Role of cationic surfactants in palladium adsorption of commercial ion exchange resins using electroless plating solutions

Global populations and economy are continually showing rising tendencies as a result of the breakthroughs made possible by science and technology. In addition, there is an increase in demand for many different resources, such as heavy and noble metals. (Mack et al. 2007; Das 2010; Morf et al. 2013). Heavy metals (nickel, arsenic, copper, and cadmium) and precious metals (platinum, palladium, silver, and gold) are the two main types of metals and their compounds are found in industrial effluents. Mines, foundries, smelters, electroplating, tanneries, and coal-burning power plants are the main businesses that release wastewater containing metals. The biological cycle of metals has been dramatically accelerated by waste management ignorance, and the deposition of heavy metals in terrestrial and aquatic environments has increased, posing risks to human health (Grandjean et al. 1999; Wong et al. 1999; Eisler 2004).

Due to this, it is crucial to eliminate metal contamination from industrial waste streams. Palladium catalysts are particularly effective because they exhibit superior chemisorption, resistance to oxidation at high temperatures, and selective activity toward reactants. In addition, precious metals play a significant role in many fields of industry. According to the most recent market analysis of supply and demand for platinum metals, a comparison of the palladium supplies over the last 3 years from South Africa, Russia, and other nations shows rising tendencies (Fleming 1992; Schubert et al. 2015). The recovery of platinum group metals (PGM) is a matter of significant concern because platinum-rich natural resources are few and sources are depleting (Mishra & Rhee 2010; Morf et al. 2013).

The importance of speciation in affecting adsorption and desorption characteristics could be extrapolated from such systems. As a result, solutions with a moderate level of complexity must be sought as references when evaluating adsorbent effectiveness in the control of industrial and complex waste streams. Synthetic electroless plating (ELP) solutions, which contain ethylenediaminetetraacetic acid (EDTA) and ammonia (NH₄OH) in addition to Pd, can be considered complex (Nagireddi et al. 2020).

Because noble metal levels are typically fairly low, recovering PGM from the electroplating or ELP industries is restricted (Butewicz et al. 2010; Sharififard et al. 2013). Therefore, recovering noble metals by adding chemical reagents does not make economic sense when employing conventional ion exchange, solvent extraction, or reduction and precipitation methods. Surfactant effects on the removal characteristics of Pd (II) ions should also be explored. Surfactants are being used to minimize industrial sludge and disperse heavy metals. By reacting with hydrophilic oxygen-containing functional groups, disrupting the colloidal network, and releasing bound water, cetrimonium bromide (C-TAB, a cationic surfactant) can neutralize negative charge (Pujari et al. 2014). Therefore, it is vital to discover and apply low-cost, environmentally friendly techniques for recovering precious metals (Rajesh & Uppaluri. 2016).

According to Nagireddi et al. (2018), this might be accomplished by using ion-exchange resins (Chitosan, Lewatit TP-214, Purolite S920, Chelite S, and Duolite GT 73) on used ELP solutions (Kanai et al. 2008; Baba et al. 2011; Bratskaya et al. 2011; Sopena et al. 2011). The majority of the metal removal work has focused on aqueous and acidic solutions. The author chose Lewatit TP-214 as an effective resin after researching the majority of ion-exchange resins in the literature because of its inexpensive cost and excellent metal removal efficiency. Surfactants also make surfaces more wettable and reduce surface tension in solutions, which activates adsorbents and increases the effectiveness of metal removal. To increase plating rates and achieve acceptable surface finishes, cationic surfactants like C-TAB are added to the electroplating solutions. This study works on commercial ion-exchange resins (LewatitTP-214) from ELP solutions to examine the role of cationic surfactant (C-TAB) and its effects on palladium adsorption.

The affinity series of palladium (II) complexes for the chelating and cationic ion exchangers is as follows based on the calculated values of working ion-exchange capacity: Purolite S 920 > Chelite S > Lewatit TP 214 > Duolite GT 73. Table 1 displays a summary of ion-exchange resins. The Pd (II) complexes in aqueous solutions can be exchanged with Lewatit TP 214 most effectively, according to Table 1. TP-214 Lewatit, a monospherical, macroporous chelating resin containing thiourea groups, Lewatit MonoPlus TP 214 has a high selectivity for PGM metals like platinum, gold, and silver. Polystyrene makes up the resin's matrix.

