The Effect of Cation and Anion Species on the Transition and Adsorption Behaviors of Thermosensitive Sulfobetaine Gel-based Adsorbent

This study was conducted to examine the ion absorption behavior of zwitterionic sulfobetaine type N,N-dimethyl(acrylamidopropyl) ammonium propane sulfonate (DMAAPS) gels. In addition, it also examines the effect of salt solution concentration and ionic strength on the poly(DMAAPS) transition behavior in different salt solutions. The adsorption behavior of the gel in nitrate, chloride and sulfate solutions was found to be a strong function of the valence. The number of ions adsorbed onto the DMAAPS gel increases in line with the increase of cations valences, with the order of adsorption being Zn²⁺ > Ni²⁺ > Na⁺, K⁺. The level of ion adsorption onto the DMAAPS gel in various salt solutions with common cations of Na⁺, K⁺ and Zn²⁺ shows a tendency in the order of NO₃^? > Cl^?  > SO4^2?.  Comparing the effect of the anion and cation of the same charge on the number of ions adsorbed, the anion species effect is greater than that of the cation species at all concentrations. At first, the transition temperature of the poly(DMAAPS) within the salt solution increases. However, it then decreases when the ions adsorbed onto the DMAAPS gel increase and reach a maximum level. On the other hand, the level depends on the species and ion concentration. Therefore, it can be concluded that the maximum level of ions adsorbed can be simply ascertained by determining the transition temperature. 

Ion-adsorption; Sulfobetaine; Transition temperature

The use of absorbent in the form of thermosensitive gel based on zwitterionic betaine is one of the most recent ways of separating heavy metals ions in aqueous solution. Such absorbent has selectivity toward the ions as the consequence of the interaction between the positive and negative charges of the chain, making it the most effective way of overcoming the heavy metal contamination in the aquatic environment. In addition, zwitterionic betaine also has the ability to simultaneously bond the anions and cations in liquid waste. A wide range of thermosensitive polymers has been developed and evaluated for their ability to recover metal ions from aqueous media, most of which are copolymers containing poly(N-isopropylacrylamide) (polyNIPAAm), and monomers bearing complexing groups. The thermosensitive moiety used is usually polyNIPAAm, and the monomer-bearing complexing groups have sorption properties (Graillot et al., 2012). 

A number of papers have been published by Nonaka et al. (2003) concerning the use of thermosensitive copolymer hydrogels with phosphonium groups. These hydrogels have been proven to be able to absorb a great quantity of water, as well as to conduct a high level of antibacterial activities, which are affected by the solution temperature. The capacities of the divalent and trivalent heavy metal retention of two cross-linked polyzwitterionic carboxybetaine structures based on 4-vinylpyridine: divinylbenzene macromolecular supporting gel and porous type were investigated by Neagu et al. (2010). In aqueous solution, both zwitterionic ion exchanger types synthesized were found to have retained metal cations and anions; however, no retention capacities have been obtained in the alkaline earth metal solutions. Another study by Ning et al. (2013) was conducted to examine the poly(DMAAPS) synthesis condition on the polymer yield, intrinsic viscosity, molecular weight and gel fraction through the variation of monomer concentration, with and without the use of an organic cross-linker.

Kusrini et al. (2018b) separated textile dyes from aqueous solution by using adsorbents. Heating, acid activation and impregnation methods were used to prepare graphite/Fe₃O₄ composites with different graphite to Fe₃O₄ ratios. The graphite/Fe₃O₄ (1:1 w/w%, 2M HCl) composite was optimized as the adsorbent of the textile dyes. The capacity of the dyes adsorbed is affected by the electrostatic interaction of the adsorbent, which is built between the dyes and the functional groups. Kusrini et al. (2019) have also reported that the utilization of the adsorption technique in separating mercury from liquid hydrocarbon is cost-effective and economical. They investigated the maximum capacity of the mercury adsorbed from the liquid hydrocarbon was as high as 0.93 ng/g using the SnOx-impregnated clinoptilolite. This amount is higher than the use of natural clinoptilolite, which achieved a maximum amount of mercury adsorption of 0.43 ng/g. In addition, based on the structure and morphology of SnOx, an analysis was also made of the SnOx coated on the surface of the clinoptilolite, making the surface rougher and with a smaller pore size and volume than with the use of natural clinoptilolite Kusrini et al. (2019). This adsorption technique was also used by Kusrini et al. (2018a) to remove commercial lanthanide ions from aqueous solution by using adsorbent in the form of activated carbon obtained from a banana peel, which has the highest iodine absorbance of 572.2 mg/g.

Our previous study explored an interesting correlation between the transition behavior of sulfobetaine N,N-dimethyl(acrylamidopropyl)ammonium propane sulfonate [poly(DMAAPS)], swelling behavior and the adsorption behavior of ions onto DMAAPS gels (Ningrum et al., 2014). The adsorption ability of DMAAPS towards metal ions was found to be strongly dependent on the charge groups forming an ionic association (intra- and/or inter-chain association) with the metal ions, which also significantly affected their swelling and transition behavior. The intra- and/or inter-chain association was disrupted and dissociated by the addition of salt solution, resulting in chain expansion and an increase in swelling degree of DMAAPS gel. It was also found that the higher the valence of cations of the salt solution, the lower the level of swelling due to the formation of cations between the polymer chains. Furthermore, cross-linker concentration also determines the type of chain association that dominates in the DMAAPS network. Considering the sensitive characteristic of sulfobetaine DMAAPS to the ions, in this work DMAAPS gel adsorption behavior in the presence of various salt solutions is investigated. The study also describes the salt solution concentration and how ionic strength affects the transition behavior of poly(DMAAPS) in different salt solutions.

The transition and ion-adsorption behavior of DMAAPS gel and polymer have been investigated in nitrate, chloride and sulfate solutions. The aqueous solution of the poly(DMAAPS) exhibited an upper critical solution temperature (UCST) in a salt solution, and with the increase in salt concentration, the UCST first increased and then decreased. The adsorption behavior of cations in the nitrate, chloride and sulfate solutions was found to be a strong function of the valence, with the order of adsorption being Zn²+ > Ni²+ > Na⁺, K⁺. The number of ions adsorbed onto the DMAAPS gel in various salt solutions with a common cation of Na⁺, K⁺ and Zn²⁺ shows a tendency in the order of NO₃^? > Cl^?  > SO4^2?.

This research was supported financially by a Penelitian Kerjasama Luar Negeri 2018 (952/PKS/ITS/2018) research grant for ten consecutive months from Direktorat Jenderal Penguatan Riset dan Pengembangan Kemenristekdikti.

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