Water contamination is one of the most significant issues the globe has faced in recent years. According to the United Nations, approximately 80% of all industrial and urban wastewater is discharged into the environment, particularly in developing countries [1], often without any treatment. Among the wide variety of pollutants entering the environment, heavy metals are one of the most dangerous due to their toxicity, persistence, and biomagnification [2]. Some elements, belonging to the heavy metals Co, Mo, Se, Cu, Cr, Fe, and Mn, perform important biochemical and physiological functions in living organisms, and their deficiency can result in numerous disorders or syndromes. At the same time, it is important to know that for elements such as, for example, Cr, Se, and Cu, there is a very narrow range of concentrations between beneficial and toxic effects [3].

The intensification of human activities and the active application of metals in agriculture, the plating and electroplating industry, the chemical industry, mechanical engineering, the textile industry, metal smelting, the petrochemical industry, etc. ^[2][3][2,3], has considerably increased the release of metals such as Al, Pb, Sb, Hg, As, and Cd, which have no biological functions and are toxic even at 1–100 µM levels, into natural water bodies ^[3][4][3,4]. Gastrointestinal and renal failure, neurological disorders, skin lesions, vascular damage, immune system problems, birth deformities, and cancer are a few outcomes brought on by the harmful effects of heavy metals. Metal ions also provoke the generation of reactive oxygen species and oxidative stress as well as enzyme inactivation ^[5][6][7][8][5,6,7,8].

Great efforts are required to reduce the negative impact of toxic elements on natural ecosystems. One of the important tools for addressing the problem is the careful treatment of wastewater before its discharge into the environment or further use. Currently, a wide variety of techniques, including biological treatment, coagulation, ultrafiltration, coagulation, flocculation, membrane separation procedures, chemical precipitation, ion exchange, enhanced oxidation, reverse osmosis, membrane filtration, ion exchange, electrochemical treatment, irradiation, extraction, and adsorption are applied to reduce concentrations of heavy metals in wastewater to maximum admissible levels ^[1][9][10][1,9,10]. However, the numerous drawbacks associated with these techniques, such as high cost, energy consumption, the frequent need for reagents, the unpredictable removal of heavy metal ions, and the production of hazardous sludge, make them unpopular ^[9][11][9,11]. For instance, precipitation produces a large amount of sludge, while membrane filtration, ion exchange, electro-deposition, and filtration are expensive techniques [12].

Adsorption has demonstrated great potential among the technologies available for metal removal due to its economic feasibility, ease of handling, the accessibility of sorbents, affordability, large surface area, high adsorption capacity, and environmental sustainability, as well as its potential selectivity for the target metal and low sludge generation. Heavy metal ions can be captured and removed from wastewater using commercial and natural adsorbents, which are often characterized by high removal capacity ^{[13][14][15][16][17]}[13,14,15,16,17]. Nowadays, metal–organic frameworks ^[18][19][18,19], anionic clays ^[20][21][20,21], nanomaterials ^[22][23][22,23], activated carbon ^[24][25][24,25], hydroxyapatites ^[26][27][26,27], and natural minerals ^[28][29][28,29] are applied for wastewater treatment. It is worth mentioning that some drawbacks of these materials have already been identified, including their frequently low surface area and water stability as well as low adsorption capacity. Therefore, the design of new porous adsorbent materials with enhanced adsorption capabilities and great structural robustness is still necessary for the development of efficient remediation systems [4].

Functional mesoporous materials are now seen as intriguing compounds for a variety of applications, including drug delivery, biomedicine, catalysis, and the adsorption of pollutants of different origins. These materials are characterized by large pore volumes, exceptionally large specific surface area, homogeneous pore size distributions, and tunable pore sizes, which contribute to their technological advantages ^[30][31][30,31]. Additionally, adding appropriate functional groups to the surface can increase their affinity for the target metal ions [32].

