removal percentage was examined as follow:
By using (Lagrange Equation) kinetic investigations for dyes (ARS and AYR):
Ln(qe – qt) = lnqe– Kad (8)
Where, qe and qt are the capacities (mg/L) of adsorption at equilibrium and time (t).
Kad is the pseudo first-order kinetic adsorption rate constant (min-1) .
The adsorption kinetics seems to follow a pseudo first-order kinetics models, as illustrated Figure 14 and the adsorption rate constant was calculated in Table 5.
And the second-order kinetic study for dyes (ARS and AYR):
Where, qe and qt denote the equilibrium adsorption capacities (mg/L) and time (t), respectively.
Kad2: pseudo second-order kinetic adsorption rate constant (mg/L)-1min-1.
The adsorption kinetics seems to follow the pseudo second-order kinetics models, as indicated in Figure15, and the rate constant of adsorption was calculated in Table 6.
The result showed that the removal of (ARS) dye by (PAA) obeys the first-order reaction and the removal of (AYR) dye by (PAA) obeys the first-order and second-order reaction.
Polyacrylic acid (PAA) (Beads form) available in local markets can be used to treat polluted water with high efficiency and low economic cost in the case of low concentrations. The adsorption pattern according to Giles classification for the three alizarin dyes (ARS, AYR, AGG) was (S3) at temperatures (20Co, 30Co, 40Co, 50Co) where the molecules were arranged perpendicularly or obliquely on the surface. The results of adsorption isotherms were interpreted according to the (Langmuir, Freundlich, Temkin) equation for adsorption of the three alizarin dyes on the surface of (APP), whereby (AYR) dye is more compatible with Langmuir's model than other dyes, while the dyes (ARS, AGG) are in great agreement with Freundlich's model. The study showed that the enthalpy value (ΔH) of the three dyes is positive (that is, the adsorption process is endothermic). The value of (ΔG) was positive for all dyes where the adsorption process is accompanied by the absorption process of any process (sorption). Adsorption follows the illusory first-order law for the three dyes (ARS, AYR, and AGG).
The authors are grateful to Department of Chemistry, College of Education for Pure Science- Ibn Alhaitham, University of Baghdad for an achieving the practical part of this work.
Conflict of Interest
The authors declare that there is no conflict of interests regarding the publication of this manuscript.
Salwan Sanad Akrab: https://orcid.org/0000-0002-3081-3956
Ammar Jasim Mohammed: https://orcid.org/0000-0002-4613-0930
How to cite this article: Salwan Sanad Akrab, Ammar Jasim Mohammed*. Removing some alizarin dyes from an aqueous solution using a polyacrylic acid hydrogel. Eurasian Chemical Communications, 2023, 5(1), 63-72. Link: http://www.echemcom.com/article_155437.html
Copyright © 2023 by SPC (Sami Publishing Company) + is an open access article distributed under the Creative Commons Attribution License(CC BY) license (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.