# Nitrate-Polluted Waterbodies Remediation: Global Insights into Treatments for Compliance
## 4 Nitrate Removal Technologies in Waterbodies
### Separation-Based Technologies
#### 4.1.6 Adsorption  
Adsorption refers to mass transfer surface processes in which a substance (adsorbate) from a fluid phase becomes incorporated onto a solid phase (adsorbent). The adsorption process has been widely considered for pollutant removal in waterbodies as a consequence of its technical feasibility, simple operation, affordable costs, and the possibility of using multiple adsorbents of the most diverse nature [4; 43; 97]. An adsorbent must stand out for its surface area, together with its porosity (pore size and volume). Particle size also affects sorption capacity, but in porous materials, this impact is minor compared to the enhanced retention observed with an increase in specific surface area and/or a reduction in pore size [98]. The presence of easily ionizable groups on the sorbent surface will also enhance its adsorptive capacity [99]. The chemical and physical characteristics of the adsorbate (molecular size, polarity, composition, and solubility) also determine whether it is strongly or weakly retained. The specific features of nitrate ions such as their negative charge, ionic size (179 pm), charge density, and the presence of unpaired electrons on the oxygen atoms are key factors influencing the availability to be retained by a given adsorbent [100]. Ionic strength, temperature, and pH of the medium are also crucial parameters of the adsorption process [97]. The adsorption approach has been tested for the remediation of nitrate-polluted waterbodies with a wide range of adsorbents, including granular activated carbon [101], zeolites [102], sepiolite [103], layered double hydroxides [104], pumice [105], chitosan [106], fly ash [107], and other biosorbents and biochars [108; 109]. In application of the guidelines for a circular economy, strategies for the recovery of biomass from agrifood and industrial wastes are multiplying in terms of sustainable technological management and green approaches [110]. In this field, there are many studies that propose to transform these wastes into valuable sorbent material to be used in the bio-removal of nitrate ions in waterbodies [111; 112; 113; 114; 115]. Nanomaterials have recently gained a reputation as an excellent alternative to conventional sorbents for nitrate removal because, due to their extremely small size, they have a large specific surface area and, therefore, a higher sorption capacity [116; 117]. Conventional adsorbents, in general, do not have an elevated uptake capacity, but they can be subjected to surface modifications or activations, both physical and chemical, to significantly increase their removal efficiency [4]. These activation processes will depend on the nature of the adsorbate, and in the case of nitrate ions, the following have been described as the most effective: protonation [118], grafting of amine groups [119], impregnation with metals or metal oxides [120], or the incorporation of cationic surfactants [121] (Figure 4). The simple heat treatment of the sorbent has also been found to achieve, in some situations, a significant activation of its nitrate retention capacity, especially in the case of layered double hydroxides [104]. The mechanism of nitrate adsorption will be dependent on the structure, chemical composition, and surface properties of adsorbents and adsorption conditions. Thus, depending on the situation, NO3\({}^{-}\) may be fixed to the sorbent by electrostatic interactions, complexation, ion exchange, and/or hydrogen bonding [65; 122]. Other anions present in the water that tend to bind to the reactive surface of the adsorbent will reduce its nitrate retention efficiency. Thus, it is reported that, in selected adsorbents, the competing anions (mainly PO4\({}^{3-}\), SO4\({}^{2-}\), HCO3\({}^{-}\) and F\({}^{-}\)) significantly affect the nitrate uptake performance [123; 124]. A variety of adsorbents of very diverse origin have been reported to remove nitrate ions from waterbodies. Figure 5 displays the sorption capacity values of selected adsorbents.