Palladium chloride (PdCl₂) was used as a source of palladium ions with ethylenediaminetetraacetic acid (Na₂EDTA) as a stabilizer and cetrimonium bromide (C-TAB) as a surfactant. The experiment was conducted using Millipore water. Table 3 provides a detailed breakdown of the composition of the ELP solution (Pujari et al. 2014; Rajesh & Uppaluri 2016).

The commercial ion-exchange resin (Lewatit TP-214) was characterized with Fourier Transform Infrared Spectroscopy (PerkinElmer, PE-RXI, range: 500–4,000 cm⁻¹) for the identification of existing functional groups over the surface of the resin. BET analysis is to conduct a nitrogen adsorption isotherm, and measurements of surface area and pore volumes were made using a surface area analyzer (Beckman Coulter, SA-3100).

Based on the adsorption characteristics of metal uptake and removal efficiency, the optimal conditions were reported by Nagireddi et al. (2018) as pH of solution 8, contact time 300 min, and dosage of adsorbent of 100 mg. Batch adsorption studies were carried out at these optimal conditions on 50 mL of synthetic ELP solutions with varying palladium concentrations for each critical micelle concentration (CMC); details are mentioned in Table 3. The studies were conducted using 250 mL flasks holding 50 mL solutions of specified Pd (II) concentrations (50–500 mg/L) at room temperature for a duration of 5 h at 200 rpm in a wrist action shaker (Make: Lab Tech., India). To achieve mass transfer equilibrium, continuous mixing was offered throughout the experimental time with a constant agitation speed of 200 rpm through a shaker. Using an atomic absorption spectrophotometer (AAS; Varian Spectra, FS240, India, equipped with an air-acetylene flame detector) operating at a wavelength of 247.6 nm, the concentration of Pd (II) in the filtrate was determined (Rajesh et al. 2017).

Based on the adsorbent properties mentioned in Table 2 and available functional groups on Lewatit TP-214, resin provided an ideal removal efficiency and adsorption capacity of 86.7% and 137.8 mg/g at pH 8, respectively.

The mechanism of Lewatit TP-214 is a chelating resin with thiourea groups that exhibit a high affinity for Pd (II) recovery from model ELP solution adsorbate system, considering the efficacy of sulfur containing functional groups with respect to nitrogen- and oxygen-containing functional groups. Pd (II) can exist in a variety of forms in synthetic ELP solutions, including [Pd (edta)]²⁻, [PdCl₄]²⁻, [PdCl₃]⁻, PdCl₂ (aq), [PdCl]⁺, [Pd(NH₃)₄]²⁺, and [Pd(NH₃)₃]²⁺. In lower pH ranges (1–6) and higher chloride concentrations, only [Pd(edta)]²⁻is present. On the other hand, Pd (II) occurs as [Pd(NH₃)₄]²⁺ at higher pH (basic medium). Given Pd (II) adsorptive capacity being maximum at optimal pH of 8 for the Lewatit TP-214 resin, at lower pH, it is hypothesized that stronger bonding of -NH and S groups in the resin with [Pd(NH₃)₄]²⁺ but not [Pd (edta)]²⁻ is responsible for non-optimal adsorption capacity (Nagireddi et al. 2018). However, for Pd (II) recovery, the optimal performance of N- and N–O-containing functional groups needs to be affirmed along with cost competitiveness to ensure the sustainability of commercial resins. Therefore, considering the ease of availability, promising Pd (II) uptake/selectivity values, and lower cost of alternate resins containing nitrogen and nitrogen–oxygen functional groups.

