## 1 Introduction  
One of the major threats to natural ecosystems is the environmental pollution arising from urbanization and industrialization [1, 2, 3]. In particular, metal ions are released as a result of various human activities, causing a variety of negative environmental consequences [4, 5, 6]. In recent decades, there has been a significant increase in the use of toxic heavy metals in various industries, many of which enter the natural environment and damage flora and fauna [7]. These heavy metals are non-biodegradable [8] and damaging to human health when present above certain levels, including raising the risk of cancer [9]. Once released, heavy metals can persist in the environment for long periods; thus, researchers, scientists, and environmentalists have pursued the development of viable methods for water purification [10; 11; 12]. Conventional methods of water purification based on the precipitation of heavy metal ions have disadvantages in terms of the need to maintain a specific temperature, pH, and ion concentration. They also produce colloids and sludge at quantities much higher than the original heavy metals [13]. For this reason, a number of adsorbents have been developed for the removal of heavy metals from industrial effluent, including activated charcoal [14], biomass [15; 16; 17], polymers [18; 19; 20], fly ash [21], zeolites [22], and graphene oxide [23]. Other methods that have been used to decrease the metal ion concentration in water include ultra-filtration, electrolysis, phytoextraction, and reverse osmosis. However, many of these methods are difficult to implement and are not very cost-effective. Therefore, a treatment strategy that is both simple and compatible with local resources and constraints is required. Recently, ion-exchange resins have been considered as a potential solution to the sequestration of metal ions from industrial wastewater [24; 25]. The removal of toxic metals via ion-exchange methods were proven to be effective in previous studies [26]. In addition, these resins can be easily recovered and reused using regeneration processes. Ion-exchange resins can act as a cation exchanger, an anion exchanger, or a chelating exchange resin, based on the functional groups involved. Ion exchangers can be effectively employed for the adsorption and separation of several toxic metal ions, such as cobalt [27], zinc [28], nickel [28], copper [29], lead [28; 29], cadmium [30; 31], and iron [32]. These ion exchangers can be produced from synthetic or natural resins. Synthetic ion exchangers based on petroleum products (e.g., DVB styrene) [33] and polymers [34] are widely used, but due to their high costs and the increasing social demand for sustainability, ion exchangers based on natural resins have attracted increasing attention [35] as inexpensive and high-quality alternatives. Several natural-resource-based ion exchangers have been developed, and a prominent one among them is the biomass-derived carbon-based ion exchanger [24]. Examples include Indigifera chloria plant-derived carbon incorporated with phenol-formaldehyde resin [24] and a natural cation exchanger from the waste biomass of pineapple peel [25]. These ion exchangers have shown better wastewater treatment results. However, considering the increasing demand for ion exchangers with increasing water pollution, it is very necessary to develop more ion exchangers that can be easily produced at an economical cost. Effort was made in this study in this regard. In this respect, the tamarindu tree (_Tamarindus indica_), which is naturally found in forests in India, Sri Lanka, Bangladesh, and Burma but which can also be easily cultivated, has the potential to be used to develop eco-friendly and sustainable products. In particular, tamarind seeds, which are generally thrown away rather than eaten, can be employed in a sustainable circular economy in which waste products are repurposed [36; 37]. The seeds of _T.