Use of Hydroxyapatite Nanoparticles to Reduce Cd Contamination in Agricultural Soils: Effects on Growth and Development of _Chenopodium quinoa_ [PERSON]  
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Footnote 1: email: {c.c. have greatly aggravated their presence. Cadmium (Cd), in particular, is a highly toxic and mobile heavy metal with no known biological function. Its anthropogenic sources include mining, smelting, industrial discharges, and the widespread use of phosphate fertilizers containing Cd as an impurity [4; 5; 6]. Cadmium contamination in agricultural soils is of particular concern due to its mobility, which facilitates its uptake by plants and its entry into the food chain [7]. Once absorbed, Cd impairs plant physiological functions, causing chlorosis, reduced photosynthetic activity, and stunted growth [8]. In humans, chronic exposure to Cd is associated with kidney damage, osteoporosis, carcinogenesis and the infamous \"tai-tiai\" disease, which emerged in Japan due to the long-term consumption of Cd-contaminated rice [9; 10]. The global extent of heavy metal contamination is alarming, with over than 22 million hectares of agricultural soils affected, a figure likely underestimated given the intensification of industrial and agricultural activities worldwide [11]. Addressing this contamination is vital for ensuring food safety and environmental health. Conventional remediation methods, such as soil excavation, washing, and chemical stabilization, are often costly and environmentally damaging. For instance, soil washing generates large volumes of contaminated leachates, while excavation and dumping permanently deplete soil resources and disrupt ecosystems [12]. These limitations have prompted the search for innovative, sustainable, and cost-effective approaches to soil remediation, including nanotechnology-based interventions. Nanotechnology offers a promising alternative due to the unique physicochemical properties of nanoparticles, such as high surface area, increased reactivity, and the ability to interact with contaminants at molecular levels [13]. Among several nanoparticles, hydroxyapatite nanoparticles (HANPs) have garnered significant consideration for their effectiveness in immobilizing heavy metals [14; 15]. Hydroxyapatite (HA), a calcium phosphate mineral, occurs naturally in rocks and soils and has been recognized for its capacity to immobilize heavy metals through mechanisms such as adsorption, ion exchange, and co-precipitation [16]. The HA has improved reactivity and surface area in its nanoparticulate form, making HANPs particularly effective for environmental applications. In addition, HANPs act as slow-release phosphorus fertilizers, improving soil fertility and favoring plant growth, which increases their applicability in agriculture [17]. Studies have shown that HANPs significantly reduce the mobility and bioavailability of Cd, mitigating its toxic effects on plants. For instance, the application of HANPs to Cd-contaminated soils reduced Cd concentrations in rice tissues by 50% while enhancing root and shoot biomass [18]. In addition, the degradation nutrients of HANPs, i.e., calcium and phosphate ions, make them an environmentally friendly alternative to conventional amendments [19]. Despite these advantages, integrating HANPs with biological remediation approaches, such as phytoremediation, remains practically unexplored. Phytoremediation is a sustainable approach to soil remediation that has gained attention for its environmental compatibility and cost effectiveness. However, its effectiveness is often limited by the toxicity of contaminants, which can impair plant growth and reduce biomass [20; 21]. Combining phytoremediation with HANPs may offer a synergistic solution, where nanoparticles reduce the bioavailability of heavy metals, allowing plants to thrive and enhance soil recovery [19; 22; 23].