## 2 MOFs
### Most Useful Types of MOF for Wastewater
#### 2.1.4 HKUST  
Concerning the last type of MOF most commonly used for water treatment, it should be noted that it is based on a copper atom, specifically the copper (II) ion. Only HKUST-1 (Cu-BTC or MOF-199) has been published in scientific articles about HKUST [70]. The basic structure of this MOF is composed of dimeric metal units interconnected by four BTC ligands (Figure 1) [71]. Some of the characteristics of this MOF are that it has an acceptable \begin{table}
\begin{tabular}{c c c c} \hline \hline
**ZIF** & **Pollutant** & **Processes** & **Reference** \\ \hline CZC600-30 & TC and methylene blue & Photocatalytic & [46] \\ MoS\({}_{8}\)@ZNC-2 & phenol & Electro-catalytic & [47] \\ N@ZIF-67 & ciprofloxacin & Catalytic activation of PMS & [48] \\ ZIF-67@WA & TC & Adsorption & [49] \\ ZIF-9@CEL and ZIF-12@CEL & p-nitrophenol & Catalytic activation of PMS & [50] \\ ZIF-8 & TC and oxytetracycline & Adsorption & [51] \\ ZnCDs/ZnO@ZIF-8 & TC & Photocatalytic & [52] \\ Zn\({}_{8}\)Sn\({}_{4}\)/SnO@2 IF-8 & TC & Photocatalytic & [53] \\ ZnO/ZIF-9 & TC & Photocatalytic & [54] \\ ZnO@ZIF-67 & TC & Adsorption & [55] \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Applications of ZIF-MOF in wastewater treatment. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**UiO** & **Pollutant** & **Processes** & **Reference** \\ \hline Acid-modulated UiO-66 & 2,4-dichlorophenoxyoxyacetic acid, ciprofloxacin and naproxen & Adsorption & [62] \\ C\({}_{3}\)N\({}_{x}\)-TET@TiO/UiO-66 & TC & Photocatalytic & [65] \\ In\({}_{3}\)S/UiO-66 & Methyl orange and TC & Photocatalytic & [66] \\ MWCNT/N-TiO/UiO-66-NH\({}_{2}\) & Ketoprofen & Photocatalytic & [67] \\ NiCo\({}_{2}\)O\({}_{4}\)/HP-UiO-66 & TC & Photocatalytic with PMS & [68] \\ UiO-66(Zr) and NH\({}_{2}\)-UiO-66(Zr) & Pyrene & Adsorption & [69] \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Applications of UiO-MOF in wastewater treatment. surface area, pore volume and a good thermal stability, and reversible adsorption and desorption properties without signs of damage to the crystal structure [72; 73]. In fact, it is reported in the study of [PERSON] et al. [74], that it is considered as one of the most studied MOFs for this type of synthesis and has been tested under a variety of conditions in traditional solvents. One point to note is that there is a lack of information on the wastewater treatment of the removal of persistent organic pollutants in aqueous media with HKUST-1. Thus, the unmodified HKUST-1 was not used between 2018 and 2022 for this type of contaminant. In fact, in this period, only studies have been found, such as that of [PERSON] et al. [75], which used this type of MOF for rhodamine B removal by photocatalytic degradation. Therefore, to remove persistent organic pollutants it is necessary to make modifications to HKUST-1. In this sense, [PERSON] et al. [76] modified HKUST-1 by carbonization at a temperature of 350 \({}^{\circ}\)C under an oxygen-free atmosphere, which they called HDC-350. Upon characterization, this new material showed very different properties from those of the starting MOF. In fact, the great difference in the hydrophobicity of each of the MOFs was highlighted. In this case, HDC-350 had a higher hydrophobicity than HKUST-1, which may have been due to the loss of hydrophilic functional groups on the surface. Therefore, with these new characteristics of HDC-350, a great improvement in the adsorption of tetracycline was obtained. Another function of this type of MOF is coating nanoparticles, wherein the MOF gives the nanoparticles new properties. A very clear example is the work carried out by [PERSON] et al. [77] by the addition of this MOF to nanoparticles, which in this case were cubic. Through this procedure, the nanoparticles were able to increase the carrier density and accelerate the interfacial charge separation and transfer. As a result of this procedure, the concentration of tetracycline hydrochloride was reduced up to (removal of around 93.40%) in 1 h under visible light irradiation.