|Suprapto||Department of Industrial Chemical Engineering, Faculty of Vocational Studies, Institut Teknologi Sepuluh Nopember, Kampus ITS Sukolilo, Surabaya, 60111, Indonesia|
|Takehiko Gotoh||Department of Chemical Engineering, Graduate School of Engineering, Hiroshima University, Kagamiyama 1-4-1, Higashi-Hiroshima, 739-8527, Japan|
|Nurlaili Humaidah||Department of Industrial Chemical Engineering, Faculty of Vocational Studies, Institut Teknologi Sepuluh Nopember, Kampus ITS Sukolilo, Surabaya, 60111, Indonesia|
|Renna Febryanita||Department of Chemical Engineering, Faculty of Industrial Technology, Institut Teknologi Sepuluh Nopember, Kampus ITS Sukolilo, Surabaya 60111, Indonesia|
|Muhammad Sa’i Firdaus||Department of Chemical Engineering, Faculty of Industrial Technology, Institut Teknologi Sepuluh Nopember, Kampus ITS Sukolilo, Surabaya 60111, Indonesia|
|Eva Oktavia Ningrum||Department of Industrial Chemical Engineering, Faculty of Vocational Studies, Institut Teknologi Sepuluh Nopember, Kampus ITS Sukolilo, Surabaya, 60111, Indonesia|
This study explored the ion adsorption of heavy metals using a copolymer gel containing zwitterionic betaine N,N'-dimethyl(acrylamidopropyl)ammonium propane sulfonate (DMAAPS) as the ion adsorbent agent and N-isopropylacrylamide (NIPAM) as the thermosensitive agent. We investigated the effect of ion monomer concentration and type and of temperature on the adsorption, desorption, and swelling properties and their correlation. A free-radical polymerization reaction was performed to prepare the thermosensitive NIPAM-co-DMAAPS gel using accelerators such as N,N,N',N'-tetramethylethylenediamine, ammonium peroxydisulfate as the initiator, and N,N'-methylenebisacrylamide at a concentration of 10 mmol/L as the cross-linker. An analysis was then performed on the gel’s adsorption, desorption, and reversible adsorption-desorption properties using atomic absorption spectrophotometry. The results showed that the swelling degree and adsorption values increased as the temperature decreased in the gel with NIPAM:DMAAPS ratios of 9:1 and 8:2. In contrast, in a 7:3 ratio, the swelling degree increased significantly, and the adsorption ability decreased as the temperature increased. The higher the temperature, the smaller the quantity of Zn2+ and Pb2+ ions adsorbed and desorbed. The results indicate that in nitrate solution, Pb2+ ions are more easily adsorbed than Zn2+ ions.
Adsorption; Desorption; Swelling; Thermosensitive
Industrial development leads to increasing metal concentrations in the environment. This is a serious problem, given that heavy metals are non-biodegradable and persistent. Therefore, above certain concentrations, they harm aquatic ecosystems and human health. Various technologies have been successfully developed to decrease the heavy metal contents of industrial liquid waste.
One of these technologies is a conventional method of chemical precipitation and neutralization (El Samrani et al., 2008; Amaral Filho et al., 2016) . Although this method is commonly used, it can produce other waste in the form of sludge containing high heavy metal ion concentrations. Other methods are reverse osmosis (Chan and Dudeney, 2008), and nanofiltration (Cséfalvay et al., 2009), which involve separating the heavy metal content in liquid waste using membranes. However, such methods require high operational costs. In addition, using adsorbent-containing ligands, such as ion-exchange or chelating groups, also requires a strong acid or base in the process of cation or anion resin regeneration. This is a disadvantage because if the process is unsuccessful, it produces secondary waste in the form of a strong acid or base (Qdais and Moussa, 2004).
Kusrini et al. (2018) applied an adsorption technique with maximum iodine absorbance of 572.2 mg/g to remove commercial lanthanide ions from an aqueous solution using an adsorbent in the form of activated carbon extracted from a banana peels. Olufemi and Eniodunmo (2018) reported the adsorption of nickel (II) ions from aqueous solutions using banana peels and coconut shells. The optimal conditions were an adsorbent dose of 4.5 g, 120 minutes of contact time, and 25°C for the banana peels and an adsorbent dose of 4.5 g, 30 minutes of contact time, and 25°C for the coconut shells. The adsorption was better determined by the Langmuir isotherm, with coefficient of correlation values of 0.9821 for the banana peels and 0.9744 for the coconut shell.
Another adsorption method involves the use of a thermosensitive gel in the form of zwitterionic betaine, which can bind the anions and cations in liquid waste simultaneously (Ningrum et al., 2015 ; Ningrum et al., 2019a; Ningrum et al., 2019b). An interaction between negative and positive charges in the same repetition unit in zwitterionic sulfobetaine causes ion selectivity, making this method more attractive than others (Neagu et al., 2010). In general, zwitterionic betaine polymers are thermosensitive in water. They do not dissolve even at temperatures above the upper critical solution temperature (UCST). This means that when they are below the UCST and in water, they undergo coil collapse due to inter- and intra-chain interactions. Above the UCST, the inhibition caused by the inter- and intra-chain interactions can be overcome by the thermal energy produced. The increase in zwitterionic polymer concentration causes an increase in intra- and/or inter-chain interactions in the polymer, and thus higher thermal energy is needed to overcome the interaction, causing the polymer’s UCST to increase (Takahashi et al., 2011). Zwitterionic betaine properties is also affected by interactions between zwitterionic containing charged groups and aqueous salt solutions (Kudaibergenov et al., 2006). Poly(N-isopropylacrylamide) (poly[NIPAM]) is a polymer with thermosensitive properties characterized by a low critical solution temperature (LCST) of 32°C (Ningrum et al., 2017a). NIPAM has a neutral charge and swells at low temperatures and shrinks at high temperatures because it changes from hydrophilic to hydrophobic.