The cationic and anionic surfactants in synthetic solutions form micelles, which are roughly spherical, dynamic aggregates with a highly anisotropic interface between their hydrocarbon cores and the surrounding bulk aqueous phase (Nagireddi et al. 2018). This interface is made up of head groups, counter ions, solubilizates, and water. Plotting of the fluctuation in surfactant concentration was done at a fixed palladium ion concentration of 300 mg/L. The graph showed that from 0 to 5 CMC, increasing the surfactant concentration increased both the metal uptake from 112.9 to 137.8 mg/g and removal efficiencies from 75 to 86.7%, respectively.

Lewatit TP-214's ability to absorb metal has been investigated with increasing metal ion concentrations ranging from 50 to 500 mg/L. The author notices that at both low and high concentrations of C-TAB, the efficiency of resin in absorbing metal ions increases continuously. At high surfactant concentrations, no change in the trend in removal efficiency patterns has been seen in Figure 4 and similar information reported (Nagireddi et al. 2019).

For the purposes of designing sorption systems as efficiently as possible, equilibrium sorption data analysis is crucial. The interaction between the adsorbed species and the adsorbent surface determines the adsorption process. Van der Waals forces, hydrophobic forces, chemical bonds, and hydrogen bonds could all be involved in the interaction (Ramesh et al. 2008). The adsorption isotherms show the relationship between the amount of solute absorbed by a unit weight of solid adsorbent and the amount remaining in the solution at equilibrium. The Langmuir and Freundlich isotherms models are frequently used to explain the adsorption of palladium metal ions on Lewatit TP-214 adsorbents. Both the Langmuir and Freundlich isotherms are applied to depict the equilibrium data of adsorbents and indicate that either monolayer or multilayer adsorption may occur depending on the type of adsorbent surface (Nagireddi et al. 2017b).

The Freundlich isotherm posits that the metal ion adsorption happens as a heterogeneous process on the adsorbent surface, in contrast to the Langmuir model, which predicts that sorption occurs on the homogenous surface of the adsorbent and a saturation monolayer is created. The Temkin and Redlich models are two further reliable representations of adsorption equilibrium. In this study, the suitability of each of the four models to accurately depict the experimentally observed adsorption properties of Lewatit TP-214 adsorbents was evaluated. As evidenced by experimental data, the Temkin and Redlich-Peterson models displayed poor fitness toward isothermal fitness. Because of this, the author has included the data from the Langmuir and Freundlich models in the following sections.

The experimental results (Table 5) have been compared to the most relevant systems (ELP solutions) and adsorbent systems in the finest literature that is currently available (chitosan, chitosan-derivatives, and commercial resins). The findings from our research group are solely pertinent for the direct comparison of resin performance with ELP adsorbate systems because other pertinent literature focuses mostly on aqueous solutions.

In this work, the author has used commercial ion-exchange resin Lewatit TP-214 as an adsorbent and to investigate the impact of cationic surfactants on the adsorption of palladium ions from synthetic ELP solutions. The highest metal uptake (201.7 mg/g) and removal efficiency (90.16%) are obtained at the lowest Pd concentration of 300 mg/L. The Pd (II) metal uptake of Lewatit TP-214 resin is highly dependent on stronger bonding of -NH and S groups in the resin with [Pd(NH₃)₄]²⁺ and the donor atoms connected to cationic surfactant confirmed with FTIR characterization. Freundlich isotherms indicate that heterogeneous adsorption is the most likely mechanism during Pd (II) adsorption with functional groups containing N and N–O. This is because the surfactant molecules alter the interface's charge density and increase activity at the resin's surface. Therefore, both %removal efficiency and metal absorption are raised with increasing cationic surfactant concentration. Additionally, as the initial concentration of palladium ions increases, the adsorbent's removal efficiency keeps decreasing while Lewatit TP-214's metal-absorption capacity increases. In the near future, cross-linking and impregnation techniques will be used to synthesize hybrid resins from the reaction between Lewatit TP-214 and chitosan.

The author thankfully acknowledges Prof. Dr. Ramgopal V. S Uppaluri sir and Mrs. Amit Kumar (B. Tech) of the Department of Chemical Engineering and Central Instrumental Facility, Indian Institute of Technology Guwahati for providing necessary facilities for carrying out this research.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.