# A Review of Adsorbents for Heavy Metal Decontamination: Growing Approach to Wastewater Treatment
## 6 Factors Affecting the Adsorption Process
### Aerogels  
[PERSON] used the term \"aerogel\" in 1932 to describe gels in which the liquid has been replaced by a gas [231]. The solid has a very low density, a large active surface area, and a low thermal conductivity. Aerogel is a substance that is made up of 99.8% air. Aerogels feature a porous solid network that contains air pockets, which take up the majority of the material's interior space. The preparation of aerogels consists of three steps: sol-gel reactions, aging, and drying [232; 233]. Carbon aerogels have recently piqued the interest of researchers due to their unique properties. Carbon aerogels can also be utilised to remove organic and inorganic contaminants from water/wastewater due to their unique qualities such as low density, three-dimensional (3D) interconnected porosity, high surface area, and extraordinary wettability [234]. The effect of carbon aerogel on the removal of Cd (II), Pb (II), Hg (II), Cu (II), Ni (II), Mn (II), and Zn (II) from aqueous solution was investigated by [PERSON] et al. [235]. They found that heavy metal ion removal effectiveness is affected by concentration, pH, contact time, adsorbent dosage, and temperature. Their findings revealed that the maximum adsorption capacity of 400 mg/g for Cd (II) and 500 mg/g for Cu (II) was attained at an equilibrium time of 48 h, an optimal pH of 6.0, an adsorbent dosage of 10.0 g/L, a concentration of 3 mg/L, and a temperature range of 20 to 60 degrees celsius. On the other hand, the effects of variable parameters such as agitation time, metal ions concentration, adsorbent dosage, and pH on Hg (II), Pb (II) removal from aqueous solutions were investigated by [PERSON] et al. [236]. [PERSON] and his co-workers studied equilibrium sorption in single, binary, and tertiary systems of three heavy metals Pb (II), Hg(II) and Cd(II). The maximum adsorption capacity, Q\({}_{0}\) (mg/g), for adsorption of Pb (II), Hg(II), and Cd (II) on the CA in a mono-component system was 34.72, 34.9, and 15.53, respectively, according to the Langmuir isotherm. They used the Langmuir isotherm to simulate the adsorption behavior of metal ions in binary and ternary systems [Cd (II) + Pb (II)], [Pb (II) + Hg (II)], [Hg (II) + Cd (II)], and [Pb (II) + Hg (II) + Cd (II)]. They discovered that each metal's maximum adsorption in binary and ternary systems is lower than in mono-component systems [237]. [PERSON] et al. [238] employed carbon aerogel for the adsorptive removal of inorganic chemicals, particularly Cd (II) metal ions, in another work. The maximum adsorption capacity (Q\({}_{0}\)) determined for Cd (II) in the adsorption isotherm using the Langmuir equation was 15.53 mg/g. Microcrystalline cellulose was used as starting material for the preparation of cellulose-based aerogel using hydrothermal carbonisation. For the metal ions Cr (VI) and Pb (II), the adsorption capacity was determined to be 68 mg/g and 240 mg/g, respectively. [PERSON] et al. [239] employed rescorcinol-formaldehyde (RF) aerogels modified with amine groups to adsorb Pb (II), Hg (II), and Cd (II) ions from aqueous solutions. Using the batch adsorption approach, they investigated the effects of contact time, solution pH, temperature, adsorption concentration, and initial metal concentration. After one hour, the highest adsorption capacity of metal ions Pb (II), Hg (II), and Cd (II) was discovered at pH 6, 6, and 5, respectively. For Hg (II), Pb (II), and Cd (II), the greatest value of the Langmuir maximum adsorption capacity (Q\({}_{\text{max}}\)) was determined to be 158.73 mg/g, 156.25 mg/g, and 151.52 mg/g, respectively. [PERSON] et al. [240] studied the effect of pyrolytic temperature on the sorption ability of Cu(II) ions in carbon xerogels based on 3-aminophenolformaldehyde polymer. They provided a reasonable explanation for how pyrolytic temperature affects Cu (II) sorption.