**1. Introduction**

Lead and arsenic are known for their toxicity and are widely distributed in the environment, particularly in water sources [1]. These elements, under low concentration, can also be hazardous to aquatic and non-aquatic creatures and plants. Lea and arsenic are carcinogenic, mutagenic, and teratogenic [2,3]. The ingestion of these elements can cause severe adverse effects that include hypertension, neurological complications, cardiovascular disease, intestinal disorders, hematopoietic dysfunction, mental impairment, organ failure, and malfunctioning of the immune and reproductive systems [4–9]. Lead and arsenic are listed as global priority pollutants because of their high toxicity, stability, and non-biodegradability [10]. It has been reported that worldwide more than 200 million people have been affected by the consequences of these elements [4]. The primary source of exposure to these lethal elements is water through regular activities like drinking, cooking, and irrigation. Water sources are getting contaminated with arsenic by both natural processes and anthropogenic activities like weathering processes, geochemical reactions, biological activities, combustion of fossil fuels, volcanic eruptions, mining activities, leaching

**Citation:** Neelgund, G.M.; Aguilar, S.F.; Kurkuri, M.D.; Rodrigues, D.F.; Ray, R.L. Elevated Adsorption of Lead and Arsenic over Silver Nanoparticles Deposited on Poly(amidoamine) Grafted Carbon Nanotubes. *Nanomaterials* **2022**, *12*, 3852. https://doi.org/10.3390/ nano12213852

Academic Editor: Muralidharan Paramsothy

Received: 23 September 2022 Accepted: 27 October 2022 Published: 1 November 2022

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of artificial arsenic compounds, smelting of metals ores, desiccants, wood preservatives, agricultural pesticides, and related anthropogenic activities [11–14]. In nature, arsenic exists in organic and inorganic forms. However, the organic form of arsenic is not of importance as it undergoes biotransformation and detoxifies through methylation [15]. At the same time, the inorganic form of arsenic exists in four oxidation states viz., −3, 0, +3, and +5. Out of these, −3 and 0 oxidation states are scarce, so +3 and +5 usually exist in water. Nevertheless, the presence of either a +3 or +5 state of arsenic depends on the redox and pH conditions of water [15,16]. Accordingly, trivalent arsenite, As(III), and pentavalent arsenate, As(V), commonly occur in water. In particular, As(III) transpires in reducing conditions, and As(V) exists in oxidizing conditions of water [4]. However, both As(III) and As(V) species are highly toxic and non-biodegradable [6]. Comparatively, As(III) is more highly mobile and stable than As(V) due to its stable electronic configuration [11]. In addition, As(III) has higher cellular uptake ability and binding affinity to vicinal sulfhydryl groups that react with various proteins and inhibit their activity [6,11,17]. In specific, As(III) is about sixty times more toxic than As(V) [6].

Another toxic element, lead, contaminates the water through industrial and anthropogenic sources. The primary sources of information pollution are industries such as battery manufacturing, printing, fuels, photographic materials, ceramic, glass, and explosive manufacturing [18]. In the atmosphere, lead persists as bivalent, Pb(II). It is also likewise toxic as As(III) and widely distributed. Because of their toxicity and detrimental effects, Pb(II) and As(III) are unsafe, specifically their association with water. Therefore, eliminating these toxic components present in water is a mandatory need. For the removal of toxic Pb(II) and As(III), several methods like chemical precipitation [19], ion exchange [20], membrane technology and reverse osmosis [21], electro-dialysis [22], and adsorption [23] have been developed. Nevertheless, chemical precipitation makes it difficult to remove low-concentrated arsenic like 10 mg/L from water [24]. In addition, chemical precipitation is less effective in the removal of As(III), and it requires the pre-oxidative conversion of As(III) to As(V) during its abolition [25]. Similarly, ion exchange is less effective in removing As(III), and the development of ion exchange resins is expensive [24]. Additionally, the membrane technology-aided reverse osmosis is costly, requiring external pressure to pass the contaminated water through the membrane. Besides, the discharge of the concentrate, membrane fouling, and flux decline is inevitable in reverse osmosis [26]. In electro-dialysis, several insoluble coagulants generate and deposit over the cathode [27]. Overseeing these drawbacks, the better alternative and promising technique for removing Pb(II) and As(III) is adsorption. Adsorption is a facile, efficient, accessible, low-cost, low-energy technique [28]. Additionally, using the adsorption technique, it is possible to remove trace amounts of Pb(II) and As(III) from water. Overall, adsorption has played a significant role in eliminating various contaminants from water. Consequently, due to its exceptional benefits, adsorption is a gifted technique for efficiently removing Pb(II) and As(III).

