Synthesis, characterization, and performance of chitosan/nylon 6/polyurethane blend for the removal of chromium (VI) and lead (II) ions from aqueous solutions for enhanced kinetic adsorption studies

Water has a dominant role in all types of activities on Earth. However, human activity leads to causes of pollution. Freshwater is now in a dangerous condition and it is difficult to research this completely. Research is conducted worldwide to establish the management of various pollutants in companies' wastewater. A hazardous metal is also a type of poisonous pollutant. Some of the recognized noxious metallic elements are arsenic (As), iron (Fe), lead (Pb), chromium (Cr), cadmium (Cd), copper (Cu), nickel (Ni), and mercury (Hg). They are also toxic in nature and non-biodegradable with accumulation in living organisms (Ghorai et al. 2014). Stream pollution has received serious attention due to its augmented impact on different lifeforms. Recently, the removal of substantial amounts of metal particles from water bodies has received increased attention. The contaminated water can cause cancer and other harmful impacts for humans and the environment (Sekar et al. 2004; Al-Omair & El-Sharkawy 2007; Ren et al. 2021). Chromium and lead contamination in water bodies is mainly due to industries such as tanneries, textiles, metal processing, pigment, paints, batteries, and electroplating (Ong et al. 2007; Minisy et al. 2018). The non-bio decomposable nature of these heavy metal particles will cause a few destructive, intense and deadly impacts. Various techniques such as ion exchange, invert assimilation, adsorption, complexation, and precipitation have been utilized to dispose of harmful metals from streams. However, these techniques are not economic; subsequently some cost-effective adsorbents have been created for the removal of poisonous metal particles (Bayramoglu et al. 2005; Parthiban & Sudarsan 2021a, 2021b).

A blend is a mixture of monomers in the form of immiscible compounds and it possesses a hydrophilic nature and insolubility. Highly reactive sites are available which are utilized to remove heavy metal ions from the industrial effluent wastewater. This is mainly based on the nature of interaction of metal ions and blend (adsorbent). Chitosan has been prepared by the deacetylation of chitin. The polymer of glucosamine from oceanic biomass has been discovered to be a competent adsorbent for different heavy metal particles in the wastewater (Wan Ngah et al. 2002; Kandile et al. 2009; Rathore et al. 2020). Hydroxyl and amino functionalities of chitosan may be involved in chelation to trap the harmful metal particles (Guibal et al. 2002; Parthiban et al. 2022), proteins, and humic acid, among others. In order to enhance the adsorption capabilities of chitosan it can be physically or chemically modified. These modifications will improve pore size, mechanical properties, chemical inertness, hydrophilicity, and bioadaptability (Maruca et al. 1982; Wu et al. 2002; Sirshendu & Rekha Panda 2015).

Nylon 6 is the most crucial polymer with respect to the fiber business. The reagent e-caprolactam is an essential material for the process of polymerization. It is produced from exceedingly low-cost materials including cyclohexane, benzene, and phenol. The excessive cost of nylon 6 may be decreased by way of the introduction of blends with decreased cost for polymers, including polyolefins. Blend introduction is broadly taken into consideration as an economically feasible and flexible approach for enhancing residences or cost–benefit relationships in polymers without the need for synthesizing new polymers (Camila Alves de Rezende et al. 2006).

Polyurethane (PUF) has a wide range of physical and chemical properties and versatile applications, due to its augmented flexibility, great elasticity, hardness, and ability to withstand extreme pH and temperature conditions (Ma et al. 2012). Blends of polyamide have gained importance due to their limited pore size conveyance, enhanced mechanical properties and chemical inertness (Ibrahim 2010; Parthiban et al. 2019). However, it has low ligand compactness and its framework displays unreliable adsorption. To conquer these issues, it is blended with common macromolecules, such as chitosan and cellulose, which increment responsive destinations in the grid, and has been utilized successfully for the removal of contaminations from wastewater frameworks (Darko et al. 2012).

The present study was used to assess adsorption competence of Cr⁶⁺ and Pb²⁺ ionic particles on 2:1:1 blends of chitosan, nylon 6 and polyurethane foam (CS/Ny 6/PUF), by considering the influencing parameters such as contact period, dosage of adsorbent and pH. An adsorption isotherm (Langmuir and Freundlich) was utilized to contemplate the adsorption harmony of metal particles on the adsorbent. The kinetics of adsorption were analyzed and resolved quantitatively by pseudo first- and second-order equations.

Deacetylated chitosan (92%) was purchased from Sea Foods in India. Nylon 6 (DuPont) and polyurethane was bought from Star Foams, India. AR grade SD Fine Chemicals were used for the other purposes. The whole reaction was done using double distilled water.

