## 1 Introduction  
The secure and sustainable supply of raw materials has become a fundamental pillar of economic and global development. Metallic elements like copper, lead, cobalt, and nickel are increasingly critical due to their essential roles across numerous sectors, notably in lithium-ion battery production and electric vehicles. Excluding lead, these metals are classified among the 32 raw materials designated as Critical Raw Materials (CRMs) in Europe, as they are scarce or difficult to source locally and securely [1]. Noteworthy, for example, China dominates global rare earth production [2], while the Democratic Republic of Congo controls around 70% of cobalt supply [3]. This geographical localization of ores/mines and the increasing demand by the industry have the potential to disrupt supply chain and spur significant price volatility, underscoring the need to harvest, reclaim, and reuse these materials [4]. While not presently included on the European CRM list, lead is also a crucial energy material and its recovery is crucial to aid in sustaining the booming battery industry driven by electric mobility and the battery energy storage system (BEES). On the other hand, the presence of the same metals in industrial wastewater is a real threat and major environmental challenge. Therefore, there is a twofold opportunity to pursue the sequestration of relevant metals to safeguard environment and health, while recovering them for reuse in industry. Significant quantities of precious metals are estimated to be lost to the environment each year through industrial wastewater [5; 6; 7; 8]. The sustainable management of this resource requires an integrated approach, one that mitigates water pollution and simultaneously meets the demand to recover valuable materials, thus aligning with sustainable development goals and circular economy principles. Technological advances in heavy metal recovery from aqueous solutions have evolved considerably in recent years, striving for greater efficiency and sustainability, while minimizing environmental impact. Among the various methodologies developed to counteract heavy metal pollution, flotation is widely employed. This technique relies on the generation of air bubbles to separate metals from the aqueous matrix [9]. Over time, several variants of this process have been developed, including dissolved air flotation, ionic flotation, and precipitation flotation. In the case of dissolved air flotation, the operating principle is based on the creation of bubbles. The success of this technique depends on various factors, such as the bubble generation method, their size, and the characteristics of the particles to be removed. The ionic variant uses surfactants to bind to metal ions, forming hydrophobic complexes that adhere to air bubbles and are removed as surface foam. This approach offers advantages in terms of energy consumption, ease of management, and operational costs by using surfactants with charges opposite to the target metals. Regarding precipitation flotation, it utilizes specific precipitating agents that combine with the target ions, causing their precipitation [10]. White flotation has proven effective in specific scenarios; other techniques such as ion exchange resin (IER) technology have garnered particular attention for their effectiveness in heavy metal sequestration in water treatment applications [11; 12]. Ion exchange processes allow for the selective removal of ions from the aqueous medium, and specific resins enable selectivity based on ion charge and size. These methods have proven especially effective for removing metals such as lead, copper, nickel, silver, and others [13; 14; 15; 16; 17]. Conventional resins are often based on polymer matrices functionalized with sulfonic and/or carboxylic groups for cation exchange and amine groups for anion exchange. Their removal efficiency can vary significantly depending on operating conditions, such as pH value, temperature, and metal competition in the treated water [18].