Zwitterionic polymers have great potential in a wide range of biological and medical applications, such as antifouling coatings (Guo et al., 2015), blood contacting sensors (Yang et al., 2011; Joshi et al., 2015), drug delivery in vivo (Fang et al., 2011; Cao et al., 2012), separation membranes (Hadidi and Zydney, 2014; Tu et al., 2015), marine coatings (Aldred et al., 2010; Zheng et al., 2017), catalysts (Ajmal et al., 2015), and absorption dyes (Sahiner and Demirci, 2017). Liu et al. (2005) used a hybrid zwitterionic polymer for ion exchange membranes synthesized by sol–gel process on N-[3-(trimethoxysilyl) propyl] ethylene diamine, with 3-glycidoxypropyltrimethoxysilane, and by a reaction with ?-butyrolactone.
Ningrum et al. (2017b) conducted a study on the effect of monomer concentration on the adsorption and desorption properties of thermosensitive NIPAM and N,N'-dimethyl(acrylamidopropyl)ammonium propane sulfonate (DMAAPS) gel, comparing NIPAM:DMAAPS monomer ratios of 2:8, 1.5:8.5, and 1:9. The temperatures used during desorption and adsorption were 10, 30, 50, and 70?, while the solution used was NaNO3. The results showed that the higher the concentration and temperature of the NaNO3 solution, the lower the NIPAM monomer concentration in the NIPAM-co-DMAAPS gel. In another study, Ningrum et al. (2020a) investigated the properties of an NIPAM-co-DMAAPS polymer gel, such as transition temperature, molecular structure, viscosity in water and Zn(NO3)2 solutions, and adsorption behavior. The poly (NIPAM-co-DMAAPS) in both water and the Zn(NO3)2 solution demonstrated a transition period of LCST. The higher the NIPAM monomer ratio, the lower the polymer’s LCST. In addition, its transition temperature with a lower NIPAM concentration was not verified either in water or in the Zn(NO3)2 solution. Moreover, the higher the NIPAM concentration used in the preparation, the higher the polymer’s viscosity, and the higher ion adsorption onto the gel, the higher the polymer transmittance.
Several studies have explored the copolymerization of polymers and thermosensitive zwitterionic sulfobetaine. The use of zwitterionic polymers can improve the selectivity of ions against adsorption since the cations and anions in the solution bind through both negative and positive charges (Ningrum, 2019; Ningrum et al., 2020b). These studies, however, generally focused only on the synthesis of gel and its properties. Therefore, this research aimed to improve the adsorption capability of gel by employing NIPAM and DMAAPS and copolymerizing them in various molar ratios such as 9:1, 8:2, and 7:3. DMAAPS was used as the ion adsorption agent because of its charged groups, while NIPAM acted as the desorption agent because it can change from hydrophilic to hydrophobic. NIPAM fills the space between DMAAPS molecules in the copolymer and allows further distances between them. Consequently, the interaction between charged groups of DMAAPS is weakened. The weak interaction between the charged groups causes the ions in the solution to pair easily with charged groups, as they are not engaged in inter- or intra-chain or intra-group interactions. Since they can adsorb and desorb ions, the gels can be used in reverse. In addition, at higher NIPAM concentrations, the gel is expected to be have higher swelling ability that will maximize the desorption and adsorption efficiency of the gel. In light of this, the effects of time, temperature, and monomer concentrations on the swelling degree, adsorption, and desorption in Zn(NO3)2 and Pb(NO3)2 solutions were investigated. The reversible adsorption-desorption of Zn2+ and Pb2+ in an aqueous solution with an NIPAM-co-DMAAPS gel were also examined.
The copolymer gel took more than 15 hours to reach the swelling degree equilibrium and desorption and adsorption points. In the NIPAM:DMAAPS ratios of 9:1 and 8:2, the swelling degree increased as the temperature decreased. In contrast, the 7:3 ratio resulted in a higher swelling degree as the temperature increased. In the nitrate solution, the swelling degree and adsorption ability of the 9:1 and 8:2 ratios decreased with an increase in temperature. Conversely, in the 7:3 ratio, the swelling degree increased significantly, while the adsorption capability decreased with a temperature increase. Furthermore, in the 9:1 ratio and a temperature of 10°C, the gel’s adsorption ability reached its highest values of 22.16% (Zn2+) and 26.24% (Pb2+). It can be concluded that the higher the temperature, the lower the quantity of Zn2+ and Pb2+ ions adsorbed and desorbed. In nitrate solution, Pb2+ ions are more easily adsorbed than Zn2+ ions. An ion concentration of 0.0018 mmol/g of dry gel was obtained from the reversible adsorption-desorption test using an 8:2 NIPAM:DMAAPS ratio in the Zn(NO3)2 solution, while the Pb(NO3)2 solution reached an ion concentration of 0.0675 mmol/g of dry gel.
This research was supported by Penelitian Dasar 2019 research grant under contract number 853/PKS/ITS/2019 from the Ministry of Research, Technology, & Higher Education of Indonesia for ten consecutive months.
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