In the process of adsorption, selecting an adsorbent is a vital step in the success of the process. For this purpose, insoluble solid materials with high specific surface area and active functional groups are elected adsorbents. In this context, several adsorbents have been developed and tested for their efficiency in removing different pollutants [6,12,29]. The dynamic adsorbents are based on activated carbon [30], activated alumina [31], inorganic minerals [32], biomass adsorbents [33–38], polymer [39–41], carbon nanotubes (CNTs) [42–44] graphene oxide [45,46], metal-organic frameworks [47], microplastics [48] and more. Among these, CNTs based adsorbents are particularly interesting because their unique physical and chemical properties are relevantly suitable for adsorption [42–44]. During the CNTs, aided adsorption process, strong interactions between CNTs and pollutants alter contaminants' mobility, bioavailability, and environmental risk [44]. The high surface area, well-defined structures, and uniform surfaces of CNTs facilitate the adsorption mechanism and process. Abundant active sites and functional groups exist over CNTs supporting the adsorption process. Furthermore, adsorption over CNTs transpires with different mechanisms, viz., ion exchange, coordination interaction, electrostatic interaction,

and physical adsorption, which augment the adsorption [44]. Beyond its exceptional properties, the employment of CNTs in adsorption technology is still fragmentary, so further exploration is required. The main hindrance that restrained the application of CNTs in adsorption technology is their hydrophobic nature, which reduces the adsorption rate. However, the functionalization of CNTs introduces reactive functional groups over their surface that can significantly increase their selectivity and sensitivity toward pollutants [43]. In this fashion, CNTs can be modified to be hydrophilic for the adsorption of pollutants and more reactive by functionalizing with secondary materials. The functionalization of CNTs generates chemically active sites around defective segments such as pentagons, oriented against a tube body which generally consists of only hexagons; and it may be this that gives CNTs their remarkable ability to interact with pollutants [43,49]. Accordingly, the functionalization of CNTs is the critical step to improving their absorption efficiency. For the functionalization of CNTs, dendritic polymers are particularly interesting because of their unusual structure and properties [50]. Among different dendrimers, poly(amidoamine) (PAMAM) is predominantly interesting because of its symmetrical structure, controllable molecule chains, vast internal cavities, and abundant functional groups [51]. Based on these special structural characteristics, PAMAM persists with unique properties, such as high hydrophilicity, high dispersing ability, high bio-affinity, and ease of modification [51]. Therefore, PAMAM is especially interesting for the adsorption of heavy metal ions and plays an important role in the functionalization of nanomaterials [51–54]. A large number of amino and amide functional groups of PAMAM dendrimers can strongly chelate heavy metal ions, thus improving the enrichment efficacy [51–54]. Considering its importance, herein, CNTs were functionalized by grafting with fourth-generation aromatic PAMAM and sequential deposition of Ag nanoparticles. Thus produced, CNTs–PAMAM–Ag was explored in evaluating the adsorption efficiency of two important toxic pollutants, Pb(II) and As(III). The controlling mechanisms and dynamics of the adsorption of Pb(II) and As(III) over CNTs–PAMAM–Ag were estimated by implementing the pseudo-first and secondorder kinetic models. The reaction pathways and the rate-controlling step in adsorption were evaluated using Weber–Morris intraparticle pore diffusion model. Furthermore, the adsorption equilibrium was estimated by fitting the experimental results with Langmuir, Freundlich, and Temkin isotherm models.