1 g each of chitosan, nylon 6 and polyurethane foam was liquefied in HCOOH (formic acid) independently. The polymeric liquids were homogenized in the ratio of 2:1:1 mass proportion and with constant stirring for 1 h. After the mixing was completed, the obtained solution was dispensed on to Petri dishes. The samples were vacuum dried to eliminate any traces of solvent as portrayed in our past work (Jayakumar & Sudha 2013). The samples were labeled for further uses.

The sorption capacity of CS/Ny 6/PUF (2:1:1) blends was determined by adding adsorbent dose of 1 g to 100 mL solution of Cr⁶⁺ and Pb²⁺ ions with differing strengths ranging from 10 to 200 ppm prepared from 200 mg/L stock solution of potassium dichromate and lead nitrate respectively. Introductory adsorption tests indicated that this time frame was sufficient to guarantee harmony among adsorbed and unadsorbed metal particles. After equilibrium, the concentration of metal was found using an atomic adsorption spectrophotometer.

When the pH is lower, the amine functionalities of the blend are effectively protonated (Equation (1)) which contend with Cr⁶⁺ and Pb²⁺ for sorption sites resulting in minimal uptake of metal ions because of repulsion of ions. Conversely, as the pH builds, negative charge is developed and sites become accessible for the uptake of metal ions (Equation (2)). At greater pH, the adsorption diminishes because of decreased dissolvability and precipitation of both ions (Cr⁶⁺ and Pb²⁺) occurs (Wan et al. 2010; Parthiban & Sudarsan 2021b). This kind of interaction has been clearly explained: initially as the pH increases the removal tendency of ions is also enhanced followed by decreases. This process mainly occurs because of increases in electrostatic interaction of ions and adsorbent.

Both the Langmuir adsorption isotherm and Freundlich adsorption isotherms fit the model impeccably. This is due to the performance of the heterogeneous blend adsorbents. Every adsorbed metal ion has followed various adsorption isotherms. This overall effect cannot be elucidated by a single Langmuir or Freundlich model (Weber et al. 1992; Olu-Owolabi et al. 2014). The comparison of room temperature adsorption capacity for the CS/Ny 6/PUF blend with different adsorbents reported in the literature is shown in Table 2.

Based on the calculated correlation coefficients, the adsorption of Cr⁶⁺ and Pb²⁺ ions follows second-order kinetics rather than first order. The correlation equation has been established for the adsorption behavior and it is because of the interaction of adsorbent (blend) and metal ions in the wastewater.

The Cr (VI) and Pb (II) hazardous metal ions were eliminated from the industrial wastewater and the adsorption mechanism was also analyzed using different pH solutions. As the pH range increases from 8 to 10, the percentage of Cr (VI) and Pb (II) metal ion adsorption also progressively increases. The highest adsorption was displayed at pH 10. At lower pH, weak Van der Waals force of attraction leads to an increase in the porosity of the structure that causes the precipitation of metal ions, whereas, strong affinity of metal ions has been observed at higher pH range, which is due to the strong electrostatic attraction in a medium of acidic pH. The effect of the CS/Ny 6/PUF blend has been observed as a definite removal of Cr (VI) and Pb (II) metal ions from the aqueous solution (Habiba et al. 2017).

The recovery of adsorbent is more important for reusability studies. It can be reused more than three times without loss of efficiency. Figure 8 shows the absorption, the above problems, the CS/Ny 6/PUF adsorbent blend was recovered and reused for further recovery studies. Different concentrations of desorption chemicals were used for desorption or reusability studies such as acidic, basic and neutral medium (Rosli et al. 2022). The CS/Ny 6/PUF blend (1 g) was occupied by chromium and lead particles in 50 mL samples at room temperature, an interaction period of 6 hours, pH 2, and agitation speed 300 rpm. Based on the above studies 92 and 90% of adsorbent was recovered under acidic media desorption process and reusability can also be adjusted at pH 2.0–4.0.

The present adsorption characteristics of Cr⁶⁺ and Pb²⁺ ions with the ratio of 2:1:1 of CS/Ny 6/PUF blends were studied at room temperature by batch process. The obtained outcome has been demonstrated in an ideal adsorption process at pH 5. The quantity of Cr⁶⁺ and Pb²⁺ ions adsorption has also been observed with different dosages of adsorbent and contact period. The removal percentage of Cr⁶⁺ is more prominent than that of Pb²⁺ ions because of the smaller ionic radii of Cr (0.58 Å) than Pb (1.33 Å) which rapidly diffuses through the adsorbent pores. The adsorption isotherms of both metal ions have been shown to best fit the Freundlich model, indicating the heterogeneity of sorption sites. The kinetic investigations have revealed that chromium ion follows the pseudo-second-order and lead follows the pseudo-first-order reaction. This present work confirmed an adsorbent blend to treat different types of industrial wastewater containing multiple toxic and heavy metals.

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

The authors declare there is no conflict.