2. Application of Mesoporous Materials for Metal Removal from Wastewater

The ability of mesoporous bifunctional magnetic NZVI-SH-HMS to remove Cd and Pb from solutions was investigated. Low adsorption at pH 3.0 was replaced by a drastic increase with the increase in pH from 3.0 to 4.0, indicating that the removal process was pH-dependent. For both elements, adsorption equilibrium was reached in a short time: 30 min for Pb and 50 min for Cd, and the kinetics of the process were presented by a pseudo-second-order model. Adsorption was better described by the Langmuir model with a maximum adsorption capacity of NZVI-SH-HMS 487.8 mg/g for Pb and 330.0 mg/g for Cd. Additionally, NZVI-SH-HMS demonstrated good adsorption capacity and sorption recyclability in the case of real wastewater of different origins ^[33][63]. Zhai ^[34][49] studied the effect of experimental conditions on Cu removal by the SBA-15 molecular sieve. The maximum adsorption capacity of 11.39 mg/g was achieved under optimized adsorption conditions (pH 3.5, adsorbent dosage 0.0050 g, time 40 min). It was shown that the kinetics of the process were described using the pseudo-second-order kinetic model, while equilibrium applied the Freundlich model. From a thermodynamic point of view, the process was spontaneous and exothermic. At the same time, Knight et al. ^[35][94], studying the nano-scale confinement effects on the adsorption of Cu on mesoporous silica with pore sizes of 8, 6, and 4 nm showed low metal adsorption onto the surface of SBA-15, with the maximum measured surface loading of 0.020 ± 0.001, 0.019 ± 0.002, and 0.039 ± 0.002 μmol/m² for SBA-15-8, SBA-15-6, and SBA-15-4, respectively. The effect of contact time and pH on Zn adsorption onto SBA-16 and SBA-15 modified with APTES (3-aminopropyltriethoxy-silane) and then with EDTA was investigated by ^[36][95]. For both adsorbents, equilibrium was reached within the first 30 min and maximum removal was obtained at pH 6.0. Equilibrium data were better described by the Freundlich isotherm model and the maximum adsorption capacity amounted to 184.1 mg/g for SBA-16 and 108 mg/g for SBA-15. The removal of As(III) and As(V) using amidoxime resin embedded into mesoporous silica was highly affected by the solution pH. The maximum removal of As(V) was attained at pH 3.0 and of As(III) at pH 8.0. The time required for equilibrium attainment was 3–4 h. The maximum adsorption calculated from Langmuir models constituted 3.8 mmol/g for As(III) and 3.1 mmol/g for As(V) ^[37][96]. EDTA-modified magnetic mesoporous microspheres (Fe₃O₄@nSiO₂@mSiO₂/EDTA) showed high adsorption capacity for Cr(III), and the maximum adsorption of 30.59 mg/g was obtained at pH 3.0 and 25 °C. The equilibrium data fitted well to the Freundlich isotherm model. Studying the effect of cations (Na⁺, K⁺ and Ca²⁺) and complex agents (EDTA, citric acid and formic acid), the authors showed that the adsorption capacity of Fe₃O₄@nSiO₂@mSiO₂/EDTA for Cr(III) did not change significantly ^[38][67]. The Fe₃O₄@SiO₂@m-SiO₂ microspheres showed a high sorption capacity of 834.18 mg/g for Cd at pH 6.0 after 20 min of contact. The adsorbent maintained high adsorption capacity during six sorption–desorption cycles ^[39][66]. Antimony adsorption on nano-Fe₃O₄-modified high-iron red mud (HRM@nFe₃O₄) was not dependent on the pH of the solution. The maximum adsorption capacity of Sb(III) on HRM@nFe₃O₄ computed from the Langmuir model was 98.03 mg/g, while both the pseudo-first-order and the pseudo-second-order models were suitable for the explanation of the experimental data. Studying the effect of co-existing ions, it was shown that Na⁺, NH₄⁺, K⁺, Ca²⁺, Mg²⁺, SO₄²⁻, and Cl⁻ did not affect Sb(III) adsorption; however, it was inhibited by the presence of SiO₃²⁻ and PO₄^{3− [40]}[68]. A set of SBA-15 adsorbents functionalized with ethylenediaminetriacetic acid, primary amine, and quaternary ammonium were applied for Cr(III, VI), Mn(II, VII), Pb, Cd, and Cu adsorption. The adsorbent functionalized by ethylenediaminetriacetic acid showed the highest adsorption capacity for the studied metal ions, which amounted to 195.6 mg/g for Pb, 111.2 mg/g for Cd, 57.7 mg/g for Cr(III), 58.7 mg/g for Cu, and 49.4 mg/g for Mn(II). The presence of organic matter and major water cations did not affect the efficiency of the studied metal ions’ removal. The maximum adsorption for all studied cations was attained at pH 4.0–6.0, while elements in anionic form were more efficiently removed in the pH range from 5.0 to 8.0. The efficiency of metals’ removal from artesian, urban river, and lake water was at the level of 96% for all metal ions ^[41][97]. A new Zn-based MOF (IUST-2), showed maximum Pb and Hg adsorption at pH values of 5.0 and 4.0, respectively. The theoretical maximum adsorption of Pb and Hg ions was computed to be 1430 and 900 mg/g, respectively ^[42][57]. The maximum adsorption capacity of the Cu–MOF toward Cd at 219.05 mg/g was attained at pH 4.0, a contact time of 60 min, and an adsorbent dosage of 0.5 g ^[43][82]. Nabipour et al. ^[44][98] discussed in detail the use of MOFs for Cd removal from wastewater, while Shellaiah and Sun ^[45][99] described their application for mercury removal. The guidance for the synthesis of novel MOF adsorbents and their application for U adsorption was provided by ^[46][100]. More examples of mesoporous materials’ applications for metal removal are presented in Table 12.